Saeed Akbari, Amir Nourani, Jan K. Spelt
University of Toronto
Toronto, ON, Canada
The effect of the joint length on the fracture of coppersolder-copper joints were investigated using doublecantilever-beam (DCB) specimens of various joint lengths. It was found that for solder joints shorter than a certain characteristic length the strength increased with increasing joint length. This characteristic length was found numerically using a finite element model to plot the maximum peel stress of the solder joint versus the joint length.
The length at which the plot reaches a plateau was considered as the characteristic length. Characteristic lengths obtained from experiment and finite element analysis were in close agreement, indicating the validity of the numerical method. The practical importance of the characteristic length is that solder joints longer than the characteristic length have a maximum peel stress that remains unchanged with joint length, causing the joint strength to become independent of the joint length. In other words, the use of joints longer than the characteristic length provides no additional strength to the joint.
The fracture strength of mode-I copper DCB specimens were measured with SAC305 solder joints of various lengths. Joint strength increased with increasing solder length before reaching a plateau value of constant strength. The corresponding finite element model showed that the maximum peel stress became independent of the length of the solder layer beyond a characteristic length which was very close to the solder length corresponding to the initiation of the measured joint-strength plateau.
This supported the hypothesis that solder joints reach a maximum strength at the characteristic solder length, and that further increases in length do not result in stronger joints. The concept of the characteristic length was illustrated in a hypothetical chip resistor assembly loaded by board bending. The characteristic length of a typical chip resistor solder joint was predicted to be about 1 mm, as determined from the finite element predictions of the maximum solder peel stress as a function of the joint length.
Initially Published in the SMTA Proceedings