Tutorial #45
Dr. Jennie S. Hwang
October 2004
As the required deadlines for converting electronic assembly to lead-free solder approach, a growing swirl of books, papers, articles, classes, conferences, and announcements all deal with some aspects of lead-free implementation. In this intellectual whirlwind, a few important points sometimes get lost, or at least misplaced. Some key perceptions about lead-free materials and manufacturing processes are not fully accurate, or are accurate only with qualifications. Here are ten selected examples, and the requisite qualifications.
1. The higher melting temperatures of lead-free solders always lead to higher process temperatures.
A drop-in lead-free manufacturing process requiring temperatures no higher than for SnPb has proven to be successful for both reflow and wave soldering of a variety of products at several production sites.
2. PCB board material, layout, and attached components must be changed when converting to lead-free soldering.
None of these require change, if the process temperature is not elevated.
3. Binary alloys are more stable than ternary alloys; ternary alloys are more stable than quaternary alloys.
Lower-element alloys are not necessarily more stable than the higher-element alloys in the actual production environment. For example, compare the composition sensitivity of binary SnCu eutectic solder with that of three-element and four-element alloys. The binary eutectic composition, 99.3Sn / 0.7Cu, melts at 227 oC. A deviation of only 0.5% of Cu in either direction from this eutectic composition may significantly change the melting temperature. This melting temperature change in the molten wave pot is enough to affect the quality and the reliability of resulting solder joints.
4. Lead-free is superior to SnPb; lead-free is inferior to SnPb
Lead-free in total is neither entirely better nor entirely worse than SnPb. The comparison depends on the fundamental physical and mechanical behavior of the specific lead-free alloy under a given set of conditions. Some lead-free solders are expected to perform better, and some worse, than SnPb.
5. Lead-free is harder on wave soldering equipment
Lead-free solder in itself should not damage a wave-soldering machine. However, if a higher process temperature is adopted, the elevated wave pot temperature creates harsher operating conditions for the wave soldering equipment.
6. Mixing different lead-free alloys in the same system always leads to problems.
Although using different lead-free compositions together in a system requires careful consideration, mixing need not cause a problem. The fundamental compatibility and the compositional tolerances of the specific solder alloys needs to be understood before using them together.
7. Any level of Pb contamination in a lead-free system degrades operating performance.
The effects of lead contamination have been studied; the results are reported in the literature. A trace amount of Pb (less than 0.5%) in an acceptable lead-free solder was found to be absorbed without causing a practically detectable level of deleterious effects.
8. All lead-free soldering processes require a nitrogen atmosphere.
In process terms, a nitrogen atmosphere plays the same role and has the same potential benefits for both lead-free and SnPb soldering. A properly formulated solder paste and flux can perform well without a nitrogen atmosphere, both in wave soldering and in reflow soldering.
9.Lead-free soldering always lowers the moisture sensitivity rating of BGA plastic packages.
If the process temperature is not elevated, the moisture sensitivity rating of plastic packages will not be lowered.
Lead-free solder is available in only a few physical forms.
Well-designed lead-free alloys can be fabricated in all of the physical forms available for SnPb solder, including powder, paste, sphere, rod, pre-form, and wire of various gages.
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