Two thoughts

1) I have heard of many people doing reflow at home who use loc= ating pins on a precisely drilled support block that positively locates the= pcb itself and the overlying stencil with matching holes when applying the= paste

2) It could be th= at your apertures in the stencil are not optimal. Smaller apertures may all= ow more precise results. IIRC gEDA PCB solder mask aperture shrinkage relat= ive to the pad shape can only be defined, if at all,=C2=A0 on a whole board= basis.=C2=A0

pcb-rnd tr= eats each pad as a padstack,=C2=A0 where layer apertures on each layer can = be defined individually,=C2=A0 or autogenerated, in the :padstackedit actio= n.

Identical pads, i. e.= in a QFN,=C2=A0 can use the same padstack prototype for all the pads,=C2= =A0 once defined. This gives you very granular control,=C2=A0 down to the i= ndividual padstack if required. Slots and arbitrary simple polygonal pad sh= apes are also supported in padstacks.

How are you generating your gerbers for the stencil?=C2=A0

Regards,=C2=A0

Erich

On Wed, 17 Aug 2022 21:23 Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com], <geda-user AT delorie DOT com> wrote:

Hello,

My apologies for this off-topic question, as this has not much to do
with gEDA - but I could do with some expert advice on PCB assembly for
reflow, and perhaps some people here have good ideas.

I've been working with SMD reflow technology for many years now, and very step of the process goes smooth except one: applying solder paste
for small-pitch components.

My main problem is that even after hundreds of PCB's, the result is unpredictable. Only 2 out of every 10 PCB's have a good crisp result, <= br> like this: http://www.linetec.nl/electro= nics/paste_crisp.jpg

The other PCB's often look like this:
http://www.linetec.nl/electronics/paste_m= ess.jpg

So applying paste is rather a hit-and-miss affair, and it sometimes
takes half a dozen attempts to get one PCB right.

This is my set-up and work procedure:

* I only use flash gold finished PCB's (the tinned ones have bumpy pads= ,
hugely increasing the error rate).
* For positioning of the PCB's, I use an L-shaped piece of PCB taped down with masking tape.
* The SMD stencil is stainless steel, 100 microns thick, taped into
position along the bottom edge and at a top corner.
* The wiper is also stainless steel, but of course rather thicker.
* I apply the paste slowly with the wiper at an angle of ~30=C2=B0 relative=
to the horizontal, with small sideways movements in addition to the main movement downwards.
* I use moderate force -- difficult to estimate, but I'd say between 50= 0
and 800 grams, so roughly 1 - 1.5 pounds.
* After each single paste application, I wipe the underside of the
stencil with an acetone cloth, to prevent any paste on the underside
getting squished, causing shorts etcetera.

Now admittedly, those 'shorted' pads often turn out OK due to the <= br> solder's high surface tension and tendency to collect at the metal part= s
of the components, but I still get rather a lot (ca. 1 in 5) of PCB's <= br> with nasty shorts, especially underneath those QFN housings.

There is of course also the problem of the solder paste degrading and
getting tougher and 'dryer' over time; however, that is not the pro= blem
here. If anything, fresh solder paste is more runny and thus tends to
short out between pads faster.

So the question is if someone has any tips to improve the process, or
maybe point out what I'm doing wrong. (Yes, of course I can have a PCB =
house do the assembly, but that typically costs > $1000 per run, and that is not always an option.)

Thanks in advance,

Richard

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