Board Level Drop Impact—Fundamental and Parametric Analysis

Abstract

A fundamental understanding of the dynamics of the PCB assembly when subjected to a half-sine acceleration has also been obtained through analyzing the PCB as a spring mass system, a beam, and a plate, respectively. The magnitude of stresses in solder interconnection due to flexing of the PCB is two orders higher than the magnitude of the stresses induced by acceleration and inertia loading the IC package. By ignoring the inertia loading, computational effort to evaluate the interconnection stresses due to PCB flexing can be reduced significantly via a two-step dynamic-static analysis. The dynamic analysis is first performed to evaluate the PCB bending moment adjacent the package, and is followed by a static analysis where the PCB bending moment is applied around the package. Parametric studies performed suggest a fundamental difference in designing for drop impact and designing for temperature cycling. The well-known design rules for temperature cycling—minimizing package length and maximizing interconnection standoff—does not work for drop impact. Instead, drop impact reliability can be enhanced by increasing the interconnection diameter, reducing the modulus of the interconnection materials, reducing the span of the PCB, or using either a very thin or a very thick PCB.