The best surface finish meets the needs of the board design and the assembly process, while being easy on the wallet.
Selecting the right final finish should not be a puzzle, but it can seem like one. Often, there is conflicting information about different finishes and their intended applications. Depending on one’s point of view, bias enters into the equation, and sometimes, conclusions can be reached that outweigh the data. The best surface finish meets the needs of the board design and the assembly process, while being easy on the wallet.
From a fabrication house perspective, the ideal surface finish is: inexpensive to apply and to stock (think pounds of gold or tons of solder); easy to set up, control, maintain and run; reworkable; and operational with little experience.
If you are a contract manufacturer, your focus and needs are a little different. You might be looking for a surface finish that provides good-looking solder joints (regardless of the assembly materials or the process used), is easy to rework, can sit in process for as much time as needed without fear of future problems and has a long shelf life. The OEM, on the other hand, is most interested in cost, reliability and availability. Chemical vendors may have a very narrow point of view highlighting the advantages of their latest system at the expense of all other surface finish options. When you couple all these different points of view with some personal experiences (set in time) and bias, the resulting confusion can be understandable.
Hot air solder level (HASL) came into the industry to replace fused, tin-lead solder. By using solder mask over bare copper (SMOBC), a fabricator could easily protect base copper and have a solderable surface finish. HASL was introduced just prior to the advent of SMT. We should give credit to the equipment manufacturers and engineers who found a way to improve air knife geometries and fluid dynamics, keeping this a viable option for the last two decades. Solder-coated copper has taken much of the work out of soldering at the assembly house because the tin-copper inter-metallic layer is already formed. The heat required at assembly is just enough to fuse HASL solder with solder paste. HASL boards have excellent shelf life and are easy to rework at both fabrication and assembly. If properly assembled and cleaned, they will have very little field failure concerns that are more common with other finishes.
However, HASL does not lend itself well to high-density boards. Once SMT pitch goes below 16 mils or pad co-planarity is an absolute requirement, HASL will not perform well. Solder thickness can vary between 35 micro inches to 1500 micro inches. Lead-free solders have higher surface tension then standard 63/37, so pad planarity will be better, but pad wetting will suffer. Small, solder mask-defined features will be difficult to wet with one pass. One major issue with lead-free HASL is copper dissolution. If standard SAC 305 is used, expect to have more copper plate required to overcome what will be lost in the solder pot, typically 20% to 30% more then standard. Proprietary, lead-free solders consume much less copper, so less copper plating is required. HASL can be applied with vertical or horizontal equipment, providing better pad wetting and flatter pads – but the cost of equipment can easily outweigh the benefits. Shelf life is typically 12 months from the date of application. Although HASL is readily available, some US companies have removed their equipment, opting to subcontract this process.
Organic solderability preservatives (OSPs) have also evolved from simple anti-oxidants to thicker, more thermally stable coatings able to withstand multiple, lead-free reflow cycles (FIGURE 1). OSPs cost the least to purchase and to operate but, perhaps, place more burden on the assembly house. The OSP coatings are commonly available on the market, ranging in thickness between 0.10 microns and 0.50 microns. Ideally suited for SMT applications, they also produce excellent solder joint strength. Handheld devices commonly use a combination of electroless nickel/immersion gold (ENIG) for key contact areas with OSP on soldered pads because the solder joint strength is superior and will stand up to multiple shocks. OSP can be found on the simplest of designs, for companies with the lowest expectations for reliability, to the most difficult designs, from OEMs that require absolute confidence for products with 10-year to 20-year life cycles. OSP is an insulator, making it unsuitable for key contacts and difficult for in-circuit testers. It requires fluxes that are active enough to dissolve it before solder can react with copper. Flux must be applied directly to the OSP in order for solder joints to form. If solder paste is applied to the board, any area the paste contacts will remove the OSP, so rework and assembly must begin right away. Once applied, the coating’s shelf life ranges from 6 months to 12 months, depending on the supplier’s recommendations. OSPs can run vertically or horizontally, and the cost of equipment is low. They are available from many board fabricators, and the surface finish covers nearly 40% of PCBs that are produced today.
