Transcript Electronics Cooling MEP 635

• Part-D
Main topics
Packaging of electronic equipments
• Components of electronic systems
• Surface mount technology
• Printed Wiring board (PWB) types
• Electronics packaging levels
• Wire bonding packaging
• Flip-Chip packaging
• Chip Scale Packaging
• Ball Grid Array packaging
• Conduction cooling for chassis and circuit boards
21-24. Packaging of Electronic
Equipments
Introduction
• Electronic packaging is the art and science of
connecting circuitry to perform some desired function
in some applications. Packaging also provides ease of
handling and protection for assembly operations. We
will devote this partition for engineering technologies
include mechanical, thermal, electrical, materials, and
components for electronic systems.
• In mechanical design concerns the supports, frames,
etc. to withstand the mechanical stresses due to
vibration, shocks, etc to which the electronic package
may be subjected.
• While the thermal design to ensure that the
electronic systems are amply cooled and would not
over heat to a point where they become unable to
function properly.
Components of electronic systems
• Electronic components for airplanes, missiles, satellites,
and spacecraft.
• Electronic components for ships and submarines.
• Electronic components for communication systems
and ground support systems.
• Personal computers, microcomputers, and
microprocessors.
Surface mount technology (SMT)
SMT process flow:
• Solder paste application on the lands of a suitable
substrate (e.g., a PWB)
• Adhesive deposition {not always required)
• Component preparation (if required)
• Component placement
• Soldering
• Cleaning
• Inspection
• Clean prior to conformal coat (if required)
• Conformal coat (if required).
• Test
Surface mount technology (SMT)
Surface mount classification
According to:
Assembly types
Type 1
assembly:
The
components
only on its
top side.
Type 2
assembly: The
components on
both its top
side and its
bottom side.
According to:
Assembly classes
Class A:
Assembly is
entirely
through hole
technology
(THT).
Class B:
Assembly is
entirely
surface mount
technology
(SMT)
Class C:
assembly is
a combined
THT and
SMT
assembly
Surface mount classification
• Type 1A assembly (all
through hole)
• Type 1B assembly
(single-sided pure
SMT)
• Type 2B assembly
(double-sided pure
SMT)
• Type 1C assembly
(single- sided mixed
technology SMT)
• Type 2C(S) (simple)
assembly

