Jump to page content

Materials for keyboard components

Contents

Contact materials

This section covers the conducting surfaces of switch contacts. See under spring materials for the body material of movable switch contacts.

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.”

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.

Usage

Silver–carbon

A mixture of silver and carbon appears to be a common material for the conductive tracks on flexible printed circuits, and thereby also used as the switch contacts where exposed pads are pressed together. In US patent 4467150, DEC referred to the conductive material as the confusingly-named “conductive silver (carbon)”. For their desktop membrane and notebook keyboards, NMB give the switch contact material as “Mylar with silver-carbon overlay”.

Mercury

Normally associated with relays, Mechanical Enterprises used mercury in their Mercutron line of keyboard switches. These had the advantage of zero bounce: as the two halves of the mercury came into contact, the liquid would join together instead of bounce apart. This simplifies the matrix scanning process at a cost of placing toxic material into every switch.

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.

Spring materials

Phosphor bronze

Phosphor bronze is an alloy of copper with 0.5–11% of tin and 0.01–0.35% phosphorus. One of its applications is springs. In keyboard switches, it can be found used for click leaves, movable contacts and return springs. Himake and Xiang Min abbreviate it to “PBS”, and use it for the click leaf (hence its distinctive copper colour) as well as the movable contact. “Spring temper phosphor bronze with gold alloy inlay” was used in Hi-Tek Series 725 for the switch contacts.

Return springs tend to be stainless steel, but the occasional copper-coloured return spring can be found.

Keycap materials

ABS

ABS is Acrylonitrile butadiene styrene. The chief advantage of ABS for high-end keyboard production is its ability to be double-shot moulded, shared with Tenite. Modern ABS keycaps demonstrate poor wear resistance, developing shine within a few months.

Vintage ABS

There is a tendency for older computers and terminals from the 1970s and early 1980s to retain the surface texture on their keycaps. As this is not characteristic of modern ABS, it suggests that old double-shot keycaps were made of a different material from that used in the late 80s onwards. For example, Acorn BBC Microcomputer keycaps hold up well with age. Inspection of the structure of BBC Micro keycaps shows that three of the four types came from the same OEM. The SMK-made keyboards have SMK-sourced keycaps (with a solid first shot), while the other three types (AWC/Futaba, AWC/SMK and unknown/Philips) use keycaps from the same source:

View full-size image

Signature Plastics have confirmed that Wong’s were a major customer of Comptec, but also that they did not make these keycaps; they are significantly different from real SA family keycaps. The actual manufacturer remains a mystery. The same keycap family was mated with Style D Cherry M7 switches in some SAGEM TX-20 Telex machines.

The only suggestion that Signature Plastics offered to explained the greater wear resistance of older keycaps is that the grade of ABS could have been different (such as being changed subsequently due to industry regulation).

Tenite™

Tenite is a trademarked cellulosic plastic produced by the Eastman Chemical Company from “100% renewable softwood materials”. Tenite was chosen by Cherry-Mikroschalter for the keycaps to M7 and M9 switches, while M8 switches used ABS, according to the 1982 German catalogue. It is more likely that it is simply the 12 mm keycaps that are Tenite, and the 6 mm keycaps that are ABS, since the keycap mounts were shared between switches. US-made M4/M5/M6 keycaps used ABS instead, per the 1973 and 1979 US catalogues.

As with ABS, Tenite can be double-shot moulded, which Cherry offered.