From: liz@poppyrecords.invalid.invalid   
      
   Dave Platt  wrote:   
      
   > In article ,   
   > Cursitor Doom   wrote:   
   >   
   > >>Micro crack in the solder?   
   > >   
   > >It'd have to be a pico crack given the hi-mag examination I gave it   
   > >and found nothing suspicious.   
   >   
   > Possibly a bad wave-soldering job?  I've heard of cases where   
   > bad/incorrect solder or flux caused a mostly-failed joint... the   
   > solder flowed up over the component lead, formed a decent-looking   
   > meniscus, but didn't actually bond to the lead properly.  The   
   > lead could eventually crack away from the solder _inside_ the   
   > joint, leading to an open circuit which is invisible to the eye.   
   >   
   > You might want to try solder-sucking away the existing joints,   
   > re-fluxing, and re-soldering, and see if that fixes it... although   
   > I'm not sure if this would teach you anything more than you'd   
   > learn by just unsoldering the resistor and measuring it out of   
   > circuit.   
   >   
   > An internal crack in the resistor is probably more likely, though.   
      
   If it were a large heavy component, I would look for circular cracks in   
   the rings of solder around the terminals; the solder blob in two parts   
   with one attached to the pin and the other as a circle surrounding it.   
   This used to happen mainly where the dip-soldering conveyor was vibrated   
   to shake off solder splashes and mass of the large component stressed   
   the cooling solder blob.  The centre of the blob was cooled by the pin,   
   the periphery was cooled by the track and the last bit to solidify was   
   stressed into a ring of 'dry joint', which later failed almost   
   invisibly.   
      
   I can't imagine that happening to anything as small as a surface-mount   
   resistor, but include this historic information for the benefit of   
   anyone repairing through-hole boards with large heavy components.   
      
   --   
   ~ Liz Tuddenham ~   
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