Author Topic: Surface mounting RasPi Pico.  (Read 528 times)

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Offline phil from seattle

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Surface mounting RasPi Pico.
« on: April 17, 2021, 12:49:11 am »
I am designing a Pico based board and looking at providing a version that allows surface mounting the Pico.  I have never designed for a surface mount module. I've been working through the footprint including the test points specified in the datasheet.

One thing I don't understand - is it safe to route underneath the Pico on the top layer of the base PCB?  The bottom of the Pico module has routed traces. Are 2 layers of solder mask sufficient to provide insulation between those traces and ones on the top layer of the base board?  I assume that the solder will hold the Pico module in place and any mask voids will still have 2 solder mask thicknesses of separation between the two copper layers.

Obviously, I will avoid test points TP1-TP6 and the 4 USB shell ground pads.  I think I will make pads for them in the footprint and but not connect them to anything.

Are there any issues with "untented" vias? The Pico looks to have fully tented vias.  Do I need to tent my vias as well?

Any issues with the programming header?  I assume that I will need to have a separate header on the base PCB. I thought about making a cutout in the base PCB to allow a pin header to be soldered onto the Pico after it was mounted. Probably not a great idea.

Are there any other things I should be aware of?
« Last Edit: April 17, 2021, 01:02:10 am by phil from seattle »
 

Offline Mariko

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Re: Surface mounting RasPi Pico.
« Reply #1 on: April 19, 2021, 06:20:41 am »
Hello, I haven`t held RPi Pico in my own hands, but from what I can see on images 2 layers of solder mask will be more than enough to isolate Pico from your board. But if you want to be sure you can just put a thin piece of kapton tape between the boards. Just be aware of TH mounted USB port.
 

Offline phil from seattle

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Re: Surface mounting RasPi Pico.
« Reply #2 on: April 19, 2021, 07:34:45 am »
The datasheet talks about the USB connector issue and the footprint shows pads for the 4 TH legs.
 

Offline asmi

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Re: Surface mounting RasPi Pico.
« Reply #3 on: April 19, 2021, 02:24:53 pm »
You can always declare the space under the module as keep-out. This is very common for RF-related modules, like BlueTooth or WiFi.
Or just use 4 layer board and keel all routing to other layers. This will probably allow you to make your module more compact too.

Offline MarkR42

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Re: Surface mounting RasPi Pico.
« Reply #4 on: April 20, 2021, 08:38:20 am »
If you are manually assembling them, you could put a layer of kapton tape underneath to discourage shorts. That's what I do with my product which I hand-assemble using a castellated module. I think shorts are unlikely as 2 layers of solder mask (plus the height of silkscreen etc) will be enough to create a little gap.

Is there any reason to treat the swd / debug pins differently from the others? On the Pi Pico, as far as I can see, every pin has both a throughhole and a castellation on the board edge, including the debug pins - you can just put pads on the main board and solder them in the usual way, then add throughholes on your main board if desired.

On the other hand, if you're not likely to need the swd pins on a specific board, you could route-out a slot underneath them to allow for optional installation of pin headers directly on the Pico, and leave them unconnected when not needed.
 
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Online Berni

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Re: Surface mounting RasPi Pico.
« Reply #5 on: April 20, 2021, 08:44:00 am »
The many kinda of RF modules like this all do it:


Notice how these modules have the same sort of half holes with plating down the sides? This is called castellated holes and is designed for SMD soldering PCBs together.

So by the looks of it the RPi Pico was designed to be soldered like this. Otherwise they would use just regular pin header holes. Those castellated holes actually cost extra in PCB production to make since they introduce extra production steps.
 


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