Homebrew PCBs

Hi group

I wonder if members of this group do their own stencils to apply solder 
paste for subsequent reflow? Any lowcost methods? I mean not everbody 
owns a laser cutter (or is having easy acess to one).

TIA

Markus

See http://www.picbasic.org/forum/showthread.php?t=8744

Norm

----- Original Message ----- 

From: "Markus Zingg" <homebrew-pcb@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Monday, January 07, 2008 7:16 AM
Subject: [Homebrew_PCBs] Homebrew stencils anyone?


> Hi group
>
> I wonder if members of this group do their own stencils to apply solder
> paste for subsequent reflow? Any lowcost methods? I mean not everbody
> owns a laser cutter (or is having easy acess to one).
>
> TIA
>
> Markus
>
>
>
> Be sure to visit the group home and check for new Links, Files, and 
> Photos:
> http://groups.yahoo.com/group/Homebrew_PCBs
> Yahoo! Groups Links
>
>
>
>

Hi Markus,

Tymm has made an Instructible on this at
http://www.instructables.com/id/Making-stencils-for-solder-paste-at-home/

Personally, I have used a brass stencil & rubber squeegee to put solderpaste
onto panels of boards, in my garage. I then reflowed in a toaster oven.
Someone else made the brass stencil for me, though.

By the way, I also made built a home made computer controlled C02 laser
cutter in my garage too. I just used it for cutting shapes in sheet acrylic,
but I often thought about trying to make Solderpaste stencils with it too...

Kind regards,
Anthony Burch http://www.SuperSolderingSecrets.com

-----Original Message-----
From: Homebrew_PCBs@yahoogroups.com [mailto:Homebrew_PCBs@yahoogroups.com]
On Behalf Of Markus Zingg
Sent: Tuesday, 8 January 2008 12:16 AM
To: Homebrew_PCBs@yahoogroups.com
Subject: [Homebrew_PCBs] Homebrew stencils anyone?

Hi group

I wonder if members of this group do their own stencils to apply solder 
paste for subsequent reflow? Any lowcost methods? I mean not everbody 
owns a laser cutter (or is having easy acess to one).

TIA

Markus

http://www.delorie.com/pcb/brass/

Yep, that's the kind I was after. I admitt though that it would be very 
cool to own one of these laser cutters mentioned by the other poster. I 
called the rep here but the price tag they have is as expected quite 
impressive. At least I know what to whish from Santa next year :-)

Were you using dryfilm resist laminate? What's the thikness of the brass 
sheet? I heard the stencil should be 3mil for very fine pitch stuff and 
4mil less fine pitch material. From looking at your pictures I got the 
impresson yours is much thiker. Does this not lead to too much 
solderpaste on the pads?

Markus

DJ Delorie schrieb:

>
>
> http://www.delorie.com/pcb/brass/ <http://www.delorie.com/pcb/brass/>
>
>

Markus Zingg <homebrew-pcb@...> writes:
> Were you using dryfilm resist laminate?

No, toner transfer.  Packing tape on the back side.

> What's the thickness of the brass sheet?

It varied from 2 to 5 mil.  I've been favoring the thinner stuff.

> Does this not lead to too much solderpaste on the pads?

Yes, thicker means too much paste.  A 5 mil thick sheet vs a 8 mil
wide opening is a big chunk of paste.

The other issue is thickness of the brass vs the space between pins.
The undercutting is significant in this case; the holes end up being
wider on one side than on the other; sometimes the undercutting can
connect between openings.  In general, an N mil thick sheet will be N
mils wider on the "near" (toner, photomask) side.

Hmmm,

I intend to use dryfilm laminate on both sides. The idea is to etch from 
both sides which I think might help keeping the undercuts at about half 
your rate. Just guessing though, and again, I definately will try this soon.

