how important is polishing the led-board/pill interface and the use of heatsink paste?

Flashlight Modding and DIY Parts
1

I am now regularly using a small home-made hotplate for reflowing emitter onto boards (a block aluminium coupled to a power-adjustable solder iron) and what surprises me is how fast the board heats up. The block is not too much hotter than the melting temperature of solder, its surface has been sanded flat but not polished or anything, I sand the board flat before the reflow but only with 800 sanding paper, nothing near shiny. And no heatsink paste there. Still the solder melts in 10 seconds after I place the board on the heating block, watch this:

Or am I just thinking that 10 seconds to heat up the board from room temp. to 180 deg C. is fast, while it actually isn't fast enough for led heatsinking?

(on a side note: because there are those small extra wire solder pads near the emitter pads on the BLF 16mm Sinkpads, a XM-L led sometimes floats to the side onto one of them wire pads as can be seen on the video. Nothing that can't be solved with a small touch with the tweezers, but still a bit annoying)

2

Moving heat from a large mass to a much smaller one is easy. If you were trying to heat that huge block with a hot Sinkpad, you'd find it a lot harder.

3

I generally try to sand flat with a light polish. The surface is generally pretty shiny but I don’t fully polish as sometimes I don’t have the ability to put enough pressure. I’d assume some “lines” would allow the paste to flow into those areas.

Yea, since the copper sinkpad is so small it probably heats up quite fast. Doesn’t need a lot of energy to reach a high temp.

If that’s solder paste, isn’t that way too much? and I think it might be better to slowly heat for the reflow.

4

I guess it depends on how fast you need to transfer the heat. I’m sure that in most instances it’s not that big of a deal. However if you are trying to push as many amps as possible I think it’s just wise to do everything you can.