PCB design with reverse mount LEDs - Page 1
Forum section: electronics / projects

author: Rutger
author_rank: Regular Contributor
timestamp: February 26, 2016, 08:18:04 pm
content: I am designing a re-flow oven controller that fit inside a standard box. I have used some reverse mount leds in the design, but run into a PCB fab problem.The PCB company Elecrow can not router 2x2mm holes. Instead of re-designing the board with either larger reverse mount leds or putting a small slot in the PCB does anyone have suggestions on how to fix this issue? or are there PCB places in the USA/China that can router 2x2mm square holes?

author: German_EE
author_rank: Super Contributor
timestamp: February 26, 2016, 08:33:06 pm
content: Why do you need square holes?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 26, 2016, 08:45:38 pm
content: Define "square" - the corner radius is the critical thing. The usual way to make a hole for a square object is to dogbone it.The default router size for PCBs is typically 1.6 or 2mm, with sizes down to 1.0 or even 0.5 available, but at extra cost

author: ataradov
author_rank: Super Contributor
timestamp: February 26, 2016, 08:45:59 pm
content: Quote from: German_EE on February 26, 2016, 08:33:06 pmYeah, make them round and all your problems will be solved.

author: Rutger
author_rank: Regular Contributor
timestamp: February 26, 2016, 08:52:17 pm
content: Below are the LED's I picked for my design, they have a square part that fits into the PCB.They don't have enough leg to make a round hole work.Digikey link:https://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/LH%20R974-LP-1/475-1415-1-ND/1802604Looking at the specs again, Elecrow told me they can go as small as a 2mm hole, but I need a small size cutout, about 1.5x1.5mm

author: marshallh
author_rank: Unknown
timestamp: February 26, 2016, 08:57:24 pm
content: Easy, just use an oblong round slot. The slot will extend above and below the LED a small amount, but you can do micro-usb mounting holes like this, should be no problem.Nobody can route square holes, unless you want to buy tooling for a punch/diecut stamp.

author: Rutger
author_rank: Regular Contributor
timestamp: February 26, 2016, 09:00:53 pm
content: Quote from: marshallh on February 26, 2016, 08:57:24 pmThanks, I was thinking the same, but wanted to check if there are other options.Do you think most PCB fabs can do a 1.5mm wide slot cutout?

author: IconicPCB
author_rank: Super Contributor
timestamp: February 26, 2016, 09:19:45 pm
content: Rutger we make prototype PCBs.It is near impossile to make a hole with a 90 degree sharp corner. You are limited by the radius of the cutter.We can and do make cutouts whihc are over compensated by drilling a small hole at the very corner typicaly 0.4mm is our limit.We then rout out the bulk of the hole with a 1mm or a 2 mm tool followed by a clean up cycle with a 0,4mm router.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 26, 2016, 09:30:40 pm
content: Quote from: Rutger on February 26, 2016, 09:00:53 pmI'd make it 1.6 as this is a more common size. Smaller sizes can be done but they need to run slower and so cost more, and may not be available from pooled services.

author: KL27x
author_rank: Super Contributor
timestamp: February 27, 2016, 05:13:31 am
content: FYI, there is such a thing as a reverse mount gull wing LED. There doesn't seem to be any easy way to actually search for them, but I bought some by accident, once. Maybe I can find them, eventually.

author: ataradov
author_rank: Super Contributor
timestamp: February 27, 2016, 05:18:33 am
content: Quote from: KL27x on February 27, 2016, 05:13:31 amOn DigiKey they are called Z-Bend in the Package / Case column.

author: KL27x
author_rank: Super Contributor
timestamp: February 27, 2016, 05:45:23 am
content: Good to know. I'm a Mouser guy, myself.I see Digikey also has a "mounting type" category where you can select "surface mount - bottom entry."I figured out Mouser has a package style "reverse gullwing." But only 3 part numbers pop up, and none of them the kind I accidentally bought, lol.They are easy enough to find on Digikey though. They look like this:http://www.digikey.com/product-detail/en/lumex-opto-components-inc/SSL-LXA228SRC-TR31/67-1395-1-ND/229238Easy enough to use a round hole for these, methinks.

author: krish2487
author_rank: Frequent Contributor
timestamp: February 27, 2016, 05:47:54 am
content: Actually, there is a fairly easy way to do this.There is a add-on boosterpack for msp430 boards. capacitive touch sensor board.http://www.ti.com/tool/430boost-sense1I have one such board and the leds are mounted on the "bottom layer" with a hole for the leds. Maybe this addresses your issue.

