It’s hard to imagine now, but seat belts weren’t required in cars at one-time. However, the lap and shoulder-harness seat belt became the industry and cultural standard in most places after many injuries. Now you can’t drive too far down the freeway without seeing a “Click It or Ticket” sign posted alongside the road. And in the same way that a little extra prevention goes a long way in saving lives on the road, so does an added measure of protection improve the performance of a printed circuit board.
PCB edge plating, which is also referred to as border plating, side plating, castellation, side metal, and plated contour, is a strip of protective metal fabricated on the narrow side of the board. A circuit board may have just one side edge plated or all around it, depending on the needs of the design. This extra metal hasn’t been used as much in the past, but it has grown in popularity due to the protection it provides for the electrical performance and physical integrity of a circuit board. If you are considering using PCB edge plating in your next layout, read on to explore what it is, how it is fabricated, and some of the design constraints you should know.
The Purpose of PCB Edge Plating
Printed circuit boards have several vulnerabilities that can be better protected using PCB edge plating. Here are the four main areas that the edge plating can help with:
- Electromagnetic compatibility: On multilayer circuit boards, the current carried by the power and ground planes can generate EMI (electromagnetic interference) at their edges. This interference can radiate outside of the system, causing the circuit board to exceed the EMC standards it is intended to meet. By enclosing the board’s edges with a barrier of metal plating, the EMI is prevented from radiating.
- Signal and power integrity: By reducing the EMI, the signal and power integrity of the board will also be improved. The extended grounding afforded by the edge plating will improve current-carrying capabilities and provide additional shielding for high-speed circuitry.
- Structural integrity: The addition of metal on the sides of a circuit board reinforces its strength and rigidity. The boards are less likely to suffer physical damage through their regular use. The metal sides of the PCB also provide better support for the board to slide into the metal frames and casings of their intended electronic systems. Edge plating will also protect the PCB from accidental physical damage in its handling or usage.
- Thermal dissipation: Circuit boards can generate a lot of heat, especially as signal speeds increase. This heat is typically conducted through thermal vias into an internal ground plane to be distributed evenly throughout the board. With the ground plane connected to the edge plating, the heat can easily dissipate outside the board through the metal edge.
As you can see, there are several advantages to using edge plating on a circuit board. Next, let’s look at some methods used by manufacturers to fabricate this edge plating onto a PCB.
How Circuit Board Edge Plating is Fabricated
Circuit board edge plating is a fairly standard process and typically an extension of the normal plating procedures used in circuit board fabrication. The outer edges to be metalized must be milled before plating the surface and the thru-holes of the board, allowing the edges to be plated simultaneously. However, the adhesion of the metal plating to the board edges can be a challenge. The board’s edges will have to be prepared by smoothing out any imperfections to ensure that the plating process works as intended. It is also essential to design a metal strip into the top and bottom layers of the board to serve as an anchor for the edge plating to bond.
Copper electroplating can be used for metalized edges, which will minimize the cost of edge plating since that process will already be in use on the rest of the board. Other fabricators might use immersion gold or silver or electroplated nickel and gold. A surface finish can also be applied to the plating, which can be done with Immersion silver or tin, hot air solder leveling (HASL), and electroless-nickel immersion gold (ENIG).
There are some limitations to PCB edge plating, which are important to understand before committing to this process:
- For those boards processed in a manufacturing panel, there will be gaps in the metal plating where the breakout tabs are used.
- Circuit boards that will have edge plating applied can not be broken out of their manufacturing panels with V-scoring.
- If the surface of the board’s edges is not adequately prepared for the plating process, the metal edge could peel due to inadequate adhesion strength.
- Any burrs left over from the milling process can cause the plating process to fail, and they must be carefully smoothed over before the metal edge is applied.
There are also some PCB layout recommendations you should know about, and we’ll look at those next.
Layout Recommendations for PCB Edge Plating
Laying out a circuit board for edge plating is not complicated, but there are a few things that the designer needs to know. First, the plating requires a metal strip on the surface layers with which to bond. Not only do the metal strips need to be included in the layout data, but the designer must also remove the solder mask in that area to leave the metal strips exposed for plating. Another important design consideration is planning the configuration of the internal plane layers. While the ground plane is usually connected to the edge plating, it is essential to pull the boundary of the power plane back from the edge of the board. Pulling back the power plane boundaries will remove any chance that the plane will connect with the PCB edge plating during fabrication and create a direct power to ground short.
PCB designers must communicate their intent for the edge plating through the manufacturing drawings. The documentation needs to specify the sides of the board to be plated, the overlap of the edge plating with the metal strips on the surface layers, and the type of plating and surface finishes required. A good PCB contract manufacturer can help you with these decisions. At VSE, our design and manufacturing engineers regularly work with our customers to resolve these and other layout questions. Let us know how we can help you with PCB edge plating or any other design and manufacturing considerations on your next circuit board project.
