Lead-Free Solder Pull-Off Stress Comparison of a Novel Bump Pull Method with Conventional Hot/Cold Bump Pull Methods

Abstract

A novel method for directly testing the adhesion strength of three lead-free solders was developed and compared with conventional methods. The Isotraction Bump Pull method utilizes a combination of favorable qualities of the Cold and Hot Bump Pull tests. Solder bumps were generated onto copper printed circuit board substrates using an in-house-fabricated solder bump-on-demand generator. The method uses polymer epoxy to encapsulate solder bumps under uniform tractions, and tested under tension for pull-off stresses. Maximum pull-off stresses for the novel method are: 18MPa (Sn-3.5Ag), 16MPa (SAC 305) and 22MPa (Sn-0.7Cu) and fall at the low end in the literature comparisons. It is suggested that since the copper substrates used in the current work were untreated, that the lower pull-off stress values resulted. Energy Dispersive X-Ray Spectrometry of the newly created faces after fracture shows that brittle fracture of the Intermetallic Compound layer was the mode of failure..