Sunday, April 18, 2010

Fun in the shop etching Printed Circuit Boards

I started a project of documenting and organizing all the various IC's I have acquired over time. Up till now they were in a little plastic box that I consulted time to time.

I stopped by Staples and picked up a handful of report binders with pockets - they conveniently had them on sale for $.50. The binders I was looking for had a insert for three hole punched paper and pockets. I printed out the data sheet for each part, 3 hole punched it and stuck the part into the pockets. Now I don't have to run to the computer when I pick up an IC I don't remember.

Being of an age where seeing very small parts is getting harder, I tended over time to accumulate a lot of soic parts, and they sank to the bottom of the sample box.

I finally purchased an inexpensive adapter board from dipmicro electronics that let me mount most of them for prototyping. I was suitably impressed of the quality of the board and @ a quarter a piece I got a bunch. One side lets you mount soic and the other ssop.

However there were a few parts that had wider spacing and no one seemed to have a break out board for them. Or should I say in my price range - I am, after all, a cheap son-of-a-gun.

So I decided to whip up one of my own. I bit the bullet and set aside the time to finally get proficient enough at Eagle CAD to make a passable design. After a few false starts I managed to get the libraries loaded and Eagle set up, thanks in no small part to a Spark Fun Tutorial.

First I could not find an outline for the part I wanted to use (DS2321 RTC chip). Google found a person that had designed a board with the part, and I set out to modify his layout. I got all hung up on making a library out of it for later use, after all this was just supposed to be a board for any so16wide parts. A forum post showed me how to make a library from the original. I was now drawing a hobbyist quality, wildly inefficient, but functional design. I had one in an hour or so. Now it was ready for toner transfer.

The last time I made a board I had purchased some wildly expensive coated picture paper. I remembered reading someplace someone using magazine covers, I grabbed a copy of Information week - the cover seemed a bit thin to me - but I was in a hurry and decided to try it. I found a back cover that was mostly white and it printed just fine in the laser. It took a bit to realize I was not enough of a pro at Eagle - I had managed to get some stuff on the top layer and some on the bottom - But after I set it to print both the top and bottom together, everything worked out swimmingly.

I now was on familiar turf ( I thought). I tossed a pieced of freshly scrubbed PCB stock onto an old re-purposed waffle iron to heat up - and ran around to locate that darn roll of masking tape. Finally taped it to a very warm board and ran it through the laminator a few times.

Looking through the cover at the toner - there were some supicious spots that I though had not stuck sufficiency, grabbed the Clothes Iron and cranked it up as far as it would go - folded a paper towel to made a pad, and pressed hard as I could to make sure the toner was re-melted onto the board.

When I put the board into a hot water bath I was very pleasantly surprised, the last time it took almost an hour to get the photo paper soft enough to get it away from the toner and off the board. This time the paper just peeled away in a few minutes, and came away very cleanly. Just a few wipes with a sponge was all it took to rid the toner traces of any paper residue. I guess it had to do with its thinness.

I guess photo paper is meant to last and was considerably thicker. It took forever to get soft enough and stuck perniciously to the toner. I guess I will renew that Information week subscription they keep hounding me about - if for nothing than to have lots of cover paper :-)

I could see where I had pushed too hard with the iron - it looks like the toner had smeared onto the board, a few pokes with a Xacto knife and and some touch up with a fine point Sharpie and it was ready for the etchant.

Then came the fun part where I get to dress up like a serious dork. Donning lots of protective clothing, chemical safety goggles, breathing mask, and nitrile gloves. I got out some Hydrochloric acid/Peroxide etchant that had to be almost a year old, and dropped the board into the cold solution. It went slowly, I had seen in some video where someone used a little sponge brush to rub off the disolved copper and let fresh etchant at the board. So I brushed and brushed and swore to next time I would take time to heat the etchant and I would flood fill that unused part of the board so that not as much copped had to be removed.

Pretty soon the etchant was so dark it almost looked like ferric chloride. Just about then, the door bell rang and I went to the door worrying what the caller would think with me a dressed up like a toxic spill worker. Just my Brother-in-law and my sister. The board sat there in the etchant for the better part of an hour as we discussed pleasantries.

When I got back nothing seemed to have happened to the board much, so I quickly whipped up a new batch. WOW. A new batch of etchant really works fast - like in another minute the board was completely etched.

A quick wipe with some acetone and the bare copper was exposed, and the family was suitably impressed by the little lines of copper running all over the board.

This morning when I got back down to the shop - the dark murky old etchant was a bright green and almost clear. I goggled around and found this weighty tome explaining how, just by exposing the etchant to air, it re-generates it given enough time. This was about 3/4 cup in a closed plastic container overnight- not much air - but evidently enough. I grabbed an old coffee container, for it's opacity, and poured both batches together for next time (have to get an acquarium pump)

Now its time to clean up all the spills and wash down the bench-top so I can get out the drill press and Scroll saw to do the necessary machining. Etchant is very, very, very corrosive and it will rust up any tool that it touches - last time I managed to spill some unknowingly onto my favorite set of needle nose piers - that will never happen again.

Time to go break lots of little expensive carbide drill bits.

So lessons learned, and notes for the next time
  1. Spend some time with Eagle cad futzing around - just to get comfortable.
  2. Use magazine covers to print.
  3. Don't press so hard with the clothes iron, or try not using it at all.
  4. Make sure to regenerate your etchant with a cap full or two of Hydrochloric acid before starting to etch.
  5. Heating it up first wouldn't hurt.
  6. Look at the yard sales for an aquarium or other small air pump to regenerate the etchant.
  7. Find really, really, cheap place to get all those tiny carbide drill bits that are so fragile.

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