The IBM buckling rubber sleeve switch was a tactile switch developed by IBM in the early 1990s as a quiet, low-profile but seemingly equivalent force alternative to IBM membrane buckling springs. It was first employed on the IBM Personal System/2 L40SX laptop and its Model M3 numeric keypad option, then subsequently reused on the discrete Model M4 and M4-1 Space Saver Keyboards and finally modified for use on Model M6-1 ThinkPad keyboard assemblies.1
The buckling rubber sleeves switches are membrane-driven and were seemingly derived from the same patents and/or copyright as membrane buckling springs as their host devices often cite 1984 as the copyright date. The sleeves are very tactile and are of medium-heavy weighting like buckling springs but do not feature part-way actuation as a property. Like membrane or conductive rubber dome switches, the user is required to bottom out to actuate, but the experience of doing so is different to rubber dome switches.
As seen, the rubber component sits externally of the assembly on top of a barrel plate and is used for providing tactility and return force for the keycap only. A rod on the keycap (for the Model M3/M4 variant) or a slider in the barrel (Model M6) instead provides a solid interface to the membrane to close the circuit, meaning the mushy bottom-out feeling typically expected from low-end rubber dome switches is avoided.1
This implementation is the original version introduced with the March 1991-launched2 IBM PS/2 L40SX laptop and its Model M3 numeric keypad option. It is perhaps most well known for being used in the IBM Model M4 and M4-1 Space Saver Keyboards released in 1993 and produced by Lexmark. Key Tronic briefly produced Model M4s shortly after Lexmark's exit from keyboard production and Unicomp finally continued producing and selling a keyboard based on them called the Mighty Mouse until 2010. During the 19 years of production and sale, the switch design remained unchanged. The principal feature of this earlier type of IBM buckling rubber sleeves switch is the keycap mount, which features two plastic clips on either side of the keycap that retains the keycap on the barrel plate. As described in Design, the keycap's central rod is used for actuation.1
This implementation was a slightly later revision to the original design intended for a plethora of early IBM ThinkPad laptops that swapped the clip mounting system for a central rod mounting system and a slider in the barrel itself. The alphanumeric keys' sleeves remain unchanged and are interchangeable with the earlier Model M3/M4 variant, so the key feel is also very similar. However, the function and navigation keys make use of a smaller variant of the sleeves. The Model M6 variant as a whole was exclusively produced by Lexmark and Key Tronic1, however, it should not be confused with Key Tronic's own take on the buckling rubber sleeves design used on the ThinkPad 701C/701CS TrackWrite keyboard3.