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[Advice Request]: managing thermal pads for SMD components in custom PCB (sh.itjust.works)

submitted 9 months ago by [email protected] to c/[email protected]

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cross-posted from: https://sh.itjust.works/post/13637559

Hello everyone, I need some advice.

I am making custom PCBs for a project of mine. It's basically for a little remotely controlled robot using little DC motors. I chose the Seeed Studio XIAO ESP32C3 as the uC since it has inbuilt wifi/bt, 3.3V regulator that I can use to power the motors (can source up to 700mA) and lipo charging management (the robots will run on battery). As you can see from here, the microcontroller is surface mounted and the pads for the battery are on the bottom layer. Same story goes for the thermal pad of the microcontroller and the thermal pad of the motor driver (datasheet). I have worked with SMD components in the past and can solder them by hand, but I have never worked with SMD components that have thermal pads on the bottom layer. My question is: how to manage (route?) them? My PCB is 2-layer and I was planning on having both layers filled with a ground plane. Do I just connect thermal pads to the ground plane and call it a day? Wouldn't that make the components hard to solder with hot air? Do I make an isolated polygon that only acts as a thermal pad?

Speaking of soldering is even hot air the way to go in this case? My PCB has components on both sides, and I was planning on ordering stencils together with the boards and using solder paste, placing the components and then using hot air to solder the components in place. I thought a hot plate would be better but I don't have access to one and I don't know how that works with components on both sides.

I attached some photos of the PCB in Kicad, and here's the git repo. If it is of any help, I'm planning of having them manifactured by JLCPCB. It is also my first time using KiCad, so go easy on me :)

Thanks!

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[–] [email protected] 2 points 9 months ago

Connecting thermal pads to a plane will make them harder to (hand) solder, but it should be manageable with hot air. Be sure to be patient and give the heat time to sink into the extra thermal mass. You should probably pre-warm the underside to give yourself a head start. You can move the hot air around a little bit as you wait for things to heat up to avoid locally overheating anything. Carefully observe the components with the thermal pads to ensure they fully "settle" as they reflow, since you want to make sure the solder on the thermal pad is fully melted.

Some design rules also reduce the aperture of the window in the paste mask over the thermal pad so that the amount of paste on the underside pads is somewhat reduced - just be careful you don't have a big ol glob of solder paste there otherwise it could squeeze out and make shorts with your other pins.

Consider adding thermal spokes to the vias, pads, and thru-holes connected to the heatsinking GND plane if those pads/holes are not intended to conduct heat, high current, or mechanical support - it will make them easier to solder by reducing the thermal conductivity to the plane. Don't spoke anything in the thermal paths.

Side note, just eyeballing the pictures but the motor traces could be widened to the width of the pads of the driver IC - motors can pull a lot of current in locked rotor conditions. I'm sure you're current limited by the driver and battery and it's probably no big deal but it doesn't hurt to have the insurance. Always good to do a once-over checking for any potential high current paths. You're not especially space-constrained in this design so there is no reason to go with the smallest settings on everything. You can check out some PCB trace width calculators to make sure your worst-case scenarios are covered.