The IBM catastrophically buckling compression column switch and actuator (commonly known as the IBM buckling spring) is a clicky 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, then popularised by the IBM 5150 Personal Computer from a month later. The 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 capacitance sensing and another with a membrane sensor. All Model Fs use the former, whereas most common Model Ms 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. Buckling spring is commonly abbreviated as "B/S".
Specifications correct for IBM membrane buckling springs employed by Unicomp as of 2000.
The idea of buckling springs by IBM was 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 switch that IBM widely employed in the 1970s on its Model B keyboards. The original 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:
More information: Model F keyboards
The IBM "buckling spring torsional snap actuator" switch was the first implemented version of buckling springs, patented in 1977 and also invented by Richard Hunter Harris. The design is essentially a much-refined version of the third proposed buckling spring implementation specified in the 1971 patent using capacitance sensing. As such, this version of buckling springs is commonly referred to as capacitive buckling springs or simply CB/S. The design is centred around a single helical compression spring that is designed to buckle in a "rapid and catastrophic manner" that causes a rocker (also referred to as a 'flipper') made from a capacitive material to make/break a circuit as the buckled spring moves the rocker from a pivoted rest position to a parallel and flush position atop of an underlining PCB. Unlike the earlier beam spring switch where the switch actuates when a circuit is broken by the switch's fly plate being lifted from the PCB, the switch actuates once the rocker hits the PCB and makes a circuit. One of the unique features of buckling springs is that the keycaps are an integral part of the switch design as the keycap directly pushes the spring down. The keycap is returned by the spring returning to rest after force has been lifted. 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 PCB and barrel plate. Since capacitance sensing was employed, this switch design enjoys N-key rollover.
Capacitive buckling springs were employed by the entire Model F family of keyboards and IBM Electronic Typewriter Models 65, 85 and 95.
More information: Model M keyboards
The IBM "rocking switch actuator for a low force membrane contact switch" was the second implementation of buckling springs, patented in 1983 and invented by Edwin T. Coleman. Most of the switch is the same as it was before - a spring in a buckling state rocks an actuator atop of some sensor - except the capacitive elements are all removed in favour of a simple plastic rocker and a dual-sheet membrane circuit. Due to this change, this version of the switch is commonly referred to as membrane buckling springs or simply MB/S. Whilst the rocker element is also smaller and the spring itself has fewer coils, the way it clicks and provides tactility is the same. That said, the change in sensing mechanism leads to a lot of changes under the hood such as the downgrade from N-key rollover to 2-key. Since the rocker is now designed to apply real pressure on the sensing mechanism, membrane buckling spring rockers can also be known as "hammers" or pivot plates as per Unicomp.
Membrane buckling springs were employed by most of the Model M family of keyboards and IBM Actionwriter, Quietwriter and Wheelwriter typewriters.