Markus

DJ Delorie schrieb:

>
>
> Markus Zingg <homebrew-pcb@... 
> <mailto:homebrew-pcb%40shdesign.info>> writes:
> > Were you using dryfilm resist laminate?
>
> No, toner transfer. Packing tape on the back side.
>
> > What's the thickness of the brass sheet?
>
> It varied from 2 to 5 mil. I've been favoring the thinner stuff.
>
> > Does this not lead to too much solderpaste on the pads?
>
> Yes, thicker means too much paste. A 5 mil thick sheet vs a 8 mil
> wide opening is a big chunk of paste.
>
> The other issue is thickness of the brass vs the space between pins.
> The undercutting is significant in this case; the holes end up being
> wider on one side than on the other; sometimes the undercutting can
> connect between openings. In general, an N mil thick sheet will be N
> mils wider on the "near" (toner, photomask) side.
>
>

Markus Zingg <homebrew-pcb@...> writes:
> I intend to use dryfilm laminate on both sides. The idea is to etch
> from both sides which I think might help keeping the undercuts at
> about half your rate. Just guessing though, and again, I definately
> will try this soon.

I thought of that.  In my case, I couldn't align the toner sheets
accurately enough for that to work with the hole sizes I needed (0.5mm
pitch TQFP).  I'm guessing with photoresist that's not a problem.

Working from one side also gives you a nice trapezoidal hole shape,
which helps release the paste.  Big side down, of course ;-)

Yep, that could be an advantage of the "one side" method.

As far as the aligning is concerned, I'm mostly doing 4-layers here, 
hence aligning layers is something I have to have under control anyways. 
I do this using diagonally placed shafts, printing on clear films and 
using a home made puncher to punch out the shaft holes in the film using 
magnifier googles. The printed black rings I use to position the hole 
are just about 1mil bigger than the physical hole in the puncher. I thus 
simply align the hole so as a very tiny ring of the punchers metal can 
be seen through the magnifier googles. This gives a very high precision 
in alignement.

I have no personal experience with the toner transfer method but since 
it relies on lasers I'm not sure if one can achive a reproduceable match 
on multiple printouts as it would be requiered for the purpose. Don't 
get this wrong, it very well might be that todays lasers can do that. I 
just decided on the traditional way long time ago and never had a reason 
to switch.

I'm looking foreward on trying this out anyways and if you are 
interested will report my results back here.

Markus

DJ Delorie schrieb:

>
>
> Markus Zingg <homebrew-pcb@... 
> <mailto:homebrew-pcb%40shdesign.info>> writes:
> > I intend to use dryfilm laminate on both sides. The idea is to etch
> > from both sides which I think might help keeping the undercuts at
> > about half your rate. Just guessing though, and again, I definately
> > will try this soon.
>
> I thought of that. In my case, I couldn't align the toner sheets
> accurately enough for that to work with the hole sizes I needed (0.5mm
> pitch TQFP). I'm guessing with photoresist that's not a problem.
>
> Working from one side also gives you a nice trapezoidal hole shape,
> which helps release the paste. Big side down, of course ;-)
>
>

Markus Zingg <homebrew-pcb@...> writes:
> As far as the aligning is concerned, I'm mostly doing 4-layers here, 

Woot!  Another one!  http://www.delorie.com/electronics/usb-gpio/

How are you managing vias and stackup details?

> The printed black rings I use to position the hole are just about
> 1mil bigger than the physical hole in the puncher.

What I've been doing is adding a registration target, and using an awl
to mark a dimple for a 13 mil drill in the PCB stock.  I use that to
line up the two sides.  For four layer boards, the outer layers are
see-through anyway, so I just line them up visually when I tape them
on.  Or, I drill all the 13 mil registration points and use spare
drill bits or wire to line them up.

> I have no personal experience with the toner transfer method but
> since it relies on lasers I'm not sure if one can achive a
> reproduceable match on multiple printouts as it would be requiered
> for the purpose.

The size is not 100% scale, but it is very predictable.  I've added a
correction factor in my output specs, and can usually get the two
layers lined up within a few mil of each other.

I'm guessing that photomask alignment would be easier; just draw
registration marks outside the pcb area and line them up visually.  At
least, if you're exposing both sides at once.

I saw your post on this lately and found it quite intersting. Here, I'm 
using a fairly "industrial" aproach in that I use an almost identical 
process than they use.

