Reverse Mount SMD LEDs - Page 1
Forum section: electronics / general

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: December 26, 2019, 11:38:16 pm
content: Perhaps this is a dumb question....Some SMD LEDs, say in a 1206 package, are specifically listed for reverse mounting – mounted upside-down, shining through a hole in the circuit board. I've seen other data sheets showing LEDs reverse-mounted, without this feature being specifically documented.Looking at 1206 LED packages, the contact seems to wrap around the end of the package, bringing the connection from the top of the package to the bottom. Does this mean any, or at least most, 1206 LEDs can be reverse mounted as long as the hole through the circuit board is large enough to clear the LED body?

author: ataradov
author_rank: Super Contributor
timestamp: December 27, 2019, 12:05:19 am
content: Yes if you do manual assembly. No if you do automated assembly. And you will not have a good time assembling that manually and getting consistent results.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: December 27, 2019, 12:18:54 am
content: Hmmm....are they positioned differently on the SMD tape for pick and place?

author: ataradov
author_rank: Super Contributor
timestamp: December 27, 2019, 12:25:51 am
content: That's part of it. Regular mount LEDs are with a lens on the top, obviously. So you would need to turn them all over.But another problem is having them sit straight in those holes. Reverse mount LEDs are designed to sit flat on the board. The regular LEDs will not align properly in a tight hole of the PCB. They will all point to a slightly different direction. This will be very noticeable if you have a row or a matrix of those LEDs.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: December 27, 2019, 01:02:20 am
content: I did a quick bit of research to see that reverse-mount LEDs are indeed "bottom up" in the SMD tape. I didn't even think about that.There is a series of Everlight reverse-mount LEDs on LCSC, but nowhere on the data sheet is this detail mentioned. Some of the datasheets for this series show a PCB footprint with a hole and the LED reverse-mounted. Others in the series don't show the hole or the reverse-mount positioning, despite being positioned in the tape properly to do so.Thanks for the info.

author: ataradov
author_rank: Super Contributor
timestamp: December 27, 2019, 01:42:11 am
content: It might help if you provide some links. There are two types of reverse entry LEDs. The first type absolutely needs a hole. Those are essentially your normal SMD LEDs with minor modifications to the design. The lens has to go somewhere for pins to reach the PCB.Others can be mounted flat even if there is not a hole. They can illuminate things right though the PCB material. This allows for nice effects if you block some of the light with a copper layer.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: December 27, 2019, 09:39:40 pm
content: Let's call theseLED 1,LED 2andLED 3with links to the datasheets. All of these are identified as reverse mount on the manufacturer's web page and the LEDs are shown in the correct orientation in the SMD tape. But nowhere in the datasheets is reverse mounting specifically called out.The datasheet for LED 1 shows the the LED reverse-mounted with the PCB footprint. The datasheet for LED 2 shows a hole in the PCB footprint, but doesn't show the mounting. The datasheet for LED 3 doesn't even show a hole in the PCB with the footprint. It's just a bit confusing! Correct positioning in the tape is the key factor for having boards assembled.

author: ataradov
author_rank: Super Contributor
timestamp: December 27, 2019, 09:46:58 pm
content: In this case it is just a matter of what they chose to show. Obviously all those LEDs need a hole.

author: German_EE
author_rank: Super Contributor
timestamp: December 28, 2019, 10:50:27 am
content: Build your circuit on 1mm FR4 and use green or yellow LEDs, then you don't even need to drill holes as they shine quite nicely through the green PCB material.

author: wraper
author_rank: Unknown
timestamp: December 28, 2019, 11:03:18 am
content: Quote from: German_EE on December 28, 2019, 10:50:27 amFR-4 is not green.

author: Fred27
author_rank: Unknown
timestamp: December 28, 2019, 12:11:28 pm
content: I've used a reverse mount RGB LED shining through FR4 and it worked really well.