Evaluating Desorption Baking Cycles for Moisture Sensitive Integrated Circuits
Baking moisture sensitive ICs requires matching package thickness and carrier heat ratings to balance desorption speed against intermetallic lead oxidation risks.
Purple Plague
Intermetallic Degradation
A destructive intermetallic compound formed at the junction of gold and aluminum conductors leads to mechanical failure and electrical opens in semiconductor devices. The phenomenon commonly called purple plague occurs when the two metals diffuse into each other at elevated temperatures, forming a brittle compound known as Au5Al2. This compound has a distinct purple coloration and is highly resistive.
Diffusion Phase
The formation of the purple alloy is driven by thermal energy, with diffusion rates increasing rapidly at temperatures above one hundred and fifty degrees Celsius. As the gold and aluminum atoms migrate, they create Kirkendall voids along the bond boundary. These microscopic voids coalesce over time, weakening the mechanical attachment and increasing electrical resistance.
Failure Prevention
To prevent this degradation, packaging engineers limit the exposure of gold-aluminum junctions to high temperatures or use alternative bonding materials. Monometallic systems, such as using aluminum wire on aluminum pads or copper wire on aluminum pads, eliminate the dissimilar metal interface entirely. Keeping operating temperatures low also slows the diffusion process to acceptable levels.
Structural Impact
The loss of mechanical integrity at the bond interface results in wire lifts and open circuits during thermal cycling or mechanical shock. In high-reliability applications, this failure can render an entire system inoperable without warning.