After a week in San Diego, Las Vegas seemed a strange old world.
A new location, a new lease on life.
Three tepid years in Las Vegas left many observers wondering if, when it came to the IPC Apex Expo trade show, Elvis had left the building.
But the move to the San Diego Convention Center was a revelation. It was as if all it took was a little sunshine (and a few fewer slot machines) to breathe new life into the doddering show.
From the opening bell, attendees swarmed keynote speaker William Shatner, some sporting Trekkie gear and other paraphernalia as they lined up by the hundreds to get photos and autographs from the famed Star Trek actor.
On the show floor, the vibe carried over, with attendance and attitudes perked up. Was it the improving economy, a different mix of attendees, or the new site? Hard to say, but exhibitors weren’t complaining.
Attendance seemed strongest the first day, and the momentum carried over through the middle of the second day. Things slowed a bit that afternoon, but picked up at the show-floor reception. Day 3, as usual, crawled.
We noticed the show itself was not as well organized as in the past. Previous shows grouped fabrication suppliers more or less in one place and assembly suppliers in another. That all changed this year. While industries (fabrication, assembly) were delineated (sort of) by the color of carpet in the various exhibitors’ booths, there was a nearly random nature that made trying to visit all the suppliers of a certain product inconvenient. Having the show split across two floors didn’t help either. Most of the products shown at Apex Expo were introduced at Productronica and have already been covered by this magazine both in print and online; we will focus here only on new releases, with a couple exceptions as noted.
The most interesting developments included a new inline printed circuit board assembly cleaner from Aqueous Technologies. Named Typhoon, the short (12' linear length) machine is considered safer than many traditional models, thanks to magnetically conductive pumps that permit it to run (at reduced cleaning performance, of course) up to 36 hr. without water, whereas most models’ pumps freeze up after a few minutes under such conditions. The machine features rotating air knives, a 24" wide conveyor, and zero or filter discharge.
Speaking of cleaning, we also viewed our first full demo of Speedline’s new Aquastorm batch cleaner. Speedline’s engineers came up with the novel idea of firing water at the board straight from the rack’s spindles, providing ample power, always the bane of batch systems.
Two companies (SMT and A-Tec) are now supplying inline convection reflow ovens containing vacuums, which are said to help reduce voiding. Matt Holzmann at Christopher Associates proudly displayed the MEK S1 SPI, reportedly with 50µm resolution and 5D vision, claimed to be the fastest SPI on the market.
At Juki, the biggest news came just ahead of the show opening, when the placement company inked a deal to distribute JT’s reflow soldering equipment in the Americas. Now armed with a printer (courtesy of January’s deal with GKG), and a complete line of soldering machines, Juki seems armed to do battle in the full-line fight. (Heller, with which Juki had previously teamed on an informal basis, seems the odd man out.) CEO Bob Black also privately showed a startling development that, while we cannot yet reveal, is likely to shake up the process equipment market down the road.
Yamaha made for an interesting presence, with a large booth and several machines, not all of which were for placement. The company, which is slowly separating from its longtime Americas distributor Assembléon, also had an AXI machine on display. (Assembléon, we should note, did pick up an NPI Award for its new iFlex placement platform, debuted at Productronica, as well as a Service Excellence Award – its tenth in a row.)
Several AOI OEMs, Koh Young, Mirtec, CyberOptics and Viscom among them, are doing a much improved job of tying SPI and post-reflow inspection data to the screen printer. The efforts in this area to make the data captured during inspection meaningful for in-process production were noticeable.
Fears of a lingering slowdown seemed overstated: Soldering oven manufacturers such as BTU and Speedline Technologies, as well as CyberOptics and BPM Microsystems, indicated strong demand, especially since January. Automotive seems to be driving much of the recovery. BTU is planning an expansion at its Shanghai-based campus.
Aegis Industrial Software has a slick new tool called inForce, a touchscreen and I/O unit that monitors and controls boards as they proceed down the conveyor, and feeds the data back to the Aegis MES factory software, aiding traceability.
We spent considerable time with the materials vendors. AIM noted sales were up 41% year-over-year in 2011, while Cookson saw sales at its Alpha unit rise about 18%. (Raw metals price inflation contributed strongly to the increases.) AIM will open a plant in Poland for blending powder, liquid flux and bar solder, and plans to expand its Montreal and Juarez operations as well. Henkel rolled out four products, including underfill brands for jetting (seeing more of that, too) and traditional dispensing. Many materials companies noted the huge potential in the LED market, although one suggested some lower-energy, higher-brightness alternatives such as ITO should not be ruled out.
There’s some disagreement as to how much the conversation over low-silver solders will translate into actual use, however. Some vendors said that low-silver is starting to transition from the discussion phase into actual production. However, other vendors say it’s mostly hype, adding that the cost-savings tradeoff versus the amount of process development work that needs to go into characterizing each alloy makes change questionable at best. We also are hearing SAC 305 is losing popularity. An iNEMI project team led by Dr. Greg Henshall of HP is evaluating 16 different alternate solder materials; it could be that once that group has completed its work, the picture will become clearer for users.
The EMS companies exhibiting, including STI Electronics and Divsys, were generally bullish in 2012, saying most of the pain from military spending delays should be worked through by the second quarter.
Apex Expo is a misnomer: The “Expo” part is a throwback to when the show was aimed at board fabricators. Those days went the way of Gene Roddenberry. Finding fabrication equipment on the show floor is unusual, and vendors said new sales are going to replace old machines, not to add capacity. Dow, Oak-Mitsui, RBP Chemical, Ventec, Park Electrochemical and Rogers were among the materials companies with new or soon-to-be released products. Isola revealed it completed the beta testing and internal qualification for I-Speed, a Pb-free compatible, low-loss, high-speed digital laminate. The company has also teamed with Circuit Foil on Ultrathin, a 40µm glass-reinforced laminate said to eliminate the thermo-mechanical mismatch issues commonly associated with unreinforced films. The Tg tops 170⁰C, and Young’s modulus and CTE are said to be similar to high-Tg systems.
