SURFACE MOUNT SOLDERING
makezine.com/16/primer
available from Madell or Zephyrtronics (
zeph.com),
but a $7 Mr. Coffee hot plate works for small, single-sided boards.
» You can reflow a board in a toaster oven. Look
for one that can heat up to 480°F (250°C) in less
than 5 minutes, which will let it reflow all the solder P
without baking the board. Since toaster ovens
don’t have their 0-to-480°F speed marked on the
outside of the box, I’d advise using a small one, or
a large one that’s more than 1,400 watts.
As an alternative,
sparkfun.com has tutorials
and blog entries that recommend using a skillet
instead of a toaster oven for boards that carry
both plastic and large metal connectors. The RD
downside of a skillet is that it only works with
1-sided boards.
Q
S
»
Stencilsunlimited.com sells temperature-indicating
markers that change color when a particular temperature is reached, to let you know when to stop
applying heat. You can also monitor temperature T
with a thermocouple.
U
» An acid brush, isopropyl alcohol, and lint-free
wipes clean up flux residues. I keep the alcohol in a
pump bottle that dispenses as needed and prevents
the rest from evaporating.
» A vacuum pickup tool can help place larger
components that tweezers can’t hold, although
fingers do a decent job, too.
Install a QFN (Quad Flat No-Lead)
The recommended method with these chips is to
use a stencil with solder paste, but you can also
get by with regular solder and hot air.
You needn’t apply solder to a chip’s bottom-side
heat sink, which is present on many motor amps
and voltage regulators, but if you do, it shouldn’t
exceed 0.01" in thickness.
Also, you’ll probably need to reflow it individually
with a direct shot of hot air or solder it through a
hole drilled underneath.
1. Flux and tin the bottom connections on the QFN
(Figure P).
2. Flux and tin just the outer pads (Figure Q).
SMD PROTOTYPING
Prototyping with SMDs is more difficult than
quickly plugging through-hole components into
a solderless breadboard, but SchmartBoard
(
schmartboard.com) carries breakout boards
that port any SMD to standard 0.1"-spaced
through-hole pins.
For prototyping, you still have to solder the
chip onto the breakout board and then remove
it later to install on the final board (unless you
just solder in the breakout board, which takes
up space). But the breakout boards are perfect
if you have a limited number of SMDs that you
need to interface with through-hole components, and you aren’t making your own PCB.
In my experience, it’s faster to skip the
breadboarding stage and go straight to a PCB
prototype of the whole circuit. You can fix mistakes by scraping traces and jumpering with
small, 30-gauge “green” wires. I’ve found that
drawing schematics on a computer is more
reliable than dealing with a million breadboard
wires, although it’s less immediate.
138 Make: Volume
16
