Metal stencil – first impression

Over the past summer I have been designing my SDI-12 data logger. So finally I was ready to send my design out to JLCPCB.com for a small batch (10 boards). I thought, why not trying out their $7 stainless steel stencil for a change? I’ve used Kapton stencils for over four years and have grown tired of the floppy stencils and frequent need to clean under the stencil while stenciling the boards. Bigger holes like crystals often leak solder paste under them and make the printing a bit messy. Overall leakage causes printing to degrade after a few board and you might have to wash it clean after a dozen boards if you can’t stand the degraded prints. Plus, alignment with Kapton stencils is not great since the film can be stretched very slightly to lose alignment across a slightly larger board (a few inches long).

Since I reviewed the quality of a batch of printed circuit boards made by JLCPCB.com in comparison with a popular PCB service a while ago, they were pretty happy with the details I went into with my favorable review and promised to print another batch of boards for free. I took the offer and asked for a stainless steel stencil. So the following story is my first impression of stainless steel stencils, which is very positive after some minor struggle. Hope it is as entertaining as it is useful.

It took me approximately one week to get the board and the stencil in a DHL envelop. Here is the package inside the DHL envelop:

It’s actually pretty big due to the large size stencil. You can make a rather large (non-framed) stencil with their $7 service. Let’s see what’s inside the package:

The package has two wood/paper/cellulose boards that sandwich the “gigantic” stencil. The stencil is protected (on both sides) by a polymer film. By the way, I used half of one of the cellulose board (probably a sacrificial base board for PCB routing) and built a home sprinkler controller on it. Details of that project will be forthcoming when I clear my backlog of “things to be posted”.

The above is a close-up. The exact side is 28cm by 38cm (11inch by 15 inch). The thickness is 0.12mm or between 4mil and 5mil, just what I wanted. My other stencils made with Kapton are only 3inch by 3inch or 5inch by 5inch. The actual stencil area is no more than 4inch by 4inch. Here is how 4*4 area looks like on an 11*15 area:

Here are the PCBs (10-qty) and a ball pen with paper stem, very environmental!

Now I have to find a good way to cut out the stencil from the whole sheet of stainless steel into something around 5inch by 5inch. I asked around online and was suggested to use a knife:

I tried several times as hard as I could with a ruler over the right side as a test but only was able to scratch a line without any hope to cut through. What was I thinking, cutting steel with steel blade? Let’s try scissors now that I have a line :

Wait a minute, let’s bend along the line to see if it breaks. LOL it won’t break! Back to scissors.

I made the cut but the edge was not flat. I can’t use this on printed circuit boards! I won’t even use these as decent shims.

I started looking for tools around my lab and office area. How about this paper cutter?

It seems to cut just fine although I need to push hard to get it started. The cut edge is flat and smooth:

A few more cuts later, just the right size (with PCB on left):

 

One things worth showing is that the surface of the stencil is not as smooth as the rest of the surface. There are fish-scale shaped marks (see the photo above for overall):

Here is the surface around a 0805 resistor under a macro-zoom lens I use for board inspection:

The reason for the fish-scale marks is that the surface has been brushed by a metal brush to remove the burr from laser cut edges. This was expected. Let’s get started stenciling. The photo also shows how nice the square corners are. The width of the pattern is a little over 0.05 inches or 1.27mm. I secured a board (black) with some older phi-panel PCBs (green):

I used a a “credit-card” squeegee with a new syringe of SAC-305 solder paste:

Here is how nice the print looks (please click and zoom in):

I’ve never see clean prints like this before! The ADS1115 (0.5mm pitch) on the top left side is super clean! And you know the end result. I posted this photo in the announcement of my data logger prototype:

I didn’t have a single solder bridge or spot that needs rework. A list of components I put on the board: ATMEGA328P-AU, ADS1115, ESP32, MicroSD card slot, DS3231, battery, crystal, LM1117-5.0, various 0805 resistors and capacitors, polyfuse, transistors (FET and BJT), and a number of thru-hole components.

So the end result was pretty darn good, for $7 extra only! My Kapton stencils of smaller sizes cost more (including shipping). I would highly recommend this stencil service if you’re already considering getting PCB from them, JLCPCB.com.

By the way, the PCB printing quality was good as well. The black one on top is from JLCPCB.com while the purple one on the bottom is from a reputable service provider in the US (guess who uses purple exclusively?). Tye quality is comparable. The pitch of the pins is 0.5mm:

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