The IBM buckling rubber sleeve switch (or simply IBM buckling sleeves for short) was a tactile switch developed by IBM as a quiet, low-profile but almost 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 both released in 1991, then subsequently reused on Model M4-family Space Saver Keyboards and finally modified for use on Model M6-family PS/Note, PS/55 Note and ThinkPad keyboard assemblies. Its use on portable computers was phased out by the late '90s, however, Unicomp continued to produce this switch for its Model M4-based Mighty Mouse keyboards until the late 2000s1.
Specifications correct for IBM buckling sleeves employed by Unicomp as of 19992.
IBM buckling 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/M4-1 type) or a slider in the barrel (Model M6 and M6-1 type) 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 avoided1.
The original implementation of IBM buckling sleeves was introduced with the March 1991-launched3 IBM PS/2 L40SX laptop and its Model M3 numeric keypad option in exclusively beige/off-white colours. The following year, a black version of the keyboard was used with the IBM PS/2 CL57SX laptop. In 1993, IBM reused the design again for the Model M4 (without TrackPoint) and M4-1 (with TrackPoint) Space Saver Keyboards, now produced by Lexmark for IBM, available in beige or black, and includes a PS/2-based controller inside. 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 sleeves 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
The second implementation of IBM buckling sleeves was introduced in October 19924 with the IBM ThinkPad 700 series and PS/55 Note C52 (a Japanese-exclusive variant of the 700). Along with the keyboards used for the ThinkPads 720 and 350 series, and PS/Note 425 series, these keyboard assemblies were designated "M6" by IBM and primarily feature a radical change in the switch's actuator design. Namely, the membrane is now pressed on by a barrel-fixed slider instead of a rod on the keycap, and the keycap itself now has a more traditional form of mounting (crosspoint) that doesn't require clips on the edges. In the M6 version of these later types of IBM buckling sleeves, the slider colour is brown and the slider mount is quite minimalistic/skeletal. For the alphanumeric and modifiers keys, the sleeve itself is unchanged from M3/M4/M4-1 type and is fully interchangeable. However, for the smaller function and navigation keys, a likewise smaller variant of the sleeve is employed. Finally, M6-type IBM buckling sleeves were exclusively produced by Lexmark.
The third implementation of IBM buckling sleeves was introduced in September 19934 with the IBM ThinkPad 750 series. This version is only a slight revision compared to the Model M6 type, with only the barrel slider and membrane's flex connectors being altered. Instead of a skeletal brown slider, M6-1 employs a black and more 'filled-out' slider. The M6-1 also become much more widely adopted, with the ThinkPads 360 series, 370C, 355 series, 755C series, Power Series 850, 365 series and RS/6000 Notebook 860 all employing Model M6-1 keyboards. M6-1 was produced by Lexmark and Key Tronic after April 19961, 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 keyboard5.