As more and more solid state logic found its way into crypto machines, it became obvious that the single-sided phenolic printed circuit board would not serve NSA's long term needs.  Naturally, the solution was to start designing circuit boards with a pattern of foil on each side.

During the same time frame, industry designers experimented with another modular concept called Cordwood Modules. As the term implies, discrete components were stacked like sticks of wood and their leads were soldered to a pair of single-sided boards. The design became so popular that it was used on many of NSA's voice coding devices. Known as "Flyball Modules" to the NSA designers, the components were soldered in place, wire leads attached, then the entire assembly was potted in an epoxy-like substance.

As the Flyball Module technique gained popularity, the thirst for a double-sided printed circuit board continued. As material suppliers provided double-sided phenolic material, PCB designers created circuitry on both sides of the board. Wherever  required a piece of very short wire was used to interconnect the foils at that point. Known as 'Z' wires, they were not always easy to install. When one side was being soldered,  the solder on the other side became a liquid and partially melted. Many boards had to be retouched in order to make good connections.  Even if everything was hand soldered it still required a good deal of dexterity.

Eventually the Z wire problem was solved by using an eyelet to create the interconnection between foil layers. The United Shoe Company built a machine that could install an eyelet into a hole, swage it on both sides and melt the solder coated on the eyelet to form a reliable connection. The U.S. military thought very highly of the double -sided circuit board with eyelets as the interconnection method. Many pieces of military hardware were successfully designed using these boards.

"FLYBALL" modules were modules comprised of discrete components set up as logic element circuit groups such as AND gates, OR gates, FLIP FLOPs, Inverters etc.  Once tested, the modules were potted in a compound whose colour indicated functionality. The coloured potting compound was extremely hard and any attempt to penetrate it resulted in damage to the internal circuitry.

In a KG-13, the following colours are confirmed: Pink, Yellow, Green, Blue, Red, Orange and Black. Purple and Brown modules were probably used but those colours are unconfirmed as of this time.

One board in the KG-13 had a black module which was a noise generator. Repair depot technicians were told it was the only module which was classified because its noise was used to generate a random key stream used in the encryption process.

Flyball modules were used extensively in the HY-2, KY-3,  KY-8, KG-13s and  KW-7s. Technicians of that era did not refer to these as "flyball  modules" but simply as "modules". Does anyone know the origins of the word "flyball" and why it was chosen?

By 1967,  the first RTL, DTL and then TTL digital ICs were appearing in equipment designs but the Flyball Module technique was still used in high performance Operational Amplifiers for several more years until silicon IC technology eventually rendered them obsolete.

This is the Flyball Module exhibit at the National Cryptologic Museum. (Photo by  Doug Eyre)

An example of pink flyball modules and one yellow one in the KG-13.(Photo courtesy Bill Rhoads)

These are not crypto modules but serve to illustrate the flyball technique nontheless. (Photos by George Mace) 

Credits and References :

1) Doug Eyre, Hewitt, TX
2) A History of Secure Voice Coding
3) Bill Rhoads, BGR101(at)
4) George Mace <gmace8(at)>

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Apr 10/09