Soldering with new TOMOV 3D Digital Soldering Microscope

My employer was kind enough to gift me with some Award points at Amazon for 40 years of service.  So, I decided to spend it on a Digital Soldering Microscope, as one does.  The full description on Amazon is: "TOMLOV 3D Digital Soldering Microscope: Magnetic Dual Lens Micro Scope TM3K-3D Max For Adult, Coin Microscope with Flex Arm, 3K 10.1-Inch IPS Screen, HDMI/USB Output, Gooseneck Light, Repair Mat, 16GB".

 

 

 

After getting it set up in my shack, beside all the other clutter of pre-existing magnifier, solder stations, hot air, and solder sucker, it looked like this:

 

 I wanted to try a microsoldering repair to build some skill.  As it happens, I had a DFM-17 radiosonde kicking around that I had damaged.  I was putting on a programming header with a soldering iron (bad idea) and managed to rip out a few traces when I bumped it after a few solder joints had grown cold.

 

That header is tiny.  It's a 0.05-inch (1.27mm) pitch, 5-by-2 (10-pin) header.  For scale, that resistor above it is an 0402 size.  In the image above, you can see that I accidentally ripped off the upper and lower right hand pad (SWDIO and nRESET respectively).

To begin, I scraped a little conformal coating off each of the traces, and soldered a dab of solder on the exposed copper.  You can see that in the image above as well.  I then stripped a stranded copper wire, and took one strand to use as my substitute trace.

 

 

In the image above, you can see the piece of wire soldered into place.  Next, I intended to use some conformal coating to "glue" the wire in place, before forming the remaining length into a pad.  This was my first mistake.  As you can see, the (very fine) nozzle on the conformal coating was HUGE for this application.  To make matters worse, it's UV Cured, but apparently does not cure well if you do too much at one time.  After covering the wire, and hitting it with UV light, it basically didn't set up.  After a bunch of fighting with it, I ended up removing the attempt and starting over.

 

 

For the next attempt, I used a very fine pick to dab on conformal coating in fine layers, and hitting it with UV, and then adding more layers.   Above you see an "in progress" shot of layering the conformal coding with that really sharp pick as a "paint brush".  That worked fairly well.  After fully securing and covering the wires, I then scraped off the dried conformal coating over the pads, to expose the copper wire.

 

 

Here's the result after a bunch of fighting with it.  It was ready to add some solder, so I fluxed it and added a dab of solder to each.

 

As I did the repair, I did confirm that I had connectivity back to the SWDIO and nRESET pins on the MCU.  Note, in this picture, you can see yet ANOTHER pad missing on the left.  I forgot to get a decent image prior to my first (failed) attempt to install the header.  In the act of removing THAT failure, I accidentally ripped off that pad on the left.  Fortunately, that one was unconnected, so other than for stability of the header, it's not really needed.

 

S0, about soldering on the header.  The root cause of this problem was using an iron, rather than hot air, to install the header.  It got on crooked, and then I ripped traces while trying to heat and straighten it.   Hot air is a challenge with this part, as it has lots of plastic.  You can't just blow hot air on it.

The solution I settled on was to heat the board from beneath.  Aside from a battery holder, there are no components on the back side in that area.  So, I desoldered the plastic battery holder so it wouldn't melt.  I was then able to heat the board from both above and below.  Once I got the solder to melt, I held my hot air on the bottom while placing the part from above.  After a few adjustments, it was in place.

 

Note, the header still had a plastic retainer installed, holding the pins in position.  That was removed once I was satisfied with the placement of the header.

Once I reinstalled the battery holder and cleaned it up, I tested it by flashing firmware onto the DFM17 with my standard cabling.  It worked like a champ.

 

 

 Lessons learned:

• I should not attempt to install these headers with an iron. Hot air is definitely the way to go.  It takes just a moment to remove the plastic battery caddy that's closest, and the work is very simple after that by heating from the top, and then from below.
• I bought cheap conformal coating.  I need to get better stuff that doesn't have to be painted on in thin layers.  In the meantime, thin layers DO work OK.
• I had also bought some thin copper "trace repair" things that could theoretically be used for this kind of repair, vs just a strand of wire.  The kit of copper traces I bought were tragically small, even for THIS repair.  The pads were sized for 0402 components and were too small for this repair.  I'll keep them, but if I want to use pre-cut copper trace repair pads, I need to get bigger ones.
• Using tiny lint-free swabs to frequently clean the board was really handy.  It's tough to get small enough portions of the conformal coating, and even flux, into place.  At first, I had some leftover flux on the board, and that inhibited the conformal coating from sticking.  So, note to self, clean the board thoroughly before using the coating.