circuits and is best suited for building digital circuits. These techniques can FEATURE also be applied to analog or RF designs. The basic prototyping material is ARTICLE , such as that sold at Radio Shack. This material has holes on 0.1² (2.54-mm) centers and is available in Stuart Ball several sizes. The holes match the pin spacing on DIP ICs and many other components. You can get perfboard with plated- through pads on every hole, but I pre- fer the type without pads. It’s cheaper Breadboarding and, as you’ll see, more flexible when using the techniques I’m going to tell you about.

GROUNDING A key area where many digital and microprocessor designs have problems is grounding. Early digital designs often used two-layer printed circuit boards, ometimes the with power and ground traces mixed s best way to test a with the signal traces on the top and With this offering, Stuart new circuit is to pro- bottom layers. pumps new life into the dy- totype it. If you are plan- As clock speeds and edge rates go ning to build only one, the up, simple grounds such as this are less ing technique of may be the only one you ever put to- effective. A modern production digital prototyping. For gether. Unfortunately, prototyping is design will typically use one or more slowly dying out in the electronics ground planes between the signal lay- experimenters, the cost of industry. Modern CAD tools let you ers to get a low-impedance ground. It is making a circuit board is too produce a circuit board layout in a few difficult to duplicate this with most hours. And, rapid prototype circuit prototyping techniques. Ground high for building only one. board shops can get you boards faster connecting the ground leads of the There is another choice. than you can get the parts to populate parts work for low-frequency designs, them. Compared to the time and labor but the overall impedance using this One-of-a-kind involved in hand-wiring a prototype, technique is just too high for many fast can be produced without making a circuit board is inexpensive components such as ACT logic. for most companies in the electronics The heart of my prototyping tech- having to lay down the cash business. nique involves the use of adhesive for printed circuit boards. However, the electronics experi- tape to make a ground plane, menter has a different economic scale. running the copper strips between the Hand-wiring may seem la- Paying $600 or more for tools for IC leads and around the edge of the something you will build only one of is board. This arrangement effectively borious, but you’ll end up a difficult decision to justify. makes a copper grid. The copper tape with an effective low-cost In this article, I will look at a way to makes a low-impedance connection produce one-of-a-kind prototypes even at high frequencies. alternative. without resorting to printed circuit Figure 1 shows two sketches of a boards. This method is suitable for prototype board using this technique. high-speed logic and microprocessor The cutaway view shows 8- and 14-pin www.circuitcellar.com/online CIRCUIT CELLAR ® ONLINE December 2000 1 DIPs mounted on a perfboard with through Newark Electronics). This technique with aluminum or stainless copper tape on the bottom of the tape (3M’s 1181 and 1194 series) comes steel tape. Solder won’t stick to it. board run between the IC leads and on a roll with a peel-off liner. The tape For a low-impedance ground, you around the edge of the board. is available in widths 0.25² and wider. can cover the entire board with tape The vertical ground strips are sol- 3M also makes a tinned copper tape, on one side, then cut out slots where dered to the horizontal strips where the 1345 series. component leads will go. This is more they cross at the edges of the board. Many hobby stores sell copper tape tedious and requires more care to The ground pins on the ICs are con- with the metalworking supplies. The avoid shorting, but it is nearly as good nected directly under the IC by run- 3M tape doesn’t seem to tarnish as as a ground plane. ning a bare from the IC lead to the much as the hobby store tape, but the Although the sketches so far have copper strip. hobby store tape is considerably less shown vertical copper strips connected The full drawing in Figure 1 shows a expensive. Whichever tape you use by horizontal ones along the edges of complete board with several DIP ICs will need to be cut to the proper the board, other layouts are possible. If and a few discrete components width. You can do this with a sharp you are working with a lot of logic on a mounted on the board. The copper knife or scissors. large board, you might want to add tape attached to the bottom of the Don’t try to use the copper tape to more horizontal