Electronic packaging solder joint reliablity assessment with a mechanics-based StrainGage methodology

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 by component suppliers and original equipment manufacturers (OEMs) 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. An analytical methodology is developed to determine the solder joint deformations from strain gage readings at various locations. This paper also proposes a new strain matching criterion. Most OEMs have been using maximum principal strain to match stress levels and define critical 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.