CH.3

1. CH.3 Layout Planning and Design Eng.MohammedAlsumady

2. Reading Drawings and Diagrams • Block Diagram Systems may be too complex to be analyzed in detail. It is therefore, necessary to divide system into sub-systems and then integrate them. Each sub-system would then represent a functional block, and the combination of all the blocks would constitute the functional ‘Block Diagram’ of the equipment. A block is only a ‘black box’ with certain inputs and outputs, but performing a definite function. The lines interconnecting these blocks indicate the signal flow from block to block or circuit to circuit. Understanding of the circuit function becomes easy with a block diagram. The integrated circuits such as microprocessors, counters, etc. are represented as individual blocks. These blocks are labelled with pin numbers, signals and associated interconnecting wires.

3. Schematic Diagram A schematic diagram is a graphical representation of interconnections of various electronic, electrical and electromechanical components of an equipment. The schematic is the first step in an electronic circuit design because it displays and identifies the components that make up the equipment. Further, the first step in designing a printed circuit is to convert the schematic diagram in to an art master. Therefore, for any printed circuit designer, it is important to learn to read and interpret the schematic diagram. However, the schematic diagram does not show any of the mechanical details of the printed circuit board.

4. The schematic provides the most broadly used view of the design and includes all components. In addition: • It gives visibility into the status of all parts of the design process; • Schematics are the primary source for developing deliverables to product design and manufacturing groups; • Design variants are built around slightly differing schematics; • Test departments rely on schematics; • Field service relies on schematics; and • Bills-of-materials are generated from schematics. In short, a schematic is the focal point for a product’s electronic data and can be viewed as a set of crucial business documents that capture the decisions affecting all aspects of the product.

5. The schematic diagram shows various components by means of symbols which are so arranged that they show the working of the circuit in a clear way. The component symbols are usually governed by various standards, which do vary widely. Therefore, it is advisable to first find out which standard has been followed before attempting to read a schematic diagram. The schematic diagram is also called the ‘circuit diagram’. • In a schematic diagram, the symbol represents either what the component does in the circuit or how it is physically constructed. • All electronic components have been designated when represented on a schematic diagram. Classification from ANSI (American National Standards Institute), IEEE (Institute of Electrical and Electronic Engineers) and IEC (International Electrotechnical Commission)

6. Guidelines have been developed over the years for drawing schematic diagrams. The main features of these guidelines are: • Signal flow moves from left to right across the page with inputs on the left and outputs on the right. • Electronic potentials (voltages) should increase as you move from the bottom to the top of a page. For example, +12V supply is shown upwards while the –12V is indicated downwards. • Use the ‘unit number’ convention for assigning a unique IC package identification. For example, U1 with its internal gates identified by letter suffixes; U1A, U1B, etc. Only one of the common gates need show the power connections. Power connections are often omitted, but it is better to include as a reminder as well as to make your schematic complete.

7. General PCB Design Considerations • The basic function of a printed circuit is to provide support for circuit components and to interconnect the components electrically. In order to achieve these objectives, various printed wiring types have been developed. They vary in base material (laminate), conductor type, number of conductor planes, rigidity, etc. It is therefore expected that the printed circuit designers are adequately familiar with the variations and their effect on cost, component placement, wiring density, delivery cycles and functional performance. No finished product is ever better than its original design. The manufacturing process, at best, can reproduce the design. The same is true with printed circuit boards. The need for formalizing design and layout methods and procedures thus assumes critical importance.

8. Design and layout broadly includes the perspective of total system hardware, which includes not only the printed circuit but each and every component in its final form. Design and layout considerations must also address the relations between and interactions of the components and assemblies throughout the system. Board design is an extremely important aspect of printed circuit board technology. • The technical requirements that are likely to affect the design of an electrical equipment are mechanical, electrical, functional and environmental. • Mechanical design requirements include size, shape and weight; location of components and their mounting, dimensional tolerances, shielding and equipment marking. • Electrical design requirements as circuit function and wiring distribution, component selection with respect to electrical ratings, size and tolerance. • Functional design parameters include reliability, maintainability, and accessibility. • Environmental design factors such as mechanical shock and vibration, temperature extremes, salt spray, and operations in space or underwater. • by careful design, proper selection of materials and manufacturing techniques, it is possible to optimize most of the above parameters.

