commonly used for DIP packages. However, with less metal and shorter lead lengths to transfer heat to ambient air, more heat is typically transferred to the circuit board itself. Several board thermal analysis software packages are available, and are highly recommended for boards that are expected to develop high thermal gradients Flotherm, 1995 Since all electronics components generate heat in use, and elevated temperatures negatively affect the reli ability and failure rate of semiconductors, it is important that heat generated by SMDs be removed as efficiently as possible. The design team needs to have expertise with the variables related to thermal transfer nction temperature: T thermal resistances: Oi, O a,e temperature sensitive parameter(TSP)method of determining Os thermal characteristics of substrate material SMt Packages have been developed to maximize heat transfer to the substrate. These include PLCCs with integral heat spreaders, the SOT-89 power transistor package, the dpak power transistor package, and many others. Analog ICs are also available in power packages. Note that all of these devices are designed primarily for processing with the solder paste process, and some specifically recommend against their use with wave solder applications. Heat sinks and heat pipes should also be considered for high-power ICs. In the conduction process, heat is transferred from one element to another by direct physical contact between the elements. Ideally the material to which heat is being transferred should not be adversely affected by the transfer. As an example, the glass transition temperature T, of FR-4 is 125.C. Heat transferred to the board has little or no detrimental affect as long as the board temperature stays at least 50oC below Tx. Good heat sink naterial exhibits high thermal conductivity, which is not a characteristic of fiberglass. Therefore, the traces must be depended on to provide the thermal transfer path [Choi et al., 1994]. Conductive heat transfer is also used in the transfer of heat from IC packages to heat sinks, which also requires use of thermal grease to fill all air gaps between the package and the" flat"surface of the sink. The previous discussion of lead properties of course does not apply to leadless devices such as Leadless Ceramic Chip Carriers(LCCCs). Design teams using these and similar packages must understand the better heat transfer properties of the alumina used in ceramic packages, and must match TCEs between the LCCC and the substrate, since there are no leads to bend and absorb mismatches of expansion Since the heat transfer properties of the system depend on substrate material properties, it is necessary to understand several of the characteristics of the most common substrate material, FR-4 fiberglass. The glas transition temperature has already been noted, and board designers must also understand that multi-layer FR 4 boards do not expand identically in the X-, Y-, and Z-directions as temperature increases. Plate-through holes will constrain z-axis expansion in their immediate board areas, while non-through-hole areas will expand further in the z-axis, particularly as the temperature approaches and exceeds Ts [Lee et al., 1984]. This uneq expansion can cause delamination of layers and plating fracture. If the design team knows that there will be a need for higher abilities to dissipate heat and/or a need for higher glass transition temperatures and lower coefficients of thermal expansion(TCe)than FR-4 possesses, many other materials are available, examples of which will follow. Note in Table 26.1 that copper-clad Invar has both variable Tg and variable thermal conductivity depending on the volume mix of copper and Invar in the substrate. Copper has a high TCE and Invar has a low TCE, so the TCE increases with the thickness of the copper layers. In addition to heat transfer considerations, board material decisions must also be based on the expected vibration, stress, and humidity in the application. Convective heat transfer involves transfer due to the motion of molecules, typically airflow over a he and depends on the relative temperatures of the two media involved. It also depends on the velocity of air flow over the boundary layer of the heat sink. Convective heat transfer is primarily effected when forced air flow is provided across a substrate, and when convection effects are maximized through the use of heat sinks. The rules that designers are familiar with when designing ThT heat-sink device designs also apply to SMT design. The design team must consider whether passive conduction and convection will be adequate to cool a populated substrate or whether forced-air cooling or liquid cooling will be needed. Passive conductive cooling c 2000 by CRC Press LLC© 2000 by CRC Press LLC commonly used for DIP packages. However, with less metal and shorter lead lengths to transfer heat to ambient air, more heat is typically transferred to the circuit board itself. Several board thermal analysis software packages are available, and are highly recommended for boards that are expected to develop high thermal gradients [Flotherm, 1995]. Since all electronics components generate heat in use, and elevated temperatures negatively affect the reli￾ability and failure rate of semiconductors, it is important that heat generated by SMDs be removed as efficiently as possible. The design team needs to have expertise with the variables related to thermal transfer: • junction temperature: Tj • thermal resistances: Qjc , Qca , Qcs, Qsa • temperature sensitive parameter (TSP) method of determining Qs • power dissipation: PD • thermal characteristics of substrate material SMT packages have been developed to maximize heat transfer to the substrate. These include PLCCs with integral heat spreaders, the SOT-89 power transistor package, the DPAK power transistor package, and many others. Analog ICs are also available in power packages. Note that all of these devices are designed primarily for processing with the solder paste process, and some specifically recommend against their use with wave￾solder applications. Heat sinks and heat pipes should also be considered for high-power ICs. In the conduction process, heat is transferred from one element to another by direct physical contact between the elements. Ideally the material to which heat is being transferred should not be adversely affected by the transfer. As an example, the glass transition temperature Tg of FR-4 is 125°C. Heat transferred to the board has little or no detrimental affect as long as the board temperature stays at least 50°C below Tg. Good heat sink material exhibits high thermal conductivity, which is not a characteristic of fiberglass. Therefore, the traces must be depended on to provide the thermal transfer path [Choi et al., 1994]. Conductive heat transfer is also used in the transfer of heat from IC packages to heat sinks, which also requires use of thermal grease to fill all air gaps between the package and the “flat” surface of the sink. The previous discussion of lead properties of course does not apply to leadless devices such as Leadless Ceramic Chip Carriers (LCCCs). Design teams using these and similar packages must understand the better heat transfer properties of the alumina used in ceramic packages, and must match TCEs between the LCCC and the substrate, since there are no leads to bend and absorb mismatches of expansion. Since the heat transfer properties of the system depend on substrate material properties, it is necessary to understand several of the characteristics of the most common substrate material, FR-4 fiberglass. The glass transition temperature has already been noted, and board designers must also understand that multi-layer FR- 4 boards do not expand identically in the X-, Y-, and Z-directions as temperature increases. Plate-through￾holes will constrain z-axis expansion in their immediate board areas, while non-through-hole areas will expand further in the z-axis, particularly as the temperature approaches and exceeds Tg [Lee et al., 1984]. This unequal expansion can cause delamination of layers and plating fracture. If the design team knows that there will be a need for higher abilities to dissipate heat and/or a need for higher glass transition temperatures and lower coefficients of thermal expansion (TCE) than FR-4 possesses, many other materials are available, examples of which will follow. Note in Table 26.1 that copper-clad Invar has both variable Tg and variable thermal conductivity depending on the volume mix of copper and Invar in the substrate. Copper has a high TCE and Invar has a low TCE, so the TCE increases with the thickness of the copper layers. In addition to heat transfer considerations, board material decisions must also be based on the expected vibration, stress, and humidity in the application. Convective heat transfer involves transfer due to the motion of molecules, typically airflow over a heat sink, and depends on the relative temperatures of the two media involved. It also depends on the velocity of air flow over the boundary layer of the heat sink. Convective heat transfer is primarily effected when forced air flow is provided across a substrate, and when convection effects are maximized through the use of heat sinks. The rules that designers are familiar with when designing THT heat-sink device designs also apply to SMT design. The design team must consider whether passive conduction and convection will be adequate to cool a populated substrate or whether forced-air cooling or liquid cooling will be needed. Passive conductive cooling