Unless something has changed in the last four years, tdracer is correct, I too was surprised to find that while avionics equipment is exempt from the requirement to use lead-free solder, the use of leaded is not mandated.
The main concern always quoted was 'tin whisker' growth but brittle joints are, in my experience anyway, a much bigger issue, especially in a relatively high vibration environment.
If this was one of the causal factors in this event, you would like to think the powers that be will look at the subject again.