Homebrew PCBs

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