Dynamic Analysis to Confirm Lead Failure of a 352-Pin CQFP Under Shock

During qualification testing of an electronics module, several leads in one corner of a 352 pin ceramic quad flat pack (CQFP) component failed. The module was exposed to several different environments including sine vibration, thermal cycling, random vibration, and shock. The last test environment applied was seven consecutive shocks normal to the printed wiring board. Given the severity of the shock response spectrum, it was believed that the shocks normal to the board were the culprit. Therefore, a finite element model (FEM) was created of the module to diagnose the cause of the failure. The FEM modeled all 352 CQFP leads using quadratic beam elements. Besides the CQFP, the FEM also included the aluminum frame, the printed wiring board, and several adjacent components. It was validated by comparing the board’s mode frequencies and shapes computed in ANSYS to those imaged by optoelectronic holography on the test hardware. ANSYS was also used to rule out sine vibration, random vibration, and thermal cycling as causes of the failure. To evaluate the stress levels in the leads during the shock pulse, the actual acceleration experienced by the hardware during a shock pulse was recorded and used in an explicit dynamic analysis in LS-DYNA. In addition, a bilinear elastic-plastic material model was used for the kovar leads. The analysis showed that the suspect leads reached their ultimate tensile strength by the fourth consecutive shock. These results confirmed that the leads failed due to the consecutive shock pulses. The FEM was subsequently used to evaluate a redesign of the module to mitigate the risk to mechanical shock.