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 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 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: 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.
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 Acrylic coating Epoxy coating Polyurethane coating 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 Rigid PWB Flexible PWB Rigid-Flexible Thin Film Cofired Direct-Bond Copper
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
Wire bonding steps with thermocompression bonding. 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 100-150 oC Three wire bonding processes
Wire bonding techniques 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.
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) 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.
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 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 Where Then
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
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
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 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
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 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.
Chassis with nonuniform wall sections Series flow resistor network Rt= R1+ R2+R3+ ………… Parallel flow resistor network.
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 - 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
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
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.
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 Where: R= 6 oC in/watt (U guide) Q=10/2 =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