A composite temperature is a value based on multiple threshold temperatures as determined by sensors on the device. SSDs, like other electronics, are prone to overheating and will throttle if they get too hot. Reported to the user by S.M.A.R.T. is one or more temperatures that indicate the drive’s current thermal state. The SSD has several components that must be carefully maintained including the controller, the flash, the PMIC, and the PCB, with a composite temperature measuring the different between current and maximum temperature values of all combined.
Consumer NAND flash is typically designed to operate at up to 70C. The controller itself, which is an ASIC often built on ARM Cortex-R5 technology, could have a very high maximum temperature (e.g. 120C) but will usually throttle at a composite temperature below 85C. Warning and critical temperatures for any given drive can be determined through smartmontools (smartctl) and the active temperature is reported through S.M.A.R.T. which can be monitored in multiple ways.
Typically, users are recommended to cool the drive’s controller for optimal results as that is the component that pulls the most power and dissipates the most heat. However, it’s ideal to cool all components due to the composite nature of throttling, and spreading heat can prevent inadvertent overheating of one specific element due to how the temperature thresholds work.
It’s popularly believed that flash “likes heat” but this is not especially accurate with consumer flash and some drives will even throttle if the flash gets too hot, and ultimately it’s best to focus on keeping such drives from throttling first. Consumer flash is rated to operate up to 70C (junction 85C) but could handle a lot more. It’s true that flash generally has better data retention characteristics if programmed hot and left to idle cool, but the mechanisms of operation are more complicated than that due to a number of factors such as cross temperature and self-repair. Check the additional sources below to get a wider picture of how it works.
Any environment that puts undue thermal stress on the SSD is one in which cooling the entire drive is therefore advisable. The normal rules of cooling apply as far as they go with material choice, surface area, and airflow. M.2 slots can be located in tight and/or hot spots on the motherboard which can impact cooling decisions. Modern motherboards come with their own M.2 thermal solutions which will increasingly be the case with Gen5 and future SSDs. Check the Seagate FireCuda 540 product manual for more information on Gen5 temperature management (p. 25).