PCB design with reverse mount LEDs

[Rutger]

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?

[German_EE]

Why do you need square holes?

[mikeselectricstuff]

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

[ataradov]

Why do you need square holes?

Yeah, make them round and all your problems will be solved.

[Rutger]

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/1802604

Looking 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

[marshallh]

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.

[Rutger]

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.

Thanks, 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?

[IconicPCB]

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.

[mikeselectricstuff]

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.

Thanks, 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?

I'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.

[KL27x]

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.

[ataradov]

There doesn't seem to be any easy way to actually search for them

On DigiKey they are called Z-Bend in the Package / Case column.

[KL27x]

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/229238

Easy enough to use a round hole for these, methinks.

[krish2487]

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-sense1


I have one such board and the leds are mounted on the "bottom layer" with a hole for the leds. Maybe this addresses your issue.

[georges80]

Yeah, oblong holes. Easy.

cheers,
george.

[ASowa]

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

[Rutger]

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 0805
2) They were the cheapest I could find
3) 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.   

[mikeselectricstuff]

Osram also do some nice round reverse-mount LEDs
http://www.osram-os.com/osram_os/en/products/product-catalog/led-light-emitting-diodes/pointled/index.jsp

[bson]

Just to ask the obvious question - why not mount the LEDs on the other side?

[Zero999]

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.

[matseng]

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

Dirtypcbs does 0.8mm slots at no extra cost...

[tooki]

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.

[Rutger]

Osram also do some nice round reverse-mount LEDs
http://www.osram-os.com/osram_os/en/products/product-catalog/led-light-emitting-diodes/pointled/index.jsp

They 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.

[Rutger]

Just to ask the obvious question - why not mount the LEDs on the other side?

Besides aesthetics, I wanted the surface mount parts on only one side, because of re-flow issues.

[Rutger]

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.

What leds would you use for that? You want them to point up!

[Rutger]

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.

Actually, 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.