LPKF has a new, versatile UV laser (Protolaser U3) said to be for depaneling, drilling, marking and surface etching on FR-4, ceramic or other high-frequency RF substrates. PCB cleaning equipment OEM Teknek will double its headcount in Asia and has added a small number of staff in Germany as well.
We had a nice long talk (for the show) with iNEMI president Bill Bader, who provided a rundown on the consortium’s programs – and there are a lot of them. iNEMI officially launched three new projects in medical in January, with three more in “definition.” UL Research is leading a project to update its standards and specifications methodology. Also, a white paper on harmonization of environmental data management is underway.
On the move. At Universal Instruments, George Westby has semi-retired from the UIC Consortium, and Dave Vicara has been named to run the lab. Also, Jeff Mogensen, last seen with Speedline, has joined Parmi, and his former colleague Greg Lefevbre has joined Cardinal Circuit. And another Speedline alum, Shean Dalton, has launched his own rep firm.
Brian O’Leary, formerly of KIC, has joined Trans-Tec (Yamaha’s distributor in the Americas) as general manager, and Lino D’Andretti has joined him in sales. Seho appointed Alexander Riedel director of customer service, and Europlacer named Chris Round global marketing manager. Rogers named Jeffrey Grudzien vice president of its Advanced Circuit Materials division, replacing Michael Bessette, who is retiring after 37 years with the company.
IPC has narrowed its search for a new president to three candidates, none of whom is reportedly from the IPC staff. All have association backgrounds, but some are said to have various degrees of indirect industry experience. A decision is expected in the next few weeks. (Ed: Update: http://www.circuitsassembly.com/cms/news/12678-ipc-names-mitchell-president-and-ceo)
Longtime MacDermid engineer Denny Fritz was inducted to the IPC Hall of Fame, and Dr. William Coleman of PhotoStencil and Mike Bixenman of Kyzen received the President’s Award for their contributions to the organization.
Overheard. Spokesmen from Nordson Dage and OK International acknowledged their respective companies are seeking acquisitions in the coming quarters.
A bid has been made for a major ($500 million-plus) publicly traded US-based EMS company, but so far the company’s asking price is higher than market value.
There was some chatter that the US Department of Defense is considering lifting the ITAR ban on PCBs, which would dramatically shake up (read: eviscerate) the US PCB market. TTM, the largest PCB supplier to the Pentagon, and others are said to be pushing back on the DoD.
All in all, a good week. The show remains in San Diego next year, then back to Las Vegas. So much for boldly going where no one has gone before.
In San Diego last month, CIRCUITS ASSEMBLY revealed the winners of its 20th annual Service Excellence Awards at IPC Apex Expo. Electronics manufacturing services providers and electronics assembly equipment, materials and software suppliers were recognized for top-notch customer service. Each firm’s own customers rated them in an online survey process during the months leading up to the trade show.
EMS winners with the highest overall ratings included EPIC Technologies in the large company category (revenues between $101 million and $500 million), Applied Technical Services in the medium company category (revenues between $20 million and $100 million), and Burton Industries in the small company category (revenues under $20 million).
EMS companies with the highest scores in each of five individual service categories were honored as well. (Overall winners were excluded from winning individual categories.) In the large company category, last year’s overall winner Mack Technologies swept all five individual awards (dependability/timely delivery, manufacturing quality, value for the price, responsiveness, and technology).
Electronics Systems took first place in dependability and value, and tied ACD for responsiveness in the medium company category. ACD also scored the technology award. Western Electronics took home the quality top prize.
In the small company category, Spectrum Assembly took top honors for dependability and responsiveness, and tied Accu-Sembly for first place in quality. Accu-Sembly also won for technology and value.
Electronics assembly equipment award winners were Assembléon America for pick-and-place; DEK International for screen printing; Kyzen Corp. for cleaning/processing materials; Nordson YesTech for test and inspection; Nordson EFD for materials; Nordson Asymtek for dispensing, and KIC for soldering equipment. Aegis Industrial Software received top honors in the automation/manufacturing software category.
CIRCUITS ASSEMBLY and PCD&F also announced the winners of the 2012 New Product Introduction Award for electronics assembly equipment, materials, software, and PCB fabrication at the same event, held on the show floor.
The NPI Award, in its fifth year, recognizes leading new products during the past 12 months. An independent panel of practicing industry engineers selected the recipients.
The winners included Cognex in the process control tools category for the DataMan 500 and Speedprint in the screen/stencil printing equipment category for Speedprint SP700avi. Seika Machinery won the screen/stencil printing peripherals prize for the Sawa Ultrasonic Stencil Cleaner SC-AH100F-LV Low-VOC Model. Aqueous Technologies was honored for cleaning equipment with its Trident XLD, while Data I/O took home an award for device programming with its RoadRunner 3 with FIS.
For dispensing equipment, GPD Global took top honors for PCD Dispensing on MAX series platform. CyberOptics scored the number one spot for AOI test & inspection with the QX100. ICT test & inspection went to Datest for the SPEA 4060 Flying Probe Tester with Goepel Boundary Scan. Test & inspection – functional test was awarded to Agilent Technologies for TS-8900. ViTrox Technologies won for its V810 In-Line 3D AXI in the AXI test & inspection category.
AIM was honored for soldering materials with NC259 solder paste, and LPKF Laser & Electronics was awarded for its LPKF MicroLine 1120 P in the automation tools category. Multifunction component placement went to Assembléon America for iFlex, and Juki snagged the award for high-speed component placement for its Sentry tool. Speedline Technologies’ Closed Loop Nitrogen Control was honored for reflow soldering, and Seho Systems’ AOI system to be embedded in a selective soldering machine was recognized for selective soldering.
