LED polarity marking -- really?! - Page 1
Forum section: electronics / projects

author: ebastler
author_rank: Super Contributor
timestamp: May 03, 2022, 10:49:00 am
content: I just had a "WTF?!" moment, courtesy of Kingbright. Have a look at the package drawings below for two of their 0603-sized SMD LEDs. The nice, intuitive polarity mark on the bottom of each package deserves special attention...These are two LEDs from the same APDT1608 series. Why is the polarity mark the exact opposite on them?! Is there any rhyme or reason to this, or are they doing it just to mess with their customers?I guess I mainly need to vent... But if anyone knows of an actual reason behind this, historical or otherwise, I'd be curious to learn!

author: Twoflower
author_rank: Frequent Contributor
timestamp: May 03, 2022, 11:00:07 am
content: WOW! That seems a real nightmare for the PCB assembly house!I only can imagine that they use the same package substrate, including the marking and bond-pads, for both types. But the LED dies emitting the light from different sides or one is a P semiconductor N doped while the other is a N semi P doped depending on the used materials targeted for the emitting color.

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 03, 2022, 12:06:37 pm
content: Quote from: ebastler on May 03, 2022, 10:49:00 amMaybe they let the die fall as it may on the substrate, and then simply test and bin the devices into separate products?Sounds exactly like a "solution" by a "marketing manager" to me, you see.

author: Psi
author_rank: Super Contributor
timestamp: May 03, 2022, 12:32:23 pm
content: It will be down to either the LED die chemistry, the machines that package them up, and time vs money.They probably chose to save money by using the same machines for both lines, or save money by not reconfiguring them when changing between LED colors.Whenever you mess with the machines you have to do test-runs to get them dialed back in and that produces faulty-parts/wastage.In reality it's only an inconvenience for prototyping and hobby stuff.For PCBA each part is checked and compared to pcb markings. They would never assume anything.

author: wraper
author_rank: Unknown
timestamp: May 03, 2022, 12:43:12 pm
content: One has InGaN die, another AlGaInP. Most likely this is a result of how different dies are attached to the same package.

author: ebastler
author_rank: Super Contributor
timestamp: May 03, 2022, 03:08:26 pm
content: Thanks all! That's a few plausible reasons, all of them better than doing it just out of spite.I did, of course, fall into this trap when populating a few prototype boards. Looked at one of the datasheets, and of course happened to grab one where the arrow is nicely aligned with the triangle in the schematic symbol. Thought "now that's a nice way to mark it" and went ahead to populate all LEDs that way -- only to find that half of them would actually light up...

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: May 03, 2022, 03:41:04 pm
content: Sure, happens to the best of us. Welcome to the club.What a nightmare. Getting everything else right, but the pretty front panel won't light up ...It's usually little stuff like this that gets under the radar "just two pins what can go wrong".I've done my share of mistakes like this. Only solution is to really check every component, even just two pins.I've had some boards where all I had to do is invert the silkscreen for the next rev.With special instructions for the first rev, which then where applied to the second rev because the board looked the same, so the LEDs ended up inverted again! Oops!

author: GLouie
author_rank: Regular Contributor
timestamp: May 03, 2022, 05:02:50 pm
content: Has this been confirmed? The schematic shown for both is the same (pad 1/pad 2).Perhaps it's a typo with the polarity marking triangle on the spec sheet - or the pad numbers.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: May 03, 2022, 05:44:05 pm
content: Quote from: GLouie on May 03, 2022, 05:02:50 pmYes. Yes it has.Everyone agrees, LEDs are the worst. I've been on panels with assemblers where we just bitched about LEDs.LEDs suck.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 03, 2022, 05:58:39 pm
content: Yes, LED markings totally suck, you just need to spend considerable amount of effort in production and prototyping. Further, unlike almost all other parts which are MSL3 and can, in practice, forgive small violations of shop floor time, LEDs tend to be either MSL3 and totally unforgiving, or MSL4, and popcorn in prototyping or small batches where you keep the bags open for a week.

author: eugene
author_rank: Frequent Contributor
timestamp: May 03, 2022, 06:05:21 pm
content: If LED's suck when it comes to markings it's only because manufacturers make a mess of things, not because of anything inherently troublesome about LED's.In the case highlighted in the OP, Kingbright unarguably fucked up.

author: b_force
author_rank: Super Contributor
timestamp: May 03, 2022, 06:20:32 pm
content: I have also seen all kinds of strange markings with LEDsFrom the teeny tiniest little dot that can barely been seen, to some even without any marking at all or datasheets that are just completely wrong.I have also seen LEDs with a clearance so small, that even some very good EU assembling houses couldn't do.Go figure.....

author: free_electron
author_rank: Super Contributor
timestamp: May 03, 2022, 06:29:13 pm
content: wait till you will have to deal with side-looking leds... the orientation in the reel matters ! if you flip the part upside down the cathode and anode reverse sides ...so when designing the footprint check the positioning in the reel !

