Crack Growth Rate Measurement and Analysis for WLCSP Sn-Ag-Cu Solder Joints

Abstract

Thermal cycle fatigue models for lead free solder joints are still in their infancy. Most of these models can predict fatigue life under specific conditions. A more general model requires a fundamental investigation of the phenomena. While some published data on the constitutive behaviour of lead free solder alloys exists and while several empirical models are available for the fatigue life prediction based on finite element inelastic strain analysis, there has been very little focus of measuring the actual crack growth rates in solder joints during thermal cycling. In this paper we have attempted to measure crack growth rates in Sn/Ag/Cu solder joints subjected to thermal cycling with dye penetration tests and crack area image analysis. The devices used were stiff wafer level CSP's which were daisy chained for in-situ electrical monitoring. The solder joints were ∼0.2 mm in height. The crack growth rate measurements have been conducted for 3 temperature profiles, −40°C to +125°C, 0°C to +100°C and −65°C to +100°C with a frequency of 1 cycle per hour, resulting in an estimated strain range of 0.01 to 0.1. For each imposed strain value, a total of 8 joints were analyzed. In all the test conditions, increase in crack area was observed to be a linear function of number of thermal cycles while crack initiation was observed at less than 5% of the total fatigue life. We have correlated the crack growth rate results with the average inelastic strain and strain energy density accumulated in a single cycle with the help of finite element analysis.