9. Important Design Elements Are the design inputs which should be provided by the equipment designer to the PCB: • Type of circuit (analog or digital, etc.); • Board size • Number of layers • Pad stack sizes • Hole sizes • Layer thickness • Board thickness • External connections • Mounting holes • Supply and ground layer thickness and • Component details with specifications.

10. Important Performance Parameters: • Tensile strength; • Flexural strength; • Shock and vibration; • Thermal shock and temperature cycling; • Moisture resistance; • Fungus resistance; • Salt spray; • Warp or twist; • Dielectric breakdown voltage; • Solderability and re-solderability; • Insulation resistance (surface and bulk); • Flame resistance; • Conductor temperature rise; • Machinability .

11. Mechanical Design Considerations • Types of Boards: • Single-sided Boards: These are mostly used in applications where costs have to be kept at a minimum.When designing layout, to jump over conductor tracks, either components or jumper wires are used. If their number is too large, double-sided PCBs should be considered. • Double-sided Boards: Double-sided boards can be made with or without PTH. Since PTH boards are expensive, they are chosen where the circuit complexity and density necessitate their use. • In the layout design, the conductors on the component side must be kept minimum in number to ensure easy maintainability. • In PTH boards, via holes should be utilized only for through contacts and not for component mounting. The number of via holes should be kept minimum for reasons of economy and reliability.

12. In order to take a decision on the number of sides, single-sided or double-sided, it is important to take into consideration the component surface area (C), which is a fairly constant percentage of the total PCB area (S), useful for mounting components. • the usual range of the ratio S:C for the most common types of PCB is 3:2. • In general, the decision about the number of sides must be fully cost-effective. As a thumb rule, the double-sided PCB with plated through-hole costs 5 to 10 times more than the single-sided PCB. Also, the cost for component assembling is another important consideration. The approximate cost of assembling (manual) the components on a PCB is 25–50 per cent of the total cost of a single-sided PCB and 15–30 per cent of the total cost of a double-sided PCB with PTH.

13. The PCB provides mechanical support and connectivity to the components mounted on it. The following are the main mechanical design considerations for the PCB: • Optimal board size compatible with the PCB manufacturing process; • Position of board mounting holes, brackets, clamps, clips, shielding boxes and heat sinks; • Proper fixation arrangement for heavy components; • Proper hole diameter for component mounting; • Assembled board to withstand the mechanical stress and vibrations occurring in transportation; • Type of installation of the board (vertical/horizontal); • Method of cooling; and • Specific locational requirements of components like front panel operated components such as push buttons, variable resistors, etc.

14. Determining the Component Area • The component area on the board is calculated by adding the contribution of each single component. Each component is considered in its orthographic projection on the board. The dimensions of the component are obtained from the manufacturer’s catalogue or by measure. • The components are considered as simple geometrical figures, for example, an integrated circuit in a DIP package is a rectangle and so is an axial component. However, for an axial component mounted horizontally, the width of the rectangle will be its diameter whereas the length will be its body plus a portion of the leads. Similarly, a TO-18 packaged transistor will be represented as a circle. • Part manufacturers provide data sheets to be used by the circuit designer to select parts for the circuit. For designing the PCB, these sheets should also have the physical dimensions of the part included.

15. Volume Computing • Some equipment may have to fit into an existing enclosure, which can limit the board size or leave the designer with a choice from only a small number of preferred board sizes. • The enclosure or case should be designed to fit the system to avoid forcing the system into an enclosure size that may dictate the use of a non-standard or non-optimum board size. In such cases, the volume available for an electronic assembly is calculated with care. Rather than the actual volume, it is important to know the maximum volume that the board can occupy in the worst condition, including the safety clearances.

