My first SMT stencil

Thread Starter

cmartinez

Joined Jan 17, 2007
7,404
Well, @Wolframore, a deal's a deal. I promised I'd open a new thread about this subject, and here it is. I hope it's worth your while.

You mentioned you've seen people make SMT stencils out of soda cans, and so I looked around using those keywords. And sure enough, I found a youtube video describing that technique and more.

  • The video recommends using aluminum from a soda can (I'd rather use aluminum from a beer can, though :p ) What I did is I measured the thickness of a soda can's aluminum, and it turned out to be 0.004". After a little browsing around, I found that Amazon sells #36 gauge aluminum foil (0.005" thick) which is normally used for arts and crafts at only $13.00 bucks apiece. Using this aluminum instead has the advantage of not having to remove the paint and plastic layers that soda cans normally have.
  • I used a mix of muriatic acid and peroxide of one to three parts, just as the video suggested, but it took a lot longer than the couple of minutes that the video mentioned for the acid to completely eat through the foil... something in the order of 10 minutes or so. Also, I had to constantly move the piece around inside the container I used because the bubbles produced by the reaction tended to stick to the foil's surface. The bubbles interfered by preventing the reaction from continuing in an efficient manner. @jpanhalt, is there a particular reason for using said mix? Wouldn't using only muriatic acid be enough? Or is the peroxide there to prevent too fast a reaction?
  • On a final note, I had some trouble with my laser printer. The paper I normally use with my transfer technique began to get stuck in the printer because its plastic film melted as it went through the fuser roll. It's very strange because it was working just fine... and all of a sudden this small disaster happened... So what I did is I began to experiment with different paper types. I even tried using the adhesive vinyl film (Con-Tact brand) recommended in the youtube video, but it didn't work as expected because the plastic gets distorted as it goes through the printer. In the end, I discovered that the perfect paper for this application is the vinyl film's back surface paper itself! ... it's a very thin sort of paper, so feeding it into the printer was a little tricky, but it all worked out when I decided to adhere a piece of masking tape to its front edge. That helped it remain straight and flat as it went into the printer.

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The printed stencil ready to be press transferred onto the aluminum foil.



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A successful transfer. With a few spotty imperfections here and there which I protected using a sharpie pen. It seems that transferring onto aluminum is not as easy as transferring onto copper.




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Bubbling reaction ...




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The stencil right after being etched.




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Looking nice and clean.




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I think I used too much soldering paste ... next time I won't be so wasteful.




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This is probably the neatest, cleanest-looking SMT PCB I've ever assembled!

 
Last edited:

jpanhalt

Joined Jan 18, 2008
11,088
@cmartinez
I have very little experience etching aluminum. I have read about the recipe you mention, and muratic acid alone will also etch it. However, you may get more "smut" (black) deposits with the pure acid. The peroxide probably aids in complete oxidation. Ferric chloride will also work.

If you search on etching aluminum, you will find some sources recommending using alkali (caustic soda, K or Na hydroxide). Aluminum is amphoteric, that is it reacts with or acts like both and acid. The problem with a strong alkali is that your resist may be removed too.

Very nice work. Thanks for sharing.
 

Wolframore

Joined Jan 21, 2019
2,342
Hey nice job @cmartinez! I’m sure you will perfect it. That’s a lot of paste. I use a tiny metal spatula and before that a piece of old credit card. it’s amazing how far that stuff goes. Ive made hundreds of boards with and just now buying a new tiny container.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,404
Just tried to etch a stencil, but this time using my FeCl3 etching tank... I think I over did it...

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The great disadvantage of FeCl against muriatic acid is its lack of transparency, one cannot so easily see how fast the reaction is taking place.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,404
So I needed to make another stencil for one of my projects. And remembering the previous process, I used muriatic acid straight from the bottle to etch it, and failed miserably. The acid ate too quickly the material on the large orifices, and too slowly on the small ones. The result was that it also ate through the borders of the large orifices, enlarging them so much so as to make them merge with neighboring ones, spoiling the piece.

