Whisker formation on matte Sn influencing of high humidity

With the 1st of July 2006 as deadline for the European RoHS directive approaching fast, the need for conversion of SnPb plated components to Pb-free alternative is increasing. Most component suppliers are looking at pure Sn plated or Sn-alloys electroplating as an alternative for the SnPb plating. However, electroplated Sri and Sn-alloys are associated with spontaneous whisker growth. In these past 4 years the E4 (Freescale Semiconductor, Infineon, STMicroelectronics and Philips) have extensively studied matte Sri in relation to whisker formation, exceeding the most common whisker assessment tests in considered time for bare parts, parts soldered on board and parts exposed to simulated reflow. The purpose of this study was to identify the main causes for whisker growth and to establish its mechanism, while at the same time identifying test conditions that can be used for an industry wide acceptance test for whisker formation, since this test has never been established. At the beginning of the study, we found different causes for whisker growth depending on the leadframe material; in case of Cu leadframes irregular growth of Cu6Sn5 was identified as the main cause for whisker growth during isothermal storage, while on FeNi42 leadframe the mismatch of coefficient of thermal expansion caused whisker growth in temperature cycle tests. Within the E4 postbake (1h, 150 degrees C) was introduced as a countermeasure for whisker growth for Cu based leadframes. Since last year many different test proposals for whisker testing have seen the light of day, resulting in a variety of test conditions. The E4 have submitted their solution, this postbake at 150 degrees C for 1 hour within 24 hours after plating, to most of the proposed tests. For all test conditions the results were positive (no whisker growth) up to 3000 hours. In this paper the test results are presented and discussed. Recently, whisker growth was discovered in some cases after prolonged exposure to 60 degrees C/93%RH on bare components, regardless of preconditioning. These whiskers were observed in combination with severe corrosion. However, on board assembled units no whiskers or corrosion were observed. In order to understand the influence of humidity and corrosion on whisker growth the E4 started an extensive Design of Experiments (DOE). Parameters in this DOE include: plating, solderpaste, flux, substrate material, preconditioning and condensation. In this paper we will present the outcome of this DOE and furthermore give an update on the previously reported ongoing experiments to discover the main factors influencing whisker growth.