16. Accessibility for Adjustable Components • Adjustable components, usually variable resistors, are common in many printed circuit board assemblies. These components should be mounted on a PCB in a such a manner that there is an easy access to such components. • Board Size: • A functional printed circuit board is not a product in itself. It always requires connections to the outside world to get power, exchange information, or display results. There may be a need to fit it into a case or slide it into a rack to perform its function. Tooling holes and keep-out areas may be required in the board for assembly or manufacturing purposes. All these factors need to be defined before the board can be designed, including the maximum dimensions of the board and the locations of connectors, displays, mounting brackets or any other external features. • In order to avoid interconnections between different PCB boards through terminations, cables or connectors, it is preferable to accommodate all the circuitry on a single board.

17. accommodate all the circuitry on a single board in many cases, can result in large-sized boards, requiring more and more space for interconnections, thus leading to more functional disturbances. On the other hand, too many small boards forming one complete circuit can lead to higher cost. • In general, an equipment or system consisting of smaller boards is easier to repair and service because of its modular structure and convenience of isolating signal flow paths.

18. working on the board size, locational constraints in respect of the following components are encountered: • Connectors or connecting tabs; • Fixtures or anchoring areas; and • Control or adjusting devices such as switches and potentiometers. • Besides these, mounting holes along with the safety areas around them are fixed. • Several factors affect the selection of board size and shape and therefore, the final choice is probably a compromise amongst conflicting factors.

19. Board Mounting Techniques • Various techniques are available to mount the printed circuit board to the chassis or to the next assembly. For providing good mechanical stability. As a general practice, boards between (0.785 and 1.57 mm) thick should be supported at intervals of at least 10 cm. The choice of board mounting technique would depend upon the following factors: • Board size and shape • Input/output terminations • Board removal requirements • Heat dissipation requirements • Shielding required • Type of mounting hardware • Available equipment space and • Type of circuit and its relation with other circuits.

20. Input/output Terminations • The most common method of providing electrical interface between the PCB and the associated equipment is by the use of connectors, terminals and cables. The type of interface to be used for any particular board is generally decided during the mechanical design of the equipment. • Usually the male part is mounted on the printed circuit board and the female part to an interconnecting board such as a motherboard or back plate. • Single or double part connectors usually have a limited number of contacts and can be mounted generally at the edges in particular directions.

21. Testing and Servicing • Depending upon the product complexity, the quality of incoming components and the manufacturing process, there is a likelihood that a certain number of finally assembled boards may not work. Therefore, the board design must take into consideration the level at which the test must be performed and to make such testing as simple as possible. Smaller boards are preferred to achieve efficient testing and repair of PCBs. • In large boards, the isolation of defective parts becomes difficult because it is usually not possible to interrupt or influence the signal flow. If the complete circuit is realized on different smaller cards, it is easier to isolate the defective card, repair it or exchange the same with a working card. Another advantage in having an electronic circuit split into sub-units on different boards is the possibility to use the common subunits like power supply for other equipment, if it is so designed.

22. Mechanical Stress • Comparatively small PCBs with a size of less than 100 ×150 mm and the standard thickness of 1.6 mm will hardly pose any problem in mechanical strength if assembled with the usual electronic components. Care is needed with bigger board sizes or thinner laminates or if heavy parts like transformers have to be mounted on the board. As a general guideline, heavy parts should be mounted near a supporting device like a card guide, connector or stiffener.

23. Board Thickness • There is no standard rule for the optimum thickness neither for the printed wiring nor for the number of multilayer conductive layers. Occasionally, the limiting factor for printed wiring thickness is the diameter of the smallest hole, especially when the holes are plated though. • The final board thickness will depend upon the number of conductor layers and on the electrical layer-to-layer spacing requirements of the design. In multi-layer boards, the increase in cost is not directly proportional to the increase in the number of conductor layers. For example, doubling the number of layers from four to eight will probably increase cost by only 30 per cent. However, if the number of conductor layers exceeds 10, the extra layer costs increase at a rapid rate. • Printed board thicknesses can vary from 0.50 to 6.5 mm, but most rigid boards have thicknesses of 1.5 mm.