strips for lower board is shown with dashed lines. hold a lead on the board. The adhesive ground impedance. One of the advan- Bypass are mounted near will eventually let go. If you have a tages of this method is that you can the ICs, and one lead can be connected component with some stress on it, put the grounds wherever you need directly to the copper ground strip, punch a lead through the perfboard and them. just like the IC grounds. Any discrete bend the wire around the top, as shown component leads that are connected to in Figure 1. POWER WIRING AND SUPPLY ground can be bent over and soldered Don’t try to use this grounding BYPASSING to the ground strip. In most digital designs, there are one or two digi- Adhesive side PLACING COPPER Peel-off backing tal supplies that need to STRIPS Copper tape be low-impedance. Usu- When you place the ally, the power leads can copper ground strips, be wired between the ICs you should burnish using 26- or 28-gauge them with a hard, insulated wire. Bypass round object such as 8-pin capacitors are connected 14-pin DIP the back of a spoon to DIP between the supply leads force the adhesive into and ground strips. Typical contact with the board. bypass values Don’t use anything for digital/microproces- with an edge or you sor circuits are 0.001 to will tear the copper, 0.1 µF. I usually use red Copper tape on bottom of board. and don’t apply so Overlaps soldered together. wire for the logic power much heat and yellow or orange wire that the adhesive lets for other supplies. This go from the board. Be makes it easy to trace the careful when working wiring later. with the copper; it Bypass capactior For low-impedance works like tape, but the power distribution, cop- edges of the foil can be per strips can be run on sharp. the top of the board, be- This technique is tween the IC leads and Discrete components effective and produces directly over the copper a low-noise ground that ground strips. This pro- is suitable for most vides a quality power digital/microprocessor connection, but it is more Push lead through the copper tape to the projects you’re likely top of the board and bend the lead over difficult to wire and you to encounter. 3M on the top. Solder the lead to the copper have to be sure you don’t tape on the bottom of the board. makes copper tape Relieve stress on the copper tape so it short the upper and lower doesn't pull away from the board. intended for EMI ground strips simulta- shielding that will neously. work (also available Figure 1—Copper tape grounding provides low-impedance ground connections. In some cases, you

2 December 2000 CIRCUIT CELLAR ® ONLINE www.circuitcellar.com/online Figure 2—Here, a The easiest way to handle surface- surface-mount mount packaging is to convert it to Copper ground strip capacitor is used to get bypassing close 0.1² spacing. If you can get your parts to an IC power in plastic PLCC packages, you can get lead. 1206-size sockets with holes on 0.1² centers that Surface-mount capacitor IC power pin capacitors are will accommodate them. easiest to work with. For dual inline SMT packages such as SOICs, you can get adapters that consist of a SOIC pattern on top and 0.1² spaced pins on bottom. You solder the SOIC to the top and plug the pins want to locate a bypass capacitor close working because you broke a too-tight into a standard IC socket. These adapt- to the IC power lead, with no interven- wire when you bolted the board into ers are expensive, so you won’t want to ing wire. An example of this would be the case. fill an entire board with them. Aries a processor or other component run- It’s a good idea to socket the ICs on makes a typical line of these parts, ning at a high clock rate or generating any prototype. The cost of the sockets series 35000x. fast (read, high switching noise) edges. raises the cost of the prototype some- Finally, you can get perfboard with Figure 2 shows a way to handle what, but sockets permit you to solder holes on 0.050² centers instead of 0.1² these cases. In this example, a surface- the parts without danger of damaging centers. This will let you prototype mount capacitor is soldered directly to the ICs and then plug the ICs in when with SMT devices, as long as the leads the IC lead and copper ground strip. you’re done. are no closer than 0.050². I have found You will typically use a 0805- or 1206- that the best way to wire this size surface-mount capacitor. Be care- TEST POINTS perfboard is to bend the IC legs so they ful not to apply so much soldering heat When debugging a , you fit into the holes, insert the wire-wrap that the end caps detach from the ca- are always looking for a place to wire leads into the same hole as the IC pacitor. ground your oscilloscope or logic ana- leg, and solder them in place. However, lyzer probes. One advantage of the it is difficult to do because you need a LEAD WIRING copper tape grounding scheme is that small soldering iron tip and a steady I like to mount the ICs with a you can punch through the tape with a hand. couple rows of holes between them sharp tool and solder component leads There is no doubt that surface- (see Figure 3). The signal wires are then directly to the copper. mount parts are harder to breadboard run on top of the board using 30-gauge You can add ground test points in with, and many of the smaller ones are wire-wrap wire. The wire is passed the same way. You can use a piece of simply impractical to use without a through the holes next to the IC or lead, bend it into a “U” shape, . component leads, and about 0.25² is push it through the copper, and solder stripped and wrapped around the lead. it. For a more rugged test point, use the OTHER TIPS This is then soldered. The holes in Keystone 5000 series parts. The leads Sometimes you have to wire a con- most are large enough to on these parts can be squeezed to fit nector such as an IDC header, where accommodate two wires. through the holes on the perfboard, there are two rows of pins but all the The wires are run in the channels and they come with wires come off the same side. With a between the ICs. Placing the wires on plastic standoffs in the top of the board in this way re- different colors so you duces the chance of accidentally break- can make the ground ing wires off the board, something that test points black. 8-pin 14-pin is always a problem with point-to- DIP DIP point wires routed on the bottom of SURFACE-MOUNT the board. For areas where a lot of PARTS wires have to be run (such as a micro- The electronics processor data/address bus), I like to world is moving to leave three or four rows of holes be- surface-mount parts Wire-wrap wire passes through the hole next tween the ICs. for most products. to the IC leg, about 1/4" is stripped. Bare wire is There is often a temptation for new Many parts are still wrapped around the IC leg and soldered. builders to pull the wires tight when available in both DIP making these connections. It is better and surface-mount to leave a little slack in the wires to packages, but many prevent wire breakage and avoid warp- newer parts are avail- IC lead ing the board. It’s really frustrating to able only in surface- Figure 3—Insulated wires are run through the board, stripped, and sol- dered to the component leads. check out everything then have it quit mount packaging. www.circuitcellar.com/online CIRCUIT CELLAR ® ONLINE December 2000 3 PLCC or PGA socket, you have a simi- the board with adhesive, not bonded. be careful not to cut your fingers on lar situation—inner and outer rows of Too much heat will soften the adhe- the edges of the tape. pins, with the wires of the inner pins sive, and the tape will let go. Any me- And finally, place the IC sockets and having to pass through the outer pins. chanical stress will eventually lift the connectors (soldering the ground pins Figure 4 shows the best way to tape unless it is clinched to the board to the copper tape), wire the IC power handle such situations. Wire the outer (as described earlier). connections (be sure to wire the bypass rows first and solder the connections. Because the copper tape can be capacitors), and wire the remaining Then wire the inner rows, passing the placed wherever you want it, you can connections. wires between the pins of the outer have two independent ground grids. rows. This way, you don’t damage the For example, you can have separate OTHER TECHNIQUES insulation on the inner wires while digital and analog grounds connected Wire-wrap is a similar prototyping soldering the outer pins. where power comes into the board. technique, using IC sockets with long Also shown in Figure 4 is a tip on pins and wrapping stripped wire-wrap wiring discrete component leads such PUTTING IT ALL TOGETHER wire around the pins. Wire-wrap can be as and transistors. When wir- The procedure that I follow when used with the copper ground strip ing these parts, wrap the wire around building a breadboard using this technique, but the wire-wrap sockets the component lead close to the board. method is to first design the circuit. are relatively expensive. They often If you leave a lot of component lead This seems obvious, but you can get cost more than the ICs themselves! protruding above the perfboard, the into trouble if you start assuming Wire-wrap has an added disadvan- leads are more prone to bend and adja- placement of components you know tage in that the long wire-wrap pins cent leads may short together. you’ll be using before you’ve designed make it