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Cherry switch contacts

Contents

Overview

Cherry keyboard switches have used a variety of switch contact forms throughout the company’s history. The style of switch contacts in some cases serves as a clear indication of the factory of origin or the subseries of the switch. Many of the details presented here were related by Günter Murmann, formerly the Vice President of Engineering at Cherry and the designer of the MX switch.

US gold “crosspoint” contact prism

‘Gold “crosspoint” contact’ switches were introduced seemingly in the late 1960s by Cherry Electrical Products as a way of adapting their existing portfolio of switches for low-energy circuits. Specifically, this adaptation allowed switches to operate within circuits running at low voltages and very low currents. Switch contacts comprising a pair of opposing gold alloy metal edges arranged at right angles ensured that the contact surfaces would not oxidise and would make contact with sufficient force as to conduct as reliably as possible.

The gold alloy chosen was Western Electric Alloy #1, composed of 69% gold, 25% silver and 6% platinum (AuAg26Pt6). This alloy was adopted in 1935 by Western Electric for telephone switches. Western Electric filed three patents in 1930 for the construction of gold alloy crosspoint prism contacts, a design that Cherry would later adopt along with Western Electric’s alloy. If the manufacturing process is the same as that in Western Electric’s patents, then the contact material is a solid wire formed into the surface portion of the switch contact. Cherry catalogues in the 70s and 80s illustrated the contact cross section as follows:

Hirose used the same diagram in their datasheets, despite using a completely different style of prism.

This contact style has flat shoulders on either side of the prism, which is specific to the US product range. Although US-made M6 and German-made M7 switches can already be readily differentiated by the shell style, the contact shape acts as another element of proof as to which factory produced the switch.

US gold alloy prism contact points can be found in M4, M5 and M6 switches.

German prism

Cherry-Mikroschalter in Germany found that importing Western Electric Alloy #1 into Germany was prohibitively expensive, and they sought to find a suitable replacement supplier for their switch contacts. They were unable to source a European manufacturer of the shoulder-profile contact prism type used in the US. Instead, they found a supplier who could produce contacts of purely triangular cross-section. These triangular prism contacts are used in M7 switches as well as some versions of M8 and M9.

Cherry Mikroschalter offered multiple contact options for their switches. M7 switch contacts used AuAg26Ni3 (71% gold, 26% silver and 3% nickel). M8 and M9 offered AuAg26Ni3 gold-alloy contacts (M81 and M91) as well as AgPd30 (70% silver, 30% palladium) for higher voltage capacity (M82 and M92). M9 was also available with Western Electric Alloy #1 contacts (M93), as was M8 in Japan (M83), although Hirose had their own contact arrangement again.

The prisms are largely silver or silver alloy, capped with gold in the low power versions. The upper block of gold provides a corrosion-free contact surface suitable for reliable switching of very low loads (as found in keyboard electronics), while the silver body will withstand a higher load. The base of the prism is nickel for better welding onto the body of the switch contact. Omission of the gold alloy allows the switch to handle higher loads. Examination of an M9 switch showed the top third by height as gold, while M7 and M8 switches only have a very small amount of gold alloy. The diagram below gives a rough guide of the structure, as much as can be determined with a 40× loupe:

The following photos show the gold alloy and silver alloy German prism contacts in M8 switches:

View full-size image M81G-0100 left, M82E-1500 right (with cover removed)
View full-size image M82E-1500 left, M81G-0100 right
View full-size image M81G-0100 left, M82E-1500 right

The photos above depict sample parts preserved by Cherry USA, who graciously provided them to UncleFan for his collection.

Hirose

Hirose Cherry Precision in Japan created their own contact system. The contact material is again Western Electric Alloy #1, as per the US switches. However, the contact itself is formed from a trapezoidal prism of solid copper or copper alloy, which passes through a hole in the body of the contact itself. The top portion of this prism is a bright metal, possibly silver alloy. The prism then appears to be plated with gold alloy, which can cover anywhere from the entire body to just the top surface. The cross sections of German and Hirose M8 stationary contacts are given in the following diagram:

The cross section was determined by filing away the end of each contact from a Hirose M8 switch, and then inspecting the result with a 40× loupe. A microscope would be required to observe the result more accurately, as well as a better filing technique to maintain a perpendicular face as the material is removed. It appears that the movable and stationary contact prisms are also fitted differently to their respective contact bodies.

The following photos depict Hirose MX and M8 contacts:

View full-size image Hirose M8 and MX stationary contacts (front and back views of each)
View full-size image Close-up of the Hirose M8 stationary contact design, showing the gold-plated copper core of the contact points

Hirose used these contacts for their M8 switches, as well as MD and MJ. When Hirose introduced their version of the MX switch, these too used Hirose contacts. As there is presently so little understanding about Hirose M5 and M6, there is currently nothing known about what contact prisms these switches used.

Plated wire

Although the European-made prisms were only half the cost, Cherry-Mikroschalter still sought to bring costs down further, and this resulted in the introduction of gold-plated nickel wire contacts. The plated wire approach was offered to Cherry as alternative by a supplier, and they considered and accepted it. This new contact design was introduced shortly after MX was introduced in Germany, perhaps in mid-1984, so the earliest MX switches would have had the German prism contacts. American MX switches also use this contact type. Further, due to the considerable cost savings that these contacts brought (costing only 1 pfennig per switch) they were adopted also for both M8 and M9 series, as M84 and M94 respectively.

The part number chart for M94 indicates that the wire contacts use a 3 µm plating of AuAg10 (90% gold, 10% silver); the wire itself is made of nickel. Such contacts are only suitable for low current and voltage. High load would burn away the gold plating and leave the nickel, which is not a suitable contact material, and there is no silver body to pass high current. Adoption of plated wire contacts required down-rating MX’s electrical specification, but for their intended market this was irrelevant. M8 and M9 switches continued to offer the existing contacts for higher rated loads, but the wire contact models were used wherever possible due to their lower cost. (Silver, copper, gold and nickel have electrical conductivities of 62.1×10⁶, 58.7×10⁶, 44.2×10⁶ and 14.3×10⁶ Siemens/m respectively.)

The diagram below shows the various contact types in MX: the gold-plated wire from US and German switches, the later solid prism type in German switches (as used in current switches) and Hirose copper or copper-alloy core.

The wire diameter is much thinner, necessitating the stationary contact posts in M8 and M9 switches to have raised areas stamped into them to make up for the difference in contact depth.

Later German prism

More recent MX switches have reverted to a solid gold alloy prism, as depicted in the diagram above. Cherry indicate that this change took place around 2006, but this seems a little unlikely as that would make some seemingly-vintage switches in fact modern production runs.