Electronic Techniques--PCB HARDWARE AND COMPONENT ASSEMBLY (part 2)

<< cont. from part 1

4. DEVICE ASSEMBLY

Assembling devices to pc boards presents special problems in terms of lead bending, aligning, inserting, crimping, and protecting the device before and during soldering. All of these points need attention if the finished pc board is to be reliable. Devices that are considered in this section for direct mounting on the pc board are transistors, stud-style zener diodes and SCRs, FETs, TO-5-style integrated circuits (ICs), dual-in-line plastic pack ICs (DIPs), and flat-pack ICs. Unique assembly problems for each of these devices will be considered separately. For a complete understanding of the various assembly techniques, continual reference should be made to Appendix II, which provides a listing of device outlines, dimensions, and lead orientations for some of the more common case styles used to package solid-state devices. A large number of solid-state device sizes and case configurations have been standardized by the Joint Electronic Devices Engineering Council (JEDEC). Several diode and transistor case outlines are identified by numerical designations prefixed by DO or TO to indicate a specific outline. Although many power transistors and rectifiers do not follow any established standard for style and size, such as the JEDEC package identification system, the manufacturer’s case numbers and specifications will readily provide the necessary device outline and spacing dimensions for assembly.

Before the devices are assembled, an important consideration regarding their protection will first be discussed. Solid-state materials are extremely temperature sensitive and, as such, precautions must be taken during assembly that will protect them from excessive heat while soldering. This heat is transferred from the soldering iron tip through the lead to the device and affects the lead bond. This thermal problem often dictates the choice of specific mounting arrangements for the device. Manufacturers often provide assembly information together with maximum allowable time at specific temperatures that the device can tolerate without ruining the lead bonds. For the 2N3053 transistor used in the amplifier, the manufacturer specifies that the maximum allowable thermal conditions while soldering is 10 seconds at 491°F (2550 C). If these conditions are expected to be exceeded, some method of heat transfer is necessary. The most common device used is a lead heat sink. This aid is mechanically connected to the lead between the device case and the soldering iron tip. The heat sink absorbs and draws off excessive heat from the device during soldering. A common method of heat sinking is shown in FIG. 20. This is a commercially available heat sink that is made of aluminum and is spring-loaded. Aluminum, copper, or brass heat sinks are more efficient than steel pliers because these materials are better conductors of heat. As can be seen from FIG. 20, some space must be provided between the device case and the insulation side of the pc board, where sinking is necessary, to attach the heat sink. This obviously is a disadvantage in terms of mechanical security because it prevents the device from being flush-mounted to the pc board. Heat sinking of leads is not normally necessary when dip or wave soldering techniques (discussed in Section 15) are employed but should be considered for hand soldering. Therefore, when assembling devices the technician must consider the necessity of lead heat sinking and provide for it where necessary.

A popular case configuration used for low-power applications below 1 watt is the TO-5 style shown in FIG. 21a. As will be shown, this case configuration is relatively simple to assemble to pc boards. The wide sealing plane provides excellent mechanical security when mounted flush against the insulated side of the board. This type of mounting requires that lead holes in the board have the exact spacing and orientation as the transistor leads. Straight feed-through connections to the pc board are made, allowing for either service bends or fully clinched leads onto the foil side of the board. This mounting arrangement is shown in FIG. 21b. One possible disadvantage could result from this type of mounting. Flush mounting will cause contaminants to be trapped between the device body and the board while cleaning and removing flux. To avoid this, an insulated plastic spacer having riser buttons is placed between the device case and the board. Several styles of these spacers are shown in FIG. 21c. This mounting arrangement is shown in FIG. 21d. The spacer allows solvents to be used to remove contaminants from under the de vice case.

FIG. 20 Spring-loaded lead heat sink.

FIG. 21 Methods of assembling the TO-5 case style to pc boards: (a) TO-5-style case configuration; (b) flush mounting of T0-5-style case: (c) insulated device spacers; (d) mounting technique using spacer.

