Early whisker detection through intermetallic compound (IMC) grain size

Sn whisker has been a concern in the semiconductor industry. The whiskers will grow within few hours to months with a diameter of 1 micron up to a length of several millimeters. It is difficult to access the whisker impact for Sn plated lead frame. There are no test acceleration factors that can be established to promote a faster growth rate for the whisker. The countermeasure that generally practices is post-bake at 150degC for 1 hour on product. In this paper, an early whisker detection method is introduced. It is observed that the intermetallic compound (IMC) for Sn plated copper lead frame will be developed with different IMC grain size at different annealing hours. The IMC grain size will be developed along the grain boundaries of the Sn deposited and at the Sn-Cu interface layer. It is shown that the larger the IMC grain size will results in reduction of the whisker growth. An IMC grain size grading was developed and it can be used as an early detection method to judge on whisker impact.