Journal of SMT Article


Author: Tong Yan Tee et al.
Company: STMicroelectronics
Date Published: 7/1/2003   Volume: 16-3

Abstract: Reliability performance of IC packages during drop impact is critical, especially for handheld electronic products. Currently there is no detailed test standard in the industry to advise on the procedures for board level drop test, nor is there any model that provides good correlation with experimental measurements of acceleration and impact life.

In this paper, detailed drop tests and simulations are performed on QFN (quad flat non-lead) packages at board level using testing procedures developed in-house. The packages are susceptible to solder joint failures, induced by a combination of PCB bending and mechanical shock during impact. The critical peripheral solder joint is observed to occur at the corner lead. Failure is initiated along the solder fillet/lead edge, and it propagates through the top solder/lead interface at the component side.

Various testing parameters are studied experimentally and analytically to understand the effects of drop orientation, drop height, position of component on board, PCB bending, solder fillet, mass-scaling, and element size. Drop height, felt thickness, and contact conditions are used to fine-tune the shape and level of shock pulse required. Board level drop test can be better controlled; compared to system or product level test, such as impact of a mobile phone, which sometimes has rather unpredictable results due to higher complexity and variations in drop orientation.

At the same time, dynamic simulation is performed to compare with experimental results. The model established has close values of peak acceleration and impact duration as measured in actual drop test. The failure mode and critical solder joint location predicted by modeling correlate well with testing.

For the first time, a life prediction model is proposed for board level drop test to estimate the number of drops to failure for QFN. The average normal peeling stresses of critical solder joints are correlated with the mean impact lives measured during the drop test. With this new model, a failure-free state can be determined, and drop test performance of new package designs can be quantified and further enhanced through modeling. This quantitative approach is different from traditional qualitative modeling, as it provides both accurate relative and absolute impact life prediction.

Key words: QFN, drop test, life prediction, finite element modeling.

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