Electronics Assembly Knowledge, Vision & Wisdom
Intermetallic Crystals and Role in Microbridges
Intermetallic Crystals and Role in Microbridges
Several short circuit failures caused by copper tin intermetallic crystals, the root cause, and rework techniques that can be used to prevent its occurrence.
Analysis Lab

Authored By:
Jeff Kukelhan
BAE Systems Electronics, Intelligence, & Support
Fort Wayne, Indiana USA
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Summary
Wave soldering is a mature manufacturing process that metallurgically joins component and PWB termination features by passing them together across the flowing surface of a molten solder reservoir. During this exposure, copper from through holes, surface mount lands, and component leads, continually dissolves into the molten solder. Unless the solder in the reservoir is regularly changed, the dissolved copper eventually reaches a point of saturation, and orthorhombic Cu6Sn5 crystals begin to precipitate out of the molten solder, causing it to become gritty and sluggish. Solder drawn from such a saturated wave solder pot can solidify into joints whose surface finish exhibits many needle like metallic protrusions.

These protrusions are in fact orthorhombic Cu6Sn5 crystals. Recently, BAE Systems has determined that this same phenomenon is responsible for the formation of nearly invisible intermetallic microbridges between fine pitch surface mount component leads. They form when a solder bridge from a surface mount paste reflow operation is hand reworked with a soldering iron and copper desoldering braid. This paper documents several short circuit failures caused by this phenomenon, the investigation that identified the root cause of the problem, and the rework techniques that can be used to prevent its occurrence.
Conclusions
A mechanism has been identified that details the steps by which nearly invisible short circuit paths form between adjacent fine pitch gull wing leads on Sn63/Pb37 and Sn62/Pb36/Ag2 solder paste reflow assemblies. These short circuit paths, which have been named microbridges, are comprised mainly of orthorhombic Cu6Sn5 intermetallic crystals. The crystals initially form during the original solder paste reflow operation, and upon solidification, can be found within the solder joints of fine pitch leads, as well as any reflowed solder paste bridges that might join them. Cu6Sn5 crystals continue to grow and concentrate when exposed to the conditions associated with a common hand rework process meant to remove solder bridges.

Under the right rework conditions, a single large Cu6Sn5 crystal, or multiple small Cu6Sn5 crystals linked by a thin film of solder, can lodge between adjacent fine pitch gull wing leads, forming a microbridge that short circuits the affected component terminations.

The hand rework method used to remove fine pitch solder paste bridges was reviewed and modified. The revised method minimizes the chance that a microbridge will form when hand reworking a solder bridge between adjacent fine pitch gull wing leads on a solder paste reflow assembly. An additional hand rework method was developed that allows for the effective elimination of an actual microbridge should one form while reworking a fine pitch solder bridge.
Initially Published in the IPC Proceedings
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