Tuesday, April 26, 2011

Making PCBs using presensitized boards

Here's my current procedure for making single-sided printed circuit boards. 

1. Draw the schematic and PCB layout. I use DesignSpark PCB since it's full-featured and free without any limitations whatsoever. Getting the layout right is the most time-consuming part. But it's all on computer so I really have no problem with this part of the work. In fact I kind of like it--optimizing a design on the drawing board is my kind of game.

Instead of (just) pouring copper all over the empty parts of the board, I put in as much relevant text including what it is for, creation date, what major ICs populate it, etc. Since I don't have the luxury of having a silkscreen on the component side, placing information on the copper side is a fairly good substitute.

2. Print PCB layout on transparency film. Since I'm using an inkjet printer, the transparency is one made specially for inkjet printers--one face of the sheet is rough to take on the ink. Scale of printout is of course is 1:1. This is very important. And to make sure that component lead pitch on the printout is correct it may be wise to print a draft copy on plain paper and check.

The printer is set to black-only printing. Contrast and other controls (eg. gamma) are set to maximize ink deposition and darkness. I'm using an Epson T10 and the print quality leaves much to be desired. It isn't solid black--there are minute cracks, gaps, lines which allow light to seep through. Because of this exposure time has to be at its minimum else copper traces will be compromised.

3. Cut the presensitized board to size if necessary. I use the score and snap method: Score both sides of the board where the cut will be made using an X-Acto type of knife. Use a ruler/straightedge as a guide for the knife. Run the knife several times to deepen the groove. Insert the board into or lay it underneath a thick book (I use a telephone directory) all the way until the score line. With one hand pushing down on the book, use the other hand to push down on the part of the board that's sticking out. If the scoring is deep enough, the cut should be fairly clean.

Peel off the protective backing. Center and secure the transparency film to the board. Do this under subdued lighting and as quickly as possible. There are various ways of securing the film to the board. Taping the transparency and board to a glass sheet is one, albeit crude, way of doing it. With my current setup I have a large foam sponge (approx 15" x 24") I use as a base on which the presensitized board is mounted (photoresist side facing up). I then lay the transparency on top of the board and align it. Printed side of film touches the presensitized side of the board. Next a large glass pane (3mm thick) goes on top. Finally the fluorescent lamp is placed on top of the glass. The lamp doubles as a weight to compresses the board-film-glass sandwich. The foam at the base evens out the pressure. Placing additional weights on the glass may be necessary because it's important that the transparency is firmly touching the photoresist for the traces/tracks and pads to be distinct.

4. Expose to UV light or fluorescent light or sunlight. Exposure time depends on light source, its intensity, and its distance from the board. It will usually be anywhere from a minute to ten minutes. With Kinsten phenolic paper single-sided boards and the 2 x 11-watt Toshiba daylight compact fluorescent lamp set up that I use with the lamps approximately 2 inches from the board, it takes between 1 to 2 minutes for boards less than 4x4 inches.

5. Develop the PCB until light-exposed photoresist is washed away. Agitate the developer and board while developing. If developer isn't included with the presensitized board when purchased, then one can make it by using a sodium hydroxide (NaOH) solution or better yet sodium metasilicate solution. If the board isn't underexposed, developing time will usually be under a minute.

6. Rinse developer off the board under running water.

7. Etch the board in a mixture of (by volume):
1 part tap water
1 part 6% (20 volumes) hydrogen peroxide (H2O2)
1 part 29% hydrochloric acid (HCl).
Etching time will depend on the thickness of the copper cladding. Etching will usually be complete within  three minutes. The etchant will turn green as the acid reacts with the copper and eats it away. The mixture is exothermic so expect it to get warm. Bubbles may also form. It's probably oxygen being liberated from the peroxide. 

8. Rinse etchant off the board under running water.

9. Drill holes into pads.

10. Remove photoresist from the the pads using Q-tips (cotton swabs) dipped in acetone. Photoresist on tracks and any copper pour area and anywhere where there won't be any soldering can be left on. After using a solvent to remove the photoresist the copper cladding may have to be further cleaned using a scouring pad to prepare the surface for soldering. 

11. Solder components to the board.

The great part about the etchant chemicals used above is that they're readily available in supermarkets, hardware stores and drugstores. I've chosen not to use ferric chloride not only because it can only be obtained from electronic parts and chemical suppliers but also because it's dark opaque color detracts from visually inspecting the progress of the etching process. The HCl+H2O2 mixture on the other hand is readily available, transparent, quick-acting, and cheap. I even use the spent mixture as toilet cleaner.

Safety notes: Sodium hydroxide and particularly hydrochloric acid are nasty stuff. You don't want them on your skin, your eyes, or making their way into your respiratory tract. Needless to say they are toxic when ingested. Here are safety guidelines that must be observed when using these substances:

* Always don protective eyewear, face mask, and gloves when using the chemicals
* Use only plastic (eg. polypropylene, polyethylene) containers for the chemicals and any implement that will come in contact with them
* Always pour water and H2O2 into a container first before adding HCl
* Always mix the chemicals outdoors or in well-ventilated area
* Dispose of them properly.

Some references which I found useful for PCB making:

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