Electroplating Bath Organic Impurity Degradation and Intermetallic Interface Voiding Dynamics
Organic bath impurities codeposit in plated copper, accelerating Kirkendall voiding at intermetallic boundaries during reflow and degrading joint strength.
Planar Microvoid Coalescence
Meaning
Degradation mechanisms where small cavities align along a single plane define planar microvoid coalescence. This process occurs primarily at the interface of solder joints during thermal aging. It represents the final stage of void development before a complete mechanical fracture happens.
Void Initiation
Diffusion imbalances at the boundary between copper and tin create individual vacancies known as Kirkendall voids. These small holes appear when copper atoms migrate into the solder faster than tin atoms can move into the copper substrate. The planar microvoid coalescence begins when these scattered vacancies grow and begin to touch.
Spatial Alignment
Environmental stress causes the voids to organize into a continuous horizontal line across the bond interface. The planar microvoid coalescence is most common within the Cu3Sn intermetallic layer. This alignment reduces the effective load bearing area of the metallic connection.
Fracture Mode
Brittle separation occurs when the assembly is subjected to mechanical pulling or shearing forces. Because the planar microvoid coalescence creates a pre-existing path for cracks, the joint fails at a much lower force than a healthy bond. Microscopic inspection of the fracture surface reveals a smooth, dimpled appearance characteristic of this failure.
Prevention involves the use of high purity copper and optimized plating additives to reduce the initial vacancy flux.