Circuit Board Photo Resist Etching - Fail.. What went wrong?

Thread Starter

Qsilverrdc

Joined Aug 3, 2014
34
Hi, I hope someone can tell me what went wrong?
The image is of a board I etched. The photo resist is still on.
I had this in the Ferric CL bath for a very long time > 30 minutes.
It seems that it did not etch where small detail is.
Thoughts please?
Etch Board.jpg
Negative UV dry film photo resist.

Thanks, Richard
 
The good part is that it's fixable with a hobby knife or Dremel.

How did you hold the positive/negative onto the board during exposure? Glass plate? Nothing?

I've used polyester paper and a drop of water. The surface tension holds it really tight to the board.

On the etching side, the warming the Ferric Chloride a bit and agitating (moving it) the board can improve etching. Agitation is very important.

Etching can be done in a vertical tank with a bubbler.

A few theories - anyway.
 

Thread Starter

Qsilverrdc

Joined Aug 3, 2014
34
....How did you hold the positive/negative onto the board during exposure? Glass plate? Nothing?
UV lamp box -> glass->2 layer negative->clad board->foam->weight.
A lot of the board etched pretty quickly, but all around the traces and between were very slow.
Like the large areas etched clean in 5 minutes, another 40 minutes to etch to this point...
BTW this is my first board attempt.
I was trying laser toner transfer method, but could not get a complete transfer.
I changed to a dry film photo resist.
 

jpanhalt

Joined Jan 18, 2008
8,508
It's negative resist, so my first two suspects in order of probability (highest first) would be inadequate removal of the unexposed resist after the exposure and light creep between your two transparencies.

Judging from the appearance of the etched areas, your transparencies are plenty dark, so I would omit the duplicate transparencies. Although light creep is not my first choice, that is easy to do. Getting rid of the extra layer will allow more light through and your your exposure time might be changed, but I would not change that variable right off.

Addressing my first suspect, the exposed resist is pretty durable. I use a small sponge that is wet with developer to rub the plate and get unexposed resist from all the little nooks and crannies.

A third option is the viscosity of your ferric chloride (too viscous). I have only found that to be a factor when using saturated ferric chloride (about 42%) or ferric chloride that has become slightly muddy from multiple uses. Adding a little water will decrease the viscosity greatly.

In case I have confused you, my prime suspect is failure to adequately remove the unexposed resist. Rubbing with a sponge will help a lot. As for choice of sponge, I use a synthetic sponge with relatively large pores that is a light tan. When wet, it should feel soft and pliable and not abrasive or stiff.

John
 

Thread Starter

Qsilverrdc

Joined Aug 3, 2014
34
.....
Addressing my first suspect, the exposed resist is pretty durable. I use a small sponge that is wet with developer to rub the plate and get unexposed resist from all the little nooks and crannies.

In case I have confused you, my prime suspect is failure to adequately remove the unexposed resist. Rubbing with a sponge will help a lot. As for choice of sponge, I use a synthetic sponge with relatively large pores that is a light tan. When wet, it should feel soft and pliable and not abrasive or stiff.

John
I will try again, this week end some time.
I will try and develop the resist longer and use a sponge.
I will see if that works...
 

Lestraveled

Joined May 19, 2014
1,946
I mainly use dry film resist. Here is a you tube video of me going through the process.

I suspect three things that may have caused your problem.
- Excessive exposure time. Experiment with less exposure time.
- Something wrong with your developer. 10 g (1/2 tablespoon) of sodium carbonate (cleaning soda) in 1 liter of HOT water. The developer should never be room temperature.
- You need to almost scrub the developed board. (See video.)

There is a whole thread devoted to photographic PCB making in the "Completed Projects" section.

http://forum.allaboutcircuits.com/threads/project-photographic-methods-of-making-pcbs.107696/

Edit: corrected mg to g and amount of sodium carbonate in tablespoons.

@Qsilverrdc
 
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les:

That's pretty cool stuff, but you need a laminator and a toaster oven, I guess.

I did some boards in our work's photolithography lab. The process and materials I never wrote down. There was one or two bake steps and I applied the resist with a "spin coater". Which is basically a high speed motor with accelerate/deaccellerate controls and timing with a vacuum chuck.

At the time, we were rarely using the lab.

