RoHS, Lead Solder, and Vice-Versa - Page 1
Forum section: electronics / general

author: TheRuler8510
author_rank: Regular Contributor
timestamp: August 30, 2014, 03:12:35 pm
content: Forgetting regulatory and compliance issues for the moment, what happens when lead-free RoHS parts are used with leaded solder? And vice-versa, what happens when leaded parts are used with leaded-free solder?What about the solder joint integrity, reliability and longevity?Any other issues of interest in this area worth bringing up?Thanks,

author: sacherjj
author_rank: Frequent Contributor
timestamp: August 30, 2014, 03:29:16 pm
content: There are legal and practical considerations.Legally, there are a few components that you can have non-RoHS and still be "lead-free" assembly. These are typically <= 0.65mm QFN or SOIC that can have a leaded finish, but are exempt. But almost everything else is any lead, RoHS is dead.On a practical standpoint, you will want to drive your soldering process a little higher than you might with only tin-lead process and tin-lead components. This is to assure that the lead-free portion will fully melt and you get a somewhat homogeneous joint. (This is obvious with lead-free solder, but less obvious with lead-free components and tin-lead solder.) The down side of this is if your tin-lead flux isn't comfortable at those higher temperatures and you degrade it and allow more oxidation.Leaded parts with lead-free are less of an issue, as the surface finish of the parts will certainly melt at lead-free temperatures. But this is opposite of what you will normal encounter, because most parts are RoHS these days, and many of us still prefer Tin Lead for hand assembly of prototypes.For a long term side of things, you can't eliminate having a different alloy form between the two types. There will be some transition layer where the concentrations of metals in the alloy is different. This can affect properties of the joint. It is possibly that it would focus structural stress due to thermal cycling, leading to premature physical failure. If we are talking hand soldering with Tin Lead, then there may be more of a factor in variability of the hand soldering process than this alloy transition for longevity.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: September 01, 2014, 03:54:49 pm
content: Tangentially relevant to your question but I see you're an RF engineer. The last place I worked we were making high Q coils to pick up MRI signals. This was formed by a single turn coil track in a large piece of flexi-PCB with lots of large series capacitors. We were still using lead solder as we were making medical devices. I did some investigations into transitioning to lead free. I found that the use of lead free solder reduced the Q of the coils by an appreciable amount.

author: nctnico
author_rank: Super Contributor
timestamp: September 01, 2014, 09:10:28 pm
content: Quote from: TheRuler8510 on August 30, 2014, 03:12:35 pmNo problem at all. The amount of lead or absence of it on the lead is so minimal that there are no issues. It does help to find an assembler who can do lead free properly. They are most likely to have better control over their soldering processes.

author: Whales
author_rank: Super Contributor
timestamp: September 02, 2014, 02:02:06 am
content: I'm assuming we're talking about the mostly tin style alloys, not the more expensive LF bretherin containing silver.Using lead free is trickier and requires more patience and flux. It is not less capable however -- everything you can do with leaded you can do LF, and it shows you're better at solderingI have read that it degrades tips faster, but my tips are all horribly cheap anyway (and I clean them with a scourer) so I cannot comment here. Apparently the transition zones between mixtures of LF and leaded can be fragile, but I'm not sure to what degree. I've repaired lots of leaded electronics with LF for personal use, but none of these get physically stressed much anyway, so I have not seen any of my joints fracture.What are you intending to use LF for? Large scale-runs, work for other people, or just try it out personally?

author: TheRuler8510
author_rank: Regular Contributor
timestamp: September 02, 2014, 02:10:47 am
content: Quote from: Whales on September 02, 2014, 02:02:06 amWhales,For now I am just learning the ins & outs of LF. I'm planning a lot of projects in the future and figured I better learn it now.Regards,

author: Whales
author_rank: Super Contributor
timestamp: September 02, 2014, 02:28:49 am
content: Quote from: TheRuler8510 on September 02, 2014, 02:10:47 amThe best thing would be to give it a tryA spool of thin LF solder should not be too expensive -- I picked up 200g of 0.7mm at brick-and-mortar store in Aus for about $14I notice you are an "RF engineer". Completely out of my league, but I assume small changes in resistance and the transitions to/between traces with a different alloy could potentially change the behaviour of higher frequency circuits. It's probably nominal compared to the variation caused by normal hand-soldering, however.

author: TheRuler8510
author_rank: Regular Contributor
timestamp: September 02, 2014, 03:16:22 am
content: Whales,All the lead free questions are about my hobby projects dealing with MCUs, gadgets, my DIY reflow oven, and (hopefully) small scale production.Cheers,