A review of best-in-class controls for maintaining factory worker health.

The National Institute for Occupational Safety and Health (NIOSH) received a confidential employee request for a Health Hazard Evaluation (HHE) at an electronics manufacturer specializing in printed circuit board fabrication, assembly, and testing for different end-user applications (sidebar). At the time of our evaluation, approximately 2,300 employees worked at the facility in either the medical section plant or the defense and aerospace section. Employees were primarily concerned about exposure to solder paste and fumes, as well as dust and noise. They reported coughs, burning eyes, nosebleeds, voice loss, headaches, sinus infections, bronchitis, and respiratory problems.

At the time of our evaluation, the medical section had four wave soldering lines and eight surface mount lines. Wave solder lines 1, 2, and 3 used Pb-free solder (96.5% tin), and wave solder line 4 used solder composed of 63% tin and 37% lead. The defense and aerospace section had five wave solder lines and six surface mount lines. It also had ruggedization, conformal coating and bonding operations, where the finished PCBs are fitted with additional structural supports and hand-brushed or sprayed with an acrylic copolymer to provide increased environmental and mechanical protection. Spraying during conformal coating was conducted in a ventilated, open-face, bench-top spray booth. Employees wore safety glasses and air-purifying, elastomeric, half facepiece respirators with organic vapor cartridges while spraying. At least once per shift, wave solder operators clean solder dross by using a ladle to remove dross floating on top of the molten solder (Figure 1). Residual molten solder inadvertently collected during this operation is separated with a sieve, and the remaining dross disposed of in a drum sealed with a lid. Employees are required to wear heat-resistant gloves over disposable nitrile gloves, a face shield, and an apron when performing dross cleaning. In addition to dross cleaning, employees periodically clean and maintain the wave solder machines. Finally, the facility has an auto insertion (AI) operation, which involves machine insertion of components onto a PCB by punching through it.

Assessment. During our first visit to the facility, we conducted confidential medical interviews with 40 employees, observed work processes and practices, collected general area air samples for volatile organic compounds (VOCs), and collected surface wipe samples for lead and tin. We also reviewed air sampling records, the respiratory protection program, injury and illness records, and material safety data sheets. After sharing our findings with employee and management representatives in an interim letter, we returned to the facility to collect personal breathing zone and general area air samples for lead and specific VOCs. We also measured noise levels at the AI stations, evaluated room acoustics near the air rotation units (ARUs), collected hand wipe samples to assess lead contamination on skin, and evaluated the effectiveness of the local exhaust hoods for the wave solder and surface mount machines (Figure 2).

Occupational exposure limits. NIOSH investigators use both mandatory (legally enforceable) and recommended occupational exposure limits (OELs) for chemical, physical, and biological agents as a guide for making recommendations. OELs have been developed by Federal agencies and safety and health organizations to prevent the occurrence of adverse health effects from workplace exposures. They suggest levels of exposure to which most employees may be exposed up to 10 hr. per day, 40 hr. per week for a working lifetime without experiencing adverse health effects. However, a small percentage of employees may still experience health effects due to personal susceptibility, a preexisting medical condition, and/or hypersensitivity (allergy). In addition, some hazardous substances may act in combination with other workplace exposures, the environment, or with medications or personal habits of the employee to produce health effects, even if occupational exposures are controlled at the level set by the exposure limit. Also, some substances can be absorbed by direct contact with the skin and mucous membranes in addition to being inhaled, which contributes to the individual’s overall exposure.

Results and Discussion

One of six personal breathing zone air samples for lead exceeded the Occupational Safety and Health Administration action limit (OSHA AL) of 30 micrograms per cubic meter (µg/m3) and was close to the OSHA permissible exposure limit of 50 µg/m3. This employee, a wave solder operator in the DAS, was cleaning the wave solder machines without wearing a respirator. The employee was exposed to an airborne lead concentration of 49 µg/m3. However, with this one exception, wave solder operators had lead exposures well below the OSHA AL.

