Pressure Induced Tin Whisker Formation on SnCu Finish by Nanoindentation Creep

This paper presents pressure induced tin whisker formation by means of nanoindentation creep. Bright tin-copper finish was plated on a rectangular coupon made of phosphor bronze with nickel underlayer. Applied load was from 1 mN to 80 mN. Small tin whiskers were formed along a contact edge after the dwell time of 3000 seconds with the application of more than 4 mN. Since tin whisker growth produces new surface area, there is a threshold stress to generate tin whiskers. Based on the balance of energy, the threshold stress is related to the size (or diameter) of whisker. The finite element analysis revealed that a pressure to induce whiskers is 40 MPa for observed whiskers. This threshold stress corresponds with a proposed tin whisker formation model based on a balance of energy.