The technique is called "Contact Etch". Without a contentional 'etching tank' and just our novel discovery, you can cut your etching time down by a factor of 12!
We've been etching circuit boards the wrong way for 60+ years. Read up on how this simple trick came to be then try it for yourself. We think you'll be amazed at the difference!
This semi-revolutionary technique was discovered one day in early 2003 when we drove our car through one of those spray-only "no brush/no touch" automatic car wash places. As I'm sure you're all aware of the fact that these car washes do a minimal job at best to really clean a dirty car.
Anyway, the car went in dirty and came out almost as dirty. The only thing that came off was the top layer of some dirt and the top layer of dust! That's when inspiration struck asking the question, "Is this why PCB's take so long to etch?"
In the spray-only car wash, nothing touches the paint to break the surface tension, which is after all, what's holding the dirt to the surface of the paint in the first place. Compare this to the proper way to wash the car making physical contact with a soft sponge to break the surface tension (with or without the girls.)
Well folks, sad to say this but the same is true for etching a circuit board. So for the past 60+ years we've been putting our circuit boards through a "no-brush/no-touch" car wash!
No tank needed? Really?
Regardless of the type of etching 'tank' being used...
- The $0 "rocking tray method"
- The $99 "bubble tank"
- The $500 ~ $10,000 "spray tank"
"How can a simple technique be better than a super expensive "spray tank"?
...they are all terribly inefficient and they all miss the #1 objective, "break the surface tension to let fresh etchant at the copper". They all try real hard, but none of them do it the right way.
Whenever you etch a board, "surface tension" is holding fresh etchant at the microscopic point of contact on the surface of the copper. That's fine for about 10 seconds until this layer becomes totally saturated with copper ions so the etching action immediately starts grinding to a halt if it's allowed to sit there undisturbed. Ahh, but you're thinking if we rock the tray (humorous), or have air bubbles rising up past the board (getting funnier) or hit the board with a heavy spray action (like the drive-through car wash above - extremely funny for $500) THEN our agigation will keep the etching action going! Right? Sure, it does to a small degree. But let me ask you this, do you enjoy wanting 10-12 times longer than necessary? Sure these tanks with their various degrees of efficiency do eventually get the job done, but there is a much better approach. The fact remains, if you don't break the surface tension by physical contact, the copper-ion saturated etchant prevents fresh etchant from attacking the copper efficiently. This is the root of the problem. Now that you have a grasp on the problem, here's our take on the current methods followed by how we think it should be done.
Here's Our Quick Synopsys of the 3 Tanks Currently Used:
The zero-cost "rocking tray" which obviously holds the distinction of being the worst technique ever. Because the etchant is sloshing back and forth, the middle of the board generally doesn't get much agitation to speak of so the ends of the board seem to always etch before the middle so you end up with 'overcooked' ends that are highly undercut! This is a total worthless method of etching a board. I think we all agree on that. It's sometimes convenient in a rush and you can't beat 'free' for a container.
Second on the hit list is the common "bubble" tank". The most common and over-priced tank design blowing air bubbles for agitation from an aquarium pump! Let's get real... this is a dumb idea. Check it out... the heater is trying to heat the chemical and the air bubbles are constantly cooling the solution! I've seen better ideas out of a 6yr old child.At best it's only slightly better than the rocking tray. Even with bubble tanks, the highest point of the board always etches slower than the lowest point of the board (which is due to wake turbulence for the most part!)
If you use Ferric Chloride (FeCl) as your etchant, you have bursting air bubbles on the surface putting out a corrosive micro-atmosphere into your shop... the number one reason why businesses don't like FeCl as an etchant.
Most of these tank designs require 1-1/2 gallons of solution which gets pretty expensive. (Better designs are very thin like the one pictured here). Because of the problems of using FeCl, many users switch over to the much SLOWER acting echant like Sodium or Amonium Persulfate. Now we're really eating up time! A/P and S/P both suffer their own big problem of self-distruction! Persulphates are an oxidizer (vs. FeCl being an corrosive) and when a fresh batch is made by disolving crystals into water, it immediately begins eating itself up. A fresh working solution will be dead in about 30 days. Great, huh?! This is an improvement?
Lastly there is the truly expensive "spray etcher" at around $500 for an 'ok' one. This design is a bit better than the bubble tank, however, even with high powered pumps with faster acting FeCl, you are just up to the efficiency of a "no-touch/no-brush" car wash. WOW! We've now gone full circle and back to the worst type of efficient "dirt remover"! These units are obviously not cheap and designed more for the higher end user to be able to knock out several large panels of boards in one shot. Any way you look at it, that's a lot of money to shell out when the efficiency is relatively poor. In the final analysis, all 3 of these etching tanks do little more than move etchant around in the tank.
HERE'S HOW TO MAKE A FAST ETCH!
The board etching problem is completely solved with a sponge, dispoable gloves and an ounce or two of Ferric Chloride etchant. (That's 1 or 2 OUNCES!) There is no need for an etching tank... period! To prove to you how fast this works, go get a box of cheap disposable gloves and a soft sponge from your local drug store and a small bottle of liquid Ferric Chloride from Radio Shack (or electronic supplier who carries "MG Chemicals" brand.)
