Re: Drilling solder paste stencils

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 *** 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.

--
Chris



"oparr" <oparr@xxxxxxxxxxx> wrote in message
news:1159569449.073558.86450@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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 *** 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.

Boris Mohar wrote:
On 29 Sep 2006 12:15:08 -0700, "oparr" <oparr@xxxxxxxxxxx> wrote:




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