There was a time when the very idea of building a complex circuit with the intention of destroying it would have been anathema to any electrical engineer. The work put into designing a circuit, procuring the components, and assembling it, generally with point-to-point wiring and an extravagant amount of manual labor, only to blow it up? Heresy!
But, such are the demands of national defense, and as weapons morphed into “weapon systems” after World War II, the need arose for electronics that were not only cheap enough to blow up but also tough enough to survive the often rough ride before the final bang. The short film below, simply titled “Potted and Printed Circuits“, details the state of the art in miniaturization and modularization of electronics, circa 1952. It was produced by the Telecommunications Research Establishment (TRE), the main electronics R&D entity in the UK during the war which was responsible for inventions such as radar, radio navigation, and jamming technology.
The first bit of the film below focuses on circuit potting. The circuits shown are built “cordwood style”, meaning that axial-lead resistors, capacitors, and inductors are mounted between two flat plates and wired together with short jumpers. It was a tedious and time-consuming construction method, but had the virtue of mechanical strength and low material cost. The potting process that followed was just as tedious, with mica-impregnated polyester resin being added to the circuit after mounting it in a mold. The resulting brick was un-molded and active components, which at the time meant vacuum tubes, would be mounted externally and wired up separately.
Where things get really interesting is in the printed circuit production process, which at the time took the “printed” part very much literally. Rather than etching copper from a pre-clad board to create traces, a die of the traces was built up from steel tooling, referred to as “type” in a nod to the printing industry, and used to press silver powder into traces onto a phenolic substrate. On the other side of the board, resistors were created by etching an even layer of graphite powder using shot blasting. And if all that doesn’t pique your interest, wait until you see the glass boards — not fiberglass, but actual glass.
For a construction method intended to make circuits cheap enough to blow up, everything shown here is fantastically labor-intensive. Then again, it was just after the war, and labor was probably pretty cheap, and when have governments ever been shy about throwing money at arms makers? Plus, it was more likely that robustness and reliability were the true imperatives driving these methods.
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- Source: https://hackaday.com/2023/06/15/retrotechtacular-circuit-potting-and-pcbs-the-hard-way/
