Effect of latching force on socketed BGA packages with Ni-Au coated solder spheres

摘要

Solder BGA packages, when used in sockets, face multiple reliability challenges such as damage to the solder and increase in contact resistance due to surface oxides on the solder and intermetallic formation at the interface with socket paddle. To overcome these challenges, a new socket to package interface technology was proposed, which involves the surface modification of solder spheres with diffusion barrier-noble metal coatings, to provide a stable mechanical contact with the socket paddle and mechanical damage resistance. This paper evaluates the effect of socket latching force on the mechanical responses of BGA packages fabricated with solder spheres coated with Ni-Au coating. Effect of different paddle geometries, flat and curved, was also evaluated and results were compared with those for packages with uncoated solder spheres. It was understood for both paddle geometries, that for latching forces beyond 0.2 N per sphere, the equivalent stress generated in the Ni coating layer was greater than the compressive strength of Ni, which would lead to cracking of the Ni layer. It was also confirmed that the contact diameter changed marginally within a range of 2–3 um with the increasing latching force, which was attributed to the very high modulus of Ni which resisted deformation of the sphere.