IBM buckling spring

Contents

Background

The IBM catastrophically buckling compression column switch and actuator (commonly known as the IBM buckling spring) is a clicky key-switch employed by IBM from 1981 onwards (and later by Lexmark and Unicomp) as the direct replacement for IBM beam springs as IBM's main keyboard switching technology for the next decade. Buckling spring switches were originally employed on the Model F keyboards for the IBM 5322 and 5324 System/23 Datamaster computers from July 1981^[1][1]
Wikipedia - IBM System/23 Datamaster [accessed 2022-01-04]., then popularised by the IBM 5150 Personal Computer's keyboard from a month later. The key-switch is most famously associated with the Model M family of keyboards which Unicomp still produces to this day. IBM developed two main variants of this key-switch technology - one with a capacitance-sensing PCB and another with a membrane assembly. All Model Fs use the former, whereas Model Ms that use buckling springs use the latter. Whilst the core principle of how the switch provides feedback remains the same for both, there are notable technical and perceptible differences between the two. Other companies that have worked with IBM have also produced their own takes on the design. Buckling spring is commonly abbreviated as "B/S".

Specifications

These are general specifications applicable to the entire IBM buckling spring family. See Implementations for specifics.

Design

The idea of buckling springs by IBM was originally patented as the catastrophically buckling compression column switch and actuator in 1971 and was invented by Richard Hunter Harris, who was also partially responsible for designing the IBM beam spring key-switch that IBM employed in the 1970s on its Model B keyboards. The original buckling spring patent as-is was never employed on an actual production keyboard but it establishes the basic premise of a buckling spring used in a keyboard switch design and the goal of combining tactility, actuation, pre-travel and return force in one spring. The patent shows three takes on such a switch design:

The first buckling spring key-switch to market would be 1977's IBM buckling spring torsional snap actuator - also by Richard Hunter Harris - which is based on developing the third idea above with capacitive assemblies^[3][3]
IBM - Buckling spring torsional snap actuator [accessed 2022-01-04].. All buckling spring designs are centred around a single helical compression column spring that is designed to buckle in a "rapid and catastrophic manner", causing a plate attached to the lower end of the spring to move. This produces a very unique key-force curve.

The plate on the end of the spring is typically referred to as a rocker in IBM's patents or as a pivot plate by Unicomp in their marketing^[6][6]
Unicomp - Pivot Plate & Spring Assembly [accessed 2022-01-05]. although the keyboard enthusiasts have also embraced nicknames such as flippers (typically capacitive-type buckling spring context) or hammers (typically membrane type). The spring and plate as a collective is typically called a "spring and pivot plate assembly" or simply "plate-spring couple". On the upper end of the spring is a keycap, which for buckling springs is an integral part of the key-switch design unlike most Alps and Cherry key-switches that have sliders that can actuate the switch without a keycap. When fitted, the keycap secures the buckling spring into a specific rest position that ensures the plate is pivoted up from the sensor below.

The click is provided by the spring buckling and striking the barrel itself. Buckling springs are non-discrete switches - all switches on a given board share a communal sensor such as a capacitance-sensing PCB called a "pad card" in IBM's terminology or membrane assembly, and barrel plate used for holding and guiding various key-switch components into their correct places.

Implementations

IBM capacitive-type

The aforementioned IBM buckling spring torsional snap actuator based key-switch was the first implementation of buckling springs to make it to market. The design patent was applied for in August 1977 and describes a key-switch system that uses the torque of a buckling spring to rock a small plate made of capacitive material that can be registered by a capacitance-sensing PCB known as a pad card underneath^[3][3]
IBM - Buckling spring torsional snap actuator [accessed 2022-01-04].. This is a departure from earlier also-capacitive beam spring key-switch where the switch actuates when measured capacitance decreases to a certain threshold, as this buckling spring key-switch actuates when the opposite event occurs (capacitance increased). This version of the key-switch is commonly referred to as capacitive buckling springs or "CB/S". The use of capacitive assembly allows keyboards with key-switch design to support N-key rollover and are rated for a lifetime of 100 million key-presses.

IBM membrane-type

Specifications correct for IBM membrane buckling springs employed by Unicomp as of 2000^[10][10]
Unicomp - The Customizer Keyboard [accessed 2022-01-02]..

