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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 a 1968 advertisement for Cherry’s gold crosspoint contact switches in January 1969; this advertisement (listed on the Cherry S31 Series page) covered series E31, E53, E63 and S31 switches, of which only S31 was used in keyboards. The same chart was later used in the Cherry Switches & Keyboards Catalog C-73. The chart 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 noted:

“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 datasheet for Hi-Tek Dovetail Series, finally allowing 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”.

In a write-up in of Oak Full-Travel Membrane in Electronic Design magazine in September 1980, the following comment was made about their conductive material:

A proprietary process for making conductor inks blends 20-Ω/sq carbon with 20-mΩ/sq silver to provide good adhesion at low cost.

The article goes on to note:

The membrane-switch contacts provide an overall contact resistance of 85 Ω typical and 100 Ω maximum. These switch contacts can handle from 250 µW to 0.5 W with resistive loads, or voltages from 1 to 30 V and currents from 50 µA to 20 mA.

Mercury

Normally associated with relays, Mechanical Enterprises used mercury in their Mercutronic 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.

Terminal materials

L49

Some Cherry M9 models use unplated L49 for the stationary contact terminal (on which the contact prism is placed). L49 is a Wieland designation for CuNi9Sn2 (a copper alloy with 9% nickel and 2% tin), which is also known as CW351H or C72500. This is a dull, pinkish metal. In Wieland’s L49 datasheet states that L49 has:

… good corrosion resistance in industrial atmosphere and resists very well to tarnishing even at prolonged storage.

This stands in contrast with some other terminal alloys and platings which darken with time, especially silver. The typical applications given are relay springs and connectors. L49 is not used for the movable contact in M9, which is a yellowish material whose name or composition is not given in the Cherry M9 chart.

Spring materials

Stainless steel

Stainless steel is specified as the spring material for Cherry M8 and MX, likely in reference to the return spring. It is given as the return spring material of both Himake and Xiang Min keyboard switches, as SUS in both cases (“steel use stainless”, the Japanese term).

Datanetics DC-60 uses stainless steel for both of the switch contacts.

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. ABS also has a severe tendency to yellow with age, especially with exposure to sunlight.

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:

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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.

PBT

PBT—polybutylene terephthalate—is a hard-wearing and heat-resistant plastic highly suitable for computer keycaps. American PBT manufacturer Celanese Engineering Resins advertised Celanex 2000-2 unreinforced thermoplastic polymer in the 15th of June 1984 issue of Computer Design magazine. The advertisement states:

Printed Celanex® keytops eliminate costly, time consuming two-shot molding processing, providing wear resistant, multi-colored lettering capability. These features, combined with outstanding processing characteristics, are why Hewlett-Packard selected Celanex® 2000-2 for the terminal keytops of their HP 150 Touchscreen Personal Computers. Quality demands quality. Celanex® 2000 series thermoplastics offer a unique combination of printability, chemical and wear resistance, dimensional stability, strength, stiffness and surface gloss to withstand the tests of time. In fact, the molding experts at Hewlett-Packard report Celanex® 2000-2’s internal lubricant system facilitates mold release without mold plate-out–maximizing productivity.

The keyboard in question is, notwithstanding the missing moulded-in HP logo, visually identical to keyboard models 46010A and 46011A.

Double-shot moulding offers limitless colour combinations, but producing new legends is expensive (as new injection moulds are required) and detail resolution is low. Hi-Tek in particular switched over to dye sublimation printing with the introduction of Series 725 to gain a cheaper, more flexible and more expressive legend production method (as reported by D’Milo Hallerberg of Hi-Tek), but the significant heat required means that the plastic must have a much higher melting point than ABS. PBT offers not just the heat tolerance, but greater wear resistance, especially in comparison to the modern soft ABS formulations.

PBT is however notoriously difficult to mould, with larger parts being prone to warping as they cool. For this reason, the space bar is typically ABS instead of PBT to improve yields. This can be seen on older keyboards where the ABS case and space bar have yellowed, but the remaining keycaps have retained their original colour from the factory. Even Topre Realforce keyboards follow this practice; a group buy at Deskthority to produce replacement PBT space bars suffered likewise with a lot of warped mouldings, due to the sheer difficulty of the task. IBM are one of very few manufacturers to produce keyboards with no yellowing at all: the plastics used for the case and all the keys (including space bar) of most Model F and Model M keyboards are free from yellowing.

PBT double-shot

The comment in the Celanex advertisement about the “time consuming two-shot molding processing” indirectly highlights a major difference between PBT and ABS keycaps. ABS was traditionally paired with the double-shot injection process, where the key legends were moulded out of solid plastic, and the remainder of the keycap was then moulded around the legend. ABS suits this process well, but it is not typically used with PBT. PBT double-shot keycaps have been introduced in recent years, but the inner shot is not necessarily PBT. One contemporary keycap vendor reports that their PBT double-shot keycaps use ABS for the legend shot, and PBT for the remainder of the keycaps, for both backlit and non-backlit types alike; in their view, ABS+PBT is the most common arrangement. Vortexgear reported that they now use PBT for both shots; previously, they used POM for the first shot and PBT for the second shot, as can be observed in product listings such as Vortex Black Doubleshot backlit PBT + POM Keycaps (Amazon) and 104 Key Black Translucent PBT+POM Double Shot Keycap Set (Mechanical Keyboards Inc).

Mounting plate materials

Keyboards with discrete switches often have the switches held in a mounting plate. Seb Zeppelin has written a detailed Introduction to Materials guide to help enthusiasts select the correct material for a mounting plate.

Steel

The vast majority of switch mounting plates are made of steel. To prevent rusting, they are typically painted. The metal is typically thick enough that even when the plate does develop rust patches, it can be cleaned up and repainted.

Aluminium

Aluminium is a popular choice for enthusiasts. It was also used occasionally by commercial manufacturers. Wong’s Electronics of Hong Kong used aluminium mounting plates on BBC Micro keyboards, which reduced the weight of the computer in comparison to batches produced with steel-plate keyboards from SMK and PED. Aluminium is less rigid than steel, which some people feel makes keystrokes a little softer. No examples of a double-blind test of steel versus aluminium plates are known, that would verify whether aluminium does offer any kind of perceivable improvement, but it does have the advantage that it will not rust.

Plastic

Some low-cost keyboard manufactuers opted for plastic plates in the 1990s. These included Acer (some 631x keyboards had a fully discrete plastic plate), Tai-Hao (as found in some models of TH-5539 keyboard) and Nan Tan Computer, as found in their cheaper KB-625x models. Plastic plates cut down some of the noise produced by mechanical switches, which some people may find preferable, and they will not rust.