Tracking the story of the PCB - part 2

Just who invented the printed circuit? And how? Considering the crucial importance of printed circuitry to modern electronics, the story is surprisingly obscure. Peter Brownlee concludes a convoluted and controversial history.

Paul Eisler (1907-1995), an Austrian scientist working in England, has been credited as being the 'father of the modern printed circuit' and with making the first recognisable printed circuit board.

However, this is being more generally questioned as earlier designs become better known and patent decisions in the US have not supported some of his claims.

Eisler graduated in engineering from Vienna University in 1930. After employment in Belgrade where he installed radios in trains, he returned to Vienna to work as a printer.

Forced out of work by the political changes in 1934, Eisler left for England with some of his patents in 1936. Living in a Hampstead boarding house, he began to fabricate a radio using a printed circuit board while trying to sell some of his ideas.

An Eisler-designed radio with a recognisable crude printed circuit is on display in the Science Museum in London.

Today, more than six decades after Eisler came up with his proposals for a 'printed circuit' to replace the point-to-point wiring used to interconnect the components making up an electronic product or system, the technology developed at Technograph - the company set up to exploit his ideas - remains the foundation of the modern PCB fabrication industry.

Although other methods of producing printed circuit boards were investigated, it was Eisler's etched foil concept that prevailed.

His basic idea involved printing the required circuit pattern onto copper foil bonded to an insulating substrate. The printing was to be undertaken using a special ink which, on subsequent immersion of the printed substrate in an etching chemical, would act as an etch-resist.

The unwanted copper would be etched away leaving behind the etch-resist-protected circuit pattern. The commercial non-availability of copper-foil-clad insulating materials was the first of many difficulties that had to be overcome.

By 1942, Eisler had produced the world's first radio with components interconnected by means of a printed circuit and in February 1943, the first application for a British printed circuit patent was granted.

This attracted little interest until the technology was used in a proximity fuse which was vital in countering the German flying bomb.

Getting some form of printed circuit accepted was no easy task. Not only had development work to be undertaken but the electronics industry had to be persuaded to drop point-to-point wiring in favour of an interconnection medium that, although inherently more consistent and more reliable, was, at least initially, considerably more expensive.

During, and immediately after, World War II efforts had been made to interest British government departments in the etched foil process. Indeed, some interest was generated, but no business of any great significance resulted.

World War II brought circuit developments that took a different turn. The need for extremely robust microelectronics for military ordnance spurred development of ceramics. Secret projects developed highly reliable ceramic substrate and conductive inks, called cermets - ceramic-metal.

This process, now widely practised in the ceramic hybrid industry, involved screen printing or stenciling circuit inks, followed by high-temperature firing. The process was used to produce tens of thousands of electronic ordnance fuses.

The war efforts resulted in both the development and optimisation of high-volume thick film printed circuit manufacturing.

After the war, the US government under the auspices of the National Bureau of Standards disseminated printed circuit technology. Conferences were held and publications described virtually all the circuit making concepts, including subtractive etching. A Circuit Symposium sponsored by the US Aeronautical Board and the National Bureau of Standards was held in Washington, DC, in October 1947, attended by dozens of speakers and hundreds of visitors.

More than two dozen processes were reduced to six distinct methods:

• Chemical deposition - electroless (non-electric) and electrolytic plating are included. Dozens of early patents described electroless, electrolytic and combination plating. Chemical deposition remains an important process in many circuit-making schemes;
• Die stamping - many of the early patents claimed cutting and die stamping as the process for patterning conductors. Modern methods simultaneously bonded the weakly adhered metal foil to the substrate during the die-cutting process. This was accomplished by using B-staged adhesive and a heated die bed. The method, although low cost and environmentally friendly, has become all but obsolete as tolerances become tighter and density demands increase;
• Dusting conductive powder over adhesive ink - application of graphite or metal powder over wet ink or adhesive is one of the earliest processes reported. Some of the later patents apply solder to the dusted conductors. The process does not appear to be in use today;
• Painting (really printing) - metallic inks are applied and cured or fired, includes ceramic thick film (CTF) and polymer thick film (PTF) technologies that remain important today;
• Spraying - molten metal or composite conductor material is sprayed through a mask or stencil. The mask can be a resist applied to the substrate. (No longer used);
• Vacuum deposition - sputtering and evaporation through a mask were the key processes mentioned. Thin film circuits are made by vacuum depositing copper, gold and other metals. The method is still used today.

Subtractive photolithography, borrowed from the ancient printing industry, still remains the most widely used circuit-making process. Fully additive copper methods have not really succeeded although ceramic and polymer thick film remain popular.

The etching process has also weathered storms stirred up by environmentalists as better waste recovery methods evolved. However, subtractive etching is under attack by semi-additive. This process requires a thin conductor layer, or 'seed' metal, to serve as a temporary plating bus.

A typical process involves applying a plating resist over the thin metal, followed by imaging, developing, electroplating, stripping resist and etching away the thin background layer. Although etching is still involved, the process does not define the conductor shape and very little metal is removed. The semi-additive process is gaining momentum since it can produce very fine lines (<10 µm), build straight-wall conductors (no etch factor), and generate minimal waste.

Some of the earliest ideas for printed circuitry such as Hanson's have echoes in modern flexible circuitry where electronic devices are mounted on flexible substrates such as plastics or advance polymers and typically used as connectors in applications where flexibility, space savings, or production constraints limit the use of rigid circuit boards or hand wiring.

A very common application of flexible circuitry is in computer keyboard manufacturing and today most keyboards use membrane-based flexible circuits for the switch matrix.

While it is possible that we will continue to use century-old processes to make circuits, it is conceivable that advances in nanotechnology and organo-electronics could provide a profound change in the way circuits are built.

Carbon nanotubes have been used to make transistors and other solid-state devices, and future circuitry could be based on molecular-level processes that might appear to us as strange as the circuit design submitted to the London Patent Office in 1903.

"This attracted little interest until the technology was used in a proximity fuse which was vital in countering the German flying bomb."

"Fully additive copper methods have not really succeeded although ceramic and polymer thick film remain popular."

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