Tracking the story of the PCB - part 1
Just who invented the printed circuit? And how? Considering the crucial importance of printed circuitry to modern electronics, the story is surprisingly obscure.
By the first years of the twentieth century, electronic communication technologies such as the telegraph, telephone and radio were transforming advanced societies, creating new industries and, in the process, fuelling demands for efficient, reliable circuitry that could also be mass produced as economically as possible.
The electric telegraph had been developed and patented in the US in 1837 by Samuel Morse. His assistant, Alfred Vail, had developed the code signalling alphabet with Morse and America's first telegram had been by Morse on 6 January 1838 across a bit more than 3 km of wire.
The first transatlantic telegraph cable was successfully completed on 27 July 1866, allowing transatlantic telegraph communications for the first time and encouraging scientific interest in problems involving electrical transmission and equipment manufacture which was further accelerated by the completion of telegraph lines from Britain to India in 1870.
Alexander Graham Bell's patented telephone apparatus was completed on 2 June 1875, and on the same day he succeeded in transmitting audible signals by magneto-electric currents with the aid of a battery.
The first long-distance telephone call was made on 10 August 1876 by Bell from his family home in Brantford, Ontario, to his assistant in Paris, Ontario, about 16 km away.
Nikola Tesla and a host of other scientists and inventors demonstrated the usefulness of wireless telegraphy (radiotelegraphy or radio) beginning in the 1890s. Aleksandr Stepanovich Popov demonstrated to the public his receiver of wireless signals (also used as a lightning detector by brave or foolhardy experimenters) on 7 May 1895 and it is generally considered that Guglielmo Marconi sent and received his first radio signal in Italy up to 6 km in 1896.
Around 1900, Marconi reportedly broadcast signals across the English Channel and shortly afterwards he radiotelegraphed the letter "S" across the Atlantic from England to Newfoundland.
In 1898, Popov conducted successful wireless communication experiments between a naval base and a battleship and in 1900, the crew of the Russian battleship General-Admiral Apraksin (as well as stranded Finnish fishermen) were rescued in the Gulf of Finland after an exchange of distress telegrams between two radio stations constructed under Popov's direction.
Each of these technologies was created and developed in an environment that encouraged secrecy, controversy and frequent sharp practice, often leading to disputes which remain of considerable interest to lawyers specialising in patent law.
In many cases, the names of one or two leading 'inventors' have achieved a certain immortality - arguably in many cases because of business capacity and patent law expertise than outstanding technological innovation or experimental priority.
The creation and development of printed circuitry was to be no different - although the story and its controversies are even more obscure and the leading lights are virtually unknown.
Printed circuitry appears to have been developed from the electrical connection systems that were in use as far back as the 1850s. Metal strips or rods were used to connect large (sometimes very large) electric components mounted on wooden bases.
Over time, the metal strips were replaced by wires connected to screw terminals. Wooden bases were replaced by metal chassis but smaller and more compact designs were needed due to the increasing operating needs of the products that used circuit boards.
In 1903, Albert Parker Hanson, a German citizen living in London, submitted a British patent application for flexible printed wiring circuits intended for use in telephone exchange interconnections.
Based on flat parallel copper conducting strips bonded to paraffin waxed paper, the design used a double layer construction. The copper strips were to be arranged in alternate layers forming a rectangular grid. Interconnections were crimped through holes in the paper. Hanson's patent application also described double-sided and multilayer boards.
Hanson may have originally developed this design in 1898 and had certainly submitted a German patent application in 1902 that was not published until 1905. The British Patent Office placed the Hanson patent application in the public domain in 1903 and granted it in 1904.
Hanson was far ahead of his time and had already realised that high component density was a crucial design feature. His designs suggested that conductors could be formed by electro-deposition or by applying metal powder in a suitable medium.
In 1904, Thomas Edison offered some novel ideas on replacing wire assemblies. His concepts included selectively applying glue and dusting the wet circuitry base with conductive graphite or bronze powder and 'printing' a dielectric with silver nitrate solution and reducing the salt to metal and applying gold foil to patterned adhesive.
Edison was preoccupied with other technologies and did not get around to working directly on these circuits although he did invent the electrolytic copper foil process still used today.
Other ideas emerged over the next few decades as electronics continued to rapidly expand. Radio soon became the most important driver as it captured the attention of the world. The first public radio station, KQW in San Jose California, had gone on air in 1912, and by the end of the second decade of the 20th century, radio had been introduced to most advanced countries.
Ships, including the Titanic, now carried the Marconi radio system and 'the wireless' was saving lives.
The earliest circuit assembly processes were based on additive or build-up methods, where conductors were deposited onto a dielectric base.
While this is the most straightforward approach, the commercial printing industry had long used subtractive etching techniques. For centuries wood had been carved to form raised letters and graphic images. Later, metal was cut and finally etching became the standard process for printing plates.
In 1913, Arthur Berry filed a patent claiming a method of making circuits where unwanted metal was etched away. He described the process of coating metal with a resist, before etching, as an improvement over die cutting. Berry appears to be the first to describe the circuit etching process.
Later, the American Ellis Bassist gave specific details of photo-engraving including the use of photosensitive chromium salts. Although his patent dealt with print plate-making, the process could easily be adapted for circuitry since Bassist described preparing flexible plates by electrodepositing copper foil onto dielectric laminate.
One successful inventor, Max Schoop, commercialised a metal flame spraying process that was used for many years. Early electronics was very power-hungry (compared with modern solid-state components) with valves requiring heated filaments and high voltages so that practical circuits needed to be able to handle substantial currents.
The Schoop process deposited thick patterns of metal by flame spraying through a mask and produced the robust circuits and coils that were required. However, the system never resolved problems of cost and wasted metal although subsequent inventors added improvements.
In 1925, Charles Ducas submitted a patent application for a method of creating an electrical path directly on an insulated surface by printing through a stencil with electrically conductive inks.
This method led to the coining of the terms 'printed wiring' and 'printed circuit'. Ducas described both etching and plated conductors with one version involving electroplating a copper, silver or gold pattern onto a low temperature metal alloy through a contact mask. Heating allowed the conductor (typically a coil) to be separated from the fusible bus plate and mask.
Another Ducas process involved forming grooves in dielectric such as wax and filling them with conductive paste. The paste was then electroplated. Conductive paste could also be printed, or stencilled, onto dielectric and then electroplated.
Both sides of the dielectric layer could be used for circuitry. Ducas went on to describe multilayer circuits and a means of interconnection layers.
In 1926, Cesar Parolini came up with improvements in additive plating. He described printing patterns with adhesive onto dielectric followed by dusting copper powder onto the wet ink. The excess copper particles were shaken off and the ink hardened with heat. This is the basic Edison concept but Parolini produced it in practical form.
Many inventors followed these developments but most offered variations and small improvements on existing designs. For example, in 1933, the Erwin Franz made a printing paste with carbon filler that could be screen-printed or stencilled onto cellophane.
The hardened ink, although stable, had high resistance compared with metal. Franz, perhaps aware of the Parolini process disclosed seven years earlier, added a copper electroplating step. Franz also described a flexible folding circuit as a replacement for windings in transformers.
However, it was to take the involvement of another key figure - like Hanson, a European living in England - and the Second World War to drive the development of the printed circuitry we know today.
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