Not much on TV so I thought I would investigate solders.
A charity shop purchase book on Metallurgy bought some years ago ("Metallurgy", E. Gregory, Blackie & Son, 1949 revised reprint of the 1932 original) only deals briefly with Tin-Lead solder, but does have a drawing showing the behavior for different proportions. More interesting is the comment on Tin itself, which is apparently unstable at room temperature and prone to disintegrate into powder below 19° C. I wonder if this is a factor in the difficulty often experienced in soldering old NOS components such as solder tags? If the "tinned" finish is in fact pure tin rather than a tin-lead solder, the change to the alpha tin allotrope would not be beneficial.
I wonder what proportion of other alloys need to be added to inhibit the formation of the alpha phase? The common lead free solders seem to consist of around 96% Tin. Possibly the reported unreliability is in part due to the progressive formation of the undesirable alpha allotrope if the mix isn't right.
When checking the composition of lead-free solders (I don't have any myself), I found the following in information about Multicore Lead-free solder on the RS web site.
"What are the differences between lead-free solder and leaded solder?
Lead-free solder is generally regarded as having a more positive environmental impact than leaded solder, and also as being safer for human use. However, lead-free solder also has potential production advantages. Lead-free solder can offer better lead spacing, which makes it more suitable for high-density components where pitches are tight. This means potentially better performance where space saving is a concern.
The advantages of lead solder are that it has a lower melting point, which is sometimes preferable for hand working components. Also, lower working temperatures reduces the risk of damage to components and circuit board.
Unlike lead-free solder, lead solder does not have a shelf-life and is less prone to losing quality after prolonged exposure to oxygen. However, many electronic application requirements exclude the use of lead solder, due to the toxicity of lead."
This is the first information I have seen that lead-free solder has a shelf life, and an admission that it does deteriorate on exposure to oxygen. I'll be sticking to my 60/40!
https://uk.rs-online.com/web/p/produ...-PRODUCT_GROUP