Solid Logic Technology (SLT) was IBMs method for packaging electronic circuitry introduced in 1964 with the IBM System/360 series and related machines. Because monolithic integrated circuits were not considered to be mature enough at the time. 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. The circuits were either encapsulated in plastic or covered with a metal lid. Several of these were then mounted on a small multi-layer printed circuit board to make an SLT module. Each SLT module had a socket on one edge that plugged into pins on the computer's backplane (the exact reverse of how most other company's modules were mounted).