Homebrew PCBs

The biggest problem, IMHO, with doing surface-mounts by hand is aligning 
the pins with their corresponding pads.  The Schmartboard line of 
prototyping boards solves this problem by putting pads between ridges. 
With an arrangement like that, you can't help but have the pins rest right 
on their pads.  Driving home tonight, I thought how wonderful it could be 
if one could put similar ridges onto homemade (or pro-made) PCBs.  I came 
up with two approaches:

1) Spread a photoresist over the board about .2mm thick.  Mask as 
appropriate to harden the resist where you want Schmartboard ridges.

2) This one came later after more thinking.  Come up with a liquid resin 
that is repelled by metal and attracted to photoresist, bare epoxy, and 
bare phenolic.  Spray it over the whole board.  Resin will bead up between 
pads creating ridges for surface-mount pins to snuggle against.

When I came up with #2, I quickly discarded #1 as too convoluted.  Can I 
get your ideas on this?  Would it work?  Would you use it?  Who would we 
talk to about mixing some of this stuff up?

-- 
David Griffith
dgriffi@...

A: Because it fouls the order in which people normally read text.
Q: Why is top-posting such a bad thing?
A: Top-posting.
Q: What is the most annoying thing in e-mail?

--- In Homebrew_PCBs@yahoogroups.com, David Griffith <dgriffi@...> wrote:
>
> 
> The biggest problem, IMHO, with doing surface-mounts by hand is aligning 
> the pins with their corresponding pads.  The Schmartboard line of 
> prototyping boards solves this problem by putting pads between ridges. 
> With an arrangement like that, you can't help but have the pins rest right 
> on their pads.  Driving home tonight, I thought how wonderful it could be 
> if one could put similar ridges onto homemade (or pro-made) PCBs.  I came 
> up with two approaches:
> 
> 1) Spread a photoresist over the board about .2mm thick.  Mask as 
> appropriate to harden the resist where you want Schmartboard ridges.
> 
> 2) This one came later after more thinking.  Come up with a liquid resin 
> that is repelled by metal and attracted to photoresist, bare epoxy, and 
> bare phenolic.  Spray it over the whole board.  Resin will bead up between 
> pads creating ridges for surface-mount pins to snuggle against.
> 
> When I came up with #2, I quickly discarded #1 as too convoluted.  Can I 
> get your ideas on this?  Would it work?  Would you use it?  Who would we 
> talk to about mixing some of this stuff up?
> 
> -- 
> David Griffith
> dgriffi@...
> 
> A: Because it fouls the order in which people normally read text.
> Q: Why is top-posting such a bad thing?
> A: Top-posting.
> Q: What is the most annoying thing in e-mail?
>

no we can use this method "Spread a photoresist over the board about .2mm thick.  Mask as 
> appropriate to harden the resist where you want Schmartboard ridges."

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

From: "David Griffith" <dgriffi@...>
To: <Homebrew_PCBs@yahoogroups.com>
Sent: Wednesday, June 03, 2009 5:32 AM
Subject: [Homebrew_PCBs] Homemade Schmartboard ridges


> 
> The biggest problem, IMHO, with doing surface-mounts by hand is aligning 
> the pins with their corresponding pads. 

I use a microscope, it's not very difficult.

Leon

I've played with a schmartboard, and do fine-pitch smt by hand, and I
don't see it as much of a benefit.  Heck, if you're doing reflow, the
parts mostly align themselves anyway.

As for a film, use soldermask - anything thicker than your copper will
do.  Two layers if needed.

At 12:32 AM 6/3/2009, you wrote:

>The biggest problem, IMHO, with doing surface-mounts by hand is aligning
>the pins with their corresponding pads.  The Schmartboard line of
>prototyping boards solves this problem by putting pads between ridges.
>With an arrangement like that, you can't help but have the pins rest right
>on their pads.  Driving home tonight, I thought how wonderful it could be
>if one could put similar ridges onto homemade (or pro-made) PCBs.  I came
>up with two approaches:

I've been making a lot of boards with qfn parts,  which I find very 
hard to place by hand. What I've been doing is taking a piece of thin 
(20 mil) fr4 and milling out openings for the smt parts. I allow 10 
mil extra space in each direction for the qfn parts to "float" into 
place. This "placement stencil" drops over the board positioned by 
using screws through some of the existing holes (or vias) in the board.

Before mounting I apply solder paste via a simple stencil that I make 
just by drilling appropriate size holes in 9 mil fr4.

Then I make a sandwich consisting (from the bottom) of:

1 - A mounting board with screws in the right places - I usually use 
2 screws, on opposite ends of the board. This is oversize, and while 
not really necessary, it makes things more convenient to handle since 
many of my boards are <1 inch square.
2 - The pcb it drops on top of the mount board, sliding over the screws.
3 - The stencil drops on top of the pcb, using the same positioning screws.

I the simply drop the parts into the correct openings and pop it into 
my temperature controlled toaster oven. My current board has 3 qfn 
parts - 32, 10 and 8 pin - and is .9 x .9 inches in size. Some boards 
will have one or two small solder bridges across adjacent pins, but 
they are quickly and easily removed with solder braid. Works great for me.

Mark