The leads of the TO-5-style case initially are inserted most easily into both the spacer and the pc board by positioning the center lead over its respective lead hole. The case is then tilted at an angle to raise the other two leads above the surface of the board or spacer. This procedure is shown in FIG. 22. Once this first lead is properly positioned, the case is rocked to align and insert another lead. The last lead is inserted in a similar manner. Once the leads have been inserted through the spacer and the pc board, the assembly can be firmly seated. The leads are bent over toward the conductor pad, cut to the desired length, and clinched similar to component leads, as discussed in Sec. 3. The transistor is now ready to be soldered.

FIG. 22 Case is tilted for insertion of second lead.

Another popular case style is the TO-92 package shown in FIG. 23a. These in-line leads are best assembled by first orienting an end lead over its lead hole on the pc board. The middle lead is inserted next and finally the remaining lead is fed into its hole. The hole centers on the board are usually drilled farther apart than the lead orientation on the transistor to provide sufficient spacing among terminal pads because the leads are so closely arranged on this type of device. The transistor is mounted so that the height of the sealing plane is approximately inch from the board surface. Lead heat sinking can then be easily employed. The completed assembly of a TO-92 transistor is shown in FIG. 23b.

FIG. 23 Typical mounting technique for small plastic transistors: (a) TO-92 case style; (b) final installed position.

FIG. 24 Hard ware arrangements for mounting stud-type de vices to pc boards.

The stud-mounted zener diode or similar-style SCR is provided with a threaded stud for mounting. Since the electrode electrically connected to the stud is usually the anode, the use of an insulating washer may be necessary when mounted to a metal frame. For pc board mounting, a hole large enough to clear the threads of the stud is formed. Sufficient copper foil at the terminal pad must remain after drilling this clearance hole to ensure a sound electrical connection between the stud and a flat metal washer used for mounting. The arrangement of this device and hardware is shown in FIG. 24. The mounting nut should be tightened onto the lockwasher with a torque wrench to prevent fracturing the insulation around the edges of the access hole. A torque wrench allows a specific pressure to be applied to the nut. Calibrated in inch-pounds, the torque wrench is rotated with an even force until the desired pressure is reached. The beam-type torque wrench is rotated in a clockwise direction until the pointer has moved to the scale marking that indicates the desired torque value. Zero torque position is at center scale, allowing torque measurements in both left- and right-hand directions. Another style of torque wrench is the dial- indicating type, which allows the operator to determine the applied torque in either direction of a finely calibrated dial indicator.

Torque screwdrivers are also available. These tools have bit holders that allow screwdriver blades, hex keys, or hex sockets to be interchanged quickly and have micrometer style scales that are set to the desired torque. A barrel scale mark is aligned with the desired micrometer setting on the handle. This tool is extremely useful where assembly accessibility is limited. When the set torque value is reached, the tool disengages with a clearly audible “click.”

Manufacturers’ torque specifications for mounting devices must be consulted to ensure that the device or pc board is not damaged. On stud-mounted devices, excessive torque can deform the sealing plane enough to form a gap in the thermal circuit that exists between the device material and the case. This would cause an increase in the internal thermal resistance that could damage the device.

The cathode of the zener diode, normally connected to a lug extending from the top of the device, can be electrically joined to its respective terminal pad with a piece of insulated wire. This connection is also shown in FIG. 24.

Some insulated-gate field-effect transistors (IGFETs), or metal oxide semiconductor field-effect transistors (MOSFETs), require utmost care during assembly. This device is extremely sensitive to the most minute static voltage at its gate terminal. For this reason, the manufacturers of these devices supply them with their leads packed in conductive foam, which shorts them together. This arrangement prevents any static charge from building up at the gate terminal, which could easily destroy the device. The leads should never be touched with the fingers and should be soldered only with a grounded soldering iron.

FIG. 25 Method of assembly for 10-99 case style: (a) T0-99-style IC package; (b) staircased leads simplify their insertion into pc board.