So there I go again, and print transfer another one, but this time I use a 1 to 3 solution of muriatic acid and hydrogen peroxide... it worked out well enough in the end, but it took almost 1-1/2 hours to etch the stencil!

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What I learned is that the problem most likely has to do with the bubbles formed by the gases produced during the reaction. Said bubbles tend to stick to the orifices being etched, practically stopping the reaction at those spots. Another thing is that maybe the solution's surface tension makes it harder for it to flow into the smaller orifices. I also observed that some small orifices took much longer than others, even though they were of the exact same size and shape. Maybe that had to do with impurities on the aluminum's surface, but I did clean it thoroughly using isopropyl alcohol prior to printing.

I've been thinking about using an ultrasonic jewelry cleaner that I have, instead of rocking by hand the plastic container that I've been using. Maybe that should help break or release the bubbles from the foil more quickly. But I'm afraid that the solution might attack the machine's stainless steel container.

Any thoughts?
 

Wolframore

Joined Jan 21, 2019
2,342
Acid will etch stainless it’s used to passivate and remove oxidation from stainless surface. It still sounds like a good idea if you can somehow contain the acid, that or some sort of vibration or mechanical agitator. I have no idea whether ultrasonic will displace the bubbles or not, let me know if it works.
 

jpanhalt

Joined Jan 18, 2008
11,088
I got my first stencil from OSHStencil (https://www.oshstencils.com) (no relation to OSHPark, but the two companies are friendly). Ordered late Thursday, got to post office on Saturday, received Monday morning. Laser cut stainless steel. Very nice and reasonable price including shipping. Don't know whether the price includes shipping to Mexico or how long that shipping would take. Comes in a simple padded envelope.

As for etching, I believe the chloride in muriatic acid or FeCl3 is important for the process, but I think the process with H2O2 will be fraught with problems from bubbles as you observed.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,404
I got my first stencil from OSHStencil (https://www.oshstencils.com) (no relation to OSHPark, but the two companies are friendly). Ordered late Thursday, got to post office on Saturday, received Monday morning. Laser cut stainless steel. Very nice and reasonable price including shipping. Don't know whether the price includes shipping to Mexico or how long that shipping would take. Comes in a simple padded envelope.

As for etching, I believe the chloride in muriatic acid or FeCl3 is important for the process, but I think the process with H2O2 will be fraught with problems from bubbles as you observed.
Thanks for the info, John. It's 99% sure that I'll be using those guys services in the near future (and yes, they do ship internationally). In the meantime, I'm trying to get this thing under control so as to get results as fast as possible on a project that I'm currently working on.
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,404
Update:

I found two things that make life the hell of a lot easier:

  • Adding one drop of a defoamer agent to the mix makes the bubbles much easier to detach with a little shaking while the etching is taking place. @jpanhalt, what brand or type of defoamer would you think is best for this application?
  • The minimum recommended size for any hole in the stencil (using this process, of course) is 0.018" x 0.040". A hole smaller than that takes much longer to etch, in detriment to the larger holes that etch a lot faster.

Also, I plan to build a small motorized shaker table with a lighted bottom to make it easier to tell when the etch reaction is complete. I'll post its plans and pictures here when it's complete.
 

DarthVolta

Joined Jan 27, 2015
521
OP that looks cool. How much does the setup cost to make PCB's like that at home ?

Soon I should order some SMD PCB's of stuff I've made, and solder it myself for fun. It's pretty cheap to order PCB's online, I shouldn't be thinking about getting all the stuff to make PCB's, not until it's smaller and even more compact I guess, what did it cost you ? LOL
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,404
OP that looks cool. How much does the setup cost to make PCB's like that at home ?