Our surface sampling showed the presence of lead and tin on work surfaces in both sections of the plant. Currently, there are no OELs for surface metal contamination in occupational settings. We also sampled tables in two of the break rooms and found detectable levels of lead and tin in one of the break rooms. This suggests that workplace contamination is being tracked into the break rooms by employees’ footwear, clothing or hands, and that these areas should be kept cleaner.

Additionally, despite hand washing prior to sample collection, three of seven hand wipe samples tested positive for lead.

Full-shift noise exposures for the AI operators in the medical and defense and aerospace section were well below the NIOSH recommended exposure limit. Because telephone communication is not required in the production areas, and communication between employees is minimal, the louder noise levels experienced by employees with work stations near the ARUs were within the criteria specified by the balanced noise criteria (NCB) 55-70 curves. The NCB curves are a set of noise criteria for occupied interior spaces, devised to limit noise to levels at which speech can be reasonably understood. Refer to “Resources and Links” for more information on the NCB curves, as well as Appendix B of the HHE report (referenced in this section).

Our ventilation evaluation revealed that several local exhaust hoods were not effectively capturing process emissions. These included three hoods in the medical section and two hoods in the defense and aerospace section. This could have been due to local exhaust ventilation systems being imbalanced or improperly maintained. Air sampling results for specific VOCs indicated that employee exposures were well below all applicable OELs.

Although both the medical and defense and aerospace section share the main workspace and have similar tasks and equipment, the health concerns originated exclusively from employees in the medical section. Of the 40 medical section employees we interviewed, 23 did not report any work-related symptoms. The most commonly reported symptoms were upper respiratory, including runny nose, cough, and sinusitis; fatigue (frequently related to overtime work), and voice loss. However, these symptoms are also common in the general population, and we could not attribute them to the exposures documented.

Last, we found inconsistencies between the facility’s written respiratory protection program and employee practice. The written respiratory protection program required respirators to be worn when cleaning wave solder machines. However, it did not identify the appropriate type of respirator that should be worn for this task, and we did not observe employees wearing respirators when performing this activity. In addition, employees were voluntarily wearing respirators during spraying in the conformal coating area.

Recommendations

We made a number of recommendations to the facility to improve employee health and safety. Many of these recommendations may also apply to readers’ facilities:

• Use engineering controls such as portable exhaust hoods when removing solder dross and cleaning wave solder machines. Periodically evaluate the local exhaust hoods to ensure they are performing per manufacturer’s specifications. Evaluate their effectiveness at least every three months, and within five days of a production, process, or control change.
• Comply with all requirements of the OSHA lead standard (29 CFR 1910.1025). This includes instituting an industrial hygiene monitoring program to assess lead exposure for employees working with leaded solder, or for employees having the greatest potential for lead exposure. Additionally, institute a maintenance program for evaluating enclosed local exhaust hoods used to control lead exposure.
• Improve general housekeeping practices to ensure break rooms and work surfaces are clean. Use commercial cleaning wipes that are made for removing lead and other heavy metals every shift to keep work surfaces such as control consoles clean. Use high-efficiency particulate air vacuum cleaners in work areas to minimize dust in the air.
• Practice good personal hygiene. Thoroughly wash hands before eating, before and after using the restroom, and before leaving work. Eating, drinking, smoking, and applying cosmetics in work areas should be prohibited.
• Ensure that the respiratory protection program meets all applicable requirements of the OSHA Respiratory Standard (29 CFR 1910.134). The written respiratory protection program should identify job tasks requiring a respirator to be worn, the type of respirator needed, and the locations where respirator use is required. Evaluate respiratory hazards for job tasks currently requiring respiratory protection to ensure protection is adequate. The program should also note any job tasks for which employees voluntarily wear respirators.
• Encourage employees to report work-related health concerns. Consult a physician trained in occupational medicine (see “Resources and Links”).
• Train employees about potential exposures and how to protect themselves from hazards, and promptly communicate any health and safety changes to employees.

