Solid Logic Technology (SLT) was IBM's method for hybrid packaging of electronic circuitry introduced in 1964 with the IBM System/360 series of computers and related machines. IBM chose to design custom hybrid circuits using discrete, flip chip-mounted, glass-encapsulated transistors and diodes, with silk-screened resistors on a ceramic substrate, forming an SLT module. The circuits were either encapsulated in plastic or covered with a metal lid. Several of these SLT modules (20 in the image on the right) were then mounted on a small multi-layer printed circuit board to make an SLT card. Each SLT card had a socket on one edge that plugged into pins on the computer's backplane (the exact reverse of how most other companies' modules were mounted). IBM considered monolithic integrated circuit technology too immature at the time. SLT was a revolutionary technology for 1964, with much higher circuit densities and improved reliability over earlier packaging techniques such as the Standard Modular System. It helped propel the IBM System/360 mainframe family to overwhelming success during the 1960s. SLT research produced ball chip assembly, wafer bumping, trimmed thick-film resistors, printed discrete functions, chip capacitors and one of the first volume uses of hybrid thick-film technology. SLT replaced the earlier Standard Modular System, although some later SMS cards held SLT modules. Details SLT used silicon planar glass-encapsulated transistors and diodes. SLT uses dual diode chips and individual transistor chips each approximately square. The chips are mounted on a square substrate with silk-screened resistors and printed connections. The whole is encapsulated to form a square module. Up to 36 modules are mounted on each card, though a few card types had just discrete components and no modules. Cards plug into boards which are connected to form gates which form frames. SLT voltage levels, logic low to logic high, varied by circuit speed: High speed (5-10 ns) 0.9 to 3.0 V Medium speed (30 ns) 0.0 to 3.0 V Low speed (700 ns) 0.0 to 12.0 V Later developments The same basic packaging technology (both device and module) was also used for the devices that replaced SLT as IBM gradually transitioned to the use of monolithic integrated circuits: Solid Logic Dense (SLD) increased packaging density and circuit performance by mounting the discrete transistors and diodes on top of the substrate and the resistors on the bottom. SLD voltages were the same as SLT. Unit Logic Device (ULD) use flat-pack ceramic packages,