Moving Ahead in Lead-Free
Peruse the conference program of any electronics assembly trade show, and you'll find one subject dominates all others: lead-free solder. And for good reason.
By July 1, 2006, lead must be eliminated from many electronic devices produced in or imported to the European Union. Circuit boards used in automotive, aerospace, military and high-reliability telecommunications equipment are exempt from the rule, but all other products must be lead-free. In addition, most of Japan's major electronics manufacturers, including Hitachi, Panasonic, Sony and Toshiba, have already voluntarily eliminated lead from many of their products.
If U.S. electronics assemblers want to remain competitive during the next 5 years, they will have to overcome the challenges of lead-free solder. Yet, by some estimates, only 5 percent of North American electronics assemblers are currently using lead-free solder.
"The big contract manufacturers...are evaluating lead-free solder in low-volume product lines, but it isn't mainstream yet," says Jim Slattery, vice president of global technology support at Indium Corp. of America (Utica, NY). "Everybody is going to wait until the last minute to implement it."
Peter Biocca agrees. The senior market development engineer for lead-free solder at the Kester Div. of Northrop Grumman Corp. (Des Plaines, IL), Biocca estimates that 30 percent to 40 percent of the solder his company sells in Asia is lead-free. "In North America, everyone is still wondering which alloy to use," he observes. "There's a lot more interest in lead-free here now, but we're not the leaders, we're the followers."
The transition to lead-free solder involves more than just choosing a new alloy. Engineers will need to re-evaluate components and board materials, as well as the finishes on the board and component leads. Engineers may even need to redesign the size and spacing of lead pads, vias and through-holes.
Several lead-free board finishes have been studied, including nickel-gold, immersion tin, immersion silver and bare copper with organic solderability preservative. "The predominant one seems to be nickel-gold, because you get slightly better wetting on that surface," says Biocca.
A number of lead-free component finishes have also been evaluated. These include silver-palladium, nickel-palladium, tin, tin-copper and tin-silver-copper.
One concern with the use of tin finishes is a phenomenon called "tin whiskering." At certain temperatures (usually below 10 C), tin can grow tiny dendrites, or whiskers. These whiskers can flake off, weakening the joint and potentially causing shorts. The occurrence of tin whiskering is related to the stress placed on the finish, as well as its purity, thickness and crystalline structure. Highly stressed deposits of 2 to 10 microns of pure tin are the most susceptible to whiskering. Electrolytically plated deposits are less prone to whiskering than immersion deposits, which are thinner and have a more polygonal structure.
When selecting a lead-free solder, board finish and component finish, engineers should be mindful of incompatibility between materials. In particular, engineers should avoid mixing lead-free solder with boards and components that have tin-lead finishes. Because lead-free and tin-lead alloys have different melting points and different coefficients of thermal expansion, mixing the two could cause fractures under excessive thermal cycling. "If you go lead-free, you should eliminate lead altogether to ensure the highest reliability," Biocca warns.
The components themselves should be tested to ensure they can withstand the higher reflow temperatures associated with lead-free solder, says David Suraski, market manager for AIM Inc. (Cranston, RI). A key concern is the moisture sensitivity level of the components. Plastic components can lose one to two levels of moisture sensitivity when exposed to temperatures near 260 C. To prevent delamination, or "popcorning," during reflow, components should be stored in a low-humidity environment. Components may even need to be baked prior to reflow.
Solder and Flux
For surface-mount assembly, tin-silver-copper has emerged as the most popular lead-free alloy, though the exact composition of the alloy varies with the manufacturer. The silver content ranges from 3 percent to 4 percent, while copper content varies from 0.4 to 0.8 percent, says Doug Dixon, product manager for Multicore soldering products with the Electronics Div. of Henkel Loctite Corp. (Industry, CA). "We recommend an alloy of 3.8 percent silver, 0.5 percent copper and [95.7 percent] tin," he says. "Japanese companies prefer 3 percent silver, 0.5 percent copper and [96.5 percent] tin. We believe the alloy with 3.8 percent silver is the only true eutectic [in that family], but that is disputed in the industry."
Tin-silver, tin-silver-copper-antimony, and tin-zinc-bismuth are also being used for surface mount assembly.
Even though surface mount alloys containing bismuth melt at substantially lower temperatures than other lead-free alloys, Dixon does not recommend them. These alloys have limited wetting performance because it's difficult to get good flux activity at low temperatures. In high-reliability applications with extreme thermal requirements, the bismuth alloys do not perform as well as other lead-free alloys. Moreover, if bismuth is exposed to lead from board and component finishes, it can form a low-temperature intermetallic that can reduce the thermal reliability of the solder joint.
For wave soldering, the most common alloys are tin-copper, tin-silver-copper, and tin-silver-bismuth-copper. Tin-copper is the preferred alloy, because it is the easiest to balance when dross is removed.
