Advice on adding missing parts to an assembled board

[Bansci]

I'm trying to start up a small business selling scientific equipment. I'm in the prototype stage, and with my current prototypes I order PCBs from JLCPCB and get them to populate all passives.

I then hand solder the ESP32-S3 MCU, a STUSB4500, it's power MOSFETs, and the ADC as JLCPCB doesn't have these parts and my scale doesn't yet justify (or afford) sending these parts to them.

Has anyone tried using a pick and place machine to populate missing components on a PCBA order, then reflow the board a second time? The main issue I can see is getting paste on the unpopulated pads, dealing with reflow issues I assume will be easier.

Any advice on this? How would you add the missing paste, with syringes or very small stencils? Is there another approach for small scale?

I'm considering placing the whole board myself, but parts are much more expensive here, and I have about 50 components so reliable feeders seem very expensive. I've ordered a pandaplacer to mess around with openpnp for fun, and to place some smaller daughter boards, but realise I might need to invest in a production line if successful.

[SMTech]

Building SMT in two passes on the same side with a reflow in between isn't really a thing done at any serious level. As you note, getting paste on is a problem and also a pick & place has no concept of avoiding collision with already placed parts.
Mini rework stencils and templates are a thing, but they are not the solution for you, you could try fiddling with syringes but its much harder than you might think, and might not work at QFN scale such as that PD chip. ?You choices include:

Change your design so that it more closely fits JLC parts library surely they have a USB PD chip in there somewhere.
Carry on as you are.
Sub contract the assembly to someone in the UK who sources all the parts for you  - someone like @Jackster or.. me (other choices exist). This frees up your time for testing and validation & revisions instead of fiddly rework level assembly.
Find that PD chip as a pre assembled module - it probably exists.
You can play with small placers, but this is something you should only do because you really want to get into that side of things and at some level want that toy. Feeders are the most expensive part of an SMT setup, most places will have spent considerably more money on their feeder stock than the placer itself. In small volumes you will frequently find yourself supplied with parts that a pick 'n place can't handle without a custom tray or some chip straightening or other process first.

[mikeselectricstuff]

Adding a few parts parts is not hard - assuming pads already have solder from the original paste, plenty of flux, and reflow with hot air.

[SMTech]

Adding a few parts parts is not hard - assuming pads already have solder from the original paste, plenty of flux, and reflow with hot air.

Do JLC paste unpopulated pads? It's not a service I have ever used. If they do, I agree hot air and a flux pen (or goop of your choice) is the easy solution and those cheap hot air station like AYOUE are surprisingly OK.

[Psi]

Hand populating pcbs is totally fine if you have the time and don't mind doing it.
Depending on board complexity you can do quite a few per day if you just sit at your work bench with music/podcast on and just do that all day.

There's some techniques to it that saves time and prevents issues.

I recommend production lining the process. 

Take advantage of the fact some parts look very physically different. You can tip those out onto your work bench all at once and hand place them without any fear they will get mixed up.
You can divide all parts to be placed into groups that can't be confused with each other. Then populate each group for the entire batch before starting the next group.

Always clean your work area between batches so no leftover parts from other groups can get into the next group.

Avoid using the iron to hand solder small passives, there is a chance you will occasionally go too fast and it will look like it's soldered but actually wont have wicked down and joined the part to the PCB at one end.  Its just more risky doing it this way. It's more reliable to use hot air and flux.
Ideally you use a solder paste stencil and vacuum pickup too, but that is only possible if it's a blank pcb to start with.
Failing that I would probably either;
- Put dabs of solder paste on each pad with a syringe and dispensing needle, place parts on top, and then reflow entire pcb at once using hot air or reflow oven.
- Put a small amount of solder on all pads with the iron and then reflow the parts on with hot air and flux one by one.


The problem of passives being up the wrong way to pickup with tweezers can be solved by having a piece of paper under them. You can tap the paper and they flip over randomly.

[mikeselectricstuff]

Adding a few parts parts is not hard - assuming pads already have solder from the original paste, plenty of flux, and reflow with hot air.

Do JLC paste unpopulated pads? It's not a service I have ever used. If they do, I agree hot air and a flux pen (or goop of your choice) is the easy solution and those cheap hot air station like AYOUE are surprisingly OK.

I would expect them to paste as per your supplied stencil - they may not even know which parts are to be populated when the stencil is made, and it would be hard to automatically tally stencil aparetures against the BOM spreadheet. 
So by default I'd expect unpopulated pads to have solder on them - just checked a PCB I had done by them & unpopulated parts have solder.

If it's just 3 ICs then this shouldn't take more than a couple of minutes per board including any tidy-up.
The OP mentioned one part is a MOSFET - it's highly likely you can find an equivalent in the JLC/LCSC range.

ISTR seeing that either JLC or PCBway, maybe both, will order parts in from Digikey & Mouser

[Bansci]

Thank you for all the replies.

I think the message is clear that the halfway adding more components via stencil/syringe, PNP, and reflow is a dumb idea. I needed this sanity check!

I've already built 5 prototypes up by hand as you describe, using existing solder, hot air, flux, and a steady pair of tweezers to add to partially populated boards. I didn't have a great time but its managable and I'll get faster.

You suggested I alter my parts to suit JLCPCB and this is a very good idea I needed reminding about. Since my first production run a couple of months back they seem to now have STUSB4500 and ADC (ADS124S08) now in stock. I just need to change my mosfet which is indeed straightforward.

Thank you! And @SMtech, I hope to one day be successful (and funded) enough to outsource production, here's hoping I'm getting a quote from you in the near future!

[Matt-Brown]

We have used tacky flux on pads that have already been soldered. Then we use the P&P to populate and reflow. Works well.
Would never do more than 100 boards that way though

[lutkeveld]

JLC now also does sourcing from stores like Digikey and Mouser.
You can contact their livechat if you have questions, but I am quite sure they can assemble your PCB completely nowadays.

[pastaclub]

Why not have one side assembled by JLC and you do the other one yourself?

That way you can place everything you want them to do on one side and only the components they don’t have on the other side. You just stencil paste, place (either manually or automatically) and reflow.

Small components won’t fall off the bottom when reflowed upside down. Big and heavy chips: either use a fixture or move them to “your” side.

This seems the easiest, and it could even result in a smaller PCB. The space under even just the ESP32 fits tons of passives.

[Kasper]

You could make an extra layer for a custom stencil.  Copy the paste layer and add cutouts around the componets that JLC will place so the stencil won't hit them.