New evaluation method of CSPs board level reliability using straingauge

摘要

The CSP is a package with external dimensions that have closelyapproached those of its chip in recent years. For this reason, it isincreasingly being used in cellular phones. Unlike conventionalpackages, however, because the CSP does not have external lead terminalsit tends to lose reliability after mounting. With cellular phonemanufacturers increasingly requesting mechanical stress testing inaddition to temperature cycle tests and other thermal stress tests,emphasis is growing on evaluating the reliability of the CSP aftermounting. The evaluation of mechanical stress is greatly influenced bythe PCB on which the CSP is mounted. Because mounting densities areincreasing and specifications and production techniques differ greatlybetween cellular phone manufacturers, testing in relation to a fixed setof parameters such as displacement and load often leads to reliabilityproblems when the CSP is installed by the customer under conditionsdifferent from those tested for by the semiconductor manufacturer. Toaddress this situation, we herein proposes a method whereby a straingauge is used to measure the strain profile under various testconditions. The profile is then analyzed to establish correlationsbetween the results of various mechanical stress tests. Furthermore, byusing a heat-resistant strain gauge, this method can also be used forthermal stress testing, such as temperature cycle testing. This reportdescribes an analytical method developed by we that is based on strainprofiles generated in a mounting PCB when mechanical stress was appliedespecially