Magnetic Drive Etchant Pump

The classic way to pump corrosives is with a "roller" pump with Tygon tubing.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

| 1962 | America: Land of the Free, Because of the Brave

AKA peristaltic pump.

Best regards, Spehro Pefhany

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speff@interlog.com             Info for manufacturers: http://www.trexon.com
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That sounds familiar...

I think that's the pump often seen on medical dramas. It's used to pump blood.

D from BC

Yep, and NaK and other such fun stuff ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave

Is this for agitating the etchant while etching? Pumping is only one way to agitate, others are stirring (titanium stirring rod with plastic vanes on the end) and bubbling (hook an air pump to a perforated nest-of- tubes on the bottom of the tank).

If your pump has enough pressure, you can spray-etch (park the boards over a sump, and spray directly onto the surface, let etchant drip off). That causes some mist, though, and EVERYTHING in your workroom will rust. Spray-etching is not an application for a 'mini' pump. With suitable (titanium) heaters to keep the etchant at the right temperature, it IS quite efficient, though.

It's for agitation and flow... I'd like to try to get a river of etchant flowing down the board. I think I read somewhere that there's a risk of over-etch. However, I'm not making brain implants.

Spraying would be more pro but I don't think I need my boards that good.. A little rework is tolerable.

I'm interested in using minimal amounts of etchant that way I don't have to play with a tank. It's safer and quicker to handle small amounts of etchant..

I believe a tank is required for a bubbler set up.

The flow(fountain) idea I have just fills the tray enough to prevent the pump intake pipe from sucking air.

~^-----[pump] D from BC

This outfit makes lots of different kinds and are fairly low cost:

Cool... This pump is 4.5" x about 4" wide.

3 gpm @ 0 head feet. D from BC

The problem will be nonuniform etch. Wherever the flow thins the boundary layer, etching will speed up. But you will have a hard time getting a fairly uniform boundary layer everywhere. Little corners will be more stagnant, and traces that run along the flow will have lots thinner boundary layers than traces than run across the flow. Spray etchers try to overcome this by producing a violent, random impact pattern that has as uniform an average density as possible. But they are never perfect, especially considering the effect of run off drool and hanging drops.

You might try convection etching. The board is supported right on the surface of a shallow puddle of etchant, etching taking place on the under side, only. As the copper loads the etchant, the density of the etchant increases and that loaded etchant falls to the bottom and fresh, lower density etchant rises to make contact with the boundary layer. No additional agitation is needed or is helpful. Usually I can find a plate or saucer that will support the four corners of the board over a quarter inch or so deep puddle. On a few occasions, I have taped flexible copper laminates to a slab of polycarbonate (sold in building supply stores as safety glass for screen doors), all around the edges, to etch this way. It provides enough buoyancy that the whole assembly floats like a raft.

The trick is to get the board in contact with the surface of the puddle without trapping any bubbles between copper and etchant. I dunk the board in etchant to wet it and brush etchant on any spots that seem to resist wetting, till the whole surface is coated. Then I hinge the board very slowly onto the surface of the puddle so the air is expelled by the surface tension of the etchant. After a couple minutes, I slowly lift one edge and have a quick look, to make sure the whole surface is etching. Then repeat the slow hinging dunk.

The convection process is so efficient at delivering fresh etchant to the ceiling of the cell that it etches faster in the upward direction than it does sideways, because the sides of the etch slots and holes are coated with a sliding boundary layer of used etchant. This effect produces almost vertical walls and can etch slots as narrow as the thickness of the copper.

If your board material is translucent, you can watch the etch finish through the back side. Otherwise, every 5 minutes or so, you have to hinge the board back and take a look. The process is gracefully slow, but surprisingly uniform. This process also produces minimal splashing and mist formation, so is more suitable to kitchen table operation than things that make fine droplets that corrode everything within a couple meters of the etching.

Doh! Long ago and new to etching, I used to attach the PCB to long pieces of masking tape and then suspend the PCB upsidedown near the surface of the etchant. I only did that because every other way didn't work as well without agitation.

