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Products > Packages > Thermal and electrical characteristics >

Mechanism of heat dissipation


A major property of packages is how they dissipate the heat generated by the semiconductor devices they house. Let us take a look at the heat dissipation mechanisms on which countermeasures are based:

Heat generation affects safety, reliability, and performance.
Heat is generated when a current flows through a resistor in an electric circuit.

A semiconductor device may be regarded as a type of resistor that generates heat in proportion to the ON resistance (internal resistance when a current flows through the device) as current flows through.

Heat can adversely affect the semiconductor device itself as well as the electronic system that uses that device. In particular, it may seriously impair safety, performance, and reliability.

An electronic system may emit smoke or catch fire if its device generates more heat than anticipated. Excessive heat may also degrade the performance of the device by lowering its operating speed, and in the worst case, damage the device, rendering it inoperable. Even if the worst case can be avoided, reliability is adversely affected through device malfunctions and a shorter system life.

To eliminate these adverse heat influences, countermeasures against heat are essential for semiconductor packages.

Heat is released in three ways: conduction, convection, and radiation.
The fundamental mechanism that must be understood when considering heat countermeasures is heat dissipation, i.e., how a semiconductor device releases the heat it generates.

Heat is transferred in three ways: conduction, convection, and radiation. Thermal conduction means movement of heat in a solid, convection refers to transfer of heat from a solid to a fluid (such as a liquid or gas), and radiation means heat transfer through the emission of electromagnetic waves. When the mechanism of heat dissipation is considered in the context of an actual operating environment that includes a printed wiring board and an atmosphere, heat flows from the source (i.e., the chip), to the final destination, the atmosphere, as shown in the figure below.
Heat Dissipation Paths and Causes of Thermal Resistance
Figure 1: Heat Dissipation Paths and Causes of Thermal Resistance
???? Heat radiation: Stefan-Boltzmann law (electromagnetic waves)
  Radiative thermal resistance Rrad = 1/(4ε σfAT^3)
    ε : Radiation rate
    σ : Stefan-Boltzmann constant
    f : Shape factor
    A : Surface area
    T : Average temperature of heated surface and surrounding area
blue arrow Convection: Newton's law of cooling through fluid movement
  Convective thermal resistance Rconv=1/αA
  α : Heat transfer coefficient
  A : Heat dissipation area
red arrow Conduction: Fourier's law (conduction by molecular vibration)
  Conductive thermal resistance Rcond=L/λA
  L : Length of path
  λ : Thermal conductivity
  A : Heat transfer area

Heat dissipation is done mostly through the printed wiring board.
Because heat radiation effectively occurs only when the surface area of the package is extremely large, heat is actually dissipated from the package via the following three paths shown in the figure below.
  • From the surface of the package into the atmosphere
  • From the external pins to the printed wiring board and then into the atmosphere
  • From the heat source to the sides of the package

Of these three paths, the heat dissipation path via the printed wiring board is the most effective and according to some calculations accounts for 80% of total heat dissipation. Actual analyses of heat dissipation indicate that 90% of the heat is released via the printed wiring board when a 352-pin PBGA is mounted on a 4-layer printed wiring board, and only 10% of the heat is dissipated from the package surface.
Heat Flow Paths
Figure 2: Heat Flow Paths
θja(°C/W): Thermal resistance from junction to atmosphere (junction to ambient air). Reduce this value when a package is used alone.
θjc: Thermal resistance from junction to package surface (junction to case). Value determined by the thermal conductivity of the materials of the chip and package surface, thermal conductivity length, and area.
θjb: Thermal resistance from junction to solder balls (junction to ball). Value determined by chip adhesive, thermal conductivity of the printed wiring board, and layout of the solder balls.
θbp: Thermal resistance from ball lands to printed wiring board surface (ball to PWB).
 
Dependence of package substrate on the number of layers and wiring pattern, and equivalent thermal conductivity of the substrate calculated from the thickness of the wiring layer
Equivalent thermal conductivity of printed wiring board= Σ(ki x ti x Pi)

Total thickness of the substrate
ki:Thermal conductivity of layer i,  ti:Thickness of layer i,  Pi:Survival rate of wiring
θca: Resistance composed of heat convection and heat radiation from package surface to atmosphere (case to ambient)
θpa: Resistance composed of heat convection and heat radiation from printed wiring board to atmosphere (PWB to ambient)
θjs: Thermal resistance from junction to side of package (junction to side)
θsa: Thermal resistance from side of package to atmosphere (side to ambient)