The influence of test parameters and package design features on ball shear test requirements

Ball shear testing is the most common test method used for assessing solder ball attachment quality on area array packages. Area array ball shear test methods and equipment are modified from those originally developed to test the bond between the gold ball and semiconductor pad. New package designs have a range of package pitch, pad sizes, solder ball diameters, and surface finishes that can complicate the basic ball shear testing and subsequent data analysis. The current area array standard, generalized for the case of typical 1.27 mm pitch EGA packages, needs to be expanded to address many of the issues and problems associated with the rapidly evolving area array technology. One persistent problem in the industry is the random occurrence of poor attachment quality, manifested as catastrophic brittle fracture. Although failure mode has been added to the ball shear acceptance criteria, the optimum parameters to test for this defect are not defined or specified. Many investigators recommend thermal preconditioning in conjunction with ball shear to expose latent brittle defects. Isothermal aging and exposure to multiple reflow profiles are used commonly as thermal preconditioning but there is no universal set of test parameters. This investigation identifies critical test parameters and package variables and evaluates their effect on ball shear test data. Shear tests are conducted as a function of the test displacement rate and two key thermal preconditioning variables: 1) the number of reflow cycles used for reflow preconditioning and 2) the time and temperature for isothermal aging. The package variables of interest are the bond attachment pad diameter and the solder ball diameter, which are related to the pitch of the package. The results are discussed in terms of accurate interpretation of shear test data and suggestions are made for additions to the existing bail shear test standards.