Kyzen’s Aquanox A4638 took top prize for cleaning materials, and Cogiscan’s LabelScan Automated Vision System was number one in labeling equipment.
Juki’s second award was for production software for its Juki IS NPI+ Bundle. Microscan was awarded in the process control software category for its AutoVISION machine vision software.
In the soldering – other category, EVS was given a plaque for its EVS 7000LFHS solder recovery system redesign. Nihon Superior was honored in the cored wire category for (SN100C (551CT)) lead-free flux-cored solder wire, while Semblant won for its SPF (Semblant Plasma Finish) in the coatings/encapsulants category.
Christopher Associates took home a plaque for the Magnus HD Trend in the rework & repair tools category, and KIC was honored in wave soldering for its KIC 24/7 Wave.
Finally, for surface treatment, Dow Electronic Materials won for its Circuposit Hole Prep 4126 Sweller.
Congratulations to all of 2012’s honorees. Join us again next year, when IPC Apex Expo returns to San Diego, where we’ll recognize a new batch of industry standouts during our 2013 event. Check back to circuitsassembly.com and pcdandf.com for more details soon.
A 30-year-old idea is coming in vogue, with stunning implications for design engineering.
“3D printing is going to play an increasing role in the future of electronics,” says David ten Have. “I’m not predicting an immediate revolution, but printed electronics will start to fill various niche applications, and that’s going to lead to some ground-breaking technologies.”
Ten Have is CEO and cofounder of Ponoko, the company behind Personal Factory, a web-based platform for the creation of custom goods. Ponoko users can make their own products using a variety of digital manufacturing methods, including laser-cutting, CNC routing, and 3D printing. More than 100,000 consumer-designed products have been made, and demand for 3D printing is on the rise.
3D printing is an additive manufacturing technique that translates digital models into physical objects by building up layers of material. A number of competing 3D printing technologies variously use chemical-, heat- or light-based methods to fuse material together layer-by-layer. Resolution depends on the printer type, but some of the more advanced machines can achieve a resolution in the micron range. For instance, the Objet Connex desktop printer has a minimum layer thickness of 16µm.
While many of these technologies have been in use since the 1980s, 3D printing has become much more accessible in recent years as a result of increased functionality and decreased cost. Companies like Ponoko, Sculpteo and Shapeways have emerged to provide online 3D printing services. Open source hardware projects like the MakerBot, RepRap, and Fab@Home have brought 3D printing technology within reach of the home hobbyist, with kit sets available for under $1,000. These machines in particular have been enthusiastically embraced by the DIY electronics communities.
There are two distinct advantages of using 3D printing over traditional manufacturing that could lead to some dramatic changes in many areas of industry: The first is it undermines economies of scale; it is now possible to make one-off objects that would otherwise have required mass-production. The implications both for prototyping and customization are profound. The second advantage is it makes it possible to produce objects that could not be made in any other way, as a result of complexity or internal features. Complex assemblies of moving parts can be designed to come out of the machine preassembled.
3D printing circuit boards. PCB production is an application for 3D printing being pursued both by industry and hobbyists. Developments are primarily focused on material research at present. 3D printing has been demonstrated with many different kinds of metals, plastics, ceramics, polymers and organic materials. With the right combination of materials, modern multi-material printers would have no trouble printing PCBs. Being able to produce high-quality, multilayered one-off boards would be a huge advantage for design engineers and home hobbyists alike, greatly simplifying the prototyping cycle and removing the time and material expense of masking and etching.
Hobbyist-led advancements. In the hobbyist realm, early signs of development in this area came in 2009 when Bath University mechanical engineering student Rhys Jones revived the old idea of making PCBs by depositing metal into pre-formed channels. He modified his RepRap 3D printer to extrude molten solder into a pre-printed ABS plastic substrate. While the melting point of solder is higher than that of ABS, the specific heat of the metal is so low that the plastic doesn’t melt. After manual assembly, the completed circuit was then installed on the actual machine that printed it. This was an early milestone for the RepRap project, whose stated aim is to create self-replicating machines.
While this process is still in its infancy, it could soon prove to be popular with home 3D printing enthusiasts that want basic custom circuit boards, without dealing with the hit-or-miss home PCB etching process. Another bonus to 3D printed circuit boards is the integration of mechanical features. For example, it would be elementary to include mounting clips in the design.
Another open source project, Fab@Home, has demonstrated the use of conductive and non-conductive silicone to print flexible objects with internally embedded circuitry laid out in three dimensions.
The rise of DIY electronics. Hobbyist 3D printing and DIY electronics are two major forces democratizing technology, and their development is entwined. In 2005, the launch of yet another open source project, the Arduino, marked a turning point in the world of hobby electronics. Arduino rapidly gained a reputation as the first truly user-friendly microcontroller development platform.
This platform opened up the world of embedded computing to a much larger audience, and it remains popular among artists, educators, and hobbyists, with an ever-increasing number of extension modules (e.g., sensors, driver boards, communication interfaces, etc.) available. These modules, known as “shields,” take the form of traditional PCBs that can be plugged into the Arduino board with stacking headers.
3D printing creates the possibility for these modules to take any shape, a feature that could further functionality and user-friendliness of these shields. For instance, one can imagine a modular robotics system with an Arduino processor embedded into a printed “torso,” a camera or rangefinder embedded into a printed “head,” and interchangeable “limbs” containing other sensors and actuators, with all associated electronics integrated seamlessly into each piece.
With the increasing popularity of DIY electronics and 3D printing, it is fair to assume that progress will continue to empower the home hobbyist with ever more capable and user-friendly fabrication techniques. One immediate area of development to look out for is a standardized file-format – a common protocol for defining designs. This will be the precursor to any serious activity in the open source community.