Soon I should order some SMD PCB's of stuff I've made, and solder it myself for fun. It's pretty cheap to order PCB's online, I shouldn't be thinking about getting all the stuff to make PCB's, not until it's smaller and even more compact I guess, what did it cost you ? LOL
The answer to that can be found in this post.

And btw, of course ordering PCBs online is cheap and all. But I had to develop this method because of speed. Most of the time I cannot wait the week or so it takes for a PCB ordered online to get home. And besides, sometimes mistakes are made and one has to build a board several times before it works right. My method has saved me lots of time throughout the years.
 
Last edited:

jpanhalt

Joined Jan 18, 2008
11,088
@cmartinez
I know almost nothing about defoamers. However, foam is a problem with agricultural sprayers as it interferes with the pumps. Ag distributors might be one place to look. Out of curiosity, I found this short article by BASF : https://insights.basf.com/files/PracticalGuide-Defoamers.pdf
The interesting part begins about page 5. I would not expect your etchant to be particularly incompatible with any of those chemicals shown on page 11 (8/12 of the PDF).

As for minimal size hole, your 0.018" x 0.040" is pretty good. That will work for TQFP packages. I think "spray etching" versus just agitation might reduce that minimal size a little. But then, that requires a chamber. There are several DIY designs as examples, such as this: https://hackaday.com/2012/02/11/create-pcbs-in-just-minutes-with-this-awesome-spray-etching-machine/ . I never got into that, as I was perfectly happy with agitation (I use a magnetic stirrer and Teflon coated stir bar.) and ferric/cupric chloride etchants.
 

DarthVolta

Joined Jan 27, 2015
521
Well I was soldering up a double sided through-hole proto-type PCB yesterday, and it looks super cool to make a dense forest of parts and use their own leads as much as possible, it's a nightmare to go back and rework. Next time I'll stick to single sided PCB, and just cut off whole parts, if needed, even if the design is right, solder balls go flying.

My stereo I'm working on also uses double sided, and it's real easy to short to the GND area
 

Thread Starter

cmartinez

Joined Jan 17, 2007
7,404
I think I've finally got the hang of it. The key to etching a good stencil in aluminum lies in two things:

  • Use a 50/50 mix of muriatic acid and hydrogen peroxide
  • Dip, don't shake, the stencil into the mix.

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The second point is extremely important because on my previous tries I vigorously shook the entire container with the stencil in it in order to let loose the tiny bubbles that formed on the exposed aluminum. With very little success. I even thought about building a high frequency shaker for the container so as to automate the process... But what worked was dipping the stencil into the mix, wait for about 30 seconds until the micro bubbles built up on top of the exposed aluminum, and then pull the stencil out of the container and wait for about 5 seconds for it to completely drain itself from the mix (and hence the bubbles), and repeat the process. This worked far better and much faster than on previous occasions.

What's next for me is to design and build a small tank with a reel mechanism that will dip the stencil and pull it out at pre-programmed intervals.

Stay tuned.
 
Thanks for sharing!! I haven't tried stencils yet, but have found HP Premium Presentation paper works good for PCBs. I does leave fibers behind that need to soak to get them to rub off between traces, but otherwise I've never had a problem due to the paper itself.
 

jpanhalt

Joined Jan 18, 2008
11,088
Here's an little background. A few weeks ago out of curiosity, I searched on what alloy was used for kitchen/Reynolds aluminum foil.

Aluminum foil is made from an aluminum alloy which contains between 92 and 99 percent aluminum. Usually between 0.00017 and 0.0059 inches thick, foil is produced in many widths and strengths for literally hundreds of applications. ...
Every source I checked had pretty much the same blurb. However, I stumbled on a site that mentioned 1235 and on further searching for that alloy specifically, it is said to be the most common alloy used for aluminum foil. It is 99.35% aluminum , 0.05% Mn, 0.06% Ti, and 0.65 % Si or Fe. Those add to more than 100% and are the maximum amounts of each it can contain. 36 gauge aluminum is about 0.005" thick.
 
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