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Upon completion of this section on printed circuit hardware and component assembly, the student should be able to:
After a circuit design has been processed into a pc board, two major operations are necessary in order to produce a functional circuit. These are (1) assembling individual components and hardware that comprise the finished board, and (2) soldering component leads to the etched copper foil conductors. This section is devoted exclusively to assembling components and hardware to pc boards.
In recent years, an emphasis has been placed on the development of reliable miniature devices and hardware that are adaptable to pc applications. Consequently, during the planning stages of a pc packaging problem, the technician is faced with the task of selecting the most suitable components and hardware for his/her specific purpose from an overwhelming variety of types manufacturers. Coupled with this is the necessity of having available the technical information for mounting these parts onto the pc board. This section discusses the applications and assembly techniques of many of the common components and hardware associated with printed circuits.
Specific information for solder terminals and methods for their insertion into the board are considered. Also included are eyelets in addition to information on connectors and associated hardware for board-to-board or board-to- chassis interconnections. Basic techniques and tools necessary for mounting components such as resistors and capacitors are discussed. Included are methods of mounting devices such as diodes, transistors, ICs (integrated circuits), FETs (field-effect transistors), and SCRs (silicon-controlled rectifiers) with and without sockets. In addition, techniques for heat sinking these devices and the thyristor are discussed.
1. STANDARD SOLDER TERMINALS
One of the major problems encountered when designing and assembling a pc board is the means by which external connections are made. For this application, the selection of suitable hardware is almost unlimited and is determined by many design factors, some of which were discussed in Section 1.
One of the most economical and easily installed terminal types is a metal stud, mechanically and electrically secured to the board, on which clips for testing or wires for final assembly may be attached. These metal studs are termed solder terminals. These terminals are mounted through predrilled holes in the pc board’s insulating material and copper conductor. They are then swaged to form a solid mechanical connection between the insulated side of the board and terminal pad. Finally, they are soldered to the copper conductor to provide a sound electrical connection. Some typical configurations of solder terminals that will satisfy many packaging requirements are shown in FIG. 1 a. The terminals shown are classified as turret, fork, tubular, taper pin, and minipin and are available in either single-ended or double-ended styles.
A typical double-turret, single-ended terminal with its associated nomenclature is shown in FIG. lb. These terminals are designated by post style and size (available in hollow or solid posts), shank diameter, and shank length. The shank length must fit the pc board thickness for which it is intended. These thicknesses generally range from to inch.
Solder terminals of the type shown are usually formed from tinned brass and are recommended for use with pretinned pc boards. This combination results in the proper flow of solder between the circuit pattern and the terminal, when heated with a soldering iron, to form a sound electrical connection with a minimum amount of solder applied to the joint.
To secure the solder terminals to the pc boards, they are swaged by one of two methods: hand or pressure staking. Both methods require that a hole be drilled into the pc board that is approximately 0.002 to 0.005 inch larger than the shank diameter of the terminal.
For hand staking, an anvil, such as that shown in FIG. 2a, is held firmly with a toolholder secured in a vise. The terminal to be swaged is placed, post down, into the anvil with the terminal crown seated. The hole in the pc board is positioned onto the terminal shank. When properly positioned over the terminal, the shank rim should protrude slightly out of the hole through the terminal pad. A simple toolholder can be fabricated from an appropriate-sized hex nut by first drilling out the threads to accommodate the tool diameter. A second hole is then drilled and tapped through the center of one of the flats for securing the tool with a set screw.
To swage the terminal, a setting tool, such as one of those shown in FIG. 2b, is used. For single-ended terminals, a setting tool with a solid face that will “cup” the shank rim down firmly against the terminal pad is required. The swaging of double-ended terminals requires that both the anvil arid setting tool be hollow. The setting tool hole diameter should be slightly larger than that of the terminal post for clearance. With the correct setting tool positioned directly over the terminal shank, it is firmly tapped with an 8-ounce hammer (FIG. 2c). This will cup the shank rim over against the copper conductor, forming a sound mechanical joint. Figure 2d shows the result of hand staking on a terminal. A properly swaged terminal shank will have the appearance of a perfectly smooth cupped eyelet with no fractures around the shape rim. In addition, no “play” should be detected when pressing down or pulling up on the terminal post.