Industry-led advancements. Looking to the industrial realm, Dutch research institute TNO (tno.nl) has pioneered a number of different circuit printing technologies. It recently printed a simple circuit in copper, along with flexible integrated housing, all in one step. This hints at another exciting and disruptive possibility of 3D printing; it allows the designer to do away with the circuit board altogether, integrating the electronics with other functional parts of the product.
As these technologies become established, 3D printed circuits could lead to tearing down several design constraints. For starters, there’s no reason a 3D-printed board would have to be a typical rectilinear planar surface (Figure 1). Disrupting the 2D PCB design paradigm could have some radical implications for design engineering. Traditional flat PCBs force design constraints on the layout of components and inefficiencies on space and material usage, constraints that will disappear when the PCB can take any conceivable shape.
Yet another benefit of using a 3D printer to build circuit boards is that many types of printers share most of their mechanical characteristics with a pick-and-place component assembly machine, meaning an all-in-one PCB printing and assembly unit could be just around the corner.
Beyond simply changing the way we make circuit boards, developments in printing technologies are set to potentially revolutionize the production of all kinds of electronic components. 3D printer manufacturing firm Objet recently mused on its company blog about the likelihood of printing both circuit boards and integrated semiconductor components at a point “not too far in the future.”
Academia-led advancements. Some of the bleeding-edge technologies at the R&D stage today indicate some very exciting developments for printed electronics in the longer term.
Printed solar panels are flexible and significantly lighter and cheaper than their traditional clunky glass-mounted counterparts. Developments in this field over the last few years have substantially reduced the cost, with the Holy Grail being small, disposable power for applications such as smart packaging. Researchers at MIT1 recently demonstrated a highly flexible and robust solar panel printed on paper “almost as cheaply and easily as printing a photo on your inkjet.”
This comes in the wake of Stanford University’s research in the field of nano-material technology, proving it is possible to store energy in super-capacitors made by printing carbon nanotubes onto treated paper.2
The flexibility of printed electronics has the potential to be hugely influential in other applications, especially mobile devices where a printed flexible OLED display could be unfolded to a size larger than a handset. Flexible interactive screens are in the prototype stage, and printed e-ink displays already have been demonstrated playing color video on a paper-thin plastic substrate.
Other recent advancements include printed sensors, transistors, and low-density data storage, meaning entire consumer devices could one day be conceivably printed in one continuous process on a single machine. Aggressive research efforts in industry, academia, and the growing community of sophisticated hobbyists make the field of 3D printed electronics a fascinating and important one to keep an eye on.
References
1. Miles C. Barr, et al, “Direct Monolithic Integration of Organic Photovoltaic Circuits on Unmodified Paper,” Advanced Materials, vol. 23, no. 31, Aug. 16, 2011. 2. Katherine Bourzac, “Print on Demand Power, MIT Technology Review, April 27, 2009.
Richard D. Bartlett is an engineer, artist, writer and practicing futurist based in Aotearoa, New Zealand.
A rundown of electrical and physical considerations.
In my previous columns, I’ve stressed the importance of the partnership between OEMs (or ODMs) and flex manufacturers throughout the product lifecycle, to properly apply flex technology. A good flex design has the right blend of performance, manufacturability, and cost: three factors that ultimately impact the success of a product launch.
This month, I focus on how and when OEMs and flex manufacturers can use material selection to optimize design for performance. Material selection plays a critical part in improving manufacturability and offering performance gains. Material selection begins with a thorough understanding of the application requirements, including the components to be used and the physical constraints for the flex assembly.
Flex design engineers, mechanical engineers and electrical engineers determine the required outline, stackup and layer construction to achieve the necessary connectivity of the flex circuit. To illustrate the different stackup options, Figures 1 to 3 show available choices for a two-layer flex circuit, depending on bending requirements. After engineers determine the stackup and layer construction, the flex manufacturer can start selecting materials to meet the requirements for bending, flexibility, dimensional stability, components, circuit density, impedance characteristics, electrical properties, thermal resistance, tear resistance, low moisture absorption and chemical resistance.
Simulation analysis and empirical knowledge enable correct material selection to achieve optimum mechanical and electrical performance. Simulations also offer fast, thorough insight into the potential conflicts during component assembly. OEM engineering teams should engage with their flex manufacturer/assembler to review simulation data, to make better decisions on the tradeoffs between cost, performance and manufacturability. Different materials available for flex circuits are:
Dielectrics. Dielectrics for flex circuits (compared to glass-reinforced dielectrics for rigid PCBs) are characterized by their thinness, flexibility, mechanical strength, thermal properties, and dielectric properties. The suitable dielectric film depends on operating environment, electrical and mechanical properties, assembly requirements and cost. Polyimide films are typically used for applications requiring high tensile strength and excellent flexibility. Polyester films are used for lower-cost applications. High-performance fluoropolymer films offer superior electrical properties and are considered for low insertion-loss requirements for high-speed applications. The thickness of dielectric materials commonly ranges from 12 to 50µm. Dielectrics can be used as a base laminate and a coverlayer when combined with adhesive.
Conductors. Choosing a conductor material depends largely on the material’s performance during specific applications. In particular, dynamic applications require the right choice of copper type and thickness. Commonly used copper thicknesses are 12µm, 18µm and 35µm. Electrodeposited copper, rolled annealed (RA) copper and high tensile elongation (HTE) copper are commonly used. While the most common solution for dynamic applications is HTE, RA copper is still in wide use. In addition to copper, other conductive materials include copper-nickel alloys and conductive inks.
Solder masks. Liquid photoimageable (LPI) solder masks are solder resist inks applied onto a flex circuit, then cured with a thermal or ultraviolet process. The result is a permanent, durable insulation. The preferred insulation material is coverlayer. In case of small component areas, solder mask may be required for insulation.
