245C when Sn is plated directly on Cu. This result was also independent of the post plate bake and alloy type. There were, however, rather large variations in the probability of whisker formation between leads on any given device, between devices from any plating lot and between devices from different plating lots. The probability of finding whiskers on any given device from any given plating lot varied from 100% to 0%. And the probability of any lead on any device having a whisker varied from 50% to 0%. The variation was not tied explicitly to any of the variables studied. The statistical variations in the data would suggested that there is a high probability of finding a false positive whisker test result (no whiskers) when a small set of samples taken from a small number devices from a small number of plating lots are used for the evaluation. However, when Ni is inserted between the Cu lead frame and the Sn plate we have not observed the "needle-like" whisker growth on any device independent of post plate anneal, plating lot and simulated solder reflow process.">

SMTA International Conference Proceedings


TIN WHISKER MITIGATION: APPLICATION OF POST MOLD NICKEL UNDERPLATE ON COPPER BASED LEAD FRAMES AND EFFECTS OF BOARD ASSEMBLY REFLOW

Author: J. W. Osenbach et al.
Company: Agere Systems
Date Published: 9/26/2004   Conference: SMTA International


Abstract: In spite almost 6 decades of research on Sn-whisker formation and growth, neither atomistic and empirical models that can be used to predict when, where, and at what time Sn-whiskers might occur on electrical devices that use Sn on Cu metallization for electrical interconnection are available. Furthermore, results reported by different researchers vary significantly. Because predictive models for Sn-whisker growth are not yet fully developed, we choose to take a statistical approach to the problem. To do so, we evaluated the propensity for whisker formation and growth on trim and formed devices that had matte- Sn plated leads where the Sn-plate was varied via: i)different Sn plating chemistries and bath suppliers; and ii) different suppliers of plated leads frame devices. In addition, different lead frame Cu alloys (7025 and 194) and different package types, as well as devices with and without Niunderplate between the Cu and Sn were used in the study.

Finally, devices with and without post plate 150C/1hr anneals, with and without a simulated board attach solder reflow processing were evaluated. We studied the propensity for whisker growth on the leads of these devices after for extended periods of time at 60C/93%RH. Devices from all of the plating chemistries, bath suppliers, and plating suppliers show some propensity for "needle-like" whisker growth after simulated solder reflow with a peak temperature > 245C when Sn is plated directly on Cu. This result was also independent of the post plate bake and alloy type. There were, however, rather large variations in the probability of whisker formation between leads on any given device, between devices from any plating lot and between devices from different plating lots. The probability of finding whiskers on any given device from any given plating lot varied from 100% to 0%. And the probability of any lead on any device having a whisker varied from 50% to 0%. The variation was not tied explicitly to any of the variables studied.

The statistical variations in the data would suggested that there is a high probability of finding a false positive whisker test result (no whiskers) when a small set of samples taken from a small number devices from a small number of plating lots are used for the evaluation. However, when Ni is inserted between the Cu lead frame and the Sn plate we have not observed the "needle-like" whisker growth on any device independent of post plate anneal, plating lot and simulated solder reflow process.



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