Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 154 of 707

BONDING PROPERTIES (WIRE PULL TESTING) Figure 8: Crack classification rating 1 to 5, with 1 and 5 showing insufficient surface bonding. Ratings 2, 3, and 4 are acceptable. Gold-wire loops (25-µm diameter) (Au HD2) were used to determine any loss in bonding properties after Betatec post-dip treat- ment. A pull tester (DAGE 4000) and TS- bonder (Delvotek 5410) were used for this study. Wire pull testing deter- mines the quality of the gold wire bond to the sur- face. It consists of apply- ing a specified upward force under the gold wire attached to the surface. The crack mechanism is studied to evaluate the bond quality. Figure 9: Thin gold wire crack rating distribution (as % of population) for Betatec-treated nickel/gold-plated samples. All samples passed the test. The crack ratings in Figure 8 highlight the crack at the wire/surface interface. Ratings 1 and 5 indicate an insufficient bond with the surface, whereas ratings 2, 3, and 4 are acceptable. Sample processing Figure 10: Strong discoloration of untreated samples compared to Betatec treatment (above) showing no discoloration after an 8- hour pressure cooker test. involved nickel sulphamate (Ni-Sulphamate HS) and pure-bond gold (Aurocor K 24 HF) plating, followed by Betatec post-dip treat- ment. To confirm mea- surement reproducibility, 4× samples were measured and compared to 2× untreated references. All samples passed the test with no crack ratings of 1 or 5 evident. The majority of samples provided a rating of 3 (central crack), considered the best bonding quality (see Fig. 9). TIN SURFACE PROTECTION STUDY To maximize cost-effectiveness (mentioned in the introduction) many connec- tors are plated with gold only at the functional contact region with the "solder flank" receiving a tin coating. However, the processing sequence does not allow for selective treatment of each region with different post-dips. Hence, the ideal gold post-dip candidate must ensure no detrimental effect on the properties of 153

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