Effect of Surface Roughening of Temperature-Cycled Ceramic-Metal-Bonded Substrates on Thermomechanical Reliability of Sintered-Silver Joints

We studied the reliability of power chip die-attach by sintered-silver on two types of ceramic-metal-bonded substrates: 1) aluminum-nitride direct-bond-aluminum (DBA) and 2) active-metal-brazed (AMB) silicon nitride-copper. The samples underwent a temperature-cycling test from -55 °C to 250 °C. Consistent with what we reported earlier, the surface roughness on both substrates grew with number of temperature cycles. The roughening rate on the DBA substrate was two times faster than that on the AMB substrate. To evaluate the effect of surface roughening on the sintered-silver joint reliability, we measured the transient thermal impedance of the bonded power chips. Despite the higher roughening rate, the sintered-silver bond on the DBA substrate had a longer lifetime - defined by a 20% increase in the transient thermal impedance of the bonded device - than that on the AMB substrate. Cross-sectional scanning electron microscopy showed the formation of vertical cracks in the sintered bond-line on the DBA substrate, as opposed to horizontal cracks on the AMB substrate. Formation of the vertical cracks in the bond-line relieved stresses without significantly impacting the thermal performance.