SMT007 Magazine

SMT-Oct2015

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64 SMT Magazine • October 2015 rability 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 pal- ladium. The changes are expressed in tin-based sol- ders 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 com- pounds 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 J- STD-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 con- cern 3. A new warning that gold embrittled sol- der terminations can be present when the sol- der volume is low (i.e., very small components) or when the dwell time is not sufficient in the soldering process Impact on the user community The changes to the J-STD-001 specification will primarily impact the EMS and OEM build communities performing class 2 assembly work where their customer base has specified and routinely uses gold-plated components, or for those building class 3 products who have hand- soldered components onto the PCB. This im- pact will be felt in terms of having to outsource these tinning operations or bring this capability in-house. In addition, the contract assemblers will have to educate their OEM partners on these changes and how that may impact lead time and costs of their finished assemblies. If this process were to be brought in-house or purchased from a service provider there are several key attributes to ensure successful, re- peatable, and consistent tinning processes: Flux choice must match the activity level needed to remove the old finish (as well as the end customer's specifications). It is prudent to test the various fluxes using a wetting balance test to determine the wettability of the solder prior to jumping into production mode. Also it is smart to make sure that the flux does not harm or impair the component body or seals. The right amount of flux volume is also key, as both the components and the fixtures need not get unnecessarily contaminated. After process- ing it is advantageous to have XRF analysis per- formed in order to confirm the elemental analy- sis of the re-tinned component leads. Fixturing needs to be done very accurately and be flexible with the numerous variants of different lead and component styles. The fix- ture's job is to make sure the component bodies and leads are exposed to the solder and the flux " 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. " knOCking dOwn The bOne Pile GoLd eMbrIttLeMent MItIGAtIon

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