Retinning RoHS components with SnPb to avoid tin whiskers for CubeSats - Page 1
Forum section: electronics / projects

author: TileSeeker
author_rank: Newbie
timestamp: August 15, 2022, 10:29:38 am
content: 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?

author: Fraser
author_rank: Super Contributor
timestamp: August 15, 2022, 10:44:44 am
content: 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

author: Fraser
author_rank: Super Contributor
timestamp: August 15, 2022, 10:51:09 am
content: 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

author: TileSeeker
author_rank: Newbie
timestamp: August 15, 2022, 11:47:56 am
content: 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.

author: SeanB
author_rank: Super Contributor
timestamp: August 15, 2022, 12:15:18 pm
content: 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.

author: mawyatt
author_rank: Super Contributor
timestamp: August 15, 2022, 12:48:37 pm
content: 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,

author: TileSeeker
author_rank: Newbie
timestamp: August 15, 2022, 01:10:54 pm
content: Do you know if that technique is normal/standard practice?

author: TileSeeker
author_rank: Newbie
timestamp: August 15, 2022, 01:12:41 pm
content: That's good to know. Thanks for the input

author: jmelson
author_rank: Super Contributor
timestamp: August 15, 2022, 05:32:54 pm
content: Quote from: Fraser on August 15, 2022, 10:44:44 amThe whiskers have been proven to pierce the conformal coating, at least some of the time.Jon

author: jmelson
author_rank: Super Contributor
timestamp: August 15, 2022, 05:35:03 pm
content: 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

author: wraper
author_rank: Unknown
timestamp: August 15, 2022, 05:38:39 pm
content: Quote from: mawyatt on August 15, 2022, 12:48:37 pmIt had nothing to do with whiskers. It's called electrochemical migration which results in dendrites, not whiskers.

author: jmelson
author_rank: Super Contributor
timestamp: August 15, 2022, 05:41:48 pm
content: Quote from: SeanB on August 15, 2022, 12:15:18 pmAnd, 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

author: rancor
author_rank: Contributor
timestamp: August 15, 2022, 05:46:05 pm
content: https://nepp.nasa.gov/whisker/background/index.htmhttps://nepp.nasa.gov/whisker/reference/tech_papers/2010-Panashchenko-IPC-Tin-Whisker.pdfAcrylic 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.

author: IgorIgor1233123
author_rank: Newbie
timestamp: August 15, 2022, 07:24:51 pm
content: 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

author: ConKbot
author_rank: Super Contributor
timestamp: August 16, 2022, 12:17:19 am
content: Quote from: TileSeeker on August 15, 2022, 01:10:54 pmFrom NASA hand soldering standards NASA-STD-8739.3 CH7.2QuoteThat 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)

author: jonpaul
author_rank: Super Contributor
timestamp: August 16, 2022, 11:29:34 am
content: ALL Lead free solder is tin whisker suspect.See work of Steve SMITH in 1980s..1990sUse only 63/37 SnPb leaded solderj