author: georges80
author_rank: Frequent Contributor
timestamp: February 27, 2016, 06:11:15 am
content: Yeah, oblong holes. Easy.cheers,george.

author: ASowa
author_rank: Newbie
timestamp: February 27, 2016, 01:24:38 pm
content: I have used round holes with kingbright LEDs. They have longer tabs than what you used.http://www.kingbrightusa.com/category.asp?catalog_name=LED&category_name=KCReverse+Mount+SMD+LED&page=1&OrderBy=4

author: Rutger
author_rank: Regular Contributor
timestamp: February 27, 2016, 03:31:53 pm
content: Thanks all for your feedback.There were 3 reasons why I picked the square face leds over the rounds ones;1) They are are the smallest I could find 08052) They were the cheapest I could find3) They had the most color selection: red, green, blue, and yellow.None of the other reverse mount leds had all these features. The only downside was that they where square, but going with a slot cutout should fix my problems.THanks.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 27, 2016, 04:31:43 pm
content: Osram also do some nice round reverse-mount LEDshttp://www.osram-os.com/osram_os/en/products/product-catalog/led-light-emitting-diodes/pointled/index.jsp

author: bson
author_rank: Unknown
timestamp: February 27, 2016, 08:35:36 pm
content: Just to ask the obvious question - why not mount the LEDs on the other side?

author: Zero999
author_rank: Super Contributor
timestamp: February 27, 2016, 08:59:47 pm
content: Ah bugger ay?I take it this was a budget PCB house? A more expensive place would've told you if they couldn't do something, rather than just attempting to manufacture it anyway.

author: matseng
author_rank: Frequent Contributor
timestamp: February 28, 2016, 02:46:02 am
content: Quote from: mikeselectricstuff on February 26, 2016, 08:45:38 pmDirtypcbs does 0.8mm slots at no extra cost...

author: tooki
author_rank: Super Contributor
timestamp: February 29, 2016, 11:02:59 am
content: Is a hole actually even necessary? Or would it work to simply have the LEDs shine right through the FR-4, using the copper as a mask?Or wouldn't a 0.5-1mm hole work? I mean, you don't need to see the LED housing, just the light from it.

author: Rutger
author_rank: Regular Contributor
timestamp: February 29, 2016, 04:26:30 pm
content: Quote from: mikeselectricstuff on February 27, 2016, 04:31:43 pmThey do look nice, but for low volume (I only needed < 10), they are a bit expensive. The squared ones I used are priced are around $ 0.28 for each color, but the round ones are between $ 0.41 - $ 0.98 (blue).I guess for my first PCB run, it ended up costing me time, because I had to drill them out myself and use a square file, which wasn't fun. A good lesson learned though.

author: Rutger
author_rank: Regular Contributor
timestamp: February 29, 2016, 04:29:58 pm
content: Quote from: bson on February 27, 2016, 08:35:36 pmBesides aesthetics, I wanted the surface mount parts on only one side, because of re-flow issues.

author: Rutger
author_rank: Regular Contributor
timestamp: February 29, 2016, 04:32:52 pm
content: Quote from: tooki on February 29, 2016, 11:02:59 amWhat leds would you use for that? You want them to point up!

author: Rutger
author_rank: Regular Contributor
timestamp: February 29, 2016, 04:38:04 pm
content: Quote from: Hero999 on February 27, 2016, 08:59:47 pmActually, that was also my mistake, I thought I had checked and checked the gerber and drill files, but I missed the fact that the 'square' holes where not included.I find it really hard to find a good gerber viewer that would show those mistakes and in the PCB software (Altium) it showed ok in the 3D viewer.I contacted Elecrow about the missing cutouts and I send them the correct files, and they told me then they couldn't do anything less than 2x2mm.In short I am happy with Elecrow and would try them again.

author: tooki
author_rank: Super Contributor
timestamp: February 29, 2016, 04:42:07 pm
content: Quote from: Rutger on February 29, 2016, 04:32:52 pmReverse-mount LEDs, like you said you're using.Or do you mean that in your original post, when you said "reverse-mounted LEDs" you didn't actually mean reverse-mount LEDs (where the light shines down toward the PCB) at all, but actually meantordinarySMD LEDs mounted upside-down on the bottom of the PCB, with the LED housing in the hole??