The IBM rocking switch actuator for a low force membrane contact switch was the second implementation of buckling springs whose patent was applied for in 1983 and invented by Edwin T. Coleman. The key-switch system operates on the same mechanical principle as its capacitive forebearer, however, the capacitive assembly is replaced with a membrane one. The membrane contact switch assembly is a type of printed circuit made of three electrically insulated layers said to be made of mylar. The upper and lower layers sport many circular contact points that can be bridged together when force is applied, connected via a network of screened traces to support the key matrix circuit. The middle layer is designed to keep the upper and lower layers separated at rest and has many cut-outs to allow the aforementioned layers to bridge when force is applied. The membrane assembly was said to approximately half the manufacturing cost compared to its capacitive predecessor^[12][12]
IBM - Rocking switch actuator for a low force membrane contact switch [accessed 2022-01-04]. at the expense of reducing the expected lifetime to 25 million key presses and lowering the circuit's maximum capability to 2-key rollover.

IBM membrane-type ("cushionless" variant)

For [generally early] Model M-based IBM electronic typewriter keyboard assemblies such as those used on IBM Wheewriters and IBM Quietwriters, the rockers are slightly different in that they are missing two plastic extensions when all known IBM PC and terminal Model M keyboards have them. IBM capacitive-type buckling springs also lack these on their flippers. These extensions or cushions (if you will) squeeze the stem of the above keycaps inwards, granting a softer bottom-out feel. The typewriter keyboard assemblies without the cushion are said to have a firmer bottoming-out feel as result, and the desirability of this trait has led to the development of the "cushionless mod" where one can remove the cushions to replicate the feel of a factory-cushionless buckling spring keyboard^[13][13]
Bitteneite @ Deskthority - Model M Cushionless Mod [accessed 2022-11-05]..

Keycaps

The keycaps found on Model F and Model M buckling spring keyboards are known for being made of [high-quality] polybutylene terephthalate (PBT). Compared to the most common keycap material, acrylonitrile butadiene styrene (ABS)^[14][14]
WASD Keyboards - Mechanical Keyboard Guide [accessed 2022-12-16]., PBT is more durable, does not degrade/yellow with age, UV or heat exposure and will keep its texture for longer without shining^[15][15]
Switch And Click - ABS vs PBT Keycaps: What’s the Difference? [accessed 2022-12-16].. However, the shape of buckling spring keycaps and how the legends are printed are subject to variance.

Standard (most colours)

For all Model Fs and the majority of buckling spring Model Ms, the keycaps have text dye-sublimated onto them. This method of printing is very durable, more so than pad-printed or silk-screen legends but only bested by multi-shot keycaps. Buckling spring keycaps exist as a single one-piece design or the more famous two-piece design consisting of a keystem that remains in a given barrel at all times and a keytop that can be transplanted to other keystems.

The standard keycap colours are pearl [white] (a 'creamy' white) and pebble [grey] (a chromatically warm brown-tinted grey). Lexmark and Unicomp have additionally produced keycaps with a variety of colours. As of February 2023, Unicomp produces the following:

Standard (raven black)

There are only a handful of examples of IBM buckling spring keyboards that had black-coloured keycaps, namely the OPTIONS by IBM TrackPoint II Keyboard (Black), the early raven black Unicomp Customizer and the early raven black Unicomp EnduraPro. Such keyboards used pad-printed legends instead of dye-sublimated legends due to the contrast limitations of non-inverse dye-sublimation techniques - you cannot sink dye into plastic when the dye is lighter than said plastic. The legends of such keycaps are therefore prone to being easily worn. Nonetheless, their relative rarity has made them desirable in their own right as well. For a while, Unicomp also had such keycaps produced but they eventually ran out of stock and cited prohibitive minimum order quantities from suppliers as the reason for their discontinuation as of July 2020^[18][18]
/u/funkmon/ - Unofficial and unplanned Unicomp Q&A on 2020-07-08 [accessed 2023-02-06]..

Host devices

IBM capacitive-type host keyboards

IBM membrane-type host keyboards

Further reading & resources

External

Sources

ASK. Admiral Shark's Keyboards original content. License/note: CC BY-NC-SA 4.0.

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