Sidebar:

The Health Hazard Evaluation Program

Based on a federal law, NIOSH conducts Health Hazard Evaluations (HHE) to investigate possible workplace health hazards. Employees, employers or union representatives can ask our comprehensive team of experts to investigate their health and safety concerns by requesting an HHE. Our team contacts the requestor and discusses the problems and how to solve them. This may result in sending the requestor information, referring the requestor to a more appropriate agency, or making a site visit, which may include environmental sampling and medical testing. If we make a site visit, we prepare a report of our investigation that includes recommendations specific to the problems found, as well as general guidance for following good occupational health practices. HHE reports are available on the Internet (http://www.cdc.gov/niosh/hhe/).

Resources and Links

1. NIOSH HHE program information, cdc.gov/niosh/hhe/HHEprogram.html.
2. Link to this HHE report: cdc.gov/niosh/hhe/reports/pdfs/2007-0201-3086.pdf.
3. Code of Federal Regulations (CFR),  29 CFR 1910.95, US Government Printing Office, Office of the Federal Register.
4. L.L. Beranek, “Criteria for Noise and Vibration in Communities, Buildings, and Vehicles,” Noise and Vibration Control, Rev. ed. Cambridge, MA: Institute of Noise Control Engineering; 1988:554-623.
5. L.L. Beranek, “Balanced Noise Criterion (NCB) Curves,” J Acoust Soc Am., 1989; 86(2):650-664.
6. Acoustical Society of America, ANSI S12.2-1995, “Criteria for Evaluating Room Noise,” 1995.
7.  Association of Occupational and Environmental Clinics (aoec.org).
8. American College of Occupational and Environmental Medicine (acoem.org).

Srinivas Durgam was an industrial hygienist at NIOSH (cdc.gov/niosh) during the time of this evaluation; This email address is being protected from spambots. You need JavaScript enabled to view it.. Chandran Achutan, Ph.D., is assistant professor at the University of Nebraska Medical Center (unmc.edu); This email address is being protected from spambots. You need JavaScript enabled to view it.. Carlos Aristeguieta, M.D., was a NIOSH medical officer during the time of this evaluation; This email address is being protected from spambots. You need JavaScript enabled to view it.. Maureen Niemeier is a freelance technical writer; This email address is being protected from spambots. You need JavaScript enabled to view it..

Every dimension poses a risk to yields or time. Make sure they are worth it.

This month’s question: What is the typical industry standard for tolerance on the length and width of a flex circuit and also for stiffeners?

Answer: When it comes to dimensioning and tolerancing flexible circuits, less is usually better. Too many designers try to dimension a flex circuit as if it were a machined piece of steel. Keep in mind that the materials used to fabricate a flexible circuit are flexible. In addition to standard manufacturing tolerances, factor in the flexibility and dimension instability of the materials used in the construction.

A lot of factors cause flex circuit materials to grow or shrink before, during and after the circuit is constructed. Major physical factors that contribute to the dimensional instability of a flexible circuit are temperature and humidity. Like any material, polyimide film and acrylic/epoxy film adhesive (by far the most common building blocks for a flexible circuit) will expand when heated and shrink when cooled. While these changes may be small, over many inches of circuit length, they can add up. The other culprit is humidity. Flexible circuit materials are extremely hygroscopic and will not saturate until they have absorbed nearly 3% of their weight in moisture. As the circuits absorb moisture, they expand. The combination of temperature and humidity can cause a flex to change up to 0.001" per inch of length. So you can see that putting +/-0.005" on a 12"-long circuit can cause some problems.

