Issue link: https://iconnect007.uberflip.com/i/1527952
OCTOBER 2024 I PCB007 MAGAZINE 27 ness to be statistically controlled to between 1.5 and 4.0 microinches. Restricting gold thickness to a maximum of 4.0 µin is not ideal for many OEM assembly tasks, including spe- cialty wire bonding, touchpad, and non-sol- dered pin connection applications. OEMs that specify higher immersion gold thickness oen find their designs under deviation or not bid. ey continue to request thicker immersion gold and many PWB manufacturers have cus- tomer specifications at the upper limit of the IPC spec in their manufacturing queue. 3. Building gold thickness on ENEPIG. Immersion gold deposition on nickel is a straightforward and strongly-driven electro- chemical reaction. Conversely, immersion gold deposition on electroless palladium is a much lower-driven reaction. is results in two undesirable scenarios: slow and incon- sistent gold-on-palladium plating rates, and underlying preferential electroless nickel attack, known as "tunneling." RAIG is a mixed potential chemical reac- tion—a hybrid reaction. e initial gold- nickel reaction is an immersion reaction that is followed instantaneously by an autocatalytic electroless gold deposition reaction. e latter provides the final thickness. e autocatalytic gold deposition process receives its electrons from a reducing agent (i.e. HCHO), effectively precluding further electro- less nickel corrosion. is RAIG feature elimi- nates the opportunity for excessive nickel cor- rosion and corresponding IPC corrosion level 111 and black pad reliability issues. is new generation immersion gold plating process is quickly becoming an industry "best practice" and is commonplace in new and upgraded final finish plating lines. As evidenced in Figure 4, RAIG thickness uniformity virtually mirrors that of ENIG in Figures 2 and 3. An equally important feature of the RAIG process is its ability to plate thicker gold on both electroless nickel and electroless pal- ladium without any risk of nickel corrosion. Specifically, by taking advantage of the auto- catalytic gold reaction, these electrolytes pre- dictably build gold thicknesses significantly above the IPC threshold of 4.0 microinches. e effective range for RAIG thickness is 1.5 to 8.0 microinches, which more than meets the needs of the OEM specialty assembly pro- cesses referenced earlier, while allowing the process to be used successfully in mainstream IPC designs. As per the IPC-4552 Rev B speci- fication, it calls out the use of RAIG for thicker gold deposits. Table 1 shows "level zero" nickel corrosion for both ENIG and ENEPIG in RAIG deposits. What applications and PCB technologies are using direct immersion gold? DIG is a final finish alterna- tive to ENIG. Direct immersion gold is a nickel-free final finish that eliminates electroless nickel from the ENIG deposit stack. e DIG process is a straightforward immersion reaction between copper and gold. By removing the nickel, we eliminate the pri- mary copper diffusion barrier of the final finish stack and extend Figure 4: Gold thickness distribution vs. pad ID-ENEPIG.