The TO-99 style of case and lead configuration, shown in FIG. 25a, presents uniquely difficult assembly problems. Integrated circuits of this style have at least eight leads attached to a case with a diameter of 0.200 inch. The mounting problems arise when attempts are made to align and insert all eight leads simultaneously, since the manufacturer normally cuts all device leads to an equal length. Assembling these devices can therefore be simplified by staircasing—t hat is, by cutting each lead, starting at the key lead, succeedingly shorter than the next in a descending direction toward the sealing plane. The key lead is then used as the reference for lead alignment and placement into the pc board. The shortest lead must be cut to a length that will allow it to pass through the pc board and be easily soldered to the terminal pad. With the leads cut in staircase fashion, the longest lead is inserted first into the proper hole without aligning the others. Each succeeding shorter lead is then inserted, one at a time, until all the leads are inserted and the component is properly mounted. The assembly of an IC with staircased leads is shown in FIG. 25b. An IC assembled with fully clinched leads and soldered is virtually impossible to remove without damaging the device. For this reason, service bends are recommended if there is any chance that the device will have to be removed at a later time, which is sometimes the case in prototype work.

Assembling 14-pin dual-in-line packaged (DIP) ICs (TO-116 package) presents equally difficult problems in terms of lead insertion. Since the leads on these devices are too short to staircase, an alternative approach is necessary. The DIP is first tilted back with its leads positioned above the pc board and approximately over the lead holes. This step is shown in FIG. 26. Finger pressure is applied evenly to the outside edges of the first two opposite leads closest to the board until they align with their respective holes. These leads are partially inserted into the board. The device is then tilted down closer to the board surface until the next two adjacent pins are against the board. Again, finger pressure is applied to align and insert these leads. This technique is continued until all the leads have been inserted into the board. Because the leads provided with the DIP are usually too short for full clinching, they are normally soldered while extending straight through the board. The DIP remains positioned during assembly as a result of the slight pressure exerted outward by the leads against the edges of their access holes. However, the package position should be checked just prior to soldering the board to ensure proper mounting.

FIG. 26 Assembly of dual-in-line (DIP) package.

The final device package to be considered for assembly is the flat-pack configuration shown in FIG. 27a. This device may be mounted using one of two methods of assembly. These are (1) mounted onto the insulated side of the pc board with leads bent at 90-degree angles to the base of the package for insertion into the access holes, or (2) mounted directly to the foil side of the pc board and directly soldered to foil conductor fingers.

FIG. 27 Methods of assembling flatpack-style cases: (a) flatpack configuration; (b) lead-bending arrangement for installation into pc board through insulation; (c) lead-bending arrangement for soldering directly to conductor pattern; (d) flatpack soldered to foil without double 90-degree bend.

The first method of mounting is similar to those already discussed. How ever, the leads in this device must be bent as shown in FIG. 27b. This configuration is necessary when the leads are to be soldered to terminal pads. Since the lead spacing on the device is so small, the terminal pads in the conductor pattern must be staggered, to avoid short circuits. The second assembly method makes use of a lead configuration such as that shown in FIG. 27c. However, if the leads extend straight out from the side of the package and the double 90-degree bend is not provided by the manufacturer, they can be pressed down against and aligned with the appropriate conductor fingers and soldered (FIG. 27d).

For large-volume production, automatic insertion equipment is used to rapidly assemble axial, radial, and dual-in-line components and devices to pc boards. As discussed in Sec. 11.3, automatic insertion is feasible only if the component layout is acceptable in terms of arrangement and spacing. Refer to Figs. 11.9 and 11.10. Sufficient space between components must be allowed iii the design for the insertion machine’s head driver and guide fingers. With computer control, the pc board is positioned so that lead access holes are sequentially positioned under the insertion head. As each component is belt-fed into the head, its leads are formed just before the driver presses the body flush against the board surface. Automatic insertion of an axial lead component is shown in FIG. 28.

FIG. 28 Automatic insertion of axial lead component: (a) cross section of driver head and lead forming guide fingers; (b) appropriate guide finger clearance.

(a) Cross Section of Driver Head and Lead Forming Guide Fingers; (b) Appropriate Guide Finger Clearance

5. DEVICE SOCKETS

An alternative method of assembling devices to pc boards is with the use of commercially available device sockets. These sockets are obtainable in all the standard case configurations. Device sockets have two decided advantages over direct device soldering to the foil. First, the use of sockets allows devices to be removed and replaced easily. Second, lead heat sinking is unnecessary. The only disadvantage of sockets is the additional cost, a factor that must be emphasized throughout the fabrication of a prototype. Where cost is of prime importance, sockets should only be used for devices that may need occasional replacement because of aging or other anticipated design changes.

