Metal stencil problems...

[alank2]

I'm having the problem of bridges on a TQFP 32 0.8mm pitch and also a USB mini connector.

This is my first time using a metal stencil and what I used before was the polymide ones from oshstencil.  The metal seems to want to bow or not lay totally flat - is this a normal problem for anyone?

[zapta]

Use a metal squeegee and press hard to make sure the stencil touches the board.

Also, the cuts in metal stencils are well defined and thus may pass more paste, and then the thickness may be different (did you use oshstencil 3mil or 5mil version).  Once you get consistent results you can adjust the pads sizes in your library.

[charlespax]

Make sure to support the part of your stencil that overhangs the PCB. You should put PCBS of the same thickness along the outer edges to get the perfect height.

[Kjelt]

yes the stencil should be supported on the edges of the board, by using scrap pcb or what I always do is use a large scrap piece of floorboard (laminate) and mill the size of the pcb 1,6mm deep out of the floorboard. Place the pcb in lay the stencil aligned on top and tape it to the floorboard. Works a charm.

[alank2]

I'm learning...  My problem with the metal stencil that was my pad sizes were too large (IPC most) and also that it was too thick (5.7mil) and I had way too much paste on the board.  I was also mistakenly squeezing paste over a single board multiple times instead of just doing it one time.  Next time I'm going to use the 0.12mm thick metal instead of 0.15mm thick and I've moved to IPC nominal pad sizes.

So, calculating the actual amount of paste being deposited, I found that a 0.12mm 0.0% aperture reduction is right around the same amount of paste as 0.15mm 8.0% aperture reduction.  Which would you rather use?  Thicker, but the on pad coverage doesn't fill the whole pad because of an 8% reduction all around the pad, or the thinner with full pad coverage?  Does it matter?

[Kjelt]

Squeezing more than once is definitly a no go. If it is not correct the first time you have to wipe it and clean with IPA and retry. But you already know that and know you know why 

If the soldermask is ok and the amount of paste is ok then it shouldn't matter, the reflow process will let a bit too much paste flow on the pins of the component, unless you really do have an excess of paste.
The problem is probably one of the following:
- the reflow (timing) process (for instance wetting phase is too long the paste can start too move on top of the flux)
- the soldermask is not good enough between the pads

but very tight pitch components can be a PITA 

[zapta]

I found that a 0.12mm 0.0% aperture reduction is right around the same amount of paste as 0.15mm 8.0% aperture reduction.  Which would you rather use?

I would go with thinner stencils and standard size holes. It makes it easier for the paste to penetrate small holes.

[zapta]

yes the stencil should be supported on the edges of the board, by using scrap pcb or what I always do is use a large scrap piece of floorboard (laminate) and mill the size of the pcb 1,6mm deep out of the floorboard. Place the pcb in lay the stencil aligned on top and tape it to the floorboard. Works a charm.

That's a neat idea if one has access to CNC machine. I am using custom size 3D printed brackets. Like this http://www.thingiverse.com/thing:530949

[alank2]

What type of floorboard/laminate?

[Kjelt]

Just what i had leftover from the floor will make pictures tomorrow if youwant and allthough i do have a cnc machine i route it manually its done faster then first make the drawing and program the machine 

[alank2]

I couldn't find any laminate samples large enough so I went with a vinyl tile for 88 cents.  This is a quarter of that tile (22 cents!)  Here is what I cut: