Risks of Mixing Tin-lead and Lead-free

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall, the Assembly Brothers, pick and place - or place and pick. We're here to answer your questions on SMT and electronic assembly process procedures, methodologies and dilemmas. 

Today we've got a lead-free question. It says, what are the risks of mixing tin-lead and lead-free components on a board that does not have to be RoHS compliant? What are some of the common pitfalls that one should be aware of?

Jim
This question surprises me, but since it's basic I think it's important if people are still learning the basics of the lead-free environment. To this specific question, unless you are just starting out opening a manufacturing facility, if you've been building boards over the last few years, you have been doing this.

If you've been building boards with a tin-lead process, you have been using lead-free components. Most manufacturers have eliminated tin-lead finishes on their components years ago. Panasonic, for instance, eliminated all leaded finishes in 2004 to be an aggressively green company in anticipation two years ahead of RoHS requirements.

Phil
Right. And they didn't ask your permission to do it, either.

Jim
And so in general terms we call this backwards compatible, when you get a lead-free component, and you're doing a tin-lead process. Virtually all components have no problems. They typically have a pure tin finish on the component leads. It solders perfectly well with tin-lead solders at tin-lead temperatures.

The tin diffuses into the tin-lead and you get essentially a typical tin-lead joint. The Hi-Rel people have some concerns about residual pure tin exposed in the leads for whiskering, but that's only in the very exotic Hi-Rel products.

The one issue that remains today that's still a problem is BGA components with lead-free balls on them. And there isn't any real resolution about this, is there Phil?

Phil
No, there isn't. And this is one of the most common areas of concern Hi-Rel customers deal with.

Jim
It's a huge topic. And we spent a lot of time on it in our normal lectures.

Phil
In our "Lead-free for the Exemptee" workshop, it's a major topic. People are coming looking for a silver bullet, and there are no silver bullets. In our humble opinion. The answer is nobody really knows yet. There's a lot of studies being done.

There's a lot of conflicting data. Stay tuned because more and more work is being done because it's a situation we have to deal with. There's still a lot of you out there that are not only not having to be RoHS compliant, but are restricted from using lead-free materials. So we've talked about before.

Jim
So in general, for most components unless you have an ultra Hi-Rel issue, there's no problem with your lead-free components in your tin-lead assembly except for your BGAs with SAC balls.

Phil
Your best bet in the meantime, if you're working with BGAs with SAC balls, to be absolutely safe, is reflow them at lead-free temperatures.

Jim
If you want to be absolutely safe, you have to re-ball them too. But whatever you do, however you solder, with whatever alloy and whatever temperature.

Phil
Yeah, don't solder like my brother.

Jim
And don't solder like my brother.

When we look into the IPC topics, we can find a .1% of lead contamination could be acceptable. Even this limit when we could be in a situation more complicated that .1% lead, analysis have shown that until a 0.5% of lead can be acceptable provided no is exposed the connection to an environment sharp of temperature. The percentages of the metals that contains the solder alloy has to be most accurate possible. If the solder connection will be exposed to an cold environment it will be breakable.

A small amount of lead introduced into an otherwise lead-free solder joint is not going to significantly compromise reliability. A larger amount, such as derived from SAC solder balls from a BGA into a solder paste that is Sn63 MAY compromise reliability in that the lead-rich regions can cause problems. But the bigger problem there is that countries with a strict RoHS requirement may not allow the product into the country.

As for "the Hi-Rel people have some concerns about residual pure tin exposed in the leads for whiskering, but that's only in the very exotic Hi-Rel products." Try telling that to a consumer electronics company that cannot sell 5 million laptops, Ipads, cellphones, or other products because the solder contains greater than .05% lead, or the reliability issues caused by the tin whiskers renders their products unusable.

If a BGA is only available with SAC balls, reballing the BGA with Sn63 alloy is an option if your product requires tin lead soldering. Running a hotter profile for the sake of SAC BGA's is generally detrimental if the solder process requires tin lead paste.

Soldering other lead free parts using a tin lead process is not a problem.

The article discusses the seemingly simple lead free termination in an otherwise SnPb joint. How about the opposite - where a trace amount of SnPb is introduced to an otherwise Pb-free joint? There are a variety of rework scenarios that can result in this combination. I've seen concerns raised that small amount of SnPb "impurity" in a otherwise lead-free joint can weaken it dramatically. Perhaps that is what the original question was referencing...?

Robert, the only issue you will have is that the balls on the BGA will not flow as it has a higher melting point. The lead paste will reflow and the BGA will sit in it like a 'cup' giving very unreliable joints. You will either need to use a Pb version of the part or change the whole PCB over to lead free.

The main concern is a Pb free BGA in reflow at a leaded temperature profile which may cause defects such as voids or cracks. Pretty much all other components can be mixed with lead solder paste and a leaded profile used.