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My “secret” technique for making double sided PCBs in the home electronics lab.

Double Sided PCBs

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Intro:


The "Holy Grail" of DIY PCB making to me is:  The Double Sided Board With Fine Pitch SMD Parts.


In this "How To" I'll show you my special technique for making great two sided PCBs in the home electronics lab.


My technique uses presensitized PCBs. These presensitized PCBs are a little like photographic paper. They come in black plastic bags to protect them from ambient light, and once exposed they both get developed in a liquid solution.  One turns into a photo the other into an etchant mask.

Compared to other typical home electronics lab PCB fabrication techniques such as ironing photocopies, stencil drawing etc... The presensitized PCB produces amazingly fine results, and allows for my special two sided technique.

Brief:


To use this technique: Print out your PCB files onto acetate sheets.  Sandwich the presensitized PCB between the two sheets of acetate.  Expose both sides to UV light, develop and then etch the board.

Drawing your PCB:


There are plenty of resources around for instructions on how to draw PCB circuits. One of the best that I've found is over at Sparkfun. Sparkfun uses Eagle CAD to draw PCBs.  They also allow you to download their libraries of parts which can be a huge time (and frustration) saver. 


Whichever PCB drawing software you decide to use here are some tips to keep in mind when making DIY double sided PCBs:


- Make your vias as large as possible. (vias are holes that connect the top and bottom layer together)


Unlike a professional PCB fabrication service, you will have to drill the vias yourself. Since you are drilling them yourself, they will not be plated. (covered in conductive material from top to bottom). You will have to insert a thin wire and solder the top and bottom.


- Do not place vias under other objects on the PCB. You will manually solder your vias (see above).  If you hide them under another part, it may be impossible to solder on one of the sides.

Notice that in this illustration the via is under the IC. This would be fine for a job being sent to a professional board fabricator, but can cause problems with the DIY technique.

In this illustration the via has been moved out from under the IC and also the size of the drill hole and pad have been increased to make it easier to solder by hand. (I know a via isn’t required here, it’s just to illustrate the idea.)

- Make your through hole component pads as large as possible. You will be manually drilling the holes for through hole parts having larger pads makes it easier to aim the drill from top to bottom and not cause a short.


- Make your traces as large as possible. This just makes things easier.


- If you are using a fill, set the ***space around the fill to as large as possible. This just makes things easier.

Preparing your transparencies:


Getting the drawing out of your PC:

Most PCB drawing software allows you to save, export or produce a set of job files from your circuit drawing in postscript.


In Eagle CAD, either click on the CAM processor icon or choose CAM processor from the file menu.

Next; once the CAM dialog box is open make the following settings then press the Process Job button:

  1. 1.Choose PS (for Postscript) in the Device list.

  2. 2.GIve your top layer a name. (On my computer I have to manually add the .eps extension or computer does not open the exported file.)

  3. 3.Uncheck Fill pads. This way the center of the pad will have a hole which will help aim your drill.

  4. 4.For the Top layer, choose (Top, Pads and Vias) for the Bottom layer choose (Bottom, Pads and Vias).

  5. 5.Click the Process Job button to save your file.

After you have produced the top layer, do the same again but deselect "Top" in the right column and choose "Bottom" and change the name before you hit the Process Job button again. (No need to mirror any layers, you can flip the transparencies once printed.)


Print your files on a laser printer:

I like to print onto regular paper first. With a paper copy, I can take the through holes parts and push them though the paper to test the placement. Very important, this also tests to make sure that the printer and or computer has not changed the scale of the drawing. Once I am satisfied with the placement and size of the paper printout I then print on the transparencies.

Printing out the PCB file on paper allows me to test the fit of the components and test to make sure the printer has not changed scale of the drawing.

Tuning up the transparencies:

The type of transparencies that I use come from the local office supply store.  They are called overhead transparencies. Very important, they must be marked safe for laser printers. Laser printers contain a fuser that will melt regular plastic and destroy your printer!


I start by printing out two copies of both the top layer and bottom layer. The reason that I print out two copies is because all of the laser printers that I tested the black is not completely black on the transparencies, which allows a bit of UV light through. This makes the development and etching more difficult and less precise.

Single print from a laser printer onto a transparency is not completely opaque (black).

The next step in the transparency tune-up is to fix any holes (areas that are not supposed to be transparent) in the black areas with a felt tip pen like a Sharpie.

Light Table:

For the next few steps a light table comes in very handy. I made one out of a wooden box. The glass from an old scanner on top, as well as a sheet of milky acrylic to diffuse the light and a CFL light bulb. (This type of light does not get very hot).

If you don't have a light box and don't feel like making one, you could simply support a glass sheet between two stacks of books with a light source underneath.

Getting your transparencies ready to print your board:


The reason that I print out two copies of each layer is so that they can be stacked on top of each other to produce completely opaque black areas.


