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First things first - you're not a factory

A project log for Assembling dense SMD PCBs - lessons learned

I've assembled many dense(-ish) PCBs with small components (down to 0201) during the last three years. A summary.

ChristophChristoph 09/28/2022 at 10:530 Comments

You want to assemble a prototype. Maybe even 3 or 5. But you're not a PCB assembly line.

That's a simple but important fact to keep in mind. You may do things that an assembly line may not do, and some things that can be taken for granted in an industrial setting are pretty different at home.

This log covers:

What quantities are we talking about?

Oshpark and Aisler sell PCBs in sets of 3, JLC and PCBWay start at 5. That's a handful of boards, and usually that's about as many as one might be willing to do by hand. You're not planning for 100, and even 10 can feel more like work than like a relaxing hobby. When the first prototype works, usually on of these things happen:

Your design will probably have between 10 and 100 components in it, definitely not 1000.

Manual assembly takes time

Be well rested. Don't rush. Slow down, take your time, and you'll have it done faster. Take a break in between when you need one. 

Get started

So for a board with 25 different components that's half an hour of preparation before we even touch the PCB. Of course you can touch it earlier but that won't get you anywhere.

Paste

No, clean your board first!

If I have a stencil:

Usually the paste is on after 5 to 10 minutes.

If I don't have a stencil:

Manual application takes a lot of time. Also depends on the application technique (more on that in a later log). So that's a large unknown, and sometimes you just have a bad day. 

Place Parts

I need 30 to 60 seconds per component. A lot of factors go into this. Most of my parts are on cut tape in zip lock bags. That means that for every BOM entry I need to open the bag, get the tape out and un-tape the required number of components. That's pretty much the same overhead no matter how many I need of a specific type.

If I need just a single part of a given type, that's one minute including placing. All additional parts take maybe 10 seconds.

On average, I need 40 seconds per component for a board with mixed component quantities (some singles, some multiples). So for a board with 100 parts, expect 67 to 100 minutes just for placing stuff. 

Reflow

Again, depends. Each side of a PCB is reflowed separately. If you build in steps, there will be even more reflow runs (unless each complete side is one of your build steps). Hot air gun, toaster oven and hot plate all have different timings - you know if it's done, when it's done. Let's say 10 minutes and another 5 to let it cool down.

I'll cover some techniques in a later project log, for low-temp, high-temp and mixed boards. Yes you can mix alloys, which might be quite expensive if you just went for contract assembly.

The sum of all that

If I use a stencil and do each side in one go, no additional assembly steps:

For a single sided board with 50 components: 30 + 7.5 + 33 + 15 = 80 minutes or 1 hour 20 minutes.

For a double sided board with 100 components: 30 + 2*(7.5 + 33 + 15) = 141 minutes or 2 hours 20 minutes.

Again, take a break in between. Maybe even two or three for double sided. 

Don't have or can't use a stencil? It will take longer then.

Build in steps? That will also take longer.

Touch-ups are expected and ok

You'll need to correct your own work. In an assembly line, this would barely be acceptable - but at home it's absolutely ok. That applies to paste application, misaligned parts, shorted pads and every other detail that could go wrong. Don't expect every assembly step to be perfect every time. You'll get better, and you'll find your own way of getting it right with the tools you have at hand.

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