24. Electrical Design Considerations • Conductor Dimensions: • In general, conductor width is determined by: • Component packing density; • Minimum spacing between conductors and components; and • Geometrical constraints due to component outlines. • In former times, the current carrying capacity of PCB conductors was often disregarded because conductor dimensions were usually much larger than needed for carrying the currents involved. However, with higher packaging density and thermal considerations, the conductor width has to be determined or at least checked according to the required current carrying capacity. • In most electronic circuits, comparatively small currents are normally flowing for which the conductor resistance can practically be neglected. However, when we deal with supply and ground lines, especially in case of high speed signals and in some cases, digital circuitry, much broader conductors than ohmically necessary have to be provided between the supply and ground lines.

25. Factors govern the choice of appropriate conductor width and determines its current carrying capacity: • Resistance of the conductor is problem when conductive paths are long. • Capacitance is a parameter of considerable importance, particularly in the design of PCBs at high frequency. The capacitance comes into play in the following two situations: • Capacitance between conductors on opposite sides of the PCB; and • Capacitance between adjacent conductors. • Inductance of PCB Conductors: • In designing the conductor patterns for fast signal or high speed logic circuits, the inductive couplings are also of major concern. In logic circuits operating at a clock rate of only 10 kHz, high frequency components of the rectangular shaped signals can often cause problems. Therefore, in such situations, it is important to know the inductance of a conductor arrangement.

26. Conductor Patterns • The manufacturability and reliability of a PCB depends, upon the basic design of the PCB in terms of conductor width, thickness, spacing, shapes and routing, etc. The design can be done manually or with a computer, but the basic rules in both cases are fairly constant. The three basic rules for layout design are: • No interference between the components; • Conductors not to cross each other; • Sufficient spacing between any two close conductors. • As a general rule, in each hole, only one lead will be present and each lead has to pass through a hole. It cannot be soldered to another lead, regardless of how well this joint can be made. Axial components will generally have both leads parallel to the body axis, unless specified otherwise.

27. Component Placement Rules • Following are the rules for component placement: • In a highly sensitive circuit, the critical components are placed first in such a manner as to require minimum length for the critical conductors. • In a less critical circuit, the components are arranged exactly in the order of signal flow. This will result in a minimum overall conductor length. • In a circuit where a few components have considerably more connecting points than the others, these key components have to be placed first and the remaining ones are grouped around them. • The general rule is to place first components, whose position is fixed for the final fitting and interconnections, e.g. connectors, heat sinks, etc. Then place the components which are connected to these fixed components. • Components should be placed on the grid of 2.5 mm. • Among the components, larger components are placed first and the space in between is filled with smaller ones. • All the components should be placed in such a manner that disordering of other components is not necessary if they have to be replaced. • Components should be placed in a row or a column, so that it gives a good overview.

28. Conductor Width and Thickness • The conductor width is determined by: • The component packing density; • The minimum spacing between conductors and/or components; • Geometrical constraints due to component outlines or fan-out. • The width of a conductor is basically a function of the current carried and the maximum allowable heat rise due to resistance. Narrow conductors result in high resistance. Wide conductors are, therefore, desirable for low impedance signals where series resistance and inductance are to be minimized and stray capacitance is unimportant. • in case of where stray capacitance is required to be limited to a low value, narrow conductors are used. • Normally, the conductor width should be as generous as possible to take care of any variation in the etching process as well as any scratches in the artwork caused unintentionally. • If a trace is made smaller than minimum width, there is a chance that it will open when manufactured. • If two traces are closer together than the minimum spacing, there is some chance that they will short when manufactured. These parameters are usually specified as X/ Y rules, where X is the minimum trace width and Y is the minimum trace spacing. For example, 8/10 rules would indicate 8 mil minimum trace width and 10 mil minimum trace spacing. Typical modern process rules are 8/8 rules with values as small as 2/2 rules being available.

