Point-to-point construction refers to the method in which electronics circuits were constructed before the 1950s. 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 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.
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Point-to-point construction uses terminal strips (also called "tag boards"). A terminal strip is a stamped strip of tin-plated loops of copper. It is mounted in a way that electrically insulates each loop from the others. The metal loops are mounted on a cheap, heat-resistant material, usually synthetic-resin bonded paper (FR-2), or bakelite reinforced with cotton, or sometimes paxolin. The insulator has an integral mounting bracket, sometimes shorted 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 wires of electronic components were physically looped through the terminals and soldered to them. Small electronic components were mounted by twisting their wires around terminals and soldering.
Professional electronics assemblers used to operate from books of photographs and follow an exact assembly sequence to assure that they did not miss any components. Although this process is error-prone and nearly impossible to automate, it is quite good for building small numbers of units when labor costs are low.
Point-to-point construction continued to be used for high quality tube electronics even after the invention 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. The American manufacturer Zenith continued to use point-to-point wiring in its tube based television sets until the early 1970s.
Today, audiophile equipment, such as amplifiers, may be point to point wired using terminal pins. Whilst many of these units are produced in very small quantities, it is perfectly possible for the public to produce commercial quality printed circuit boards using free design software and online prototyping printers. The point-to-point style is retained instead as mark of quality, as the connections between components are as thick as their lead out wires will allow and usually short, involving no tracks and few jumpers between points. This is desired as a common goal of audiophile equipment is to have the absolute minimum resistance possible where it is not controllable or needed, which is provided by the thick "solid" wiring method; a common complaint against PCB layouts being that the tracks have a very small cross sectional area. Short, straight leads also produce the lowest values of inductance. With the leads being suspended in air, they have a minimal amount of capacitance, where as PCBs can suffer from capacitive effects between the layers. As silver is a slightly better conductor than copper, very expensive capacitors and resistors will be made from pure silver and feature silver lead outs. The slight increase in conductivity is not commercially worthwhile, or necessary, for the majority of electronics and so PCB manufacturers do not offer it as a standard option for the conductive material. But the point to point method allows the producer to have an essentially pure silver path between components.
As with the gain in conductivity when using copper, the electrical gains with regards to resistance, inductance, capacitance and so on are often of little importance to most electronics. However, they are extremely important at high frequencies, as frequency is a parameter in defining how currents and voltages interact with these properties. The effects can be so pronounced as to cause high frequency equipment to catastrophically fail with regards to performance if improperly laid out, regardless of component choice and the circuit's logic. As a result, it is still possible to see a direct, point-to-point, straight, heavy line layout used in high performance radio frequency equipment, particularly as the frequencies approach the gigahertz band. A blend of the two methods if used in many of these cases, where a ceramic board is used featuring tracks, but with the point-to-point layout logic. The difference between the values is small, and the audio spectrum is orders of magnitude below the radio frequencies, making the measurable difference between a PCB and point to point terminal method vanishingly minute in many cases. When the current is constant DC, the differences become essentially zero (bar the effect of DC resistance) as this has frequency of zero, meaning the inductances and capacitances are being multiplied by zero, which will give a zero result.
Despite its often superior electrical properties (and disregarding its bulky, often messy appearance), this method of wiring is by far and a way one of the most simple to put together and solder, as it has a basic nature to it and features large, easily handled and heated components and posts. More difficult than the actual assembly of a point-to-point board is usually the design, as boards are usually laid out as a "ladder" of side by side elements, which may need connecting to others that aren't adjacent to themselves; requiring a jumper between the two. The art in a layout is the minimization of these jumpers (the wiring complexity).
This method of wiring (be it between terminals, components or leads) has one well known disadvantage in that the parasitic resistance of PCBs, poorer conductors and longer leads can actually add stability to a circuit. By reducing it close to zero, capacitive and inductive elements within the circuit may go into resonance, with the designer having to purposefully add a resistor at a later date.
Placing the completed unit in an enclosure protects it from mechanical damage when the chassis is mounted in a piece of furniture or an equipment rack, prevents foreign objects which may alter the electrical characteristics of the circuits and interfere with proper operation (such as by causing short circuits or altering electromagnetic fields), and protects humans and animals from contact with potentially dangerous voltages which may cause an electric shock while the machine is operating and from sharp edges and points on the assembly which could cause physical injury such as cuts.
Some large brand names still use point-to-point boards, but usually for special product lines. Marshall, who famously produce guitar amplifiers, have recently released reissues of their older models featuring point to point boards. At audio frequencies, it is highly debatable whether or not the any of the electrical gains of using this method are in any way tangible, but the method carries a weight of originality and solid, hands-on construction that implies more care and higher quality components have been used. Marshall's standard product line has been for many decades, and still is, PCB based. When it was realized mounting thermionic valves directly to the PCB caused heat issues, manufacturers of audio equipment like this began mounting the ceramic connectors for the valves on the metal chassis, running lead out wires to the PCB below. This allows them to mass produce the complex boards and still create a product that will last with a minimal amount of manual soldering involved.
For hobbyist work, 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. This is sometimes called "dead bug style" as the ICs are flipped upside-down with their pins sticking up into the air, evoking the image of the upturned body of an expired insect. While it is often messy-looking, error-prone, and difficult to repair, this 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. One advantage is the ease with which wire wrap connections may be made to components when resistor and capacitor leads are bent to a U shape and also glued in place.