Pressure staking is the preferred method of securing solder terminals to pc boards because there is least chance of fracturing the board by this method. Hand staking imparts mechanical shocks to the board material and unless extreme care is exercised, the base material could fracture. Pressure staking imparts a steady pressure to the terminal, eliminating the mechanical shocks and thus reducing the possibility of board fracture.
Pressure staking utilizes an arbor press such as that shown in FIG. 3. The terminal is inserted into the anvil exposing the shank onto which the hole of the pc board is positioned. With the appropriate setting tool attached, rotating the operating handle forward will bring the tool in contact with the terminal shank rim. Additional rotation will apply pressure to the terminal to complete the swaging. This operation is shown in FIG. 3. Because excessive pressure could damage the anvil, setting tool, terminal, and the pc board, most arbor presses are equipped with a threaded anvil. This anvil is threaded into the press table to a height that will provide adequate clearance for the thickness of the pc board. Adjusting for the correct travel of the setting tool is made by trial and error. Optimum anvil setting is achieved when full forward rotation of the operating handle properly swages the terminal without excessive pressure being applied to the insulating material or terminal shank.
When a terminal shank is formed by a V- or funnel-type swage instead of being fully cupped, a solder ring may be first slipped over the shank where it protrudes through the board prior to the swaging operation. For this type of swaged shank, solder rings are recommended to ensure a uniform, reliable solder joint, especially if a board is to be hand-soldered.
Terminals with test leads attached and leads soldered are shown in FIG. 4a and b, respectively. Techniques for properly orienting leads and soldering hem to terminals will be discussed in detail in Section 24.
2. PRINTED CIRCUIT BOARD CONNECTORS
The preceding section introduced a simple means of providing terminal points on pc boards for external connections to chassis- and panel-mounted components. This section introduces basic and commonly available hardware and methods for interconnecting boards. Selecting the most appropriate connector for a specific application will depend on such factors as (1) cost, (2) mounting limitations, (3) required number of contacts, (4) board thickness, (5) vibration, 6) current and voltage requirements, and (7) required insertion and removal operations. These considerations must be examined before an intelligent choice of connector can be made.
Connectors are available for board-to-board, board-to-wire, board-to-flexible cable, and board-to-chassis interconnections. These can be separated into two general categories: board edge plugs and receptacles. The edge-receptacle or finger-receptable-type connector consists of a molded insulator containing a number of female-type contacts that accept the edge of the pc board and mate with foil fingers to make the electrical connections. A typical edge connector is shown in FIG. 5a. An edge plug-style connector is shown in FIG. 5b, which contains either in-line or staggered contact pins swaged into the pc board and soldered to the terminal pads at the edge of the board. Assembly time is in creased with this connector, but it is recommended when the unit is subjected to high vibration. A two-piece connector consists of a male plug and a female contact arrangement. One section is mounted to the pc board and the other section is attached to a chassis or another pc board. Electrical contact is made when these two connectors are mated.
Connectors are also specified in terms of female contact design. For the edge-receptacle type, which must contact the foil fingers extending to the edge of the board, two contact arrangements are necessary. The first is the type with contacts that mate with a row of fingers on a single-sided board, whereas the second mates with rows of fingers on both sides of a board. Several contact designs for single- and double-sided requirements are shown in FIG. 8. The tuning fork design shown in FIG. 6a has low contact resistance and is used for single-sided board applications because it has only on terminal point that extends through the rear of the insulator. The ribbon-type contact design shown in FIG. 6b provides for contact to double-sided boards. Each upper and lower pair of contacts have individual terminal pins that may be use independently or soldered together with a jumper wire. The cantilever contacts shown in FIG. 6c are preformed and preloaded so that a specific force is applied to the foil fingers when a pc board is inserted into the connector. Finally, the bifurcated contact arrangement shown in FIG. 6d furnishes two points of contact on each foil finger. Each section of the individual bifurcated contacts is different in width, imparting to each its specific resonant frequency. During a period of vibration, it would be expected that at least one leaf will always be in contact with the foil. These split contacts also ensure positive contact if there should be any nonuniformity in finger thickness.
The connectors just discussed are typically constructed of either brass, phosphorous bronze, or beryllium-copper. When cost is a major factor, brass is the least expensive. It does, however, degenerate spring tension, owing to aging and continual insertion and removal of the pc board. Phosphorous bronze contacts are superior in spring-retention characteristics, but they are slightly more expensive and have a higher contact resistance than brass. Beryllium- copper contacts, although more expensive, overcome all the disadvantages of the brass or phosphorous bronze type. Since the materials used in the construction of the contacts for these connectors are relatively soft, they tend to wear under continual insertion and removal of the pc board. For this reason, they are plated with a gold alloy to a thickness of 20 to 100 millionths of an inch.