Some commercially available sockets for transistors and ICs are shown in FIG. 29. These sockets are provided with round pins for direct soldering into pc boards or with long square pins that serve the dual function of pc board mounting and wire-wrap lead assembly.

Another method of assembling devices that has all the advantages of sockets plus the additional benefit of versatility in positioning involves the use of socket terminals such as those shown in FIG. 30a. These individual terminals consist of a wire-wrap stake for interconnections at the base, a four-leaf contact socket arrangement for device lead insertion, and a barb on the side of the socket for positive positioning and securing the terminal when it is inserted into the pc board. To mount this type of socket, the appropriate diameter mounting holes are first drilled into the pc board in accordance with the socket lead orientation. The socket terminals are then inserted into the board and swaged into place with an arbor press. If desired, the pins can later be soldered to the copper foil of the pc board. High-density DIP test panels using the socket terminals are shown in FIG. 30b.

FIG. 29 Transistor and integrated circuit sockets: 3-, 4-, 8-, 10-pin socketsforTO-5 case style; (b) 14-pin DIP socket with wire-wrap terminals; (c) 24-pin socket for large-scale integration (LSI). Courtesy of Augat, Inc.

FIG. 30 Socket terminal nomenclature and DIP panels: (a) wire-wrap socket terminal; (b) high-density test panels using wire-wrap socket terminals. Courtesy of Augat, Inc.

6 DEVICE HEAT SINKING FOR PRINTED CIRCUIT BOARD APPLICATIONS

Heat sinking devices on pc boards is usually less difficult than heat sinking chassis-mounted devices. When devices and heat sinks are assembled on the pc board, insulators are not necessary because the board itself is an insulator. This is not the case with chassis-mounted heat sinks (as was discussed in Section 1).

Several commercially available heat sinks used in low- and medium-power applications and the case styles to which they are adaptable are shown in FIG. 31. These heat sinks are fabricated from aluminum or beryllium-copper alloys. They are secured to the device case by either a press fit resulting from the spring tension of the sink material, or by mechanical clamping techniques. Silicon grease (thermal joint compound) used where the device case contacts the heat sink can reduce the thermal resistance between the two by as much as 100%. These sinks are finished in either a black anodized or natural unaltered metal surface. The black finish is more efficient in radiating heat away from the device case.

FIG. 31 Printed circuit heat sinking for low- and medium-power devices; (a) sink for TO-5 and 10-18 case styles; (b) dual T0-5 sink. Courtesy of International Electronic Re search Corporation.

FIG. 32 Economical method of dual-mount heat sink.

FIG. 33 Methods of heat sinking DIP integrated-circuit packages: (a) sinking for socket assembly, courtesy of Astodyne, Inc., Wilmington, Mass., subsidiary of Rockwell Corp.; (b) tab for direct soldering to large copper-clad area of pc board for heat sinking; (c) assembly technique for dual surface heat sinking. Courtesy of Thermalloy.

A less expensive method for heat sinking plastic devices is to construct a sink from sheet aluminum to the dimensions required to provide adequate heat sinking. (These dimensions are determined by the designer of the circuit.) One such arrangement is shown in FIG. 32 for the power amplifier pc board. The devices are mounted to the heat sink with machine screws and nuts. The optimum position for the devices is as close to the bottom edge of the sink as possible but allowing some heat-sink surface below the base of the device for more efficient cooling. Since the sink is vertically mounted, it allows for excellent heat dissipation with normal air convection. After the devices have been se cured, the sink is mounted to the pc board through its mounting tabs. Since these plastic-type devices have their collectors connected to a metal surface on the case for heat-dissipating purposes, an insulating mica washer must be used to electrically separate them when mounted on a common heat sink. When securing these plastic cases to a heat sink, a torque screwdriver should be used. Care must be taken not to exceed 5 inch-pounds of torque to maximize thermal transfer between the metal surface of the case and the sink.

