Flat Sections in LED Leads -- Why? - Page 1
Forum section: electronics / general

author: rediv
author_rank: Newbie
timestamp: November 27, 2021, 09:49:43 pm
content: Possibly dumb question, not sure where/which forum to ask...I needed some white LEDs and ordered these without looking at the mechanical diagram:https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiV_pacwbn0AhWQRDABHRNbCA8QFnoECAcQAQ&url=https%3A%2F%2Fcree-led.com%2Fmedia%2Fdocuments%2FC512A-WNS-WNN-942.pdf&usg=AOvVaw3QdQwqfQ8r9sg_H6kaspxOThere are flat spots on the leads that prevent fully inserting the LEDs into my PCB. Never seen that before. What purpose do they serve?Thanks.

author: flolic
author_rank: Frequent Contributor
timestamp: November 27, 2021, 09:53:13 pm
content: Exactly that.

author: Infrared_Fred
author_rank: Regular Contributor
timestamp: November 27, 2021, 10:10:58 pm
content: I think the point of the flat parts is that if the LEDs get fully inserted to the epoxy case topuching the circuit board, overheating is more likely, especially during something like hand soldering, where the time of heat applied to the leads may be longer than the heat it gets during wave soldering. Also, having air space with exposed leads would probably give a lower operating temperature for the LEDs, which gives a longer lifespan since heat kills LEDs.

author: eti
author_rank: Super Contributor
timestamp: November 28, 2021, 05:22:34 am
content: Crude but effective stand off

author: Gyro
author_rank: Super Contributor
timestamp: November 28, 2021, 01:27:22 pm
content: It also allows for some differential expansion between the epoxy package and the PCB.I bought some LED car lamp replacements (as a light source for a microscope), where the LEDs were flush to the board. Even under-running them, they didn't last long - they started the flashing behaviour characteristic of detached bond wires (package stress).

author: wraper
author_rank: Unknown
timestamp: November 28, 2021, 01:42:38 pm
content: Leftowers of leadframe used during production.

author: langwadt
author_rank: Super Contributor
timestamp: November 28, 2021, 02:24:28 pm
content: Quote from: eti on November 28, 2021, 05:22:34 amand looking at the datasheet it is also the only specified distance from the top of the led

author: tooki
author_rank: Super Contributor
timestamp: November 30, 2021, 09:22:59 am
content: Quote from: rediv on November 27, 2021, 09:49:43 pmHuh? It’s rare for through-hole LEDsnotto have those wider sections. Their size varies significantly (with some having them trimmed to almost nothing), but they’re basically always there, and wraper’s image explains why.

author: rediv
author_rank: Newbie
timestamp: November 30, 2021, 06:15:41 pm
content: Thanks to all for the explanations.

author: EPAIII
author_rank: Super Contributor
timestamp: December 07, 2021, 12:46:59 pm
content: Actually that photo does not explain "why". It only shows the origin of the wider part of the leads. There is absolutely no reason in the world why that horizontal metal buss can not be trimmed flush with the rest of the lead's length to a very high degree of accuracy.The other posts indicate two excellent reasons why it is there in the first place and why some of it is left after the part is complete. One, since it is the basis for the specified height of the LED, it can be used both in the production process to mold the plastic around the working end in a precise manner. And secondly it can be used to space the LED a precise height above the PCB for mechanical assembly as the circuit board is assembled. Many products will have an outer case with a hole which the LED protrudes through. It needs to be high enough to be seen but low enough to not interfere with final assembly.It can also be used to automate the precision bending of the leads if that is needed for assembly.This is a feature, not a problem. Really it is.Quote from: tooki on November 30, 2021, 09:22:59 am

author: wraper
author_rank: Unknown
timestamp: December 07, 2021, 01:06:00 pm
content: Quote from: EPAIII on December 07, 2021, 12:46:59 pmDunno how it is supposed to help, neither I've ever seen the plastic molded around the LED.EDIT: seems I understood what was meant wrongQuoteIt has little use for such application since does not help to keep the LED straight on the board.QuoteI don't see how it will help with this either. It only bakes bending harder, since bending close to wider area will cause the bend area to have a sharp radius where the width changes and make it prone to cracking.

author: langwadt
author_rank: Super Contributor
timestamp: December 07, 2021, 06:28:14 pm
content: Quote from: wraper on December 07, 2021, 01:06:00 pmthe plastic that makes up the plastic part of led is molded ...

author: TimFox
author_rank: Super Contributor
timestamp: December 07, 2021, 07:05:43 pm
content: When LEDs were a novelty, I remember through-hole parts (low-power red LEDs) in a metal case similar to TO-18 (with clear plastic top) that had non-rigid wire leads (again, similar to a 2N2484 transistor) without the steps. I haven't seen them in decades, however.

author: wraper
author_rank: Unknown
timestamp: December 07, 2021, 07:51:58 pm
content: Quote from: langwadt on December 07, 2021, 06:28:14 pmI understood it as if after the LED was produced someone molds the plastic over it. Leadframe in a way to keep its shape not only during plastic molding but during the whole manufacturing process, such as die attaching, wire bonding.

author: Infrared_Fred
author_rank: Regular Contributor
timestamp: December 10, 2021, 09:03:15 pm
content: Quote from: TimFox on December 07, 2021, 07:05:43 pmJSYK you can still get some from Digikey though they are expensive, apparently they are for rather exotic sounding applications.Digikey Metal Can LEDs