The Effect of Temperature Cycling on Tin Whisker Formation

Tin platings on component finishes may grow whiskers under certain conditions, which may cause failures in electronic equipment. Although the thermal mismatch of tin and FeNi42 is well known and tin whiskers have been reported after thermal cycling of this material combination, no systematic investigation on the effects of thermal cycling is available. In this paper we describe the influence of various cycling conditions on the whisker growth rate of tin on FeNi42 and attempt to correlate these tests to service life conditions. We demonstrate that the whisker length has a linear relationship with ∆T. In addition, the whisker growth rate appears to decay as a function of number of cycles and/or whisker length. Furthermore, the maximum whisker lengths appear to be reduced on plated components that have been assembled to a printed circuit board. Introduction Recent activities of component manufacturers to introduce electroplated, pure tin as the lead-free alternative to SnPb plating for the solderable finish on leadframe-based devices draws new attention to the well known phenomenon of whisker growth. Since most components have copper-based leadframes much effort has been put into investigations of tin whiskers formed on copperbased materials. A mechanism has been proposed for tin whisker growth on copper-based materials and viable countermeasures have been identified. For tin electroplating on copper-based materials, the focus has been on isothermal storage conditions in order to investigate the propensity for whisker growth during storage in distribution centers or at the end customer before assembly. It is believed that second level assembly will slow the process of whisker formation and growth. Although tin plated copper leadframes tend to grow whiskers under isothermal storage conditions, matt tin plated FeNi42 leadframes do not typically grow whiskers under these circumstances. FeNi42 leadframes are not as popular as Cu-based leadframes, but they are still widely used in the electronics industry. In temperature cycling, tin electroplate on FeNi42 typically forms whiskers, while tin plated copper typically exhibits minimal whisker growth during temperature cycling. The explanation for this different behaviour is presumably related to different mechanisms of stress induction. For tin plated on copper, it is believed that compressive stress is formed in tin finishes due to the excessive irregular growth of the intermetallic Cu6Sn5 at the copper-tin interface. On the other hand, for tin plated on FeNi42, stress may be induced by the large mismatch of the coefficients of thermal expansion between FeNi42 (cte = 4.3 *10 K) and tin (cte = 23 *10 K). This mismatch can cause stress in the constrained tin layer, when temperature cycling is applied. Since temperature cycles due to the environment or operation occur during the service life of most electronic equipment, the risk of failure caused by whisker growth on tin plated FeNi42 based components should be investigated. Further, it must not be assumed that all of the tin plating fuses during the board assembly processes, so portions of the aselectroplated finish will still exist during operation. Based on some experimental findings and the above considerations, the current study was initiated to address the various parameters that influence whisker growth in tin electroplated on FeNi42. Experimental The test packages used for the experiments were fully processed TSOPII-66 with a memory chip inside. All packages are from the same production lot and are electroplated in the standard conditions for mass production. The thickness of the tin finish has been measured by X-ray fluorescence analysis as 8.5 ± 0.1 μm. The tin finish is characterised as a matt pure tin finish with an average grain size of 2 μm to 6 μm. The electrolyte for the electroplating process was an MSA-based chemistry of the latest generation. The plating process was performed in a continuously operating belt line. SnPb plated components of same type were plated and used as a control. These Sn-Pb parts have also been made during standard production and underwent the same conditions of temperature cycling and inspection as described below. The plating thickness for the SnPb control group was 8.2 ± 0.1 μm and the lead content was 10 ± 1 %. For every test condition and test interval (e.g. 250, 500, 1000 cycles), five components of both plating types have been evaluated. Thus 330 leads are inspected for each environmental condition and test interval Hence – if not explicitly mentioned – 5 different components have been inspected for every data point in the diagrams. Three types of temperature cycling chambers have been used to apply the various conditions. In most cases with so called air to air or liquid to liquid shock conditions a two chamber system was used, so that the transfer from the hot to the cold chamber and vice versa occurs within a few seconds. The dwell time began with the completed transfer of the components from one to the other chamber. Temperature cycles of less than 10 K/min ramp rate have been performed by the use of single chamber systems. Due to the limited thermal mass of the components the test samples followed directly the temperature of the chamber and dwell time started after reaching the temperature limit with a tolerance of 5 °C. Inspection has at first been done with an optical microscope at a magnification of 50x. This method was chosen because many leads can be evaluated quickly, unlike in SEM, and the whisker lengths measured with an optical microscope are consistent with whisker lengths measured in SEM. Thus every lead was fully inspected in live bug position and the longest whisker identified. After noting the position of the longest whisker every component was inspected in scanning electron microscope (SEM) and a picture taken from the longest whisker of every component at 1000x magnification. The length of these whiskers have been measured in SEM again for comparative reasons and for higher accuracy. A maximum whisker length for a particular environmental condition and test condition is calculated by averaging the maximum whisker length measured on each of the 5 components exposed to the same condition. Additionally pictures of the area without influence of trim & form tools or any bending and from the tip of the leads at 300 x magnification were taken to get an overall impression of the whisker density and the length distribution. During the design phase the parameters of influence have been identified as • the temperature range • the absolute temperature