Several contact terminations through the top or rear of the connector are shown in FIG. 7. The eyelet and dip solder types afford the most sound electrical connections and are recommended for use where vibrations are expected. Wire-wrap styles allow for more rapid assembly, with a resulting cost savings over the solder types. The crimped tapered pin style allows for easy and rapid modification or repair of assembled connectors.
Connectors have contact spacings that vary from 0.100 to 0.200 inch, the most common being an on-center spacing of 0.156 inch. The number of contacts per connector may vary from 6 to as many as 100 and will, naturally, depend on the number of contacts required on the pc board.
Most manufacturers design their connectors to function equally well with either - or i-inch-thick pc boards. Insertion-type connectors for thicker boards are also available.
To eliminate the possibility of incorrectly inserting the pc board into the connector, especially in the case of double-sided boards, some method of connector polarization should be employed (FIG. 8). To insert the board into the connector in the correct position, a plastic or metal polarizing key is inserted either between pairs of contacts or beside them in slots provided for this purpose. For the board to be inserted into the connector when this type of keying is used, a notch in the leading edge of the board at the fingers must align with the key. The type of key that is positioned on the insulator beside contact pairs is sometimes preferable because it does not reduce the number of available contacts by one set. The leading end of the board that is inserted into the connector is chamfered at a 45-degree angle on both edges (FIG. 9). This configuration allows the board to be inserted easily into the connector and reduces the possibility of the foil fingers lifting on boards that are frequently removed and replaced.
3. COMPONENT ASSEMBLY
The smaller components such as resistors arid capacitors are first assembled. Under no circumstances should swaging be done on the board after any components have been assembled because of fragile components that might be damaged.
The components to be first considered are axial lead type (tubular shape) having leads at each end). On single-sided boards, these components are normally mounted parallel to the pc board surface with the body of the component lying flush against the insulated side. The leads are bent at right angles to the component body and passed through the predrilled clearance holes to the foil for electrical connection. Once through the board, the leads are bent in the direction of the terminal pad entry. This operation keeps the components from slipping out of position as others are installed and ensures sound electrical connections after soldering. The proper methods of component assembly are shown in FIG. 10. Several considerations need to be taken into account b fore the components are assembled. First, only components that weigh 0.5 ounce (14 grams) or less and/or dissipate less than 1 watt should be considered for flush mounting against the board. Heavier components should be provided with some mechanical clamping arrangement, which will be discussed later in this section. Components dissipating more than 1 watt should be mounted above the surface of the board (also discussed later in this section), since the heat generated from these components tends to weaken the foil bond.
Second, to minimize stresses on the components and their leads, the following minimum specifications should be observed: (1) the minimum bend radius for all component leads should not be less than twice the lead diameter, and (2) this bend should begin no closer than inch from the body or end bead of the component. These dimensions are shown in FIG. 10a, labeled R and X, respectively.
Finally, the leads must be properly bent after passing through the insulating material. One of two types of bending procedures may be adopted; these are the fully clinched lead arrangement, as shown in FIG. 10a, and a service bend, shown in FIG. 10b. The fully clinched lead is used when maximum rigidity of the component is desired. This technique also requires less solder to complete the electrical connection. Service bends, as the name implies, are adopted if components may have to be removed later on. This lead arrangement is made by bending the component lead to within approximately 30 degrees of the terminal pad. This technique makes it less difficult to remove a component after it has been soldered than one with fully clinched leads.
Although the specifications provided do not conform exactly to any standards, they are realistic values that have proven to be effective in pc boards not exposed to severe environmental or physical stresses.
The leads of tubular components must be accurately bent before being inserted into the pc board. Two simple bending tools are shown in FIG. 11. For nonuniform and nonstandard terminal pad center dimensions, the combination caliper and lead bender, shown in FIG. 11a, is first positioned with its pointed jaws aligned with the centers of the proper holes in the pc board. This automatically sets the correct spacing between the bending tabs, which are located be tween the jaws and the tool handle. A knurled thumb screw is provided to maintain this setting. With the component centered in the bending jig, the leads are sharply bent around the tabs until they are at right angles to the component body. This procedure is shown in FIG. llb. When this bending operation is completed, the component leads are ready for insertion into the pc board.
