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Switch materials

Contact materials

Overview

Mechanical keyboard switch contacts typically use gold alloy for the surface material. Gold alloy is advantageous for preventing contact oxidisation, providing good switch reliability. It is also implied to have good resistance against contact bounce. Another option is silver; Mechanical Enterprises noted of DN series:

“DN switches work in your product’s environment because the contacts are sealed in a silicone rubber tube. And since they are sealed, the contacts can be silver with its lower cost and cleaner electrical characteristics. That’s why DN reliably tests to over 15 million cycles.”

This comment suggests that gold is not the optimal choice of material for switching, but that its use is selected for its freedom from oxidisation in unsealed switches. Since keyboard switches are also found in non-keyboard applications, manufacturers found it advantageous to offer materials and configurations beyond that of simple terminal and teleprinter keyboards. For example, although logic circuits are switched at low voltage, there are situations where higher voltages need to be switched, and the choice of contact material depends in part on the intended operating load. With M8 and M9, Cherry offered a choice of contact shape and material, with both gold and silver alloys on offer. The silver-palladium alloy contacts of Cherry M82 and M92 permitted an increase in maximum switching voltage to 60 V, up from the 28 V of the gold alloy contacts of M81 and M93 (all at a maximum current of 100 mA).

The following material selection chart was given in Cherry Switches & Keyboards Catalog C-73 (page 33). It indicates that gold alloy crosspoint contacts are suitable for switching loads up to 30 V and up to 100 mA. For higher voltages, silver is indicated, and for higher currents, one is advised to consult Cherry’s factory. Cherry also notes:

“This chart is intended only as a general guide. Application of information with respect to electrical and mechanical life requirements and type of actuation may require deviation from suggested contact materials.”

Western Electric created their own gold alloy for switching purposes (Western Electric Alloy #1, below). In time, this was replaced by silver; per the reference below:

“In 1935, Western Electric Alloy #1 (69% gold, 25% silver and 6% platinum) found universal use in all switching contacts for AT&T telecommunications equipment, an application that would later be overtaken by pure silver.”

Computer magazine noted the following in its September 1982 issue, on page 107:

“Mechanical Enterprises has implemented some trade-offs in order to produce a low-cost, full-travel keyboard. This was accomplished largely by tolerating a slightly longer bounce, which the company feels is acceptable for most applications. More expensive keyboards often use gold contacts to reduce bounce. Mechanical Enterprises’ Sabrecoil uses a specially coated silver plate contact that is “almost as good as gold” but allows a reduction in cost.”

Other materials

RFT TSS reed switches offered a choice of gold and rhodium switch contacts, with rhodium presumably being provided—like with silver—for higher current and voltage capacity (and lower cost), but a lower rated lifetime.

Western Electric Alloy #1

Cryptically referred to by Cherry as “W/E Alloy #1” in their catalogues, Western Electric Alloy #1 is a gold alloy introduced in 1935. A longer name of “Western Electric #1” was found in the Hi-Tek Corporation Dovetail Series Arrays and Keypads datasheet, finally allowing for Cherry’s mystery alloy to be identified.

Western Electric Alloy #1 comprises 69% gold, 25% silver and 6% platinum (AuAg25Pt6). It was intended to be used in solid (rather than plated) contacts, previously described in three patents all filed in November 1930, each one confusingly entitled “Process of manufacturing electrical contact members”:

The patents describe how the contact surface starts out as wire, before being formed into a shape suitable for applying onto the switch contact using one of several means. This processing appears to be how the “another CHERRY design first” gold crosspoints were formed, with a solid contact block formed over a solid core of base metal. (It is interesting to note that this “design first” uses an alloy that one assumes was supplied by the company notable for inventing the manufacturing process decades earlier.)

It appears that the alloy’s intended purpose was switch contacts, being used in AT&T telecommunications equipment, Western Electric being their primary supplier.

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