Thermo-mechanical investigations on the effects of the solder meniscus design in solder joint lifetime for power electronic devices

Abstract

The crack and delamination of solder joints between base plates and DCB substrates of power modules is one of the most frequently encountered failure mode and studied in literature. In this paper we present numerical effects of solder meniscus design in solder lifetime prediction. Especially, we show the effect of singular points, which appear in the border edges or corners of DCB or solder joints geometries, in mechanical stress, strain and plastic work evaluations. Furthermore the effect of mesh density on mechanical stress and strains is shown. Finally, we will see the effect of the meniscus design of the solder joint on lifetime estimation using both plastic strain-based and energy-based models. IntroductionThermal cycling is often responsible for thermomechanical damages of power electronics devices [1]. Such constraints lead to crack initiation followed by crack propagation inside solder attach materials. In case of IGBT power modules, the weakest attach layers are the solders between DCB (Direct Copper Bonding) and component base-plates due to their large areas [2].In previous works [1,3], scan acoustic microscopy (SAM) analyses have shown that DCB substrates are rarely attached to baseplates with homogeneous layer thickness. The most frequent case is an over-thickness in one of the substrate corner and an under-thickness at the opposite one. In such a case, it has been experimentally and numerically shown that the thinnest corner is the weak area for crack initiation when it is subjected to thermal cycles [4]. Nevertheless, too much thick solder layers are not desirable because its results in higher thermal resistances. Generally, after thermal cycles, crack initiation of the solder occurs at the corner of DCB metallization as shown in figure 1 and then propagates from this point. Thus, the geometrical shape and the solder amount at these locations should have some importance for the solder lifetime. As illustration, figure 2 shows a typical solder joint meniscus shape.Many authors have published results concerning lifetime evaluation of such solder joints for electronic devices. Unfortunately, geometrical singularities like border edges or corners make the simulation results delicate and very sensitive to the mesh density. So, these models are quite good for comparison of different design from each other but are not necessarily well-suited for realistic lifetime prognosis.

Comparison of stress distributions and failure modes during thermal cycling and power cycling on high power IGBT modules

“…Another interesting observation is the location of crack initiation. In a previous paper [6], we have shown with simulation of realistic solder shape that location of maximum mechanical stresses in the solder is not near the solder edge but rather in a solder region in contact with the corner of the DCB metallization. Experimental results presented above confirm these calculation results.…”