Examination of nickel underlayer as a tin whisker mitigator

Abstract

Tin (Sn) whiskers are electrically conductive crystal structures that may spontaneously erupt from Sn-finished surfaces. Tin whiskers present a reliability hazard in electronics, because of their potential to create unintended short circuits. In the past, researchers have suggested that the addition of a nickel (Ni) underlayer between the copper (Cu) base material and Sn plating may act as a mitigation strategy for whisker formation. To evaluate this claim, samples with Sn plated directly on Cu, and samples with a Ni underlayer between the Sn and Cu, were subjected to environmental exposure to induce whisker growth. Prior to the test, all samples were stored in an office environment for 2.5 years and little to no whisker growth was observed. Testing consisted of 1000 temperature cycles (−55°C to +85°C, 10 min dwells) followed by two months of elevated temperature humidity exposure (60°C and 85%RH). During the test, whisker length sand densities on the samples were periodically measured. All whisker lengths were measured in accordance with the JESD201 standard, taking the effective shorting distance of the whisker (between the root of the whisker and the point farthest away). We present a simple methodology for whisker length measurement as an alternative to the JEDEC-suggested technique of varying the observation angle of the whisker to see its maximum length.