On Fri, 2007-05-18 at 09:41 +0100, keith.jamieson@bt.com wrote:
FWIW I'd leave the router always on because I've known power supplies
to die when being
switched on/off all the time, maybe someone who knows electronics can
say why.
It's the thermal expansion / contraction which puts stress in the soldered joints / chips etc.
Thermal cycling plays a big part in these failures but not generally at the solder joint level. Dry joints are generally formed by either bad soldering technique (or in the case of commercial production a fault on the process line) or by vibration / top heavy components with no other means of support.
It is also possible to get a situation where the energy flowing through the joint is more than should really be allowed for that pad size..although technically this isn't a "dry" joint
The main exception to that would be where screening metalwork is soldered to the PCB...they always crack due to thermal expansion
There are several other reasons why Switch Mode Power Supplies fail after being power cycled.
One is that you get inrush current, for example the mains input of a standard SMPSU is mains almost directly into a bridge rectifier (AC-DC converter) followed by a nice fat smoothing capacitor across the resulting 300V of DC. The cap is "empty" when the device has been left off for a bit and therefore has a low internal resistance. So for the first few milliseconds this circuit is powered you have a low resistance path across the supply (via the Bridge) until the cap has charged.
This same sort of thing happens all around many electrical circuits, generally a lot of them are stressed more at switch on than they will be during their normal operating cycle. So any weakened components are more likely to fail at this point.
Another reason is that SMPSU's have a number of components that are critical only for startup, typically at least one of those is an electrolytic capacitor. Generally in consumer grade equipment they use cheaper parts that are only designed to survive 85C, so in a tiny wall wart with no vents they soon fail.
But because some of these parts sit unused until startup they sit technically faulty but working well until the next time you try and turn them on.