Study on Board-Level Drop Impact Reliability of Sn–Ag–Cu Solder Joint by Considering Strain Rate Dependent Properties of Solder

Board-level drop reliability test and analysis require dynamic characterization of the high strain-rate properties of the solder, along with solder joint failure tests. Relying on just the drop impact analysis of board-level dynamic response (i.e. G-levels and board bending strains) and an oversimplification of deformation response of solder joints (i.e., assuming elastic stress criteria) can lead to misleading conclusions in the physics-of-failure understanding in drop impact tests. In this paper, impact tests were conducted with a split Hopkinson pressure bar test system to study the dynamic response of bulk solder materials. A finite-element analysis of drop impact was conducted by considering different solder constitutive models such as the elastic and strain rate-dependent plastic model to investigate the effect of the solder constitutive model on the dynamic response of the solder joint. The important finding of this study is that the constitutive model used has a major impact on the dynamic response of solder joint stress and strain results. Theoretically, it is predicted that the strain rate-dependent plastic model gave better correlation results than the simple elastic model. A solder joint reliability characterization using the drop impact test with a clamped-clamped boundary condition was investigated for a plastic ball grid array assembly with Sn-Ag-Cu solder and two board surface finishes. In order to assess drop reliability of solder joint with different surface finishes, Charpy test was conducted to evaluate the dynamic strength of the soldered specimen using Sn-Ag-Cu solder and with or without Ni/Au plating.

I. Introduction

Portable electronics devices like mobile phones, personal digital assistants (PDA), and laptops have to be designed to withstand repeated drops because it is common for these products to be subjected to accidental drop impacts resulting in damage and hence failures. Therefore, the reliability characterization of electronic assembly subjected to drop impact is a major concern. Lead-free solder usually exhibits less drop impact life than lead-based solder from many test results [1]– [4]. The study of impact reliability for solder joint by Date et al. [4] showed that the Sn–Ag–Cu solder was more prone to fracture at the interface than SnPb solder under impact test because of the higher bulk solder strength. Solder joint failures during drop testing is a complex failure interaction process between low cycle impact fatigue crack growth versus the brittle fracture of the intermetallic interfaces. During a drop test event, dynamic hardening causes the yield stress in the solder to rise several times above the normal monotonic tensile test yield stress. The increase in dynamic strength in the solder joint can cause dynamic strain cycling in the solder material and lead to progressive low cycle impact drop fatigue failures. On the other hand, when the drop loading is excessive, impact failure strength of the intermetallic compound (IMC) interface will result in a brittle fracture of the solder joint. Charpy test was commonly used for evaluating dynamic strength of solder joint and assessing IMC interface strength and failure mechanism.