Improving heat conduction when using a reflow hotplate

[skelly]

I have a reflow hotplate setup using a cheap hotplate designed for warming phone screens for repair. It'll go well over 200C and has overall worked pretty decently for reflow except when it comes to when the boards are slightly warped (or perhaps the hotplate itself isn't perfectly flat...), which causes the reflow to occur unevenly and honestly I have a hard time getting boards to reflow completely at all. I've thought about using a thermally conductive silicone pad but those are usually rated to about 200C and don't want to run close to the max of their rating like that. I'm wondering perhaps if enclosing the setup would work better but at that point I might as well just get a cheap toaster oven and a reflow controller for it.

Are there any methods to improve conduction or is there perhaps something else at fault here?

[fourfathom]

I've heard of people putting sand in the hotplate to deal with uneven surfaces, and putting a steel disk (such as a circular saw blade) to even out the heat.  One problem with the steel disk is that it slows the cool-down phase.

I quickly gave up on the hotplate/skillet and moved to a toaster oven.  I built my own controller, and left the oven essentially stock other than drilling a small hole for the thermocouple cable.  I use this for reflow soldering.

[Siwastaja]

I use fiberglass enforced silicone sleeve (you know, the white stuff used to protect wires in hot environments, or put over fusible resistors to protect surrounding components, pretty cheaply available in different diameters), cut it in pieces, and support the PCBs on the top of those pieces.

This leaves room for air circulation to provide some thermal decoupling. The board supports itself on top of these flexible pieces. Never had warping issues.

I preheat the hotplate to just 140degC or so (because I still use a 3D printer headbed, was supposed to be a temporary hack but working too well to be replaced; the FR4 material used in it can't take much more than that) and let 350degC hot air from above do bulk of the job. Obviously a tad more temperature from below would be even better, but this is working for me.

Anyway, those glassfiber sleeves might survive some 300degC, so maybe try it out. But, I do recommend combining hot air from above, limiting the temperature and time required to do the job.

[Peabody]

I had a similar problem using the $10 $15 Walmart hotplate, which has a spiral heating element.  I mounted a circular saw blade a centimeter above the heating element, which equalized the heat pattern.  If your phone warmer is hot enough, you could put something under the four corners of your board, so that none of the board touches the surface.  Or as suggested above, a layer of sand might work to get everything back in contact.  Basically, the whole board needs to be one way or the other - all of it touching, or none of it touching (except at corners).

Here's a video on the saw blade option: