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

Ball shear testing is the most common test method used to assess solder ball attachment quality on area array packages. New package designs have a range of package pitch, pad sizes, solder ball diameters and surface finishes that can complicate basic ball shear testing and data analysis. The current area array standard, generalized for typical 1.27 mm pitch BGA packages, must be expanded to address issues and problems associated with evolving area array technology. One persistent problem is random occurrence of poor attachment quality, manifested as catastrophic brittle fracture. Although the failure mode has been added to ball shear acceptance criteria, optimum parameters to test for this defect are not defined or specified. Many studies recommend thermal preconditioning in conjunction with ball shear to expose latent brittle defects. Isothermal aging and exposure to multiple reflow profiles are used commonly for 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 package pitch. The results are discussed in terms of accurate interpretation of shear test data and suggestions are made for addition to existing ball shear test standards.