EMI shielding. EMI shielding protects against electromagnetic inference. Copper layers provide the best shielding effect, while flexible silver epoxy paste is the next best option. Silver or aluminum film materials 9 and 22µm thick are commonly available shielding solutions for most applications.
Stiffeners. Stiffeners are used to reinforce flex circuits under areas where components are assembled. When selecting a stiffener, consider overall thickness, thermal properties, shielding, adhesion method and cost. Another consideration is the process to singulate the stiffener to the required shape, and the process needed to attach it. Common materials include polyimide, polyester, FR-4, metal and plastic.
Adhesives. Adhesives fall into three groups: thermal set adhesives create permanently bonded laminates; pressure-sensitive adhesives bond stiffeners or mounting hardware to flex, and conductive adhesives connect shielding stiffeners and flex to various structural components inside a device.
For instance, the outlined approach can be used to select the right materials for a flex circuit to meet dynamic bending requirements to exceed 100,000 cycles. Simulation analysis shows HTE copper on 12µm polyimide dielectric adhesiveless laminate with epoxy adhesive coverlayer provides best results. Copper thickness depends on the fabricator’s ability to meet the characteristic impedance and insertion loss requirements. Simulation analysis also shows the impact of solid ground versus hatch ground to meet the impedance requirements. To meet the dynamic bending requirements, shielding film is the preferred option. Shielding film thickness depends on the flex manufacturer’s capabilities. Even though this example is for a consumer electronics product, it can be easily adapted to other products in any other market.
Designing flex circuits is a collaborative process. The above methodology is one approach for flex manufacturers and assemblers to effectively guide designers toward structures and circuit layout using the right materials to optimize design for performance and improve new product time to market by decreasing ESI and NPI times.
Jay Desai is director of marketing at MFLEX (mflex.com); This email address is being protected from spambots. You need JavaScript enabled to view it..
Pb-free alloys can shorten tip life – but so can poor practice.
During the introduction of Pb-free, solder iron tip damage was very common due to the variability of the plating on some tips. Damage also has been caused by poor control and incomplete operator training; some operators do not recognize the impact of the solder alloy and are reluctant to switch off irons when not in use. Figure 1 shows erosion of the copper core due to lack of protective plating, which may have become damaged due to incorrect use of the iron tip.
If the base material of any soldering tip on irons or desolder stations is exposed to high temperatures and alloys prone to dissolution, this can still occur. If tips are being consumed faster than expected, examine the tip quality. Examine samples before and after use to ascertain the impact on soldering and wetting.
These are typical defects shown in the National Physical Laboratory’s interactive assembly and soldering defects database. The database (http://defectsdatabase.npl.co.uk), available to all this publication’s readers, allows engineers to search and view countless defects and solutions, or to submit defects online. To complement the defect of the month, NPL features the “Defect Video of the Month,” presented online by Bob Willis. This describes over 20 different failure modes, many with video examples of the defect occurring in real time.
Chris Hunt is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); This email address is being protected from spambots. You need JavaScript enabled to view it.. His column appears monthly.
New systems are more robust, faster curing and require less energy than conventional chemistries.
With each passing day, use of silicone rubber is becoming more and more pervasive; products made from silicone are showing up in hospitals, cars, the kitchen and bath, even golf bags. Flexible bakeware and cooking utensils, fuel-resistant hoses and gaskets, airbags and touch pads, shampoos, skin care products and much more are all made possible with new silicone technologies. Dr. Fredrick S. Kipping, the father of silicone chemistry, could never have envisioned the plethora of uses into which his “sticky mess” has grown. From the earliest days of commercialization, silicones have always been selected for any high-performance specialty application that requires durability and outstanding physical properties.
Silicone chemistry is a unique combination: neither purely organic (i.e., carbon-based) nor purely inorganic (i.e., silicon-based), but a molecular-level hybrid of both. The nature of the Si-O bond in the polymer backbone creates low rotation barriers and large bond energies. This inorganic polysiloxane backbone provides the foundation for building materials with superior thermal properties, environmental resistance and flexibility, even at temperatures below -70oC. The methyl groups pendant to the Si-O-Si chain provide for low surface energies, and the low rotation barrier along the backbone permits the polymer to freely orient these groups to the exposed surfaces. The hydrophobic character and soft-touch feel of silicone-based materials is a direct result of this combination.
Silicone elastomers are used as sealants, adhesives and coatings, where they are fluid-applied and cured in place, but may be also molded into a multitude of shapes and used to produce functional products in a range of applications. There are two predominate cure types: condensation-cure and addition-cure. Condensation cure products may be either single-component or dual-component, generally use tin catalysts, require moisture to react and liberate alcohol or other monomers as byproducts during cure. Addition cure materials may also be either single-component or dual-component, are either free radical cured or rely on precious metal catalysts, but perhaps the biggest distinction is that there are no byproducts generated during cure.
Certainly nearly every homeowner is familiar with single-component, condensation cure silicones, perhaps even using some to caulk the bathtub or kitchen sink: the familiar smell of vinegar as acetic acid is liberated during the condensation reaction. Single component, moisture cure, RTV silicones (room temperature vulcanizing) are valued for their ease of use, adhesion, physical properties and environmental resistance. The products are manufactured under dry conditions and stored in moisture-proof packaging. Once applied, the material draws humidity from the atmosphere and begins to cure. The cure proceeds from the outside inward, with the rate of cure determined by the amount of catalyst in the formula, but more important, by the amount of moisture available. The byproduct liberated is dependent on the leaving group of the cross-linker used in each specific formulation (Figure 1).