Here is the technique that I use to layer two transparencies to produce one darker one.

This video shows my technique for layering two laser printed copies of acetate to produce a darker one.

At this point if you want to make a single sided PCB you could skip down to Preparing the “presensitized PCB.” Otherwise do the same layering process with the second layer and then continue.

Making the transparence sandwich:


To make a double sided PCB you need two sides. Take the two sides that were prepared in the previous step and follow the instructions in the video.

This video shows my technique for lining up both top and bottom printed sides of the acetate into a hinged version to easily make two sided PCBs.

Preparing the Presensitized PCB:


Presensitized PCB are shipped in light proof envelopes with a layer of peel-able protective film. Leave them covered until you are ready to cut and expose them.

Presensitized PCBs come in light proof bags and also have a peel-able protective layer. (In this photo the white material is the peel-able protection.

Pre-Prep:

In the next few steps you will be exposing and developing your PCB. Make sure your work area is set-up with the UV exposure bulb and that you have your developer solution mixed and at more or less the right temperature recommended in your developers instructions.

Exposure Light:

For an exposure set-up I used a florescent UV bulb mounted in an old kitchen counter light fixture.  I made this “ghetto" light stand with three pieces of scrap wood.

Board Prep - Steps:

  1. 1.Measure your design, mark the size on both sides of your board.

2. Using a ruler, score (a few passes may be required) the board with a sharp utility knife.

3. Snap the board on the score line.

4. Using a file, remove the bent edge of copper that the scoring produced. Removing this edge will ensure that the PCB lays completely flat against the transparencies, producing a better PCB. Remove the dust once you are done filing.

5. Carefully remove the protective layer on the PCB. Handle the board by it's edges, don't touch the presensitized resist and do this in lower lighting away from florescent light sources.

6. A. Two sided board: Place the board between the two acetate layers and then sandwich the entire assembly between two sheets of glass. I use two ~3x5" pieces of glass out of an IKEA picture frame. Use rubber bands to hold the glass, acetate and board sandwich together. Make sure your elastics do not cover your printed circuit drawing.

Hinge area (1) of the two sided acetate PCB drawing (5). Rubber bands (2) used to hold the glass (4) tightly against the acetate (5) and PCB (3). The light rays are there to show that both sides of the PCB can be exposed (I only do one side at a time).

6.B. One Sided board: Place the board down on your exposure surface, with the acetate on it with a single layer of glass on top. You may need to weigh down the edges of the glass to keep everything flat. The glass is required, it helps by keeping things flat and protects the acetate and resist from the heat of the lamp.

    I like to buy copper boards that are larger than I need and cut them up. Cutting them can be a pain, but it is a great way to save money, and if you make a mistake you can cut off another piece.

7. Exposure: Expose each side of the PCB. Check with the instructions that came with your presensitized boards to determine the length of time to expose your boards and height of the lamp from the board. You may have to do some test exposures and developments to see which setting work best with your brand of boards. Too short an exposure (or with the light too far,) all of the resist will come off when you develop the board. Too long and you will have to develop the board much longer and will loose fine detail.

  1. 8.Development: As soon as possible, after your board has been exposed, immerse the board in the developer solution. I've had good results using a disposable foam paint brush to gently brush over the exposed surface until the non-desired area melts off the board. Once the exposed areas are clear (you see copper) remove the board from the developer and rinse under running water.

Note: Examine the board closely in the areas that there is no resist.  Make sure that there is no more resist film in those areas or you will have a very hard time etching the board. If you see a film then return the board to the developer. Rub a little with the foam brush and rinse once again.

9. Etch how ever you usually etch you PCBs. Clear the remaining etch resist with alcohol and a paper towel. Drill holes, solder short lengths of wire from top to bottom in the via holes and trim. Solder the remaining components. You’re done!

Note: The etch resist is very thin and easily scratched. When working with two sided boards take special care not to scratch the layer that is on the bottom of the developer or etching tray. If the board that I am working on is small enough I'll simply hold it off the bottom by the edges with a pair tongs.

Caution


Nearly every step in making PCBs is dangerous. The chemicals used to develop and etch the boards can stain anything, damage surfaces, produce toxic fumes and can damage your eyes or skin if splashed. Cutting, filing and drilling the PCB material will produce dust that can damage your lungs. Cutting and drilling the PCBs can cause parts to fly into your eyes or of those around you. The UV light from the exposure lamp may damage your eyes or skin.


Wear splash proof googles and chemical proof gloves while handling liquid chemicals. Wear safety glasses while cutting and drilling the PCBs. Avoid breathing the dust produced while cutting, filing and drilling, wear a dust mask. Avoid exposure to the UV light. Avoid breathing the fumes from the chemicals and work in a well ventilated area.


Follow all cautionary information and instructions provided by the manufactures of boards, chemicals, lights, equipment and supplies you use.

(This is the 2 sided PCB created with the acetates in the videos.)

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