Mechanical and Thermal solutions

The iPhone as in the case of many other phones by other makers use an exoskeleton of the case and glass screen.

No phone has an interlace of girders to stiffen the structure of the phone internally. Many plastic cased phones require an inner metal case to offer the needed strength (i.e. iPhone 5c). But thats due to the softness of the plastic.

The iPhone 6 & 6 plus do not use any heat pads or heat pipes. I don't know of any smaller screened phone that has either (upto 5.5").

I'm sure some larger phones "Phablet" with super HD displays may need them as the GPU processing needed can push the heat. Even the iPhone 5s has been known to get quite warm when running very aggressive visual games for a long time.