solder to wick around the back of each pin. I like to
lay a length of 0.02" solder along the pins and then
press it into each pin with the iron (Figure N).
SOLDERING NO-LEAD SMDs
The following tools let you handle IC packages without leads, like QFNs (quad flat no-lead) and BGAs
(ball grid array), that defy soldering with an iron.
I
J
» You can buy a hot air station with temperature-and flow-controlled air for under $300 from
Madell (Figure O, background;
madelltech.com).
Instructables.com also has a wonderful array of DIY
hot-air machines. If you’re feeling less adventurous, K
a $25 arts and crafts embossing heat tool (Figure O,
foreground) also gets the job done. Avoid ordinary
heat guns; their nozzles are too big and they’re too
hot for SMD work.
L
» Solder paste consists of tiny solder balls floating
in flux gel. It comes in 2 forms: in syringes, for applying to contacts individually, or in jars, for applying
en masse with a mylar stencil and squeegee (see
sidebar). Some distributors require fast shipping on
solder paste, since its lifespan decreases without
refrigeration.
M
N
» A hot plate can preheat the board to 212°F–250°F
in order to limit the time and energy required when
applying solder or hot air. This is optional, but it
mimics the large-scale manufacturing process and
reduces the risk of damaging boards or components. Preheating is especially helpful if you’re
using lead-free solder or if the board contains large, O
heat-absorbing ground planes. Preheaters are also
SOLDER PASTE TYPES
Solder paste comes in either syringes or jars. With a syringe, you should apply small, Hershey’s
Kiss-shaped drops to individual pads on the PCB, and thin lines on packages with rows of pins. I like
a 22-gauge needle. In the oven, the paste will wick to the connections and avoid bridging (for the
most part). Don’t bother trying to put paste on every little contact individually, because it will slump
(spread out) anyway when it heats. You can buy solder paste syringes from Chip Quik, Zephyrtronics,
smtsolderpaste.com, and others.
Paste in jars retains its form, and you can quickly apply it to all the pads on a board using a squeegee
and a laser-cut mylar stencil. Getting the right amount of paste — between having too little solder and
bridging leads — takes some trial and error. For stencil material, try
stencilsunlimited.com.
Both types of paste come in either “no-clean” or water-soluble formulas. With water-soluble paste,
the flux residues are corrosive and must be removed.
Make: 137
