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November 20, 2006

Lead levels in lead-free process

There is a lot being written on controlling the lead in lead-free soldering. Contamination is introduced in the solder process by way of component plating, board surface finish, migration from soldering iron tips/tooling and the original lead content in the solder. The RoHS Directive has and allowance of 0.1% for compliance with its standards.

From a manufacturing process control standpoint, I am interested in whether the industry has established a level/standard at which the percentage of lead results a significant reduction in solder joint strength. Has this been identified specifically for the SAC alloy, family of lead-free solders?

Richard Neblett

Experts Comments

The introduction of lead does not, in of itself, reduce the inherent strength of the solder joint. The primary concern is where and how that lead is present in the solder. If the lead is segregated, especially preferentially in the interfacial regions, you can have a Pb-rich region which is very weak. If the Pb is well-mixed, Pb levels as high as 5, 10 ,15% do not seem to result in a reduction in solder joint reliability under temperature cycling. Less data is available on the behavior under mechanical loading.

Dr. Craig D. Hillman
CEO & Managing Partner
DfR Solutions
Dr. Hillman's specialties include best practices in Design for Reliability, strategies for transitioning to Pb-free, supplier qualification, passive component technology and printed board failure mechanisms.
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