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Lattice Semiconductor
195
Data Sheet
December 2002
ORCA Series 3C and 3T FPGAs
Package Thermal Characteristics
There are four thermal parameters that are in common
use: ΘJA, ψJC, ΘJC, and ΘJB. 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 airow.
Table 76 contains the currently available thermal speci-
cations for FPGA packages mounted on both JEDEC
and non-JEDEC test boards. The thermal values for the
newer package types correspond to those packages
mounted on a JEDEC four-layer board. The values for
the older packages, however, correspond to those
packages mounted on a non-JEDEC, single-layer,
sparse copper board (see Note 2). It should also be
noted that the values for the older packages are con-
sidered conservative.
Θ
ΘJA
This is the thermal resistance from junction to ambient
(a.k.a. 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 ther-
mal 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 convection measurements. A controlled
amount of power (Q) is dissipated in the test chip’s
heater resistor, the chip’s temperature (TJ) is deter-
mined by the forward drop on the diodes, and the ambi-
ent temperature (TA) is noted. Note that ΘJA is
expressed in units of °C/watt.
ψ
ψJC
This JEDEC designated parameter correlates the junc-
tion 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
thermal resistance, and it is dened by:
where TC is the case temperature at top dead center,
TJ is the junction temperature, and Q is the chip power.
During 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/watt.
Θ
ΘJC
This is the thermal resistance from junction to case. It
is most often used when attaching a heat sink to the
top of the package. It is dened by:
The parameters in this equation have been dened
above. However, the measurements are performed
with the case of the part pressed against a water-
cooled heat sink so as to draw most of the heat gener-
ated by the chip out the top of the package. It is this dif-
ference in the measurement process that differentiates
ΘJC from ψJC. ΘJC is a true thermal resistance and is
expressed in units of °C/watt.
ΘJB
This is the thermal resistance from junction to board
(a.k.a. ΘJL). It is dened by:
where TB is the temperature of the board adjacent to a
lead measured with a thermocouple. The other param-
eters on the right-hand side have been dened above.
This is considered a true thermal resistance, and the
measurement is made with a water-cooled heat sink
pressed against the board so as to draw most of the
heat out of the leads. Note that ΘJB is expressed in
units of °C/watt, and that this parameter and the way it
is measured is still in JEDEC committee.
ΘJA
TJ
TA
–
Q
--------------------
=
ψJC
TJ
TC
–
Q
--------------------
=
ΘJC
TJ
TC
–
Q
--------------------
=
ΘJB
TJ
TB
–
Q
--------------------
=