Type 2C (C) (complex)
assembly
 Type 2C (VC) (very
complex) assembly
Conduction cooling for the
components mounted on PCBs
Conduction heat flow path from component to heat sink.
Example
Several power transistors, which dissipate 5 watts each, are mounted on a
power supply circuit board that has a 0.093 in (0.236 cm) thick 5052
aluminum heat sink plate, as shown in Figure. Determine how much lower the
case temperatures will be when these components are mounted close to the
edge of the PCB, as shown in Figure b, instead of the center, as shown in
Figure a.
Power transistors mounted on an aluminum heat sink plate. (a) Old design (b) New design
Solution
Since both plug-in PCBs are Symmetrical about the center,
consider each half of the board for the analysis.
Q = 3 x 5 = 15 watts
La = 3.0 in = 7.62 cm (length of old design)
Lb= 1.0 in = 2.54 cm (length of new design)
k = 143.8 W/m K (5052 aluminum)
A = (5.0) (0.093) = 0.465 in2 = 3.0 cm2 (area)
The temperature rise at the old design location:
ta 
(15)(0.0762)
 26.5 o C
(3/10000)(143.8)
The temperature rise for the mounting position near the edge of the PCB,
new design:
tb 
(15)(0.0254)
 8.83 o C
(3/10000)(143.8)
This shows that moving the transistors closer to the edge of the PCB can reduce the
component surface mounting temperature by 26.5 – 8.83 = 17.7 °C.
Chassis design
• Electronic systems normally consist of many different electronic
component parts, such as resistors, capacitors, diodes, transistors,
microprocessors, and transformers, which are enclosed within a
support structure called the chassis, such as a chassis used in a
space craft as shown.
Chassis design procedures
1- Formed sheet metal electronic assemblies
2- Dip-brazed boxes with integral cold plates
Dip-brazed electronic chassis
chassis with side wall heat exchanger
Chassis design procedures
3- Extruded sections for large chassis
• Extruded sections must be designed to withstand the Navy shocks and
vibrations for large chassis.
• Extruded sections with hollow cores, are capable of providing
a rigidity for large chassis also relatively lightweight electronic enclosure.
• Extruded sections is very convenient for ducting cooling air to various parts of
the cabinet with fans or blowers.
Chassis design procedures
4- Humidity considerations
Offset drain holes in bottom of chassis
Chassis design procedures
5- Circuit board conformal coatings
There are five popular types of conformal coatings to protect the
circuit boards from moisture
a. Acrylic coating
b. Epoxy coating
c. Polyurethane coating
d. Silicone coating
Chassis design procedures
6- Sealed electronic chassis
• The sealed box is used to prevent the loss of air, which is
required to cool the electronic components.
• O-ring seals are the most popular and easy to use for
large or small boxes.
O-ring for electronic box
Printed wiring boards (PWBs)
• It contains the wiring required to interconnect the component
electrically and acts as the primary structure to support those
components.
• In some instances it is also used to conduct away heat
generated by the components.
Cross-sectional view of typical multiwire printed board
Printed wiring boards (PWBs)
Printed wiring board types
Ceramic PWB
Developmental PWB
Organic PWB
Thick Film
Thin Film
Cofired
Direct-Bond
Copper
Rigid PWB
Flexible PWB
Rigid-Flexible
Different packaging and
interconnection techniques
1- Flip-Chip packaging
• The concept of Flip-Chip process: where the
semiconductor chip is assembled directly face down onto
circuit board with small, solder-coated copper balls
(electrically conducting bumps) sandwiched between the
chip and the board.
Cross sections of Flip-Chip joints without and with underfill material
1- Flip-Chip packaging
Flip-Chip with underfill material: The underfill material is
applied by dispensing along one or two sides of the chip, from
where the low viscosity epoxy is drawn by capillary forces into the
space between the chip and substrate.
The underfill application by dispensing.
1- Flip-Chip packaging
Flip-Chip joining
The Flip-Chip joining mainly by thermocompression or
thermosonic bonding.
2- Wire bonding packaging
• In wire bonding (chip-and-wire) packaging, the IC chip is bonded
directly on an interconnecting substrate (board) using thin wire and
protected with a top encapsulant against moisture.
Chip interconnection using wire bonding technology
2- Wire bonding packaging
Wire bonding steps with thermocompression bonding.
2- Wire bonding packaging
Wire bonding types
Thermocompression
wire bonding
Ultrasonic wire
bonding
Thermosonic
wire bonding
Wire bonding
Pressure
Temperature
Ultrasonic energy
Thermocompression
High
300-500 oC
No
Ultrasonic
Low
25 oC
Yes
Thermosonic
Low
100-150 oC
Yes
Three wire bonding processes
2- Wire bonding packaging
Wire bonding techniques
Ball bonding
The surface of the molten metal forms
a spherical shape or ball and the
second bond having a crescent shape.
Wedge bonding
The wire is fed at an angle usually 30-60o
from the horizontal bonding surface through
a hole in the back of a bonding wedge.
2- Wire bonding packaging
Application of ball bonding
Application of wedge bonding
3- Chip scale packaging (CSP)
• CSPs combines the best of Flip chip assembly
and surface mount technology.
• CSPs are often classified based on their
structure. At least four major categories have been
proposed. These are: Flex circuit interposer, rigid
substrate interposer, custom lead frame, and
wafer-level assembly. As shown in the next slide.
3- Chip scale packaging
Chip scale packaging classification
4- Ball Grid Array Packaging
The BGA taking advantage of the area under the
package for the solder sphere interconnections in an
array to increase both the numbers of I/Os and pitch.
4- Ball Grid Array Packaging
Types of BGA packages
PBGA (Plastic ball grid array)
• a die is mounted to the top side of substrate,
double-sided PWB.
• The over-molded or glop-top encapsulation is
then preformed to completely cover the chip,
wires and substrate bond pads.
TBGA (Tape or Tab ball grid array)
• TBGA gets their name from the tape
(a flexible polyimide conductor film with copper
metallization).
•The back of the chip can be direct contact to
heat sink which easily dissipate 10 to 15 W.
Conduction cooling for chassis and
circuit boards
• Introduction
• Conduction cooling is important method used in many practical
electronic systems such as spacecraft system as shown below.
Shows a conduction-cooled electronic box designed to carry the
heat down the vertical walls to a base cooled plate
conduction-cooled electronic box designed to carry the heat from vertical
walls to a base plate, for mounting in a spacecraft.
• Uniformly distributed heat
sources, steady state conduction
• Identical electronic components are often placed next to
one another, on circuit boards, as shown below. When
each component dissipates approximately the same
amount of power, the result will be a uniformly distributed
heat load.
• Uniformly distributed heat
sources, steady state conduction
L
L
Uniform heat input
dQ1=qdx
dQ2
dQ3
dx
dx
x
t
t+dt
Temperature rise
3
t
4
t
L=a/2
L/2
x
a
Parabolic temperature distributions for uniform heat load on a circuit
board.
• Uniformly distributed heat
sources, steady state conduction
When only one side of one strip is considered as shown in section
AA, a heat balance equation can be obtained by considering a
small element dx of the strip, along the span with a length of L.
Then
dQ1  dQ2  dQ3
Where
dQ1  qdx  heat input
dQ 2  - kA
dQ 3  - kA
dt
 heat flow
dx
d(t  dt)
 total heat
dx
Then
qdx - kA
dt
dt
d
 -kA
- kA
(dt)
dx
dx
dx
• Uniformly distributed heat
sources, steady state conduction
Then
d2t
q


