V
3. I recommend preheating, especially if you’re
soldering the heat sink.
4. Apply hot air about ¾" away in a circling motion
until you feel the chip drop. Surface tension from the
molten solder should pull the chip into alignment.
You can also nudge the chip with tweezers to
make sure it’s correctly seated; it should spring
back into position (Figure R).
5. Check the sides with a loupe to make sure the
markers line up with the pads (Figure S).
SOLDER A DOUBLE-SIDED BOARD
If the board has components on both sides, you
need to use a toaster oven rather than a skillet.
1. Apply solder paste, using either a syringe or a
stencil and squeegee (see “Solder Paste Types”), to
whichever side of the board has lighter components
(Figure T; PLCCs are the heaviest).
2. Place the components using tweezers, fingers,
or a vacuum tool. It’s alright if the smaller components aren’t perfectly aligned; they’ll snap into place
during reflow (Figure U).
3. Reflow the board in the toaster oven. I use binder
clips to suspend it above the rack (Figure V). Paste
and component manufacturers recommended a
precise 3-phase sequence:
3a. Preheat and evaporate solvents in the paste
at 300°F ( 150°C).
3b. “Soak” between 300°F and 350°F ( 150°C–
180°C) for 1–2 minutes to let the flux remove
the oxides.
3c. Run up to about 425°F (220°C) for 1–1½
minutes to melt the solder.
What I do is simply turn the oven on max, wait for
all the solder to melt, then count to 15 and open
the door.
More complex boards and BGAs might require
greater precision. A thermocouple or temperature-indicating marker lets you see when you’ve
reached your target temperature.
For more control, sites like
articulationllc.com
and
thesiliconhorizon.com sell controllers that plug
into toaster ovens and let you program and run
time-temperature sequences, although most toasters don’t heat up fast enough to give a controller
much to work with.
4. After the first side is cooled, apply solder paste,
place components, and cook the other side. Surface
tension will hold the lighter bottom components
in place.
My results with the project photographed here
were about 25 bridged connections on the 208-pin
QFP with 0.5mm pitch, and a couple here and there
on the other packages, but the majority turned
out OK.
Scott Driscoll (
scott@curiousinventor.com) is an IPC-certified
soldering specialist, has master’s degrees in mechanical
engineering and music technology from Georgia Tech. He
researches and writes how-to guides at
curiousinventor.com.
Make: 139
