IBM pioneered the arrangment of vacuum tubes in pluggable modules such as the one shown here on the left. The IBM 650 was a first-generation computer. The era of miniaturization begins.
Transistors are much smaller than vacuum tubes, draw less power, and generate less heat.
Discrete transistors are soldered to circuit boards like the one shown, with intereconnections accomplished by stencil-screened conductive patterns on the reverse side. The IBM 7090 was a second-generation computer.