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

"Tim Shoppa" wrote in message
...

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

Thanks for the reply.

You are correct, a 1206 is a large SMD component and I will try to use them
wherever possible. However, smaller (0402 and larger) may also be used. It
is encouraging to hear about successful fine pitch results with hand solder
techniques. In addition, it will certainly be useful for parts placed on
both sides of the PCB.

On Jun 30, 4:33*pm, "numeric" wrote:
You are correct, a 1206 is a large SMD component and I will try to use them
wherever possible. However, smaller (0402 and larger) may also be used. It
is encouraging to hear about successful fine pitch results with hand solder
techniques. In addition, it will certainly be useful for parts placed on
both sides of the PCB.

For my purposes, 0603 is the "standard" size SMD part to use when I
have a choice. Of course power parts are gonna be bigger.

I mean, just so you don't scare yourself, look at the bottom of a
TO-92 transistor that you've been soldering for 30 years or more. The
pitch there 0.05 inches or 0.127 mm. And you do those all the time.
Your LQFP is finer pitch, but not ridiculously so. So don't set
yourself up with the mental attitude that it's gonna be hard, because
it isn't going to be hard!

I have occasionally used 0402 and even 0201 parts when they weren't
available in 0603... I struggled with 0201, and 0402 required a lot of
concentration. By comparison the 0.65mm pitch parts were a piece of
cake.

And finally, don't ignore the option of doing "dead bug" with SMT
parts. Very fun for low-lead-count analog parts... and possible though
questionable for digital stuff with lotsa leads.

Tim.