At present...I use a foam brush and pamper the PCB with fresh ferric chloride.

I like the float idea... Perhaps sticking styrofoam on the PCB with double sided tape might work to. D from BC

D from BC wrote: (snip)

It doesn't take all that much. Ferric chloride etchant is so dense that single sided boards almost float. Like I said, polycarbonate has a low enough density to float a board the same size and thickness, especially if there is a little bit of air between them. The trouble with Styrofoam is that it has lots of cracks and pores, making it soak up etchant and making a good tape seal difficult. I like a rectangle of polycarbonate just a little larger than the board, so I can scotch tape the board flat against it. And I can watch the etch finish very clearly through it.

All that said, choosing the right size plate, bowl or saucer that holds up the corners at the right height, eliminates the need for flotation. I just start light and add etchant till the board just rests on the top surface.

Ok..just thinking of an alternate to polycarb in case I don't have it. I do have pink insulation styrofoam...It's not like that white packaging styrofoam.

Is this agitation by reaction unique to ferric chloride?.. Does Ammonium P do the same?

D from BC

Just want to add my very positive experience with "convection etching". I saw the technique somewhere on the net and gave it a try. It works very well and I found it reduced the etching time considerably. As I recall, I put a little handle in the middle of my pcb using masking tape, then just floated the board on the etchant. I was very impressed with this method. Mike

A method I used with good results when I made my own boards long long ago was the heated rocking tray board etcher.

A pyrex baking tray was temperature controlled with a heater and thermocouple glued to the bottom, using a cheap temp controller from Omega. Something over 100 F is required for decent etch time with ferric chloride, ISTR using 110 or 120. The tray was glued to a piece of plywood with pins in the ends supported in grooved blocks on a base so it could rock, and a small motor with eccentric roller used to rock the tray at about 1 rock per second, perhaps 3/8" peak to peak at the edge of the tray. You want a wave of etchant to slosh neatly from side to side with no splashing so some experimentation on amplitude and speed may be required. Board supports were cut from rubber erasers and glued to the bottom of the tray to hold the board at about

1/4" from and parallel to the bottom of the tray. 1/2" of ferric chloride etchant was preheated (a glass cover reduces evaporation losses), the board placed on the supports, and the tray set to rocking for something on the order of 5 min for double sided 1 oz copper. With the board height set just right both sides etch at the same rate; otherwise flip it over mid-etch.

This method is probably not suited for fine pitch boards, but for occasional use in a small shop I like it better than the equally effective bubbler circulation in a vertical tank, because it does not tend to fill your shop with etchant mist. And you can use the heated tray to start seedlings in the early spring :-).

Glen

I have no experience with Ammonium P but I suspect that when it is loaded with copper it is denser than when it is fresh. Does it produce any gas when it reacts with copper. That would spoil the convection method.

Years ago I build a pump completely made out of PVC except for the motor (which is far away from the etchant anyway).

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I'm guessing those magnetic drive pumps are the centrifugal type. It's tempting to DIY.

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I'll guess at the design.. Rotor has embedded neodymium magnets. The motor head has matching magnetic positions (not shown). All parts can be laser cut acrylic. Dunno about an easy rotor blade shape. Paddles? Curved? I'm imagining that the magnetic pull on the rotor could cause friction. But I suspect there's a way to get the rotor to "ride" on a fluid layer for just about 0 friction..

Heyyyy....I'm supposed to be doing electronics design! :( D from BC

Bummer...I can't remember.. Last time I used Ammonium P I had both copper and aluminum in the etchant..(It wasn't a normal PCB project.) Dunno if one or both metals made little bubbles or if the bubbles were just trapped air. D from BC

[...]

Almost float?. I've never had one sink. Surface tension keeps 'em aloft. I put 'em in dry having pre popped any stray floaters with a cocktail stick. Loop of tape on the back for launching/retrieving and preheat the etchant in the microwave. (40degC about 4 minutes etch). Yes. Floating is the best method. (moreso for the occasional PCB)