1. THERMODYNAMIC IN ELECTRONICS
THERMAL MANAGEMENT IN ELECTRONIC
PROBLEMS IN THERMAL MANAGEMENT

2. What process involved in this problem??
QUESTION 1:
A square silicon chip (k = 150 W/m.K) is of width w = 5mm on a side of thickness t = 1 mm. The chip is mounted in a substrate such that its side and back surface are insulated, while the front surface is exposed to coolant. If 4 W are being dissipated in circuits mounted to the back surface of the chip, what is the steady state temperature difference between back and front surface?
What process involved in this problem??
CONDUCTION

3. = 0.004/0.00375 = 1.07 °K SOLUTION: Equation of CONDUCTION: Or
So, the temperature difference is:
= 0.004/ = °K

4. What process involved in this problem??
QUESTION 2:
A square isothermal chip is of width w = 5mm on a side is mounted in a substrate such that its side and back surface are well insulated, while the front surface is exposed to the flow of a coolant at Tinfinity = 15 °C. From reliability considerations, the chip temperature must not exceed T = 85 °C. If the coolant is air and the corresponding coefficient is h = 200 W/m2K, what is the maximum allowable chip power? If the coolant dielectric liquid which h = 3000 W/m2K, what is the maximum allowable power?
What process involved in this problem??
CONVECTION

5. From the lecture notes: q = hA (Ts – Tf)
SOLUTION:
Equation of CONVECTION:
From the lecture notes:
q = hA (Ts – Tf) or
Temperature conversion:
Ts : = 358 K, Tinfinity : = 288 K
For air cooling :
P = hA (Ts –Tinfinity) = 200 x (0.005)2 x ( ) = 0.35 W
For dielectric liquid cooling :
P = hA (Ts –Tinfinity) = 3000 x (0.005)2 x ( ) = 5.25 W
Comparison between air cooling and dielectric liquid cooling shows that air cooling is poorer than liquid heat transfer. But in terms of cost, dielectric liquid cooling is expensive.

6. What process involved in this problem??
QUESTION 3:
An instrumentation package has a spherical outer surface of diameter D = 100 mm and emissivity, ɛ = The package is placed in large space simulation chamber whose walls are maintained at 77 K. If operation of the electronic components is restricted to the temperature range 40 ≤ T ≤ 85 °C, what is the range of acceptable power dissipation for the package?
σ = 5.67 x 10-8 W/m2.K4
What process involved in this problem??
RADIATION
Temperature conversion:
Ts1 : = 313K, Ts2 : = 358 K

7. SOLUTION:
Because the electronic component in a large enclosure, then the geometrical factor , f = 1. Then the radiation heat transfer is:
Equation of RADIATION :
Q = εσA(T41 –T42)F12
ε = emissitivity σ = Stefan-Boltzmann constant (5.67 x 10-8 W/m2K4)
F12 = radiation view factor between surface 1 and 2.
T1 and T2 are the temperatures of the respective emitting and receiving surfaces
Q T40 = (5.67 x 10-8 ) (0.25) (1) (4π [ 0.1/2]2) ( ) =4.27 W
Q T85 = (5.67 x 10-8 ) (0.25) (1) (4π [ 0.1/2]2) ( ) =7.3 W
The power range 4.27 ≤Q ≤ 7.3 W

8. ii) Show effect of variations in the emissivity by considering the values of ɛ = 0.20 and 0.30 respectively.
iii) Deduce total thermal resistance, Rtot if the power dissipation, Q = 9.3 W
Rtot = (Tmax –Tref)/Q
= (358 – 77)/ 9.3
= K /W 40 85 C Q
ɛ = 0.25
ɛ = 0.2
ɛ = 0.3

9. QUESTION 4:
A square chips of width L = 15 mm on a side are mounted to a substrate that is installed in an enclosure whose walls and air are maintained at a temperature of Tinfinity = T surr = 25°C. The chips have an emissivity, ɛ = 0.6 and a maximum allowable temperature of Ts = 85°C.
If heat is rejected from the chips by radiation and natural convection, what is the maximum operating power of each chip? The convection coefficient depends on the chip to air temperature difference and may be approximated as h = C (Ts-Tinfinity)0.25, where C = 4.2 W/m2K5/4
If a fan is used to maintain air flow through the
enclosure and heat transfer is by forced
convection with h = 250 W/m2K,
what us the maximum operating power?

10. SOLUTION:
The maximum operating power is the summation of heat transfer by radiation and convection:
Maximum operating power if fan is used:
Temperature conversion:
Tinfinity : = 298 K, Ts : = 358 K

11. QUESTION 5:
A transistor that dissipates 10W is mounted on a duralumin heat sink at 50°C by a duralumin bracket 20 mm wide as shown in the opposite figure. The bracket is attached to the heat sink by a rivet. With convective cooling negligibly small, estimate the surface temperature of the transistor.

12. SOLUTION:
The heat source is the base of the transistor and the rivets connecting the bracket to the heat sink.
The heat flow path length is
L = = 60 mm
The base transistor area equals the heat flow area is
A = width x thickness = 20 x 5 = 100 mm2
The duralumin thermal conductivity could be obtained from appendix is k = 164 W/mK
The conduction formula
The surface temperature is:
Temperature conversion:
Tinfinity : = 323 K
Ts – 50 = qL/kA
Ts = [(10 x 0.06) /(164 x 100 x 10-6) ] + 323K =