Placement of LEDs on same or opposite site of mechanical switch (for keyboards) - Page 1
Forum section: electronics / general

author: eigenvektor
author_rank: Contributor
timestamp: March 31, 2022, 03:37:11 pm
content: Hi,I want to design a PCB for a mechanical keyboard with neopixel LEDs underneath each switch. The way I see it I have 2 options:option 1: place small LEDs on the same side of the board as the mechanical switches.cons: will have SMD components on both sides of the board (possibly higher failure rates due to thermal stress of 2x reflow?) + small size of LEDs will make rework hard.option 2: place LEDs upside down on backside of the board within a cutout. (see adafruit neokeyhttps://www.adafruit.com/product/4978as an example)cons: milling cutouts will require extra manufacturing time. Will need to request the LED manufacturer to place the LEDs upside down in the tape for pick & place machines.pros: rework would be really easy on SK6812 mini-e.Can anybody advise me which option makes more sense for manufacturing?Thanks in advance :)

author: thm_w
author_rank: Super Contributor
timestamp: March 31, 2022, 08:44:01 pm
content: How many of these are you making, if its not many, the LEDs can be placed by hand.For Option 1, are you sure there is space available underneath the key switch?Having SMD components on both sides of the board is a non-issue.For Option 2, milling cutouts is a non-issue as well.

author: eigenvektor
author_rank: Contributor
timestamp: March 31, 2022, 09:18:36 pm
content: If all goes to plan, enough that I'd like to have it automated. Since there are ~80 LEDs per keyboard, this can turn into a lot of work really quick.I plan on using Kailh Box switches, which have a space of 3.1x3.1mm for the LED, 1.6mm high. I'd use neopixels in a 2020 package.I guess the only difference then is going to be convincing the manufacturer to ship "inverted" tape reels vs being harder to rework.Thanks for your help! :)

author: thm_w
author_rank: Super Contributor
timestamp: March 31, 2022, 09:34:46 pm
content: SK6812 mini-e is 3.2x2.8x1.8mm:https://cdn-shop.adafruit.com/product-files/4960/4960_SK6812MINI-E_REV02_EN.pdfSo probably 2020 package is a better idea to fit under thereWS2812-2020APA102–2020

author: eigenvektor
author_rank: Contributor
timestamp: March 31, 2022, 09:40:17 pm
content: Quote from: eigenvektor on March 31, 2022, 09:18:36 pmIf going with option 1, yes, I'll be using using something in a 2020 package. The SK6812 mini e would only be if going with option 2.

author: thm_w
author_rank: Super Contributor
timestamp: March 31, 2022, 09:40:42 pm
content: Quote from: eigenvektor on March 31, 2022, 09:40:17 pmedited my post after I understood what you wrote

author: eigenvektor
author_rank: Contributor
timestamp: March 31, 2022, 09:48:09 pm
content: theoretically even the SK6812 mini E would fit under there, it has an interesting configuration where half the LED's height is sunk into the board. :PI guess my last question would only be how hard it is to rework an LED in a 2020 package. Can those type of LEDs even be reworked? From what I understand if they've acquired any humidity they can go pop under hot air, or am I misinformed there?

author: thm_w
author_rank: Super Contributor
timestamp: March 31, 2022, 10:04:03 pm
content: Quote from: eigenvektor on March 31, 2022, 09:48:09 pmIt can be tough due to temperature sensitivity of the LEDs.- If you are building your own prototype and don't have a lot of soldering experience, you can use low temperature solder for assembly, which means lower hot air/soldering iron temp required and less chance of melting the LEDs.- As they say below, to remove a LED can use two soldering irons, or an appropriately sized hot tweezer. Hot air would work but you might need to mask off keyswitches, etc. to prevent melting plastic there.- Good chance LED will be destroyed, but thats ok you can always use low-temp solder when replacing it (or regular leaded solder), which will be much easier task.I have not personally reworked these 2020 packages, just the larger sizes.From datasheet:Quote

author: eigenvektor
author_rank: Contributor
timestamp: March 31, 2022, 10:22:49 pm
content: I have access to a vapor phase reflow oven, so building the prototype should be doable. Guess I'll contact the SK6812 mini e manufacturer about the inverted tapes and decide then. Thanks for the help!

