The history of the printed circuit board

The age of the modern printed circuit board began 80 years ago with the filing of patent US2441960A by Paul Eisler. Today, it is impossible to imagine electronics without it. Eisler's invention is one of the technologies that changed the history of mankind. In this article, engineer Manfred Hummel provides an overview of its complex history.

The 19th century marked the beginning of an age of incredible inventions. To name just a few examples: The invention of telegraphy made it possible to spread information quickly around the world. Electric motors replaced steam engines. Entire regions were supplied with electricity. Entertainment was enriched by radios, films and cinemas. Sewing machines, the internal combustion engine and the first cars also emerged. In 1880, Tomas Alva Edison's light bulb became popular, and towns and villages were illuminated.

The beginning of the 20th century marked the start of an enormous growth spurt in communications technology. Telephone networks and subscriber numbers grew. They required a large number of electrical connections. Radio equipment became increasingly complex and required individual components to be wired by hand. Many errors occurred during assembly and wiring. At that time, the components could only be soldered by hand. Mass production was difficult.

The rapidly growing electronics industry was looking to simplify wiring work. As early as 1903, the first patents were filed that made it possible to automate wiring. However, the improvement was not achieved with just one patent - it took many years of development.

However, the first patent from 1903 is one of the most important inventions and represented a key technology for possible automation, which was followed by further patents and ultimately led to today's printed circuit board. At that time, the Berlin engineer Albert Parker Hanson filed patent number 157882[1]. It describes a new wiring technique that makes it easier to manufacture the switching equipment for telephone systems. Punched or cut-out copper or brass strips are glued to paraffin-impregnated paper. An insulating layer is applied between the individual layers, with openings at the joints. Longitudinal and cross wires could then be connected to each other at these openings by soldering. The number of layers was variable. Additional layers of insulation provided more layers. The metal strips were arranged in alternating layers to form a rectangular grid. With this system, Hansen was able to make many connections in a very small space.

This was the first publicly documented invention in the field of printed circuit boards and ultimately led to today's plated-through circuit boards and multilayers.

Tomas Alva Edison also received a patent in 1904 for the production of galvanic connections by applying metal powder to insulators, which was used in the development of printed circuit boards.

In 1913, Artur Berry applied for a patent in Great Britain, in which the conductive path was drawn on metal with an acid-resistant lacquer and then the exposed metal was etched off. Bitumen was used as the lacquer. This patent was the first to describe the etching of a printed circuit board.

In 1918, the Swiss Max Schoop received an American patent for a metal flame spraying process that sprayed liquid metal through a mask. This system was used to apply very thick conductor tracks, which were necessary for the electronics of the time. The process was very expensive, but was used as standard. The thick conductors were able to transport the high voltages and currents of the time.

All previous patents brought improvements to electronics, but no process was suitable for the production of large quantities.

The demand for radios and gramophones had grown considerably during this period. In the 1920s, all devices were already being manufactured using products similar to printed circuit boards. Bakelite, paper laminates, cardboard, fiberboard and even wooden boards were used as a base into which holes were drilled. The components and tubes were fastened into these holes with screws. The components were connected with wires.

The work process could only be carried out by hand and with precise knowledge of the wiring diagram.

The industry continued to work on improvements in assembly technology. Significant developments were made in America in particular and better methods for large-scale production were developed.

In 1925, the American Charles Ducas was awarded US patent 1563731, which for the first time described a printed circuit board design and the etching and electroplating of conductor track structures [2]. A stencil was used to produce the conductor tracks, which was placed on the insulating board and the conductive paste was pressed through the recesses (screen printing). After removing the template, the conductors were galvanically reinforced. In his patent specification, he also describes how grooves in wax are filled with conductive paste and then galvanically reinforced. The process could be carried out on both sides of a dielectric.

At around the same time, another patent was received by the US Patent Office (US patent 1 582 683). Francis T. Harmon described the creation of radio coils using etching technology. A metal sheet is coated with a chemically resistant layer, which is exposed in defined areas and the metal that is now exposed is etched.

In March 1925, M. Cesar Parolini filed a patent in France with the number FR patent 608 161. According to this process, an insulating plate is printed with a conductor pattern. Shellac-based printing ink was used for printing. The special feature was that at the crossing point of two conductor tracks, one conductor track was interrupted. While still wet, the conductor pattern was dusted with copper powder. It adhered to the damp bonding agent ink. The rest was brushed off. At the crossing points, the interruption was pressed into the metal powder with U-shaped metal wire bridges. The board was then electroplated with copper to reinforce the conductive paths. A subsequent thermal process strengthened the adhesion of the copper.

Also in 1925, there is a patent from an American named Bassist for the production of flexible printing plates. He worked with light-sensitive chromium salts as photoresist. The conductive pattern was galvanically reinforced and etched.

During this period, many patents were filed on the manufacture of 'printed circuit boards', but these only related to deviations or minor changes to earlier patents. The breakthrough to automated production had not yet come.

In 1926, companies in Germany began to improve their capacity and production quality by wiring components with brass strips. The punched-out strips were riveted to phenolic resin and the components were attached to the rivets. This was the first concept of a printed circuit board in Germany.

In 1930, a special feature was created at the Hescho-Keramische-Werke in Hermsdorf. Conductor tracks were printed onto a ceramic substrate using screen printing technology and the ink was then baked on. The application of metallic conductors to ceramics made it possible to design radio components more economically. This was the basis of today's thick-film technology. A special feature arose in this context: all connections were joined to the plate by a common dip soldering process.

In 1933, the American Franz produced a screen printing paste with a carbon filler, which was printed onto cellophane. After curing, the varnish had too high an electrical resistance to metal. This was improved by an additional galvanic reinforcement. He also described a flexible, foldable circuit as a replacement for the windings in transformers.

From the 1930s onwards, radio receivers became the most important entertainment and information medium. In radio sets, the resistors, coils and electronic tubes continued to be connected by individual wiring. The tangle of wires could only be soldered by hand. The high demand for devices was therefore only possible with a large number of workers.

In 1936, a huge improvement was on the horizon. The Austrian engineer Paul Eisler emigrated to London as a Jew. He worked on putting electronic circuits on an insulating basis. The first printed circuit board was developed. A telephone company showed interest, but did not pursue it further as manual wiring was cheaper. His first patent in 1936 described today's printed circuit board. A milestone. But English radio companies, with a large number of solderers, also rejected his patent for cost reasons. The British Ministry of Defense Industry was also not interested. Eisler continued to work on his concept and applied for further patents in 1943. Patent 63911 (three-dimensional printed circuits), patent 639178 (foil technology of printed circuits) and patent 639179 (powder printing).

Eisler proposed using a plate-shaped phenolic resin paper laminated with copper foil as the starting product for printed circuit board production [3]. The copper surface is then printed with a mask corresponding to the conductor tracks and the exposed copper is etched off. Holes are drilled at the ends of the traces and used for component placement. The soldering eyes were also enlarged in this area. All solder connections were soldered together in a dipping process. Automation was now possible. Eisler's manufacturing process still corresponds to screen printing etching technology today. Eisler used this printed circuit board technology to build the first radio, which is on display in the London Museum.

The patents registered by Eisler in England were automatically reported to America, where they were immediately taken up. By 1941, the first production facilities for printed circuit boards had already been built here. They were used in weapons systems, particularly for the manufacture of proximity fuses.

After the war, the development of printed circuit board technology was greatly accelerated in America, many conferences were held and publications written. In 1947, a huge symposium was held in Washington which reported on all existing printed circuit board processes.

In 1948, the Americans released all processes for the production of printed circuit boards for commercial use after the American patent no. 2,756,485 was approved. This described the first method for the manufacturing process and automatic PCB assembly. To create the circuit board, the developer drew the conductor pattern and photographed it on a zinc plate, which was used to make a printing plate for an offset printing machine. This was used to print the layout onto the copper-coated material.

Production of printed circuit boards did not begin in Germany until 1956, with Fritz Stahl (Ruwel-Werke), Dr. Hermann Seul (Isola-Werke) and Werner Peters (Lackwerke Peters) [4] among the pioneers.

Ruwel's first series panels were delivered to Metz in Fürth in 1957. At the same time, the company Dr. Johannes Heidenhain also supplied printed circuit boards to Metz.

From this point onwards, conventional component wiring was replaced by printed circuit boards. All solder joints could be soldered in one plane.

