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18 The PCB Magazine • January 2017 the electroless Au thickness is between 0.1 to 0.2µm. Past studies have examined the per- formance of ENEPIG [1,2] . The current IPC EN- EPIG specification (IPC-4554 Amendment 1) calls for gold thicknesses between 1.2 µin minimum at 4 sigma below the mean and 2.8 µin maximum, and some specific customer requirements call for even thicker gold. It has been found that thicker deposits of immersion gold might lead to nickel corrosion with EN- EPIG if the dwell time in the immersion gold solution is too long. The goal of this study was to examine the performance of ENEPIG using three different types of immersion gold: standard displacement immersion gold, high efficiency immersion gold, and reduction assisted immersion gold. The three types of immersion gold dissolve cor- responding different amounts of nickel while the gold deposits as shown in Figure 1 using ENIG. The high efficiency immersion gold dis- solves approximately half the amount of nickel from that of the standard displacement immer- sion gold while the reduction assisted gold dis- solves approximately 60 percent of the nickel dissolved from the high efficiency immersion gold solution. Later studies will compare results for these immersion golds with ENIG only. Examination methods in this study included solder wetting balance tests, gold sire bonding tests, cross section analysis for nickel corrosion examination and for intermetallic formation af- ter solder testing. Experimental and Results The coupons used in this study consisted of a copper metallized and pattern plated (to 25 µm) test board which was subsequently coated with soldermask as shown in Figure 2. For wetting balance tests, standard wetting balance coupons with 36 mm wetting area were used. Specific BGA pads of 0.7 mm diameter connected to a ground plane were used for cross-section examination of nickel corrosion. Two BGA pads in each array were connected to a ground plane containing 0.375 in2 area giving a 629:1 area ratio which has been shown to help cause excess nickel corrosion in the past [3] . A wire bonding coupon was used for wire bonding tests. The test coupon is shown in Figure 1. This substrate was plated with ENEPIG by using plating chemicals commercially available from C. Uyemura & Co., Ltd. The ENEPIG plating process is shown in Table 1. STUDY OF IMMERSION GOLD PROCESSES USED FOR BOTH ENIG AND ENEPIG Figure 1: Depiction of nickel concentration in the immersion gold solutions after approximately 0.25 MTO comparing the three different types of gold plating processes used with ENIG. Figure 2: Test board showing wetting balance coupons, BGA array and wire bond array. Table 1: ENEPIG plating process.