Eliminate Tin Whiskers on Component Leads

Phil
Welcome to board talk. This is Phil Zarrow and Jim Hall, pick and place of the assembly brothers. Sometimes known as ITM consulting consultants. Today we've got an interesting one from an aerospace guy.

Jim
This question is from EB. We work with an aerospace manufacture on PCB assembly that includes lead free components, but no BGA's. We process using tin lead 60/40 solder. But this does not solve the issue with tin plating on the leads and the risk of tin whiskers. According to information on the nasa.gov whisker website (http://nepp.nasa.gov/whisker/), tin whisker formation is an issue on the exposed component leads outside the solder region.

Is there a process that can be used to re-plate component leads to add lead? Well, this is a very common topic which we discuss regularly in our technical workshops entitled lead free for the excemptee where we have people dealing with backward compatibility.

Phil
Typically your brethren from aerospace military avionics, medical, and all you other guys. I thought you were going to get it easy when you didn't have to comply with lead free but we've found just the opposite.

Jim
The NASA website is absolutely correct. When you solder tin plated leads, even matte tin over nickel, the solder cannot be counted on to wick all the way up the lead so you end up with a small section of lead close to the component body that still has tin coating on it. And it can cause whiskers.

If you look at the INEMI (http://www.inemi.org) recommendations for surface finishes, they say that matte tin over nickel is acceptable, it's all under the caveat not for high-rel applications.

Phil
That's right, so Jim, what's an engineer to do?

Jim
The standard procedures are affectionately called strip and dip. Basically a chemical or a metallurgical process is used to remove the tin coating and replace it with a tin lead coating. Some use chemistry and acids to strip the tin off and then usually they dip it to replace it. Some use sequential dips in a tin lead pots so each time you dip it you get a purer and purer tin lead coating on the leads. The bottom line is you end up with all of the lead completely coated with tin lead.

Phil
This has become quite a little bit of a cottage industry.

Jim
If you're doing a significant volume, there's automated equipment you can buy to do this yourself.

Phil
Well, great, that hopefully answers EB's question. This had been Jim Hall and Phil Zarrow, the assembly brothers. Talking to you on board talk and we thank you. Keep those questions coming in. And whatever you do in the meantime ...

Jim
Don't solder like my brother.

Phil
And don't solder like my brother.

There is a process that can prevent tin whisker growth, but it requires a willingness on the part of the engineer to employ a process that is foreign to circuit assembly: electroless metal deposition. This process, which has been used for circuit fabrication for a half century, deposits on most conductive surfaces (including tin and solder), but not on non-conducting surfaces. (Conducting surfaces that must be protected are masked, as they are for conformal coating.)

Like cleaning, it requires immersion of a fully soldered assembly in a series of aqueous solutions. The details are available at ldfcoatings.com.

I plan to make a presentation on Wednesday, June 15 and June 25 (11 AM to 1 PM Eastern Time) on the weekly tin whisker telecon that has been going on since 2002. Anyone who is interested can call in at 646-364-1285. (At the prompt, enter the code 1322078#.)

The presentation viewgraphs are available in PowerPoint show and .pdf formats at dbicorpora-tion.com/twteleco.htm. The first presentation will cover facts about tin whiskers and deficiencies of present �mitigation� practices; the second will discuss the process itself.

Tin Whiskers can be a tough topic because everyone has their own opinion and they come up with their own stories. But I have seen it for 37 years that using a hard Urethane conformal coating will completely stop Tin Whiskers.

Related to tin-lead dipping part leads, beware that you don't damage the internal structure due to thermal shock. There have been studies showing that this can happen. Sometimes you have to dip at controlled rates, sometimes damage is unavoidable. Depends on die size, materials, package size, etc.

Currently we use selective solder (Kiss 104) to solder mixed technology boards with bottom side surface mount parts. I have been running into customers using 5 and 6 one ounce copper layers as ground planes and tying them all to the same pins (10 mil spokes for thermal relief are used) on connectors and sockets. Subsequently there is a lot of heat and solder time need to even get 50% hole fill. Is this a normal practice for the pcb design now?

SnBi shows great promise as a lead finish to mitigate tin whiskers. This lead finish has been used by several Japanese companies without any issues. While there have been concerns that brittle intermetallics could be formed if a SnBi plated lead is used with conventional Pb-based solder, this has not been a problem when a small amount of Bi is used in the plating solution. Celestica published some studies on this topic and a new study is underway at Auburn University.

Robotic hot solder dip with eutectic tin lead solder has been used successfully on many package styles. There is an industry standard that we use in aerospace and defense, GEIA-STD-0006, it covers qualification of this process. There are several vendors who offer the service including Corfin industries and Six-Sigma. This process completely removes the tin and replaces it with tin-lead.