how bad are unleaded/leaded joints?

[nrxnrx]

Hi!

A friend has asked for help modifying one of his music boxes (korg volca beats) by replacing some resistors and caps and maybe soldering some components in parallel with the existing ones. As far as I can tell from pictures, it's 0805/0603 surface mount components.

I don't have any lead-free solder. How brittle is a joint on a 0805 component  made of 63/37 mixed with "Sn96.5 Ag3.0 Cu0.5"¹ in equal proportions ? How would two such resistors on top of each other (ignoring power dissipation) fare being thrown around in a bag  for a few hours?

Is it worth the trouble of removing all solder and maybe replacing components instead of soldering resistors/capacitors in parallel?

--
1. I don't know what solder Korg's contractor used. The above is just a lead-free solder mixture Pace's website told me is common.

[NEYi]

Personally, I prefer removing the component, wicking up all the old solder and applying new leaded one.
This way you'll get mirror-like joints that won't crack in like forever.

Though, in general all these "aerospace rules" on soldering aren't that important, even crappy looking joint with mixed solder most certainly will do the job and won't crack.
Just make sure it's was soldered with some kind of flux, personally I prefer rosin diluted in isopropanol.

P.S. You don't have to throw away the removed part, unless it's obviously damaged.
Components on top of each other work okay.

[helius]

According to http://publications.npl.co.uk/npl_web/pdf/matc85.pdf, mixtures of traditional tin-lead and lead-free alloys do not result in brittler joints. They can cause separation of the joint as it cools, which can in extreme cases destroy pads on the board. A 50%-50% mixture of SAC305 and Sn63 is actually less brittle than a Sn62Pb36Ag2 pure joint.

[David Hess]

According to http://publications.npl.co.uk/npl_web/pdf/matc85.pdf, mixtures of traditional tin-lead and lead-free alloys do not result in brittler joints. They can cause separation of the joint as it cools, which can in extreme cases destroy pads on the board. A 50%-50% mixture of SAC305 and Sn63 is actually less brittle than a Sn62Pb36Ag2 pure joint.

Sn62Pb36Ag2 is pretty tough so I wonder what the story is there.  I suspect its tensile strength and adhesion was so high that it was ripping the end terminations off.

If I am reading that paper correctly, all but one of the mixtures were better than the two pure alloys they tested.  Unfortunately they only tested the pure alloys of SnCu and SnPbAg and did not include SnPb or any of the others which makes me question all of their results.

[stj]

i'v never seen it cause a problem,
what you do want to avoid is mixing different fluxes.
so clean the area before you solder it if there is any old flux around.