A mechanics-based strain gage methodology for solder joint reliability assessment

It is essential to understand solder joint strains to improve package reliability. However, it is often difficult to measure the true solder joint strains directly. Strain gages have been increasingly used to indicate package mechanical stress levels. One of the most used strain gage locations is on the component side, right next to the package corner. A concern with this location alone is that the strain gages mounted close to the package pick up the local stress concentration near the package edges or corners. In this paper, appropriate strain gage locations are suggested based on mechanics principles and finite element simulation results. The solder joint deformations are determined from strain gage readings at various locations. This paper also proposes a new strain matching criterion. Most original equipment manufacturers (OEMs) have been using maximum principal strain to match stress levels and define strain limits. A problem with maximum principal strain is that it does not indicate the direction along which the maximum bending occurs. Experimental data demonstrates that the new strain criterion has much better correlation to solder joint failure than maximum principal strains in various bend modes. The conclusions apply to any strain gage mounting metrology on packages attached to printed circuit boards (PCBs).