Questions about making my own PCB.

[jdraughn]

pcb make uv

Sorry for such a long post here, hopefully anyone who knows enough to help me does not run out of patience before they finish reading this. I know I will figure this out via trial and error but would prefer to save some money by having an experienced person point out some mistakes.

I finally got to the point where I tried to make my own PCB. I learned a ton but have a few questions, mainly how to tell if your over/under exposing or over/under developing.

I will describe my setup and procedures in as much detail as I can and describe my symptoms, maybe someone has some ideas.

I printed the schematic on overhead transparencys. I read they suck though on some of the website tutorials that I came accross and that I should use vellum or paper soaked in peanut oil. It seems there were a couple of other materials mentioned too that would be better.

I had 3 decent prints as I was experimenting with contrast and other print settings built into the printer itself to try and make it as dark as could be.

I printed it using a multi $100k xerox full color laser printer. When I went to expose my boards I overlayed the 3 good images I had over each other and taped the sides so there was no chance of them slipping. It is a perfect alignment as far as I could see. Bit I am afraid that this cost me some resolution when I exposed the board since the top sheet would cause a slight blur on the board, and I think that may actually be pretty much be the whole problem that I am generally having.

I made a custom exposure unit. I bought some clearance bug zapper UV bulbs from walmart a few months ago during the winter months just for this use. They are 40 watt, about 4" wide and around 12" long, one bulb folded in a tight "U". I wired them to a electronic ballast from home depot. It's just temporary right now, I plan on having two switches, one built into the lid and one outside so it won't run unless the lid is shut, and also having a built in timer so once I get this figured out I will be able to get perfect exposures every single time.

I know im putting out a lot of UV rays compared to just using normal flourescents, but I don't know if it's comparable to a real "exposure" fixture or somewhere inbetween. I covered the whole thing with a thick towel to protect my eyes and I could swear when I poke my hand under I could feel tingling, but it could be my imagination. I didn't keep it under long enough to find out for sure, I don't need skin cancer on my hand

My first 2 or 3 exposures I didn't have enough weight on the glass or something so I had messy exposures. Took me about 3 trys before I realized this was the problem (I was also using crappy PCB's that had been pre-exposed to light because my dogs had chewed up the corner of the board while it was still brand new in the packaging.) I was using these crappy PCB's even though I knew they probably wouldint work so I could figure out what the heck I was doing.

After I realized what was going on I taped the transparency image to the glass and layed it on top of a thick paperback book. That made a huge difference but the problem that I am having is that my copper traces are slightly merging together, which was still occuring even after sandwhiching the image real nice and tight.

I think that it's from a combination of having 3 overlayed transparencys and im either under or overexposing.

The last try I did was my best yet. I had about 3" between bulbs and PCB and I exposed it for about 90 seconds. The presensitized boards I was using said 8 minute exposure with flourescents and 60 to 90 seconds with real exposure units.

I accidently turned the image slightly though on my last try so my exposure was crooked and off the board but I went ahead and developed and etched it anyway just to see how it turned out (traces slightly running into each other).

I bought some real "etcher" that you mix with water but just for the heck of it I mixed my own using muratic acid and hydrogen peroxide and it appears to work ok. I am hoping I can just take the etcher back to where I bought it and buy more developer and/or PCB's instead.

The developer instructions said to mix the whole package with 1 liter of water which is about 4.2 cups, and me being the cheapy I am, I visually split it up into 8 parts and bagged them seperately, each one being mixed with 1/2 cup of water when I go to use it. (could also be my problem is my ratios are quite a bit off since visually splitting it up isin't the most accurate way of measuring them out).

I found the process for the developing and etching was taking quite a bit longer then what I read it should take, but the water I used was probably a little cool. I was afraid of using water that was too warm so I think i went the extreme oppisite. I wish I had kept records via a thermometer as I was writing down my trial and error exposure times and whatnot.

I ended up putting my developer and etcher in a bigger tub which had an inch of super hot tap water to speed things up and it made a huge difference in how fast it developed and etched the boards. I could actually see the "wisps" of color after heating up the solutions a little.

I am sure I probably left out some important details and instead gave a lot of unnessary information.

My last try which turned out the best, I accidently ruined when I thought I was done developing it. After I etched it. I used some laquer thinner to clean the remaining developer off thinking it was done before I realized it still had a ways to go in the developer so i dropped it back in the developer (but had already removed the photoresist with the laquer thinner) so i accidently erased all the traces.

The one board I have that actually looks like a semi usable board is not worth taking a picture of since I left it in the devloper for wayyyy to long when I realized my image was upside down and useless and I forgot about it while I worked on cutting the next trial PCB.

Thanks for any advice or tips, and again, sorry for the novel length post I just wrote. (and bad spelling and grammar).

 

[wp100]

pcb making

Hi,


Well that sure is a bit of a tale, difficult to say in a simple word that 'this is wrong' as the whole process is a balancing act.

What does stand out, is using 3 copies, the chances of registering them when using fine tracks is slim - why 3 ?

To me it sounds is your UV source is very strong and hence you need the densisty from 3 copies to preserve the image ..?

Most safety glasses from diy stores etc are UV safe and are very cheap, so protect your eyes at least. ( check the packaging states UV protection)

Have a look at this recent thread and the detail in the UV box pdf - you might find some usefull pointers.

 

[jdraughn]

pcb making using xerox

Hi,


Well that sure is a bit of a tale, difficult to say in a simple word that 'this is wrong' as the whole process is a balancing act.

What does stand out, is using 3 copies, the chances of registering them when using fine tracks is slim - why 3 ?

To me it sounds is your UV source is very strong and hence you need the densisty from 3 copies to preserve the image ..?

Most safety glasses from diy stores etc are UV safe and are very cheap, so protect your eyes at least. ( check the packaging states UV protection)

Have a look at this recent thread and the detail in the UV box pdf - you might find some usefull pointers.

Thanks for your response. I printed a new version with slightly smaller traces and this time I just used one sheet instead of stacking them, and it came out perfect. I am going to try soldering it up in a day or two once I have time.

The one thing I found out about all this is the hardest part is drilling the holes. Easy to drill, time consuming and hard to get them all lined up perfectly. Just a pain if you have a ton of straight in a line pins like what are on sockets and headers. I can see where using the larger surface mount components would actually be easier and faster in the long run.

Is there any reason why all the tutorials I have read always say to use plastic containers for your solutions rather then plastic or glass?

 

[wp100]

all about pcb making

Hi,

Is there any reason why all the tutorials I have read always say to use plastic containers for your solutions

Well sounds like you haven't spilt any fluids yet !! - when you do you will soon realise how corrosive and staining they are to metals and fabrics, not to mention your skin, thats why you should always wear safety glasses and rubber gloves - one wrong splash and that could be your eyes gone !!
Be aware that the etchant you are using must be mixed in a certain way to prevent a violent bubbling reaction and that it also gives off heavy fumes so use outside etc.

this is the hardest part is drilling the holes.

Don't know what you are using to drill with ..? although it is possible to drill the holes with a hand held drill, its much better to use a bench stand and drill.
Many folk use the dremmel type drills but I get good enough results with a standard old fashioned bench pillar drill.

Using 0.8mm or 1mm drill bits should suit most holes on a typical pcb, normally your ic pads or round pads have a drill hole in them, are these holes being etched away properly ? - they act as a natural guide to the drill bit so something like a 40 pin ic socket should drill reasonably straight.