IRONS, HOT AIR, AND TOASTER OVENS
We’ll look at 3 methods of SMD soldering. The easiest components have feet or other accessible contacts
that lay flat on the board’s pads. These you can connect with a soldering iron. A quick touch of the tip, and a
bit of solder will naturally flow under the foot and make the connection. This is the magic of SMD soldering —
capillary action does most of the work for you.
Other SMD packages have their contacts on the underside, out of reach. You can solder these in 2 ways:
individually, using solder or solder paste and a jet of hot air, or en masse by positioning all components on
the board with solder paste between each contact and its pad, and then heating the board on a skillet or in
a toaster oven to “reflow” the board (melt the paste) and make all the connections.
BASIC SMD SOLDERING
Each method has its own tools and supplies. Here
are the ones you’ll need for iron-soldering the
simplest SMDs: resistors, capacitors, and IC
(integrated circuit) packages with leads.
» Soldering stations include a sponge, but a dry tip
cleaner lets you clean a soldering tip without lowering
its temperature.
» Fine-tipped industrial tweezers let you pick
up and align small components. Also helpful are
hemostats, dental picks (for fixing bent leads),
and an X-Acto knife.
» Soldering tip selection is a matter of personal
preference. I prefer a small e" (0.8mm) chisel or
screwdriver tip because it can hold a bit of solder
at its end. I don’t recommend tips smaller than
0.6mm, as solder tends to draw away from the
point. Bevel/spade/hoof tips are designed to hold
a small ball of solder at the end, which is useful for
the drag-soldering technique explained later.
» Flux is the secret sauce in surface-mount soldering. It removes oxides from the connections so that
solder can bond to them, and also helps to distribute heat. During normal through-hole soldering, you
heat the joint with an iron and then melt solder wire
against it, which lets the flux in the solder’s core
melt out and clean the joint. With surface-mount
soldering, solder is often melted on the iron and » Desoldering braid or wick is a fine mesh of copper
then transferred to the joint — a mortal sin in regular strands that you can use to remove excess solder.
soldering. The flux tends to boil off during this
transfer, so you need to add more to the connection
directly. Flux comes in 3 types of container: felt pen,
brush bottle, and needle bottle.
» Use 0.02" or 0.015" diameter, flux-cored solder.
To get the hang of SMT, I’d recommend starting off
with lead-based solder, which is slightly easier to
work with.
» You can solder all but the most finely pitched
components using a lighted magnifying glass, and
you can use a $10 loupe with 10x magnification for
the finest. If you expect to do a lot of SMD work,
get a stereo zoom microscope with up to 30x
magnification (try eBay).
» For removing SMDs without a hot air station and
myriad special nozzles, use the Chip Quik SMD
removal kit (item #SMD1, $16 from
chipquik.com).
The kit contains a low-melting-point metal that when
mixed with existing solder causes it to remain molten
for a couple of seconds — long enough to flick off
the component.
» A small vise such as a PanaVise.
» I recommend getting a temperature-controlled
soldering station, at least 50 watts, which will probably cost $50–$120. A cheap 15W iron will work on
some things, but will be slower and more frustrating.
A good soldering iron is especially important if you’re
using lead-free solder, which requires higher heat.
Install a 1206 Resistor
Now we’re ready to install a surface-mount resistor.
Note that the resistive element in an SMT resistor
is exposed and colored, and it should face upward
to dissipate heat. The number 1206 means that the
package measures 0.12"×0.06". A 603 package is
0.06"×0.03", and so on. Let’s get started.
Make: 135
