22 Jan 2006, 21:53
According to my engineer friends, the kind of protective coating you're referring to here is called "encapsulation". In theory, it has a twofold function; structural reinforcement, in that it physically immobilizes the wires and keeps them from bending, and as a barrier to moisture and chemical contamination. I have seen this in two variations, one a hard, dark epoxy-like material [all clock chips on P2 and P3 Pulsars are encapsulated in this way], and a clear, silicone rubber-like material. These materials are highly specific to the application; in particular, they must have a zero coefficient of expansion or contraction as they cure [such a dimensional change during cure would put potentially bond-breaking tension on the wires]. Once cured, the material is unremovable without causing more damage to existing bonds, so such modules are essentially unrepairable. In either case, sometimes it worked, and sometimes it didn't. Moisture and corrosive chemicals [probably from leaking batteries], are insideous and very patient, and a lot can go on in a sealed watch case in 25 or 30 years. All Pulsar P4s, Calculators, and Ladies' models [except Touch-Command] have unprotected or "open" [to the air] wire bonds [still protected by a removable plastic shield]. While it may make them a bit more vulnerable, it also makes them vastly more repairable [whether or not this was a design consideration by the Time Computer engineers is something we may never know, but, having fixed a fair number of these, I'm grateful they decided to do it the way they did].
