Effect of Intermetallic Morphology Evolution on Void Formation in Ni/Sn/Ni Micro Joints

A common approach to 3D assembly is to stack flip chips with micro bumps or 5-20μm thick caps of Sn or SnAg on Ni or Cu pads or pillars. The use of Ni leads to much slower growth of the intermetallic bond layers during and after assembly than Cu, but a thermal history with many reflows and perhaps deliberate annealing may still lead to complete reaction to form an intermetallic (IMC) joint. This has been shown to lead to the entrapment of Sn and eventually severe voiding when the uneven Ni
₃
Sn
₄
layers from the opposing pads impinge on each other. A relationship between the morphology of the Ni
₃
Sn
₄
and the formation of voids would seem credible as this IMC grows by diffusion of Sn through it to the Ni surface. A systematic study was therefore conducted to ascertain which, if any, design, materials, or process parameters might affect the morphology.As expected, the combination of alloy, solder thickness, Ni properties (high purity vs. electroplated Ni with and without additives), reflow profile, and subsequent thermal history did in fact have strong effects on both the initial IMC morphology and the growth rate. However, a new approach to the quantification of the IMC morphology of actual concern led to the conclusion that the only parameters that affect voiding are the solder thickness and the total pad area. This assessment was validated through measurements on joints between two Ni surfaces.