Category: Electronics Workbench

The next step in the process for this board: toner transfer masking.

What you see here follows the basic process given by the folks at Pulsar, so go there for the instructions & supplies. The overall flow is:

• Print the PCB trace pattern on the special paper
• Align to the circuit board
• Tape in place
• Fuse toner to PCB
• Apply green sealant film
• Etch
• Repeat for the other side

Some tricks:

An Eagle CAM file (found there in Useful Stuff) creates Postscript files for the top & bottom copper (and the silkscreen, although I don’t use that).

I load those PS files in The GIMP at 600 dpi with mild antialiasing, crop (autoshrink!), combine into a single image, then print on a single sheet of paper using an HP Laserjet 1200 (obsolete, but pretty nearly any laser printer should work).

Turn off all the toner saving features; you want a really dark image with plenty of toner!

I run a sheet of paper through to find out exactly where the images will wind up, then tape the transfer paper atop the images (shiny side up!) using laser address labels (because they can take the heat) on the first-to-enter end of the transfer paper.

Vital step: run that whole assembly through the printer to print a blank page. This cooks the moisture out of the transfer paper and pre-shrinks it for the next step. This might not matter for very small circuit boards, but if you’re doing anything over an inch or so, it makes a big difference.

Run it through the printer again to print the trace patterns. They should wind up exactly in the middle of the transfer papers, although you’ll be surprised at how far off they can be from the patterns on the sheet underneath. Lasers have great dot resolution, but are not particularly accurate at locating the paper relative to the printing drum. That doesn’t matter for ordinary documents, but be sure you leave more margin on the transfer paper around your patterns than you think necessary.

Use the printer’s manual-feed feature to queue up all three prints at once. My printer is in the basement laboratory and my comfy chair is upstairs, so that saves several trips up & down the stairs. Not that I can’t use the exercise, mind you, but it’s the principle of the thing.

Cut the transfer paper off the backing sheet. Don’t bother trying to un-stick the labels; they’re fused solid. Stick another label on the back side of the transfer paper along shortest side.

Alignment trick: now lay the paper pattern-side-up on a light table (I use an old fluorescent fixture with a frosted-glass lens), lay the board atop it, and adjust for best hole alignment over the whole board. The trace patterns on the paper form very nice bright spots that shine up through the holes in the circuit board. You can do it with the board on the bottom, but this way works better.

You’ll be dismayed at how far off some of the holes are, but you should get within perhaps 10 mils all across the board. Squish the board down on the sticky side of the label and fold the label over the top of the board. That anchors the paper to the board and, because the label is fairly wide, keeps the paper from twisting relative to the board.

Flip it over, verify that the holes still line up by looking through the paper, then apply labels to the sides to hold the transfer paper flat and in alignment. I’ve tried it without the side attachments and the result is, ahem, not quite as good. You can see the label residue on the front side in the photo above; obvously, you’ll be cleaning the gunk off before doing that side.

I’ve tried the clothes-iron technique with little success, so I have one of the heated roller fusers. Works pretty well, although it requires some fiddling to get the proper combination of heat and spacing.

Add water, let the paper soften, peel it off.

Run it through the fuser again with the green sealer film. I get better results with a layer of ordinary paper atop the green film, perhaps because that prevents the film from touching anything inside the fuser. Without the paper, the film sometimes transfers lengthwise scratches to the toner.

Peel off the film and touch up any imperfections with an Ultra-Fine-Point Sharpie; I use orange to make the corrections easy to see against the green background. Those are the highly visible ugly marks on the bottom mask.

Having masked one side, etch it. Then you mask the other side and etch that. Don’t try to get clever and do both sides at once; it doesn’t work. Ask me how I know.

Next step: etching. More on that later…

Suggestions: after you etch the first side, leave the mask in place to protect the copper. When you run the board through the fuser, add a sheet of paper on the masked side to keep the film and toner from coming off on the rollers.

Memo to self: always pre-shrink the transfer paper.

While writing a column for Circuit Cellar, I managed to utterly botch the choice of a boost converter inductor. The inductor (the rightmost one in the lineup) was a common-mode choke from a power supply, so I figured it had enough meat for 300 mA of boost current.

Bzzzzt! Wrong choice!

So it’s time to document how to measure this stuff. The intent is to plot a BH curve: core flux density B versus magnetizing force H.

