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Current Trends in Lead-free Solder
December 31, 1969 |Estimated reading time: 3 minutes
The Restriction of Hazardous Substances (RoHS) directive will ban the use of lead in electronics assemblies in Europe effective July 1, 2006. Electronic and electrical waste is the fastest growing category in the municipal waste stream, and legislators are concerned that lead may leach from these disposed devices into ground water.
One study using the EPA's test method has shown that lead does not appreciably leach into water, while lead-free alloys containing silver do. Whatever the truth, it is too late to stop the industry's momentum toward lead-free. The European directive places the responsibility on the "brand owner."
Lead is an excellent diluent for tin, and reduces the melting point, cost and surface tension. It also is a cheap addition that provides good wetting and minimal voiding.
The preferred substitute is tin-silver-copper (SnAgCu). A number of different SnAgCu compositions exist, and there essentially are two overlapping patents, held by Iowa State University and Senju Matsushita. Because the scope of a patent cannot be established until the patent holder defends it in court, most solder suppliers are conservatively holding licenses for both patents in hopes that users of solder products may sell their products worldwide.
The spreading/wetting of SnAgCu is less than that of tin-lead (SnPb) because:
- Surface tension is greater for the high Sn alloy than the SnPb solder.
- Components limit peak temperature so there is less superheat.
- Instead of SnPb to SnPb, SnAgCu is soldered to immersion Ag or Sn, OSP copper, or NiAu, etc.
Even if wetting out does not extend to the corners of the pad, joint strength appears to be good. With higher surface tension, it is more difficult for occluded gas to escape from the solder, making voiding levels higher. The higher surface tension also can lead to tombstoning.
In a real process, the cooling rate produces a microstructure that is not truly eutectic. This causes tin dendrites to be surrounded by solidification/contraction, causing surface shrinkage, a purely cosmetic problem. A duller joint is simply an indication of different metallurgy (Figure).
On a typical PCB used in cell phones, 0201s are soldered using lead-free SnAgCu alloys.
Numerous published studies benchmark SnAgCu against SnPb (published by IDEALS, the NCMS 1 & 2 projects, and the Boeing study). Fatigue life studies on bulk solder indicate that the lead-free alloy has a superior fatigue life by a factor of 10. But because this is not supported by data from soldered assemblies subjected to thermal cycling, it seems reliability cannot be predicted from bulk alloy properties.
The main failure mode for sound joints is low cycle fatigue during temperature/ power cycling. The reliability of the lead-free alloy must be at least as good as that of SnPb. Originally, lead-free alloys were expected to have better reliability and allow higher operating temperatures, largely due to the superior bulk alloy properties. However, joint design, assembly compliance, fillet size, standoff, voiding and joint microstructure also affect reliability.
The NCMS 2 project investigated high-temperature/high-stress conditions. Under reliability test conditions of -55°/160°C, SnPb was better than all lead-free alloys. Under less extreme conditions, lead-free alloys were superior. The Boeing study, which was over a -55°/125°C range with 15-minute dwell periods, also found that SnPb was superior to SnAgCu.
Under extreme test conditions, SnPb has up to twice the fatigue life of lead-free materials. But under lower temperature and strain ranges, SnAgCu has up to double the fatigue life.
If materials existed that offered the performance of SnPb, yet were nontoxic and less expensive, the industry would be using them already. Lead-free raw materials cost more, and there is no economy of scale because lead-free comprises less than 5 percent of world lead-free consumption.
Steve Dowds, global product manager — Solder Products, may be contacted at Henkel Loctite Corp., Kelsey House, Wood Lane End, Hemel Hempstead, Hertfordshire HP2 4RQ, UK; 44-1442-418-252; E-mail: steve.dowds@henkel.com.