tin whisker - Page 1
Forum section: electronics / general

author: bkubicek
author_rank: Newbie
timestamp: September 11, 2013, 06:05:41 am
content: Hi!I think that some more info on tin whiskers, and these things could be quite interesting.http://nepp.nasa.gov/whisker/video/index.htmlI stumbled on this topic, while watching this NASA soldering tutorial:It would be quite cool if the showed soldering methods are still "up to date".

author: Stonent
author_rank: Super Contributor
timestamp: October 17, 2013, 03:43:25 am
content: Quote from: bkubicek on September 11, 2013, 06:05:41 amThose NASA soldering videos make me want to tear my hear out when watching."Looks like Bob on the other side of the room didn't wash his hands after coming out of the bathroom, we're going to have to wipe and sterilize our solder again"

author: Monkeh
author_rank: Super Contributor
timestamp: October 17, 2013, 03:50:33 am
content: Quote from: Stonent on October 17, 2013, 03:43:25 amAnd this attention to detail is what keeps spacecraft, whether manned or otherwise, working..

author: Stonent
author_rank: Super Contributor
timestamp: October 17, 2013, 05:33:11 am
content: Quote from: Monkeh on October 17, 2013, 03:50:33 amI wouldn't think they'd be hand soldering everything for that though.Now, two things I did get from the video that I liked. The resistor bending tool and bending the pins in the direction of the trace.Also did not think about stressing the component. I've been known to take a component that is slightly askew and melt the solder on one side and pull it down by the pin while leaving the other side soldered down to straighten it. I do see how that could be a problem now and will be more mindful of it.

author: Monkeh
author_rank: Super Contributor
timestamp: October 17, 2013, 05:34:39 am
content: Quote from: Stonent on October 17, 2013, 05:33:11 amThere's more hand work involved than you might think, it's not as if they produce large quantities of some of this stuff.. Plus repair (kinda worthwhile when the board cost six figures), modification..

author: Stonent
author_rank: Super Contributor
timestamp: October 17, 2013, 05:41:08 am
content: I hear they're looking into that new fangled 74 series logic now.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 17, 2013, 06:46:45 am
content: Interesting that he bends the resistor leads one way to retain the part in the board, then after cropping, bends them a completely different way. He even applies quite a bit of twisting force with that wooden stick during the clinch process, which I can't see being a particularly good technique for ensuring the component and its end caps don't get over-stressed.

author: NickS
author_rank: Unknown
timestamp: October 18, 2013, 06:06:04 am
content: Quote from: Stonent on October 17, 2013, 05:33:11 amI do that too and couldn't care less. Never had a failed resistor from it.Mind you I'm not building spacecraft.....

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 18, 2013, 07:54:13 am
content: Do keep in mind that components on a spacecraft may be subjected to very serious levels of thermal extremes. A component assembly may have to survive repeatedly being frozen down to the temperature of liquid Nitrogen. This could happen for instance if there is a temporary fault condition with the attitude control, or something similar. Same thing for literally being baked in the high noon Sun for a while...

author: Stonent
author_rank: Super Contributor
timestamp: October 18, 2013, 07:02:56 pm
content: Quote from: ElectroIrradiator on October 18, 2013, 07:54:13 amPossibly the temperature of solid nitrogen too.

author: ddavidebor
author_rank: Super Contributor
timestamp: October 18, 2013, 07:37:07 pm
content: The fact is if a satellite turn around earth 40 times a day, you have 40 extreme temperature changes.For everyday. For 15 year.

author: grumpydoc
author_rank: Super Contributor
timestamp: October 18, 2013, 08:23:56 pm
content: Quote from: bkubicekThe NASA video was interesting but doesanyoneoutside NASA call a PCB a "PWB"??

author: Monkeh
author_rank: Super Contributor
timestamp: October 18, 2013, 08:59:01 pm
content: Quote from: grumpydoc on October 18, 2013, 08:23:56 pmSometimes, it's an old term.

author: Kjelt
author_rank: Super Contributor
timestamp: October 19, 2013, 07:53:21 am
content: Pffff now at least I know why those products from nasa are so expensive, they take ages to assembleCome on use a semi cleanroom, gloves, bent it the right way the first time and you,re done in 10 seconds.Are they using SMT nowadays or is that still too risky? If you have such thermal shocks, I wonder how SMT will hold up.

