Leila Jannesari Ladani
Department of Mechanical and Aerospace Engineering
Utah State University, Logan, UT, 84341
Phone: 1- 435-797-0480, Email: Ladani@engineering.usu.edu
Department of Mechanical Engineering
University of Maryland, College Park, MD, 20742, USA
This study examines damage initiation and propagation in solder joints with voids, under thermo-mechanical cyclic loading. An accelerated thermal cycling test is conducted on Printed Wiring Assemblies (PWAs) containing 256 input/output (I/O) Plastic Ball Grid Arrays (PBGAs) with voided solder joints. Destructive and non destructive failure analysis of the solder balls are used to detect the presence of voids and to relate the extent of damage propagation to the number of thermal cycles. Particular cases of voided and damaged joints are selected from these tests, to guide the development of a strategy for modeling damage propagation, using a three dimensional (3-D) global-local Finite Element Analysis (FEA). The displacement results of the global FEA at the top and bottom of the selected solder balls are used as the boundary conditions in a local FEA model which focuses on the details of damage initiation and propagation in the individual solder ball. The local model is error-seeded with voids based on cases selected in experiment. The damage propagation rate is monitored for all the cases. The technique used to quantify cyclic creep fatigue damage is a continuum model based on Energy-Partitioning (E-P). A method of Successive Initiation (SI) is used to model the growth and propagation of damage in the selected case studies. The modeling approach is qualitatively verified using the results of the accelerated thermal cycling test. The verified modeling technique described above is then used for parametric study of the durability of voided solder balls in a ChipArray Thin Core BGA with 132 I/O (CTBGA132) assemblies, under thermal cycling. The critical solder ball in the package is selected and is error seeded with voids with different sizes and various distances from damage initiation site. The results show that voids in general are not detrimental to thermal cycling durability of the CTBGA132 assembly, except when a large portion of the damage propagation path is covered with voids. Small voids can arrest the damage propagation, but generally do not provide a significant increase in durability because the damage zone deflects around the void and also continues to propagate from other critical regions in the solder ball.
Keywords: Pb-free solder, Voids, Thermo-mechanical durability, Simulation, Energy Partitioning, Successive Initiation, Damage initiation, Damage propagation.
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