Epoxy mounting of 10W LED - Page 1
Forum section: electronics / projects

author: Seekonk
author_rank: Super Contributor
timestamp: November 04, 2019, 02:23:44 pm
content: I've been asked to relamp an outdoor lighting fixture. This will see the normal extremes of weather from -20F to 120F. Want to use epoxy for ease. These 10W chips will only see about 3W so thermal transfer isn't an issue. Mounting to aluminum so thermal expansion could be an issue. Likely use JB WELD because they have a lot of this on hand. Could also use a Silkiflex adhesive product instead. Looking for ease because there is well over a hundred of these to mount. Thoughts?

author: DaJMasta
author_rank: Super Contributor
timestamp: November 04, 2019, 05:32:40 pm
content: Hard to say without really seeing the fixture/LED, but if you end up going the epoxy route, I'd use a thermal epoxy to be sure about the heat concerns (lower die temp will be longer lifetime), and then maybe try to develop a jig or something to hold them in place. With lots to do, you're going to lose most of your time redoing them waiting for the epoxy to get tacky enough that you can leave it be, so having something to hold/press it in place could be useful in speeding things up.Alternatively, if the LED module can be screw mounted, you could look for a way to drive the screws into the existing housing (drill and self tappers if there's a spare layer before you get to the outside), or maybe find something that could serve as a mounting bracket/plate. For example, if you could epoxy your plate in place or otherwise affix it that way, you could come back and screw in the LED module - could simplify attaching wires, make it possible to replace the LED module itself, or just make the mounting depth better.Since these are retrofits, if they were incandescents before, you may find that an LED module sits too deep in the fixture, which gives the light a narrower beam (and any LED optics could do this on their own), so having the LED a little farther from the back wall could end up being better lighting for your situation.

author: ConKbot
author_rank: Super Contributor
timestamp: November 04, 2019, 05:37:27 pm
content: Mounting carrier MCPCBs and not the bare LEDs I assume? Given the low flux density (w/cm^2)as long as the bond line is thin, just about anything is OK. I've done testing with Cree XM-Ls at 10W on 20mm star PCBs, and the quality of the MCPCB made way more difference than attachment glue to the heatsink. IIRC different PCBs made up to a 10% change in lux@1m measurements for me. Arctic Silver thermal epoxy vs JB weld made no measurable difference beyond LED to LED variation. I was keeping the bondline thin by using a socket and C-clamp to keep the star compressed against the heatsink surface until it cured. Not exactly production friendly, but I was too lazy to drill and tap holes for the test.

author: SeanB
author_rank: Super Contributor
timestamp: November 04, 2019, 06:51:06 pm
content: I have done quite a few mounting LED star PCB's to heatsinks using Pratley quickset steel epoxy, because it has a fast gel time ( around 5 minutes, unlike Araldite rapid steel, which takes around a half hour) and the biggest thing is to first solder the wires to the small star PCB first, as once mounted you have to either use a soldering gun ( 200W Black and Decker) and risk delaminating or destroying the epoxy, or else the solder will never melt on the star board. I use the same to stick small heatsinks to hot chips, it works as well as the thermal epoxies for the job, and is very cheap and available all over.if just bare LED dies then use a star PCB first, unless you have one of the more recent all in one integrated LED units, which have the LED's, mains bridge rectifier and current controllers all on the same aluminia sheet. There solder the wires on first, then use the epoxy, as the aluminia does spread the heat out nicely.Major thing is to first make sure the heatsink is flat, and use a surface block and waterpaper ( 100 grit then 400 grit) to get the surface absolutely flat and burr free, and then wash both sides of the heatsink and LED with a solvent to remove any grease or impurities. Epoxy amount is just enough to form a thin film when pressed down firmly, and held there till it cures. Typically for a star PCB a matchhead size works perfectly, placed in the centre and pressed till itr come out the sides evenly.

author: Seekonk
author_rank: Super Contributor
timestamp: November 29, 2019, 06:23:57 pm
content: I finished the first one of seven and it is approved Replaced 12 3W LED each side with 10 10W 5S2P arrangement. Set current limit to 800ma and added a 5 ohm resistor in series. This is in case they replace the power supply and don't set the current.