[sdiy] stupid solder

[Grant Richter]

>> I have a lead-free solder alloy (Sn96%) that is real pain to use.
>> It melts in higher temperature than leaded one, and it's almost
>> impossible to achieve shining mirror finish with it. After many
>> solder points I came to not bad results, i.e. solder surface is
>> not matt, but it's not so shiny as Sn60 either.
>> 
>> Now my question goes - how bad is it when solder joint surface
>> is matt, i.e. oxidized? Or is it those few % of silver that
>> come out to the surface and make it look matt? Does it influence
>> anything? Will it break sooner than shining one?

You may also consider the current flow in the solder joint. If the joint is
not perfectly formed it may demonstrate a higher resistance. The voltage
error produced by the added resistance will be I x R. So for a 1 milliamp
current flow, the voltage difference between say 100 milliohms and 500
milliohms will only introduce and error voltage in the microvolts.

Much greater variations will be produced by the 5 and 10 percent tolerances
of the components, than a few hundred milliohms resistance change in the
connecting solder joint.

For high current circuits (1 amp or more) this would be a much greater
issue. You may see those in a power supply for example. Signaling currents
are usually kept as low as possible to minimize circuit heating, which
causes drift in analog circuits.