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References:	<66c88bcb-820c-9a1d-1698-d0b36f32e3f3 AT linetec DOT nl>
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From:	"Erich Heinzle (a1039181 AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Date:	Wed, 17 Aug 2022 22:29:08 +0930
Message-ID:	<CAHUm0tOM07EoJjh0UC5_e8mKp_XZcM+Mh0ixHHzdp35emvX9yA@mail.gmail.com>
Subject:	Re: [geda-user] [OT] Solder paste woes
To:	geda-user <geda-user AT delorie DOT com>
Reply-To:	geda-user AT delorie DOT com

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Two thoughts

1) I have heard of many people doing reflow at home who use locating 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 that your apertures in the stencil are not optimal. Smaller
apertures may allow more precise results. IIRC gEDA PCB solder mask
aperture shrinkage relative to the pad shape can only be defined, if at
all,  on a whole board basis.

pcb-rnd treats each pad as a padstack,  where layer apertures on each layer
can be defined individually,  or autogenerated, in the :padstackedit action=
.

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

How are you generating your gerbers for the stencil?


Regards,

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,
> like this: http://www.linetec.nl/electronics/paste_crisp.jpg
>
> The other PCB's often look like this:
> http://www.linetec.nl/electronics/paste_mess.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 relati=
ve
> 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 500
> 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
> solder's high surface tension and tendency to collect at the metal parts
> of the components, but I still get rather a lot (ca. 1 in 5) of PCB's
> 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 problem
> 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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<div dir=3D"auto"><div>Two thoughts</div><div dir=3D"auto"><br></div><div d=
ir=3D"auto">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</div><div dir=3D"auto"><br></div><div dir=3D"auto">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</div><div dir=3D"auto"><br></div><div dir=3D"auto">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.</div><div dir=3D"auto"><br></div><div dir=3D"auto">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.</div><div dir=3D"auto"><br></div><div=
 dir=3D"auto">How are you generating your gerbers for the stencil?=C2=A0</d=
iv><div dir=3D"auto"><br></div><div dir=3D"auto"><br></div><div dir=3D"auto=
">Regards,=C2=A0<br><div dir=3D"auto"><br></div><div dir=3D"auto">Erich</di=
v><br><br><div class=3D"gmail_quote" dir=3D"auto"><div dir=3D"ltr" class=3D=
"gmail_attr">On Wed, 17 Aug 2022 21:23 Richard Rasker (<a href=3D"mailto:ra=
sker AT linetec DOT nl">rasker AT linetec DOT nl</a>) [via <a href=3D"mailto:geda-user AT de=
lorie.com">geda-user AT delorie DOT com</a>], &lt;<a href=3D"mailto:geda-user AT delo=
rie.com">geda-user AT delorie DOT com</a>&gt; wrote:<br></div><blockquote class=3D=
"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding=
-left:1ex">Hello,<br>
<br>
My apologies for this off-topic question, as this has not much to do <br>
with gEDA - but I could do with some expert advice on PCB assembly for <br>
reflow, and perhaps some people here have good ideas.<br>
<br>
I&#39;ve been working with SMD reflow technology for many years now, and <b=
r>
very step of the process goes smooth except one: applying solder paste <br>
for small-pitch components.<br>
<br>
My main problem is that even after hundreds of PCB&#39;s, the result is <br=
>
unpredictable. Only 2 out of every 10 PCB&#39;s have a good crisp result, <=
br>
like this: <a href=3D"http://www.linetec.nl/electronics/paste_crisp.jpg" re=
l=3D"noreferrer noreferrer" target=3D"_blank">http://www.linetec.nl/electro=
nics/paste_crisp.jpg</a><br>
<br>
The other PCB&#39;s often look like this: <br>
<a href=3D"http://www.linetec.nl/electronics/paste_mess.jpg" rel=3D"norefer=
rer noreferrer" target=3D"_blank">http://www.linetec.nl/electronics/paste_m=
ess.jpg</a><br>
<br>
So applying paste is rather a hit-and-miss affair, and it sometimes <br>
takes half a dozen attempts to get one PCB right.<br>
<br>
This is my set-up and work procedure:<br>
<br>
* I only use flash gold finished PCB&#39;s (the tinned ones have bumpy pads=
, <br>
hugely increasing the error rate).<br>
* For positioning of the PCB&#39;s, I use an L-shaped piece of PCB taped <b=
r>
down with masking tape.<br>
* The SMD stencil is stainless steel, 100 microns thick, taped into <br>
position along the bottom edge and at a top corner.<br>
* The wiper is also stainless steel, but of course rather thicker.<br>
* I apply the paste slowly with the wiper at an angle of ~30=C2=B0 relative=
 <br>
to the horizontal, with small sideways movements in addition to the main <b=
r>
movement downwards.<br>
* I use moderate force -- difficult to estimate, but I&#39;d say between 50=
0 <br>
and 800 grams, so roughly 1 - 1.5 pounds.<br>
* After each single paste application, I wipe the underside of the <br>
stencil with an acetone cloth, to prevent any paste on the underside <br>
getting squished, causing shorts etcetera.<br>
<br>
Now admittedly, those &#39;shorted&#39; pads often turn out OK due to the <=
br>
solder&#39;s high surface tension and tendency to collect at the metal part=
s <br>
of the components, but I still get rather a lot (ca. 1 in 5) of PCB&#39;s <=
br>
with nasty shorts, especially underneath those QFN housings.<br>
<br>
There is of course also the problem of the solder paste degrading and <br>
getting tougher and &#39;dryer&#39; over time; however, that is not the pro=
blem <br>
here. If anything, fresh solder paste is more runny and thus tends to <br>
short out between pads faster.<br>
<br>
So the question is if someone has any tips to improve the process, or <br>
maybe point out what I&#39;m doing wrong. (Yes, of course I can have a PCB =
<br>
house do the assembly, but that typically costs &gt; $1000 per run, and <br=
>
that is not always an option.)<br>
<br>
Thanks in advance,<br>
<br>
Richard<br>
<br>
</blockquote></div></div></div>

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