Incomplete PCB Etching

Thread Starter

Overlord_Manny

Joined Dec 18, 2017
5
20171216_233741.jpg 20171216_233745.jpg 20171216_173122.jpg 20171217_103905.jpg Could anyone explain what may be happening with my etching. It seems like maybe the unexposed photoresist is simply not washing away.
Here are some details of my process.

I'm using single sided boards. I scrub the copper clad board with a scotch bite pad and dish soap. I thoroughly rinse and dry with kimwipes. I then store them face down on a kimwipe in my work area to prevent dust from settling on the surface.

In a dark room (using a bug light), I apply negative resist film to the copper board and slowly peal the layer back. I typically can get no air bubbles if I take my time. I then laminate the resist on the board with an amazon branded laminator. I'll pass it through 3 times at full heat.

I then expose the board for 7 minutes with the circuit layout I've exported from eagle. I use two laser jet transparency printouts doubled up. I ensure that my image is as clean as possible, if not I print another set. I check the exposed board under the bug light if I see blurriness I remove the resist and start over.

If the exposure looks good I peel off the protective film on the exposed resist and submerge the board in a solution of hot tap water and about a tablespoon of sodium carbonate. This causes the unexposed resist to turn milky. Using an acid brush I gently wipe the board until I can no longer see the milky film on the board. This usually takes only a couple of minutes, but seems to be where my problem is. Smaller traces, thermal ground plane holes, etc are very hard to see under the bug light and don't seem to like to give up the emulsion. I have tried letting the board sit in the sodium carbonate bath longer and also being more aggressive with the brush in these ares but neither have helped.

I rinse the board in tap water after the sodium carbonate bath. If I see obvious signs of the emulsion not being removed then I just remove the resist and start over, in most cases it's hard to tell though. I then submerge the board in ferric chloride, heated enough to slightly steam, and use an acid brush to move the solution over the board. This usually only takes about 8-10 minutes. I rinse in tap water after and remove the exposed resist using about a tablespoon of sodium hydroxide in a Pyrex dish of water.

In most places I get a great etch, but some places it's like the emulsion would not come off. I was using 10mil traces and 10 mil clearance, but the clearance was not etching totally. It was inconsistent though. In the same place on one board where there was no etch, the next board would etch there but not somewhere else.

I eventually changed my clearance to 20mil and got a good etch but I'd like to get 10 mil to work if possible.

20171216_233741.jpg 20171216_233745.jpg 20171216_173122.jpg20171217_103905.jpg
 
Last edited:

Robin Mitchell

Joined Oct 25, 2009
821
Hi,

What you got there is a development problem. This is being caused by one of two things
  • UV leaking - Your mask is not close enough to your substrate and is resulting in UV undercutting of the traces. Try using a glass plate with bricks ontop of your PCB to close the gap between the PCB and the artwork
  • Incorrect development - You are not developing your PCBs for long enough - Try scrubbing or agitation
Mt credentials on the subject are that I use this stuff daily and can go down to 6 mil trace /clearance

All the best,
Robin
 

Thread Starter

Overlord_Manny

Joined Dec 18, 2017
5
Hi,

What you got there is a development problem. This is being caused by one of two things
  • UV leaking - Your mask is not close enough to your substrate and is resulting in UV undercutting of the traces. Try using a glass plate with bricks ontop of your PCB to close the gap between the PCB and the artwork
  • Incorrect development - You are not developing your PCBs for long enough - Try scrubbing or agitation
Mt credentials on the subject are that I use this stuff daily and can go down to 6 mil trace /clearance

All the best,
Robin
At first I tried putting a sheet of glass and a heavy programming book over it. My glass is picture frame glass and it seems to have a bit of flex. I though this was an issue so I swapped to using 2 sheets of glass with the transparency taped to one sheet with the printed side up and the board sandwiched between them. I also used spare boards on either side to make them flush and clipped it all together with some document clips. I think there should be little leakage that way, but I still had issues. I may try a different brand of photo resist and maybe a different dark room light, just in case I'm getting some minor exposure from the bug light. Here's a photo of my light box, I'm using UV lights.
20171126_144200[1].jpg

Here's the one that worked well. I didn't change my process for this one just the trace width. I realized I wanted my traces on the back of the board, so that's why the mirroring.

20171217_123018[1].jpg
 

Robin Mitchell

Joined Oct 25, 2009
821
Hi,

Different film will not solve this problem. You can tell that this is a film / PCB contact issue due to the local nature of the undeveloped areas. What you need to do is not use picture frame glass and instead invest in a glass chopping board (these are REALLY thick but ensure its clear and not matt). Then you need to use at least 10 ~ 20KG in weight but this will never produce perfect results. What you need is a vacuum seal that can suck all the air out and then presses onto the PCB with a load of force.

I also dont recommend LEDs for a UV source. Fluorescent bulbs are much better!

All the best,
Robin
 

Thread Starter

Overlord_Manny

Joined Dec 18, 2017
5
That much pressure 10kg is 22 lbs 20kg is 44 lbs? Does this have to be tempered glass for that weight? How is fluorescent light better? What does it do differently. Because I've heard the exact opposite, but little explanation as to why.
 

Robin Mitchell

Joined Oct 25, 2009
821
UV Fluorescent lights have a broader spectrum whereas LEDs are very narrow. This means that if your LEDs are shifted on the spectrum slightly or the UV sensitive ink is slightly different then the ink wont cure too well. Fluorescent essentially guarantee exposure.
 

philba

Joined Aug 17, 2017
959
As long as we are talking alternates - I used to do home etching of PCBs but chinese PCB production has gotten so cheap and fast that it doesn't make sense to me to continue at home.

With AllPCB, it costs $5.49 delivered for a 2 layer PCB up to 4"x4". Solder mask and silk screen legend. I got two boards today - one I sent off on the 14th - yes, that took 4 days. Though usually, it's more like a 6 or 7 days. It's just insane how fast, cheap and good it is.
 

Robin Mitchell

Joined Oct 25, 2009
821
@cmartinez Firstly, you are bang on about the randomness of the films (that can be an issue sometimes). Secondly, they do have a short shelf life but i have used them well over a year in storage
 
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