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lead free platings and tin lead



lead free platings and tin lead | 31 August, 2004


Any body have any issues soldering tin lead solder paste with lead free terminations ?

Components are leaded devices and chip components

have read the theoritical discussions on the forum, need to know if any body has seen any real issues todate

appreciate everyones help


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lead free platings and tin lead | 31 August, 2004

Kris, this practice has gone on for years. I have never heard of any problems associated with this. I have heard "grumblings" about tin lead parts being processed with lead free paste, but have seen or heard of no data to support.


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lead free platings and tin lead | 31 August, 2004

NO-LEAD COMPONENTS WITH LEAD SOLDER ISSUE: We agree with Russ. We've dicussed this several times on SMTnet. For instance, look here

LEAD COMPONENTS WITH NO-LEAD SOLDER ISSUE: We disagree with Russ. As with many things, there appears to be two distinct industry camps on the topic of lead contamination of no-lead solder joints. * One camp is of the opinion that the presence of lead is detrimental to no-lead solder joints and significantly reduces the thermal cycle fatigue life. * The opposing camp is of the opinion that the presence of lead does not impact or only slightly degrades a no-lead solder joint thermal cycle fatigue life.

Many solder suppliers recommend against using lead components with no-lead solder, because of weaker solder connections [discussed in the next paragraph].

Unfortunately, in the past the presence of lead in lead-free alloys has been presumed to be acceptable. The logic behind this was that tin and lead are soluble in a lead-free system. However, what has been overlooked is that the intermetallic crystalline structures in lead-free systems are not soluble and will precipitate at lead boundaries. Thus, when using a lead-free alloy to solder to Sn/Pb coated component leads, Pb can actually create voids in the solder joint that can result in joint failure. [from an AIM Solder presentation]

Dynamics of Lead Contaminated Solder Joint Failure [from an AIM Solder presentation] * It is important to note that lead that contaminates a lead-free solder joint is not distributed uniformly through the joint; rather, the Pb localizes in the last point to cool. * This is similar in dynamics to �zone refining�, a process utilized to refine high-purity elements. In zone refining, a heat source traverses across a billet. As this occurs, the elemental impurities are collected in the liquid phase and are condensed at the last point to cool (the end of the billet), which is then removed. * Just as in zone refining, lead as an impurity in a solder joint migrates to the last area of the joint to cool. This occurs under the middle of the component lead at the solder joint-PCB interface. * When this occurs, the joint forms pockets and the grain structure is disturbed. These Pb-rich regions are lower in melting temperature and may cause dewetting during soldering. * This is inevitably the area of a solder joint that results in a failure.

Let's not kid ourselves, we're going to be building [and dealing with field failures and having freaked-up general managers] with lead contaminated no-lead solder connections for years. [And that's why the solder suppliers are recommending not using leaded parts with no-lead solder. So that when the failures start piling-up, they can say, "See we told them not to mix materials, like that, but did they listen? Nooo!!! Our hands are clean."]

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lead free platings and tin lead | 31 August, 2004

Now see Dave, now I have some good info on why it is "bad", This must be the lead enrichment I have heard about but never got any substantiated data or explanantion. I wonder how much weaker or less reliable these are. Is there any data that you know of that shows actual numbers or case histories. I actually have a customer that requires us to use lead free paste but they can never seem to get all pb free parts for the builds. I actually have told them that I have heard that it was not a goood idea to do this for the reasons you stated but could not go any further in explanantion due to ignorance on my part.

Thanks Russ

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lead free platings and tin lead | 31 August, 2004

See Russ, that's why we're all here. => To take our turn at keeping everyone else on the straight and narrow.

For more on lead contamination of no-lead solder, look here:

[We apologize to all for appearing to be such a pimp for AIM Solder in this thread, but we gain no benefit from AIM. AIM has been very aggressive in educating practitioners in the issues of no-lead. We're sure other solder suppliers have made equal efforts, but it's just that we're unaware of their work.]

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lead free platings and tin lead | 31 August, 2004

I have experienced this directly in SMT and wave solder.

Fillet lift (can) be a direct indicator to lead enrichment (but its not the exclusive symptom).

Lead enrichment in smt joints reduces the interfacial strength shortening the time to creep fatigue failure

I read a study that indicated thet TH joints were only margionally less reliable than those without lead enrichment. However, within SMT joints there was a more pronounced affect (naturally, this is different with different types of smt components).

I think what is important here is fact its something most folks seem to miss

Lead bearing solders can be used with lead free components and produce a given reliability that will (generally) not be less than (the baseline) SnPb solders and SnPb parts.

You can use tin/lead parts with lead free solders (lets assume SnAuCu) and produce a given reliability that (possibly) will equal or be less than the baseline tin/lead assembly.

However, there are so many factors that influenc joint reliability (component design, Th/SMT, choice of LF solder etc.). The best method to determin what is best for you and your product is thermo-mechanical fatigue testing using HALT.

There is no perfect answer. Lead bearing solders have over 40 years of tried and true experience in Electronics.

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