author: Rutger
author_rank: Regular Contributor
timestamp: February 29, 2016, 06:56:49 pm
content: Quote from: tooki on February 29, 2016, 04:42:07 pmI guess I don't understand what you mean. A reverse mount led usually needs a hole, otherwise it doesn't sit flush with the PCB or are there ones that are flush mountable ones that project the opposite way?

author: ataradov
author_rank: Super Contributor
timestamp: February 29, 2016, 07:00:22 pm
content: Quote from: Rutger on February 29, 2016, 06:56:49 pmYes, there are. As I said earlier, on DigiKey they are called Z-Bend in the Package / Case column. But selection and prices are not that great.

author: IconicPCB
author_rank: Super Contributor
timestamp: February 29, 2016, 07:48:28 pm
content: A better Gerber viewer is a free version from Graphicode, a subset of their professional CAM station.Updates come out quarterly and the free viewer is well suported in terms of being compatible with their PowerStation CAM package.Powerstation will set You back 5K, Gerber viewer is free.

author: KL27x
author_rank: Super Contributor
timestamp: February 29, 2016, 07:56:07 pm
content: QuoteThese still need a hole for the lens to insert into.I would think a flush mountable reverse LED would be handy for what tooki stated. You could define the shape of the indicator with the copper and soldermask layers on the other side.But it would be of somewhat limited use, because the pcb would probably have to be relatively thin, and that would limit the size/rigidiity of a board that is basically doubling as a housing.There are also different colors of FR-4 which would transmit light, differently, and perhaps for any given pcb manufacturer, the color of substrate you are getting might be subject to change without notice. Some manufacturers don't appear to have provisions in place for selecting FR-4 color.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: February 29, 2016, 08:15:28 pm
content: Quote from: tooki on February 29, 2016, 11:02:59 amA bright LED through FR4 works quite well, but you still need a bottom-emitting LED

author: ataradov
author_rank: Super Contributor
timestamp: February 29, 2016, 08:19:15 pm
content: Quote from: KL27x on February 29, 2016, 07:56:07 pmNot all of them. Here is an example of one that does nothttp://www.digikey.com/product-detail/en/osram-opto-semiconductors-inc/LS%20T776-R1S1-1-Z/475-2699-2-ND/1938730

author: KL27x
author_rank: Super Contributor
timestamp: February 29, 2016, 08:28:55 pm
content: Ahh so. There's that.All this has got me looking for a reverse mount tac switch. I don't see the "surface mount - bottom entry" as a search option on Digikey.

author: ataradov
author_rank: Super Contributor
timestamp: February 29, 2016, 08:49:31 pm
content: Quote from: KL27x on February 29, 2016, 08:28:55 pmThere is Actuator Orientation column, but the only options are Side and Top actuated, so it looks like they simply don't exist.On the other hand, there are things like thishttp://www.digikey.com/product-detail/en/c-k-components/KMT031%20NGJ%20LHS/CKN10290CT-ND/2297281. They looks like they may work in reverse mount configuration, but I would not use them like this for a high volume product.

author: langwadt
author_rank: Super Contributor
timestamp: February 29, 2016, 09:25:08 pm
content: Quote from: KL27x on February 29, 2016, 08:28:55 pmit is probably because it isn't seen as a very reliable idea, the force of pushing the button will directly try to pull off the pads

author: Rutger
author_rank: Regular Contributor
timestamp: February 29, 2016, 10:42:56 pm
content: Quote from: langwadt on February 29, 2016, 09:25:08 pmI wanted a reverse mount switch as well and couldn't find any. I ended putting a touch sensor ic and that is working fine. I used AT42Q chip.

author: Rutger
author_rank: Regular Contributor
timestamp: February 29, 2016, 11:09:48 pm
content: Quote from: ataradov on February 29, 2016, 07:00:22 pmI looked at them, but they don't do one in blue. I might have to compromise and use yellow, orange, red and green for the reflow indication.

author: exmadscientist
author_rank: Frequent Contributor
timestamp: March 01, 2016, 12:36:14 am
content: Quote from: Rutger on February 29, 2016, 11:09:48 pmSure they do.While SunLED isn't exactly a household name (at least not around here), I've used that exact part before and it works just fine. The color wasn't fantastic, but I couldn't be bothered to track down if that was due to the LED, using the wrong current-limiting resistor on it, or just the particular make of FR4 it was shining through -- it was plenty good enough for what I wanted.