Homebrew PCBs

This is for Chris and the others that have yet to fiddle with SMD
components.
http://groups.yahoo.com/group/Homebrew_PCBs/files/Surface_Mount/

Files:
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PCB size SMD.JPG
PCB size non-SMD.JPG

These pics are to scale (to the best of my limitted graphic abilities).  The
through-hole circuit is 6" x 6" while the SMD circuit is 3.75" x 3.75".
Note that the through-hole circuit is single-sided and the SMD is
double-sided.  I used 1206 size SMD components, 0805 are even smaller but
also harder to manipulate if you have a shaky hand like me.  Your breath can
move 0805 components easily.

Both PCBs are different generations of the same circuit;  the through-hole
version uses a PIC 16F877 while the SMD version uses dual MCP23016 I/O
expanders.  This circuit receives a signal from a driver board and triggers
a keystroke on a personal computer.  That piggy-backed green circuit is
hacked from a HP USB keyboard.

I sprayed clear paint to protect the silkscreen on the through-hole circuit.
I had used Pulsar white transfer paper with mild results, the lettering
easily came off.  On the SMD circuit I sprayed engine block paint and had
masked all the pads with ordinary clear adhesive tape.  Then I applied the
silkscreen using Pulsar toner transfer paper with excellent results.  The
problems started when I used isopropyl alcohol to clean the rosin, but I'm
sure the paint would have been ok if I had cured it as the can suggested
(curing = patience = mission impossible).

I rely on a headband magnifier for SMD work, they are awesome once you get
used to them.  I have yet to master the fancy 'single stroke solder
technique', I soldered each lead of the SOIC components individually.  One
trick to hold components in place is to generously wipe rosin on the lands,
that stops parts from slipping around.  I tack solder one end of a
component, then I solder the other end, solder some more the 1st end then I
quickly dip the iron into both ends; the liquified solder and surface
tension automatically straightens out the component, that's too cool.  It
takes a bit of practice, but things go smoothly once you get this technique
down.

I'll work on that 'single stroke across the leads' solder technique some
other day.  The one time I tried it on 0.5mm pitch leads I got a bridge
across 2 leads and it was a royal pain to take it out.  I know SOIC are
spread further apart, but since I'm not doing massive production batches I
don't mind individual soldering the leads, for now.  But yeah, by the time I
had soldered the 13th MCT6 I was starting to seriously think about
practicing that trick.

Oh yeah, one last thing, SMD components are priced about the same as their
through-hole counter-parts.  In some cases they are even cheaper and more
readily available.  I expect DIP packages to become harder to get over time,
so I figured I might as well get used to them now.  One added bonus is the
lower hole count.  I had started making my vias with wire, but now I use the
leads on 1/4 watt resistors (or small capacitors, whatever falls under my
hand); they are thin, hold in your fingers easily and I've got tons of the
stuff.

Robert
:)





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