Laser drilled circuitboard with solder mask

[Keith Colson]

I made an FR4 printed circuit board today and it came out extremely well. I will quickly list the steps, but first here is a photo first!

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1) I lasered the top side of the PCB, this did three things. It cut the holes for the component legs, it cut the perimeter of the board out. It engraved the component designators. The laser did not go through the copper at all but that's ok.

2) I did a rough hacksaw cut down the two short sides of the board and it simply snapped out at that point. I also cleared the 3mm mounting holes with a drill bit

3) I double sided taped some 3mm acrylic to the engraving bed and lasered some 3mm dowel holes. I fitted 3mm dowels. This will hold the board for all the following steps and keep registration.

4) I painted the board with some "cheap as" black spray paint. I then lasered off the paint for the chemical resist. I made sure the leg holes were hidden so it would etch through the copper where the legs are.

5) Etching the board was really fast. I used some ammonia persulfate in a small zip lock bag. This way it could be really concentrated. I put the plastic bag in a sink full of hot water to accelerate it. Etched in only 10 minutes. The amazing bit afterwards was I simply rapped the board on the bench and all the carbon fell out of the holes leaving them clear.

5) I then painted the board again and then laser engraved just the pads with some line width expansion. I noticed its best to engrave the paint twice at 50% power and 100% speed rather than do it all at full power once.

I will definitely be doing more boards like this in the future. This was my first go so I am hoping it only gets better.

Cheers
Keith

[Keith Colson]

This leaves me with a question. I want to do some finer scale boards. What are the tricks with a VLS6 to get the absolute maximum resolution/detail when rastering? Does slowing the speed down help or are there some settings I can tweak?

Cheers
Keith

[Scott Shepherd]

Looks great Keith! On the quality issue, what are your settings? The highest resolution is Image Density of 7. What image density are you running?

[Keith Colson]

Thanks Scott

I looked at my quality settings which was set for 5 on this print which was adequate for this low detail job. I will turn it up to 7 and do some tests. So that's pretty much the only setting I can play with? I am wondering what the minimum line/space I can do on this machine is. With a 0.4mm laser line width I am guessing that restricts me to 0.4mm line space unless the machine does some tricky interpolation. I guess I have to generate some test patterns and get the microscope out and see what actually happens. Having a stereo zoom microscope is comiing in really handy. I purchased it just 3 months ago when I got the laser cutter.

Cheers
Keith

[Keith Colson]

I popped downstairs to the lab and did a track and space test at quality 7. Here is a picture of the artwork and results. I did concentric circles with equal line width and line spacing from 0.2mm to 0.5mm. The lines came out much thinner than the spacing. see photo. The black and white is a Corel screenshot.

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Is this just the limits of the machine or I can tweak the sliders below?
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I assembled the board. The solder-mask made for a clean easy solder and all the components fit nicely in the holes, now if could just make the board work! I think I need to tweak a couple of component values.
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Cheers
Keith

[Scott Shepherd]

What focal length lens are you using?

[Keith Colson]

Great point! my lens is a 2.0, I wonder if the 1.5 would be a big improvement with out having to spend the big bucks on the HDFO? I can't seem to find a matrix showing spot size versus focal length. My manual says the 1.5 has a spot size of 0.076mm which would be a massive improvement over my ~0.4mm spot.

Cheers
Keith

[Scott Shepherd]

Great point! my lens is a 2.0, I wonder if the 1.5 would be a big improvement with out having to spend the big bucks on the HDFO? I can't seem to find a matrix showing spot size versus focal length. My manual says the 1.5 has a spot size of 0.076mm which would be a massive improvement over my ~0.4mm spot.

Cheers
Keith

If your spot size is .4, then your focus tool is wrong. If you are using autofocus, you need to calibrate it through the control panel.

The spot size should be .15mm on a 2" lens. I suspect if you're seeing a .4mm spot size your focus is off. Try running it in .2mm increments up and down until it's the smallest just to test things. The Universal loses it's z-orgin really easily (like turning the machine off), so you have to constantly keep it in check. It doesn't move much, but over several weeks of turning on and off, it can easily find itself off by .5mm.

[Keith Colson]

Hi Scott

I had a play with the focus and the results didn't improve. I had a fiddle with all the adjustments and one had a massive change in results. When I adjusted the "tuning" the image came into focus. You can see the results in the picture below. If I had to guess the tuning has a horizontal backlash trim in it that accounts for belt stretch etc. I did the test on MDF so its not as clear is it could be but I did that for speed.

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Is there a document that explains all the parameters clearly? e.g. contrast definition density and tuning. There must be an optimum setting for all of these when you are doing black and white lines. Anyway I can get 0.2mm track and space now which is probably good enough. Learning how these machines work is quite interesting.

Cheers
Keith

[Brian R Cain]

Hi Keith, the user guide explains what those parameters adjust. If you didn't get a copy with the machine you can download one here: http://www.engraversnetwork.com/files/VLS_Platform_User_Guide.pdf

As you suggest, the Tuning feature compensates for backlash. The simplest way to set it up is to engrave a rectangle at image density 1 and look at the edges under a magnifier. Adjust the tuning until the lines are aligned. You need to do this at the speed you intend to run the job.

[Scott Shepherd]

Oh my, you're tuning was set to 0? Yeah, that's going to be an issue. 10 is pretty high, never seen that high on our machine, but you're looking under a microscope so maybe it's the spot that gives you the best results. There are two places to tune, one is in the diagnostic page where you adjust the tuning for the machine and then there's the one in material setting. I'd start by running the tuning wizard from diagnostics first, and then start with the material tuning. Once you find that spot, most materials will be within 1 or 2 of that setting.

Like Brian mentioned, there's a very good explanation of how to "tune" a material in the manual. It takes you step by step through it. Takes a few minutes, but once you dial that material in, it's set. First time through, it'll take you 30-45 minutes. As you do it more and more, you'll be able to do it in a couple of minutes.

[Ian Johnson]

After you cut the PCB out and placed it onto the dowels, how did you register the jig with the image?

I'd like to try this, but my boards are usually 2-layer. I suppose I can do a pass where I acid etch just the holes, then drill them while rastering the tracks.

[Keith Colson]

Hi Ian

To keep the registration you just print any Corel "layers" in the same place you printed your dowels. You can either print individual layers in Corel or you can print all layers and use the skip option on the laser cutter to select which layer gets printed. I did the later so I only had one print with about 4 or 5 layers. I had some really nice translucent green lacquer made up in a spray for my next board so it will look like the real thing.

Cheers
Keith