The perpetual PCB design/build/help thread!

If you want to get that part properly soldered you might want to consider something like this:


View: https://www.amazon.com/ML-851K-Hot-Pencil-Rework-Station/dp/B003UCR332


We used to have one where I work.

that's kind of a mid priced one. There are cheaper ones.

For reflow and as a hobbyist more, i kinda wanted one of those miniware hotplates for surface mount soldering. (have had the TS80 for 4+ years now and really happy with it.)
Havent gotten one yet since the pandemic hiked prices (specifically of the MCU) and locally they havent dropped/normalized yet for my liking.

I've been using Fusion for PCB design (mostly because I originally learned/suffered through Eagle, and that's what underlies Fusion's PCB module) and for one-off designs usually buy from OSHPark. Generally I do assembly myself, using either regular solder or syringe-dispensed solder paste (Digikey etc. sell tubes of the stuff for a reasonable price). I don't bother with stencils, but instead squeeze a bit out into a plate, then dab it onto the pads using a toothpick or similar. I usually use a pencil hot-air rework system like the one linked a couple comments up to actually do the soldering, though a hot plate is certainly a good alternative.

Works pretty well, though it obviously is best suited for boards with not-huge numbers of SMD pads. The nice thing is that even if you’re not super-neat and get a bit of paste smearing between pads, the solder mask coating will tend to eliminate bridging between the pads once the paste is melted. The size you can work with depends on how steady your hands are and how good your eyesight is. I can manage 1 mm pitch without a magnifier and 0.5 mm pitch with a modest binocular scope. Finer than that and positioning the chip over the right pads becomes a nuisance.

I bought a soldering station with hot tweezers for little resistors, but I don't do enough SMD work to know if that was the right option. I do have a hot air station too though, with nozzles ranging between pencil and moderately large. How do you folks with actual experience feel about hot tweezers?

Tweezers always felt like they are more of a rework/repair thing, than in assembly as soldering is better done with regular soldering irons with very fine tips. But It really makes taking off resistors/caps incredibly easy though (and comes in handy when you cant just blast an area with the hot air station).

Drizzt321 said:

Except I'm realizing that getting an ESP32 put onto it is far more expensive (relatively) than I realized. It needs Standard rather than the cheaper Economic, and ESP32 apparently is recommended doing baking (reduce risk of moisture with solder reflow process).

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For one off and prototype boards, solder it yourself. If you don't design tons of boards good odds the first one won't work anyway or you'll have to rework it because you made some simple mistake, so might as well avoid paying to have a bunch of boards made and then having to rework each by hand. Plus its faster turn around time, which again is nice if you are going to need to iterate/fix things.

And yeah, definitely agree with skipping stencils and just dabbing down paste onto pads with a syringe. If you're going to do a lot of prototyping, buy a pneumatic paste dispenser, but you don't even need that here.

For actual reflow, on single, small boards without BGA, you can use a hot air gun or something like this:

View: https://www.youtube.com/watch?v=ORVPBt1WP1A


If you're making batches, using lead free or doing more complex parts, you want to buy a reflow oven that can do more controlled reflow profiles. The T962A is less than 300 on Amazon Prime, and can be hacked to be half good.

Shavano said:

If you want to get that part properly soldered you might want to consider something like this:


View: https://www.amazon.com/ML-851K-Hot-Pencil-Rework-Station/dp/B003UCR332


We used to have one where I work.

that's kind of a mid priced one. There are cheaper ones.

Click to expand...

FWIW those are in the awkward gap between the really cheap generic Chinese hot air stations (which are honestly fine for a lot of things) and the better stuff. I'd say spend either more or less. There are lots of good mid priced hot air stations coming out of China these days. This channel reviews tons of them:

View: https://www.youtube.com/playlist?list=PLZzwMlLVLdOCgT7rYau-CqcDyeZ3Rn5qV


I picked up the Atten ST-862D as a home station based on those reviews for well under 200 and its way more hot air than any hobbyist would ever need.

Drizzt321 said:

I've had to fix/replace a few tiny SMD resistors, always a huge pain. Some paste and hot air pen probably would have been so much easier.

Maybe I'll look into it fir my next round of low volume stuff. Cheaper to get the boards and just a few SMD things like the tiny resistors/etc, and on the cheap/economic assembly.

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Paste is for assembling new boards. Once the board is assembled and you're doing rework you don't usually want to use paste. If it is a tiny resistor you want to change, quick and easy way to remove is to put a big blob of solder on a chisel tip, touch the blob onto the part from above, hold for 2 seconds, and then lift. The part will float up into the solder blob and you can wipe it off your iron on a sponge. Once it is off, you can usually reuse the existing solder with just a bit more flux.

Hot air is another way, but unless you do it a lot and get lots of experience theres a real risk of blowing off surrounding parts, melting connectors, etc. Tweezers are the best solution, but there aren't a lot of options for tweezers that are worth using that aren't going to cost double or more what your iron and hot air combined cost.

Xenocrates said:

I bought a soldering station with hot tweezers for little resistors, but I don't do enough SMD work to know if that was the right option. I do have a hot air station too though, with nozzles ranging between pencil and moderately large. How do you folks with actual experience feel about hot tweezers?

Click to expand...

Part of the reason that there are no value tweezers that enthusiasts recommend like there are for hot air is that they're a niche tool. They save a tiny bit of time if you're going to swap out small SMD resistors, but you don't actually need them. They're more for reworking dozens or hundreds of boards at a time where the extra time it takes to use hot air or a single iron would matter.

Xenocrates said:

I bought a soldering station with hot tweezers for little resistors, but I don't do enough SMD work to know if that was the right option. I do have a hot air station too though, with nozzles ranging between pencil and moderately large. How do you folks with actual experience feel about hot tweezers?

Click to expand...

they're excellent. I always try to use them for 2-pin surface mount packages. Not needed for soldering. It's easier to hold the part with plain old tweezers, solder one end, and then solder the other, for me.

I think they make pin compatible switch mode replacements for some of the LDO packages (TO-220's from what I've seen), like so https://www.mouser.com/datasheet/2/670/pxo78_500_s-3313217.pdf

Drizzt321 said:

Nice. Except it's not a TO-220, it's SMD SOT-223. Not to say that's not pretty great, useful for sure, but a bit smaller and physically on the board would be useful. I have seen some other switch mode SMD, will eventually figure out how to do their circuits, but for now I'm just going to go off of the larger 5v buck for the LED strips. Which what I wanted to do was be able to use the enable/disable on that buck board, so I could completely shut off the LEDs, including the quiescent power. Ah well. v3 design it'll have to be, perhaps next year.

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FWIW it really is not that hard to implement a buck converter that works well enough for typical low power applications, and since you're doing the super common ~12V to ~3.3V conversion, half of the datasheets you open are going to have an example schematic you can use literally unmodified. Getting high efficiency at low cost under a wide range of loads is where expertise makes a difference, but if you're starting with a linear regulator you probably don't mind if you're 5% less efficient than you could be. Just google "buck hot loop" before you do the layout and think hard about where you put that input capacitor and you'll be fine.