kA
dx 2
This is a second-order differential equation, which can be solved by
double integration. Integrating once yields to
dt
qx

 C1
dx
kA
qx 2
t
 C1 x  C 2
2kA
The constant C1 is zero because at x = 0, the plate is adiabatic
The constant C2 is determined by letting the temperature at the
end of the plate be te; Then
qL2
 t e  C2
2kA
• Uniformly distributed heat
sources, steady state conduction
The temperature at any point along the plate (or strip) is
qx 2
qL2
t

 te
2kA 2kA
q

(L2  x 2 )  t e
2kA
When x = 0. This results in the equation for the maximum temperature
rise in a strip with a uniformly distributed heat load.
qL2
t 
2kA
The total heat input along the length L is
Q  qL
Then
QL
t 
2kA
Example
A series of flat pack integrated circuits are to be mounted on a
multilayer printed circuit board (PCB) as shown below. Each flat
pack dissipates 100 milliwatts of power. Heat from the components
is to be removed by conduction through the printed circuit copper
pads, which have 2 ounces of copper [thickness is 0.0028 in
(0.0071 cm)]. The heat must be conducted to the edges of the
PCB, where it flows into a heat sink. Determine the temperature
rise from the center of the PCB to the edge to see if the design will
be satisfactory.
Note that the typical maximum allowable case temperature is about
212°F.
Solution
The flat packs generate a uniformly distributed heat load, which
results in the parabolic temperature distribution shown in Figure.
Because of symmetry, only one half of the system is evaluated. The
temperature rise at center of the PCB
t 
QL
2kA
Where:
Q= 3(0.1) =0.3 Watt heat input, one half strip
L= 3 in = 7.62 cm (length)
k= 345 W/m.K
A= (0.2) (0.0028) =0.00056 in2 =0.00361 cm2 (cross-sectional area)
Then
t 
(0.3)(0.07 62)10000
 91.7  C  197 F
2(345)(0.00361)
Solution
The amount of heat that can be removed by radiation or convection for
this type of system is very small. The temperature rise is therefore too
high. By the time the sink temperature is added, assuming that it is
80°F, the case temperature on the component will be 277°F. Since the
typical maximum allowable case temperature is about 212°F, the design
is not acceptable.
If the copper thickness is doubled to 4 ounces, which has a thickness of
0.0056 in (0.014 cm), the temperature rise will be 114.5°F (45.85°C) then
the case temperature on the component will be about 195°F so that the
system can be operated safely.
For high-temperature applications, the copper thickness will have to be
increased to about 0.0112 in (0.0284 cm) for a good design.
• Chassis with nonuniform wall
sections
Electronic chassis always seem to require cutouts, notches, and
clearance holes for assembly access, wire harnesses, or maintenance.
These openings will generally cut through a bulkhead or other structural
member, which is required to carry heat away from some critical highpower electronic component. This cutouts result in nonuniform wall
sections, which must be analyzed to determine their heat flow capability.
One convenient method for analyzing nonuniform wall sections is to
subdivide them into small units that have relatively uniform sections. The
heat flow path through each of the small units can then be defined in
terms of a thermal resistance. This will result in a thermal analog resistor
network.
Two basic resistance patterns, series and parallel, are used to generate
analog resistor networks.
•Chassis with nonuniform wall
sections
Series flow resistor network
Rt= R1+ R2+R3+ …………
Parallel flow resistor network.
1
1
1
1



