Observation of the bonding interface between ALN ceramics and metal base with the ultrasonic imaging system

During the thermal cycle test of semiconductor modules with Si chips, AlN ceramics and metal bases for heat sink connected to each other by soldering, the thermal resistance of some specimens increased in a comparatively short test period. By observing the interiors of these specimens using an ultrasonic survey imaging system, a strong reflected wave was observed at the solder connection joints of the AlN and the metal base. The area of this reflected wave was in the circumferential part of the sample at the initial stage but extended to the center with the thermal cycles. The reflected wave observed in these specimens was caused by a gap due to the breakage of solder because of the difference in thermal expansion coefficients between AIN and the metal base. It has been clarified that the reduction of the observed connection area agreed with the analytical estimation based on the thermal fatigue failure of solder, and the relationship between the reduction of the connection area and the increase of the thermal resistance also was clarified empirically. Moreover, it was confirmed that the ultrasonic imaging system is applicable to evaluation of the connecting conditions between semiconductor chips, ceramics substrate and metal bases because it can observe clearly the voids inside the solder which cause thermal resistance fluctuation.