68. Circuit board assembly tips
Assembling a circuit board can be a little intimidating if you haven't
done it before, especially one with as many parts as the Pinscape
expansion boards. But remember that the whole point of a printed
circuit board is to make it easier to build a circuit. Ideally, a PCB
turns circuit assembly into a paint-by-numbers project.
Order of assembly
For most circuit boards, the Pinscape boards included, the order of
installing the parts really doesn't matter. Most PCBs have simple
flat layouts, without any parts layered on top of others.
The only thing I'd add is that it can sometimes be a little easier if
you start with the smaller parts and save the larger ones for last,
especially the taller ones that stick out the most. A couple of
taller parts close together can create a little valley, where it might
be harder to see the board markings for the parts that go in between.
Reading the markings on the board
The markings on the board are designed to make it easy to figure out
where each part goes. Each part generally has three things marked:
- An outline showing the "footprint" of the part on the board. This
is basically the way the part looks when installed if you look at it
from straight overhead.
The outline often includes something to help you
orient the part correctly. For example, a MOSFET's outline shows a
thick bar on the side where the MOSFET's big metal heat-sink fin goes.
You just have to line up the metal fin with the bar mark, and the
part will be oriented properly. Similarly, a polarized capacitor's outline
includes a "+" sign on the pad where the "+" leg goes.
- The part's "reference designator" (or just "designator") that identifies
it in the parts list, like R1 for a resistor or C7 for a capacitor.
To find the part to install, you just look up the designator in the parts
list, and the parts list will tell you exactly which part to use.
("Reference designator" seems to be the formal term for this. I realized
when writing this material that I didn't know what this was really called
before; I didn't have a term in my mind for it, and just thought of it as
a label or name. This seems to be true for most people, and indeed, even
the engineer-focused EAGLE software just calls it the "name". But "label"
and "name" are too ambiguous when you're trying to talk about the schematics
as a language; they could be easily confused for manufacturer part numbers,
or part values like "100Ω".)
- A very concise description of the part, such as the Ohms value for a
resistor, the uF (micro Farads) value for a capacitor, or the part
number for an IC chip. This information is redundant with the
designator, since you can use the designator to look up a more
complete description in the parts list, but this is there as an
additional guide. It's also a good idea to cross-check the value
printed on the board against what you find in the parts list, to make
sure you're really looking at the right labels. Sometimes two parts
are so close together on the board that you can mistake one part's
labels for another's.
For example, here are the markings you might see for a capacitor (in
this case, a polarized electrolytic capacitor, which has designated
"+" and "-" leads):
In the chapters that follow, where we go into more detail on each of
the common component types, we'll show you the specific types of
circuit board markings for each type.
Installing through-hole parts
All of the parts on the Pinscape expansion boards are "through-hole"
parts, meaning that they have wire leads that you insert through holes
in the circuit board and then solder into place.
Here's the basic procedure to install a through-hole part:
- Identify the part's footprint on the circuit board - the little
outline showing where the part goes, which should contain all
of the holes for the part's wire leads
- Determine the proper orientation (see the chapters on the various
component types for more on how to do this for each type)
- Straight out or bend the leads on the part, as needed, so
that they line up with the holes in the board
- Orient the part properly and feed the leads through the holes
- Try to seat the part as close to the circuit board as you
comfortably can, without forcing anything. Parts should ideally sit
flat against the board, so that they can't move around or bend once
installed. But it's not necessary to force anything flush with
the board; sometimes the leads won't quite bend enough and the
part will sit a few millimeters above the board. That's fine.
- Holding the part in place (and keeping it from slipping back
further away from the board), flip the board over and solder
each of the leads to the pads on the bottom of the board
- After the solder cools, use wire snippers to clip off the
excess length from the leads, so that there's no dangling wire
past the solder joint
- See the recommendations on soldering tools in
Tools, including the recommendations on
the solder itself. I've heard so many people on the forums say "I'm
not a very good solderer", but what they often really mean (without
knowing it) is "I'm using a
crappy Home Depot soldering iron and crappy Home Depot solder". Those
soldering irons are for casual home repair jobs, not delicate
electronics work. Using them for circuit boards can be
painfully difficult. You'll be amazed at
how much your skills improve when you have the right tools.
- Keep a wetted solder-tip cleaning sponge handy while working. A
soldering station should include this. Saturate it, then squeeze out
the excess water. It should be thoroughly moistened but not dripping
- Wipe the soldering tip on the sponge from time to time while
working to keep it clean.
- Make sure the iron is at full temperature before you begin. A
proper soldering station has a readout showing the temperature and
a thermostat that holds the temperature steady.
- The default temperature setting on my Hakko soldering station
is 750° F, and that seems to work well for PCB soldering work.
- When the iron is hot, coat it with a little bit of solder, just
enough to flow over the surface. Wipe off any
excess bead on the solder cleaning sponge.
- Most newbie solderers want to use the iron to melt the solder, and
then try to drag the solder onto the parts. That's the wrong
way to do it.
- The right way to do it is to heat the parts.
- Start by putting the two parts to be joined into the desired
- With the parts held together in this position, firmly press the tip
of the iron against the point where the parts meet. Hold it there for
several seconds to let the parts heat up. Then touch the solder to
the junction point in the parts - not to the soldering iron,
but to the join point. If the parts are hot enough, the solder will
melt and flow over the parts, forming a bubble around the junction
- Don't use too much solder - just enough to flow over the parts
and form a bubble.
- As soon as the solder flows over the junction, withdraw the
soldering tip, but keep holding the parts together, keeping everything
as still as possible, for about 5 to 10 seconds while the solder
- Visually inspect the solder joint to make sure that the solder is
confined to the desired area, and that it completely surrounds and
covers the junction point. On a circuit board, it should stay within
its solder pad, and it should form a nice little droplet shape
covering the solder pad and wire lead. Make sure there are no gaps
and that the wire lead is covered all the way around. Wiggle the
part gently to make sure that the connection to the pad is