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FIGURE 16.12
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Temperature rise versus board thickness.
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Board Material The PWB material, or more precisely the PWB material thermal conductivity, has a direct impact on the temperature rise of a trace. The differences between the FR-4 material and
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polyimide material thermal conductivity, shown in Table 16.3, have approximately a 2 percent impact on trace temperature rise. The thermal conductivity reported in most data sheets is the z-axis thermal conductivity, which represents the resin in the material. The x- and y-axes are higher due to the woven fibers in the laminate. There is not much information provided on the thermal conductivity of
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TABLE 16.3 Material Thermal Conductivity Material FR-4 Polyimide Copper OFHC Air Kx w/in.-C (w/m-K) 0.0124 (0.488) 0.0138 (0.543) 9.935 (391.2) 0.000879 (0.0346) Ky w/in.-C (w/m-K) 0.0124(0.488) 0.0138 (0.543) 9.935 (391.2) 0.000879 (0.0346) Kz w/in.-C (w/m-K) 0.0076 (0.299) 0.0085 (0.335) 9.935 (391.2) 0.000879 (0.0346)
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laminate materials, and the values listed in Table 16.3 for FR-4 and polyimide are measured values from current carrying capacity test board coupons. The thermal conductivity of copper is almost 1,000 times greater than the dielectric material and is the reason that internal copper planes have such a significant effect on trace temperature rise. The thermal conductivity of air, which is 10 times less than the dielectric, helps explain why external traces generally run hotter than internal traces. 16.5.6 Environments The term environment refers to the surroundings to which the circuit board is exposed and in which it operates. The circuit board may be mounted in an electronics box in a vacuum or surrounded by still air. The board and electronics may be exposed to forced air or it could be immersed in an inert fluid. It may be necessary to estimate the impact of one environmental condition over another; therefore, it is important to know that the baseline configuration is for a still air environment. One environment that is not more conservative than still air is an environment in vacuum or in space (see Fig. 16.13). In a vacuum, the internal and external traces run approximately at the same temperature. The internal IPC chart is recommended for sizing traces
Internal, External and Vacuum 18.0 16.0 14.0 Delta T ( C) 12.0 10.0 8.0 6.0 4.0 2.0 0.0 2oz 0.07 Int
FIGURE 16.13
+54% +19%
2oz 0.07 Ext
2oz 0.07 Vac
Vacuum versus baseline environments.
CURRENT CARRYING CAPACITY IN PRINTED CIRCUITS
for both internal and external traces in vacuum environments. If the baseline chart is used, the temperature rise should be derated by 55 percent for internal traces and 35 percent for external traces.
Vias The cross-sectional area of a via should maintain the same cross-sectional area as the trace or be larger than the trace coming into it. If the via has less cross-sectional area than the trace, then multiple vias are required to maintain the crosssectional area. The cross-sectional area can be calculated based on the barrel diameter and the plating thickness. Consult with the printed circuit board manufacturer to determine the plating thickness. Figure 16.14 illustrates the cross-sectional area of a via.
2 2 Area = D d 4 4 4 4
Trace to Via to Plane If a trace is connected to a via and the via is connected to a plane, the plane will conduct heat away from the via and the via will run cooler than the trace.
(D d) = barrel plating thickness
FIGURE 16.14 Via cross-sectional area.
Microvias Microvias respond to current the same as a through-hole via. The cross-sectional area is the parameter to match to a current level and temperature rise.
16.6 ODD-SHAPED GEOMETRIES AND THE SWISS CHEESE EFFECT
High current is often applied to copper planes that deliver the current to various locations in the printed circuit. It is not uncommon for these copper planes to be odd-shaped geometries, or swiss-cheese (referring to the holes in copper planes resulting from vias, holes, and sections of copper that are etched away from the copper plane). The simple conductor sizing charts become limited in their use for these applications. A technique for evaluating the temperature distribution in copper planes of odd-shaped geometries, as well as the result of many vias and cutouts, is by using a voltage drop analysis.
Voltage Drop Analysis Voltage drop in an odd-shaped geometry can be calculated accurately only with numerical techniques. The easiest way is to use software tools (such as ANSYS Thermal Analysis System (TAS) Thermal Modeling Software) that are designed for this type of problem. There is a direct analogy between thermal resistance and electrical resistance. Because of this, thermal analysis tools can be used to calculate voltage drop rather than temperature drop. The following summarizes the analogy between thermal and voltage drop analysis: (Mho is a unit of conductance equal to the reciprocal of an ohm; mho is more properly referred to as siemens = amperes/volts.)
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