PCB007 Magazine

PCB-Aug2014

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52 The PCB Magazine • August 2014 It is clear from the SEMs in Figure 1 that there is significant corrosion on the electroless nickel surface. Obviously, this is an unacceptable situ- ation, and in order to prevent it one must work harder to optimize and control the electroless nickel-immersion gold process. Some of these factors will now be presented in detail. Critical Electroless Nickel Features 1. Phosphorous content control in the elec- troless nickel deposit. Don't underestimate this important parameter. 2. Stabilizers in the EN solution influence phosphorous content in the deposit. 3. Phos content has strong influence on sol- derability, immersion gold deposit thickness and potential for hyper-corrosion. Let's look at these points in a little more de- tail. As I have written in this monthly column on many occasions, operating the chemical pro- cesses within the process window is the key to consistent and predictable performance. After all, your customers should expect no less from you. So with respect to #1 above, it is recom- mended that the EN process be able to maintain a reasonably tight window with respect to phos content through five metal (MTOs) turnovers in the nickel solution. Figure 2 provides a good summary of what happens under certain plating conditions. Note that X (green) refers to an EN formulation with the operating pH kept at 5.3. The area of the chart outlined in red refers to an EN process that operates at a 4.8 pH, but co-deposits a higher level of phosphorous in the EN deposit (due to additional influences). The blue refers to the preferred range of phos (7–8.5%) and a pH of 4.8. Note that under these operating conditions (as well as the proprietary nature of the EN so- lution) the blue process gave a higher immer- sion gold thickness that was required to meet the IPC-4552 specification. It should be noted the thicknesses of the gold shown in the red area are much too low to meet the minimum specification as listed in IPC-4552. And this was primarily due to a higher EN solution operating pH. There is one caveat here. Quite a few OEMs and fabricators will accept lower gold thick- nesses. I am not advocating such an approach without the need for further study. The phosphorous level in the deposit of most EN processes is inversely related to the pH of the working bath; as the bath pH decreases, the phosphorous level in the EN deposit increases. Each 0.1 decrease in bath pH units will increase the phosphorous level of the deposit by about 0.5 %. Close pH monitoring will improve not only corrosion resistance of the nickel deposit, but will also improve the gold plated distribu- tion across the panel. The Use and Control of Stabilizers in the EN Solution The type and concentration of stabilizers in the EN plating solution exert a strong effect on the phosphorous content of the deposit. It has been well documented that some stabilizers, while performing the intended effect of prevent- ing nickel plate-out, may actually negatively in- fluence the amount of phos in the deposit. And as was stated earlier, the higher phosphorous content can actually reduce the amount of gold that is deposited on the nickel (via the galvanic or displacement reaction). As a consequence, some fabricators, in an effort to increase the gold thickness, will increase the dwell time in the im - mersion gold solution. The outcome is usually corrosion spikes in the electroless nickel deposit. CONTROLLING THE ENIG PROCESS FOR OPTIMUM EFFICIENCy AND PERFORMANCE continues figure 2: Influence of en solution ph and phos content of en deposit on gold plating thickness.

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