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.
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).
[1] 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.
[3]: 15 The chips are mounted on a 0.5 inches (13 mm) square substrate with silk-screened resistors and printed connections.
[3]: 15 SLT voltage levels, logic low to logic high, varied by circuit speed:[3]: 16 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: