Designing PCBs With Castellated Holes | Voltlog #335

Welcome to a new Voltlog, in today’s video we’re going to be talking about castellated holes and how you can create them in your CAD tool. If this term is new to you, it’s pretty simple, you’ve certainly come across them if you’ve ever played with a bluetooth or wifi module because those 99% of the time will use castellated holes, which are these semi-plated holes on the edges of a PCB.  Having these connection points allows these modules to be soldered on top of a main PCB which contains our main circuit.

You might ask yourself why do we use castellated holes, why not use a simple through hole header as a board interconnect or just some simple SMD pads. Well in my opinion the most important reason is the relatively low difficulty for soldering castellated holes. If you think about it, having some SMD pads that go on the bottom of the PCB makes it pretty hard to solder, at least without proper equipment, you need to deposit solder paste on those pads, you need to get it in the right amount and then you need to have perfect alignment of the module on top of the pads which reside on the main PCB. Having these connection points underneath our module makes it very hard to align because you can’t see them. Also debugging such a module is going to be a pain because you won’t have access to all of those connection points.

So this where castellated holes improve, by having the plated half-holes at the side you can solder them even with a simple soldering iron, alignment is pretty easy because you can clearly see the connection points and debugging these is much easier because once again you can access them, you can do measurements with your scope probes or whatever instrument you are using.

Another advantage of having castellated holes on a design is to think of it like a building block, you might improve this building block externally or switch to a new building block that uses the same pinout and you just drop it into your system as a simple upgrade.

And believe it or not but having a module with castellated holes can lower your BOM cost in some cases because let’s say you need to use an RF module which might be 4 layers or a complicated system on module that may be 6 or 8 layer PCB with a powerful processor. Instead of building your entire system on an 8 layer PCB and assembling that complicated BGA chip yourself, you can buy the module ready made, it has castellated holes and you just drop that module into your system which may be a 2 layer mainboard or 4 layer mainboard that costs less.

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