Plating Lead Free Parts for Tin/Lead Assembly

We work with an aerospace manufacturer on a PCB assembly that includes Pb-free components (no BGAs). The device is processed using SnPb (60-40) solder, but this does not solve the issue of Sn plating on the leads and the risk of tin whiskers.

According to information on the nasa.gov website tin whisker formation is an issue on the exposed component leads outside of the solder region.

Is there a process that can be used to re-plate component leads to add lead?

E. B.

Expert Panel Responses

If you want to eliminate any risk of tin whiskers occurring you really need to replace the tin finish with tin/lead. This involves solder dipping the device for the full length of the leads up to the component body to ensure all pure tin is removed. Note that you may have to prove that this is not harmful to the specific component you are dipping. There are other possible mitigation techniques such as conformal coating however the jury is still out on whether this is really effective. I would point you towards GEIA-STD-0005-2 for more detailed information. This is still a bit of a hot potato in the industry so good luck !

Bryan Kerr
Principal Engineer - CMA Lab
BAE Systems

Bryan Kerr has 35 years experience in providing technical support to PEC assembly manufacturing. His experience ranges from analysis of materials and components to troubleshooting and optimizing, selecting reflow, cleaning, coating and other associated processes.

The re-plate and solder dipping processes depend on the type of component. There are companies such as Corfin and Meehan that are doing this for companies today. They reduce the surface of exposed tin only to the best of the processes ability and pay attention to the post cleaning protocols that bring the components to a very low level of contamination to allow for good electrical performance.

Terry Munson
President/Senior Technical Consultant
Foresite

Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

As you are an aerospace manufacturer you may be conformaly coating the boards. If so HumiSeal has just completed a study with the NPL (National Physical Laboratory) into the interaction between tin whiskers and conformal coating. Conformal coatings will abate tin whisker growth. Various conformal coating chemistry types perform in different ways with tin whiskers but they all abate growth. Please contact us if you would like full information on this project.

Chris Palin
European Manager
HumiSeal

Chris Palin is currently managing European sales and support for HumiSeal Conformal Coatings. His expertise is in test & reliability, solder technology, power die attach and conformal coating.

There are suppliers in theindustry who will replate leaded components with a tin/lead solder, but beextremely careful when doing this as there is quite a bit of risk inherent inthis process. We recently went through the exact same issue this year with aprime A&D account (their end customer was a tier one systems integrator)with very stringent quality control processes and requirements and even thenthere were many issues with the replating supplier that subsequently resultedin production delays, additional unanticipated costs and quality issues. Eventhough the customer provided the parts in question (they purchased COTStin/lead devices and then sent them to an outside supplier for roboticreplating), parts that were eventually found to be nonconforming due to theoutside plating process were able to pass through their quality controlprocesses and make their way onto the finished product. Issues included parts re-reeled incorrectly(polarized parts were put back into tape and reel format with some inbackwards) and excess solder on J-leaded devices due to the replating processwhereby solder on the leads came in direct physical contact with the devicebody (a definite non-conformance for this customer). I would also be verycautious with respect to the cleaning process of the plating supplier to makecertain that no flux residues in the form of ionic contamination are left onthe devices after plating. Finally, make certain that liability is determinedBEFORE sending any parts to an outside supplier for replating. If expensiveand/or long lead time leaded items are sent to an outside supplier (which isalmost always the case for high-reliability A&D applications like this) andthey come back in a nonconforming state for any reason, who is responsible?Just the replating cost itself (unit price and setup charges for EACH componenton the BOM that requires replating) is very expensive, not to mention the costof the component (which is now most likely scrap) and replacement delays. Thebottom line is to avoid the process altogether if at all possible due to thehigh level of risk involved but if you absolutely have to use an outsidesupplier for the replating of leaded parts, you need to have a very good riskmanagement process, qualify your supplier and their processes very thoroughlyand have very tight internal process controls to avoid common pitfalls likethese that are inherent in this process.

David Bonito
Sales & Marketing Manager
Technical Manufacturing Corp.

David has been active in all areas of the contract electronics manufacturing industry for over fifteen years. He is currently in charge of all Sales and Marketing related activities for Technical Manufacturing Corporation.

Let me just point you to this article in CircuitsAssembly magazine. It will answer many of your questions.http://www.circuitsassembly.com/cms/magazine/207-2010-articles/9439-a-novel-process-for-automated-lead-tinning

Richard D. Stadem
Advanced Engineer/Scientist
General Dynamics

Richard D. Stadem is an advanced engineer/scientist for General Dynamics and is also a consulting engineer for other companies. He has 38 years of engineering experience having worked for Honeywell, ADC, Pemstar (now Benchmark), Analog Technologies, and General Dynamics.

Reader Comment

Is there any talk of getting rid of RoHS compliance? The more I have had to deal with and the more I read about the problems and cost that have been created by the RoHS directive and re-plating of component leads so they can work in so many of the situations and environments reliably (which is just a small part). I'm really all for clean environments but... Does anybody ever take a step back and ask the proverbial "Did we jump from the frying pan into the fire" or does our pride get in the way? Can we say that we have better cleaner processes now and drop all the "multi-different" (New word no charge) ways which are throwing so many complex variations into the electronic assembly industries. Or maybe I'm wrong or think to simply, or can we really, honestly, quantitatively say we are better off having invested all this effort into assembly instead of better more relevant technologies. Maybe we are to far into the vortex to exit without significant collateral damage to the positive aspects we need to preserve.

Timothy Croissant, L-3 Communications ASIT-MA-Ops, USA

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