| As mentioned in TFA, the most important factor for successfully joining a metal and a glass is to match their thermal expansion coefficients. Most pure metals have a much greater thermal expansion than any glass, which will cause cracks. In the nineteenth century, the first successful joinings of metal with glass were done using platinum, but that is obviously too expensive for normal applications. Eventually a special alloy of iron-nickel-cobalt was developed, which is named kovar and whose thermal expansion is matched to that of a certain type of borosilicate glass. The use of kovar was widespread in electronics, starting with the vacuum tubes and gas tubes, and then continuing with the first generations of transistors and integrated circuits, which used metal packages. All the old transistors and operational amplifiers that were packaged in metal cans had pins and package bases made of kovar. When kovar had to be joined with a different kind of glass than the type with which it is matched in thermal expansion, that glass was coated in one or more layers of different kinds of glasses, with that matched to kovar in contact with the metal and the intermediate layers having intermediate thermal expansion coefficients, interpolating between the bulk glass and kovar. Kovar is not a good thermal or electrical conductor, which is why the modern power transistors that use plastic packages (e.g. TO-247) and copper bases and pins (which are plated with nickel or tin, to avoid corrosion) can easily dissipate much greater powers than the old transistors in TO-3 metal cans, which had the same size. On the other hand, the old transistors in metal packages were pretty much immune of environmental influences. |