For standard or uniform spacing between terminal pad centers, the universal bending block, shown in FIG. 11c, may be more conveniently used. This type of bending block is usually graduated in 0.05- to 0.1-inch increments, with increasing on-center dimensions from 0.5 to 1.5 inches. The component body is positioned in the center channel with the leads resting in the appropriate pair of lead slots that most closely agree with the terminal pad on-center dimensions on the pc board. The leads are then bent down over the edges of the block to properly form both the correct radius and on-center dimension.
If neither of the described bending tools are available, tapered round-nose pliers, such as those shown in FIG. 12a, should be used to perform the lead- bending operation. Only pliers with smooth jaw surfaces should be used, to avoid scraping and nicking the component leads. A nicked lead could result in a complete fracture under operational stress or installation.
To form the bend with this tool, the lead is gripped at the proper position between the tapered jaws. Holding the pliers stationary, the lead is bent at right angles to the component body (FIG. 12b). Unlike bending tools, round- nose pliers do not allow for repeated accurate formation of on-center dimensions. With practice and care, however, the correct lead position in the tapered jaws can be determined quickly to achieve the desired bend radius and on center dimensions.
After the component leads are bent, they are inserted into the correct holes in the pc board from the insulated side. The component body is held firmly against the surface of the board with one hand and the leads are initially bent back against the foil with the other hand (FIG. 13a). The direction of the bend should be toward the conducting path entry at the terminal pad through which the lead passes. When the lead is bent to within approximately 30 degrees of the foil, it is cut with a pair of small diagonal cutters, as shown in FIG. 13b. (More information on these cutters will be provided in Section 24.) The lead should be cut even with the edge of the terminal pad so that the length of the lead is equal to the radius of the terminal pad. For service bends, the assembly is complete. If the leads are to be fully clinched, the jaws of a pair of long nose pliers are used to force the lead firmly against the foil. Leads should never be clinched before cutting because the jaws of the diagonal cutter could gouge the copper conducting path.
Pneumatic cut-and-bend tools are commercially available for pc board assembly. These tools will, in one operation, bend the lead to the desired angle and cut it to the correct length. Both sides of the power amplifier board assembled with wire-wrap terminals and tubular components are shown in FIG. 14.
The assembling of disc capacitors presents mounting problems due to their physical design. Whenever possible, they should be mounted with the edge between the leads flush against the insulation and the leads passing straight through the board before bending. Since disc capacitors are circular, care must be taken to prevent the component from rotating during the lead clinching operation, which could upset the vertical and center positioning. Figure 15a shows the proper method of mounting a disc capacitor and the incorrect mounting method. It is advisable to fully clinch the leads of disc capacitors to secure these components firmly to the pc board prior to soldering.
Vertical-mounted capacitors, such as those shown in FIG. 15b, are specifically designed for more efficient use of available area on pc boards. In mounting these capacitors, the access holes must be in alignment with the base layout of the component leads so that they may feed straight through the board. Bending the leads on this type of component from their original direction defeats the advantage of mechanical securing obtained when the base is flush against the board insulation.
When space is extremely critical on a pc board, axial lead components may be mounted vertically. This is accomplished by inserting one lead of the component, without bending, directly into the pc board. The other lead is bent 180 degrees around the body of the component and then passed through the board. This technique is shown in FIG. 16a. As can be seen from this figure, the mechanical security of even a small component mounted in this fashion is extremely poor. Under shock and vibration, stresses encountered between the component and the pc board could result in mechanical failure. To minimize this possibility, plastic supports, such as those shown in FIG. 16b are used. They significantly improve mechanical security.
Physically large components (greater than ounce) may present undue strain on their leads if not secured. A common type of support used to secure large components to pc boards is shown in FIG. 17. This clip-style holder is provided with an expansion lug that secures the clip to the board by a simple press fit. This type of component clamp is desirable since it does not require any disassembly to remove the component.
The mounting of small trim-pots (screw-adjusted potentiometers) and thumb-pots (thumb-adjusted potentiometers) shown in FIG. 18 requires special consideration since their leads are in the form of rigid pins or tabs that may not be crimped or bent. These leads must therefore be fed straight through when mounted. For mechanical security prior to soldering, the access holes for the pins are drilled for a press fit.
All the pc boards for the amplifier with their basic hardware and components assembled using the techniques discussed in the preceding sections are shown in FIG. 19.