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36 SMT Magazine • February 2014 Kester no-clean tacky flux (TST-6592LV) and printed on a silicon wafer using a glass slide and 0.007" feeler gauges as standoffs. Bumped test boards and the Intel 845 device were dipped into this powder/flux mixture and then re- flowed in nitrogen. Images of the power dipped Intel 845 device pre- and post-reflow are shown in Figure 6. In the second reflow the Cu powder was mixed with tacky flux and mixed with the sol- der joint providing a limitless source of Cu. A 60-second time above reference temperate was used since this is more representative of stan- dard assembly processes used in electronics manufacturing. This second reflow has provided adequate time for the formation of Cu6Sn5 at the electroless Ni substrate. Peak temperatures for these profiles were 213°C for SnPb and 242°C for lead-free. Intermetallic formations following the addition of Cu were unique for the lead-free cases. Morphological differences observed opti - cally seemed to have evidence of precipitation of Cu6Sn5 above the pad surface for the long attach profile samples (Figure 7 C & D) and considered when testing solder joint interme- tallics in this manner. Heating from a localized pin may also affect the Cu pad adhesion to the base dielectric and cohesion of the dielectric be- neath the pad in the cases of pad cratering. This test is best suited for testing intermetallics of solder mask defined pads due to the additional strength of a larger pad defined by mask. Test Method Intermetallic of various morphologies and compositions were created by soldering solder spheres of specific alloy to the ENIG substrate. These intermetallics were then either HBP test- ed or subjected to a second reflow with Cu pow- der and then CBP tested. ball attach All solder joints were reflowed in nitrogen environments using a Vitronics Soltec 10 zone convention reflow oven. For the first reflow process (ball attach), a short (~20 second) and long (~120 second) time above the reference temperature of 217°C was used for the lead- free solder alloys Sn/3.5Ag and SAC304. Similar times above the reference temperature 183°C were used for the SnPb 25 mil solder balls. A peak temperature was measured for the long profile to be 208°C for SnPb and 236°C for the lead-free. For the short profiles peak tempera- tures were limited by duration above the refer- ence temperature however for SnPb 185°C was achieved and 226°C for the lead-free alloys. addition of Cu by Powder Copper powder with a nominal diameter of 20 microns was purchased from Advanced Powder Products. This powder was mixed with FEATUrE TeSTING INTerMeTaLLIC FraGILITy ON eNIG uPON aDDITION OF LIMITLeSS Cu continues Figure 6: l-r: Device dipped in Cu power pre and post reflow respectively. Figure 7: 500x optical inspection lead-free alloys following addition of Cu powder (A) SnAg Short (b) SAC304 Short (C) SnAg long (D) SAC304 long.