Basically I first cut out the base material to form a stack. In case of 
a four layer, the inner two layers consist of a double side claded base 
material of one third of the usual board thikness. The outer layers are 
similar in thikness, but only cladded on one side. I decided a while ago 
to always work on a "mini pannel" and dump execss material at the end of 
the process by cutting it off. This has helped a lot in serveral areas 
to simplify the process. I "support" here two pannel sizes, but could do 
other if the need really arises. The "norming" just helps to keep things 
consistant, fast and simple. The most common "pannel" size I use is 
160x100mm (euro card size), then there is a 180x100 variant in case the 
final product does not have the mounting holes where I usually place 
them (at 5/5mm and 155/95 mm, 3mm diameter). These holes are the 
alignement holes for the shafts and in many cases at least the first one 
is also at the same time a later on mounting hole for the PCB. I drill 
the first hole through the whole stack, put a shaft in and then drill 
the second hole. I meanwhile have a CNC drill, but I did this also back 
when I did not had one (in other words the method also works with hand 
drilling).

Once the stack is defined this way, I disasemble it, make the films for 
the inner layers, laminate, expose develop and etch the inner layer. I 
then glue the outer layers using five minute epoxy and a home made press 
(which basically is a very simple wood construction with two vices to 
generate the pressure). Needless to say that for the glueing process, I 
insert the shafts to keep the layers aligned. One of the pressure 
applying plates is haveing holes for the shafts for this purpose too, so 
as the shafts do not get bent from the pressure and remain in a right 
angle compared to the PCB.

After glueing (the nice thing compared to the industrial process is that 
it's a lot faster, just 5 minutes), I drill the holes through the 
complete stack. The CNC is also having positioning holes on a 
permanently mounting plate, so this process is easy, fast, precise and 
reporduceable. I simply stick the stack onto the shafts of the CNC 
drill. No further mounting or positioning requiered. Without CNC I used 
to laminate, expose and develop (but not etch) a pattern on one outer 
side to serve as a drill gudance, then stripped it away after drilling.

The next step then is through plating the stack in my home made thourgh 
plating machine. I used to present it here some years ago, and to 
describe the hole process is a bit long for a single post. I meanwhile 
have overworked the design of this machine to reflect the experiance 
made and will document this in the near future (no time yet). Anywas, I 
may should say that throughtout all those years I had to replace the 
chemistry (which is in case of the palladium bath somewhat expensive, 
but not THAT expensive) only ONCE. But maintaining the baths in a helthy 
state is important. If you master this, making four layers is just plain 
and simply said dirt cheap and fast (4 - 6 hours depending on your 
experience and equipement).

After the throughplating, the outer layers are laminated, exposed, and 
etched. In simple cases this completes the process. In cases where the 
PCB has to be seen by external people, I also apply a chemical thin 
imersion step and then laminate, expose, develop and harden a solder 
stop mask I have available as a laminate. I also do this in cases where 
I have to reflow solder components due to their nature (BGA etc.).

Ok, that was a bit short, but I hope you get the picture.

Markus

DJ Delorie schrieb:

>
>
> Markus Zingg <homebrew-pcb@... 
> <mailto:homebrew-pcb%40shdesign.info>> writes:
> > As far as the aligning is concerned, I'm mostly doing 4-layers here,
>
> Woot! Another one! http://www.delorie.com/electronics/usb-gpio/ 
> <http://www.delorie.com/electronics/usb-gpio/>
>
> How are you managing vias and stackup details?
>
> > The printed black rings I use to position the hole are just about
> > 1mil bigger than the physical hole in the puncher.
>
> What I've been doing is adding a registration target, and using an awl
> to mark a dimple for a 13 mil drill in the PCB stock. I use that to
> line up the two sides. For four layer boards, the outer layers are
> see-through anyway, so I just line them up visually when I tape them
> on. Or, I drill all the 13 mil registration points and use spare
> drill bits or wire to line them up.
>
> > I have no personal experience with the toner transfer method but
> > since it relies on lasers I'm not sure if one can achive a
> > reproduceable match on multiple printouts as it would be requiered
> > for the purpose.
>
> The size is not 100% scale, but it is very predictable. I've added a
> correction factor in my output specs, and can usually get the two
> layers lined up within a few mil of each other.
>
> I'm guessing that photomask alignment would be easier; just draw
> registration marks outside the pcb area and line them up visually. At
> least, if you're exposing both sides at once.
>
>

Thanks for the details!