In part one of this blog series, we gave an overview of plating to accomplish a specific function, such as solderability, corrosion resistance, enhanced thermal or electrical conductivity, or appearance. Here we’ll dive a little deeper and examine two of the most common methods, and an alternative to plating that’s worth considering too.
Thickness of the coating depends on how long the parts are held in the bath (i.e. dwell time) as well as the complexity of the part geometry and features. These can also affect how even or uniform the coating is. Additional factors like bath temperature, electrical current, and the metals used also impact plating.
While a range of thicknesses can be achieved, the specific application is usually the determining factor, such as a connector pin or a clip/bracket or a battery terminal. A desired thickness can be defined as an average taken at several points on the part, an acceptable range between a maximum and minimum, or a value above or below a lower or upper limit only.
All-over and Selective Plating
Plating can cover the entire part or only selected portions. While all-over plating does increase costs and complexity, not all applications require it.
Selective plating is generally used to minimize costs (such as when plating with precious metals like gold) when some part features don’t require it. Features that don’t need to be plated can be masked with a non-conductive material like rubber so only the exposed material is coated. Controlling the depth to which parts are inserter in the bath, especially with reel to reel plating, is another option.
What about hot dip tin as an alternative to all-over plating? In this method, parts are dunked in melted tin, resulting in a thicker coating that plating provides. There is also less control over where and how evenly the tin coats the part. Again, the application is the determining factor: hot dip cannot offer the same level of precision for fine part details or tight tolerances.
Parts with Exposed Edges
Some parts made from pre-plated material have exposed metal at the edges where they’re cut. One example is thin materials where the exposed edge is less than 0.030” thick, such as a circuit board part with a soldered leg. The solder must be on the top and bottom of the leg as it is pressed to a circuit board, but the thin side edge doesn’t impact the solderability of the part, so it doesn’t strictly need to be plated.
Bare edges may be more of a concern with thicker gage materials because more surface area is exposed, or if there is potential for corrosion or contamination of the base material. In these cases, all-over plating cannot be avoided.
In barrel plating, stamped parts are placed in a basket and immersed in the plating solution and rotated for a predetermined time. Parts are agitated in the basket and tumble around, becoming completely coated with particles of the plating material in the process. It’s an efficient choice for high-volume batch processing and can be cost-effective since many parts are plated at one time. Sturdy, robust parts, such as electrical terminals, are good candidates for barrel plating.
Not all parts are suited to this process, however. It can cause distortion or damage to delicate parts or those with flatness and angle tolerances. Some parts may no longer reach or exceed acceptable quality levels (AQL) or desired roughness averages (RA). And even if the jostling of parts doesn’t cause damage, very flat parts or those that can nest may stick together, resulting in inconsistent coating.
When parts cannot stand up to barrel plating process or when certain specifications will be difficult to achieve or maintain as a result of using it, reel to reel plating or stamping from pre-plated material are excellent alternatives.
Reel to Reel Plating
In reel to reel plating, finished parts are attached to a carrier system that is fed off of one reel into the plating bath, and then taken up onto a second reel. Parts are secured to webbing that exposes the area to be plated while still supporting the part so it can be connected to the carrier bands using special tie bars. The plated strip is interleaved with a paper strip to keep the parts separated. This process can also be used to pre-plate raw material (i.e. blanks) that will later be stamped into parts.
While the size of the reel limits batch size, the trade off is highly consistent plating with limited damage or distortion to parts. Reel-to-reel plating can be a more involved and expensive process. It is often used with reeled parts that will be singulated (either by the stamper or by the customer later) or in two-stage stamping operations where a blank is created with one tool, then plated, then run in another tool to be formed and singulated.
Selective plating is also possible with this method. Delicate parts, such as automotive door lock electrodes, are an example. A part like this may require all-over plating on the “leg” feature with bare edges on the rest of the part. Only the critical feature is exposed so that plating is only applied in that area. It’s also important to remember that even if the goal is not to selectively plate parts, there will be an uncoated area where the part is attached to the carrier strip.
Stamping with pre-plated material is often a preferable option to plating. It is a simpler process because stamping a pre-plated part is really no different than an unplated part, so it eliminates any additional operations and reduces failure modes for the part. There is also a higher consistency in the plating thickness.
For example, plating a leg, rod, or other long, narrow part often leads to what’s called the “dog bone” effect. This happens where there’s a higher build-up of plating material at the ends compared to the middle of the part, kind of like the shape of a dog bone. It can be problematic in cases where the tip of the part must fit into a slot or circuit board. Pre-plating the material ensures even coverage on the critical area without additional trimming or finishing.
Barrel and reel to reel plating can cause damage to parts in different ways, so inspection for damage is different for each method. Barrel plated parts can distort due to the weight of the parts colliding or nesting can cause bent legs and surface scratches.
Reel to reel plated parts include an interleaf paper separating each wrapped layer of the carrier strip. It’s necessary to check that the winding is correct, the paper is in place, and that there is no damage or distortion on the part. In addition, there might be a winding orientation that needs to be verified so that the parts will payoff correctly in their final use.
Plating may seem like a relatively simple process, but there are many considerations that go into selecting the right method for each application. An experienced stamper can advise you on best practices and pros and cons, so please contact CEP to talk about your next project!