Analysis of Solder Fatigue on Mounted Test Assemblies under Thermal Cycling Loads

Reliability is an obvious requirement in particular for the developing use of automotive electronics towards autonomous driving. Micro-electronics must also stay reliable in mounting situations, denoted as “3
^rd
level reliability”, which has been studied systematically. A methodology combining FEA and high precision optical metrology based measuring techniques was applied to test setups, which achieve "3rd level" component loads either by screwing onto aluminum heatsinks or by superimposed four-point bending loads, as was described in [1]. This paper reports the results of both temperature cycle and temperature shock tests up to 900 cycles -40/125°C, the system level deformation effects at higher cycle loading states as analyzed by the high precision optical measurements, and related FE-analyses, which aimed at adaption of the measuring results from component to system level. From deformation measurements a complicated response of the boards to mounting has been observed, including locally different effective board CTEs as a function of cycle number and an irreversible board stretching. It turned out that standard simulation assumptions like neglecting molding compound initial intrinsic stress, the use of geometrically linear theory, and the assumption of homogeneous temperature fields across the systems do not longer hold for system analyses. Measurements revealed significantly different transient temperature fields at heatsinks and board, dependent on position. The transient temperature gradients across the system were adopted by coupled transient thermo-mechanical simulations. They reflect the transient board deformations different from those simulated by assuming homogeneous temperature changes. Solder joints fatigue loading was considered for these cases and comparison was made to previous results, depending on mounting and w/o or with transient temperature gradients. A significant increase of joint stresses was observed, also with regard to the homogeneous T-distribution assumption. Very early solder fatigue failure was predicted and observed from thermal cycle tests.