■ FIGURE 4. Board layout
using the EAGLE Layout Editor.
The ground path is highlighted.
haven’t tried it yet. Smaller
components are difficult to
handle. Try to stay larger than
the 805 case size.
yields good results if you are careful
and don’t rush. Using this design
process, your circuit is printed to an
8-1/2 x 11 inch piece of transparency
film. The film is then placed on the
copper board and heated as shown in
Figure 6. The toner on the film transfers
to the PCB, creating a mask as in Figure
7. The following steps are required:
time to learn new tricks.
• Most SMT parts are contained in
the standard parts library. If not, refer
to the device data sheet for the
recommended pad layout.
• Try to use a short, thick, direct
ground path for your circuit. The
ground path is highlighted in Figure 4.
Actually, try to use short paths for all
your circuit traces.
• Optimize the circuit layout
before turning on the
autorouter. You will find that
circuit layout is an iterative process. You
are always shuffling components to
minimize wire length and detangle circuit
paths. The autorouter will not do this for
you. It only optimizes the wire run. If
your parts placement is less than optimal,
the wiring solution the autorouter
provides will be atrocious. I’ve seen it
route a ground wire in a loop covering
half the board. This bad wiring will likely
result in oscillations in analog circuits and
random logic errors in digital circuits.
1. Print the circuit design to a sheet of
transparency film using a laser printer or
a copier. Remember, inkjet technology
will not work as it has no toner to transfer. Go to the setup page in your printer driver and select the mirror image
option. Print to the smooth side of the
transparency film. This allows the toner
to be easily transferred to the PCB. Be
sure to select “black” in the printer
properties so that you have solid lines.
• Don’t go too small. The toner
transfer process isn’t 100% accurate.
You may be able to use this process
with SSOIC (shrink SOIC) chips, but I
• You can verify layout and clearances
by printing the design to paper and
comparing the actual components to
2. Clean the PCB. You can clean the
board mechanically using a green
scrubbing pad. The rough surface
gives the toner a place to adhere and
removes any oxidation.
■ FIGURE 5. Routing a trace beneath
larger SMT components. The pads are
for 1206 SMT capacitors and the wire
width is 0.016 inches.
• Connection to the board can be
simple pads (large circles) as shown
in Figure 3. SMD connectors are
available or you can use
3. Tape the board and transparency to
a table as demonstrated in Figure 6.
Cover with a piece of paper. The
paper acts to slow the heating process
and spread the heat evenly. You do
not want to apply the iron directly to
the transparency film because it will
yield poor results.
The PCB traces are
developed using a masking
process. A mask is applied to
the PCB to “save” the
copper as shown in Figure 6.
The board is then exposed to
an etching chemical. All the
copper that is not
protected by the mask is
etched away. Our goal
is to make a good mask.
The toner transfer
method is a fast, easy
method to place the
mask onto the PCB. It
4. Heat the PCB and transparency film
with a clothes iron. Use a constant
motion and let the weight of the iron
do the work. Don’t push down on it.
5. Inspect the transparency every 30
seconds. This is the advantage of using
a transparent film. You can actually see
the toner to determine your progress.
Stop heating when the toner melts. It
will take some experimentation to get
the process correct. If you make a
mistake, you can start over again.
■ FIGURE 6. Preparations
for toner transfer.
The PCB and the
transparency are both
taped to the table.
6. Don’t move the transparency or the
board until they are cool to the touch.
If you don’t wait, you will have a black
stringy mess to clean up.
7. Touch-up may be necessary — Use
a Sharpie pen to add/modify traces or
an X-Acto knife to remove material.