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February 2014 • SMT Magazine 33 The SAC304 alloy was selected due to the ternary Cu-Ni-Sn phase diagram. SAC304 was created combining a Sn/Ag3.0wt.%/Cu0.5wt.% (SAC305) solder ball with a calculated Sn/ Ag3.5wt.% print volume knowing the solid content of the paste, density of the alloy, and calculated print transfer efficiencies for a square stencil aperture of a given thickness. A sten- cil was purchased with this aperture size and transfer efficiencies were measured to be ~95% with a Cyberoptics laser volume measurement tool. Exact Cu concentration is not critical in this case. However, we wanted to have repeat- able results with an alloy contain less than 0.5 wt.% Cu and greater than 0.3 wt.% Cu since this is what has been described in literature to be the limits for the formation of the Ni3Sn4 and Cu6Sn5 intermetallic formation at the Ni boundary 1,2 . Substrates An electroless Ni/immersion Au (ENIG) test board that was used in this work is a 12-layer PCB with a nominal copper to copper thickness of 2.12 mm (.083"). The board was constructed with Matsushita HF-FR-4 (Tg 148°C) laminate material and PSR-4000 BL01 solder mask. Pad openings were 22.7 mil diameter solder mask defined. Phosphorous concentration was de- termined to be 12.6 wt.% in the bulk Ni when evaluating cross-section by EDS. Figure 2 shows the condition of the test board as-received. In this study we compare our controlled ex- periment to a commercially available Intel SnPb device (Intel 845). The pad diameter for this de- vice was measured to be 26.2 mil solder mask defined as compared to the 22.7 mils for the test board. Although these diameters are not exactly the same the bond testing performance should be comparable. We would expect the peak load to failure to be higher for this device due to the larger pad area. Mechanical Test CBP testing is a technique often used in electronics to test the mechanical properties of solder joints and laminates 3,4,5,6 . JESD22-B115A is the standard used for ball pull testing and the "A" revision has been released in August 2010. Figure 3 and Figure 4 illustrate the pull testing apparatus and tweezer alignment respectively. A tweezer tool with a hollowed tip is used to grab the solder joint. Tweezers of similar diam- eter as the solder joint should be used in order to effectively distribute the gripping forces and minimize the deformation of the solder joint. Solder joint deformation as well as twee- zer alignment to the solder ball could affect the results of the test since the solder may not be placed under a uniform tensile stress across FEATUrE TeSTING INTerMeTaLLIC FraGILITy ON eNIG uPON aDDITION OF LIMITLeSS Cu continues Figure 2: ENIg test board; A) SEM of cross- section, b) SEM of surface inspection ("mud flat" condition is typical for electroless Ni), C) optical image of entire pad, and D) 500x optical inspection of pad surface. Figure 3: general solder ball pull apparatus 6 .