ICSR (Soldering and Reliability) Conference Proceedings

A Geometry And Size Independent Failure Criterion For Fracture Prediction In Lead-Free Solder Joints

Authors: Siva P. V. Nadimpalli and Jan K. Spelt
Company: Department of Mechanical and Industrial Engineering, University of Toronto
Date Published: 5/17/2010   Conference: ICSR (Soldering and Reliability)

Abstract: 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 hightemperature loading conditions.

Keywords: lead-free solder, fracture prediction, finite element analysis, PBGA

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