Manual labor was greatly reduced by the invention of wave soldering. On October 3, 1956, three employees of Fry's Metals applied for a patent for a wave soldering system. The assembled circuit board could now be soldered in a single operation [5].

A number of new technologies developed alongside PCB production. The layout of the PCB was still designed and drawn by hand. This was soon followed by computer programs that designed an automatic layout. Phenolic resin paper was improved by epoxy resin and high-frequency material was developed. At the same time, companies were founded that supplied machines, systems and chemical products for PCB production.

Previously, only single-sided copper-clad printed circuit boards had been used in the industry. A higher packing density was achieved with the wire-wrap process, which went into production in 1959. This technology is created by combining a printed circuit board with wire-wrap connection technology. Square pins are pressed into the perforation of the printed circuit board. A wire is wrapped around a pin using a "wrapping machine" and guided to the next selected pin and wrapped in the same way.

However, as early as 1947, methods were available that enabled through-hole plating from the top to the bottom of the circuit board. In America, the company Photocircuits produced the first series boards using this technology in 1957. Series production began in Germany in 1960.

A further advance was made in 1961, when the Hazeltyne company patented the multilayer board process in America. The normal, plated-through printed circuit board has a copper side on top and a copper side on the underside. The two sides are connected by copper-plated holes. In the multilayer, additional copper layers are pressed into the inside of the base material. All layers are also connected to the outer layers by copper-plated holes. The multilayer has a higher packing density than the double-sided plated-through PCB.

1966 saw the development of multi-wire technology, the written circuit board. The basic concept of this printed circuit board is that conductor tracks, which consist of an insulated wire, are written onto a prepared surface with the aid of a numerically controlled wire laying head [6].

Work on process and production improvements began in 1968. A small selection: The Du Pont company developed the laminable solid resist. IMS printed circuit boards went into production. Numerically controlled drilling machines increased capacity. Automatic placement machines replaced manual placement. Hot tinning systems replaced the galvanic tin-lead process. UV-curable solder resists and solder resist foils improved quality.

From the mid-1980s, bare components were manufactured. They could be soldered directly onto the circuit board. These surface mounted devices (SMD) replaced many plug-in components. The PCB could be populated on both sides. The packing density increased and the PCBs became smaller. At the same time, EDA software (Electronic Design Automation) was developed. With its help, it became possible to process the PCB and the assembly as a unit.

The development of new components changed the layout design from 1990 onwards. Ball Grid Array (BGA) housings were introduced. Micro-via technology (through-hole plating of inner layers) enabled the introduction of high-density interconnect (HDI) PCBs [7]. These PCBs can only be drilled with laser drilling machines.

Heat sink PCB technology was developed by Fela in 2002. A major technical change to the printed circuit board occurred in 2003 when the lead-containing surface was banned. The replacement was either galvanic tin, chemical tin or hot air tin. This environmental measure (RoHs) caused major soldering problems worldwide.

Today's development of PCB technology is determined by the nature of the new components. Rapidly advancing miniaturization in the field of electronics requires components to be incorporated into the printed circuit board. With the help of embedding technology, the miniaturization of printed circuit boards is being driven forward. Optical fibers are also already being incorporated into multilayers [8].

80 years have passed since Mr. Eisler's first printed circuit board patent. PCB technology has changed enormously in this short time and has become one of the most important technologies. None of our modern electronic devices could be manufactured today without the printed circuit board.

Literature

[1] Albert Parker Hanson, patent specification no. 157882
[2] Charles Ducas, US patent 1563731
[3] Paul Eisler: Gedruckte Schaltungen, Carl Hanser Verlag, Munich, 1961
[4] Günther Herrmann; Karl Egerer: Handbuch der Leiterplattentechnik, Volume 2, Eugen G. Leuze Verlag, Bad Saulgau, 1991
[5] Armin Rahn: It is not with inventions, but with improvements that fortunes are made, PLUS 8/2021
[6] Manfred Hummel: Introduction to printed circuit board and assembly technology, Eugen G. Leuze Verlag, Bad Saulgau, 2017
[7] Printed circuit board, Wikipedia entry, https://en.wikipedia.org/wiki/Printed_circuit_board (retrieved: 10.8.2023)
[8] Klaus Ritz: Handbook of printed circuit board technology, Volume 5, Eugen G. Leuze Verlag, Bad Saulgau, 2019