The study of bare-die FCBGA die damage in response to applied mechanical stress during heat sink assembly

Die chipping or cracking in bare-die FCBGA packages is occasionally but consistently reported by OEMs during system-level heat sink installation. Since heat sink assembly is often a manual process, it can be difficult to determine if the damage was the result of a latent component defect or was induced during heat sink assembly. Most heat sinks require some type of spring force to maintain sufficient mechanical contact with the target electrical component, such as spring-loaded push-pins or Z-clips. When installed properly, these spring systems will not damage the electrical component. However, if the heat sink was installed improperly (i.e., tilted, with excessive force, etc.), or if the full spring deflection is maxed out, die damage may occur. Published studies on this topic are limited, so the following analysis was conducted. In this study, a test method was developed to exert a repeatable, controllable force on the top of a bare-die BGA component. The force was varied in magnitude, load/impact angle, and load rate. In addition, criteria for systematically characterizing component damage response were developed. Based on the data collected, the authors were able to characterize and compare the propensity to create damage by different types of assembly forces, such as forces perpendicular to the component, off-angle, excessive, slow, etc. Detailed quantitative results are presented. The test methods and empirical data generated during this study are useful in identifying certain limits in procedures used for system-level heat sink assembly that should result in less component damage and higher yields.