Solder paste stencils and jigs

[David_AVD]

It seems that there are a lot of suppliers for stencils out there, but what do people use as a jig when doing dozens of the same board?

Is there some (not too expensive) standard frame that small production people use?

A DIY approach is not what I'm looking for.  I would rather just buy something that can accommodate boards up to about 200mm x 150mm in size.

[8086]

I get people to donate old LCD panels to me, and tape a frame to it made from some spare PCBs of the same thickness as I'll be using.

Then place the stencil on top and off you go, works a charm.

Edit: Sorry, I misread. I thought you were after a DIY approach

[mikeselectricstuff]

I rcently bought the Eurocircuits EC-Stencil mate
It's pretty good, and the cheapest solution I could find that had proper tensioning of arbitary sized frameless stencils. Tooling requirement is minimal - 2 holes on one edge of the PCB and  2 on the stencil
However I think it's still pretty expensive for what is basically a frame with a few adjustments.

I think there is scope for someone to design a much cheaper DIY/kit stencil printer - I was thinking maybe based on thick FR4 PCB material for the structure, and some clever details for alignment and adjustment, so most of the parts could be made by your local friendly PCB manufacturer.

[Short Circuit]

This is a low cost (15 euro) fixture sold by www.eurocircuits.com :
Works quite nicely, I use it all the time.

[David_AVD]

Mike, that one you linked to seems like what we'd want.

As for the PCB holes, can you just use existing mounting holes in the corners, or do they have to be outside of the PCB area?

[Short Circuit]

Both eurocircuits use the same aligning system;
two 3mm holes on the edge of the panel (or on the edge of the PCB), spaced multiples of 10mm apart,
and two 5mm holes in the paste stencil, equal spacing and 30mm away from the PCB holes.
The holes need to be sizes  accurately, or the stencil o PCB can wiggle too much (probably less problem on the heavy stencilmate)

http://www.eurocircuits.com/index.php/ec-registration-system

[mikeselectricstuff]

The holes can be within the print area, but not for 0.8mm or thinner as the pegs will distort the stencil

[David_AVD]

Hmmmm.... I was hoping to utilise the print jig with existing boards that have a 3mm mounting hole in each corner.

The spacing between the existing mounting holes varies a lot between board types.  So this rules that system out then?

[poorchava]

I think you could have it made of aluminum in any machine workshop. Shouldn't be too expensive if you don't overcomplicate things.

[mikeselectricstuff]

Hmmmm.... I was hoping to utilise the print jig with existing boards that have a 3mm mounting hole in each corner.

The spacing between the existing mounting holes varies a lot between board types.  So this rules that system out then?

Look at the drawing on the EC website - existing 3mm holes may be useable if they aren't too far from the edge. You need the corresponding holes in the stencil at the right distance below the PCB holes, but the only constraints on X position are fitting in the printer and beingfar enough apart to minimise any rotational error.

It is possible to use this unit with untooled stencils but a bit fiddly - what you do is insert a sheet cut from a scrap stainless stencil under the PCB supports, so you can move it slightly to tweak alignment. Not ideal but doable.
 

[David_AVD]

I still haven't gotten around to buying a stencil printer.   

Anyway, I found this one from Stencils Unlimited.

It's cheaper, but the description suggests that it's more fiddly to use than the Euro Circuits one.

[Corporate666]

I buy my stencils from Stencils Unlimited... I've tried various methods for pasting (short of buying a manual stencil printer).  After lots of trial and error, what we do is...

I have a 2' x 2' sheet of polyethylene on a table.  I put the PCB to paste on the sheet, and I put a few junk PCB's around it, or you can just use scraps of FR-4, or even get a couple of L-shaped pieces cut for you (Stencils send me some with the stencils).  I tape those sheets down with wide 2 inch or 3 inch masking tape, so the PCB for pasting is securely held in the center.  Then I just put a strip of tape on top of the stencil and line it up and press the tape down on my PCB platform.

I've bought various squeegees for applying paste, and the best one we keep going back to is ironically the cheap free one that came with the stencil.  I past the board, flip up the stencil and put a new board in.  I can do 20 or 30 boards easily before the flipping up and down of the stencil causes a little bit of misalignment.  Then I just pull the tape off and re-position it.  We panelize our boards in panels of between 5 and 30 boards/panel depending on size of the PCB, and I can easily paste 500 or 1000 boards (30-100 panels) in an hour or two. 

[David_AVD]

Do you always wipe the squeegee in the same direction?  If so, I assume it's away from the taped end?

[Corporate666]

Do you always wipe the squeegee in the same direction?  If so, I assume it's away from the taped end?

Always in the same direction. I have the stencil sized so there is about 2-3" of extra space above the apertures and between the tape. I lay out a blob of paste and I use the front of the blade to spread it out.  I hold the blade at maybe a 45 (maybe even a bit lower) angle, and I apply quite a lot of downward pressure.  As I pull it forward, it creates a "sausage" of solder paste that rolls along and fills in the apertures.  I do it in a single pass... then put the blade aside with the sausage stuck to it, facing up of course.   I carefully lift the stencil and remove the board, hold it up, and drop in the next board and repeat.  Only problem is the "sausage" of solder paste gets longer/wider with each pass and eventually is longer than my blade - so I use a spare piece of FR4 to scrape it off the blade and back onto the top of the stencil and keep going.  I pull the tape off and clean everything with acetone and a toothbrush.

It doesn't take long to master the process, and only maybe 1 in 200 panels is off and needs re-done.  I've pasted literally tens of thousands of boards with the same "prototype" stencil I got about 6 years ago   I would like to get a nicer stencil holder like have been posted in this thread, but it seems like anything worthwhile starts at $1,500-2,000USD, and considering I ship about 10,000 boards a year and spend maybe 1 hour every 2 weeks pasting on average...it's just not high on my list of things to spend money on.  I also use a toaster oven for reflow, and out of tens of thousands of boards it's reflowed - never a problem.

[David_AVD]

I don't mind spending the money on a stencil printer if it means that any of my guys can use it with repeatable (good!) results.

BTW, I use a toaster over for reflow too.  Works a treat.  Is is one of the higher end models though, not a $49 special.

[janekm]

I don't mind spending the money on a stencil printer if it means that any of my guys can use it with repeatable (good!) results.

BTW, I use a toaster over for reflow too.  Works a treat.  Is is one of the higher end models though, not a $49 special.

I use a high accuracy stencil printer I picked up on a visit to China... It's similar to the simple stencil printers but built to a much higher spec (most importantly the vertical adjustment can be fixed with the two screws at the back to allow high repeatability from print to print).
I've used it to print 0.4mm pitch QFN parts with no problem, also sot363 and the like. 0402 is trivial.
I get the stencils made ready framed in China. The only difficulty is the transport cost as they are a little bulky.

I have tried the "tape it to the desk" method in the past but it's not repeatable for fine pitch parts and I was far too likely to get smearing or inadequate paste release when lifting the stencil from the board.