How should I be mounting MCPCBs to a heatsink? Should I screw them down as tight as I can? Should I use thermal paste? If so, how much? Also, How should I mount reflectors to the MCPCB?
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I think it varies from build to build. You definitely want to use thermal paste. Pressure from the leads connected to the driver plus the optic/reflector pressed against the lens and o-ring are plenty of pressure to hold the pasted mcpcb in place. As for reflector. Ideally a good fitting emitter spacer made to perfectly fit over the emitter with a perfect fit into the reflector opening is the best but when diving into custom builds and mods you will eventually run into not finding things that fit perfectly. I will say when installing reflectors you need to be extremely mindful that the leads don’t touch the reflector at all or you can damage the driver. Sometimes it’s not a problem sometimes it’s a bear to deal with. You don’t need to glue a reflector in place. It should fit perfectly on top of the mcpcb and snug against the lens. No need to crank the engine in place. Just snug enough to keep everything in place. If you tighten to much you can cause the mcpcb to rotate on the pill and tighten, weaken, or break your emitter leads.
Hope this helps.
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What if its a DIY build? I’m currently working on a build that has no lenses (or even a housing. its all just screwed to an aluminum frame). Do you think its okay to glue the reflector to the mcpcb? I would imagine one would need heat-resistant glue.
Also, is too much thermal paste a problem? I saw somewhere that you want to keep direct metal-to-metal contact as much as possible, and to only use thermal paste for the cracks
With no lens and no housing, the reflector is gonna get scuffed up real quick. I would reconsider that. If you glue the reflector to the MCPCB, you give up the option to ever undo it if you ever need to adjust it.
Never a problem if you have sufficient clamping force–the excess will just squeeze out.
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As firmly as you can, make sure surface of the heatsink is as smooth and even as possible, metal to metal contact gives best heat transfer ratio, thermal paste is only to fill air gaps, its heat conducting ability is lower then 2 metals pressed hard together. but higher than air, if both surfaces of heatsink and mcpcb (aka star) mirror like and flat paste is not needed, but in reality it is hard to achieve, so people use paste, when I mount stars onto heatsink, I put thin layer of paste then I press and twist the star to get as much paste out as possible, only what is filling air gaps is needed, if it blocks metal to metal contact, it is not improving anything , quite the opposite. Of course design of your light dictates how to mount it, but use screws where possible.
In small lights stars often pressed down by reflectors, no room for screws, as long as it presses hard it is acceptable, more ofthen then not those heatsinks are not smooth, but have ridges from machining, so your metal to metal contact is limited, paste is needed to fill those valleys, but not be used as a gasket. there are silicon gasket designed to be used to mount stars, it is acceptable for relatively low power leds, they are not heat conductive enough for high power leds, I had tried, and had leds get so hot, they unsolder themselves, you star is the most critical part as far at heat transfer, not only you should use copper direct heat path star but how you reflow matters a lot, i have seen many premonted leds poorly reflown where heatpad only had some contact with the star, and the rest was hanging in the air, that is why I only buy bare leds and reflow myslef.