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 03, 2022, 06:41:02 pm
content: Is the taping orientation consistent?

author: ebastler
author_rank: Super Contributor
timestamp: May 03, 2022, 06:57:58 pm
content: In the tape, the package (and its arrow marking) is always oriented the same way. So the cathode (and pin 1) is sometimes on the left, sometimes on the right.

author: ebastler
author_rank: Super Contributor
timestamp: May 03, 2022, 07:01:56 pm
content: Quote from: Alex Eisenhut on May 03, 2022, 03:41:04 pmUnfortunately the LEDs were somewhat central to the function in this build: It's a poor man's spectrometer with several LEDs at different wavelengths used as the light sources. Well, at least the problem was easily fixed once I had checked all the individual data sheets.(Well, reasonably easy -- this was also my first flex PCB, so I was a bit timid to resolder anything. Worked well though; the hot air or iron temeprature can be set quite low, since the PCB has very little heat capacity.)

author: free_electron
author_rank: Super Contributor
timestamp: May 03, 2022, 07:05:52 pm
content: Quote from: ebastler on May 03, 2022, 07:01:56 pmAnother one of the many misconceptions of soldering...Let's set the iron at 600 degrees so it heats faster...

author: ebastler
author_rank: Super Contributor
timestamp: May 03, 2022, 09:41:40 pm
content: Quote from: free_electron on May 03, 2022, 07:05:52 pmNot quite sure what your message is. If you imply that my statement regarding iron temperature vs. heat capacity of the PCB is wrong -- well, yes, in a perfect world the iron would always immediately heat the soldering joint to its setpoint temperature, by supplying infinite amounts of heat. But my world, including my soldering iron, is not perfect.Specifically I can say from experience that setting my iron (an Ersa iCon) to 260°C would not work well at all on a regular FR4 PCB. But it worked nicely on the 0.1 mm flex PCB. So yes, there is a difference in temperature settings for me. Your mileage my vary.

author: b_force
author_rank: Super Contributor
timestamp: May 04, 2022, 06:21:32 am
content: Quote from: ebastler on May 03, 2022, 09:41:40 pmNo he probably means that some people think that by setting the iron at an higher temperature, the iron will heat up quicker.That can be only true when the feedback loop is extremely lousy.

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 04, 2022, 10:26:51 am
content: Yep, I've seen them. Happened once after making hundreds of boards, we had to manually fix them all.I just can't understand the logic behind such stupid decisions, same for tantalum caps.Perhabs:- Hey, Mike, it's tantalum, so for no damn reason, let's put the minus sign at the positive!- Definitely, that's a great idea!*popcorn noises*

author: b_force
author_rank: Super Contributor
timestamp: May 04, 2022, 11:21:39 am
content: Quote from: DavidAlfa on May 04, 2022, 10:26:51 amHaha yes.Although even way back you always had to double check the pinout of those transistors.For some reason they could never figure out a real standard and there was always that one sneaky little bastard with swapped pins.Could really ruin your day.Sometimes even with different brand for the same part number if I recall correctly.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 04, 2022, 11:50:14 am
content: Unfortunately this happens more often than you'd think.(Of course a sensible person assumes this would never happen in the first place).I've also seen this on small glass diodes such as the 1N4148.I once bought a bunch of them at a local store (Which don't exist anymore for some 15+ years) and they got mixed up in the same bag.If you held the two versions next to each other then you could see a slight difference in the location and width of the black stripe.Luckily diodes are easy to test with a DMM.From that I learned t never trust these markings and I always test diodes before soldering them to something.Tantalums are also easy to test for polarity. Just charge them to their normal polarity. If they turn bright red then the polarity was wrong.Another trap is the SOT23 case.For this case in particular, manurefacturers can not even agree where pin 1 is located on the package.