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Temperature Cycling with Bending to Reproduce Typical Product Loads



Temperature Cycling with Bending to Reproduce Typical Product Loads
A new methodology for solder joint reliability prediction taking into account the mechanical load as well as the thermal mismatch load between component, solder & PCB is proposed.
Production Floor

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Authored By:


Udo Welzel, Lauriane Lagarde, Yunxiang Wang, Fabian Schempp
Robert Bosch GmbH
Schwieberdingen, Germany

Summary


In automotive applications, electronic products are subject to several loads, among which thermomechanical loads are particularly affecting the product reliability. Temperature cycling tests on free PCB aim to reproduce the thermal mismatch load at solder joint level, based on coefficients of thermal expansion but does not take into account the real mechanical load applied in the product. The latter, influenced by design elements like housing or screws, has to be determined separately for each product. The new proposed approach enables electronic component design for reliability by developing an experimental and simulation test method combining products relevant thermal mismatch and mechanical loads. For this, PCBs undergo an experimental bending test under temperature cycling. For each tested component and each mechanical load level, a lifetime loss-factor is determined. The influence of component positioning on PCB, i.e. orientation is also considered.

Experimental bending test results are then combined with detailed simulation models for complex components, i.e. BGA, to establish Wöhler-like curves and lifetime models. The transfer of those generated models into relevant electronic products enables reliability prediction i.e. layout design. Moreover, bending test combined to temperature cycling allows the visualization of potential failure mode changes at high bending levels (i.e. IMC cracks). This is an advantage versus free PCB test to avoid field and test failures on product level by early detection of bending sensitive components. At long-term, this new method aims to reduce the experimental costs by using the generated lifetime models for further reliability predictions.

Conclusions


The new proposed method is based on experimental results from superimposed temperature and bending cycles, which are directly relevant to real product loads. The components can be tested in their final configuration, such as in-phase / out-of-phase or specific orientation.

The influence of PCB strain as well as component orientation on the reliability of solder joints has been demonstrated for a BGA component using the new developed experimental method.

The transferability of the board level bending test results to product level is investigated through additional passive temperature cycling test, in which the BGA components are placed in housed PCBs.

The combination of experimental results with validated simulation models is currently under development and will enable efficient design for reliability in products (in terms of thermo-mechanical loads), with the final aim to avoid product validation failures.

In the long-term, this methodology should enable costs and time savings in product development process, through reduction of the scope of component testing.

Initially Published in the SMTA Proceedings

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