Bump scaling and shear test requirements

Shear test has become an established method to assess quality and reliability of solder bump assembly techniques, also for reduced bump size. Several authors have found that median bump shear strength does not scale with the area of the shear surface, but only with its square root. This work shows that bump shear test does not progress along the theory developed for bolt shear, but follows a cutting by deformation of the bump along the shear tool edge. This difference comes as the bump is not confined in two sliding orifices as in the classical bolt shear test, but provides much less guidance and restriction. The implication of this fact, which will be substantiated with shear test results is, that minimum shear test requirements for scaled bumps is not proportional to the square of bump diameter as implied in the AEC-Q100-010A standard, but depends on its size linearly, thereby giving a much smaller rate of decrease of the shear force for scaled bumps. For bumps of smaller dimension this would result in a lower limit for shear strength requirements not scaled to the intrinsic strength and thus a too wide tolerance band for process variation.