Many years ago, I had a customer who ordered 18"-long flex circuits that had a tolerance of +/-0.005" on the overall length. Being young and inexperienced (I wasn’t always a Flexpert), I missed this, and we built and shipped the parts. About a week or so later, I received a call from the customer’s QA engineer telling me that the circuits were averaging 0.004" out of tolerance on the long side. I had the parts shipped back and when I measured them, they were actually averaging 0.001" under nominal. To make a long story short, these parts achieved frequent flyer status as they went back and forth between our facility and the customer’s. After about the third round, I realized what was happening. We were building the circuits in January in Minnesota (i.e., -20°F outside and less than 10% humidity inside). The customer, on the other hand, was in Tampa Bay, FL, where there is nearly always a fair amount of moisture in the air. All along, the parts had been growing and shrinking due to the wide variation of humidity levels between the two locations. I asked the customer’s QA engineer to give the circuits a low temp bake for an hour or so, and then measure them immediately after they cooled. Bingo! He got the same measurements as I was.

The bigger question is why had they specified +/-0.005" on a flex circuit 18" long? Did I mention that flexible circuits are flexible? I can’t imagine what application would need a tolerance that tight on a flex that long.

On the vast majority of drawings, I prefer to see just a few overall length and width dimensions that are reference, and let the CAD data drive the rest. If there is a dimension that is truly critical, by all means put in on the drawing and tolerance it accordingly. But realistically, since a flex circuit can bend, twist and flex, very few dimensions are truly critical. All dimensions on a drawing will have to be verified, which will add cost to the circuit. The designer needs to evaluate all dimensions that they put on the drawing and ask themselves if that dimension is going to add value, or just add cost.

So to answer the original question: Length and width of FPC would be +/-0.010" plus 0.001"/inch of length. This is the tightest that I would ever agree to, and I would prefer +/-0.020" plus 0.001"/inch.

Length and width of stiffeners. Since stiffeners are machined and constructed from a much more dimensionally stable material, they can hold tighter tolerances. But since you are putting them on a part that cannot support tight tolerances, are tight tolerances on a stiffener really necessary? They usually are not. For most normal-sized stiffeners, +/-0.010" is usually safe.

Always keep in mind that you pay for every dimension on a drawing either directly by potentially causing lower manufacturer yields or indirectly by requiring the manufacturer to verify each dimension. When dimensioning a new design, ask yourself if all those dimensions are adding value. Are you getting your money’s worth?

Mark Finstad is a senior application engineer at Flexible Circuit Technologies (flexiblecircuit.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. He and co-“Flexpert” Mark Verbrugge from PICA Manufacturing Solutions (This email address is being protected from spambots. You need JavaScript enabled to view it.) welcome your questions.

“Design by plug-in” comes to printed circuit board CAD tools.

Have you ever wished for a feature in your design tools, maybe even requested it from your EDA vendor, but never seem to get what you want? I hear that a lot from my customers, and I doubt I’m alone. Sometimes the requests are good ideas that everyone can use, and they make the development list. Sometimes the requests facilitate a custom flow or procedure that just isn’t appropriate for a shrink-wrapped product. Regardless of how the request is categorized, it always seems to take longer to implement than the customer would like, mainly because they’d really like it right now.

Contrast this situation with your iPhone, iPad, or Droid phone. Need some new functionality? Chances are someone has already thought about it and created an app for it. I am amazed at the abundance of apps out there for smartphones and pads. Need a scientific calculator app? It’s there. How about an alarm clock, a gas mileage manager, voice recorder, book reader, or even the Angry Birds game? They’re all available.

Wouldn’t it be nice if you could get those apps in your design tool? OK, maybe not, but I would bet you can think of a lot of more germane nice-to-haves. I had one customer ask for a feature to synchronize test point placement between their PCB and schematic. Another said it was too difficult to find specific parts or values in their library of designs, and couldn’t we make that easier? Yet another wanted to automatically add various visual cues relating to production status to their schematic. There’s a never-ending stream of wants and desires out there. Sounds like the perfect scenario for a healthy dose of apps.

As PCD&F reported earlier this year, Cadence got things started by adding an app store to its OrCAD Capture frontend tool. Remember those customer requests I just mentioned? We implemented them for the customer, and then made them more generic for the general population and released them as apps. Apparently this isn’t unique to us, as other VARs are doing the same thing, helping us to create the first app store for the PCB design community.

