Evolution of PCBs: From Eisler’s Printing Technique to Modern Miniaturization
For many people today, PCBs are an integral part of life. Many people are never more than a few feet away from a PCB – the TV, the microwave, the computer, the car, the smartphone… and the list goes on. Yet we probably all know someone who was alive before PCBs were invented. Well… maybe not! They have been around for longer than most people think.
Most people trace the origin of the PCB to Paul Eisler. In 1936, this Austrian developed the concept of using a printing technique to lay electronic circuits onto an insulating base. Prior to this, components were hand-soldered. Eisler’s concept was to lay wires down on the board, then add the components over the board. Eisler moved to England to escape the Nazis. There was varied interest in his concept, and it took a further seven years for him to apply for a patent for the circuit board. The patent was eventually granted in 1950.
Eisler wasn’t the first to consider using pre-defined electrical paths. In 1935, Charles Ducas applied for a patent for creating an electrical path on an insulated surface. At this time, boards holding circuitry were made of various non-conducting material (including wood, Bakelite and Masonite). To make the boards function, holes were drilled into the boards, and brass wires were riveted in. Others laid the ground work for Ducas. Albert Hansen, Thomas Edison, Max Shoop and Arthur Berry were all working on the concepts that would morph into PCBs as early as 1903.
In 1943 the United States realised that there was a benefit to the technology, and incorporated Eisler’s boards into proximity fuses. In 1947, the first double-sided (non-printed) board was developed. By 1948, the US established a standard procedure for all airborne circuitry to be printed.
By the 1950s, circuit boards were moving from being purely military into the commercial world. A range of design changes made the boards more practical. In 1956, a patent was filed for “Process of Assembling Electrical Circuits”. The process involved drawing the wire pattern. This was then photographed onto a zinc plate. The plate was used to create a plate for offset printing.
The 1960s saw a number of advances. Multi-layer boards (4+ layer count) began to be produced. Methods were introduced to stop corrosion. This protected the trace and the components, and kept them cleaner (and therefore more efficient).
Through each generation, the PCBs were progressively becoming smaller. In the 1970s, the issue of bridges forming between pads was addressed through the use of solder masks – a thin layer of polymer applied to the copper, or through liquid photo imageable masks or LPIs. In this process, photo-polymer is coated on the circuit, and then dried at controlled temperatures. Unnecessary solvent is then removed. This process became industry standard.
The 1980s saw a continuation in the miniaturisation process. Surface-mount parts became the preferred design option – previously, through-hole components had been preferred. With the advent of SMT, additional developments were required to make the PCBs effective. Stencil techniques improved, as processes such as laser, electroform and chemically etched plastic stencils were developed. Each of these processes allowed for progression of PCBs.
In the 1990s, computer aided manufacturing (CAM) allowed for PCBs to be designed and tested before being printed. The complexity of PCBs increased. High Density Interconnector PCBs were introduced from around 1995. These boards are reduced in weight and size. They have smaller lines and pads.
An indication of the miniaturization process over time can be seen in the specifications. Historically, the standard sizing was measured in ‘mils’, which is a thousandth of an inch. Through-hole components had pins located at 100 mils. Surface-mount components have a pin pitch of 50 mils. Small Outline Package (SOP) boards have pin spacings of 25 mils. In the 2000s, tracing separation continued to decrease, moving to 5-6mil, and later in the decade to 3.5-4.5 mil. At the same time, Flex and Rigid-Flex PCBs became widely used.