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January 2017 • The PCB Magazine 19 Thickness results were recorded using a Seiko SEA-5120 Element Monitor MX XRF. Gold deposit thicknesses were targeted for the lower and upper ranges of the IPC-4556 Amendment 1 specification. The cross-section images of BGA pads were observed using a JEOL JSM-6010LA SEM. Wetting balance coupons were tested in an as-plated condition and after 3x reflow at 255°C maximum temperature using a Heller 1088 reflow oven. Solder wetting balance testing was performed using a Metronelec Menisco ST 50 wetting balance with IPC test flux #1 using SAC305 solder at 255°C. Wire bond coupons were first baked at 175°C for 16 hours in a Fisher Isotemp 400 series oven. The wire bond coupons were subsequently argon plasma cleaned prior to wire bond tests. Wire bond testing was performed at Fast Semiconductor Packaging LLC (Anaheim, CA) using the equipment and conditions shown in Figure 3. Measured thicknesses are shown in Table 2. Solder Wetting Balance Solder wetting balance test results are shown in Table 3. Solder wetting curves were compared to the standard curves shown in Figure 4 while a typi- cal curve from the testing (Sample 4, standard immersion gold at 1.49 µin) is shown in Figure 5. Solder wetting times and final wetting forces were excellent for samples tested with good wet- ting observed as described in the depiction fig- ure. Interestingly, the wetting times decreased in all cases after the 3x reflow exposure. Final wetting forces were all above 0.20 mN/mm. The reduction assisted immersion gold samples showed some of the lowest wetting times while the standard immersion gold samples exhibited the longer wetting times. Wire Bond Testing Previous work performed by IPC committee members in the writing of IPC-4556 involved a large round robin test program whereby several STUDY OF IMMERSION GOLD PROCESSES USED FOR BOTH ENIG AND ENEPIG Table 2: Deposit thicknesses in microinches. Table 3: Solder wetting balance test results. Figure 3: Wire bond conditions. Figure 4: Wetting balance curve depictions.