Author Topic: Shining leds through the PCB  (Read 8391 times)

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Offline OM222OTopic starter

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Shining leds through the PCB
« on: May 09, 2019, 12:57:15 am »
Hello
I 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  :(
 

Offline Rerouter

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Re: Shining leds through the PCB
« Reply #1 on: May 09, 2019, 12:59:46 am »
Mount a smd led upside down

The 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.
 

Online wraper

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Re: Shining leds through the PCB
« Reply #2 on: May 09, 2019, 01:16:44 am »
Mount a smd led upside down

The 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.
Not possible unless you bodge them with soldering iron. Top facing LEDs have terminals on bottom part only.
 

Offline OM222OTopic starter

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Re: Shining leds through the PCB
« Reply #3 on: May 09, 2019, 01:17:46 am »
Mount a smd led upside down

The 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.
Light 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!

Not possible unless you bodge them with soldering iron. Top facing LEDs have terminals on bottom part only.

actually 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
 

Online wraper

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Re: Shining leds through the PCB
« Reply #4 on: May 09, 2019, 01:20:19 am »
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.
« Last Edit: May 09, 2019, 01:23:12 am by wraper »
 

Online wraper

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Re: Shining leds through the PCB
« Reply #5 on: May 09, 2019, 01:25:02 am »
actually 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
That includes what I meant by bottom part.
 

Offline OM222OTopic starter

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Re: Shining leds through the PCB
« Reply #6 on: May 09, 2019, 03:52:00 am »
isnt digikey your friend?
https://www.digikey.com/product-detail/en/sunled/XZM2DG45WT-9/1497-1338-1-ND/6615770

something 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 purpose  :P

I'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  :palm:
« Last Edit: May 09, 2019, 03:58:13 am by OM222O »
 

Offline thinkfat

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Re: Shining leds through the PCB
« Reply #7 on: May 09, 2019, 04:49:20 am »
assume the FR4 have light transmittance of 1%, your best hope of seeing anything is to use the brightest LED, which means there isnt much choice.
No, FR4 is actually quite transmissive, no problem having a led shine through.

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Offline BravoV

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Re: Shining leds through the PCB
« Reply #8 on: May 09, 2019, 04:55:22 am »
FR4 with approx. 1000 lumens white led flash light.


Offline T3sl4co1l

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Re: Shining leds through the PCB
« Reply #9 on: May 09, 2019, 08:28:55 am »
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.

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Offline MarkR42

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Re: Shining leds through the PCB
« Reply #10 on: May 09, 2019, 09:36:21 am »
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

« Last Edit: May 09, 2019, 09:44:29 am by MarkR42 »
 

Offline DuPe

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Re: Shining leds through the PCB
« Reply #11 on: May 09, 2019, 10:14:44 am »
maybe soldering a standard led on the bottom (solder) side of pcb would do the job?
 

Offline mikeselectricstuff

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Re: Shining leds through the PCB
« Reply #12 on: May 09, 2019, 10:50:16 am »
A side-firing LED, next to a hole in the copper and resist works quite well
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Offline Fred27

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Re: Shining leds through the PCB
« Reply #13 on: May 09, 2019, 11:22:46 am »
assume the FR4 have light transmittance of 1%, your best hope of seeing anything is to use the brightest LED, which means there isnt much choice.
I'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
« Last Edit: May 09, 2019, 09:17:37 pm by Fred27 »
 

Offline thinkfat

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Re: Shining leds through the PCB
« Reply #14 on: May 09, 2019, 11:28:50 am »
A side-firing LED, next to a hole in the copper and resist works quite well

Reasonably priced in 100s quantities, too :)
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Offline OM222OTopic starter

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Re: Shining leds through the PCB
« Reply #15 on: May 09, 2019, 01:17:37 pm »
A side-firing LED, next to a hole in the copper and resist works quite well
You are a genius good sir! thank you! this is exactly what I wanted  :-DD
I don't know why I thought about THT side view ones but not the SMD ones  :-//

Edit: 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.
« Last Edit: May 09, 2019, 01:21:42 pm by OM222O »
 

Offline jmelson

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Re: Shining leds through the PCB
« Reply #16 on: May 09, 2019, 08:24:26 pm »
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
 

Offline Nominal Animal

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Re: Shining leds through the PCB
« Reply #17 on: May 09, 2019, 09:00:57 pm »
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.
 

Offline OM222OTopic starter

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Re: Shining leds through the PCB
« Reply #18 on: May 09, 2019, 09:09:45 pm »
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.
it 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!
 

Offline Nominal Animal

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Re: Shining leds through the PCB
« Reply #19 on: May 10, 2019, 12:27:07 pm »
it works well for under glow, but not for an indicator led. what mike suggested seems to work the best so far!
I 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?
 

Offline Fred27

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Re: Shining leds through the PCB
« Reply #20 on: May 10, 2019, 12:36:14 pm »
 
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Offline Ice-Tea

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Offline Jeroen3

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Re: Shining leds through the PCB
« Reply #22 on: May 10, 2019, 01:38:57 pm »
Just get upside down LEDS.
1206 upside down chipled: APTR3216ZGC

Eagle library: https://github.com/sparkfun/SparkFun-Eagle-Libraries/blob/master/SparkFun-LED.lbr
Part: LED-1206-BOTTOM
 

Offline OM222OTopic starter

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Re: Shining leds through the PCB
« Reply #23 on: May 10, 2019, 03:01:09 pm »
Reverse mount and diffused:

https://www.kingbrightusa.com/category.asp?catalog_name=LED&category_name=KCReverse+Mount+SMD+LED&Page=1
Just get upside down LEDS.
1206 upside down chipled: APTR3216ZGC

Eagle library: https://github.com/sparkfun/SparkFun-Eagle-Libraries/blob/master/SparkFun-LED.lbr
Part: LED-1206-BOTTOM

both of these require holes / bodge wires since the plastic package extends beyond the pads. that's not ideal!
 

Offline tooki

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Re: Shining leds through the PCB
« Reply #24 on: May 10, 2019, 07:36:44 pm »
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=True

I think that £0.17 in quantities of 100 isn't too bad at all!
« Last Edit: May 10, 2019, 07:42:57 pm by tooki »
 


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