# Drilling solder paste stencils

O

Jan 1, 1970
0
These are usually laser cut ($$) but what's wrong with CNC drilling your own using the coordinates for the SMD pad centers? Paste doesn't have to cover every nick and cranny of the pad. It will require a few tool changes for the various pad sizes but since we're dealing with say 8 mils brass sheet, relatively cheap high speed steel bits can be used. Any other thoughts on this? B #### Boris Mohar Jan 1, 1970 0 These are usually laser cut ($$) but what's wrong with CNC drilling your own using the coordinates for the SMD pad centers? Paste doesn't have to cover every nick and cranny of the pad. It will require a few tool changes for the various pad sizes but since we're dealing with say 8 mils brass sheet, relatively cheap high speed steel bits can be used. Any other thoughts on this? 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) http://www.viatrack.ca void _-void-_ in the obvious place O #### oparr Jan 1, 1970 0 For large pads multiple holes could be tried. Even overlapping to some extent. Perhaps slightly thicker deposition could make up for the corners. Instead of an 8 mils sheet one could use 10 - 16 mils depending on the size of the smallest pads (smallest holes). I suspect at some point the paste would rather stick to the walls of the hole rather than the pad as stencil thickness increases. C #### Christopher Ott Jan 1, 1970 0 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. At145 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.

B

Jan 1, 1970
0
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. B #### [email protected] Jan 1, 1970 0 Its not about drill bits - have you tried drilling full hard stainless with a jobbing drill bit? Most SS for stencils has a hardness of 370 Vickers and top end stuff runs at over 400 Vickers (divide by 10 to get Rc). You wont get positional accuracy and the steel will distort (due to the rotation of the bit or cutter) and the stencil wont be flat. It would not be possible to do fine pitch. The mechanics of it are not possible and I doubt that i would be any cheaper as a CNC system would be no cheaper than a laser - new laser systems can be bought for around 150,000 USD now. Some one did try to sell a stencil "cutter" that was based on stamping technology. No idea if it worked and I dont know of any stencil operations that have one. Ben O #### oparr Jan 1, 1970 0 I would suggest using stainless steel sheets instead of brass. Either can be used with the paste I'm using. Brass isn't cheap The alloy I'm using is very cheap....12" X 8" sheet is less than$5.00.
Its not about drill bits - have you tried drilling full hard stainless
with a jobbing drill bit?

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.

C

#### Christopher Ott

Jan 1, 1970
0
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

B

#### Borat

Jan 1, 1970
0
Christopher Ott said:
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

C

#### Christopher Ott

Jan 1, 1970
0
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

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

M

#### Matthew Kendall

Jan 1, 1970
0
Borat said:
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.

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

O

#### oparr

Jan 1, 1970
0
If he has a CNC mill already available, it seems like it would work out alright.

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.

http://www.pbase.com/eldata/stencils&page=all

C

#### Christopher Ott

Jan 1, 1970
0
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

F

#### Fred Bartoli

Jan 1, 1970
0
oparr a écrit :
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.

http://www.pbase.com/eldata/stencils&page=all

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.

O

#### oparr

Jan 1, 1970
0
the better way is
to also have a ground plane and stitch the thermal pad to it with a
bunch of via.

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;

http://www.pbase.com/eldata/image/66582088

F

#### Fred Bartoli

Jan 1, 1970
0
oparr a écrit :
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;

http://www.pbase.com/eldata/image/66582088
Aha, OK. I missed that and just quickly skimmed through the data sheet.
Nice job.

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