THE TROUBLE WITH INKJET
Don't get me wrong, the EPSON L3110 is a great printer. Unfortunately I have discovered that it is simply not accurate enough. The accuracy discrepancy reveals itself on the X-axis where a deviation of up to 0.2mm accumulates. That means for a regular 2.54mm pitch package like a DIP40, you will see the deviation when the time comes to drill the PCB on a CNC machine.
THE FIRST PROBLEM - PRINTING DARK ENOUGH
The problem of printing dark enough is an ongoing issue with the inkjet printer. It's the same struggle I have had with laser printers.
The UV light still gets through and over-exposure is a problem, so much so that I cannot get rid of the undercut on tracks and pads.
I have been using 3M CG6000 film for printing- this is the only film that actually comes close and might be good enough for the homebrew PCB. Aside from a little bit of occasional smudge, this film allows you to lay the ink on thick.
Now there's one problem- this product is discontinued!
The UV light still gets through and over-exposure is a problem, so much so that I cannot get rid of the undercut on tracks and pads.
I have been using 3M CG6000 film for printing- this is the only film that actually comes close and might be good enough for the homebrew PCB. Aside from a little bit of occasional smudge, this film allows you to lay the ink on thick.
Now there's one problem- this product is discontinued!
Enter SpeedBall
Now apparently the screen printing people use a type of milky white inkjet film to make film negatives for exposing silkscreens. Unfortunately that stuff is sold in large rolls and it has to be cut to size (A4) so the cost alone and the hassle is not worth it. However there's a product on the market by SpeedBall that provides you with A4 sheets of that stuff. That offered me a way to try before spending a couple of grand on a roll.
So I tried the SpeedBall product. Its an utter waste of money! I can't print half as dark as I could on the 3M film and if I try and go darker, I end up with a sheet full of smudged ink and my printer (and hands) and fucking everything full of black ink. After having to clean the printer, the dining room table and my hands and clothes I decided to once again look for someone, somewhere who could do imagesetting!
Finding an Imagesetting Service
Where I live, in Cape Town, there's nobody willing to do photoplots for me.
I tried WH Circuit again and was again given the same answer: NO!
I tried WH Circuit again and was again given the same answer: NO!
I asked around on the Slack group I am in, when one of the members mentioned that a company known to them uses photoplots for etching metal parts. So I contacted the company. While this was going on I sent of an email to another PCB manufacturer in Durban. Their response met me with surprise - they were willing to photoplot for me! Enter Master Circuits!
Shortly after this, Micro Etch put me in touch with a company known as Graystonite, and the very kind people there (Barbara) asked for a sample PDF artwork. They had a look at it and confirmed they could do the film negative and positive for me. Cool!
Encouraged by this and Master Circuits indicating their willingness, I prepared a set of Gerber Layers for them to photoplot for me. Here is what I sent them:
Shortly after this, Micro Etch put me in touch with a company known as Graystonite, and the very kind people there (Barbara) asked for a sample PDF artwork. They had a look at it and confirmed they could do the film negative and positive for me. Cool!
Encouraged by this and Master Circuits indicating their willingness, I prepared a set of Gerber Layers for them to photoplot for me. Here is what I sent them:
First Films
So with December fast approaching, and with optimism from the good news received from Master Circuits, I ordered a set of photoplots. I only sent two Gerber layers; specifically for the bottom copper layer and the solder mask layer. Within 3 days they were here, delivered to my door, and because I had forgotten about something- I never specified which side the film emulsion should be, they sent me two sets, one with emulsion side up, the other with emulsion side down. What a nice company that goes out of their way to help me and knew I probably forgot about the emulsion sides.
As can be seen from the above, they are not only super accurate, they're extremely opaque! Holding these up to the sunlight you can see hardly anything comes through. So I laminated the board with dry film and decided to give these new films a spin. This revealed another problem in my process which is detailed here in a separate article.
The Second Problem - Accuracy
Now I will readily admit I used a YouTube tutorial to adjust the e-steps on my CNC machine. The author of the video uses a piece of paper, a ruler, and a vernier caliper to calibrate his machine. I printed a Gerber layer on the L3110 and used that with my vernier, all of that is as accurate as the printer is unfortunately. When the photoplot was used to etch a real PCB which maintains all the accuracy of the photoplot, that's when I was able to deduce that the EPSON L3110 was not printing accurately enough. In fact the printout was out by a constant amount in the X-axis and a similarly constant amount in the Y-axis. I discovered this when trying to align the PCB to the machine. So the CNC was significantly inaccurate- I had to recalibrate the e-steps once again and get it done in an afternoon to get the PCB drilled and CNC routed.
In the end I got it done, as close as I could. It's still inaccurate by an amount of +0.08mm in the X-axis and a bit more in the Y-axis. I noticed it with the holes on the furtherest side of the PCB, as the width of the PCB goes in the X-axis, the holes begin to move off the pad centres. It's minor enough, so I tolerated it and got the PCB fabricated.
This is why proper phototools are so important. Without them I would not have been able to get the accuracy I needed to fabricate that PCB at home and I would not have been able to resolve technical problems with my processes. The fabrication of the PCB is covered in this article.
To improve the CNC machine's accuracy, which would probably see me take an exercise in the law of diminishing returns at some point, I will be using a dial gauge to further dial in the e-steps. This will be covered in another article soon!
In the end I got it done, as close as I could. It's still inaccurate by an amount of +0.08mm in the X-axis and a bit more in the Y-axis. I noticed it with the holes on the furtherest side of the PCB, as the width of the PCB goes in the X-axis, the holes begin to move off the pad centres. It's minor enough, so I tolerated it and got the PCB fabricated.
This is why proper phototools are so important. Without them I would not have been able to get the accuracy I needed to fabricate that PCB at home and I would not have been able to resolve technical problems with my processes. The fabrication of the PCB is covered in this article.
To improve the CNC machine's accuracy, which would probably see me take an exercise in the law of diminishing returns at some point, I will be using a dial gauge to further dial in the e-steps. This will be covered in another article soon!