Homebrew PCBs

Hi all. 

I am looking in to paste screens. I have found some 0.2mm ( 8mil ) thick aluminium sheet that I can work with on my milling machine. I was wondering if 0.2mm is too thick for a screen and if aluminium was any good as a material. It seems quite nice and certainly strong enough for prototype screens. Not sure about cleaning and life of the screen but got a feeling it isnt bad. 

I dont have any paste at the moment so I can  not do any tests myself. 

Thanks all

Trev

On 27/01/2010 11:30, Trevor wrote:
> Hi all.
>
> I am looking in to paste screens. I have found some 0.2mm ( 8mil ) thick aluminium sheet that I can work with on my milling machine. I was wondering if 0.2mm is too thick for a screen and if aluminium was any good as a material. It seems quite nice and certainly strong enough for prototype screens. Not sure about cleaning and life of the screen but got a feeling it isnt bad.
>
> I dont have any paste at the moment so I can  not do any tests myself.


Laser-cut Kapton stencils are quite cheap and will work a lot better:

http://www.ohararp.com/Stencils.html

Leon
-- 
G1HSM
http://webspace.webring.com/people/jl/leon_heller/

Hi Leon. 

I appreciate the reply but I am really wanting to know about the Aluminium I found. Think it might be a stronger, more durable material and I can actually get hold of it in the UK. 

I tried milling out a stencil from Mylar but the cuts seem a little rough. Might not be a problem for pasting but was not that happy with it. I can not find a source to buy Kapton from in the UK. I had some 2mil sheet but that is just too thin I think. I milled a stencil out and the Kapton did seem to give a nice cut. 

I suppose I am just wondering if 8mil thick Aluminium will end up delivering too much paste to the pads. 

Trev

On Wed, Jan 27, 2010 at 01:05:00PM +0000, Leon Heller wrote:
> Laser-cut Kapton stencils are quite cheap and will work a lot better:

Have you used their service? How long can I expect a Kapton stencil to last? I have a few 
(possibly few tens) of the same board to make and it'll save me a colossal amount of time 
if I can use a stencil (and at $25 for the stencil it'd be mad not to).

How about for 0.4mm pitch LQFP?

Cheers.

On 27/01/2010 13:38, Dylan Smith wrote:
> On Wed, Jan 27, 2010 at 01:05:00PM +0000, Leon Heller wrote:
>> Laser-cut Kapton stencils are quite cheap and will work a lot better:
>
> Have you used their service? How long can I expect a Kapton stencil to last? I have a few
> (possibly few tens) of the same board to make and it'll save me a colossal amount of time
> if I can use a stencil (and at $25 for the stencil it'd be mad not to).
>
> How about for 0.4mm pitch LQFP?

They are really intended for prototypes and short runs, I'd ask him 
about durability for your application and whether they are suitable for 
that chip. I haven't used them myself, but I've seen some favourable 
reports from people who have. One of my PCB suppliers (PCB Pool) 
provides a free stainless steel stencil with each order.

Leon

I've done kapton, brass sheets, and aluminum foil.

The kapton's nice if you need accuracy, but I think it'll be important
to ask for vector-cut stencils instead of raster-cut for fine pitch
parts.  That stencil lasted through eight boards, you just have to
carefully cleen it with something that cleans your solder paste.

brass sheets are hard to etch because of the thickness, even a 2 mil
sheet gets 2-3 mil undercutting which makes it very difficult to get
accurate stencils for fine pitch parts, or even 0603s.

The aluminum foil method[*] works great if you can align the two masks
properly.  A 1 mil foil gives a 4 mil stencil, but it's rather flimsy
and really only good for one, maybe two boards.

As for 8 mil alumimum, I think it would work but you'd be limited to
larger parts.  Since you're milling and can get straight sides, you'll
have to experiment and see how small a hole you can mill and still
have the paste stay on the pcb when you lift the stencil.  Also, you
can make the hole *smaller* than the pad to control the *volume* of
paste put on the board.

[*] Put photomask on both sides of household aluminum foil, expose and
develop.  Etches the foil in a few seconds, leave the photofilm ON.