The cost of lead-free alloys varies, depending on their composition and form. Tin-silver-copper paste is slightly more expensive than its lead-bearing counterparts. For large consumers, tin-lead paste might cost $0.07 per gram, while lead-free paste might cost $0.08 per gram, says Biocca. However, the price differential is much wider for bar and wire solder, because it's bulk metal. If tin-lead bar solder costs $2.50 per pound, a bar of tin-silver-copper might cost $4 per pound. Lead-free wire solder might cost 20 percent more than tin-lead wire.
The flux used with lead-free solder may be more important than the composition of the alloy, says Gordon Clark, director of European support services for Koki Co. Ltd. (Glasgow, Scotland). Lead-free solder has a higher surface tension than tin-lead solder, and it doesn't wet surfaces as well. However, because lead-free alloys typically melt at a much higher temperature than tin-lead solder, long-established fluxes may vaporize before they can work. Other fluxes can discolor at high temperatures, giving boards a burnt appearance.
As a result, solder suppliers have been forced to develop new, more thermally stable fluxes specifically for lead-free solder. These fluxes are available in both no-clean and water-soluble formulations. "The secret to lead-free soldering isn't the alloy; it's the flux," says Clark.
"You want a flux with the highest activity you can get," adds Biocca. "In our tests, water-soluble fluxes tended to work the best with lead-free solder, because they're more active."
An aggressive flux is particularly important for reflowing double-sided boards, because the high temperature will increase oxidation on the opposite side of the board. "A paste that's OK for a relatively unoxidized surface may not work as well on the opposite side," Biocca points out.
As when choosing any type of solder paste, engineers should evaluate lead-free products for such characteristics as print speed, stencil life, tack time, open time and pin-testability. Tests for solder balling, slumping and spreading are critical. "You want a paste with exceptional slump properties," explains Biocca. "Because the peak reflow temperature is higher, the preheat temperature is also higher. As a result, solder pastes tend to slump, which can create problems with bridging, solder balling and shorts. A solder paste that slumps excessively will cause problems at high temperatures."
Thankfully, some parts of the circuit board assembly process will not be affected by the transition to lead-free solder. Lead-free solder will not change the pick-and-place process, and lead-free pastes print and dispense as well as their tin-lead counterparts, says Suraski. However, because lead-free pastes are significantly less dense than tin-lead pastes, the pressure setting on the printer squeegee may need to be adjusted. The stencil itself also may need to be modified to maximize the spread of paste on the pads and counteract the diminished wetting ability of lead-free solder. Finally, lead-free pastes may have a shorter shelf life than tin-lead solder, and they may require more careful storage.
At the other end of the spectrum, lead-free solder will significantly change the reflow process. The peak reflow temperature for many lead-free alloys ranges from 235 C to 260 C. Most reflow ovens should be able to reach those temperatures, but some may be unable to adequately control them. At such high temperatures, tight control over actual oven temperature is imperative, says Suraski. In addition, the higher temperatures may increase the need for preventive maintenance.
When developing the reflow profile, engineers will have to strike a balance between peak temperature and heating time. "Depending on the oven and the density of the assembly, the ramp-to-spike profile is generally recommended for lead-free assembly," says Suraski. "This profile offers superior wetting and less thermal exposure than the traditional ramp-soak-spike profile."
A nitrogen atmosphere will increase wetting with lead-free solder, lower the peak reflow temperature, and decrease the amount of time that boards spend above liquidus temperature. "Nitrogen is an advantage in any process, whether it's tin-lead or lead-free," says Slattery. "The downside is that nitrogen is expensive, and people don't want to use it if they can get away with it."
Wave soldering will require a pot temperature of 260 C to 275 C. To avoid thermal shock, preheat temperature should be higher for lead-free soldering than for tin-lead soldering. As with lead-free reflow, lead-free wave soldering requires an aggressive, thermally stable flux. An inert atmosphere will improve wetting and reduce dross.
"Tin-silver-copper and tin-copper solder don't completely fill through-holes like tin-lead solder, so insufficient solder can be an issue as the board gets thicker," Biocca warns.
Another potential problem with lead-free wave soldering is fillet lifting, though experts disagree about its affect on product reliability. Fillet lifting is caused by contraction in the joint as it cools. This causes the main solid portion of the joint to lose contact with the wetting surface before the last liquid freezes, says Dixon. The problem is related to lead or bismuth contamination in the solder bath.
Like reflow ovens, wave soldering equipment will require additional maintenance with lead-free solder. Tin-rich lead-free alloys can rapidly dissolve stainless steel pots, nozzles, impellers and other parts in wave soldering equipment. These can be replaced with parts made from cast iron and other resistant materials. In some cases, parts can be coated with a special high-temperature paint that protects them from corrosion for 2 to 3 years.
In the end, converting to lead-free solder will force assemblers to re-evaluate every aspect of the assembly process, from product design and parts procurement, to test and inspection. "Lead-free solder will increase the potential for defects," warns Biocca. "If you're not careful, you will start seeing a lot of defects that you thought you had under control. Some defects, like bridging and solder balling, will increase dramatically, so you will have to optimize your process carefully to avoid those issues."