I know some of you are shaking you head at the very thought of using that nasty, clothes staining, stinky, corrosive atmosphere generating Ferric Chloride, however, keep in mind that we're only using a micro amount that literally stays in the sponge! There is no large open quantity of etchant. Also, there is such a small amount in use, there is no noticeable odor or corrosive "atmosphere" to speak of. It's all round simple, fast and an "etching epiphany".
Technique #1: "Sponge"
First up for demonstration purposes, is our "Quick 'n Dirty" method.
Granted it's not very sexy but it's extremely efficient and faster than any other method and will demonstrate just how powerful "contact etch" really is.
By merely wiping lightly over the board without overlapping strokes, you can etch a 1/2oz copper board in about 45 seconds (and a 1oz board in about 2 minutes). Simply throw on some gloves, pour about an ounce or two of Ferric Chloride into the sponge and you're ready to etch. Use a thin sponge so the etchant stays closer to the wiping surface. The etchant will stay suspended in the sponge so you shouldn't get any drips. Of course the larger the board the more etchant you will need. The sponge is going to get very black as the copper is etched off. If you are on a sewer system, "heavy metal" is normally treated. Check with your particluar county water treatment facility and check this for your area so you can wash out the sponge when done, however, septic systems will put the heavy metal into the ground which you must never do. You can optionally put the sponge in a Zip-Lock bag and dispose of at any gas station that does oil changes since they have a "Hazardous Waste" section.
Technique #2: "Brayer"
The ultimte "throw away" etching tank!
This basic idea was submitted by Chuck Bagg (firstname.lastname@example.org). It's kind of ironic that his idea just happens to be his last name, and that being to use a bag to hold the the etchant and board for a cleaner technique with almost the same etching speed.
We modifed his idea to include a rubber roller like that pictured here, called a "brayer". This method is a big jump over the first technique above in that it is a much cleaner method - no gloves or sponge needed. Materials needed: two Ziploc bags (preferably the "Easy Zipper" type), piece of sand paper (about 220 grit) and a hand-held brayer. Simply put one Ziploc bag inside the other (for leak-proof security). Take your board and while holding it on a 45º angle, run it over the sandpaper on all edges and both sides if a double-sided board. The objective here is to round-off sharp copper edges to further prevent any leakage through the bags. Now insert your board (single or double-sided) followed by pouring in an ounce or two of Ferric Chloride. Close each bag letting as much air out as possible.
Using a standard 4" SOFT RUBBER brayer (not the hard rubber type), roll it over the board with good pressure, back and forth. This roller idea to squish-out the etchant sitting on top of the board. If the board is double-sided, flip the bag over about every 30 seconds. Key to this technique is to be consisant in the rolling with moderate pressure to squish-out the etchant. This method is just a tad slower than the physical wiping of the board, but it is still much faster than any of the conventionl etching tanks on the market!
If you can do this procedure ontop of a light table, you'll be able to see clearly when the board is done. For disposal, we like to put a few paper towel squares in the bag to soak up and suspend the etchant from sloshing around as it awaits proper disposal. You could easily stack these in a box until you're ready to dispose of all of them in one shot. Because you are using very little etchant, that which was poured into the bag is considered pretty dead by the time you are done. The "brayer" is available at most all "Art's 'n Crafts" stores. There are also a lot of resellers of this SpeedBall brand product line. You might want to try Google or Amazon.
An important area we haven't addressed
Whenever you etch a board, the etchant starts straight down, however, shortly there after the etch has gone below the "mask" the etchant starts eating sideways attacking the "wall" of the etch. Cross-sectionally is would look like a chopped off traingle. The angles of the triangleare proportional to the length of time a board is allowed to be immersed in the etchant. By using our "Contact Etch" method, you are first of all getting a very fast etch so there's not much time for the developing wall to be damaged.
Undercutting It is further reduced by the fact that you are wiping over the board so the etchant is being directed straight down. In effect, we're allowing a micro buildup of saturated etchant to sit just under the mask (in the case of a much longer etching of 1 or even 2oz copper-clad board). Both of these principles, "short exposure time" and "perpendicular etchant exposure" reduces undercutting on our 1/2oz copper board to near zero. What that means is you can image traces much thinner than we were ever able to do before. This is how we can reliably etch traces as fine as .005" using a standard 1,200dpi laser printer.
SUPER CHARGING "Ferric Chloride"
A new additive to help this etchant be more effective!
Feric chloride, as it bites away at the copper, leaves a residue of iron, which stops or slows the process. Wiping over the board "brushes" this out and keeps the process going. Check out the "Edinburgh Etch" which iscitric acid added to Ferric Chloride. The citric acid keeps the etching surface clean and you may not need as much wiping/agitation. You can Google and find lots of info on it, but here is a good page for an overview, www.polymetaal.nl.
Rick Blanchard, Youngsville, NM