Electronic – how to solve the solder joints not shiny

See this directly related paper:
Robustness of Surface Mount Aluminum Electrolytic Capacitors When Subjected to Lead Free Reflow

The important and (to me) surprising conclusion is that the case distortion does NOT affect capacitor life. YMMV.

Reflow Sensitivity Results

• Time to deformation strongly dependent on capacitor volume

• Smallest and largest more susceptible to deformation

• Moderate volume capacitors
  – 100 – 500 mm3
  – More robust

• Some capacitors experienced deformation before the 40 second hold time defined in J-STD-020C

Conclusions

• Case distortion mainly dependent on the volume of the capacitor

        – Small capacitors under 100 mm3 and   
        - large capacitors over 1000 mm3 susceptible to case distortion
        – Medium capacitors, less susceptible  
  

• Case distortion did not influence capacitor life

  – No evidence of latency in any sample

• Extended temperature or extended lifetime capacitors are more robust

Sounds like a classic cold solder joint. This happens when the solder and pad are insufficiently heated (time or temp), or the surface is not clean, and the wetting action does not occur properly.

If you use a meter to measure from one point to another on the solder itself, it should conduct, but between the solder and pad, there is not actually a good electrical connection, hence the open.

It's not that solder isn't conducting, but that it hasn't made a bond to the copper and therefore contaminants or flux are actually creating a non-conductive barrier.

As for the pin-to-pad conducting but not solder-to-pad, the pin is resting on top of the pad and likely making a physical connection (held in place by a brittle solder blob). The solder, however, may have a thin air gap left over from flux or contaminants, and isn't actually making contact with either.