Electronic – Does solder being or not being uniformly thin and shiny really indicate reliability of the circuit


I'm currently reading an article (in Russian) and the author compares two devices (hard disks) of the same model one manufactured by Company A and the other manufactured by Company B. One of the things he compares is how solder looks like on the boards.

Company A device board has soldered parts looking like this:

"bad" solder

and this is claimed to be "low quality tin-coating". What I see is that solder spots are not shiny and the solder layer is thinner near the spot edges and thicker far from the edges (a bit concave).

Company B device has soldered parts looking like this (that's exactly the same area of the board as on the Company A device):

"good" solder

and this is claimed to be "high quality tin-coating". What I see is that solder spots look shiny and the solder layer looks having uniform thickness across each spot.

So to me the first board just looks neater. From what I know about soldering once the surfaces have been properly degreased with soldering flux and the solder was melted properly the connection will be just fine regardless of how shiny and neat looking it is.

However the author claims that the Company B device is of higher quality and should be preferred because of (among other factors) the "better quality" of tin-coating. How reasonable is such claim? Can device reliability be judged based on such tin-coating analysis?

Best Answer

I've said this before, and I'll say it again: How shiny the solder looks is not a reliable indication of the quality of the solder joint.

Even before lead-free solder the shine wasn't a reliable indicator. More reliable than lead-free solder, but not reliable enough for most people.

Here are some things that I look for when evaluating the quality of the solder:

  • Consistency: Do all the solder joints look the same? If not, it indicates a variation in the solder process and thus it is more likely to have problems.
  • Wetting/Wicking: Did the solder melt evenly and flow onto the mating surfaces, or does it look like the solder beaded up like water on a newly waxed car? Solder that is beaded up could have problems, and hidden cracks under the bead.
  • Smooth finish: I'm not asking if it is dull or shiny, but rather is it smooth or are there lumps in it? Lumps are a sign of uneven or incomplete melting.
  • Conductive Flux: This one is rare, but important. Some types of flux are conductive, but not everyone is aware of this. Sometimes a board will be reworked with conductive flux but the flux will not be cleaned off correctly (water for water soluble flux, etc.). Check that the proper flux was used in the proper way. Note: Some flux leaves a residue and this is OK as long as the residue is not conductive even though it might look bad.
  • Cracked Solder Joints: Often this can only be seen using a microscope, and sometimes not even then.