Judging from the dates codes on the ICs inside, Mary’s HandiQuilter Sixteen long-arm machine is about two decades old and many of the white LEDs in the front handlebars have gone dark:
The vertiginous view looks upward into the handlebar at the top of the machine (more on this later). The PCBs run strings of three series LEDs from a 16 VDC supply with a 390 Ω ballast resistor (oddly enough, on the ground end of the string), so one failed LED takes down all three.
I decided to replace all the LEDs, on the principle they’re surely dimmer than they used to be and to take advantage of a decade or so of improvement in white LEDs (yes, I have old stock).
After discovering that the HandiQuilter engineers violated the Principle of Least Surprise by orienting adjacent LED strings in opposite directions, I found one of the strings still didn’t light up.
Pop quiz: which one of these LEDs caused the problem?
To the best of my knowledge, all 5 mm round LED packages mark the cathode lead with a flat edge. It’s easy to remember, as the cathode side of the schematic symbol has a bar: straight bar = straight edge.
Inside, the LED chip’s cathode lead is bonded to the reflective cup, with the anode lead wire-bonded to the top.
Took me a while to see what was wrong, too.
For whatever it’s worth, the backward LED works fine.
The Smell of Molten Projects in the Morning 
Comments
5 responses to “White LED QC Escape”
As memory serves (assuming nobody got a substantially better way to do it), this type of LED was assembled on a leadframe strip of several (25?) parts. Somebody managed to get at least one strip into the encapsulating mold backwards. So, there’s likely to be more of those wonky LEDs in a bulk buy. (If the leadframe setup was done as a coil, or if the molds got installed backwards, there could be a boatload of wonky parts.)
It should have failed final test, making the rash assumption that the parts were actually tested…
It’s problems like these that make me glad that most of my production responsibilities ended after we shipped tested wafers out to the assembly line. I had to deal with a few parts in packaged form. Wanted a cross and garlic to deal with those problems.
It ought not be possible to lay the leads in backwards, but maybe Mondays are that bad all over the world. Ditto for final QC …
[…] new and old white LEDs produce distinctly different colors and intensities on the practice quilt […]
[…] tedious process weeded out a couple of deaders, one with its case on backwards, and a handful of completely different white LEDs evidently from a different manufacturing batch. […]
Nice, last time I was surprised by that feature was around 2011.