Whiskering behaviour of immersion tin surface coating

Abstract

In this paper the whiskering behaviour of immersion tin surface coating was studied in different environments. 2μm thick immersion tin layer on copper substrate has been tested. Five different environmental conditions have been applied: a reference (25°C/50% RH), two elevated temperature tests (50°C/15% RH and 105°C/15% RH) and two elevated temperature and humidity tests (40°C/92% RH and 105°C/100% RH). The whisker growth was studied by using Scanning Electron Microscopy (SEM). It was observed that the immersion tin layer was capable of growing tin whiskers. Most of the detected whiskers were the so called “nodule” type whiskers, approximately 3–9μm long with 1–2μm thickness. The structure of the whiskers and the tin layer underneath were examined with Focused Ion Beam (FIB) and Transmission Electron Microscope (TEM) equipped with Energy-dispersive X-ray spectroscopy (EDX) and X-Ray Diffraction (XRD) unit. It was found that the temperature induced intermetallic (IMC) layer growth was the main stress factor causing the whiskering of the immersion tin coating. In addition under the developed whiskers the IMC layer was found to be uneven. The observed whisker grew from a grain which has a preferred [01−2] grain orientation for large grains of an immersion tin layer.