Drilling solder paste stencils

Interesting idea. For large pads multiple holes could be tried. Perhaps slightly thicker deposition could make up for the corners.

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

void _-void-_ in the obvious place

Even overlapping to some extent.

Even with really sharp drill bits, I suspect the holes would not have clean enough edges to release the paste consistently. Perhaps if you sandwiched the sheet between two boards while drilling it would help with that.

I just measured one of my thicker stencils and it is 6 mils thick. 10 - 16 mils would lay down way too much paste making the self alignment (during reflow) unpredictable. It does depend on the pitch of the parts you're using too. The smallest parts on most of my boards are 50 mil pitch IC's and 0805 passives, and 4-5 mil stencils work fine.

Success would also depend on the flux in your solderpaste. Water washable flux is somewhat tricky as it burns off quickly preventing the dots from flowing over the pad in time. RMA would flow better with round dots and give a better chance of flowing to the whole pad. While it is preferable to cover the whole pad with paste, dot's do work. I have a paste dispenser on my pick n' place which places dots. Reliability is not as good as stencils, but for low volume runs, it works out alright. The most common problem I see with dots is misaligned parts during reflow.

I use Stencils Unlimited for SS laser cut prototype stencils. At $145 each, they're not all that expensive. Also, if you're going to try making your own, I would suggest using stainless steel sheets instead of brass. Most types of soldering flux will activate the brass.

Good luck, I'd be interested in knowing how it works out.

The two previous posters have hit the nail on the head with the solder paste release issue.

On a laser cut or etched stencil the openings are usually just slightly cone-shaped to allow for easy release of the solder paste when the stencil is lifted. It may not be much but every little assist makes the job that much easier.

That said, it is possible to do and my employer used to do their own as well. I wasn't around then and have no idea how successful they were. However, how much are you saving? I have heard some comments in this NG that people are buying prototype screens for slightly less than $100USD. So what is your time and the material worth? Brass isn't cheap, nor is it easy to handle in very thin sheets and every slight kink/bend is going to adversely effect performance.

Either can be used with the paste I'm using.

The alloy I'm using is very cheap....12" X 8" sheet is less than $5.00.

I'm using brass. Also, have carbide bits of each size but see no reason to use them.

Drilled a piece of .016" brass with the five hole sizes needed using jobbers bits(.021", .032", .036", .052",.06"), here's a picture;

Squeegeed the paste at 72F on a solder tinned piece of copper and got this;

Only the .021" hole didn't release properly. I'll try a pieces of .010" and .007" brass next week. So far so good, didn't expect it to work so well with the worst case .016" brass. This has got to be better than using a syringe.

Those dots look pretty good, and so do the drilled holes. Much better than I would have expected. Since your sheet is so thick, you'll probably want to try using the smaller dots first and see how they reflow as not enough paste is usually better than too much. What type of flux is in your paste? RA, RMA, NC & WS flux will usually react with brass. Not a big deal though if you wash the stencils with the appropriate cleaner (depending on the flux type) after you're done.

Yes, it's definitely better than using a hand held syringe.

I'd be interested to see photos of your experiments with thinner sheets too...

Chris

Would polyimide work? It may be gentler on the drill bits and easier to handle as well. I had a rep visit me showing off laser cut polyimide masks for lower volume production. From memory they were 1/2 or 2/3 the cost of s/steel.

rob

I've never used polyimide stencils. My exposure to Kapton has been limited to flex circuits and tape. I might be nervous about drilled Kapton tearing easily. I also remember it being absurdly expensive. The laser needed for Kapton would be much cheaper than that needed for SS so some cost could be made up there.

Honestly the brass sheets looked quite good after being drilled, and brass is soft enough to be gentle on the bits. If he has a CNC mill already available, it seems like it would work out alright.

Chris

One of the assembly houses we use has tried polyimide stencils and says they can be torn easily by the squeegee.

Actually, it's just a CNC drill. Haven't gotten around to the mill yet. And yes, the drilled stencil worked well, goodbye to hand soldering and syringes. Some after "pasting" and after "baking" shots were added at the link below. Click on slideshow in the upper right hand corner.

Christ> I've never used polyimide stencils. My exposure to Kapton has been limited

The soldered parts look just dandy!

An aggressive flux (RA or RMA) might get the solder to flow more evenly on the QFP-32, but that's just being nitpicky. I'd ship any of those parts without concern.

Chris

oparr a écrit :

alright.

That's really impressive of a hobby outcome.

Just one remark: your IC2/MC33886 has a thermal pad, which is intended for heat spreading. As such the PCB pad underneath must *not* have thermal relief and depending on your board stack up, the better way is to also have a ground plane and stitch the thermal pad to it with a bunch of via.

This is all explained in the IC data sheet.

Freescale goes to great lengths in their datasheet in order to avoid using a conventional heatsink on the device IMO. The best way is to use an external heatsink in addition to lots of copper which my design calls for. Makes the board easier to maintain if the part should fail for whatever reason. Here are three homemade prototypes that survived all the abuse I threw at them;

Fred Bartoli wrote:

oparr a écrit :

Aha, OK. I missed that and just quickly skimmed through the data sheet. Nice job.