Making a solder paste mask or stencil

HalloweenBob

By now, anyone who was not a soldering expert with SMD parts, especially the evil U1 on the K-Ceptor has realized that it requires special skills and tools to get it done right.

One thing that will make the job infinitely easier is to have a solder paste stencil. They are usually expensive, but I have found a vendor, OSH Stencils, that I could get one from that only cost me $21.00.

I also thought some people would benefit from me sharing the file you need to upload to OSH Stencils in order to get one for yourself. You can get that file HERE.

This is a stencil for a single board. You would have to figure something out if you had your boards all printed on a large sheet and you were planning to cut them away later.

You carder many different types of stencils. I got a stainless steel 4 mil thick stencil without a frame. I also ordered a set of jigs that they offer which are supposed to hold the board in place. It is on it's way to me, but I don't have it yet. I will post pictures when it arrives. The company is in Utah, in the US, but they do ship internationally.

I hope this helps someone who is getting stuck with this part of the project.

HalloweenBob

Got the Stencil a few days ago and it looks great. Great price too. Here are some pictures.

daylanKifky

Great!!

With stencils applying the correct pressure with your squeegee (and, in consequence deposit the correct amount of paste) is crucial to get good results.

I found a couple of photos that you can use as a starting reference.

This one has just enough solder on most pins, and perhaps too little on the upper right. Even a little more paste should work

Instead, on this one there's definitively too little paste.

HalloweenBob

Thanks, that's very useful.

kmw199614

Thank you for posting this! Do you have any recommendations for what solder paste to use / how to heat it?

Thanks!

daylanKifky

kmw199614

The first thing to keep in mind is: Avoid buying cheap solder paste. A 10cc syringe won't cost less than USD22 or so if the product is good quality.

I got better results with fine pitched paste. I always get TYPE 4 or TYPE 5 if I can find them.

If the regulations in your country allows it, getting leaded paste will make everything easier. By leaded I'm referring to alloys like 60Sn/40Pb or 60Sn/37Pb.

The solder paste is conceived to be heated inside an oven, with a particular temperature curve

In your lab you will probably have a hot air station, but you should try to emulate such a profile. There's plenty of videos online that shows you the manual technique (I also linked a couple of those on another thread). The important part is to gradually heat the component PCB area in order to allow the paste to go through the SOAK stage.

The reason because Leaded paste is easier is that the reflow peak, and the overall curve is cooler.

And finally: remember that solder paste has an expiration date. Try to buy it only from trusted resellers or directly from the manufacturer. Normal solder paste should be kept in the fridge (and taken out a few hours before start working with it). I recently got a Thermally stable one that can also be kept at room temperature. But be aware of what type are you getting in order to store it right!

sully

Happy to report that I was able to solder 15 k ceptor sensors that pass the i2c test
using a 4ml stainless steel stencil from OSH.

I used type 4 80% lead paste, I would have preferred to use type 5 if I could do it again. Here are some things I picked up on after doing a bunch in a row.

As per some of the tutorials out there, I made a jig out of various other pcbs I had lying around. It made it super easy to accurately line up the board with the stencil. Make sure to not have any tape coming in contact with the underside of the stencil, as that might increase the 4ml clearance. Luckily the boards I had around were the exact thickness as my K ceptor boards which made things easy.

I also found it crucial to clean both sides of the stencil liberally with rubbing alcohol every time after using the stencil. In the beginning when I wasn't as serious about this, the gaps in the stencil for U1 and U2 began to gunk up with paste from the previous coating. I used 90% isopropyl but 99% would have been better.

I only used the stencil for u1 and u2. I got results like this consistently and was able to solder 15 functioning boards in a row without any hiccups. My first (and only extra) k ceptor didn't end up working, I wasn't being liberal enough with how much solder paste I was using.

sully

Also, as you can see, the section of the gerber files I sent to OSH also included through hole components and other unnecessary stuff, it didn't matter to me but I would advise omitting the stuff you intend to hand solder to make things easier.

zachnfine

It looks as though pcbway, the place that HalloweenBob contracted to make his PCBs, also does assembly (with customer supplied parts or their own collection of parts). I wonder how cost-prohibitive it would be to just order assembled, soldered boards as well as parts from them? Has anyone tried getting a quote from them for PCB prototyping and assembly?

I'd like to build a suit, but the talk here of microscopes and reflow work makes me think I'd probably end up with a high dead-board to functional-board ratio. Having the difficult soldering done by the PCB supplier sounds appealing.

sully

That was an option I was considering but never followed through with on a quote. For your reference, as far as my total investment in this process (not including a traditional soldering iron and basic tools I already had) The boards were extremely cheap.

30 k ceptors and 5 Hubs for around $25 shipped from jlc pcb (the difference between ordering 15 and 30 boards is comically low)

this cheap hot air station for $50

$20 for the stencil (I could have ordered a stencil through the pcb supplier and saved more money probably, although I still highly recommend OSH stencils)

around $10 for the solder paste

so bit little over $100 for me.

I didn't use a microscope or anything like that. U1 and U2 would be the only parts worth paying to have soldered in my opinion. I was able to hand solder U2 with a traditional iron on a couple of occasions with some flux and solder wick so if that part bumps up the cost it may be worth it to hand install.

U1 is the most expensive element in the whole project at almost $6 per chip in my case, so whether that part ends up being more or less expensive to have pre-installed could also have a big impact on cost. I took the risk of only buying 1 extra accelerometer chip and luckily got away with it......

HalloweenBob

I did get a quote from PCBWay for assembling the K-Ceptor boards, but I only asked about U1, U2, and U3.
Since I had everything (boards and components) the fee was $141.00 to assemble those 3 parts on 20 boards. That included an X-ray test. I ended up not using them because I got help through the college where I work which has an electronics lab. I am currently adding all the other components and hope to be testing them in the next few days, although my weekend is pretty full. I will report back.