Life-time-test investigation of solder joints formed with tin-silver-copper and micro-alloyed solders

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Supervisor:
Dr. Krammer Olivér
Department of Electronics Technology

The silver in the widely used tin-silver-copper solder alloys is used for its excellent electrical and mechanical properties. However it has two major disadvantages: higher price and in the case of lower cooling speed the formation of intermetallic compounds can become starting points of fracture, that can lead to breakage and mechanical failures. I have chosen my thesis topic to examine whether the higher silver content could be replaced by some other micro-alloyed solder which not only provides the required mechanical strength so far, but also reduces the risks associated with different cooling slopes, respectively that has a favourable price.

During my research, the literature of reflow soldering technology was reviewed highlighting the soldering process. The thermodynamic process taking place in the solder alloys was presented. The role and formation of the intermetallic compounds in the solder joints were demonstrated. The life-time-investigation tests were summarized and the effect of temperature and thermal fatigue were detailed. Based on literature, two accelerated life-tests: the temperature cycle and the thermal shock tests were compared.

An experiment was designed to examine the life of solder joints. The experimental samples were created, initial measurements were carried out and the samples were subjected to an accelerated life test. The intermetallic layer structure was investigated on cross-sectional grindings and the mechanical strength was determined by shear test.

Based on the measurements of the shear strength, the SACX0307 alloy’s initial strength was 10,5 N, while in the case of SAC305 it was 14 N. As an effect of the Thermal Shock, this difference was decreased, after the 2500 cycle this values were 7 N and 9 N. For both alloys, the largest relative shear strength decreasing were formed at the fastest cooling (2,8 °C/s). Based on the Weibull-plot of the results, the characteristic life-times were calculated for the different samples. As a result, the characteristic life-time of the samples were increasing in proportion with the decreasing of the cooling speed of the soldering, in the examined range.

Based on the measurements of the intermetallic layers’ height, the samples were comprised pairwise, and it was found that the SAC alloy’s IMC height is about 10% larger than that of the SACX. The effect of the different cooling speed appeared similarly in both alloys’ growth: the slower the cooling process, the thicker the formed IMC layer. During the TS test, the growth rate of the IML was the greatest for the SACX0307 samples with slow and medium cooling speed. The reason for this is that the initial height of the IML was the minimal for these samples. However the relative decreasing of the shear strength were the smallest also for these samples.

Based on the results of the measurements, among the examined samples, the SACX0307 with slow– 1 °C/s – cooling the most reliable with the longest expected life-time.

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