Another question about mixing leaded and unleaded solder. - Page 1
Forum section: electronics / projects

author: McBryce
author_rank: Super Contributor
timestamp: October 07, 2015, 08:53:57 am
content: Hi all,I know there are other threads that have already discussed these matters, but they don't seem to answer my questions, so I've started this thread. I have two situations where I need to mix leaded and unleaded solder and would like opinions on any issues I might encounter.Situation one is repairing modern equipment. I prefer hand soldering with leaded solder (for numerous well known reasons). I have a repair where removing the lead-free solder isn't really an option, it's a massive connector 100+ pins and to fully remove the old solder I'd have to unsolder the entire connector, when the repair only requires resoldering 4 or 5 pins.The other situation is a batch of PCBs, where the SMD components were reflowed using lead-free solder. Several other parts (connectors) need to be added by hand, but the lead-free solderpaste was applied to those contacts too.Do I really need to remove all the unleaded solder? Are the issues I've read with cracking / blooming etc really a problem?McBryce.P.s. As my "normal" electronics activities mainly involves repairing equipment from the 70's and 80's, I don't currenty own a reel of lead-free solder and would prefer not to have to buy some if possible.

author: Psi
author_rank: Super Contributor
timestamp: October 07, 2015, 08:57:02 am
content: It's supposed to make the joint weaker, and i'm sure it does, but week enough to actually care about? i doubt it.Most people just solder it and dont care.

author: McBryce
author_rank: Super Contributor
timestamp: October 07, 2015, 09:04:36 am
content: That's what I thought might be the case. I made a few test joints. Under the microscope they look fine and I certainly can't break them. We have a pretty sofisticated strain guage rig here at work, maybe I should compare all three variations on that to get numbers?McBryce.

author: Pjotr
author_rank: Frequent Contributor
timestamp: October 07, 2015, 09:11:49 am
content: What happens if you mix both, the melting point goes up as well compared to regular SnPb. So if in doubt get a roll of lead free. Modern ones are not that badBut as said before, I wouldn't bother if it is for own use. You can't stress both kinds mechanically too much if reliability is an issue.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: October 07, 2015, 09:19:20 am
content: I usually remove as much of the lead free solder as possible e.g. with braid or sucker. Then 'rinse' the joint a couple of times with leaded. i.e. re make the joint with leaded then suck the solder away again with the sucker/braid. This way I'm fairly satisfied that the final joint is 60/40 rather than 55/35 and 10% whatever alloy was on there before.

author: McBryce
author_rank: Super Contributor
timestamp: October 07, 2015, 09:21:28 am
content: Quote from: DenzilPenberthy on October 07, 2015, 09:19:20 amThat's exactly the kind of hassle I didn't want to go to. Situation two would involve doing that for 34 joints on a batch of 50 boards.Bryce.

author: MLXXXp
author_rank: Frequent Contributor
timestamp: October 07, 2015, 03:07:48 pm
content: Sorry if I'm taking this a bit off topic...There a many different alloy mixtures for unleaded solder. Do you get any of the same problems when you re-work and mix one lead free alloy with a different lead free alloy, as you would get with mixing leaded and unleaded?

author: wraper
author_rank: Unknown
timestamp: October 07, 2015, 03:22:50 pm
content: Quote from: DenzilPenberthy on October 07, 2015, 09:19:20 am"Rinsing" a few times is excessive and actually harmful for PCB. I could justify doing this only if doing lead free after leaded, not the other way.

author: DanielS
author_rank: Frequent Contributor
timestamp: October 08, 2015, 07:18:18 am
content: Quote from: MLXXXp on October 07, 2015, 03:07:48 pmIf you mix anything with anything else, you end up with an unknown alloy with unknown properties. The main reasons why applications settle for specific solder alloys are for soldering process repeatability and well documented long-term behavior. If you do not care (much) about either, then just about anything will be fine. Most solder alloys exist for highly circumstantial reasons: simply to facilitate joining different types of surface finishes, reduce wear on soldering iron tips, ease of soldering, better electrical or thermal conductivity, etc. Almost anything can be used as solder, you just need to be reasonably confident that whatever you end up with meets your application's requirements.As others have said above, obsessing over it if your application has no particular requirements is likely to cause more harm than good. You only need to use a specific alloy when the application or customer requirements say so. Unless it is a mission-critical, medical or otherwise potentially sensitive (legally-speaking) application, I would not lose sleep over it.