■ A chip
once cut for
removal.
■ MOB controller for OSMC boards (example of surface
mount and through-hole parts).
http://store.nutsvolts.com. In the June 2008 issue, there
is an article on a controller you can add to a regular
toaster oven.
The other method uses a regular soldering iron and is
what we'll focus on here. There are two slightly different
variations to this method which both provide excellent
results. The first only applies if the PCB (printed circuit
board) was made with a reflow solder finish. (This is where
the board already has a thin layer of solder applied by the
PCB manufacturer on the pads.) In this case, you just need
to heat each pin and pad so that the solder from the pad
will stick to the lead of the part. When using this method,
you should have a nicely tinned tip (important for heat
transfer) but not any excess solder. I usually only use this
when installing very fine pitch parts like the TQFP devices.
If the board doesn't have any extra solder, then you'll have
to do more traditional soldering.
NOTE: During assembly, make sure you keep the
soldering iron tip clean. Wipe it often on a wet sponge or
cloth then apply a little solder to the tip. This will protect
the tip and enable you to make good connections. When
the solder tends to "ball" or does not stick to the tip, the
tip needs to be cleaned and re-tinned.
■ Boards with pads to accept surface mount or
through-hole parts.
Do not rush! After
perfecting the techniques,
you'll naturally get faster
along the way. A good
surface mount solder
connection forms the
electrical connection
between two parts, such as
a component lead and a
circuit board foil. With
surface mount parts, it also
provides the mechanical
connection, as well. Make
sure there is enough solder to leave a nice fillet between
the component lead and the pad.
NOTE: Before installing any components, plan ahead
and think about the final placement of each part and what
the final board will look like. With that in mind, it should
help with the assembly order so that each component will
not block the next one installed.
The alignment of the parts is critical. Small
components like the 1206 capacitors and resistors are
more forgiving and are good to start out with. Others like
a TQFP microcontroller are absolutely critical in regards to
placement with their fine pitch leads. Before installing a
surface mount component, apply a small amount of solder
to one (and only one) of the pads on the PCB where the
part is going to be installed. Then, while warming the
solder on that pad use the tweezers to set the part in
place. If the alignment isn't right, you can do adjustments
while the solder is still molten. If it takes too long, let
the solder cool to ensure the part is not damaged by
excessive heat. Once cool, the joint can be warmed up
again and the alignment can be adjusted until it is perfect.
Before soldering any other joints on the part, use the
magnifying glass to verify that the alignment is okay. At
the moment, the exact orientation of the part in relation
to the pads is what we're concerned with. Next, go
ahead and solder an opposing lead on the part. As long
as everything looks good, continue soldering each one.
■ An OSMC
H-bridge board.
62
January 2009