author: jmelson
author_rank: Super Contributor
timestamp: April 01, 2022, 12:37:35 am
content: There are actually special "bottom firing" LEDs designed to send light out through a hole in the PCB. I ordered some of these by mistake once.Jon

author: 48X24X48X
author_rank: Frequent Contributor
timestamp: April 01, 2022, 02:24:58 am
content: Quote from: jmelson on April 01, 2022, 12:37:35 amThe regular LED has those but I have yet to see those for addressable ones.

author: ANTALIFE
author_rank: Frequent Contributor
timestamp: April 01, 2022, 06:03:43 am
content: If it was me I would go the reverse mount LED route. If you are buying them in bulk then pretty sure they would already be placed in the correct orientation in tape/reelAlso if you are worried about reflow cycles, have a look inside the datasheet. Usually they would say the maximum number a reflow cycles the component can withstand. Like for fancy tantalum capacitors it's generally 2-3 cycles, so pretty sure the LEDs/switches can do at least that

author: eigenvektor
author_rank: Contributor
timestamp: April 01, 2022, 01:00:29 pm
content: Alright I'm majorly confused. The datasheet on the SK6812 mini e seems to indicate that it's packaged "right side up". I went ahead and ordered some samples, turns out they're not.That's one problem solved I guess, but can I rely on this to always be the way they're packaged if the manufacturer can't even get it right in the datasheet?Bonus: check out the the flush fit of the SK 6813 mini (on the left)

author: thm_w
author_rank: Super Contributor
timestamp: April 01, 2022, 09:35:54 pm
content: The flush fit looks great.I know they show the LED facing up in the datasheet reel image, but, I don't think this was the intent of the specification. I'm guessing they just lazily used whatever photo they had, because its clearly not the same part.If you go by the footprint provided at the top, then it does match the way you received them.Here is the same thing:https://keeb.io/products/sk-6812-mini-e-rgb-leds-12-pack

author: eigenvektor
author_rank: Contributor
timestamp: April 01, 2022, 09:46:17 pm
content: oh yeah, nice find! ...also the notch on the body is opposite to the notched lead on the footprint drawing

author: DavidMenting
author_rank: Contributor
timestamp: April 05, 2022, 09:55:36 am
content: We've been using these SK6812 mini-e in our product for a few years now and are very happy with them. Previously we used the non-reverse 3528 SK6812 but the rate of succesful rework was very low. Also, the reverse mount leds allow us to place all smd components on one side of the board as opposed to both side. So far the reels we received were correctly oriented for through-pcb placement.I mention we are using the SK6812 mini-e but actually we are using the SK6805 mini-e due to their lower maximum brightness. Having 12mA per channel pointing straight at your face is way too bright and the lower current gives us more color resolution in the usable range.Unfortunately all these LED datasheets are just a cut and paste mishmash of other datasheets. If you disregard the pin numbering and just assume the notch in the body and leg is at the GND pin you should be alright (no guarantees though).

author: eigenvektor
author_rank: Contributor
timestamp: April 05, 2022, 10:11:40 am
content: Thanks for sharing your experience! Are you sure about the sk6805 mini e though? I can't find any reference to it on the internet...

author: DavidMenting
author_rank: Contributor
timestamp: April 06, 2022, 10:24:36 am
content: Yes, absolutely 100% sure but you are right in that no webshop stocks them. We buy reels directly from Shenzhen Rita Lighting (Ray Wu) and you can also purchase sample quantities there.

author: eigenvektor
author_rank: Contributor
timestamp: April 06, 2022, 02:52:07 pm
content: Yes you are correct. There's even a mention on the SK6812 mini e datasheet that I totally missedThanks for the tip!