Within electronics, moisture-cure RTVs are often employed as staking compounds, where vibration dampening and stress relaxation are important. These types of staking compounds are most often shear-thinning pastes that, once applied, resist flow and may be used to build up a protective barrier around sensitive components. When fully cured, these materials become relatively soft (i.e., Shore A 25-45), durable, low modulus rubbers. Many silicone conformal coatings are also based on this type of chemistry. The advantages are ease of application, solvent-free formulations and environmental resistance of the cured films. Silicone conformal coatings are most widely used in high-temperature environments, making them the primary choice for under-the-hood automotive applications. However, because moisture from the atmosphere is needed to catalyze the cure single-component, RTVs must only be applied in thin cross-sections, typically less than 0.5", and the assembly must be racked overnight to permit slower cure speeds.
Less familiar, but no less functional, are the dual-component condensation cured silicones where the cross linker and catalyst are contained in separate packages that must be mixed prior to application. Here, the moisture is an integral part of one component, making these formulas much less sensitive to environmental conditions. Also, because the reaction is not dependent on the transport of moisture from the atmosphere, these materials are able to cure in thicker cross-sections. However, a byproduct is still generated during cure, so shrinkage is relatively high.
High consistency silicone rubber (HCR) compounds may be processed by injection molding, compression molding, and extrusion or calendaring. These are the original single-component, addition cure rubbers: typically, ready-to-use blends of silicone rubber with fillers, modifiers and vulcanizing agents that are heat-cured to form elastomeric components. The vulcanizing agents are generally peroxides that break down with heat, generating free radicals that initiate the cross-linking reactions. Liquid silicone rubbers (LSR) are also based on addition cure chemistry. Generally, two-part pumpable materials that must be mixed and often heat-cured to form elastomers are suited for intricate designs manufactured in large, automated quantities. LSRs rely on precious metal catalysts, usually platinum, to promote cross linking. As with the condensation cure, two-component formulations, the catalyst and cross linker are contained in separate packages that must be mixed prior to application. LSRs and fully-fluorinated LSRs currently represent the fastest growing branches of the silicone family tree.
Platinum-catalyzed, addition-cure silicones may be formulated with an extremely wide range of physical properties and cure characteristics, which makes these materials very popular. One of the most exciting developments is the introduction of low viscosity, optically clear potting and encapsulating compounds. LEDs, photovoltaic subassemblies and other light-sensitive devices require radiation-resistant, non-yellowing encapsulants for protection and improved light transmission. For radiation energy to initiate any chemical changes, causing a breakdown in properties and yellowing, the polymer molecules must first absorb it. Because of the absence of any double-bonds or other ultraviolet (UV) light-absorbing groups polydimethyl siloxane-based, addition-cured silicones absorb very little ultraviolet radiation in the 300-400nm region. As mentioned, the bond strength of silicon-oxygen linkages in the polymer chain is exceptionally high, which prevents oxidation and loss of properties.
In the past, there were concerns that silicone oil migration or bleed out would contaminate sensitive electronic components. Early formulations used silicone polymers that were not stripped of low molecular weight contaminants, and electronic devices relied heavily on mechanical switches. These low molecular weight oligomers could volatize and condense on switches; because silicones are excellent dielectric materials, this could cause poor contact and device failure. Today, silicone polymers go through a much more extensive stripping process to remove and control contaminants. Device design also has improved, so the technology is not as susceptible to any potential contamination.
Heat cure, moisture cure, mixing, ovens: All are fine and effective means for curing products to generate pieces and parts for a great many applications. The real interest and some of the fascinating performance advances are in the field of UV-curing silicone rubbers, with the advantages of speed, ease of application and energy efficiencies associated with traditional UV curing to produce elastomeric materials with all of the performance enhancements of silicone.
The concept of UV curing silicones is not new. Acrylic end-capped, free-radical cure silicones were first commercialized in the early 80s. While they are technically UV cure, these dual-cure systems rely quite heavily on the traditional moisture reaction to affect full cure. The acrylated-silicones are sensitive to oxygen inhibition, require fairly significant energy input to initiate the reaction, and need prolonged exposures to complete the cure. Perhaps because they are not so robust, these materials have not enjoyed the market success of hydrocarbon-based UV cure technologies.
The thiolene “click” reaction is very powerful chemistry. Click chemistry is not one specific reaction, but more of a concept or a philosophy introduced in 2001 by K. Barry Sharpless, Ph.D. of the Scripps Research Institute.1 By definition, click reactions are simple and robust. They use only readily available starting materials, require no solvent, or chemically benign solvents (e.g., water), and proceed to high conversions. Applying these concepts to polymerization reactions yields materials that are extremely dynamic. Thiolene chemistry is the reaction between thiol groups and vinyl functionality. If the molecules involved contain higher functionality (i.e., F(x) >/= 2), then the reaction will produce polymeric materials. The process is normally photoinitiated and proceeds via a very rapid step-growth mechanism; an idealized outline of the initiation – propagation – chain-transfer reactions is shown in Figure 2.
In addition to the polymerization reactions discussed earlier, the thiolene reaction may also be used as a cross-linking mechanism. High molecular weight, vinyl functional polymers may be cured with short-chain, or monomeric, multi-functional thiolene curing agents via the same photoinitiated reaction. It is essentially an addition reaction, so no byproducts are produced, and shrinkage is low. There are hundreds of vinyl and thiol combinations. Vinyl functional polydimethylsiloxanes are abundant and readily available, and there is a fairly wide selection of short-chain mercapto-functional polysiloxanes. This combination may be employed to produce UV-cured silicones.2,3 These systems were originally investigated as possible release agents for paper coatings.
This chemistry has been extended to include a secondary moisture reaction for curing in shadow areas and to further increase cure strength, adhesion and system dynamics.4 Clear materials based on this new system are much more robust and cure to greater depths with much lower energy requirements than conventional acrylated silicones. This translates to increased processing speeds and faster turnaround. Essential for protecting electronic circuitry and significantly extending the service life of printed circuit boards, conformal coatings are an integral component of the entire sub-assembly. Eliminating moisture and contamination is a key element for protecting sensitive electronics. Silicone conformal coatings provide an effective barrier, even under the most severe service conditions. Now UV-cured conformal coatings that offer all the enhanced performance characteristics of conventional silicone-based materials, but with higher processing speeds, are available. Perhaps more important, these new UV-cure, silicone resins may be filled, which means pigmented and even electrically conductive UV-cure silicone products are now possible. Flexible and printed electronics (FPE) incorporate several new technologies and emerging processes and materials across a variety of applications. FPE may be twisted, bent or shaped without damage, enabling endless innovation and unlimited possibilities.
