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82 SMT007 MAGAZINE I JULY 2019 The resultant bond was characterized by shear tests, X-rays, and cross-sectional analy- ses of the intermetallic formed with the alu- minum. Shear tests were done to gauge the strength of the bond. They were also done on various components: resistors, capacitors, and chips. The tests were done using a production force gauge. The shear test values were greater than 15 N/mm 2 . The solder-aluminum bond was so strong that failure was often between the aluminum and PET. Figure 5 depicts such failures. The assembled boards were also inspected using X-ray. This was done to get a bet- ter understanding of the wetting of the sol- der. When examined under X-ray, the com- ponents showed good solder joints and voids were within an acceptable limit of less than 30% of the solder joint area. The key process parameters were the thickness of surface treat- ment, thickness of solder, and reflow profile of the solder. Figure 6 shows X-ray images of the components. Cross-sections were done on various com- ponents, including capacitors, battery holders, and QFN chips. These give a better view of the wetting of the solder, and the intermetal- Figure 6: X-ray images of components showed good solder joints. Figure 7: Cross-sections of capacitor, battery holder, and QFN/BTC soldered using surface treatment.

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