Figure 2 shows another prototype in which a single-sided copper-clad Vectorboard has
predrilled holes on 0.1-in. centers. Power buses are at the top and bottom of the board.
The power pins of each IC have their own decoupling capacitors. Because of the loss of
copper area due to the predrilled holes, this technique does not provide as low a ground
impedance as a completely covered, copper-clad board.
A variation of this technique is to mount the ICs and other components on the non-copper-clad side of
the board. The holes serve as vias, and you do the point-to-point wiring on the copper-clad side of the
board. To prevent shorts, you must drill out the copper surrounding each hole used for a via. This
approach requires that all IC pins be on 0.1-in. centers. You can use low-profile sockets for
low-frequency circuits, and the socket pins allow for easy point-to-point wiring.
One commercial breadboarding system has most of the advantages of robust ground, screening, ease of
circuit alteration, low capacitance, low inductance, and several additional advantages. The system is rigid,
has components that are close to the ground plane, and lets you easily calculate node capacitances and
line impedances. The product is available as "Mini-Mount" in Europe from Wainwright Instruments
GmbH (Andechs-Frieding, Germany) and as "Solder-Mount" in the United States (where the trademark "Mini-Mount" is the property of another company) from Wainwright Instruments Inc (San Diego, CA).
Solder-Mount comprises small pieces of pc board with etched patterns on one side and
contact adhesive on the other (Figure 3a). The pc-board pieces stick to the ground plane,
and you can solder components to the pieces. The board pieces are available in a variety
of patterns, including ready-made pads for eight-pin SOICs to 64-pin dual-in-line types;
strips with solder pads at intervals ranging from 0.040 to 0.25 in., including strips with 0.1-in. pad
spacing, which you can use to mount dual-in-line devices; strips with 50, 60, 75, or 100_ conductors to
form microstrip transmission lines when you mount them on the ground plane; and a variety of pads for
mounting various other components.
The conductor-strip feature of Solder-Mount at VHF is convenient. You can use these strips for
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transmission lines, impedance matching, or power buses. Glass-fiber/epoxy pc board is somewhat lossy
at VHF and UHF, but the losses are probably tolerable if microstrip runs are short. Self-adhesive, tinned
copper strips and rectangles (LO-PADS) are also available as tie-points for connections. These strips
have a relatively high capacitance to ground and, therefore, serve as low-inductance decoupling
capacitors. The strips come in sheet form, and you can cut them with a knife or scissors.
The main advantage of Solder-Mount construction over bird's nest or dead-bug construction is that the
circuit resulting from using Solder-Mount is stiffer and, if an application requires, smaller. The latest
Solder-Mounts for surface-mount devices let you construct breadboards scarcely larger than the final pc
board. It is generally more convenient if the prototype is somewhat larger than the final breadboard,
however. Solder-Mount is durable enough for low-quantity production. A 2.5-GHz PLL prototype built
with Solder-Mount (Figure 3b) is a high-speed circuit, but the technique is equally suitable for the
construction of high-resolution, low-frequency analog circuitry.
Both the dead-bug and Solder-Mount techniques become tedious for complex analog or mixed-signal
circuits. Formal layout techniques often produce better prototypes of large circuits. One approach to
prototyping more complex analog circuits is to lay out a double-sided board using CAD. PC-based
layout packages offer easy layout and schematic capture to verify connections. Such packages are
available from PADS Software (Marlborough, MA) and Accel Technologies (San Diego, CA). Although
most layout software offers some autorouting capability, this feature is best left to digital designs. After
you place the components in position, manually route the interconnections following layout guidelines
(Reference 3). After you complete the layout, the software verifies the connections against the schematic
diagram's netlist.
Many design engineers find that they can use CAD techniques to lay out simple boards or work closely
with a layout person who has experience in analog-circuit boards. The result is a pattern-generation tape
or Gerber file, which you normally would send to a pc-board-manufacturing facility, which makes the
final board. Rather than use a pc-board manufacturer, however, you could use automatic drilling and
milling machines that directly accept the pattern-generation tape. These prototype-board cutters are
available from LPKF CAD/CAM Systems Inc (Beaverton, OR) and T-Tech Inc (Atlanta). These systems
directly produce single- and double-sided circuit boards by drilling all holes and then using a milling
technique to remove copper, create insulation paths, and create the finished board. |