When an application requires performance, durability and physical properties, a silicone-based product is the superior choice. No other chemistry provides the environmental resistance and performance at thermal extremes. While it is true that silicones are critical for applications under the most severe and harsh conditions, it is also true that the enhanced adhesion, increased flexibility and UV and moisture resistance are important for many less-demanding applications.
References
1. H. C. Kolb, M. G. Finn and K. B. Sharpless, “Click Chemistry: Diverse Chemical Function from a Few Good Reactions,” Angewandte Chemie International Edition 40 (11): 2004–2021, 2001. 2. Lee, et al., U.S. Patents Nos. 4,946,874 (1990) and 5,124,212 (1992), Dow Corning Corp. 3. Viventi, U.S. Patent No. 3,816,282 (1974), General Electric Co. 4. Chambers, et al., U.S. Patent No. 7,105,584 (2006), Novagard Solutions.
Building resistive metal foils on a flex circuit to create heat.
A customer asked if a flex circuit could be made using a metal foil other than copper. They needed to heat a non-flat surface, and wanted the contouring abilities of a flex circuit combined with heating qualities of a resistive foil.
The answer is yes, resistive metal foils can be used on a flex circuit to create heat. Most manufacturers that make resistive flex circuits refer to them as flexible heaters. Many resistive foils can be used for this type of application, as can many different types of adhesive and insulation materials. It is important to understand the differences in the materials used to construct the flexible heater, so that you will know how to properly specify your heater for maximum efficiency and reliability, and also for lowest cost.
While many types of resistive metal foils are available for the construction of flexible heaters, the most common types are cupronickel, constantan (very similar to cupronickel), Inconel and aluminum. The best metal foil type for a given application is driven by the resistance density required. This is derived based on operating voltage and the temperature at which the heater will operate.
Metal foil selection is determined by the amount of resistance needed, and also by the area that that resistance needs to cover. Total heater element resistance is driven by the resistivity of the metal that the foil is made from, the metal foil thickness, and the width of the element conductors. Most flexible heaters are designed to cover roughly 50% of the total heated area with metal. This means that if the heating element traces are 0.020" wide, the spaces between element traces should also be 0.020". This will generally provide the best heat distribution and minimizes the chance for “hot spots” or overheating.
Here is a summary of some common foils and their particular characteristics:
Cupronickel. As the name implies, cupronickel is an alloy of copper and nickel. While there are several types of cupronickel available with different ratios of copper and nickel, the most common alloy for flexible heaters is alloy 715. This alloy is 70% copper and 30% nickel. This material processes during manufacturing in a manner similar to copper, and has the relatively low resistivity of 16.22047 µΩ-inch (for reference, copper is 0.661417 µΩ-inch). This alloy is typically used in applications that don’t require a high-resistance density. One advantage of cupronickel is that it is relatively easy to copper plate to this material. This makes it possible to have a flex heater where certain areas (like power leads) will not heat. Plating copper on select areas of the resistive pattern will significantly lower the resistance in that area, resulting in little or no heat generation. Copper plating also makes it possible to make connections between layers using copper-plated through-holes. When using this technique, the resistive foil typically needs to be on an innerlayer with copper pads (and possibly circuitry) on the outer layers. Cupronickel can be soldered to, making it easy to attach lead wires. Cupronickel has a low temperature coefficient of resistance (TCR), so the heater resistance will change very little as the temperature goes up and down. This makes the temperature much easier to control over a wide range.
Constantan. Constantan is a variation of cupronickel with 55% copper and 45% nickel and a resistivity of 19.63495 µΩ-inch. Constantan is typically used in flex circuit applications such as strain gauges and thermocouples. Constantan also has a very low TCR.
Inconel. There are several alloys of Inconel, but all are predominantly nickel, with chromium as a second element. Iron, molybdenum, niobium, cobalt and other metals are used to create the different Inconel alloys. Inconel 600 is probably the most widely used alloy, with a resistivity of 40.6 µΩ-inch. The high resistivity makes this foil ideal for applications that require a high resistance packed into in a small area. This is a very hard foil that is not easily copper-plated. Inconel is also virtually impossible to solder to, which means that lead wires will usually have to be brazed to the heater element. As with the previous foils, Inconel has a low TCR.
Aluminum. Aluminum foil is generally chosen as a heater element material to save money. The resistivity is roughly double that of copper, and like many other pure metals, it has a high TCR. Aluminum also etches very quickly, which makes it difficult for the manufacturer to keep tight resistance control. But, if you are looking to save pennies and unconcerned about tight temperature control, aluminum may be a good choice.
Picking the Correct Adhesive/Insulation System
The choice of insulation and adhesive is primarily driven by the operating environment of the heater (i.e., operating and ambient temperature, chemical contact, etc.), and to a lesser extent cost. Both the insulation and adhesive type must be rated for the temperatures the heater will be operating at, with sufficient margin to account for temperature excursions. Temperature excursions can be caused by variables such as fluctuating supply voltage and elevated ambient temperatures. While there are many types of flexible insulation types, and an equal or greater number of adhesive types that could be used to make a flexible heater, a few cover 95% or more of all applications. For insulations, these include:
Polyimide. Polyimide film can be used in a wide variety of flexible heater applications. In addition to being an extremely good electrical insulator (0.001" polyimide has a dielectric strength rating of 7700V), it has also been used successfully in applications at temperatures as low as -269°C (-452°F) and as high as 400°C (752°F). For this reason, polyimide heaters are in use in very low-temperature space/satellite applications, and also in high-temperature applications such as semiconductor manufacturing. The high dielectric strength of polyimide film permits use of film thicknesses as low as 0.001". This results in extremely fast response time and quick transfer of heat from the heating element to the object being heated (usually referred to as the heatsink). Polyimide film also has very good chemical resistance. While polyimide film excels in most of the physical and electrical properties required for a flexible heater, its downside is cost.