Immersion tin (ISn) has been in the industry for quite some time. Early bath versions were simple and cheap to operate but provided questionable solderability and were prone to generating tin whiskers. Modern baths perform much better and have whiskering under control, due to the incorporation of better coating morphology and anti-whiskering agents. Immersion tin baths are hot and acidic, requiring careful soldermask surface preparation and tight soldermask process control in order to survive the tin-plating bath. Immersion tin is favored by the automotive industry and is a particular favorite for press fit applications. It has shown excellent color stability throughout assembly product life and has proven to be effective in harsh environments. In-circuit testing is not an issue because the coating is conductive. It can be run vertically or horizontally, but unless production needs are high, vertical equipment is most desirable. While immersion tin is widely used, not all board shops have the process installed.
The ENIG process came into play in the early 1990s as a coplanar surface finish to replace HASL. Early on, issues with bath stability, skip plating and extraneous plating required very tight process controls and process development. Later, black pad became an issue. In large part, these issues have been overcome, but black pad still seems to come into play on occasion. This particular problem stems from the chemical attack of the nickel layer, resulting in inconsistent or questionable solder joint strength. An automatically controlled vertical line, with properly controlled baths from a reputable supplier, should not suffer from this issue. ENIG has many benefits including excellent shelf life, easy rework at assembly (but not at the board house), low contact resistance, excellent solderability and good solder spread. The downsides include cost of operation, soldermask attack, high demand for shop utilities and a tight process window. ENIG is the most expensive surface finish to apply because pricing is tied to gold values and thickness. All that being said, ENIG usage has risen dramatically over the past few years, and it is widely available in the industry (FIGURE 2).
Electroless nickel, electroless palladium, immersion gold (ENEPIG) has recently started to be used in the PCB industry, with the promise of being a universal finish. A universal finish is solderable, can be used for key contacts, edge rails and mating surfaces and is wire bondable. It has all the benefits of ENIG but eliminates the black pad phenomena by eliminating the potential for gold corrosion of the nickel. Like ENIG, ENEPIG also adds the benefit of a nickel barrier over the copper, eliminating any potential for copper dilution issues that have been seen with other surface finishes. However, it operates at a high cost, will attack soldermask and is relatively new to the industry, so failure modes have not been fully characterized.
Immersion silver (IAg) is the most cost-effective, metal, surface finish alternative to HASL (FIGURE 3). It has excellent soldering characteristics and is highly conductive. Baths are stable, with relatively mild chemistries, and dwell times are quick so it is easy to conveyorize. Although IAg seemed poised to replace HASL as the dominant surface finish, recent industry issues related to performance in harsh environments has exposed a weakness of the coating; creep corrosion. When placed in sulfur-rich environments, silver catalyzes copper sulfide formation that can lead to shorts in the final assembly. Initially, studies placed the blame solely on the immersion silver finish, but recent work has shown assembly materials are perhaps a more significant contributor to the problem. The industry has responded with new immersion silver post treatments that either suppress the corrosion reaction or keep the corrosion localized to unsoldered areas. Immersion silver can be run vertically or horizontally, and equipment costs are low. Most shops have immersion silver installed.
A new option is available to the market that can replace ENIG. Organic metals (FIGURE 4) show excellent solderability, operate at lower temperatures, have shorter dwell times and are highly conductive. They also have lower material costs than ENIG and have reduced requirements for power and water, resulting in a much lower cost of ownership for the fabricator. This type of process will generate a finish that looks closer to an OSP, but with superior hole fill and in-circuit test capabilities. Because they are new to the industry, availability is limited, but demand will force more installations in the near future.
Surface finishes can be puzzling, but with the right information, knowledgeable decisions can be made for the best fit keeping in mind the needs of the board fabricator, assembler and OEM.
Jim Kenny is global product line manager, PWB final finishes, Enthone; This email address is being protected from spambots. You need JavaScript enabled to view it..