29. Conductor Spacing • The conductor spacing considerations are generally based on voltage breakdown or flashover between adjacent conductors. The conductor spacing is determined by the peak voltage difference between adjacent conductors, capacitive coupling parameters and the use of a coating. • Conductor Shapes • While deciding the layout, sharp covers and acute angle bends in conductors should be avoided as far as possible. The rounded contours will not only minimize conductor cracking, and electrical breakdown, but also greatly facilitate solder distribution. Rounded corners at conductor bends and smooth fillets at the junction of conductors and terminal areas are desirable.

30. A trace (line) that extends straight and then turns 180 degrees back on itself looks just like an antenna. • A trace that makes a right angle turn also has the characteristics of, an antenna. It is not a really good antenna.

31. Conductor Routing and Locations • For error-free operation of the circuit, it is necessary to provide proper routing of the printed conductors and to locate various components in a manner that they do not cause electrical or mechanical disturbance. • The conductor routing rules are as follows: • Conductor length should be the shortest possible. • Conductors forming sharp angles should be avoided as this creates problems in etching. • Where one or several conductors have to pass between pads or other conductive areas, the spacing has to be equally distributed. • Minimum spacing is applied only where it cannot be avoided, otherwise higher spacing should be given. • In a double-sided PCB, it is a normal practice to draw the tracks on the component side in the direction of the Y-axis and tracks on the solder side in the direction of the X-axis. • Maximum number of tracks should be distributed on the solder side and drawn in the direction of X-axis.

32. Some other practical suggestions are: • Use minimum number of layers of wiring. • Provide maximum line width and terminal area commensurate with the density of packaging. • Avoid sharp angles and bends in conductors to minimize electrical and mechanical problems. • By careful routing of individual sections of each circuit, provide an exclusive conductor to a single ground at essentially a single point. Grounding circuitry is of major concern in determining the internal circuitry layout.

33. Supply and Ground Conductors • The primary consideration in a power or ground conductor is to provide a direct connection from the device to the power supply. • As this is not always possible, the next best step is to increase the width of the conductor. • The width of these conductors and their layout play an important role in imparting stability to the circuit voltages. In some cases, resistive losses in these conductors may result in unstable supply voltage or ground system. Therefore, it is advisable to be fully aware of the possible circuits on a PCB for designing the conductors with an adequate width. The problems are more serious in case of digital and high frequency signal circuits. • When several supply voltages are used in a circuit, it must be ensured that the ground conductor has the capacity to carry the combined load under the worst case condition.

34. A rule of thumb for deciding the width of conductors for various purposes is: • Wground > Wsupply > Wsignal Wground = conductor width of ground line Wsupply = conductor width of supply line and Wsignal = conductor width of signal line. • The fundamental rule for TTL circuits is • Wground ≥ 2 Wsupply • Wsupply ≥ 2 Wsignal • it is advisable to utilize all the unused board area for ground conductors of a ground plane.

35. The distribution of voltage or power and ground planes is therefore a critical design element in the layout of a complex or high I/O semiconductor package. Voltage and ground conductors are commonly designed as full metal planes. • The main problems associated in the design of voltage and ground conductors are: • Power losses; • Power and voltage level variation; • Efficiency; and • Interconnection cross-talk. (Electronics Undesired signals).

36. These problems are addressed in the design rules as per American National Standards Institute/trace width calculator ANSI/IPC-2221. • The analog and digital circuits on the same PCB should strictly have an independent ground network to avoid power losses. • All digital signals and components should be located away from analog circuitry. • Voltage and ground signal conductors have to be provided with sufficient width to keep resistance and inductance low and to carry the required current, otherwise the conductor will act like a fuse. • While connecting voltage and ground conductors, priority should be given to the component with the highest power consumption, so that the power consumption along the supply line should continuously decrease. • The spacing between voltage and ground conductors should be as large as possible to avoid cross-talk problems. • All I/O voltage and ground conductors should have minimum conductor length to achieve more efficiency in circuit. • To increase the performance speed of semiconductor die and packages, many power and ground I/Os are required. Normally, a PCB with 40-60 per cent voltage and ground conductors has a good performance speed.

