Shining leds through the PCB - Page 1
Forum section: electronics / projects

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 12:57:15 am
content: HelloI was wondering if there are SMD LEDs that can be mounted on the PCB, with copper areas cut out , so they shine through the PCB and are visible on the other side!I searched for reverse mount LEDs, but they require a hole to be drilled, so they can poke out. I want something that just shines through the PCB without requiring any holes (for aesthetic reasons). I searched mouser, digikey, amazon and aliexpress but couldn't find anything

author: Rerouter
author_rank: Super Contributor
timestamp: May 09, 2019, 12:59:46 am
content: Mount a smd led upside downThe reason your likely struggling is the light will be very diffuse after travilling through the board unless you constrain it with a wall of vias.

author: wraper
author_rank: Unknown
timestamp: May 09, 2019, 01:16:44 am
content: Quote from: Rerouter on May 09, 2019, 12:59:46 amNot possible unless you bodge them with soldering iron. Top facing LEDs have terminals on bottom part only.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 01:17:46 am
content: Quote from: Rerouter on May 09, 2019, 12:59:46 amLight bleed is not an issue. It's just a single on/off indicator. The main problem is that the plastic package sits on the PCB first, which raises the soldering pads off the PCB. I can bridge the distance with adding more solder but I'm not sure if there is an LED meant for this application. if there is, that would be the easiest solution!Quote from: wraper on May 09, 2019, 01:16:44 amactually I have a few which have castellated holes (plating on top, bottom and side) but they sit about 1mm off the board due to the plastic package

author: wraper
author_rank: Unknown
timestamp: May 09, 2019, 01:20:19 am
content: PC motherboards use a trick. They simply mount usual LEDs on the back side and rely on reflection from a metal plate behind it to shine through copper free areas. For example you could solder a small metal shield above it. It likely would even provide better visual effect.

author: wraper
author_rank: Unknown
timestamp: May 09, 2019, 01:25:02 am
content: Quote from: OM222O on May 09, 2019, 01:17:46 amThat includes what I meant by bottom part.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 03:52:00 am
content: Quote from: 3roomlab on May 09, 2019, 01:24:05 amsomething similar to that, but smaller (1206 or 0805). I saw those before but they waste a ton of PCB area and they're really pricy! 1.4$ per led? even in volume they're rough!I think having something to reflect the light on a different area isn't such a bad idea but then the problem becomes where can I get shileds that are small enough for this application? I'm not sure if I should get custom ones manufactured which again defeats the purposeI'm really surprised this isn't really available as an off the shelf part since I can think of so many applications where not having holes for the reverse mounting LEDs can look a lot nicer and since the LEDs aren't shining directly in your eyes, they look a lot softer which is nice IMO.I think my best option is actually side view THT leds which are just solder to some pads on the board and fixed with some sort of glue so they don't come off, but that's even more wasted PCB area

author: thinkfat
author_rank: Unknown
timestamp: May 09, 2019, 04:49:20 am
content: Quote from: 3roomlab on May 09, 2019, 04:07:47 amNo, FR4 is actually quite transmissive, no problem having a led shine through.Gesendet von meinem Nokia 6.1 mit Tapatalk

author: BravoV
author_rank: Super Contributor
timestamp: May 09, 2019, 04:55:22 am
content: FR4 with approx. 1000 lumens white led flash light.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 09, 2019, 08:28:55 am
content: FR-4 is both quite absorptive and scattering. That is, it makes a terrible window, and a slightly better diffuser. Both act to reduce the brightness of your LEDs.Standard solutions exist, fortunately: bottom-firing LEDs, placed over holes in the PCB; light pipes, and transparent (or diffused or tinted or..) overlays are excellent, and quite affordable (at least in small production quantities; YMMV for one-offs).Very thin PCB would be competitive, but is mechanically questionable, and special-order. Flex PCB is also quite available, but as the substrate is yellow to brown (depending on thickness), it wouldn't be as practical a transmission medium. But you can bend it into whatever orientation you like, again making standard top-firing LEDs quite reasonable.You can kind of combine the two and get overlays with LED dice integrated, too.Tim

author: MarkR42
author_rank: Regular Contributor
timestamp: May 09, 2019, 09:36:21 am
content: Can you just mount a normal SMD led upside down (a bigger one e.g. 1206), and route out an area for the lens? The ones I've got have a sticking up lens element. Maybe there are totally flat ones also, which could just be mounted upside down with a few holes underneath?Update: Kicad has a footprint which does just that, called LED_SMD:LED_1206_3216Metric_ReverseMount_Hole1.8x2.4mm

author: DuPe
author_rank: Regular Contributor
timestamp: May 09, 2019, 10:14:44 am
content: maybe soldering a standard led on the bottom (solder) side of pcb would do the job?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 09, 2019, 10:50:16 am
content: A side-firing LED, next to a hole in the copper and resist works quite well