 .........
R t R1 R 2 R 3
Example
•
An aluminum(5052) plate is used to support a row of six power resistors.
Each resistor dissipates 1.5 watts, for a total power dissipation of 9 watts.
The bulkhead conducts the heat to the opposite wall of the chassis,
which is cooled by a multiple fin heat exchanger. The bulkhead has two
cutouts for connectors to pass through, as shown in Figure. Determine
the temperature rise across the length of the bulkhead.
Solution
•
A mathematical model with series and parallel thermal resistor
networks can be established to represent the heat flow path, as
shown in Figure.
Bulkhead thermal models using a series and a parallel resistor network
Solution
•
-
Firstly must determine the values of each resistor.
Determine resistor R1:
L1= 2 in = 5.08 cm
k1= 158 W/m.K
A1= (5) (0.06) = 0.3 in2 = 1.935 cm2
R1 
-
(0.0508)10000
 1.6616 o C/W
(1.935)(158)
Determine resistor R2:
L2= 1.5 in = 3.81 cm
K2= 158 W/m.K
A2= (0.375) (0.06) = 0.0225 in2 = 0.145 cm2 (average area)
R2 
-
(0.0381)10000
 16.63 o C/W
(0.145)(158)
Determine resistor R3:
L3= 1.5 in = 3.81 cm
K3= 158 W/m.K
A3= (1) (0.06) = 0.06 in2 = 0.387 cm2
Solution
R3 
(0.0381)10000
 6.23 o C/W
(0.387)(158)
- Determine resistor R4:
L4= 1.5 in = 3.81 cm
K4= 158 W/m.K
A4= (1.5) (0.06) = 0.09 in2 = 0.581 cm2
(0.0381)10000
R4 
 4.15 o C/W
(0.581)(158)
- Determine resistor R5:
L5= 1 in = 2.54 cm
K5= 237 W/m.K
A5= (4.75) (0.06) = 0.285 in2 = 1.839 cm2
(0.0254)10000
R5 
 0.874 o C/W
(1.839)(158)
Solution
• After getting all resistors we can simplify the network. Where
resistors R2, R3 and R4 are in parallel, which results in resistor
1
1
1
1
R6 .



R6 R2 R3 R4
R 6  2.166 o C/W
• The total thermal resistance is
R t  R 1  R 6  R 5  4.7 o C/W
• The temperature rise across the length of the bulkhead is.
t  (9)(4.7)  42.3o C
Circuit board edge guides
• Plug-in PCBs are often used with guides, which help to align
the PCB connector with the chassis connector.
• Also the edge guide can be used to conduct heat away from
the PCB.
Plug-in PCB assembly with board edge guides.
Circuit board edge guides
Types of edge guides
G guide
B guide
U guide
wedge clamp
Board edge guides with typical thermal resistances,
(a) G guide, 12 °C in/watt; (b) B guide, 8 °C in/watt;
(c) U guide, 6 oC in/watt; (d) wedge clamp, 2 °C in/watt.
Example
Determine the temperature rise across the PCB edge guide (from the edge
of the PCB to the chassis wall) for the assembly shown in Figure. The edge
guide is 5.0 in long, type c. The total power dissipation of the PCB is 10
watts, uniformly distributed, and the equipment must operate at 100,000 ft.
Solution
Since there are two edge guides, half of the total power will be conducted
through each guide. The temperature rise at sea level conditions can be
determined as
t 
Where:
R= 6 oC in/watt (U guide)
Q=10/2 =5 watt
L= 5 in
RQ
L
(6)(5)
 6 oC
5
At altitude of 100,000 ft, the resistance across the edge guide will
increase about 30%. The temperature rise at this altitude will then be
Then
t 
t  1.3(6)  9 o C