Electronics Assembly Knowledge, Vision & Wisdom
Can You Mix Leaded and Lead-free?
Can You Mix Leaded and Lead-free?
Can you mix leaded and lead-free components on the same circuit board? The Assembly Brothers discuss this common request.
Board Talk

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Board Talk is presented by ITM Consulting
Phil Zarrow
Phil Zarrow, ITM Consulting
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.


Jim Hall
Jim Hall, ITM Consulting
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.
Transcript

Phil 
Welcome to Board Talk. This is Jim Hall and Phil Zarrow, the Assembly Brothers, also known as consultants for ITM Consulting.

We have a question today from a AS who asks, "Dear Jim and Phil. Can you mix a leaded component and a lead-free component in the same circuit board and still say the product is lead-free even when you are using lead-free?"

Jim
If we're talking about the legal definitions defined by RoHS, the question is maybe. That is, what kind of lead are we talking about. There are some kinds of lead that are exempted under the RoHS law.

For instance, if you have a fine pitch component like a QFP or a SOIC with a lead pitch of less than .65 millimeters then surface finish on those leads is exempt.

However, for most parts, chip capacitors, BGAs, and so forth, if you have any lead on one part on an assembly, that whole assembly violates lead-free as defined by RoHS.

So if we're talking about a chip capacitor that has a lead finish instead of a lead-free finish, and you put that on a board, that violates the lead-free legal status of that entire assembly.

There are also a number of sources of lead internal to the component, such as in the ceramics of capacitors and some of the internal solder joints in IC components that are exempted from RoHS.

But if we're talking about surface finishes, I think with the exception of the fine-pitched parts, any surface finish on any individual component will put you in violation of the RoHS definition of a lead-free product.

So I hope that answers your question. The bottom line is don't mix your parts. If you want to be lead-free, get all the tin lead parts away from that area and maintain your inventories carefully.

Once you get a tin lead part mixed in to your assembly anywhere, it's going to be hard to find and very expensive to correct if you need true lead-free status for RoHS or any of the other international legal requirements.

Phil
So beware of the caveats and know the rules and regulations. That should be it for Board Talk today and whatever you do and wherever you're doing it, don't solder like my brother.

Jim
And don't want to solder like my brother and keep the lead out of those assemblies.

Reader Comment

Except for the cases where forward and backward compatibility of lead-free (LF) components is critical, the answer is YES, you can mix. I remembered 2004 to 2006 when RoHS switching programs began to assure compliance with EU RoHS, it was common we did not have a clear idea about which components were and which one were not LF/RoHS. During that time, most process engineers mixed and soldered these components together on a same PCB and got very good results. Of course, if you intend to mix LF BGA with tin-lead components in SnPB reflow (tin-lead solder paste), you should think twice and avoid this, once several reliability studies are showing this is not a good idea. Leaded components with LF plating in SnPB reflow need to be checked for wettability of the terminals, mainly looking for bubbled solder surfaces and excess solder voids in solder joints. In this case, a low thermal profile normally is the major contributor for these "defects".

Glayson Figueiredo, Philips Medical Systems, Brazil
Reader Comment

You can always mix some lead-free components in leaded solder. With current industry trends towards lead-free, you face this situations with components you may not know. Problem is not all lead-free components are easy to solder with tin-lead. Not all solder fluxes for tin-lead create a good joint for lead-free components. Contact your solder provider and let them know the situation. Might help to see what finishes you are using and change flux. Try to avoid bright tin, palladium, silver, lead-free components. Mate tin, gold or pure tin dip are easy lead-free options for tin solder.

Pedro I. Zaragoza, Rockwell Collins Inc., Mexico
Reader Comment

Since we are a defense industry company, we are exempt from the RoHS requirement, but as many others may have discovered: you cannot get the new cool IC's with lead, and the old cool IC's with lead are going obsolete and replacement are RoHS compatible. Therefore we see that mixing lead and RoHS components is an interesting issue.

We have conducted some initial testing on mixing RoHS compatible components and lead components on the same PCB, but only with manual soldering using lead solder. No report or final conclusions have been made, but our initial findings is that this causes a lot of poor solder joints (cold solder) that looks like proper solder joints (proper wetted).

There is a clear difference between large and small components.

Small components, up to 1210 capacitors, achieves a correct soldering with proper mechanical strength. Larger components get solder joints which seem to be correctly wetted, but have a very low mechanical strength, and behave like a cold solder oint. SO-8 packages seem to be the limit between good and poor solder joints.

Our initial guess is that the lead solder melts at 183C, and as the temperature increases towards the melting temperature of the solder pre-applied on the RoHS components, the surface of the pre-applied solder begins to melt, but not all the way through. To the operator soldering this looks like proper wetting, and the soldering iron is removed. This causes an incomplete mixing of the two solder types, and gives a weak joint that looks strong visually.

The reason why we see a difference between large and small packages, we believe is caused by the thermal mass of the components. Smaller components have small mass, and the solder iron has a large thermal reservoir compared to a 1210 capacitor. The solder iron has a temperature of at least 300C. The operator soldering removes the soldering iron as they should, but since the time required to heat the small components to temperatures above the melting temperature of RoHS lead is so short, all the solder around small packages melts and mixes properly.

Larger packages will take a longer time to reach the same temperatures, so when the solder joint looks properly wetted (above 183, but below 220) the operator remove the soldering iron. This causes the RoHS solder to not melt completely, only on surface, and gives the weak joint.

The mini-experiment was conducted two different qualified soldering operators in a controlled environment.

Are there any others out there with similar experiences? Have you come to the same conclusion? We plan to conduct a similar experiment using automated reflow process. Do any of you have any good ideas or recommendations on how to conduct these tests?


Martin Sollien, KDA, Norway
Reader Comment

Was the question about Rohs compliance or was it about the metallurgical or processing pitfalls of mixing lead with lead-free components? I think you answered the easiest question!

Jerry Wiatrowski, General Dynamics C4 Systems
Reader Comment

What are the risks of mixing SnPb and Pb-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?

Ashwin Kasi, Tellumat, South Africa
Reader Comment

Hello R.L.,

Here is the exemption in RoHS.

*23. Lead in **finishes of fine pitch components other than connectors with **a pitch of 0.65 mm or less** with NiFe lead frames and lead in finishes of **fine pitch components other than connectors with a pitch of 0.65 mm or **less with copper lead frames.[2010] *

There is no need for component manufacturers to follow this
exemption. It is my observation that most suppliers plate all their leads with Matte Tin regardless of the pitch.

You should read the RoHS document, it isn't that long.


Jim Hall, ITM Consulting
Reader Comment

Recently you commented on using leaded and lead-free components on the same assembly. You said that fine pitch parts, those whose pitch is .65 mm or less, were exempt from RoHS requirements as far as the plating on the leads. Please tell me where I can find this exemption written.

Ralph Longton, LoJack Corp
Reader Comment

Do you have results of mixing lead and lead-free components, so that I can keep the company I work for from doing this.

Asalene Coffey
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