author: Neilm
author_rank: Super Contributor
timestamp: October 19, 2013, 01:37:23 pm
content: Quote from: grumpydoc on October 18, 2013, 08:23:56 pmIEC61010 still refers to PCBs as PWB.

author: walshms
author_rank: Regular Contributor
timestamp: October 22, 2013, 04:40:32 am
content: Quote from: Stonent on October 17, 2013, 05:33:11 amUS Military Wiring Specification WS-6536 revision C; can't remember for certain, but I think it was an Air Force spec. Stringent as hell, but very highly reliable, especially in high vibration/high stress environments. I used to assemble power supplies that had to meet the requirements of that spec and they were beasts... and yes, everything was hand soldered, inspected, tested, reworked as needed, cleaned within an inch of its life, tested a second time and then potted with a re-enterable compound before being tested yet again.QuoteAmazing how easily some things can be damaged. When it's got to be as reliable as it can possibly be, you don't mess around.

author: walshms
author_rank: Regular Contributor
timestamp: October 22, 2013, 04:46:42 am
content: Quote from: ddavidebor on October 18, 2013, 07:37:07 pmThe International Space Station circles the earth about every 92 minutes, roughly 16 orbits per day. Satellites higher up orbit slower... so it's not quite 40, but the extremes are indeed extreme. Think in terms of up to ~200C swings, depending on the emissivity of the surface in question. That's why most of the stuff up there has heaters built in.

author: walshms
author_rank: Regular Contributor
timestamp: October 22, 2013, 04:51:53 am
content: Quote from: Kjelt on October 19, 2013, 07:53:21 amSure; SMT is commonly used now, but the requirements are probably just as painful to meet. Proper centering on the pads. Solder fillets on component legs that must show the entire outline of the component leg (no excess) and make a smooth transition to the pad. Each and every one. No nicks on component surfaces, or the component legs. The slightest scratch is enough to cause an inspector to reject a board.Think that's expensive? Trust me on this -- it's far more expensive to send something up there that fails because the time wasn't spent on the ground to ensure it was right. That's why teams spend up to 10-15 years preparing hardware before sending it where it can't be repaired...

author: Kjelt
author_rank: Super Contributor
timestamp: October 22, 2013, 06:54:09 am
content: Quote from: walshms on October 22, 2013, 04:51:53 amProbably also gluedots to keep the components mechanically fixed without having mechanical stress on the solder joints?They probably still use leaded solderpaste?

author: Stonent
author_rank: Super Contributor
timestamp: October 22, 2013, 07:32:54 am
content: Quote from: Kjelt on October 22, 2013, 06:54:09 amI think given the poor reliability that RoHS has added to electronics, I'm sure of it.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: October 22, 2013, 07:39:14 am
content: Quote from: Kjelt on October 22, 2013, 06:54:09 amYes, they do. Solder containing lead is still allowed for use in critical applications like aerospace, military and medical equipment.

author: AndyC_772
author_rank: Super Contributor
timestamp: October 22, 2013, 08:23:59 am
content: I should hope so. Lead leaching out of scrapped PCBs into drinking water supplies is less of a problem in space than it is on Earth!

author: Stonent
author_rank: Super Contributor
timestamp: October 22, 2013, 09:22:21 am
content: Quote from: AndyC_772 on October 22, 2013, 08:23:59 amLead came from the earth in the first place.

author: romantronixlab
author_rank: Regular Contributor
timestamp: December 08, 2013, 01:10:56 pm
content: Since i have been reading information on tin whiskers why is formed etc. Ill post some pics for those lazy enough to search google and of course for the novices in the forum.Removing lead from solder may seem a smart idea environmentally, but the resulting microscopic growths called tin whiskers could be just as problematic. - theguardian.comCredited to the respective providers.Tin Whisker Detached from RF Shield/Bracket which then bridged exposed conductor. nasa.govMillstone nuclear generating plant in Connecticut shut down when a circuit board monitoring a steam pressure line short-circuited. theguardian.comnasa.govelectronicstheory.com

author: SeanB
author_rank: Super Contributor
timestamp: December 08, 2013, 03:22:29 pm
content: Had it on my car as well. Took the unit apart and had a little work on it with a soldering iron to check joints on connectors, and scrubbed it with IPA. Seems to have fixed it for now. Was rather annoying to drive with all the warning lights lit, including for diesel heating, EGR, ABS, SRS, overheat and oil pressure. They stayed on with key off as well.