I would have liked to move the drill into the lithography lab. I spun the boards with my front/back alignment holes which messed up the resist along the edge. It was known as the "yellow room" because of the lighting.

Exposure was done using a "mask aligner" and I think I was limited to a 4" x 4" area of exposure at at time,
The "spin coater" limited the board size even more.

I just had really good results with translucent polyester paper, a laser printer and a drop of water to hold the polyester paper.

Prior to that I did "taped layouts" when there were no PCB layout software. I learned this at an HP Boy Scout Explorer's Post. I did a "sun exposed" board too.

Jameco has a kit of parts made by an ETO member to modify an Apache laminator to do multiple passes.



I have "no experience" with "toner transfer"
 

Lestraveled

Joined May 19, 2014
1,946
Hi Les, do you think that dry film resist holds any advantage over pre-sensitized boards? It seems somewhat cheaper but maybe not that much cheaper to have to bother with applying resist yourself.
- Dry film resist has a huge monetary advantage over pre-sensitized boards. It costs about a penny per square inch for the film, so a 4 X 6 board will cost you 24 cents to sensitize. Buying a pre-sensitized board of the same size will cost you around 7 dollars. I saved enough in the first six months to pay for my laminator.

- You can examine the pattern on the board before you put it in the etchant. If there is a flaw, you remove and re-sensitize. You are only out pennies worth of film. With pre-sensitized boards you would be out the full cost of the board.

@KeepItSImpleStupid
Toaster oven?? There is nothing to bake when you use dry film.
 

Thread Starter

Qsilverrdc

Joined Aug 3, 2014
34
I suspect three things that may have caused your problem.
- Excessive exposure time. Experiment with less exposure time.
- Something wrong with your developer. 10 mg (2 tablespoons) of sodium carbonate (cleaning soda) in 1 liter of HOT water. The developer should never be room temperature.
- You need to almost scrub the developed board. (See video.)
Thanks for the thoughts..
At this point I am convinced that this is an exposure - developer problem.
Not getting the resist off enough.
When I expose, the film comes out Purple (exposed) and clear.
I am going to first try a very short exposure, say 1/2 the current time, then a long like double.
- I think I have had too much developer powder in the water also.
-- Hot water rinse I have not been doing?
Every thing else I do.
5 boards attempted = (s)crap.
 

jpanhalt

Joined Jan 18, 2008
8,508
Thanks for the thoughts..
At this point I am convinced that this is an exposure - developer problem.
Not getting the resist off enough.
Agree, but...
- I think I have had too much developer powder in the water also.
How would having too much developer lead to less removal of resist? The more you have, the more you remove.

BTW, this recipe @Lestraveled has to be wrong:
Something wrong with your developer. 10 mg (2 tablespoons) of sodium carbonate (cleaning soda) in 1 liter of HOT water.
For starters, 2 tablespoons is a whole lot more than 10 mg. A table spoon is about 15 mL. I suspect that amount won't even dissolve in cold water easily.

@TS: In other words, the solution is close to saturation, which means, there can't be "too much" in solution. Less exposure might help, but that will probably be a waste of time. Try rubbing and warming the developer.

John
 

Lestraveled

Joined May 19, 2014
1,946
BTW, this recipe @Lestraveled has to be wrong:

For starters, 2 tablespoons is a whole lot more than 10 mg. .............
John, I went back an checked and you are correct. 1 tablespoon of sodium carbonate weights 15 g. I have been using too much developer. I will be making a few boards tonight or tomorrow. I'll report back.

The first time I mixed up developer I accidentally used 100 g of sodium Carbonate in 1 liter of water and it failed badly.

Edit: corrected mg to g
 
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jpanhalt

Joined Jan 18, 2008
8,508
@Lestraveled : I think you are confusing mg (milligram) with gram (approx 1/30 of an ounce).

Warning OT:
Several years ago, I checked on the price/availability of DuPont Riston. Basically, you had to buy large amounts, so I stayed with pre-coated ones from DigiKey. Unfortunately, that was a sole-source manufacturer and they are getting scarce. It is time to change. Where do you get your dry laminate (Riston?)?

John

Edit: 1% = 1 g/100 mL That is a lot less than 1/2 tablespoon. A teaspoon is usually considered 5 mL or very roughly 5 g. In chemistry, we typically used 5% to 10% sodium carbonate to extract phenolic compounds.
 
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