On Jun 30, 1:12*pm, "numeric" wrote:
Hi,

I would like to build a PCB board with fine pitch (LQFP32) and very small
surface mount parts (1206 and smaller) parts. The board is a simple 24 bit
A/D and D/A converter with a full speed (12 Mbs) USB2 interface; although I
would prefer the USB2 high speed interface (480 Mbs). The temptation is too
hard to resist using today's microprocessors such as SiLabs C8051F350
(basically an 8051 cpu). I know that a commercial 4 layer PCB with a solder
mask would probably work for homebrew construction; but the board cost is
high.

So what is the chance of a double sided PCB without solder mask working with
very small parts? The process I would use in either case would be:

1. Place solder paste on all pads. Typically, when heated the solder will
vacate between pins and will suck up, like a magnet, under the SMD pin.

2. Place glue under parts that will possible move when the board is handled.

3. Heat the PCB in a toaster oven until soldered. Manually follow the heat
up and cool down time curves for type of solder paste used.

Comments/suggestions are appreciated.

LQFP32 is not ridiculously fine pitch (I do 0.65mm pitch parts by hand
all the time), and 1206 is just enormous in the SMT world.

I do both by hand all the time - just a Weller WTCPT with 0.015"
solder. No solder mask for me (I use ExpressPCB cheapo boards) but
pretinning does help. Sometimes flux comes in handy but is not really
necessary. Just lay the solder down across the leads, melt on with the
soldering iron, go back and clean up any blobs with solder-wick.

Toaster oven is great for BGA's but if there's only a few leaded SMT
parts it seems like way overkill.

Tim N3QE