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Fine Pitch Solder Joint Fuze Electronics Under Mechanical Shock Loads



Fine Pitch Solder Joint Fuze Electronics Under Mechanical Shock Loads
The survivability of 0.4mm pitch and 0.5mm pitch parts at acceleration levels upto 50,000g have been studied for bare test boards.
Analysis Lab

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Authored By:


Pradeep Lall, Ph.D., Kalyan Dornala
Department of Mechanical Engineering
Auburn University Auburn, AL, USA

Jason Foley, John Deep
Air Force Research Lab
Eglin, FL, USA

Ryan Lowe
ARA Associates
Littleton, CO, USA

Summary


Commercial electronics parts are increasingly being used in high-g fuzing applications. Sustainment of long-term systems requires the understanding of the survivability limits on newer fine pitch part architectures. In this study, the survivability of 0.4mm pitch and 0.5mm pitch parts at acceleration levels upto 50,000g have been studied for bare test boards, underfilled test boards, and potted test boards. In consumer electronics, survivability in mechanical shock is ascertained using the JEDEC JESD22-B111 test standard which involves the use of 1500g, 0.5ms shock pulse in a standard 132mm x 77mm, 15-parts test board. Consumer products may be designed in a number of form factors which may likely differ from the test board size and part configuration. Correlation of performance in JEDEC test to actual product performance is often weak. In order to alleviate this limitation of the JEDEC test, the test assembly in the present study has been designed in the form factor of the actual use application to be circular with an annular ring in the center.

Two categories of underfills has been used including Lord Thermoset ME-531, and Loctite UF 3811. Two categories of potting compounds have been used including Armstrong A12, Henkel Stycast 2850FT. Armstrong A12 is a low modulus material and Henkel Stycast 2850FT is a high modulus material intended for shock applications. Digital image correlation has been used to measure the strain field of the board assembly during impact. Failure of interconnects has been detected using a high-speed data acquisition system. The transient dynamic motion of the board assemblies and the strains in interconnects have been modeled using ABAQUS Explicit finite element models.

Conclusions


Fine-pitch electronics for fuzing applications has been studied with high speed video in conjunction with 3D-DIC measurements for measurement of board strains under highg mechanical shock. In addition, explicit finite element models have been used to study the transient dynamic behavior and predict the board strain and out of plane deformation of the assemblies. Model predictions have been used to extract the solder joint strains for g-levels in the neighborhood of 10,000g-50,000g in unreinforced, underfilled, and potted assemblies. Experimental data indicates that potting adds survivability margins for shock exposures upto 10,000g. However, for shock exposures higher than 10,000g the delamination failure mode between the potting compound and the printed circuit board dominates reducing the design margin. Underfilling of the electronic components added survivability margin at all glevels. The highest I/O, fine-pitch components (CVBGA360, 0.4mm pitch; CTBGA228, 0.5mm pitch) in the study showed the poorest wsurvivability in the study at all the g-levels studied. Kalman filter was able to track the damage accrued in shock- events accurately and prognosticate remaining useful life (RUL).

Initially Published in the SMTA Proceedings

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