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Lead-free and Lead Contamination
December 31, 1969 |Estimated reading time: 2 minutes
Concerns exist about lead contamination in lead-free soldering. Although more components are available in a lead-free finish, some are not yet available or not easily obtained.
By Peter Biocca
Many lead-free components contain pure tin finishes and have been assembled with leaded solder such as 63/37 for years. No issues exist with joint reliability when pure tin is soldered with leaded solder, except for tin whiskers. These are needle-like protrusions or nodules that grow out of certain pure tin finishes. They are still being studied but have little to do with lead contamination. In fact, use of leaded solders may reduce their occurrence. Pure tin-finished boards and component terminations have been available for years and also have been soldered with leaded solders, with no reliability issues.
Components also are available with lead-free finishes containing bismuth. Soldering bismuth finishes with leaded solders can create lower melting phases that can cause brittle solder and a reduction in reliability, especially during thermal cycling.
Leaded boards such as HASL and tin-lead components will contaminate the lead-free solder pot during wave soldering. The limit for lead per the RoHS Directive is 0.1 percent, which can easily be surpassed if parts to be soldered are lead bearing.
Fillet lifting due to lead contamination.
Wavesoldered leaded-component terminations also may cause issues with fillet lifting. (Figure) Dissolution of lead into the mass of the lead-free solder will create lead-bearing metallic phases, which contract and expand differently than the bulk of the solder. This results in lifting of the solder holding the annular ring around the through-hole. Although not a defect, fillet lifting may be aggravated with thermal cycling. Cracked copper tracks also have been reported to occur.
In SMT assembly, soldering leaded parts with lead-free solder paste may be less of an issue. Many electronic assemblies have been produced with leaded terminations and lead-free solder paste. Many of these have passed reliability testing without detriment. However, most assemblies are destined for consumer electronic use. Highest reliability is achieved where parts to be soldered are free of lead, as is the solder.
Hand soldering of lead-free terminations with leaded solder is a non-issue. Whether soldering leaded parts with lead-free solders can reduce reliability when compared to using leaded solders has not been determined. NPL in the UK has conducted a study that seems to indicate no loss of reliability when lead-free solder joints are reworked with leaded solders. This applies to lead-free solders such as tin-silver-copper (SnAgCu) or tin-copper (SnCu). Reworking bismuth-containing lead-free solder joints did show a dramatic reduction in reliability. Creation of lower melting phases the same as the fillet lifting phenomena in wave soldering also was an issue.
BGAs also are being offered with SnAgCu balls instead of leaded ones. Soldering lead-free balls with leaded solder paste is not recommended due to a 34°C difference in melting points. Profiling to lead-free paste's recommended thermal requirement will result in gross segregation of leaded and lead-free phases. An increase in voids can be noticed at the solder and ball interface. Using a higher reflow profile to achieve ball collapse results in charring of residues and excess voids.
Soldering leaded BGAs with lead-free solder paste results in increased voiding and reduced reliability. Lead-free BGAs should be soldered with lead-free solder paste such as SnAgCu for highest reliability.
Conclusion
Lead-free soldering makes it increasingly important to address the issue of components, board finishes and the solders used to assembly them. Keeping materials identifiable throughout the assembly process is critical, as is identification of the final assembly for reliable rework.
Peter Biocca can be contacted at Kester, E-mail: pbiocca@kester.com.