Quick inductor tutorial using (ptooie) Imperial units and avoiding Greek letters for simplicity:

Magnetomotive force mmf = 0.4 * pi * (N = turns) * (I = current)

induced voltage e = – N * (d phi / d t) * (10^-8) where phi = flux linking the N turns

Magnetizing force H = mmf / (MPL = mean path length around core)

Flux density B = phi / (Ac = effective core area)

also B = (mu = permeability) * H

You might think that B & H are lashed linearly together, but mu is most savagely nonlinear. It depends on H, temperature, core material, and a bunch of other stuff you don’t want to know about. So, by and large, plotting B against H shows you how mu varies: it’s the slope of the curve at any point. Vertical slope = high mu (good), horizontal slope = zero mu (bad).

Stirring all that together, you get the Fundamental Transformer Equation:

E in RMS volts = 4 * F * f * N * Ac * B * (10^-8)

F = form factor = rms / average

• sine = 1.11
• bipolar square = 1.00
• unipolar square = 1.41

So…

• H is proportional to mmf and thus to primary current
• B is proportional to phi and thus the integral of secondary voltage

Sounds scary, but primary current & secondary voltage are easy to measure.

The basic lashup goes a little something like this: stick a sampling resistor in the primary, run the secondary through an RC integrator, feed both into an oscilloscope that can do XY plotting, drive the primary from a Variac, turn the knob, and watch at the results. You might want a bit more finesse than this, but, eh, it works.

The primary voltage will be relatively low, so plug a 12 VAC wall wart into the Variac. This gives you galvanic isolation from the power line, finer control over the primary voltage (more of a full turn on the Variac), and will prevent you from killing yourself or burning out your basement laboratory. You want a fairly husky wart if you’re measuring husky inductors. Remember: this is an AC measurement, so you want an AC wall wart, not one with a nice filtered DC output.

The resistance of the sampling resistor should be much less than the inductive reactance of the coil. We’ll measure it at 60 Hz because that’s easy, so for a coil of inductance L, the reactance at 60 Hz is 2 * pi * 60 * L. That simplifies to 377 * L, so a 1 mH coil has about 400 m-ohm reactance. I have a nearly infinite stash of 100 m-ohm sandbox power resistors, so that’s what we’ll use; you’d want less, but this is quick & easy.

If the inductor doesn’t have a secondary, poke some magnet wire through the core. Use some simple number of turns and remember that number, just in case you want to calibrate the results.

The time constant of the RC integrator must be a lot bigger than the frequency you’re integrating. For a 60 Hz signal, you want maybe a 6 Hz integrator: 60 Hz -> 17 ms, so pick 200 ms. Having a nice 1 uF film cap in my heap, the resistor works out to (200×10^-3) / (1×10^-6) = 200 k. Anything in that range will work fine. A larger cap gives you a smaller resistor and more signal to the scope.

Wired it up about like that and here’s what happens for the inductor I picked

The calibration of H is 100 mV / amp (from the 100 m-ohm resistor) and the scope display is thus 2 A/div. The calibration for B is whatever the secondary winding provides, scaled by 1/RC from the integrator.

For this coil, the value of mu is essentially infinite (B rises vertically for changes in H), but the core saturates at very small values of H (and thus primary current). It was a common-mode choke for a power supply, so that’s exactly what you want: high inductance for very small currents and no need for much in the way of core flux because the opposing line currents in the windings cancel.

I harvested some inductors from a bunch of dead power supplies in my heap and measured them similarly, using an existing winding as the secondary wherever possible. The results look like this:

The area inside the loop is proportional to the core power loss, so it’s obvious that some inductors are better than others.

If you’re going to do this for real, you’ll want actual calibrations for both axes. You can work the numbers out from the equations and read real values for both B and H directly from the ‘scope graticule. Pretty slick, mmm?

The primary current can get surprisingly high, so turn the Variac knob up, make your measurement, and turn it back down again. The sampling resistor and the core can get scary hot if you let ’em cook while pondering the ‘scope display. A storage scope is most helpful, which comes pretty much for free with digital scopes these days.

I wrote about all this in more detail in the context of a resistance soldering unit in my February 08 Circuit Cellar column, so I should have known better.

Memo to self: measure first, write later.

And remember to turn off the WordPress Smiley option (Dashboard → Settings → Writing) so equations don’t get cutsy artwork instead of numbers. Feh!