Soldering is a skill lots of hobbyists learn out of necessity, but for most people this limits you to working with through-hole parts which were state of the art in the 70’s and 80’s. These days, anything electronic actually being professionally designed for production is using surface-mounted parts, with the solder applied using a silkscreen-style stencil, the parts robotically placed on the paste pads, and then the whole thing goes through a reflow oven to melt the paste and voila, the whole board’s done at once.

Things get tricky when you discover a design flaw during the development process and you want to rework the board to test potential fixes before committing to a new manufacturing run. Today, I had to move a few resistors to alternate locations on a board we’re designing at work. The resistors are there to configure a chip with certain “hard-coded” settings at boot time. They’re also incredibly small- SMT resistors of the 0402 variety- this means each one is .4 x .2 mm in size. That’s really, really small. Reworking parts like this evidently usually involves specialized tools like special hot-air soldering tools to heat multiple leads simultaneously, or irons with very fine pick-like points, and a binocular microscope is really the only way to see what the heck you’re doing. Luckily the lab does have most of these.

I was the only one in the lab today and this was really at the edge of my skills. There did not appear to be an SMT desoldering tweezers, available, so I tried using a very small hot air nozzle to heat up the leads. I tried higher and higher heat and air volume settings, but for some reason I just could not get the solder to reflow. I still haven’t figured out why. I then resorted to a standard iron with an extremely fine point. This melted the existing solder easily, but it was impossible to nudge the resistor off its pads without damaging one or the other of the extremely fine leads which wrap around the sides, to the bottom where they meet the pads. I’ve been able to do this with larger smt parts, but these are just too delicate. Luckily there was a tape reel available with additional resistors of the correct value and size, so I opted to just dispense with the damaged ones and use new resistors on the correct locations. Manipulating them into the correct location and orientation and then getting a good solder joint on both sides was a challenge as well, but ultimately I think pulled it off successfully.

This took quite a bit of time, and it made me wonder just where do the professional lab techs who do this kind of rework learn these skills? I sometimes see some very nicely done “blue wire” jobs with a tiny little 30- or 32-gauge wire neatly creating a new trace, and I just don’t know how these guys do it, or where they learned how in the first place. I’d love to raise my game.

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