Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Figure 3: Skip plating in RoHS bath due to excessive stabilizer. from the substrate. Quantitative tests are not commonly used in produc- tion facilities. For quality control purposes, qualitative tests are usually adequate.5 Good adhesion is largely the result of good surface preparation. There are, how- ever, some concerns that RoHS-compliant chemistries do not initiate plating as rapidly on some substrates, which may cause poor adhe- sion. This is especially true on copper and its alloys, although some hardened steels and tempered aluminum alloys can also be affected. In such situations, it is useful to employ an electrolytic nickel strike to ensure the substrate is fully active as it enters the EN bath. Roughness. The causes of roughness are not altered by the type of stabilizer used in the EN bath. However, it is possible to get some precipitation of the new heavy-metal stabilizers either in their containers or even in the solution, which leads to shelf roughness (Fig. 1). (If a precipitate forms in the storage drum, this usually represents a formulation or manufacturing issue and the product should be quarantined and the chemical vendor contacted). Tank precipitation is over- come by ensuring the bath is well managed and that filtration is used even when not plating. Typical agitation and filtration should be capable of turning over the solution at least 10 times an hour. Pitting. One of the causes of pitting in lead-containing solutions can be excess lead leading to passive spots and hence pitting (Fig. 2). This is especially true in high-phosphorus baths where there is no sulfur to counteract the effect of lead on EN plating. It is also possible to get pitting due to precipitated lead reacting with impurities in the bath, either from contamination, drag-in, or low-quality raw materials. RoHS baths tend to not have these issues, although many of them do use EDTA to complex the replacement stabilizers. This also helps prevent the risk of precipitation. Dull deposits. Historically, cadmium-based processes are much brighter than non-cadmium-brightened systems, but cadmium-based chemistries had a strong tendency to lose their brightness as the bath aged. The RoHS processes do not tend to suffer in this respect. Although they are less bright when new, the bright- ness level remains as the bath ages. RoHS processes are more sensitive to poor pretreatment especially smutting of the surface because they lack the ability to "smooth" the surface in a way that gives a nearly leveled finish. This means that pre-treatment is even more critical with the new processes. Many of the new RoHS baths give a bright deposit but 158

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