impossible to do any kind of Because the perfboard doesn’t have the rest of the circuit. low-profile mounting. Any wire-wrap any pads or traces, you can drill holes Next, lay out parts on the project will have a project board with a without worrying about shorting any- perfboard. Be sure to leave sufficient thickness of at least 1². In addition, thing. Power devices, such as TO-220 room for bundles of wires to run be- many components (such as connectors) regulators, can be bolted directly to tween the ICs and for the discrete are not available in wire-wrap ver- the board. components. Leave extra space for sions. The copper tape is about the same things like microprocessor data and Many experimenters make their thickness as 1-oz. copper on a PC address buses. own circuit boards for projects. There board, and you can use it in much the After laying out the parts, drill are drawbacks to these methods. The same way. If you are using small sur- mounting holes. If you leave this for simplest way to make a circuit board is face-mount power packages (such as later, you may find that you’ve placed to manually draw the circuit pattern DPAK and D2PAK), you can solder wires or copper where you want the on a blank PC board with an etch resist their power tabs directly to the copper mounting holes to be. pen or dry-transfer patterns (both for heatsinking. The only thing to Then, place the copper ground tape, available at Radio Shack). The prob- remember is that the tape is stuck to soldering the intersections. Remember, lems with this approach are circuit density, registration, through-board connections, and circuit quality.

Wire this row first so the wires First of all, you can only get so on the adjacent row won't be damaged dense with those traces, especially by soldering. because the circuit layers have to be shared with the power and ground connections. There is no way to get more than two layers with any manual process Edge of board Secondly, a double-sided board is difficult to keep registered, unless you Wire connectors and other devices with an inner and outer row of pins. drill all the holes first and place the pads/traces second. Also, any connection that has to be Wiring discrete component leads made through the board requires that a wire be run through the hole and soldered on both sides. Component leads with traces connected to the top If the wire is wrapped around the component and bottom pads have to be soldered Wrap the wire around the lead with significant space between the wrap and component lead close to the perfboard, the lead may bend and short to on both sides of the board to make the the perfboard. an adjacent lead. connection. Figure 4—Here are some wiring tips for devices with two rows of pins and for wiring discrete components. And finally, whether using an etch-

4 December 2000 CIRCUIT CELLAR ® ONLINE www.circuitcellar.com/online resist pen or dry-transfer resist, it is ExpressPCB difficult to make the board without Engineering Express some breaks in the resist. These result www.expresspcb.com in open traces that have to be located and fixed. Some experimenters use photoetching, which usually elimi- nates the open-trace problem but still leaves all the others. Clearly the best way to make printed circuit boards is to use a CAD package and send the resulting Gerber files to a board shop. This takes care of registration, through-hole plating, and allows multiple layers to be used. The drawback, of course, is the cost. Most circuit board houses have high initial setup fees and a minimum order re- quirement. However, there are services on the Internet that cater specifically to the hobbyist by reducing the setup fees and providing a bare-bones two- layer process. Although you wouldn’t want to hand wire a 500-MHz Pentium proces- sor, the copper-tape and wire method of prototyping allows you to experi- ment with higher speed devices and parts than traditional prototyping techniques allow. I

Stuart Ball, P.E., is an electrical engi- neer with 20 years of experience. He has written numerous articles and three books about microprocessor- based designs, all published by Butterworth-Heinemann (www.bhusa.com). You may reach Stuart at sball [email protected]

SOURCES EMI copper tape 3M http://www.3m.com/ehpd/ emi_shielding/ emi_shielding_tape.html Adapters and sockets Aries Electronics, Inc. (908) 996-6841 Fax: (908) 996-3891 www.arieselec.com Newark Electronics (800) 463-0275 Circuit Cellar, the Magazine for Computer Applica- (773) 784-5100 tions. Reprinted by permission. For subscription Fax: (773) 907-5339 information, call (860) 875-2199, [email protected] or www.newark.com www.circuitcellar.com/subscribe.htm. www.circuitcellar.com/online CIRCUIT CELLAR ® ONLINE December 2000 5