Plating Q&A: Reduced Brightness and Thickness
Q. Our parts are not as bright as before and the plating thickness is thin. Why is that?
A. We had a customer who ran an alkaline non-cyanide zinc bath and reported that the brightness of his parts was slowly degrading every week. To make matters worse, the plating thickness was diminishing without adjusting time, temperature, current density or any generic ingredients like zinc and sodium hydroxide.
Because even the most sophisticated labs do not have the means or the instrumentation to perform analysis on the proprietary maintenance additives that are added to an alkaline zinc bath, like carrier and brightener, concentration figures must be determined using less technologically-advanced methods. The conditioner or replenisher component can be analyzed typically via atomic absorption spectroscopy or inductively coupled plasma spectroscopy.
First, an explanation of additives. Carrier (also called base or initial) is used to improve deposit distribution or throwing power—evenness of plated thicknesses from high-current density area (HCD) to low-current density area (LCD)—pitting and HCD burning. Too much will cause a reduction in plating efficiency and plating rate, as well as dullness. The brightener component is designed to refine the deposit appearance, but too much will hurt plating efficiency and adhesion.
To determine the source of the dullness and thickness disparities, we ran three different Hull cell panels without agitation using fresh solution for each separate panel. The first panel was run at 5 amps for five minutes and it did not exhibit any burn or nodulation due to zinc hydroxide precipitation along the far left (HCD) area, indicating possibly too much carrier. A light burn means your carrier concentration is within range and too much burning is an indication of low carrier.
The next panel was run at 3 amps for five minutes. This panel should be clear and bright from left to right (HCD to LCD) with no burning in the high or dullness in the low, indicating a lack of either carrier or brightener. We did not experience any issues with burning or LCD dullness with our 3-amp panel, indicating at least a sufficient amount of both components, but there was some dullness on the panel itself, indicating high carrier.
Lastly, we ran a 1-amp panel for 20 minutes. This panel should also be full bright and special attention should be paid to the LCD area. If there is any dullness here you may be low in both or either component. Our panel was a little dull indicating an excess of carrier or a lack of brightener, but our LCD maintained its clarity and brightness, indicating at least a sufficient amount of carrier and brightener.
The results of the qualitative analysis led us to believe that the bath might be high in proprietary organics (carrier and/or brightener) so we ran a 1-amp panel for 15 minutes in a Hull cell without agitation. The temperature was maintained at approximately 75°F because higher temperatures will give slightly better throwing power or deposit distribution (even thicknesses from HCD to LCD), which will interfere with the procedure and the results. Lower temperatures will give slightly worse deposit distribution.
We marked the panel using a Hull cell ruler at the 40, 20 and 5 amps per square foot (ASF) areas. The actual microns or mils of thickness at these markings will differ depending on what system you are using, but many systems comport to the following designated thicknesses:
40 ASF (HCD) = 3.5–4.5 µ (0.14–0.17 mils)
20 ASF (MCD) = 3.0–4.0 µ (0.12–0.16 mils)
5 ASF (LCD) = 2.8–3.2 µ (0.11–0.13 mils)
The zinc thickness on our customer’s bath at all three designated marks was very low, indicating high brightener, but we also encountered an extreme degree of thickness uniformity indicating too much carrier. The plating thickness ratio was a lot lower than the range that is given for a well-operating bath. Since we encountered both a poor thickness ratio and low thicknesses at all marked current densities we knew that our customer was overdosing his bath with both carrier and brightener.
We determined that the dullness and the plating rate was due to a highly saturated bath overloaded with proprietary organics, so we had our customer gradually taper off the pump speed and volume of these products in 5 and 10 percent increments. Within a few weeks our customer saw major improvements in all three problem areas.
Adam Blakeley is a technical service representative at MacDermid Enthone, macdermidenthone.com.
Originally published in the June 2016 issue.
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