37. Avoid switch mode power supplies near ADCs, DACs and analog circuits. Sometimes, it is easier to use a separate 5V three-terminal regulator, near the chip, for the analog supply. A 22 mF tantalum or aluminum capacitor at the board edge helps to reduce power supply noise. • Watch out for the external magnetic field of inductors and transformers. Use electrostatic and magnetically shielded components, if necessary. RF de-coupling chokes can be mounted at right angles to minimize mutual inductance. Power transformers should be mounted off the board and oriented, with the most intense area of their external field, away from critical analog circuits. Use toroidal power transformers to reduce magnetic external fields. • Consider carefully the presence of Programmable Logic Devices (PLDs) and Very Large Scale Integrated (VLSI) logic chips on the same printed circuit boards. These chips frequently include lots of synchronous logic and generate large switching currents that can infiltrate the rest of the board. This will ensure their reliable operation.

38. In a complex circuit, it is desirable to split the ground connection to achieve optimal efficiency of the circuit. A common ground may give rise to detrimental voltage drops along the conductor, introducing noise and false signals and resulting in malfunctioning of the circuit. Therefore, the designer must either decrease the length or increase the width of the ground conductor as far as possible. • In order to provide adequate shielding, particularly in the case of high frequency shielding, it is desirable to provide a ground plane. In the PCB world, a plane is a solid sheet of copper. It is a ground plane if it is connected to ground and it is a power plane if it is connected to a power supply voltage. Usually large left out areas are converted into ground plane

39. Fabrication and Assembly Considerations • Certain limits should be taken into account in order to maximize manufacturability and thereby minimize cost: • Conductor spacing less than 0.1mm will not work with the etching process, because the etchant fluid does not circulate efficiently in narrower spaces resulting in incomplete metal removal. • A conductor width smaller than 0.1 mm will lead to breakage and damage during etching. • The land size should be at least 0.6 mm greater than the hole size.

40. The following limitations determine the layout techniques: • Size capability of reprographic camera for film master production; • Artwork table size; • Minimum or maximum board processing size; • Drilling accuracy; and • Fine line etching facilities. • The following parameters are taken into considerations for design from the point of view of assembly of printed circuit boards: • Hole diameter shall be expressed in terms of maximum material conditions (MMC) and least material conditions (LMC) limits. • clearance between a minimum of 0.15 mm and a maximum of 0.5 mm. Also, for flat ribbon leads, the difference between the nominal diagonal of the lead and the inside diameter of the unsupported hole shall not exceed 0.5 mm and shall be not less than 0.15 mm. • Properly locate smaller components so that they are not shadowed by large components. • Solder mask thickness should not be greater than 0.05 mm. • adequate attention must be given to the possibility of shorts being generated by an inserted component deviating from its theoretical position before soldering. As a rule of thumb, the maximum allowed inclination for a component lead is that it should remain within 15° of its theoretical position.

41. Multiple board assembly normally makes servicing at the field level easier as board level replacement can be easily carried out. However, this is possible only if each individual board performs a specific function. In such cases, the board replacement involves no major dismantling with minimal soldering/de-soldering. The design of the printed circuit boards must therefore take into consideration the maintainabilityaspects. • Soldering techniques and equipment for assembly also impose many restrictions on the board design and layout. For example, in wave soldering, the maximum sizes of the slots, edge clearances and handling clearances are important parameters. Also, the designers must be aware of what the final product will be and try to protect its most sensitive parts, as far as possible. For example, any high voltage circuit must be protected to prevent contact from outside. Careful location of components on the boards and of the boards in the product can help to minimize the likelihood of damage by external agents.

42. Environmental Factors • The reliability of an equipment, to a large extent, would depend upon the reliability of the basic printed circuit board. It is therefore, expected that the PCB should withstand exposure to the environmental requirements without either physical damage or change in operating characteristics. Also, besides serving as an electrical connection medium, printed wiring planes provide mechanical support for the active and passive components they are interconnecting. Thus, they become an integral part of the package or assembly and must therefore be able to withstand the environmental stresses associated with the entire structure.