author: Christopher
author_rank: Frequent Contributor
timestamp: December 08, 2013, 03:55:55 pm
content: Does thick conformal coating help at all?

author: ddavidebor
author_rank: Super Contributor
timestamp: December 09, 2013, 10:09:56 pm
content: i suppose yes, how can tin wiskers grow up on the coating?but i'm not an expert

author: KJDS
author_rank: Super Contributor
timestamp: December 09, 2013, 10:17:10 pm
content: Quote from: Kjelt on October 19, 2013, 07:53:21 amOne of the worst tasks I've ever done is assemble three boards, each with 600 components, about 400 of which were 0402 resistors and capacitors to go on a military satellite. It took nearly forever to get it right, complete with a small air gap underneath each component to ensure that a slight board flex didn't stress the part. And the paperwork, don't get me started on the paperwork.

author: SeanB
author_rank: Super Contributor
timestamp: December 10, 2013, 04:28:30 pm
content: You do know that the paperwork has to be larger than the unit mass. I only had to fill out one form per unit, and one on receipt, one on test, one on repair, one on verification and one for QC. Then the labels for the box, for the unit, for the stores and all the paperwork, in the style that stores wanted, complete with the spaces they wanted left blank for their side. If I took from stores a page for the order, a record in a register, a copy in a file for the unit, another in the general file and still another form to order the part. Carbon paper a good saving, as the forms all had to be original or carbon copy, not a photocopy. Best thing I ever did was to get a self inking stamp pad with the required data aside from my signature, so as to use on the forms. Stores looked at that and she wanted heir own.That was only for avionics, I only had to fill out modification forms once, that took a 3 month period of back and forth of piles of paper. The basic form was 4 pages with "Add as needed extra pages marked annexure XYZ" and whole sections marked as " Not affected by this modification" along with prices, suppliers and 3 quotes per part with lead times and guarantees as to availability for x months and a set of contact details and other details.

author: KJDS
author_rank: Super Contributor
timestamp: December 10, 2013, 05:00:39 pm
content: Quote from: SeanB on December 10, 2013, 04:28:30 pmI'd have been happy if it were larger than the unit, even by a factor of ten. The unit from memory weighed about 250g, that's about half a pound in old money.As one of the designers, not only did I have to sort the assembly paperwork but all of the design details as well.From memory there was the followingThe design description, which includes a full description of the design and how it works, all its schematics, parts lists etcThe process description, so if soldering component to board using Sn63 solder and weller iron with bit type xyz was on the list of approved and controlled processes that line was easy. If it wasn't then there was more work.The worse case analysis, which covers temperature, component and tolerancing variationsThe rad hardness report.The compliance matrixThe test plan, which explains what testing philosophy will be used to prove that the product meets each line item of the specThe test procedure which details how to do the testsThe test report, which contains the results of the tests. I had one friend that had put together 9000 plots on a single, complex piece of equipment. He only had another 6000 to do to finish when the project was cancelled.Each report not only had the essentials, but also a few dozen pages of boilerplate, plus indices of chapters, figures, tables. It was usually necessary to provide six copies of each on paper, not less than 90g/sqm, as well as a few more copies on CDRom.and despite all that, I still prefer working space jobs to military ones, at least the paperwork is proactive rather than reactive, even if there is fractionally more of it.

author: codeboy2k
author_rank: Super Contributor
timestamp: December 11, 2013, 06:58:11 am
content: Quote from: Christopher on December 08, 2013, 03:55:55 pmQuote from: ddavidebor on December 09, 2013, 10:09:56 pmApparently, it does help. Specifically the one they tested and I assume is approved for use. But they only needed a few mils. Anything less than that and the whiskers grew through and could penetrate outward or another outside whisker could penetrate inward.Summary:"After >9 years, no whiskers have grown through the nominal 2 mil thick Arathane 5750 (formerly known as Uralane 5750). There are extensive (high # per area) whisker growths beneath the coating that are pushing/stretching the conformal coat, but none have managed to puncture the coating where the coating is a nominal 2 mils thick or greater"From their testing here:http://nepp.nasa.gov/whisker/experiment/exp2/photos here:http://nepp.nasa.gov/whisker/experiment/exp2/exp2photo.html#ccsEDIT: one of the more interesting findings from that experiment was that the conformal coating actually increased the rate and number of tin whiskers under the coating. i.e. they appeared sooner and in greater quantities to the non-coated samples in the experiment.