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Electronics -- Creating Two-Sided PCBs

This page provides a brief description on how to create two-sided PCBs.

Local: [Overview] [Mask] [Exposure] [Development] [Corrosion] [Drilling] [Tips]

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.

PCB Mask

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 printer. 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 edges. 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 decrease sharpness.

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:

Two-sided PCB mask [6kb]
[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.

Exposure

PCB UV light exposure [7kb]
[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 20W. (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 in sequence.

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 intensity.

Note that the actual exposure times also depend on the UV transmission of the applied mask slides and the thickness of the glass plate.

PCB UV light exposure box [7kb]
[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 horicontally.

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.

Development

PCB after development [4kb]
[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 37C normally.
Note: Do not make it too warm (e.g. 50C, but YMMV) because this may also rapidly kill the non-exposed parts of the photo-active layer.

Corrosion

PCB after corrosion [4kb]
[click to enlarge: 60kb]

Corrosion takes longest. I am using 22g Na2S2O8 in 100g water (distilled) which is enough for more than one two-sided 8x10 plate with normal layout. To speed up the process, use 50-60C and keep the lid closed to keep the water. The Na2S2O8 will get increasingly blue (the color of the sulfur-copper-complex).

Drilling

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.

Tips

  • 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 device.
  • Write description of important connectors on the board (e.g. "+++" and "GND" for supply).

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Last modified: 2004-10-06 18:56:11