Techniques for heat sinking DIP-type ICs are shown in FIG. 33. The sink shown in FIG. 33a requires no hardware for mounting, since it is held by a friction fit between the base of the device and the top surface of the socket.

Some DIP types are equipped with heat-sink tabs protruding from the case (FIG. 33b). The tabs are inserted through the insulated side of the pc board and soldered to a large foil area provided on the conductor side. This technique of using a large foil area that would normally be etched away is quite economical since it eliminates the cost and assembly of a separate sink.

For those DIP-type ICs that are to be mounted directly to the pc board without the use of a socket, the heat sink with its method of assembly as shown in FIG. 33c may be used. This arrangement allows heat to be removed from both the top and bottom surfaces of the device. The mounting used to secure the heat sink to the device is also used to secure the assembly to the pc board.

The selection of the proper heat sink is dictated by design factors discussed in Section 1. However, it is the responsibility of the technician to select the most appropriate type and style to meet the packaging requirements. All the assembled pc boards for the amplifier’s circuits, using all the assembly techniques discussed in this section, are shown in FIG. 34.

The assembled boards are now ready to be soldered. The following section discusses types of solder and fluxes, and methods and techniques of soldering pc boards.

EXERCISES

A. Questions

1 What is the purpose of a polarizing key on connectors?

2 What is the difference between a full clinch and a service bend?

3 Why is it important to use pliers with smooth jaws when bending component leads?

4 What care should be taken when assembling disc capacitors to pc boards?

5 Why is a torque wrench used when assembling stud-mounted components to a pc board?

6 What is the advantage to staircasing device leads?

7 What are the advantages of using device sockets?

8 Why are heat sinks generally made with a black finish?

9 Why is heat sinking components on pc boards less difficult than those that are chassis mounted?

10 Why are lead heat sinks used?

True or False

1 Hand- or pressure staking is employed when installing turret terminals. T F

FIG. 34 Completely assembled amplifier boards.

2 Both anvil and setting tools must be hollow when swaging double-ended terminals. T F

3 Axial lead components dissipating up to 2 watts may be flush mounted on the pc board. T F

4 Service bends are used if it is expected that the component may be removed at a later time. T F

5 Socket terminals can be used in place of device sockets. T F

6 A heat sink may be applied to the device case while soldering the leads. T F

7 Some connectors are plated with a gold alloy to reduce wear. T F

C. Multiple Choice

Circle the correct answer for each statement.

1 Solder terminals are secured to the pc board by (welding, staking).

2 For maximum rigidity, components should be installed with (full-clinch, service) bends.

3 A major advantage of using device sockets is (service, system reliability).

4 A component lead bend should begin no closer than ( ) inch from the body end.

5 When a component is assembled into its mounting holes, the leads are bent (toward, away from) the conducting path at a (60-, 30-) degree angle.

6 When installing disc capacitors, their leads should be provided with (full clinch, service) bends.

D. Matching Columns

Match each item in column A to the most appropriate item in column B.

COLUMN A | COLUMN B

E. Problems

1. Cut two 1.5-inch lengths of 1/8-inch-diameter brass or copper rod. File a taper on one end and a flat side over the entire length of each piece. Re move the teeth from only the ends of the jaws of a small alligator clip and construct a lead heat sink similar to that shown in FIG. 35.

FIG. 35

2. Install turret-type terminals and socket pins into the printed circuit de vice test jigs designed in Problem 11.2 and fabricated in Problem 13.2. Soldering is performed in Problem 15.2.

3. Assemble the components onto the pc board designed for the seven- fixed-interval UJT timer of Problem 11.3 and fabricated in Problem 13.4. The assembled pc board is soldered in Problem 15.3. The indicating lamp (L on-off switch (S and battery clip are mounted to a chassis in Problem 24.5.

4. Mount the components on the pc board designed for the series feedback regulator in Problem 11.4 and constructed in Problem 13.5. The assembled board is soldered in Problem 15.4.

5. Install the components on the pc board designed for the tachometer in Problem 11.5 and fabricated and labeled in Problem 13.8. The assembled board is soldered in Problem 15.5. The tachometer pc board and meter are chassis- or panel-mounted in Problem 24.6.

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