Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 161 of 707

old bath age. Field results have proven that the RoHS baths can be used longer because they tend to show less of a tendency to dull and crack as they get older. Therefore, it is more likely that they will be used at higher MTOs than previous solutions, which may cause more frequent occurrences of this problem. The answer is, as always, to either discard the solution or to cool it, add more com- plexants, filter, and then reuse. Plate out. Plate out is a lack of stability in the plating bath, and it tends to be a more unusual occurrence with RoHS-type processes. This is because RoHS- compliant baths use a higher concentration of stabilizer, resulting in far slower depletion than if lead was used. Having a much larger reservoir of the active sta- bilizers means that there is a larger allowable error in control, so it is much less likely to reach a situation where the stabilizer is completely depleted. This lack of sensitivity is not sufficient to allow poor control and equipment issues to be eliminated; as always, increased care and control will give better per- formance. An interesting advantage of RoHS-type processes includes stabilizers having less of a tendency to be plated out on the cathode if using anodically pro- tected stainless steel, and so they are more forgiving when used in this way. A disadvantage has been hinted at during the discussion on corrosion resis- tance. The deposits are more corrosion resistant in some circumstances and, hence, when tanks and equipment are being cleaned, the nitric acid must be clean and at least 8 hours should be allowed to strip the EN. In some cases, tanks have not been allowed to strip completely. This has lead to plate out as soon as the EN is reintroduced—something that would not happen with the older processes. No plating. The only area where there may be a major difference between con- ventional EN and RoHS-type processes is in the area of plating copper alloys. Other than during this circumstance, controlling the bath parameters, avoiding contamination, and ensuring the parts are catalytic remain critical factors. With copper alloys, it is important to use a nickel strike to activate with the new RoHS processes, because the replacement stabilizers have a strong tendency to passivate copper and its alloys. This means that if the bath has a high concentration of stabilizer, or if the plater is trying to activate in the EN solution directly or by using palladium as an activator, then the new RoHS processes may not initiate. This can also be true on hardened steels and some aluminum alloys, but it is most prevalent with copper alloys. To avoid this situation, use of an external strike prior to entering the EN bath is vital. Bath decomposition. It is very unlikely that a modern EN bath formula will decompose unless the bath is badly managed, there is a problem with the equip- ment (including incorrect cleaning), or there is drag-in of particulates from the work being plated. Make sure that you do not drag-in activators, such as a pal- ladium activator, because small amounts will cause the plating solution to decompose. This is exactly what happens to a conventional bath. Experience has shown that, as with plate out, the newer RoHS baths are less sensitive to these issues in general, although no solution has yet been designed that can overcome very poor bath management. CONCLUSIONS The discovery of electroless plating was a serendipitous moment in the history 160

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