Many electronic products require thermal cooling systems to remove heat generated in the electronic boxes and to maintain proper operating temperatures in the transistor junctions within the PCBs. Several techniques have been developed to achieve thermal cooling for electronics. They include adding fans to the electronic box or using PCBs with thermally conducting cores. Several core materials are used, including aluminum and composite materials with high thermal conduction properties.
Thermal cooling systems can be overdesigned using these techniques. Cost reduction in thermal designs can be achieved by using six sigma quality principles. Statistical data such as best- and worst- case design conditions have to be gathered from different sources including current thermal designs and thermal modeling of new de- signs.
A summary of an example thermal design assessment is given in Table 8.14, In this case, the thermal cooling system for an electronic box is specified to maintain the transistor junctions (Tj) on the PCBs to less than 105°C when the inlet air temperature into the electronic box is given at 55°C, The current design meets these specifications with two cooling fans and composite core PCBA. An analysis of the variability of current boxes indicates thatσ= 1.2. Table 8.14 lists the total temperature rise starting from the inlet air, through the different heat transfer mechanisms in the box and PCBs, to the transistor junctions (Tjs). Best- and worst-case conditions are given for current design, resulting in an average Tj = 96℃ and a 7.5σ design quality based on the maximum specification of 105oC- Two assumptions are made in this assessment: the best-case condition and the variability of the current design remain the same for all scenarios. Several modified design scenarios are explored in Table 8.14. One is to substitute aluminum for the composite core, while the other is to use only one cooling fan. The first option increases the thermal core conduction through the PCB assembly from 7 to 11, while the second increases the conduction and convection air temperature rise by 4°C each. The effect of the modified designs is to decrease the heat transfer of the design. This raises the average transistor junction temperature, hence reducing the distance from the average to the 105°C Tj specification. It can be seen from Table 8.14 that substituting the composite core with the aluminum core and the current two fans will achieve six sigma quality, whereas removing one fan from the original PCB design will result in a four sigma design.
In this example, six sigma can be used as a tool by the design team to explore alternatives to reduce the cost of overdesigned systems while maintaining high quality and low defect rates.