Castellations— or castellated edges, plated holes on the board’s edge, plated half-holes — are rampart-like structures along the edges of a printed circuit board. Their main advantage is that they allow an electrical connection to another board through the side edges of the board, without additional components. Pretty neat!
Castellations are manufactured by creating ordinary plated holes and then running a sharp router bit across them, leaving half the hole in place.
Castellations have several properties that make them quite useful:
- They can be complete modules that are certified to standards that you care about, which can be quite useful for adding complex functionality like RF to a design.
- Since there are no connectors usually involved, they can be low profile and compact, manufactured to a higher spec than the board it will go on.
- They can be considered, with some caveats, like any other surface-mount component for the purposes of automated assembly and sourcing.
But, as always, there are some special cases we need to be aware of:
- A useful feature is to have both a castellated half-hole and a complete throughole for a header. If we don’t prevent solder paste wicking via the throughole, however, our re-flow joints might fail. Possible solutions are to remove the contact on the bottom side or adding a soldermask bridge between the castellation and the throughole.
- While most commercial castellated modules are certified to certain standards, the product onto which it is attached usually needs to be certified as well.
- The entire board cannot be covered with castellations since manufacturing requires break-tabs to attach the board to a panel. The amount of tabs depends on the size and shape of the board and it’s best to find out what the fab you’ll use recommends.
- Modules — castellated or not — that have gone through reflow to place components on them will have gone through additional reflow cycles than the boards they’re mounted on. This is something the designer must check in order to determine if that’s going to be an issue in terms of the components used, the effect on solder joints, and the materials used for manufacturing the PCB.
- The castellated module can only have components on one side, and the primary board cannot have components in the area the module covers. (Although I haven’t seen any, double-sided modules can exist but then the primary board needs to have recesses to accept them or hover over the board using connectors.)
How Eurocircuits helps you castellate
Manufacturing castellated holes reliably is all about making sure that there is good mechanical strength that prevents delamination and other faults during manufacturing and assembly. We’ve defined a set of recommendations on how to best achieve that, although we factored in some flexibility in case you, the designer, would like to do something different.
- We use break-tabs to connect the PCB to a panel during manufacturing and assembly. We need up to 5mm of edge-length for each tab, and normally place two per edge. As expected, though, the amount and position of tabs greatly depends on the size and shape of the board. Generally, if you have castellated holes in two of four edges of a rectangular board, there’s no problem to find space for these tabs on the other edges. However, if those two edges are entirely occupied with castellated holes and the other edges are castellated as well then the best thing to do is to contact us for advice.
- There must be a copper annular ring on both the top and bottom of the board (see the article on annular rings for why this is important). We recommend that the minimum pad size is the finished hole size + 0.70mm. For example, if you define the finished hole size as 0.80mm (our recommended minimum hole size) then the pad size should be a minimum of 1.50mm, this gives an annular ring of 0.30mm.
- Unless it’s absolutely impossible, we highly recommend that inner layers also maintain the same size pad at the annular rings in the surface layers; this increases the mechanical strength of the castellated hole. (Be aware that sometimes EDA software ‘optimises’ these away, so you’ll need to prevent that from happening.) Again, if inner-layer pads are not possible because something is in the way, get in touch.
- We can manufacture castellated holes down to 0.5mm finished hole diameter but we recommend 0.8mm or larger. ENIG surface finish is better for castellated holes of any size, although finished hole diameters above 1mm can be manufactured with other finishes, like HAL.
- We recommend that exactly half of the hole remains on the PCB, not any other split ratio because that may increase the amount of manufacturing faults.
That was a lot to take in, so here’s a summary: make sure that there’s room for break tabs; large annular rings on the surface and inner layers strengthen the structure; ENIG surface finish allows reduction of required diameters; and, it’s best to get in touch with us early in the design about your requirements.
Here’s additional resources: