Effects of PCB design variations on bend and ATC performance of lead-free solder joints

Abstract

Sn-Ag-Cu (SAC) solder alloys, such as Sn-3.0Ag-0.5 Cu (SAC305) are the popular choices of lead-free solders replacing SnPb solders. However, SAC solders are more brittle in nature due to high stiffness and excessive intermetallic compounds growth at the solder joint to pad interface. This leads to higher risks in solder joints failures. Memory module type lead-free BGA packages are constantly under dynamic stresses during handling and thermal stresses during operations. It is important to understand the dynamic performance and long term reliability of memory module lead-free BGAs. It is believed that the PCB design variations cause dynamic and long term failure discrepancies in the fields. In this study, different pad and trace designs were introduced to evaluate the effects of PCB design variations on the bend and Accelerate Thermal Cycling (ATC) performance of lead-free solder joints. Pad designs with NSMD (Nonsolder mask defined), SMD (Solder mask defined), and a unique web design were assembled and tested. Different solder alloys including SAC305, SAC105, and SnPb solders, have been evaluated in this study. Different PCB materials have also been evaluated in the test. Four point monotonic bend tests were performed to characterize the bending performance variations with different PCB designs and compared with conventional Sn-Pb solder. SMD pad is shown to have the best bend performance among all other types of designs in this study. In addition, this design also shows improvement in mitigation of PCB pad cratering with leadfree solders. Wide trace width seems to degrade the strength and is not preferred. Same as its superior shock resistance comparing with SAC305, SAC105 solder alloy shows better bend performance. There is no significant improvement of bend performance with Web Design. After aging treatment, bend performance of both SAC305 and SAC105 degraded by up to 34% and 29% respectively. However, the bend performance of eutectic SnPb was actually improved after aging. ATC tests were performed to investigate the effects of design variations on the long term reliability of lead-free solder joints; SMD design shows less reliability life than others. The implications of these results for the reliability of lead-free solder joints are discussed in this paper. IntroductionLead-free solder alloys, especially SnAgCu (SAC) alloys show higher stiffness than eutectic SnPb without aging [1-2]. As a result of the stiffness, it may induce defects, particularly under dynamic loading conditions. Dynamic performance of electronics under bend and shock conditions (mechanical loading) has been an important factor on electronics quality and reliability. Monotonic bend test represents a measure of the fracture resistance to flexural loading during operation,

A Systematic Analysis of Printed Circuit Boards Bending during In-Circuit Tests

“…To measure the strain data for each experimental test, strain gages were used. This is the most practical way to find the strains without affecting the response of the measured part [4,[6][7][8]10,14,17,18]. Three uniaxial strain gages (KFGS-1-120-C1-23) were applied in strategic positions of the PCB (Figure 4).…”

“…Several studies have been performed to analyse mechanically the bending effects on PCBs. The greatest number of works have resorted to three-and four-point bending tests, and are addressed to the packages' solder joints reliability and their main failure mechanisms [2][3][4][5][6][7][8][9][10][11]. There are also studies using drop-impact tests to understand the PCBs' dynamic bending effects on solder joint behaviour [12,13].…”