New evaluation method of CSPs board level reliability using strain gauge

The CSP is a package with external dimensions that have closely approached those of its chip in recent years. For this reason, it is increasingly being used in cellular phones. Unlike conventional packages, however, because the CSP does not have external lead terminals it tends to lose reliability after mounting. With cellular phone manufacturers increasingly requesting mechanical stress testing in addition to temperature cycle tests and other thermal stress tests, emphasis is growing on evaluating the reliability of the CSP after mounting. The evaluation of mechanical stress is greatly influenced by the PCB on which the CSP is mounted. Because mounting densities are increasing and specifications and production techniques differ greatly between cellular phone manufacturers, testing in relation to a fixed set of parameters such as displacement and load often leads to reliability problems when the CSP is installed by the customer under conditions different from those tested for by the semiconductor manufacturer. To address this situation, we herein proposes a method whereby a strain gauge is used to measure the strain profile under various test conditions. The profile is then analyzed to establish correlations between the results of various mechanical stress tests. Furthermore, by using a heat-resistant strain gauge, this method can also be used for thermal stress testing, such as temperature cycle testing. This report describes an analytical method developed by we that is based on strain profiles generated in a mounting PCB when mechanical stress was applied especially.