Guide to Designing

Guide to Designing

CPU OR COMPONENT INTERFACE	Noren Products Inc. (NPI) heat pipes can interface with all types of CPU component designs, PGA, BGA and LAN. One option is to retrofit PGA-CPU connections using Noren Products transition sockets, in the case where through holes in the PC board are not possible. Similar products are available for outer CPU interfaces.
THERMAL JOINT	The thermal joint is a critical component of the system thermal budget. Thermal resistance varies greatly among the different types of joints. The best are brazed or solder joints; the worst are "clip on" joints.
TEMPERATURE vs POWER	There are many factors that influence the ∆t in a heat pipe: the wick structure, the properties of the working fluid and the operating temperature of the heat pipe.
FIN INTERFACE	To dissipate the absorbed thermal energy in the heat pipe, the condenser end of the pipe must be linked to a suitable heat sink. This is accomplished by rejection of the heat into the air, either by natural or forced convection, or by sinking the output to an enclosure structural component.
HEAT PIPE RELIABILITY	Heat pipes have been widely accepted in military applications and have high reliability standards. Noren Products Cu/H₂0 heat pipes can be expected to operate for over thirty years, far beyond the life/usefulness expected of the electronics.
WORKING FLUIDS	The more common working fluids for heat pipe designs used for ambient temperature ranges are water, ammonia, acetone and methanol. NPI uses all of these and more, having developed over 37 fluid combinations for specific applications.
BENDING	Noren Products has developed special tube bending tools and procedures. This special tooling fits on the inside of the wick against the wall. Most tooling is quite elaborate and can only be used by withdrawing the bending tool as the heat pipe is being bent. As a result of this custom tooling, there is no degradation of performance for factory bent heat pipes, no matter how elaborate they are bent or how many bends are made.
THERMAL RESISTANCE	Heat pipes perform much differently than solid conductors that have a fixed thermal resistance. In designing heat pipes you have to consider three controlling resistances: input (getting power into the pipe), output (getting heat out of the pipe) and length of the pipe. The lower the total resistance, the cooler the CPU or device will run. In most designs the resistance down the length of the heat pipe is so low it need not be considered. The thermal resistance in and out of the heat pipe and all the joint resistances usually control the performance of the pipe. The input and output resistances of the heat pipe will vary with the inside temperatures. The designer can use some standard design criteria to determine the feasibility of a heat pipe application. The resistance in and out is inversely proportional to the amount of surface that the pipe has exposed to the input heat. The input is normally higher than the output resistance, therefore the following factors can be used for a Noren Products standard 3mm diameter heat pipe, input resistance 0.008 °C/W/mm of length and for the output 0.006 °C/W/mm of length. To determine the in or out resistance of the heat pipe, simply divide the factors by the length of the 3mm diameter heat pipe. The total resistance of the heat pipe is the sum of these two resistances.
LIGHT WEIGHT HEAT PIPES	Noren Products manufactures its heat pipes with special copper tubing to ensure lightweight, high power and long life. For reference, a 3.0mm x125 heat pipe weighs only 2.5 grams, much less than the standard heat pipe which weighs 9.5 grams.
POWER TRANSPORT	Noren Products heat pipes have a higher power capability than other designs. Due to its flat configuration, flat heat pipes are thin in profile, down to 1.6 mm and their flat surfaces allow for better thermal contact giving them increased performance. The increased cross sectional area allows for a lower ∆t and greater heat transport. These high power pipes use a hybrid wick that has excellent fluid pumping against gravity and will work in any orientation.

GETTING STARTED ON YOUR HEAT PIPE DESIGN

YOU NEED THE FOLLOWING INFORMATION

A sketch of the heat pipe on the PCB.
Sketch or drawing of PCB and component layout.
Sketch or drawing of PCB and its location in the system.
Space available for fins or heat sink.
Airflow available in LFM and CFM..
Processor or component type, power and package.
Component heat load (in temperature and watts.)
Desired operating temperatures, components and system
Heat transfer, size of input area & output area.
Orientation, horizontal or vertical.
Any special thermal requirements, Belcore etc.