piclist 1997\09\08\011638a >
Thread: SMT soldering
www.piclist.com/techref/pcbs.htm?key=smt
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face BY : tjaartEraseME@spam_OUTwasp.co.za



Andrew Russell Morris wrote:
>
> One question: If I end up having to solder parts onto both sides of the
> board, how do I keep the ones on the bottom from falling off? Are all SMT
> parts the same height, so that the oven tray will hold them in place until
> they cool off? Any suggestions are welcome.

Hey, those are two questions! ;)

The one side of the PCB is laid out to contain all the through-hole
components, and most of the SMD's. This side is soldered by a process
called
reflow. This is basically an oven with a conveyor belt running through
it (a
bit like some high volume Pizza ovens).

The rest of the SMD components (*NOT* through holes) are arranged (and
glued)
on the other side in rows so their leads are lined up. The board is held
with
this side facing down over a solder bath. A wave is created in the bath
and
this runs under the board soldering the components as it travels from
one
side to the other (See why you need glue?).

This is called wave soldering.

Never put though hole components on both sides of the board even if your
friendly manufacturer assures you it is possible.

Try to keep your component count as low a possible by using, for
instance,
resistor arrays in stead of discrete resistors. These arrays are more
expensive than the discretes, but the component cost is small in
relation
to the placement costs (per component).

Try to avoid using through hole components. The placement costs are high
and you pay per pin, where with SMD's you pay per component.

Use the smallest components your manufacturer can handle. The higher
cost
is paid for by the savings on PCB and enclosure costs. You'll see that
these last two often-overlooked aspects often consistute more than half
the total cost.

--
Friendly Regards

Tjaart van der Walt
tjaart.....spam@spam@wasp.co.za
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