
Lucent Technologies Inc.
126
Data Sheet
June 1999
ORCA Series 2 FPGAs
Package Thermal Characteristics
There are three thermal parameters that are in com-
mon use: 
Θ
JA
, 
ψ
JC, and 
Θ
JC
. 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.
The data base containing the thermal values for all of 
Lucent Technologies’ IC packages is currently being 
updated to conform to modern JEDEC standards. 
Thus, Table 29 contains the currently available thermal 
specifications for Lucent Technologies’ FPGA pack-
ages 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 (indicated as Note 2 in the 
table). The values for the older packages, however, cor-
respond to those packages mounted on a non-JEDEC, 
single-layer, sparse copper board (see Note 1). It 
should also be noted that the values for the older pack-
ages are considered conservative.
Θ
JA
This is the thermal resistance from junction to ambient 
(a.k.a. theta-JA, R-theta, etc.).
where T
J
 is the junction temperature, T
A
 is the ambient 
air temperature, and Q is the chip power. 
Experimentally, 
Θ
JA
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 (T
J
) is deter-
mined by the forward drop on the diodes, and the ambi-
ent temperature (T
A
) is noted. Note that 
Θ
JA
 is 
expressed in units of °C/watt.
ψ
JC
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 defined by:
where T
C
 is the case temperature at top dead center, 
T
J
 is the junction temperature, and Q is the chip power. 
During the 
Θ
JA
 measurements described above, 
besides the other parameters measured, an additional 
temperature reading, T
C
, is made with a thermocouple 
attached at top-dead-center of the case. 
ψ
JC 
is also 
expressed in units of °C/watt. 
Θ
JC
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 defined by:
The parameters in this equation have been defined 
above. However, the measurements is performed with 
the case of the part pressed against a water-cooled 
heat sink so as to draw most of the heat generated by 
the chip out the top of the package. It is this difference 
in the measurement process that differentiates 
Θ
JC 
from
ψ
JC. 
Θ
JC 
is a true thermal resistance and is 
expressed in units of °C/watt.
Θ
JB
This is the thermal resistance from junction to board 
(a.k.a., 
Θ
JL)
. It is defined by:
J
T
B
–
Q
where T
B
 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 defined 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 
Θ
JB
 is expressed in 
units of °C/watt, and that this parameter and the way it 
is measured is still in JEDEC committee.
Θ
JA
J
T
T
A
–
Q
=
ψ
JC
J
T
T
C
–
Q
=
Θ
JC
J
T
T
C
–
Q
=
Θ
JB
T
=