LED MCPCB footprints deviating from Cree's recomended layout?

Mike_C · 2018-12-13T15:33:01.000Z

This is the recommended footprint for Cree XP LEDs taken from the datasheet:

This is the current Noctigon XP footprint (picture stolen from intl-outdoor.com).

I’ve never thought about it before, but the Noctigon has two small tabs on the outter sides of the anode and cathode tabs, and also the middle thermal pad has rounded top and bottom. This is deviating from Cree’s recommendations. Other brands of MCPCBs have similar tabs. What are they actually there for? To gather up excessive solder paste if too much is applied?

netprince · 2018-12-13T15:44:00.000Z

I always thought they were there to gather extra solder for those of us that reflow by hand and are not as precise as machines…

Mike_C · 2018-12-13T20:15:41.000Z

They do tend to blob up with solder, so that’s what they are for then. It’s the only reason I could think off.

eas · 2018-12-14T02:42:13.000Z

Adding to what netprince said, I think I’ve seen them referred to as a “solder thief.”

DavidEF · 2018-12-14T02:51:45.000Z

CREE ratings are fairly conservative. Even a cheap, thin, non-DTP MCPCB, or in some cases FR4 PCB, will work just fine within CREE emitter ratings. So, the possibility of “extra” solder doesn’t really even matter. It’s not until you really want to push things, the way BLF members have done for a very long time, that you see gains in efficiency and/or reliability from something as simple as a “solder thief” at the ends or sides of a solder pad. Notice that is a Noctigon Copper DTP MCPCB with short, wide copper traces and gold coating on all contact surfaces.

eas · 2018-12-14T03:58:06.000Z

They aren’t just used on nice DTP MCPCBs. Check Kaidomain, plenty of cheap thin aluminum core MCPCBs with them.

CRX · 2018-12-14T05:34:17.000Z

The little side overfills are a good indicator of the right amount of solder used.
First the LED pops itself into place when reflowing then a light press down on the emitter causes a little bump to form on each side. Not that I’ve ever achieved this
Massive blobs- too much, no blobs - not enough.

Enderman · 2018-12-14T05:39:15.000Z

The extra metal out the side of the LED guides the excess solder there, which helps avoid the pads from shorting below the LED, to an extent.

kiriba-ru · 2018-12-14T07:11:32.000Z

CREE recomendations work well if you have automatic solder paste dispenser and oven for heating.
For manual soldering, most prefer put more paste on pad (but not less) that it is necessary, and noctigon`s pad shape is more suitable.

Mike_C · 2018-12-14T09:26:15.000Z

Alrighty, that settles it. Thanks all!