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Knocking Down the Bone Pile: Tin Whisker Mitigation in Aerospace Applications, Part 3
Solder Paste Stencil Design for Optimal QFN Yield and Reliability
April 29, 2015 | Ben Gumpert, Lockheed MartinEstimated reading time: 13 minutes
These two components were selected based on results of previous testing. The QFN68 package has a very high die to body size ratio, and has performed relatively poorly in other thermal cycle tests. The QFN44 package, however, has a low die to body size ratio, and in a previous test, there were no failures after 2520 thermal cycles. This testing will investigate the ability to improve results on a ‘poorly’ performing part, as well as the potential for reducing performance on a ‘robust’ part.
To develop an idea about how the chosen experimental parameters and variation would affect test results, a simulation software package was used to predict the outcomes of the testing. Figure 2 shows the results of that simulation and the prediction of the average thermal cycle survival period for the QFN68 package at the solder joint heights to be used in this test. Results of previous testing were used as a baseline to fit the prediction results to the test vehicle used.
A 5 mil stencil was used for solder paste application with apertures similar to those shown in Figure 3, but which varied by location on the board to control the volume of solder deposited at different sites across the assembly. The volume of solder was controlled by the number and size of apertures on the center pad of the QFN/BTC footprint. The volume of solder applied to the perimeter pins was also adjusted, with the objective of creating similar solder joints on all component locations (i.e. solder joint with the same amount of toe wetting and with similar fillet shape), although the height of these joints was different from location to location. Ten different aperture patterns were used for the QFN44 package, and 14 different aperture patterns were used for the QFN68 package. Reduction of solder on the center pad ranged from 10% to 90%. These values are outside of both the minimum and maximum recommendations, but are being evaluated to observe potential trends.
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