Abstract
In testing the resistance of a molding compound to popcorn failures after solder reflow, there are two basic options for shortening the moisture absorption phase: varying the severity of moisture exposure by varying the temperature and percent relative humidity or maintaining a constant humidity/temperature profile and varying the length of time the compounds are exposed. In addition, the total testing time can be accelerated by the choice of test method. There are three major measurement techniques for evaluating the extent of popcorn cracking: SAM testing; bubble chamber testing; and visual crack inspection. In this paper we will explore the effects of differing lengths of exposure time and the merits of different measurement techniques. A variety of epoxy resins, filler mixtures and adhesion additives were examined in semiconductor molding compounds for solder-reflow crack resistance (popcorn crack resistance). A limited comparison of molding compound high-temperature strength properties and extent of popcorn cracking suggests no strong positive correlation. On the other hand, factors that might have an impact on adhesion properties of the molding compound, such as low melt viscosity resins and putative adhesion additives, appear to have the greatest impact. A molding compound developed with these factors showed no internal or external cracks on 84 lead QFPs (29.2×29.2×3.68 mm) after 72 h of 85°C/85% R.H. and one 10 s solder dip at 260°C. Nor were any external cracks observed after 168 h of 85°C/85% R.H. and one solder dip. However, minor internal cracks were observed. The most data can be obtained by using bubble chamber testing for large numbers of parts and evaluating a smaller sample of the best of those compounds via SAM testing with crack extension analysis. The length of exposure time affected the parts as expected, with longer times being more severe. There did not appear to be a gradient effect or a threshold level. In general, 72 h allowed the greatest differentiation among the compounds tested for internal defects; external defects were not fully developed until 168 h of moisture exposure.
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