Silicone rubber. Silicone rubber heaters combine high operating temperature with moderate price. The most common silicone rubber substrates used for flexible heaters are reinforced with glass fibers to enhance dimensional stability. Silicone rubber heaters can operate continuously at temperatures exceeding 450°F. There is no separate adhesive system for rubber heaters. The base and cover material is generally supplied to the manufacturer as a laminate with cured rubber on one side and uncured rubber on the other. The etched resistive foil pattern is sandwiched between the two sheets of rubber and then laminated to seal.
Polyester (PET). Polyester film is a good choice if the application does not have high temperature requirements and is cost-sensitive. The maximum operating temperature of polyester is less than 225°F. This limitation makes polyester a very distant third behind polyimide film and silicone rubber in total flex heater usage. The relatively low temperatures tolerated by polyester also require special attention be paid to manufacturing processes that utilize elevated temperatures. Special non-standard adhesives may have to be used to bond the foil element to the polyester base and cover.
For adhesives, these include:
Modified acrylic (film). Modified acrylic adhesive is a popular choice for flexible heaters because it is easy to process, has excellent bond strength, and can operate at temperatures of up to 300°F. It is used almost exclusively in conjunction with polyimide film as the dielectric. When processed with good techniques, modified acrylic adhesive will stick well to just about any smooth, clean surface. For this reason, it can be used with all foil types. This adhesive can also be used to permanently bond a finished heater to a heatsink. It is difficult to use acrylic adhesive with polyester because the temperature needed to cure the adhesive would melt or significantly soften the polyester.
Modified epoxy (film). Modified epoxy adhesive is very similar to modified acrylic, and in most cases, the two adhesive systems are interchangeable. Acrylic and epoxy adhesives look the same, process the same, and perform very closely in the finished product. A notable difference is that epoxy has a very slight edge on temperature and chemical resistance over acrylic. US manufacturers tend to lean toward acrylic adhesive, while Asian manufacturers favor epoxy adhesive. Like acrylic adhesive, epoxy adhesive is difficult to use with polyester and is used mainly in conjunction with polyimide film as the insulator.
Teflon (FEP). Teflon adhesive is used in applications that require the material to tolerate either very high or very low temperatures, or both. While this material can operate from less than -300°F to nearly 400°F, its selection significantly reduces the number of vendors that can build the heater due to the high processing temperatures.
Polyimide adhesive. B-stage polyimide is typically supplied to the flex heater manufacturer in the form of a laminate in which adhesive is coated to one side of a sheet of cured polyimide film. This material is the most expensive of the commonly used adhesives, and is also the most difficult to process due to the high lamination temperatures required for curing of the B-stage adhesive. As with FEP, specifying this material will significantly reduce the supplier base. Once cured, polyimide adhesive can withstand much higher temperatures (up to 500°F) than any of the other adhesives.
Heater Design Considerations
Once the materials to construct the heater have been selected, the next step is to lay out the heater element pattern(s). Most heaters will have a single continuous element that serpentines around the heater area, but some heaters will have multiple elements that can be controlled independently from one another for precise temperature control.
The heater element artwork layout requires the heater designer to fill up the area to be heated with uniform conductor pattern(s) that will yield the necessary resistance. Heater element patterns can also be profiled with smaller traces in areas that require more heat and wider traces in areas requiring less heat. As mentioned, selective areas of the heater element can be plated with copper (on cupronickel and constantan) to significantly reduce the heat generated in those select areas.
Once the heater material selection and heater element layout are complete, manufacturing is relatively straightforward. The flexible heaters are processed much like single-sided flexible circuits. The heater element traces are defined using a photo-etch process, and then the cover insulation is laminated in place with heat and pressure. Prior to laminating the cover insulation, many heaters will have lead wires soldered, welded or brazed to the heater element.
Terminations. On the vast majority of flexible heaters, power is supplied via lead wires that are welded, brazed or soldered to pads on the heater. Connectors can be used in applications where the foil can be soldered (i.e., cupronickel, constantan, etc.). Also, insulation displacement contacts can be a cost-effective and reliable means to bring power to the heater. Because these contacts do not require soldering, they can be used on all types of resistive foil. When using insulation displacement contacts, heater foil thickness should be at least 0.002" for ease of manufacturing.
Mounting to heatsink. The final step to complete the heater assembly is to attach the heater to the item to heat (heatsink). This can be done by either the end-user or by the flex heater manufacturer. Probably the most common adhesive used to mount a heater to a heatsink is an acrylic or silicone-based pressure-sensitive adhesive. It is very important to ensure that the heater is mounted to the heatsink with no voids or air bubbles between the heater and the heated surface. These voids will keep the heat generated by the element from being transferred to the heatsink efficiently. This in turn can cause isolated hot spots in the heater element. If the hot spot is large enough, the resulting temperatures can cause the heater element to burn out like a fuse. For this reason, most heater users prefer to have the flexible heater mounted by the heater manufacturer.
And I would be remiss if I did not mention that the folks that build these products are the experts. They can assist in material/termination selection, element layout, and heater attachment methods. Engaging them early in the design offers the greatest chance of success on a heating project.
Mark Finstad is senior applications engineer at Flexible Circuit Technologies (flexiblecircuit.com); This email address is being protected from spambots. You need JavaScript enabled to view it..
