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.
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
Günter Murmann, formerly the Vice President of Engineering at Cherry, explained that Cherry-Mikroschalter in Germany was 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 gold alloy prisms are largely silver alloy; only the top edge is gold alloy. 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:
M81G-0100 left, M82E-1500 right (with cover removed)
M82E-1500 left, M81G-0100 right
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:
Hirose M8 and MX stationary contacts (front and back views of each)
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 wire contacts. This new contact design was introduced just in time for MX, and both German and American MX switches use this contact type. Gold-plated wire contacts were also made available for M8 and M9 series. The part number schema for M9 from 1985 indicates that M94 switches use instead of prism contacts. When MX switches entered production in the US, the same contact system was introduced there. M94 uses 3 µm AuAg10 (90% gold, 10% silver); 3 µm will be the depth of the plating.
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.