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From:	"John Luciani (jluciani AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Date:	Wed, 17 Aug 2022 11:30:58 -0400
Message-ID:	<CA+qhd=-L2b8t7XnsU-GTfvODujrEyXpU0ACH=HXeFCWqL6XBjQ@mail.gmail.com>
Subject:	Re: [geda-user] [OT] Solder paste woes
To:	geda-user AT delorie DOT com
Reply-To:	geda-user AT delorie DOT com

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I do the following -

  * ENIG finish
  * 4mil ss stencil with trapezoidal apertures
  * 0.062 FR4 or aluminum scrap for a squeegee
  * Chipquick (T4 particle size) or Syntech solder paste
  * 0.128" alignment holes in the stencil and pcb.
     I place these outside the board area.
     I use 1/8" ss dowels for alignment.
  * stencil footprints for all the small form-factor components.
    These footprints have slightly smaller pads. For thermal
    pads the coverage is between 40 - 60%

Also, when screening the boards I do not keep the solder paste open
for too long. Typically 30minutes but no more than 60 minutes.

John L


On Wed, Aug 17, 2022 at 9:36 AM Erich Heinzle (a1039181 AT gmail DOT com) [via
geda-user AT delorie DOT com] <geda-user AT delorie DOT com> wrote:

> 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 itse=
lf
> and the overlying stencil with matching holes when applying the paste
>
> 2) It could be that your apertures in the stencil are not optimal. Smalle=
r
> 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 :padstackedi=
t
> 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 relat=
ive
>> 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"ltr"><div>I do the following - <br></div><div><br></div><div>=
=C2=A0 * ENIG finish</div><div>=C2=A0 * 4mil ss stencil with trapezoidal ap=
ertures<br></div><div>=C2=A0 * 0.062 FR4 or aluminum scrap for a squeegee</=
div><div>=C2=A0 * Chipquick (T4 particle size) or Syntech solder paste</div=
><div>=C2=A0 * 0.128&quot; alignment holes in the stencil and pcb. <br></di=
v><div>=C2=A0=C2=A0=C2=A0=C2=A0 I place these outside the board area.</div>=
<div>=C2=A0=C2=A0=C2=A0=C2=A0 I use 1/8&quot; ss dowels for alignment.<br><=
/div><div></div><div>=C2=A0 * stencil footprints for all the small form-fac=
tor components.</div><div>=C2=A0=C2=A0=C2=A0 These footprints have slightly=
 smaller pads. For thermal</div><div>=C2=A0=C2=A0=C2=A0 pads the coverage i=
s between 40 - 60%</div><div><br></div><div>Also, when screening the boards=
 I do not keep the solder paste open</div><div>for too long. Typically 30mi=
nutes but no more than 60 minutes. <br></div><div><br></div><div>John L<br>=
</div><div><br></div></div><br><div class=3D"gmail_quote"><div dir=3D"ltr" =
class=3D"gmail_attr">On Wed, Aug 17, 2022 at 9:36 AM Erich Heinzle (<a href=
=3D"mailto:a1039181 AT gmail DOT com">a1039181 AT gmail DOT com</a>) [via <a href=3D"mail=
to:geda-user AT delorie DOT com">geda-user AT delorie DOT com</a>] &lt;<a href=3D"mailto:=
geda-user AT delorie DOT com">geda-user AT delorie DOT com</a>&gt; wrote:<br></div><block=
quote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1=
px solid rgb(204,204,204);padding-left:1ex"><div dir=3D"auto"><div>Two thou=
ghts</div><div dir=3D"auto"><br></div><div dir=3D"auto">1) I have heard of =
many people doing reflow at home who use locating pins on a precisely drill=
ed support block that positively locates the pcb itself and the overlying s=
tencil with matching holes when applying the paste</div><div dir=3D"auto"><=
br></div><div dir=3D"auto">2) It could be that your apertures in the stenci=
l are not optimal. Smaller apertures may allow more precise results. IIRC g=
EDA PCB solder mask aperture shrinkage relative to the pad shape can only b=
e 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 treats each pad as a padstack,=C2=
=A0 where layer apertures on each layer can be defined individually,=C2=A0 =
or autogenerated, in the :padstackedit action.</div><div dir=3D"auto"><br><=
/div><div dir=3D"auto">Identical pads, i. e. in a QFN,=C2=A0 can use the sa=
me padstack prototype for all the pads,=C2=A0 once defined. This gives you =
very granular control,=C2=A0 down to the individual padstack if required. S=
lots and arbitrary simple polygonal pad shapes are also supported in padsta=
cks.</div><div dir=3D"auto"><br></div><div dir=3D"auto">How are you generat=
ing your gerbers for the stencil?=C2=A0</div><div dir=3D"auto"><br></div><d=
iv dir=3D"auto"><br></div><div dir=3D"auto">Regards,=C2=A0<br><div dir=3D"a=
uto"><br></div><div dir=3D"auto">Erich</div><br><br><div class=3D"gmail_quo=
te" dir=3D"auto"><div dir=3D"ltr" class=3D"gmail_attr">On Wed, 17 Aug 2022 =
21:23 Richard Rasker (<a href=3D"mailto:rasker AT linetec DOT nl" target=3D"_blank=
">rasker AT linetec DOT nl</a>) [via <a href=3D"mailto:geda-user AT delorie DOT com" targ=
et=3D"_blank">geda-user AT delorie DOT com</a>], &lt;<a href=3D"mailto:geda-user AT d=
elorie.com" target=3D"_blank">geda-user AT delorie DOT com</a>&gt; wrote:<br></div=
><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border=
-left:1px solid rgb(204,204,204);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>
</blockquote></div>

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