Creep analysis of lead-free solders undergoing thermal loading
Lead solders, such as the popular tin-lead variety, have been used in the electronics industry for the past 50 years; thus, their long-term reliability is well understood.
However, lead is highly toxic and regulations are requiring the use of lead-free materials. The accelerated application of lead-free materials has brought new challenges in simulating the realistic physical behaviour of lead-free solder.
Electronic components are subject to a large number of thermal cycles. Mismatch between the thermal expansion behaviours of the various materials may induce severe stresses, high enough to cause plasticity and creep. After a number of thermal cycles, the accumulation of large inelastic strains may lead to solder joint failure.
Abaqus FEA software from Simulia, a brand of Dassault Systemes, provides several creep models that are used for analysing the real-world behaviour of lead-free solder.
In this ball grid array model, 36 solder balls connect the silicon die and the substrate The bottom of the substrate is fixed, and no other direct mechanical loading is applied.
The results show that the distribution of equivalent creep strain (CEEQ) is resulting in thermal expansion mismatch between the solder and the surrounding materials.
High creep strain is indicated near the top and bottom surfaces of the solder balls. The solders with the highest creep strains are those at the four corners, suggesting that these are the critical joints for the simulated device.
Abaqus FEA features a large material library, extensive nonlinear analysis capabilities, and thermal-mechanical coupling, making it a powerful design simulation tool for the electronics industry.
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Simulia/Abaqus Inc is based in Providence, RI, USA. www.simulia.com
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