The new “package of the day” requires modified paste layers.

Modules with castellated mounting holes are showing up more and more these days. I’ve also heard it called a “half-via” setup. Both seem to fit. The copper mounting pads on the bottom of the module’s little PCB wrap up the side of the PCB with a half-via. Thus the name “half-via.” If looked at from the base, the edge does look a bit like the outline of the top of a castle wall, so that makes sense too.

Whatever. The name isn’t that important. A few things are important though, such as the land pattern and solder deposition. I hadn’t heard any special requirements for using this type of part until recently, when I ran across a GPS receiver module from ublox (u-blox.com). What they say is that more solder is needed on the outside of the pad than on the underside of the pad, so it can properly wet up the half-via. That makes perfect sense. There are probably multiple ways of doing this, but you can see their take on it in their data sheet (Figure 1).

Obviously, follow the data sheet that comes with your specific part, but if you don’t have any official guidance and can’t get word from your part manufacturer’s applications engineers, this method might provide a hint. The copper land is just a standard rectangle, as is the solder mask. The paste layer, however, is not. It’s wider toward the outside of the part. This will help create a proper fillet up the via, while reducing the chance of solder balls and other messy things that can happen when you have too much solder under a part.

All that said, when did this become the package of the day? I haven’t seen what IPC has to say about it yet, but these things are all over the place. Our engineers and assembly folks are cool with it. It’s not that tough to build, fortunately. But, as I saw with the u-blox GPS part, there are new design issues to contend with.
I can certainly see the advantages of the package. The half via can permit a good solid fillet, providing good mechanical connection. They’re typically a PCB-type substrate, so the coefficient of expansion and flex strengths should be similar to the underlying PCB. On the other hand, like with an LGA (land grid array), the low profile after soldering will tend to exacerbate an expansion or flex risks.

If you happen to be in the business of creating modules that use this form factor, you can do a few things to help designers and manufacturers out. First, and most important, make sure to put a good pickup place centered on the topside of the module. It can be a metal EMI shield or a big part with a nice flat surface. Either should work, but without that pickup point, most manufacturers will end up hand placing it and that doesn’t bode well for high-volume use of the part. On the bottom, put a decent sized pad along with the half-via. I’ve seen some that just use the via’s annular ring. Don’t do that. It doesn’t give enough room for a good
solder joint.

Duane Benson is marketing manager at Screaming Circuits (screamingcircuits.com); This email address is being protected from spambots. You need JavaScript enabled to view it.. His column appears bimonthly.

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