I dont know how important is but i found this info about heat shield:
"There is disclosed a liquid crystal display (LCD) device that prevents degeneration of a displaying performance due to heat generated in the LCD. A heat shielding thin film of an aluminum-contained material, which is disposed in a receiving container, shields the heat additionally generated during a converting process of power for driving a lamp in an inverter module, thereby minimizing the heat to be transferred to a reflector panel. Therefore, the heat generated during a process of producing light is efficiently blocked and discharged, thereby minimizing the degeneration of displaying characteristic"
I have changed many middle frames for iphone 4 and there was not any problems.