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In the portable electronics market, power management integrated circuits (PMICs) are increasingly found being packaged into ball grid array (BGA) and chip scale packages (CSP) for their lower material costs, improved electrical performance (no bond wire impedances), and smaller form factors. These advantages do not come without compromise: The silicon die of CSPs are no longer in direct contact with large heat-spreading thermal paddles (E-PADs) used for electrical and thermal conduction.
This is the primary performance trade-off; because the IC substrate is not in contact with an E-PAD there is no high-conductivity direct thermal connection from the substrate to the heat-spreading copper planes on the PCB. This article will discuss PCB level methods that will lower the operating temperature of CSP devices by examining methods to transfer heat from the source and transport it to the ambient environment by lowering thermal resistance of the CSP IC. There are usually multiple ways to enhance the performance while simultaneously lowering the operating temperature that can be incorporated into new boards or revisions of existing boards.
In order to meet size and weight requirements, constraints of portable electronic designs often force PCB designers to reduce the size of components and PCB real estate area. To meet these demands, the use of CSP packages to shrink the PCB area needed is a common change in designs. As a result of the reduction of total PCB area, the available options to move heat and route high-power PCB traces is also reduced. Furthermore, the thermal performance cannot be matched when a QFN is compared to an equivalent CSP package; therefore, it is imperative that the PCB is designed to optimize heat transfer from the CSP to the PCB, which in turn dissipates it into the atmosphere. The parameter measuring the heat conductivity is the junction-to-ambient thermal resistance specification, Theta-JA (ӨJA (˚C/W)).
To read this entire article, which appeared in the January issue of The PCB Design Magazine, click here.