Fracture Prediction in Lead-Free Joints

Siva P. V. Nadimpalli and Jan K. Spelt
Department of Mechanical and Industrial Engineering
University of Toronto, Toronto, ON, Canada

Solder joint fracture due to mechanical loads, such as drop impact and board bending, is a significant reliability concern, but little attention has been paid to the development of methods of predicting solder joint fracture under such loading conditions. This paper evaluates a failure criterion that can predict joint failure independent of joint size and geometry. In the first part of the work, continuous and discrete SAC305 solder joints of different lengths were made between copper bars using standard surface mount processing conditions, and then fractured under various loading combinations: pure tensile stress, mixed tensile and shear stress.

The critical loads corresponding to crack initiation in the continuous joints were measured and used to determine the fracture parameters at initiation, Gci and Jci. This serves as a strength property for the solder-substrate system. In the second part of the investigation, fracture of discrete solder joints was simulated using elastic and elasticplastic finite element methods, and crack initiation was predicted using the measured Gci and Jci values for this solder system. The predictions matched reasonably well with the measured values. An interesting observation was that the failure of joints less than 2 mm in length can be predicted using the fracture parameters at initiation from continuous joints.

This suggests that Gci and Jci measured in this way should also provide a strength property that is applicable to failure prediction in much smaller microelectronic joints. In fact, some preliminary fracture predictions of solder balls in a PBGA package showed a good agreement with experimental observations. The scope of this study was limited to quasi-static loading, but the methodology can be extended to impact and high temperature loading conditions.

The copper bar DCB specimens made with continuous SAC305 solder layesr were fractured under pure tensile and various combinations of tensile and shear loads to measure the characteristic fracture energies corresponding to crack initiation, Gci and Jci. Using these fracture properties as a failure criterion, crack initiation loads in discrete 2 mm and 5 mm long joints under different loading combinations were predicted with finite element models. These loads were then compared with experimental data and the predictions agreed to within 14%. The same fracture criterion was then extended to predict the fracture of solder balls in a PBGA package with SAC405 tested under three-point bending.

The predictions agreed reasonably well with experimental observations, considering that Jci for SAC405 was estimated from the value for SAC305.