author: Fred27
author_rank: Unknown
timestamp: May 09, 2019, 11:22:46 am
content: Quote from: 3roomlab on May 09, 2019, 04:07:47 amI've used reverse mount LEDs through PCB. It works just fine - in fact brighter then I expected. The one I used was an RGB version of the one pictured above.UPDATE: This page contains a video where you can see how it looks. Easily visible in daylight.https://www.element14.com/community/community/project14/nfc-rfid/blog/2019/04/28/locknfc-does-the-pcb-work

author: thinkfat
author_rank: Unknown
timestamp: May 09, 2019, 11:28:50 am
content: Quote from: mikeselectricstuff on May 09, 2019, 10:50:16 amReasonably priced in 100s quantities, too

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 01:17:37 pm
content: Quote from: mikeselectricstuff on May 09, 2019, 10:50:16 amYou are a genius good sir! thank you! this is exactly what I wantedI don't know why I thought about THT side view ones but not the SMD onesEdit: I actually want the light to be diffused, I know its not the brightest but I want it for one of those "digital dice" type circuits which are obviously used indoors so the brightness isn't an issue. it looks very nice if the 6 segments are like an actual dice (circular markings which are created by a circular cut in the copper). The main problem with smd leds is that they are too harsh on the eyes when you look directly at them, since they don't have a colored plastic piece like the THT ones that act as a natural diffuser and I don't really like that.

author: jmelson
author_rank: Super Contributor
timestamp: May 09, 2019, 08:24:26 pm
content: You want "bottom firing" LEDs. I bought some a while ago by mistake, but I think that is what they are called. To get best results, you need a hole right in the center between the SMT pads. I doubt a red LED will work well shining through a green PC board, but a green one might work quite well (without the hole).Jon

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 09, 2019, 09:00:57 pm
content: You could check BigClive's recent video soldering a LED "coaster", using standard small LEDs on the underside of a normal PCB, to see what that looks like,at 36:49.

author: OM222O
author_rank: Frequent Contributor
timestamp: May 09, 2019, 09:09:45 pm
content: Quote from: Nominal Animal on May 09, 2019, 09:00:57 pmit works well for under glow, but not for an indicator led. as you can see there is a visible band in the middle of each led which is the substrate. what mike suggested seems to work the best so far!

author: Nominal Animal
author_rank: Super Contributor
timestamp: May 10, 2019, 12:27:07 pm
content: Quote from: OM222O on May 09, 2019, 09:09:45 pmI did not intend to imply otherwise, just to point out what the simplest option would look like, since I had just seen it in a video.I would like to see what the other possibilities look like, though, for future reference for myself. In particular, bodging a standard SMD LED upside-down, say a small distance (1-2mm) off the board itself, using extended "legs" (cut-offs from TH LEDs, say), bent into an ∫ or C shape, so no holes need to be drilled. I don't like the glare of direct LED light, especially in the dark, and have used e.g. electrichicken's tape as a diffuser; so, perhaps silkscreenless PCB would work well as a diffuser in some use cases?

author: Fred27
author_rank: Unknown
timestamp: May 10, 2019, 12:36:14 pm
content: Quote from: Nominal Animal on May 10, 2019, 12:27:07 pmIt does. Video on this page:https://www.element14.com/community/community/project14/nfc-rfid/blog/2019/04/28/locknfc-does-the-pcb-work

author: Ice-Tea
author_rank: Super Contributor
timestamp: May 10, 2019, 12:47:54 pm
content: Reverse mount and diffused:https://www.kingbrightusa.com/category.asp?catalog_name=LED&category_name=KCReverse+Mount+SMD+LED&Page=1

author: Jeroen3
author_rank: Super Contributor
timestamp: May 10, 2019, 01:38:57 pm
content: Just get upside down LEDS.1206 upside down chipled:APTR3216ZGCEagle library:https://github.com/sparkfun/SparkFun-Eagle-Libraries/blob/master/SparkFun-LED.lbrPart: LED-1206-BOTTOM

author: OM222O
author_rank: Frequent Contributor
timestamp: May 10, 2019, 03:01:09 pm
content: Quote from: Ice-Tea on May 10, 2019, 12:47:54 pmQuote from: Jeroen3 on May 10, 2019, 01:38:57 pmboth of these require holes / bodge wires since the plastic package extends beyond the pads. that's not ideal!

author: tooki
author_rank: Super Contributor
timestamp: May 10, 2019, 07:36:44 pm
content: search for "reverse gullwing SMD LED" and you find all kinds of results from Osram, Vishay, and LG.https://www.mouser.co.uk/Optoelectronics/LED-Lighting/LED-Emitters/Standard-LEDs-SMD/_/N-b1bb1?Keyword=reverse+gullwing&FS=TrueI think that £0.17 in quantities of 100 isn't too bad at all!