Whiskering behaviour of immersion tin surface coating

In this paper the whiskering behaviour of immersion tin surface coating was studied in different environments. 2 μm 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 3–9 μm long with 1–2 μm 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 [0 1 −2] grain orientation for large grains of an immersion tin layer.

► Immersion tin coating is also capable of growing tin whiskers.
► The temperature is the main factor that affects the whiskering in this case.
► Most of the detected whiskers were nodule type.
► Under the whiskers large intermetallic apices have been wedged into the tin coating.
► Whisker grains have a preferred [0 1 −2] orientation for this immersion tin coating.