Electronics -- Creating Two-Sided PCBs
This page provides a brief description on how to create two-sided
Overview and Purpose
PCBs (printed circuit boards) are the "natural" way to actually implement
electronic (especially digital) circuits when using CAD (such as
e.g. CadSoft's Eagle which I am
using) and/or SMD parts.
Hobbyists have long been able to create single-sided PCBs
and there are lots of descriptions on the net for the required processing.
However, for non-trivial devices, one signal layer is often not enough and
fortunately the step from one to two layers is not very hard. (In contrast,
more then two ones, i.e. PCBs with hidden layers, cannot be made as easily.)
So, this page is not meant to be a beginners' guide to PCB creation but
a quick reference on the proceedings used by including a description on how
to make two-layer PCBs without requiring special additional equipment.
First step is, of course, creating a litography masks from the PCB layout:
One for the top and one for the bottom. In Eagle, the CAM processor is
used for that purpose; I use "Mirror" and "Upside down" for top and none of
the two for bottom; this way, the masks are put with the printed side on
the board plate. It is advisable not to "fill pads" because it
makes the drilling easier.
The masks are printed on normal transparent slides using a laser
Using an ink printer is not advisable because the ink often has little
UV light absorption and the special ink slides tend to give less sharp
I Depending on the slide and the printer, you may need to layer two
identical slides (I need to use 2) but keep in mind that more of them will
The top and bottom masks need to be properly aligned so that the drills
from top to bottom match signals on both sides. In Elector, a
German electronics magazine, people suggested to build a special
alignment and exposure device from several layers of wood and glass and
some more non-trivial processing. Before creating my first PCBs, I read
that article but then dismissed it, because things can be done at least
as good using a much easier method:
[click to enlarge: 92kb]
Cut off some border of the top-layer slide and then simply lie the top- and
bottom-layer slides above each other (without the board plate in between
which will be inserted lateron) and align them properly. (Don't be surprised
to see that your printer was not creating two slides with perfectly identical
spacing...) Fix the (smaller) top-layer slide on the bottom slide with one
or two stripes of tape so that the resulting mask can be opened like a
(note)book. Verify that opening and closing it several times will not
destroy your careful alignment (it really works!).
With the mask "opened", put some (1..3) tape rolls (or alternatively
tape with two sticky sides) on places where no important layout is located.
Such places can be found on nearly any board (unused areas, future
drills in the egdes,... If not, you should re-think your design because
chances are high that you regret it afterwards.) The purpose is to keep
on both sides of the board plate fixed to the mask slides so that everything
can be turned upside down after having exposed one of the sides.
Next, the raw board plane needs to be inserted. I am using (standard
industry-quality) two-sided Epoxyd plates with 35um Cu and normal photo
layer on both sides. After removing the protective cover, carefully place
it on the bottom-layer slide and "close" the mask (like a book).
(Be sure not to touch the transparent photoactive layer on the board plate.)
The board plate is now fixed on the PCB masks.
[click to enlarge: 33kb]
The photoactive layer is most sensitive to UV light, so handling the
plate under normal dim light was not yet source of trouble for me.
Using a "black light energy saving" (gas discharge) lamp with
(Note that I recommend against using standard UV/"black" light bulbs
because their UV efficiency is very low.)
The exposure setup can be seen on the left image: The mask and board are
below a glas plate which has weights put on each border to keep the
mask slides plane. The top-layer and the bottom-layer are exposed
Detailed process parameters:
"Energy saving" UV lamp with 20W and self-made reflector
Distance lamp (bottom) to top of 3mm glas plate: 20cm.
Exposure times: 8 minutes for top layer (first), 6 minutes for bottom layer
(last) with 1 minute for turn-around in between. Lamp switched on for
exposure only (off before and during turn-around).
The reason for the different exposure times for the top and bottom layers
is that the applied lamp is slowly heating up and is thereby increasing its
Note that the actual exposure times also depend on the UV transmission of
the applied mask slides and the thickness of the glass plate.
[click to enlarge: 64kb]
The above mentioned method has some disadvantages: The reflector is
very fragile and everything needs to be installed and adjusted each
time I want to make a PCB. So, one time (after a longer PCB creation
break when I had 2 unsuccessful attempts in creating 10x8cm boards),
I discussed the issue with my brother and we finally spent an
afternoon building the exposure device which can be seen on the left
(which was made out of an old speaker box).
There are no reflectors on the side walls so that most of the light
comes from the top (in 90° angle and less light under small angles) so
that the edges of the wires on the board are projected sharper.
Furthermore, in contrast to the above method, the lamp is installed
Here, the detailed process parameters are:
"Energy saving" UV lamp with 20W and self-made reflector
Distance lamp (bottom) to top of 2mm glas plate: 16.5cm.
Exposure times: 5 minutes for one side when starting with a cold lamp.
[click to enlarge: 51kb]
Development is rather easy. I am doing it by sight (see left image) and it
takes several minutes.
As developer, I am using 1g NaOH (solid state) in 100g Water
which is enough for a 8x10cm two-sided board with much less than 50%
mask coverage. Be sure to mix the developer directly before applying it
because it cannot be stored very well in liquid form.
To speed up the process, make it hand-warm and use a pipette to produce
some circulation and "blow away" the photo-active layer. Alternatively,
a soft brush can be used. I'm using 37°C normally.
Note: Do not make it too warm (e.g. 50°C, but YMMV) because this
may also rapidly kill the non-exposed parts of the photo-active
[click to enlarge: 60kb]
Corrosion takes longest. I am using
22g Na2S2O8 in 100g water
which is enough for more than one two-sided 8x10 plate with normal layout.
To speed up the process, use 50-60°C and keep the lid closed to keep
The Na2S2O8 will get increasingly
blue (the color of the sulfur-copper-complex).
Most holes can be made with a 0.8mm drill. Only some parts need 1mm or
larger. 0.8mm turned out to be a good compromise because smaller drills
will cut the board material less well and break more easily.
I am using standard HSS drills with 0.8mm and 1.0mm and a fixed-mounted
drilling machine. Note that a crand will stay at the side where the drill
leaves the material; this side will be harder to solder.
Write "top" and "bottom" on the top and bottom layers.
Save corrosion liquid by "filling" unused parts of the layout.
Leave place for drills to fix the board once it gets built into some
Write description of important connectors on the board
(e.g. "+++" and "GND" for supply).