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I am making my own LED bulbs (for wedge-base sockets in a low-voltage application). A key construction detail requires me to cement a small (roughly 3/8" x 1") piece of FR4 circuit board (which plugs into a wedge-base socket and holds a couple of small components) to an aluminum "star" (a thermal substrate for the LED emitter itself):

https://www.mouser.com/ProductDetail/Bergquist-Company/804087?qs=jQRjkUoUCJebprw0Kn9Vjw%3D%3D

The FR4 will be perpendicular to the star, with the narrow 3/8" edge of the FR4 against the back of the star.

enter image description here

The Osram Oslon SSL emitter will be running at up to a watt (but usually closer to 3/4). They will operate continuously for up to 10 hours at a time. The maximum spec'd junction temp is 135 C.

My question, what is the best method for making this attachment? I have tried JB Weld epoxy (two part), 100% silicon caulk (specifically GE clear Silicon II), and intend to experiment with Sugru. The epoxy is quite strong, but I have managed to break the connections when pulling bulbs out of sockets when they have gotten stuck. The silicon has the advantage of flexibility, but I don't think it's as strong; it's also easier to assemble, as it has enough stiffness to hold the FR4 in position while it cures, whereas the epoxy requires bracing. I've never worked with Sugru.

Are there other methods I might consider? Factors I haven't considered?

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    \$\begingroup\$ Do you have any specs for temp rise of junction e.g 50’C and done any calculations on Rth . Most Plastics including silicone, epoxy , iron particles in JB weld , FR4 make great thermal insulators, but thinner is better. also PU subfloor structural adhesive on edges is good. Generally 99.9% silver epoxy is best with thinnest layer and structural support to prevent warp. \$\endgroup\$ May 26, 2018 at 19:38
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    \$\begingroup\$ I've recently used JB Weld: Plastic bonder to mount nylon standoffs on a zinc plated steel base. Very mild flex and a rock solid bond. Would imagine it would work well for your needs, but can't say for sure. \$\endgroup\$
    – Phil C
    May 26, 2018 at 20:35
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    \$\begingroup\$ Part of the problem is the thermal expansion and contraction of aluminum with temperature. I was able to bond a aluminum bracket to a stainless steel box only with RTV 262, which has no ammonia in it. All types of epoxies and cements failed the heat/freeze test. I do not have a better opinion than RTV. \$\endgroup\$
    – user105652
    May 26, 2018 at 21:27
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    \$\begingroup\$ My expectation is that epoxy will fail at high temperature. However, it may work a bit better if it is specifically designed for high temperature and if you are able to apply a controlled post cure heat treatment. Silicon seems a good choice. You MUST use a silicon designed for PCB's. There is also double-sided acrylic adhesive heatsink tape. Whatever you use, you may need to perform some surface treatment first. Any film residue on the aluminum or FR4 can cause the bond to fail. \$\endgroup\$
    – user57037
    May 27, 2018 at 6:33
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    \$\begingroup\$ @RustyShackleford correct. I was not suggesting PU for thermal bond but structural strength for CTE shear and warp forces. The silver epoxy only needs to fill the pits of non-coplanarity clamped to remove all air gaps so very thin. CPU use silver and ceramic filled grease under extremely flat surfaces with strong structural spring force to do the same. But here structural weakness is evident. \$\endgroup\$ May 27, 2018 at 14:43

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This should be able to be "designed" with a reasonable degree of confidence either from theory alone or with a little experimental help.

The key parameters seem to be heatsink temperature, forces involved and adhesive performance with the materials used.

Independent of the adhesive considerations, the LED mounting temperature has a max upper limit set by the LED temperature ratings. As a first approximation, modern lighting class Cree LEDs are rated at 105 degrees C operating. That's hot by conventional electronic standards - you will not usually want to operate equqipment continually much above that unless there is an extremely good reason to, and lower is liable to be wise.

You do not say what wattage LEDs you intend to use, operating periods and duty cycles, or target temperatures or overall size. These combined will control whether it is possible to build your 'bulbs' without means of exterior heat removal.

A look at the technical data sheets of a few epoxy adhesives show a wide range of temperature specs. Some basic epoxies 60 C max operating) would not work. Others with a 120 C max "maybe" rating will "maybe" work. High temperature epoxies withstand temperatures that would rapidly kill LEDs.

Examples:

  1. Here is a semi-randomly chosen epoxy technical data sheet (TDS).
    BOSTIK EPOXY BOND 5 MINUTE TWO PART ADHESIVE

They say

When fully cured, the adhesive can withstand boiling water for short periods of time.Heat resistance rating of 60ºC temperature continuously.

ie - it would perhaps meet your needs for LEDs guaranteed to be on for short periods (indicator, stop ...) but would fail in applications where temperatures rose to 105+ degrees.

  1. EVO-STIK HARD & FAST TWO PART EPOXY RESIN ADHESIVE STICK.
    Temperature: -40 C to +120 C depending on conditions of use

MAYBE!

  1. Resinstech RT323 High Temperature Epoxy Resin System
    Operating Temperature: -50 C to 200 C

Suitable.

  1. MG Chemicals High Temperature Epoxy Encapsulating & Potting Compound 832HT
    Constant Service Temperature -30 to +225 °C
    Max Intermittent Temp. 250 °C

Suitable.


Looking at TDSs for various silicone rubbers would allow comparison, but there seem to be adequate epoxies available.

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  • \$\begingroup\$ Thanks, Russell, for responding. I have edited the question (just below the figure), to add some of the info you mentioned. 135C seems awfully high, but it looks right (the "absolute" max is 160C): media.osram.info/media/resource/hires/osram-dam-2495583/… \$\endgroup\$ Jun 17, 2018 at 20:52
  • \$\begingroup\$ I have mostly experimental evidence now, which is some test bulbs I've made with silicon RTV and with basic JB Weld epoxy. The epoxy will break if I pull too hard, but I doubt I prepared the Al adequately (I gather I should wipe with solvent, buff with emery cloth, and wipe again). The RTV works better, but will break eventually. A gentlemen at Loctite recommended this 2-part polyurethane, and I've got a tube on the way: ellsworth.com/products/adhesives/urethane/… \$\endgroup\$ Jun 17, 2018 at 20:57
  • \$\begingroup\$ These test bulbs have been running up to a year. The star doesn't not get too hot to touch (highly scientific). But I believe it's correct that temperature is the big issue, for three reasons: 1. Adhesive fillet insulates star too much, allowing emitter to overheat. 2. Temperature weakens the adhesive directly (glass transition temp of U-05FL is only 48C, could be a problem). 3. Temperature expansion/contraction of materials weakens joint. \$\endgroup\$ Jun 17, 2018 at 21:04

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