OR4 datasheet(142/152 Pages) LATTICE

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142

Lattice Semiconductor

Data Sheet

May, 2006

ORCA Series 4 FPGAs

Package Thermal Characteristics

Summary

There are three thermal parameters that are in common use: ΘJA, ψJC, and ΘJC. It should be noted that all the

parameters are affected, to varying degrees, by package design (including paddle size) and choice of materials,

the amount of copper in the test board or system board, and system airflow.

ΘJA

This is the thermal resistance from junction to ambient (theta-JA, R-theta, etc.):

where TJ is the junction temperature, TA, is the ambient air temperature, and Q is the chip power.

Experimentally, ΘJA is determined when a special thermal test die is assembled into the package of interest, and

the part is mounted on the thermal test board. The diodes on the test chip are separately calibrated in an oven. The

package/board is placed either in a JEDEC natural convection box or in the wind tunnel, the latter for forced con-

vection measurements. A controlled amount of power (Q) is dissipated in the test chip’s heater resistor, the chip’s

temperature (TJ) is determined by the forward drop on the diodes, and the ambient temperature (TA) is noted. Note

that ΘJA is expressed in units of °C/watt.

ψJC

This JEDEC designated parameter correlates the junction temperature to the case temperature. It is generally

used to infer the junction temperature while the device is operating in the system. It is not considered a true ther-

mal resistance, and it is defined by:

where TC is the case temperature at top dead center, TJ is the junction temperature, and Q is the chip power. Dur-

ing the ΘJA measurements described above, besides the other parameters measured, an additional temperature

reading, TC, is made with a thermocouple attached at top-dead-center of the case. ψJC is also expressed in units of

°C/W.

ΘJA

–

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ψJC

–

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OR4 Datasheet(PDF) 142 Page - Lattice Semiconductor