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Howdy,
The primary cause for failure of solder joints is fatigue. Thermal cycling coupled with mechanical stress over time.
Other factors can cause failure, of course. But manufacturing or material defects will have presented long before now.
You've got to remember we are dealing with old components in a high stress environment. Take the RSR, for instance. It lives under the hood where temperatures can swing from sub-zero to near the boiling point. It is subjected to high moisture and oil vapor. Every power pulse, road impact, and vibration ever delivered to the right front of the car was felt by that device.
Electrical current causes chemical changes along the entire path. These chemical changes are most pronounced where two dissimilar metals touch. The RSR has several mechanical contacts that have increased resistance due to these chemical changes coupled with arcing because of the high current it carries. Increased resistance equals heat. More heat creates more degradation creates more heat. Once started, it is just a matter of time until the device fails. Solder, being the alloy with the lowest melting point, is the first to go.
Your question begs another; How did Bosch manage to make a relay that can last 25 years?
As for vehicles much older than 1980, the answer is simple; find the relay.
On my 1966 122S, the only "relay" is the starter solenoid; it has been replaced. I have no mechanical regulator but I can assure you that if I did, it would have been replaced by now. How many OD relays and solenoids are original?
My 1970 Chevy pickup also has no relays. If it had AC, it would have and I know it would have been replaced more than once by now. I have had to replace the dimmer switch and the light switch on that truck because of the current they carry. Soon I will have to remove the fuse block to repair the ravages of time on it (and it does not move).
Cadillacs were loaded with relays. How many 1970-something Caddys are there out there now? Not many. One reason; electrical "gremlins" too expensive to track down.
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Mr. Shannon DeWolfe -- I've taken to using mister because my name misleads folks on the WWW. I am a 53 year old fat man. ;-)
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