Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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have a different color/hue. The new RoHS systems tend to give a whiter deposit when compared with a more yellow-looking cadmium deposit. Streaky deposits. One of the major causes of uneven and inconsistent deposits with cadmium and lead is an imbalance between heavy-metal stabilizers and sulfur additives in the bath. The new stabilizers are less sensitive to this effect and, there- fore, are much less likely to be seen. The lack of smoothing of the deposit (which is seen in cadmium-based processes) does mean, however, that stains due to pretreatment failures are more likely to be seen after EN plating with RoHS-com- pliant baths. Step or skip plating. Because the new heavy-metal stabilizers/brighteners most com- monly used in new RoHS processes are not as effective as lead and cadmium, a higher concentration is needed to have similar effects on the plating solution. Although it is less likely, it is still possible to have step or skip plating if the add back ratios are incorrect or if the bath loading is low (Fig. 3). Generally, the effect is to have a slowdown in the plating rate before these issues are seen. There are more issues if the substrate is either aluminum or a copper alloy. This can also be seen if parts are not cleaned or correctly activated, because the new baths can be much more sensitive than lead-based systems. This is not an issue for new systems using heavy-metal stabilizers. Poor corrosion protection. It has been observed that post-RoHS processes produce deposits that perform better in neutral salt spray than their pre-RoHS equiva- lents.6 Electroless nickel is a barrier coating; its corrosion protection is inverse- ly proportional to the number of pores in the deposit and directly proportion- al to the amount of alloyed phosphorous. Neutral salt spray for cathodic coatings, such as EN on steel, is a porosity test. Nickel is not attacked when sub- jected to neutral salt fog. Hence, it is safe to conclude that the new generation of stabilizers and brighteners reduce the propensity toward deposit porosity. BATH ISSUES Slow plating rate. Most of the factors valid for conventional processes are still applicable to the new RoHS solutions. For example, checking the temperature, pH, nickel, and hypophosphite concentration, while ensuring the measurement is accurate and that the bath is not too old. However, there are some differences. The new solutions are less sensitive to low bath loading that causes over-stabilization (although it is still possible). It is dif- ficult to use skip plating as an early indicator of excess stability, so many times the first adverse effect noticed is a reduction in the plating rate. This is especially true if the product is older and some of the stabilizer has precipitated, giving the hypophosphite additives a darker appearance. This precipitated stabilizer can be added to the plating solution during normal additions, and once added it is pos- sible for it to redissolve, providing very high stability and slowing or even stop- ping the plating. In these circumstances, the bath must be worked with a large surface area or activated using current until it starts to plate again. "White out." White out is no different regardless of the stabilizers, because it is related to the precipitation of orthophosphate due to a lack of complexants or 159

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