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:

Temporary connections?

I bet once or twice in the past you wish that you could find a simple way to just hold a module with male headers like this Adafruit GPS module in the pin holes of a circuit board without soldering the pins so you can later take the module and put it elsewhere. Here I found a neat solution, wedging the module with some ESD foam. You get ESD foam pieces from buying ICs so they come for free and have some springy-ness. Then you put in between the module and the board and push in. The force from the foam will make the module form a wedge and there is enough strength to tlit the pins against the holes to make connection. Here are some photos:

The first photo shows the underside of my new phi-3 shield. The GPS module is supposed to be on the top side but unless I solder it to the shield, there is not enough space to make a wedge with the module while it is under the display. So I hang it below the shield board and stuck a piece of ESD foam between the module and the shield board. Here is a side view. You can’t see the foam, which is too dark but you do see the pins are all tilted and thus pushed hard enough to make electrical connection. I only recommend this on a temporary basis. Tomorrow the GPS module is going on another board, the SDI-12 + GPS USB adapter. I’ll solder this time.

Desoldering tricks

Sometimes you need to remove parts from a soldered board. How?

Here is my trick of the trade:

1) Buy some radioshack desoldering wick, suck out the solder from a part. If you can’t remove all solder, take your small screw driver, drive its tip of driver into the wick to make a small puncture hole. Then press the hole against a pin to be desoldered. This makes the wick surround the pin, maximizing area of contact, instead of staying above it or on its side. Then heat the wick around the hole and you will get most of the solder out. I’ve done this quite some times and each time it worked nicely. Next, if you feel you’ve removed all the solder from a pin but it won’t budge, just slightly rock/push it a bit with a flat-head screw driver so the last bit of solder is broken.

Desoldering wick/braid

Desoldering wick trick to remove solder from a pin

2) Some people also opt to destroy the plastic holding the pins together before they proceed to desolder each pin, divide and conquer.

3) You may also buy a radioshack desoldering iron to remove solder by larger amount. I found the desoldering iron very useful at times but as it ages somehow it is unable to remove solder cleanly as before.

Desoldering iron

4) You could also use solder sucker or desoldering tool but I never got hold of a good unit so I can’t say too much but a good one may work nicely.

Desoldering pump/tool

Desoldering bulb

 

 

5) If you have a higher-power (de)soldering iron like 30W or more, keep the part in your third hand, try to desolder and when solder melts, quickly use a pair of pliers to pull out the hot pin. This sometimes seals the pin hole so make sure you remove as much solder as you can with some method.

How I soldered header pins for arduino shields

While I was soldering header pins on my Phi-1 shield for arduino, I’ve developed this technique. You can use this technique on most things that have male header pins. You will need two bare PCBs, or you may photocopy your PCB, use double-sided tape to tape the photocopy on a card, poke holes where male headers go. Now use masking tape to hold together the two PCBs or the PCB and the paper copy, sandwiching the male headers in between. Just use some masking take to hold the four corners tightly together. Once you do this, soldering is pretty easy. The pins will be up right, not tilted, or rotated. You can also use an arduino board to hold the pins, then stack the bare PCB on top and tape them together. Since the arduino female headers will conduct heat away, this needs longer time than using two PCBs or PCB and card. I will then use a third hand/holder to hold the board while I solder. If you don’t have a third hand, you can lay the board on a flat surface but sometimes it will wobble when you solder on a corner.

The soldering iron I used for this project was a portable battery-powered dual-setting iron from Weller. I’m experimenting whether this is a good idea. A corded iron has potential for dissaster in a student lab (burns, melting electrical cords and getting zapped). The iron claims to be able to solder up to 150 points. As of now, with the original batteries, I managed to solder about 75 points. I’ve done 60 with a new set of batteries. I’ll solder some more to find out how many more points I can solder with the remaining battery power.

Here’s some pictures:

Male pin headers are sandwiched between two PCBs.

I taped the four corners with masking tape.

Use a 3rd hand to hold the board when you solder.

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