Electromigration in Large Volume Solder Joints

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Among the various reliability related concerns in automotive and power electronics, electromigration in solder joints might be one of the least investigated degradation effects. It requires high current densities that usually only occur in small flip chip solder joints. Even if the current densities in power electronic components, like IGBTs (TO 263), cannot be compared with that in ultra small flip chip joints, there is an enormous thermal factor that helps to promote electromigration processes even in the larger volume solder joints of power electronic components. The scope of the paper is to present the results of electromigration and thermomigration experiments on large volume solder joints of power electronic components (TO packages). Two solder alloys - SnAgCu and SnPb - were tested for electromigration behaviour. An experimental setup will be described that allows electromigration testing at very high absolute currents (up to I = 100 A) which are necessary for electromigration testing of these large volume solder joints. The numerous difficulties of transmitting such high currents to the solder joints of the relevant TO-components will be discussed. Thermographic analysis of the test structures will be presented that show accurate adjustment of test temperatures at the solder joints of the TO-components. Metallographic analysis show the migration of lead metallisation into the solder joints and therefore grow of intermetallic phases. However, the degradation of the large volume joints is different from that of ultra small flip chip. Instead of severe Kirkendall voiding there is rather changes in the microstructure of the solder. The most important degradation mechanisms are the formation of intermetallic phases in the solder and phase separation. The paper will describe the electromigation processes in the tested large volume joints. The degradation processes will be compared to that of degradation by thermomigration, in order to separate the electrical driven effects from the thermal effects. The results on large volume joints will be compared to that of small volume joints. Also a comparison between SnAgCu and SnPb solder will be presented.