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4 11 Publication Order Number:AND9596/DAND9596/DA Quick PCB ThermalCalculation for PowerElectronic Devices withExposed Pad PackagesINTRODUCTIONT hermal design of PCBs in electronic systems is criticalto maintain device operating temperatures below specifiedlimits. Although the predictions from full field CFDsimulations are accurate, the computational cost and modelgeneration time could be fairly high. Thus, it is preferable touse a quick estimation tool to design a preliminary layout ofPCBs with different heat dissipating from some of the existing cooling packagingoptions in the market today, ON Semiconductor sexposed pad packaging solution offers standard lead framebased board mounting in a fully encapsulated DFN and QFNmolded package (see Figure 1), minimizing the thermaldifference between the device and the PCB while offeringnear zero parasitic inductance with its layout 1.

5 Typical Exposed Pad Packages for power ICs (Top View)THERMAL RESISTANCE MODELS FOR DEVICESAND BOARDWith thermal simulation software available today, it ispossible to predict the temperature for a PCB board, even theentire system at any chosen location within that many designers do not have access to thermalsimulation software and thermal models for an entire PCBboard, nor even would want to spend the time on a largenumber of lengthy computations for each set of possiblecooling arrangements. So with this in mind, a fast estimationmethod of thermal dissipation for power electronic devicesand the PCB board with thermal resistance parameters isproposed, which will rapidly provide the maximumpermissible power generation by the Semiconductor devicefor any combination of substrate and cooling an exposed pad package placed on a PCB in naturalconvection (still air) environment (see Figure 2), the heatflow patterns from the package and environment can bedrawn (see Figure 3).

6 A significant point is that heat transferefficiency from the PCB to the air is often the dominanteffect on the overall temperature difference between the chip( junction ) and the dissipation happens mainly by conduction andconvection. (Radiation can be significant in still airapplications, but it is most often accommodated in simpleanalysis tools as an adjustment to the natural convection film coefficient .) For handy reference, the thermalconductivities of a number of materials commonly found inelectronics applications are listed in Table 1 [1]. The naturalconvection film coefficient h is an experimentallydetermined parameter whose value depends on all thevariables influencing convection such as the surfacegeometry, the nature of fluid motion, the properties of thefluid, and the bulk fluid velocity.

7 We can empirically take NOTEAND9596 as 15, 30, 45 W/(m2K) for air velocities of 0, , m/s [2]. (As noted above, for air velocities lower than1 m/s, a significant fraction of this coefficient, as much as35 40%, may actually be due to radiation, so surfaceemissivity may need to be considered. Shiny, metallicsurfaces tend to have lower radiation contribution;matte finish, non metallic surfaces tend to have higherradiation contribution).A four resistor simplified equivalent thermal networkdiagram of the package and environment can be drawn (seeFigure 4) with the package represented by a two resistormodel [3]. The die represents the junction node, whereas thecase top and exposed pad in the bottom plate represent the Case Node and Board Node in the diagram, 1.

8 MATERIAL THERMAL CONDUCTIVITY PROPERTIESM aterialThermal Conductivity (W/mK)Silicon145 Mold Frame277 Die Attach Colder50 Figure 2. Exposed Pad Package on PCBF igure 3. The Heat Flow Directions (Red Arrows)AND9596 4. Equivalent Thermal Resistance Diagram of the Modified Two resistor Model on a PCBIn many applications of exposed pad packages, it ispossible further to simplify this network from four resistorsdown to two resistors. Several reasonable approximationsmay be considered to justify this:1. B TCbottom and JB JCbottomThe exposed pad packages are designed to injectheat directly into the copper plane of the board viabottom pad, and the effect of solder between thepackage pad and PCB pad is neglected due to itsthin and with not bad thermal conductioncapability. So the bottom of the exposed pad canbe taken to be the board temperature, that is B TCbottom and JB JCtop >> JBTake 6 6 mm QFN package of the NCP323X ina natural air environment, for example, we have JCtop 22 C/W, while JB JCbottom, (from approximation 1) is only C/W.

9 Thus JCtop >> CA >> BABecause convection resistances are inverselyproportional to the exposed heat transfer surfacearea, as a general rule PCB convection resistanceto air is much lower than case thermal resistance toair. For example, the total area of both sides ofa 50 50 mm PCB is 5000 mm2, whereas thesurface area of a 6 6 mm QFN is only 36 , CA >> JA JB + BASimple analysis of the four resistor thermalnetwork (Figure 4) shows that:qJA (qJB)qBA) (qJCtop)qCA)(eq. 1)From the 2nd and 3rd approximations above, ( JB + BA) >> ( JCtop + CA). Thus can besimplified as:qJA qJB)qBA(eq. 2)This means for exposed pad packages thedominant heat dissipation path is through the PCBvia the bottom pad to summary, then, when JB can be read from a datasheet,as, for example, the NCP81295, then only BA remains todetermine the total package/PCB system thermal useful approximation with respect to thefour resistor package/PBC system is J the same 6 6 mm QFN example, the junction ( J)to case top (TC), thence to ambient (TA), thermal resistancescan be calculated by conduction and convection yields the result of JC 22 C/W vs.

10 CA 1300 voltage divider analogy, the difference TJC is thus onlyabout (22/(1300 + 22)) = of the total TJA. If thejunction was 100 C above ambient, then the differencebetween junction and case would be less than 2 2. THERMAL CHARACTERISTICSR atingSymbolValueUnitThermal Resistance, Junction to AmbientRqJA30 C/WThermal Resistance, Junction to Top CaseRqJCT50 C/WThermal Resistance, Junction to Bottom C/WThermal Resistance, Junction to C/WThermal Resistance, Junction to C/WAND9596 of Thermal Resistor of Board to Ambient (qBA) BA represents the thermal resistance of the PCB, from thepoint at which the package puts heat into the board, toambient. Different board sizes and properties have different BA values. Consider a simple axisymmetric model of a PCBboard as shown in Figure 5.