Point-to-point construction

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Section of a typical Australian late 1930s radio, showing the point to point construction between components.

Point-to-point construction refers to a non-automated method of construction of electronics circuits widely used before the use of printed circuit boards (PCBs) and automated assembly gradually became widespread following their introduction in the 1950s. Circuits using thermionic valves (vacuum tubes) were relatively large, relatively simple (the number of large, hot, expensive devices which needed replacing was minimised), and used large sockets, all of which made the PCB less obviously advantageous than with later complex semiconductor circuits. Point-to-point construction is still used to construct prototype equipment with few or heavy electronic components.

Before point-to-point connection, electrical assemblies used screws or wire nuts to hold wires to an insulating wooden or ceramic board. The resulting devices were prone to fail from corroded contacts, or mechanical loosening of the connections. Early premium marine radios, especially from Marconi, sometimes used welded copper in the bus-bar circuits, but this was expensive.

Point-to-point wiring is not suitable for automation and is carried out manually, making it both more expensive and more susceptible to wiring errors than PCBs, as connections are determined by the person doing assembly rather than by an etched circuit board. For production, rather than prototyping, errors can be minimised by carefully designed operating procedures.

Point-to-point construction uses terminal strips (sometimes called "tag boards") or turret boards. The crucial invention was to apply soldering to electrical assembly. In soldering, an alloy of tin and lead, or later bismuth and tin, is melted and adheres to other, nonmolten metals, such as copper or tinned steel. Solder makes a strong electrical and mechanical connection. Note that if components are arranged on boards with tags or turrets at both ends and wires going to the next components, then the construction is tag or turret construction respectively, as the components are not going from point to point.

Terminal strip construction[edit]

Point-to-point construction of military radio equipment
Underside of the chassis of a 1948 Motorola VT-71 7" television, showing the complexity of the point to point wiring.
Point to point wiring on a homemade tube amplifier circuit.

Point-to-point construction uses terminal strips (also called "tag boards"). A terminal strip has stamped tin-plated copper terminals, each with a hole through which wire ends could be pushed, fitted on an insulating strip, usually made of a cheap, heat-resistant material such as synthetic-resin bonded paper (FR-2), or bakelite reinforced with cotton. The insulator has an integral mounting bracket, sometimes electrically connected to one or more of the stamped loops to ground them to the chassis.

The chassis was constructed first, from sheet metal or wood. Insulated terminal strips were then riveted, nailed or screwed to the underside or interior of the chassis. Transformers, large capacitors, tube sockets and other large components were mounted to the top of the chassis. Their wires were led through holes to the underside or interior. The ends of lengths of wire or wire-ended components such as capacitors and resistors were pushed through the terminals, and usually looped and twisted. When all wires to be connected had been fitted to the terminal, they were soldered together (and to the terminal).

Professional electronics assemblers used to operate from books of photographs and follow an exact assembly sequence to ensure that they did not miss any components. This process is labor-intensive, subject to error and not suitable for automated production. Even after the introduction of printed circuit boards, it did not require laying out and manufacturing circuit boards.

Point-to-point construction continued to be used for some vacuum tube equipment even after the introduction of printed circuit boards. The heat of the tubes can degrade the circuit boards and cause them to become brittle and break. Circuit board degradation is often seen on inexpensive tube radios produced in the 1960s, especially around the hot output and rectifier tubes. American manufacturer Zenith continued to use point-to-point wiring in its tube-based television sets until the early 1970s.

Some audiophile equipment, such as amplifiers, continues to be point-to-point wired using terminal pins[citation needed], often in very small quantities. Point-to-point wiring is used as a design feature, not due to the economics of very-small-scale production.

