
Thermal Specification of Microelectronic Packages 123
resistance of a package. Figure 6.27 shows that junction-to-air thermal resistance
of a 132-lead PQFP package is less if it is mounted on a board with higher thermal
conductivity. It also shows that the thermal resistance decreases as the PCB area
increases. However, there is no reduction in thermal resistance for PCB to pack-
age area ratios larger than ve. The reason is that the board temperature reduces
at larger distances from the package, the heat ux which is proportional to tem-
perature difference between the board and air reduces, and those areas do not help
in heat dissipation f