Retinning RoHS components with SnPb to avoid tin whiskers for CubeSats

[TileSeeker]

I've been working on designing a CubeSat with COTS components. I've heard that it is strongly advised to avoid components that plate the terminals with high consentrations of tin, as they will generate tin whiskers relatively quickly. The problem is that this requirement greatly reduces the pool of applicable components. Would it be possible to retinn tin-plated components with an alloy better suited to space, without reducing component longevity? Does anybody have any relevant experiance in this area and/or thoughts?

[Fraser]

Sadly I have no experience in this area but just thinking about tin whiskers for a moment, could you use standard ROHS components and then apply a good quality lacquer coating to the PCB's as done when Marinising PCB's for use at sea ? The lacquer should prevent inter-pin shorting due to tin whiskers and would add ingress moisture protection to your design.

Fraser

[Fraser]

https://www.hzo.com/blog/mitigate-tin-whiskers-with-conformal-coatings/

It looks like others have the same idea about conformal coatings as used in marinisation.

Fraser

[TileSeeker]

Thanks for the reply. We are planning on coating out PCBs for this exact reason. The coating will help mitigate the formation of whiskers, but it won't stop it entirely. It's possible we decide that just coating is enough but for now, ill look into our options before we make a decision.

[SeanB]

How about simply getting a solder dip pot, and retin the legs with the 60/40 solder before assembly. For the SMD parts just using the tin lead solder paste will dilute the tin enough not to grow whiskers ever, but for the leaded parts simply dipping in solder will do a similar job as well. Cheap, and also ensures that your parts will have a fresh coating on them before assembly.

[mawyatt]

50 years ago we had an issue where our handheld radios were failing initial temperature cycling, the 12V battery inputs were shorted. Traced to a ceramic SMD cap across the Vcc to ground on the alumina substrate, which were replaced but later the replaced caps failed. Eventually the cause was discovered as the potting compound was hydroscopic (all plastics are to some extent) and during temp cycling the moisture would condense from the potting to the substrate surface where capillary action would concentrate the moisture under the SMD cap.

Silver and Tin whiskers would migrate under the SMD cap and short Vcc in the moisture, as soon as the suspect cap was unsoldered the whiskers would be absorbed in the molten solder and "disappear". Took awhile to figure this out, and the solution was to keep the potting compound in a humidity controlled environment (sealed) and apply in same environment until cured.

So message is that coatings may contribute to the whisker growth unless carefully controlled in storage and application.

Best, 

[TileSeeker]

Do you know if that technique is normal/standard practice?

[TileSeeker]

That's good to know. Thanks for the input

[jmelson]

Sadly I have no experience in this area but just thinking about tin whiskers for a moment, could you use standard ROHS components and then apply a good quality lacquer coating to the PCB's as done when Marinising PCB's for use at sea ? The lacquer should prevent inter-pin shorting due to tin whiskers and would add ingress moisture protection to your design.

Fraser

The whiskers have been proven to pierce the conformal coating, at least some of the time.
Jon

[jmelson]

There is an entire industry changing the component lead plating on commercial parts for the military/aerospace/space manufacturers.  Do a search to see if any of them describe what they are doing.
Jon

[wraper]

50 years ago we had an issue where our handheld radios were failing initial temperature cycling, the 12V battery inputs were shorted. Traced to a ceramic SMD cap across the Vcc to ground on the alumina substrate, which were replaced but later the replaced caps failed. Eventually the cause was discovered as the potting compound was hydroscopic (all plastics are to some extent) and during temp cycling the moisture would condense from the potting to the substrate surface where capillary action would concentrate the moisture under the SMD cap.

Silver and Tin whiskers would migrate under the SMD cap and short Vcc in the moisture, as soon as the suspect cap was unsoldered the whiskers would be absorbed in the molten solder and "disappear". Took awhile to figure this out, and the solution was to keep the potting compound in a humidity controlled environment (sealed) and apply in same environment until cured.

So message is that coatings may contribute to the whisker growth unless carefully controlled in storage and application.

Best,

It had nothing to do with whiskers. It's called electrochemical migration which results in dendrites, not whiskers.

[jmelson]

How about simply getting a solder dip pot, and retin the legs with the 60/40 solder before assembly. For the SMD parts just using the tin lead solder paste will dilute the tin enough not to grow whiskers ever, but for the leaded parts simply dipping in solder will do a similar job as well. Cheap, and also ensures that your parts will have a fresh coating on them before assembly.

And, of course, SAC305 seems to eliminate the whisker issue for the most part.  NASA has tons of literature on this, they have studied it a bunch, as they had several rocket launches that were lost due to whiskers.  I did have this issue with some Xilinx chips that must have had pure Tin plating on the leads, and I used SnPb solder.  Apparently, the leads were formed into the "gull wing" shape after the leadframe was plated, and that locked in stress in the plating.  Then, when I reflowed the boards, the temp was not high enough to reflow the pure tin, and we got whiskers after a year or so.
Jon

[rancor]

https://nepp.nasa.gov/whisker/background/index.htm

https://nepp.nasa.gov/whisker/reference/tech_papers/2010-Panashchenko-IPC-Tin-Whisker.pdf

Acrylic and Urethane conformal coating is better than nothing. Urethane is the type tested, it reduced whisker growth and provides a barrier to reentry. Anything coated in tin lead solder should not have a problem. TI also has NiPdAu plating on some components and other suppliers may have similar parts.

[IgorIgor1233123]

As mentioned above, a coating is not particularly helpful, but it is better than nothing. The problem is also that no one really knows how whiskers are formed and the mechanism, so it is a lot of trial and error as to what promotes and what prohibits them. If you have chemically deposited tin, you have to remelt it to reduce the risk of whiskers. In addition, components are often only tin-plated with pure tin, they are re-alloyed in a tin-lead pot, even the leads of components are always re-tinned. In addition, the gold must be removed from the soldering surfaces.
More details are often a bit proprietary

[ConKbot]

Do you know if that technique is normal/standard practice?

From NASA hand soldering standards NASA-STD-8739.3 CH7.2

5. Tinning of Conductors. The portion of stranded or solid conductors or part leads that will eventually become a part of the finished solder connection shall be tinned with solder and
cleaned prior to attachment. Immersion of conductors in a solder bath shall not exceed 5 seconds.
Liquid flux may be used. The flux shall be applied so that it does not flow under the insulation
except for traces carried by wicking. Precautions shall be taken when removing flux with a
cleaning solvent to prevent excess solvent from flowing under the conductor insulation.
a. Hot tinning of solid conductors and part leads should not extend closer than 0.5mm
(0.020 inch) to part bodies, end seals, or insulation unless the part configuration and mounting
configuration dictate. If closer tinning is required, the part body, end seals, or insulation shall be
inspected for damage after tinning and the results recorded (see also paragraph 4.1-3).
b. Solder pots or hand tinning with a soldering iron shall be used.

That was a '97 revision of the standard, so it was practice at least at one point, not sure what the most current revisions say, but it sure looks like tinning most of the component leads in a pot is acceptable, without question, and the whole lead if required and inspected.

Obviously, you're going to want the components to be baked out and moisture controlled to prevent popcorning from the heating of all the leads on one side. Look into the standard more to get a more complete understanding. Just the snippet by itself doesn't paint the whole picture. (I.e. my warning against moisture)

[jonpaul]

ALL Lead free solder is tin whisker suspect.

See work of Steve SMITH in 1980s..1990s



Use only 63/37 SnPb leaded solder

j