[R-390] Solder

[Charles P. Steinmetz]

Andy wrote:

>I did try the 4% silver solder but I didn't like it a bit - it doesn't
>form a mirror surface - it hardens into a matte finish, so it is quite
>hard to tell a cold solder joint from a proper one just by looking at it.

I, on the other hand, use 96/4 (Sn/Ag) exclusively, and have for over 
30 years.  It is substantially stronger, is free from 
electromigration, and adheres more tenaciously to a wider variety of 
substrates.  The silver is not there just to prevent silver leaching 
(although it does) -- it also prevents "tin whiskers," improves 
wetting, and increases conductivity.

You are correct about the appearance of joints -- perfect joints have 
a uniform frosty appearance -- but this is actually a symptom of an 
advantage.  96/4 has a much narrower solidus/liquidus range (i.e., it 
is more perfectly eutectic) than any other common solder alloy, so 
cold solder joints are much harder to make with it.  On the rare 
occasion that I make one, I find there is no difficulty identifying 
it by inspection.  Really, it is just a received Sn/Pb prejudice that 
all solder joints should be shiny.  Get over that, and one can enjoy 
the substantial benefits of 96/6.  [The above applies ONLY to 96/4 
Sn/Ag and 96/3.5/.5 Sn/Ag/Cu -- other "lead free" solders do not have 
these advantages and have some real disadvantages.]

Proper activated rosin flux (RA) is the best electronic flux for 
cutting through crud and wettability and for leaving a residue that 
does not need to be removed (although I usually do).  However, 
although you can still get RA flux, you cannot still get "proper" RA 
flux.  I have bottles of the real stuff I saved when flux went to 
hell (early '80s) and use it as necessary to augment the wimpy RA in 
currently-available cored solders.  You may be able to find an old 
bottle at a hamfest.

Best regards,

Charles