Q. What are some of the factors that contribute to thickness variations across a plating rack? Specifically, we have racks with four rows, each row containing 10 positions. The parts on the interior rows (two and three) and positions one and 10 on each row have the greatest plating thickness. The difference between the thickest and thinnest parts is approximately double. The plating bath is cyanide silver. Any suggestions for solving this problem? –D.B.
A. There are a number of possible causes of thickness variation in plating baths. Current density variations cause differences in deposit thickness over the surface of a part. The higher the current density, the thicker the deposit. The corners and edges of a part normally have a higher current density than other areas of the part. Hence, the corners and edges have a greater plated thickness. The same phenomenon will also cause parts that are on the ends of racks to have a somewhat higher deposit thicknesses than parts placed on the interior of racks.
Agitation of the plating bath affects thicknesses. This is particularly true in larger plating tanks. If the plating solution is not thoroughly mixed, certain areas of the bath will be “starved” of metal ions, and a thinner deposit results. Also, good agitation is required for even heating of the plating bath. Typically, cooler baths do not deposit as much metal as warmer baths.
Anode placement will also affect plating thickness. The anodes must be placed evenly in the plating bath. If, for example, your plating tank is deep, you must have anodes that will be long enough so that the bottom row or rows of the plating rack receive enough current and metal ions to plate effectively.
In the situation you describe, I would first consider anode placement and anode size. It may be that you do not have enough anode areas on the edges of your tank and too much in the middle of your tank. The fact that your rack has four rows leads me to believe that your tank is fairly deep. Perhaps that your silver anodes are not long enough.
I am sending you via email a couple of older papers that describe some of the ins and outs of anoding and racking in a plating bath.