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Comparison of Active and Passive Temperature Cycling



Comparison of Active and Passive Temperature Cycling
This paper describes the various accelerated temperature cycling tests, compares and evaluates the related degradation of solder joints.
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Authored By:


Mathias Nowottnick, Andrej Novikov, Dirk Seehase, and Chris Kronwald
University of Rostock / Institute for Electronic Appliances and Circuits
Rostock, Germany

Summary


Electronic assemblies should have longer and longer service life. Today there are partially demanded 20 years of functional capability for electronics for automotive application. On the other hand, smaller components, such as resistors of size 0201, are able to endure an increasing number of thermal cycles until fail of solder joints, so these are tested sometimes up to 4000 cycles. But testing until the end of life is essential for the determination of failure rates and the prognosis of reliability. Such tests require a lot of time, but this is often not available in developing of new modules.

A further acceleration by higher cycle temperatures is usually not possible, because the materials are already operated at the upper limit of the load. However, the duration can be shortened by the use of liquids for passive tests, which allow faster temperature changes and shorter dwell times because of better heat transfer compared to air. The question is whether such tests lead to comparable results and what failure mechanisms are becoming effective. The same goes for active temperature cycles, in which the components itself are heated from inside and the substrate remains comparatively cold. This paper describes the various accelerated temperature cycling tests, compares and evaluates the related degradation of solder joints.

Conclusions


As mentioned in the evaluation, for the thermal shock cycle test the upper and lower temperature limits should be optimized separately. The utilization of maximum deformation by the application of full temperature range, e.g. from -40 Celsius to +125 Celsius, is obviously inefficient, because this will take effect for several hours dwell time. It has to find a compromise between the maximum usable temperature difference and the shortest possible dwell time at the lower temperature limit. Fig. 14 illustrates this compromise schematically.

In addition to adjusting the temperature limits and dwell times, there is still some potential for improvement in the reduction of the warm-up and cool-down times. For small and low-mass assemblies is the heat transfer by convection sufficiently effective. For heavy assemblies, it may be useful to improve the transfer of heat by a fluid. However, the experiments have also shown that active heating of components by current does not cause the desired aging of solder joints. What certainly is a positive effect for the practical application of assemblies, affects the accelerated aging tests unfavorable.

transfer should be further investigated and evaluated in future studies. It has to be considered that the optimum for different components and materials may require different parameters. The possibility of active heating of the substrates would be also interesting, as already explained above. With standard printed circuit board that is certainly almost impossible. Nevertheless, even this approach should be pursued in research by alternative heating methods, for example by electromagnetic fields. It is also planned to analyze the evaluation of aged solder joints additionally by metallographic investigations of structural changes.

Initially Published in the SMTA Proceedings

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