We showed the apps and the store at the recent PCB West trade show, and found that users were thrilled to see that they had a new way to get more capabilities that they need, and that they could pick and choose which ones to get. As excited as I am to get this kind of response, I was probably more excited to hear other users saying they had some ideas and wanted to know how they could create their own apps. Imagine that: PCB tool users creating solutions to solve their own problems and then adding them to the pool of apps available to everyone.

This is similar to the concept of open source in that the user community has the ability to continually improve the tools they use. However, with apps, the added capabilities plug into the tool rather than modifying its base, so these new features become available the moment they are finished, rather than at the next release of the base tool. Another difference is that apps are based on scripting languages that are simpler to learn and use than C++ or many of the other programming languages used to create PCB tools. The developer has access to the database, command structure, and menu structure, making it relatively easy to implement features. Finally, problematic apps don’t ruin the whole tool; they can simply be removed.

That brings up a vital part of an app community: separating the good apps from the bad. Anyone actively downloading apps on their handheld devices knows that there are a lot of duds out there. Some just plain don’t work as advertised. We’ve built in quality assurance measures to minimize this problem, but the truth is there will still be imperfect apps. The key to this problem is the familiar rating system. Users of the apps get to rate the apps. Good apps get good ratings, and bad apps get left behind. It can be harsh, but it’s necessary to let this Darwinian approach to apps weed out the undesirable ones.

Another nice use of apps is to integrate existing tools with the schematic or PCB editing tool. We’ve come to expect the ability to launch another tool from the same vendor, but wouldn’t it be nice to build bridges between different vendors’ tools, especially if that bridge includes built-in knowledge of design? Similarly, a company with internal tools can create a more effective flow by tying the tools together. These apps can plug in to different versions of the PCB tools and can be shared between different design teams or the community at large.

I can see the user community adding some real vibrancy to our industry. It’s not too hard to imagine many apps available, because we’ve seen this with smartphones and pads, but I can also see a pool of developers looking for good ideas to implement. Some in the community will implement what they need with the scripting language, but others will take their wants and desires to an app developer for much quicker results, conforming to their specific needs. In fact, I’ll sign up to host the first app developer request line. Send me your wants and desires, and we’ll get to work building an abundance of PCB design apps.

Manny Marcano is president and CEO of EMA Design Automation (ema-eda.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. His column runs regularly.

No panacea exists, but certain coatings are getting closer.

What are the guidelines for tin whiskers in electronics assemblies, and how are they prevented? Part of the question can be answered by the Jedec and GEIA standards that delineate acceptance requirements for tin whiskers (Figure 1) in commercial and military hardware. But none of the methods outlined by the standards guarantees total mitigation of whisker growth.

No single accepted theory explains the tin whisker growth mechanism. The leading theories postulate the growth mechanism is powered by a release of internal stress in the tin finish or by re-crystallization and abnormal growth in the tin grain structure.¹ Since the growth mechanism is not fully understood, the required environmental stress tests used for qualification, such as thermal cycling and thermal aging, do not necessarily simulate tin whisker growth in an assembly. A linear growth rate for tin whiskers has not been observed during long-term studies of whisker growth. There is a latency period after the coating is applied during which tin whiskers do not grow, which can range from a period of days to years. Once a whisker starts growing, it will most likely grow at a constant linear rate, but not all whiskers on a particular finish will grow at the same rate. This unpredictable whisker growth rate, which leads to inaccurate acceleration models, makes qualification testing of high-reliability electronics assemblies a challenge.

Qualification tests. For tin whisker acceptance testing, the commercial electronics industry has used Jedec standard JESD201, Environmental Acceptance Requirements for Tin Whisker Susceptibility of Tin and Tin Alloy Surface Finishes. Due to the long service life (>10 years) required for high-reliability electronics assemblies used in the aerospace and defense (A&D) industry (Table 1, Class 3), JESD201 requirements are not adequate. While valuable as a process evaluation tool for data comparison, the environmental test conditions do not correspond with actual A&D service environments. Tin whisker testing in JESD201 focuses on humidity, thermal cycling (1,500 cycles maximum), and calendar aging for a maximum of only 4,000 hr. (5.5 months). The A&D environment includes vibration, shock, corrosion, and even probe marks in the surface finish left behind during troubleshooting. These environmental conditions induce stress on the thin tin finish, which promotes the growth of tin whiskers.