43. The important environmental factors in the design of printed circuit boards are: • Thermal Considerations: The PCB designer should ensure proper cooling of the electronic packages by: • Use of high temperature components, where possible; • Thermal isolation of temperature-sensitive components from high heat-emitting sources; • Ensuring proper conductive cooling; the heat removal can be achieved by conduction, convection and radiation. • In order to eliminate local hot spots that can damage the board or adjoining components, special consideration should be given to the placement of power transistors or high wattage resistors. In general, such components should be mounted close to the frame which serves as the heat sink.

44. In order to maintain the components below their maximum operating temperature, proceed as follows: i) Analyze the circuit and obtain the maximum power dissipation for each component. ii) Determine the maximum operating surface temperature to be expected. The maximum allowable temperature is dependent on the insulation present as well as the components themselves. The design is worked out keeping these parameters in mind.

45. Contamination • Printed circuit boards must be protected against dust, dirt, contamination, humidity, salt spray and mechanical abuse. There are many insulating compounds that can be applied as protective coatings. • The technical considerations involved in the selection of protective coating are: i) Ability to prevent corrosion of the board; ii) Flexibility — resistance to cracking during shock; iii) Easy application and processing; iv) Transparency — to enable viewing of the board’s component marking; and v) Easily removable for repairing the printed wiring assembly. • For general applications, the thickness of protective coating is typically 0.075 mm minimum and 0.25 mm maximum.

46. Shock and Vibration • Vibration, flexing and bowing are the problems usually encountered on larger boards. The effects of vibration and warping can be minimized in exactly the same way as those met in any other form of engineering and similar solutions can be used. Parts that might be susceptible to failure because of shock or vibration should be located as near to the supported areas of the board as possible. Clamping or strapping may be required for properly holding the components in place. • One of the commonly encountered problems due to vibration or bowing is the possibility of components with electrically live cases coming in contact with the soldered joints on the back of the adjoining board. Such a danger is avoided by fitting the board with spacers higher than any of the electronic component/packages on it at suitable places. • Bulky or heavy components need particular attention. Unless they have many leads, it is necessary to anchor them to the board with some device.

47. Guidelines to eliminate vibration during the design of printed boards: • The mounting height of free-standing components should be kept to a minimum. • Positive support of all components with a weight of more than 5 g per lead has to be considered if the board will be subjected to vibration. • Vibration isolators should be considered for mounting of units, whenever practical. • Board stiffeners and/or metal cores should be considered to reduce the board deflection. • If the equipment in which the PCB is mounted is subject to shocks, vibration, etc. the leads generated by it can easily lift the pads of single-sided PCBs. One method to prevent such a failure is to enlarge the pad area to cater for large components. Alternatively, it is recommended to provide two dummy pads. If one pad de-laminates, the board can be easily repaired by soldering the component to the dummy lead.

48. An even better practice is to use a funnel eyelet into the holes used for mounting heavy components. This provides much larger mechanical resistance to loads applied to the PCB through a component lead.

49. Cooling Requirements and Packaging Density • Heat Sinks: • Thermal management is an important aspect of the design of printed circuit boards. The design should accommodate the problems of heat distribution and heat removal in systems utilizing integrated circuits. For example, component density is often higher with SMT, resulting in greater power dissipation per square inch of PCB. In addition, closely spaced components make forced air cooling less efficient. Therefore, the air flow arrangement must be so designed that it can deliver the volume of air required to restrict the temperature rise of the board within the permissible limit. Reliability can thus get degraded unless special attention is paid to thermal management. For example, in multi-layer boards, all interconnections can be placed on internal layers and a heat sink of thick, solid copper or another material can be placed on the outer surfaces. Components can then be mounted directly on the metallic surface.

50. Sufficient free space should be provided around the heat sinks to improve efficiency. • No bulky component should be mounted near the heat sink which obstruct the free air flow. • Heat-generating components are raised to a higher level above the board. This prevents damage to the component and the board itself. • The air flow must always pass the heat sink in the same direction as the slots are made.