What is PCB side plating?
Circuit board side plating, also known as edge plating or castellation, is the copper plating that runs from the top to the bottom surface of a board and along (at least) one of the perimeter edges. It ensures a strong connection through the board and reduces the possibility of equipment failures, particularly for small form factor boards and sub-motherboards. Examples of such plating are commonly found in Wi-Fi and Bluetooth modules.
During the fabrication process, the outer edges to be metalized should be milled before the copper plating process. After copper deposition, a suitable surface finish is applied to the PCB edges. ENIG (electroless nickel immersion gold) is the preferred surface finish.
When is circuit board side plating used?
Edge plating is implemented when:
- There is a need to improve the conductivity capabilities of a board
- Connection is to be made across the edge of a circuit board
- A board requires protection from lateral shock
- Secondary circuit boards are connected to the mainboard through an edge connection
- Soldering of the edge is needed to improve the assembly
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Design parameters for edge plating
The metalized (plated) area is defined using overlapping copper (copper surface, pads, or tracks) in the CAD layout. It is done to ensure the fabrication of the side plating. The minimum overlap should be 500 μm.
On a connected layer, a minimum of 300 μm of connected copper must be specified. Moreover, on non-connected layers, the copper should have a gap minimum of 800 μm from the outer contour.
Types of side plating done on PCB
There are three options to consider:
- Copper up to board edge
- PTH on the board edge
- Round-edge plating
Copper up to board edge
To avoid damage to the copper during the profiling it is necessary to keep a minimum distance between the copper features and the edge of the PCB. This distance is:
- 0.25 mm on outer layers and 0.40 mm on inner layers with break routing
- 0.45 mm on all layers with V-scoring
Copper to board edge distance should only be used for planes and large copper areas where any slight damage to the copper will not affect the board’s performance. Tracks should not be located within the minimum distance of the board edge to avoid damage.
PTH on the board edge
These are known as castellated holes. These are plated holes cut on the board edge and used to join two circuit boards either by direct soldering or through a connector. There must be enough spare space on the edge of the PCB to hold the board in the production panel during manufacture.
It is necessary to have pads on the top and bottom layer to anchor the plating securely to the circuit board. A selective gold surface finish is preferred for smaller sizes.
Round edge plating
Here the edge of a board is plated from top to bottom. This is to establish a good ground for metal casing or shielding purposes. To produce a board with this plating, the board profile is milled before the through-hole plating process.
As the circuit needs to be held within the production panel during processing, it is not feasible to plate around 100% of the edge. There must be some gaps to place route tabs. For round-edge plating, selective chemical nickel-gold is the ideal surface finish.
What are the advantages of side plating?
- It is useful in high-frequency boards where signal quality is of prime importance.
- Side plating increases the electromagnetic compatibility of a PCB in a multilayer structure.
- Edge plating acts as a shield to the inner planes of the circuit board. Thus, the interference of signals emerging from external sources is minimized.
- Boards with side plating are prevented from causing electrostatic damage that can occur during handling.