Sometimes point-to-point wiring—without terminal strips—with very short connections, is used at very high radio frequencies (in the gigahertz range) to minimise stray capacitance and inductance; the capacitance between a circuit-board trace and some other conductor, and the inductance of a short track, become significant or dominant at high frequencies. In some cases careful PCB layout on a substrate with good high-frequency properties (e.g., ceramic) is sufficient. An example of this design is illustrated in an application note describing an avalanche transistor-based generator of pulses with risetime of a fraction of a nanosecond; the (few) critical components are connected directly to each other and to the output connector with the shortest possible leads.[1]

Particularly in complex equipment, point-to-point wired circuits are often laid out as a "ladder" of side-by-side components, which need connecting to ladders or components by wire links. A good layout minimizes such links and wiring complexity. Amongst complex devices, the pre-PCB Tektronix vacuum-tube oscilloscopes stand out for their very well-designed point-to-point wiring.[2]

If parasitic effects are significant, point-to-point wiring has the disadvantage compared to a PCB of indeterminate parasitic components; while the inductance and capacitance due to a PCB are the same for all samples, values may vary between point-to-point wired units, changing circuit operation.

Placing the completed unit in an enclosure protects the circuit from its environment, and users from electrical hazards.

A few large brand names still use point-to-point boards, but usually for special product lines. Electric guitar amplifier manufacturer Marshall have reissued some of their older models, using point-to-point construction as a design feature, although their standard products have long used PCBs. Thermionic valve equipment usually does not have the valves mounted on the PCB, to avoid heat damage, but uses PCBs for the wiring, achieving the economy of mass-produced PCBs without the heat damage.

Breadboard[edit]

Main article: Breadboard

Prototypes which are subject to modification are often not made on PCBs, using instead breadboard construction. Historically this could be literally a breadboard, a wooden board with components attached to it and joined up with wire. More recently the term is applied to a board of thin insulating material with holes at standard 0.1-inch pitch; components are pushed through the holes to anchor them, and point-to-point wired on the other side of the board. Some prototyping "breadboards" have this layout, but with metal socket strips into which components are pushed; all the terminals in a straight line in one direction are electrically connected. Such breadboards, and stripboards, fall somewhere between PCBs and point-to-point; they do not require design and manufacture of a PCB, and are as easily modified as a point-to-point setup.

Stripboard[edit]

Main article: Stripboard

A stripboard is a board with holes in a 0.1-inch pitch; all the holes in a straight line are connected by a copper strip as on a PCB. Components are pushed through from the side without strips and soldered in place. The strips can be interrupted by rotating a tool like a drill bit at a perforation.

"Dead bug" construction[edit]

Free-form construction can be used in cases where a PCB would be too big or too much work for a small number of components. Several methods of construction are used. At one extreme a wiring pen can be used with a perforated board, producing neat and professional results. At the other extreme is "dead bug" style, with the ICs flipped upside-down with their pins sticking up into the air like a dead insect. While it is messy-looking, free-form construction can be used to make more compact circuits than other methods. This is often used in BEAM robotics and in RF circuits where component leads must be kept short. This form of construction is used by amateurs for one-off circuits, and also professionally for circuit development, particularly at high frequencies.[3]

For high-frequency work a grounded solderable metallic base such as the copper side of an unetched printed circuit board can be used as base and ground plane. Information on high-frequency breadboarding and illustrations of dead bug with ground plane construction are in a Linear Technologies application note.[3]

See also[edit]

References[edit]

  1. ^ Linear Technology AN47 - High Speed Amplifier Techniques, p.AN47-94, figure D3, head of avalanche pulse generator. "Lead lengths ... should be experimented with to get best output pulse purity."
  2. ^ Illustration of interior of Tektronix 310A oscilloscope, with complex point-to-point wiring using ceramic, rather than bonded-paper, terminal strips.
  3. ^ a b Linear Technologies AN47 describes and illustrates dead-bug breadboards with ground plane, and other prototyping techniques. Illustrated in Figures F1 to F24, from p.AN47-98. Information on breadboarding on pages AN47-26 to AN47-29.[dead link]

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