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In-Situ measurements of the relative thermal resistance: Highly sensitive method to detect crack propagation in solder joints

The crack propagation in solder joints is detected by measuring the relative thermal resistance using transient thermal analysis. The method can be applied on high power devices which have to dissipate a significant amount of thermal load through the solder joint. The thermal load in the device is switched and the forward voltage of the junction is measured in time resolved modality. As published earlier, to obtain the relative thermal resistance the dissipated power of the device and the proportional factor (k-factor) between temperature and forward voltage are not required when the time resolved forward voltage curves are normalized [1]. Aim of the research was to investigate the concept and the sensitivity of the measuring method using high power ceramic LED packages. For prove of sensitivity, two batches of high power LEDs were soldered on PCBs, i.e. aluminum insulated metal boards (Al-IMS): one batch with SAC305 and the other one with a SAC+ (Innolot) solder. The test modules were exposed to typical temperature cycles of -40°C / +125°C as required for automotive applications. With standard test methods differences in reliability, i.e. thermo mechanical fatigue, of the two solders were very difficult to detect for the module design under these test conditions [2]. First, we investigated the relative thermal resistance after defined number of cycles at room temperature. To define a realistic failure criterion for calculation of the cumulative failure probability, we have set-up a finite element model and simulated the transient temperature curves. The increase of measured relative thermal resistance is critically compared and calibrated with transient finite element simulations. The Weibull curves were determined based on the failure criteria describing a 70% cracked solder area. Then, after resolving a significant difference in the increase of thermal resistance of the two solders, an In-Situ test system was set up and measurements were performed, i.e. a set of samples was measured in the temperature chamber under hot and cold condition.