Part-D

1
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

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21-24. Packaging of Electronic Equipments
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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.

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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.

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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

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Surface mount technology (SMT)
Surface mount classification
According to Assembly types
According to Assembly classes
Type 1 assembly The components only on its top
side.
Type 2 assembly The components on both its top
side and its bottom side.
Class A Assembly is entirely through hole
technology (THT).
Class C assembly is a combined THT and SMT
assembly
Class B Assembly is entirely surface mount
technology (SMT)
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Surface mount classification

• Type 1A assembly (all through hole)

• Type 1B assembly (single-sided pure SMT)

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• Type 2B assembly (double-sided pure SMT)
• Type 1C assembly (single- sided mixed technology
  SMT)
• Type 2C(S) (simple) assembly

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• Type 2C (C) (complex) assembly

• Type 2C (VC) (very complex) assembly

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Conduction cooling for the components mounted on
PCBs
Conduction heat flow path from component to heat
sink.
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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
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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
The temperature rise for the mounting position
near the edge of the PCB, new design
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.
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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.

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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
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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.

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Chassis design procedures
4- Humidity considerations
Offset drain holes in bottom of chassis
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Chassis design procedures
5- Circuit board conformal coatings

• There are five popular types of conformal
  coatings to protect the circuit boards from
  moisture
• Acrylic coating
• Epoxy coating
• Polyurethane coating
• Silicone coating

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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
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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
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Printed wiring boards (PWBs)
Printed wiring board types
Ceramic PWB
Organic PWB
Developmental PWB
Thick Film
Rigid PWB
Rigid-Flexible
Flexible PWB
Thin Film
Cofired
Direct-Bond Copper
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Different packaging and interconnection
techniques
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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
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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.
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1- Flip-Chip packaging
Flip-Chip joining
The Flip-Chip joining mainly by thermocompression
or thermosonic bonding.
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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
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Wire bonding steps with thermocompression
bonding.
2- Wire bonding packaging
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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
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2- Wire bonding packaging
Wire bonding techniques
Ball bonding
Wedge bonding
The surface of the molten metal forms a spherical
shape or ball and the second bond having a
crescent shape.
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.
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2- Wire bonding packaging
Application of ball bonding
Application of wedge bonding
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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.

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3- Chip scale packaging
Chip scale packaging classification
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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.
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4- Ball Grid Array Packaging
Types of BGA packages
PBGA (Plastic ball grid array)
TBGA (Tape or Tab 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 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.

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Introduction
Conduction cooling for chassis and circuit boards

• 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.
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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.

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Uniformly distributed heat sources, steady
state conduction
Parabolic temperature distributions for uniform
heat load on a circuit board.
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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

Where

Then
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Uniformly distributed heat sources, steady
state conduction
Then
This is a second-order differential equation,
which can be solved by double integration.
Integrating once yields to
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
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Uniformly distributed heat sources, steady
state conduction
The temperature at any point along the plate (or
strip) is
When x 0. This results in the equation for the
maximum temperature rise in a strip with a
uniformly distributed heat load.
The total heat input along the length L is
Then
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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 212F.
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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
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
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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 80F, the case
temperature on the component will be 277F. Since
the typical maximum allowable case temperature
is about 212F, 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 114.5F (45.85C)
then the case temperature on the component will
be about 195F 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.
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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
high-power 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.
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Chassis with nonuniform wall sections
Series flow resistor network Rt R1
R2R3
Parallel flow resistor network.
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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.

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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
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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
• - 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)
• - 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

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Solution

• - 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
• - 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

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Solution

• After getting all resistors we can simplify the
  network. Where resistors R2, R3 and R4 are in
  parallel, which results in resistor R6.
• The total thermal resistance is
• The temperature rise across the length of the
  bulkhead is.

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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.
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Circuit board edge guides
Types of edge guides
wedge clamp
B guide
U guide
G guide
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.
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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.
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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
Where R 6 oC in/watt (U guide) Q10/2 5
watt L 5 in Then
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