GEIA-STD-0005-2, Mitigating the Effects of Tin Whiskers in Aerospace and High Performance Electronic Systems, is presently the adopted standard used to manage (but not eliminate) tin whisker risk. These mitigation methods include SnPb solder dipping of component leads, conductor spacing control, and conformal coating. Unfortunately, the current knowledge base does not include decades of experience to verify the success of these mitigation methods  and their effect on system reliability.

Mitigation techniques. While pure tin and high tin content finishes are not recommended for Class 3 assemblies, the push to restrain A&D electronics costs has precipitated the use of COTS (commercial off-the-shelf) components whenever possible. Unfortunately, since pure tin finished devices are common and inexpensive, they have inadvertently made their way through A&D supply chains. Component manufacturers and distributors frequently use the lot number as the only discriminator to distinguish the Pb-free finish. Even reputable distributors have difficulty tracking lots of pure tin finished components versus the same component with an alternate finish.

As a first level of tin whisker mitigation, it is critical to screen for pure tin finished components at incoming receiving. Conformal coating, required for most military applications, is another whisker mitigation technique frequently used to help retard the effects of whisker growth. By isolating individual leads, the coating can prevent arcing due to whisker bridges. The more elastic coatings rely on elongation properties to prevent penetration. Other coatings rely on their hardness to restrain or redirect whiskers to grow back on themselves, preventing them from breaking through and potentially shorting. It must be reiterated that currently conformal coating is not a foolproof method to prevent whisker growth.

Various methods and materials are available for conformal coating. The following have been the most commonly used, with most having been qualified under IPC-CC-830. The coatings are listed in increasing order by their expected tin whisker mitigating value.

1. Acrylic: can be dipped or sprayed.
2. Silicone: can be dipped or sprayed.
3. Epoxy: can be dipped or sprayed.
4. Parylene: chemical vapor deposition.
5. Arathane 5750 (formerly Uralane 5750)2 Urethane: can be dipped or sprayed.
6. ALD-Cap3: high alumina ceramic applied using atomic layer deposition.

With research still pending, ALD-Cap high alumina conformal coating has shown encouraging signs as a reliable method for tin whisker mitigation.

References

1. Jay Brusse, Kim Jong, Michael Sampson and Henning Leidecker. “Basic Info on Tin Whiskers.” NASA Electronic Parts and Packaging (NEPP) Program, http://nepp.nasa.gov/whisker/background/index.htm.
2. Jay Brusse, Kim Jong, Michael Sampson and Henning Leidecker, “Characterize the Effectiveness of Arathane 5750 (formerly Known as Uralane 5750) Conformal Coat Material in Prohibiting Tin Whisker Formation and/or Tin Whisker Penetration.” NASA Electronic Parts and Packaging (NEPP) Program, http://nepp.nasa.gov/WHISKER/experiment/exp2/.
3. Ofer Sneh, “N04-058: ALD-Cap: Thin Film Encapsulating Coating for Hermetic Environmental Protection,” Navy SBIR Success Story, July 25, 2009. https://www.navysbirsearch.com/widgets/hyperlinking/successdetails.jsp?url=DocURL&id=90078.
4. The Lead-Free Electronics Manhattan Project – Phase I. Benchmarking and Best Practices, July 30, 2009. http://www.dodb2pcoe.org/LFEMP_book.pdf.

ACI Technologies Inc. (aciusa.org) is the National Center of Excellence in Electronics Manufacturing, specializing in manufacturing services, IPC standards and manufacturing training, failure analysis and other analytical services. This column appears monthly.