Rev. B – 22-Aug-01
1
Introduction The thermal performance of semiconductor packages is
a very importan t considera tion for th e board de signer.
The reliabil ity and functiona l life of the device is directly
related to i ts ju nction oper atin g tem perat ure. A s the tem-
perature of the device increases, the stability of its
junctions decline, as does its reliable life. The thermal
performance is also important for the board design,
because it may limit the board density, or dictate the
board location of high power dissipating devices, or
require expensive cooling method for the system. As
devices have become mo re complex a nd boards hav e
become denser, the need to account for the thermal
characteristics of packages has shifted from being a
minor consideration to being a necessary consideration.
Characteristics The thermal performance of a package is measured by
its ability to dissipate the power required by the device
into its sur roundings. The el ectrical power d rawn by the
device generate s heat on the top su rfac e of the die. This
heat is conducted through the package to the surface
and then transferred to the surrounding air by convection.
Each heat transfer step has corresponding resistance to
the heat flow, which is given the value R θ, the thermal
resis tance coefficient . S ubs cr i pts ar e add ed t o the coeffi-
cient to sp ecify the two po ints tha t the heat i s transfe rre d
betw een. Com monly us ed coef ficient s are Rθja (jun cti on
to ambient ai r), Rθjc (junction t o case) and Rθca (case to
ambient).
Model An electrical analogy can be made, as s hown in the fig-
ure below, to illustrate the heat flow of a package. The
heat tran sfe r ca n b e c har acte ri ze d m athe matically by th e
following equation:
Tj-Ta=P x Rθja
Where
P = Device operating power (Watts)
Tj = Temperature of a junction on the device (°C)
Ta = Temperature of the surrounding ambient air (°C)
Rθja = Rθjc + Rθca in °C/W
Package
Thermal
Characteristics
Technical Note
Die
Package
Heat Flow
cavity
Rθca
Rθjc
Rθja
2
Packages
Rev. B – 22-Aug-01
Graphs The graphs show typ ical values of Rθja versus pin c ount for few dice dimensions at 3
different dissipation power values, in still air; because of the difficulty to perform accu-
rate measurements, some points may diver ge from the natural trend, as in the next
graph where the natural trend should be that the Rthj-A should asymptote to 30-35°C/W
rather than seeming to be diving to 0.
R Theta ja in Still Air
Die size 76 mm²
0
10
20
30
40
50
60
70
0 50 100 150 200 250 300
Pin Count
R Theta j a
0.5 W
1.5 W
3 W
R Theta ja in Still Air
Die size 56 m m²
0
10
20
30
40
50
60
70
0 50 100 150 200 250
Pin Count
R Theta ja
0,5 W
1,5 W
3 W
3Packages Rev. B – 22-Aug-01
R Theta ja in Still Air
Die size 121 mm²
0
10
20
30
40
50
60
0 50 100 150 200 250 300
Pin Co unt
R Theta ja
1 W
2,5 W
4 W
R Theta ja in Still Air
Die size 169 mm²
0
5
10
15
20
25
30
35
40
45
50
0 50 100 150 200 250 300 350 400
Pin Count
R Theta ja
1 W
2,5 W
4 W
4
Packages
Rev. B – 22-Aug-01
This graph shows typical values of Rθja versus pi n count for fe w dice dimen sions, in
1m/s air flow.
Rθjc is no t repo rted becaus e of sec onda ry effec t in th e total Rθ value. In any case it is
below 1°C/W.
R The ta ja for 1m/s Air Flow
76mm²
169mm²
121mm²
57mm²
0
5
10
15
20
25
30
35
40
0 50 100 150 200 250 300
Pin Coun t
R Theta ja
© Atmel Nantes SA, 2001.
Atmel Corporation makes no warranty for the use of its products, other than those expressly contained in the Company’s standard warranty
which is detailed in Atmel’s Terms and Conditions located on the Company’s web site. The Company assumes no responsibility for any errors
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components in life suppor t devices or systems.
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