White Bronze, Copper-Tin-Zinc Tri-metal Process sequence The basic process sequence is depicted in Fig. 1 below and is very similar to single metal plating processes that exist in any plating facility. Pre-treatment steps are comprised of a soak cleaner followed by a standard reverse electrocleaning step. This step is followed by an acid activation before a standard cyanide copper strike. In some processes, acid copper is used because of its higher leveling capability and the fact that tri-metal plating tends to duplicate the underlying plated surfaces more closely. The tri-metal plating rate of approximately 22.9 nm/min (0.9 µ-in./min) requires a 15-min cycle at 0.5 A/dm2 (5.0 A/ft2) to deposit the preferred 3.0 µm (120 µ-in.) of alloy. Three to four tri-metal plating cells per bath normally balance a full production line, although that depends greatly on the thickness of underlying copper that is being plated. The deposits of Cu-Sn and Cu-Sn-Zn alloy processes are achieved with an alkaline solution containing cyanide and hydroxide as metal chelators. A proprietary brightener system helps to compensate and buffer any variation in the concentrations of the major components of the bath. This helps to provide and maintain a specific alloy deposit composition. The target concentrations of the components and the wide operating window for our proprietary additive system are shown in Fig. 2. One of the most important parameters to target and control is the plating current density. This is because the alloy ratio is highly dependent upon current density and will favor a copper-rich alloy at high current densities and a tin-rich alloy at low current densities. The ideal (largest operating window) current density for the proprietary process is centered at 0.5 A/dm2 (5.0 A/ft2). However, the additive system was optimized to compensate and plate a tight percentage alloy over the range of 0.2-0.8 A/dm2 (2.0-8.0 A/ft2). The value of this wide window is considerable. Within a single part and location on the rack, current densities can have a fairly high variability. The specific part design, rectifier sensitivity, number of anodes, anode spacing, size and distance from the plated part all have profound effects on localized part current density. The capability of the additive system to buffer and compensate for these variations is critical in holding the target alloy and the resulting color and/or brightness of the part.
Posted on: Sat, 28 Sep 2013 06:21:45 +0000