Piezo-induced fatigue of solder joints

Abstract

Piezomechanical loading of an adhesive joint is a very close analogue to the loading imposed by adherends with dissimilar thermal expansion coefficients under a temperature change. Using this concept, a double lap joint test specimen was developed to investigate the damage mechanisms in solder joints for electronic packaging applications under cyclic loading conditions. Analytical results are derived for the plastic shear strain at the free edges of such a specimen using a shear lag model. Results are also presented for the strain energy release rate for steady state crack growth. Experimental results and observations are presented for the damage processes in lead-tin eutectic solder joints between PZT-5H adherends. The lifetime of a specimen can be divided into two regimes: initiation and steady crack growth. Cracking was generally observed to initiate at voids and other defects in the solder joint. The time to initiate damage, the total joint life, and crack growth rates were quantified as a function of applied loading. Data for damage initiation was quite scattered, reflecting the variation in joint quality, but broadly conformed to a Coffin-Manson relationship. The data for crack growth rate approximately corresponded to a Paris law at higher applied voltages. At lower voltages, a strong dependence on frequency was observed, and there was evidence of a threshold strain-energy release rate. Crack growth rates increased with increasing temperature over the range 0- 25°C, but decreased up to 80°C. The appendices detail the specimen manufacturing techniques, the experimental set-up, and the testing methods and regime. An expanded literature review for this work is also provided in the appendices for further background and insight as to the issues surrounding adhesive fatigue.