Metal Can Package
A metal-can package is the small, sealed metal housing in which most transistors were sold from the 1950s into the 1970s: a flat metal header carrying the leads through glass insulators, covered by a cylindrical metal cap welded on to enclose the silicon or germanium chip. The can is not just a convenient shell. It was a solution to three problems that the first semiconductor devices made impossible to ignore: they were damaged by moisture and contamination, they were sensitive to light and stray electric fields, and they had to shed heat. Familiar outlines such as the TO-5 and TO-18 are metal cans, and for decades the phrase "transistor" brought to mind a little shiny top-hat with three wires.
Hermetic sealing was the point, because early semiconductor surfaces were chemically fragile. A germanium junction exposed to humid air drifted, leaked and sometimes failed outright, since a film of moisture and ions on the surface forms conducting paths across the junction. Sealing the device in a dry, inert atmosphere, usually nitrogen, kept those paths from forming and gave devices a stable life. The seal itself is a glass-to-metal feedthrough: each lead passes through a small bead of glass fused into a hole in the header, and the cap is joined to the header by resistance or cold welding, so that gas has essentially no path in. Manufacturers verify the result by leak testing, commonly with helium, which is one reason the claim of a hermetic package can be measured rather than merely asserted.
Kovar and similar alloys make the glass-to-metal seals survive heating and cooling. A seal between two materials that expand at different rates cracks as temperature changes, so the metal must closely match the glass. Kovar, an iron-nickel-cobalt alloy, was designed to match the thermal expansion of certain hard borosilicate glasses over the working range, which lets the glass bead stay tightly bonded to the pin through soldering and operating cycles. The glass fuses to a thin oxide layer on the metal, forming a chemical bond rather than a mere mechanical grip. The idea came from the vacuum-tube industry, where the same sealing problem had been solved for glass envelopes, a useful example of how transistor packaging borrowed tube-making skills.
Shielding and heat were practical reasons as well as hermeticity. Germanium devices respond to light, because photons create carriers, and an uncovered junction acts partly as a photodiode; an opaque metal cap removes that unwanted sensitivity, which is why glass-bodied early transistors were often painted black. The grounded can can also act as an electrostatic shield for high-frequency and low-noise devices, and in some types a fourth lead ties the case to ground. For heat, the metal header and cap offer a much better path from chip to ambient air than an early plastic would, so a small can can dissipate roughly a watt in free air and several watts if a clip-on heat sink or a large mounting surface is used, and larger cans such as the TO-3 carry power transistors.
Plastic displaced the metal can for most uses, but not all, and the reason is cost rather than failure of the idea. In the late 1960s manufacturers began moulding devices in epoxy, which was cheaper, easier to automate and compatible with automatic insertion into circuit boards. Early plastic parts were less reliable, since moisture could creep along the leads and corrode aluminium metallisation, but better materials, cleaner chips, and passivation layers on the die closed the gap for ordinary consumer and industrial electronics. Metal cans and ceramic-and-metal hermetic packages persist where failure is unacceptable or the environment is harsh: military and space electronics, high-temperature or high-radiation applications, and some precision parts. A common misconception is that hermetic means indefinitely perfect; every seal leaks at a very small rate, and the specified limit is what matters.
Read a metal-can package as a miniature pressure vessel for a fragile chip: Kovar and glass seal the leads, the cap seals the cavity, and the whole assembly buys stability, shielding and heat flow at a price that plastic later undercut for everything except the demanding cases.