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A significant change to the soldering assembly specifications is outlined in the recently released "F" version of the J-STD-001, Requirements for Soldered Electrical and Electronic Assemblies (July 2014) for gold-plated components. With the advent of more information from studies on the behavior of how much gold can impact the mechanical structure of the solder joint, the "washing away" of gold has become more important.
Gold is widely used in electronic package finish designs in order to enhance solderability and wire bondability. A thick, pure gold finish can be used to enhance wire bondability, but can be too thick for soldering, causing solder joint embrittlement. It does not oxidize very readily, melts at relatively low temperature, and dissolves rapidly in the soldering process. However, too much remnant gold in the solder joint can weaken the integrity of the interconnection.
If the gold dissolution is excessive during the solder alloy’s liquid phase formation, then the composition, mechanical properties and durability of the resulting joint alloy can change, compared with the original solder alloy. As the finished electronic assemblies heat up and cool down, or when exposed to stresses, the gold in the solder joint can weaken or embrittle it and fail. Given the above understandings, solder joint embrittlement is defined as a change of solder joint durability due to dissolution and/or reaction with a finish such as gold and/or palladium.
The changes are expressed in tin-based solders by the appearance of AuSn4 intermetallic compounds from gold finishes. The compounds can occur in the bulk of the solder joint, at the finish interface or in both locations. The compounds are brittle in comparison with the soft solder alloy. As a result, the ability of the joint to be robust when subjected to mechanical strains is reduced.
Limiting these localized weaknesses and maintaining a reliable solder joint is the reason, along with the corresponding testing to verify this, behind the J-STD specification change.
Changes in the J-STD-001 Specification
With the advent of the new version of IPC JSTD-001, the governing the assembly of printed circuit boards, changes to the procurement and preparation of gold plated parts for the military and aerospace markets has taken a new turn.
The following significant changes were made to the specification:
1. Gold embrittlement mitigation (gold washing), which when not done, was a process indicator for class 2 assemblies in "E" is now a defect if NOT done in "F"
2. All through-hole leads required to be hand-soldered regardless of gold thickness need to be "washed" (and hence are a reliability concern
3. A new warning that gold embrittled solder terminations can be present when the solder volume is low (i.e., very small components) or when the dwell time is not sufficient in the soldering process
Editor's Note: This article originally appeared in the October 2015 issue of SMT Magazine.