Solder Coalescence
Assembly defect classifications define microscopic cavities that form in a flat array along the interface of a solder joint as structural weaknesses. Small planar micro-voids develop within the intermetallic compound layer of ball grid array connections or surface mount joints during the reflow process. These cavities reduce the mechanical strength of the solder joint and can lead to premature mechanical failure.
Outgassing Mechanism
Volatile organic compounds from the solder paste flux become trapped during the molten phase of the reflow profile. Fast heating rates can cause the solvent to vaporize rapidly before the solder has fully solidified, creating pockets of trapped gas. When these pockets align in a single horizontal plane, they form a continuous line of weakness across the solder joint.
This specific alignment is particularly hazardous because it facilitates the rapid propagation of cracks when the board is subjected to drop tests or mechanical vibrations.
Diagnostic Inspection
High resolution x-ray imaging and cross sectional microscopy are used to identify these interface voids. Standard visual inspections cannot detect them because they reside deep within the joint structure. Measuring the cumulative area of the voids allows inspectors to determine if the board complies with ipc standards.
