Dendritic growth on PCB - Page 1
Forum section: electronics / projects

author: marc1996
author_rank: Contributor
timestamp: September 03, 2018, 07:54:40 am
content: Hello to everyone,I would like to know what factors affect in the apparition of dendritic growth in PCB.I have this question because I'm trying to use Printed Spark gaps as ESD protectors and considering that the electrodes will have to hold an intense electric field, this problem came to me.Reading on the internet basically says that it depends on the flux quality but I think there are more things to consider in order to avoid that.Thanks!

author: Zero999
author_rank: Super Contributor
timestamp: September 03, 2018, 10:57:36 am
content: I don't have much experience with this.Ideally the flux should be removed, so it's a non-issue.I think the thickness of the copper, whether it's tinned or not, the comparative tracking index of the PCB material and environmental factors such as, pollution, humidity and temperature all play a role.

author: tooki
author_rank: Super Contributor
timestamp: September 03, 2018, 12:24:29 pm
content: Quote from: marc1996 on September 03, 2018, 07:54:40 amLook at NASA’s website on tin whiskers, it’s pretty much the last word on the topic.P.S. The word you’re looking for is “appearance”. An “apparition” means something rather different! (Or rather, it’s the appearance of somethingvery, veryspecific!)(Though this did make me chuckle!)

author: Orson Maxwell
author_rank: Contributor
timestamp: September 03, 2018, 12:46:16 pm
content: What I've gathered in my research in this topic is: don't use tin plating of anything. Plate with proper soldering alloy; copper containing led-free ones are ok as well.

author: Wirehead
author_rank: Regular Contributor
timestamp: September 03, 2018, 12:51:09 pm
content: ENIG finish in this case should be chosen over HASL, no?

author: marc1996
author_rank: Contributor
timestamp: September 04, 2018, 06:43:48 am
content: Quote from: tooki on September 03, 2018, 12:24:29 pmHahaha thank you, I will remember it for ever I think.Btw, Thanks to everyone, I will try to consider as much as I can all the things that you wrote here.

author: Kjelt
author_rank: Super Contributor
timestamp: September 04, 2018, 07:50:19 am
content: Quote from: marc1996 on September 03, 2018, 07:54:40 amI guess that there should be no tinplating on those parts, hence no dendrites.So use goldplating.If you use tinplating, how can you know how it performs since each factory and processing will result in different deposition of tinlayer (different thickness) ?Or is the application non-critical ?

author: marc1996
author_rank: Contributor
timestamp: September 05, 2018, 08:43:42 am
content: Quote from: Kjelt on September 04, 2018, 07:50:19 amThe electrodes of the spark gap will be made of tin or copper (preferable tin). Talking with my superior he explained that the machine that will weld every component will do it with the same thickness everytime, so the thickness is supposed to be the same.

author: Kjelt
author_rank: Super Contributor
timestamp: September 05, 2018, 09:23:55 am
content: So the sparkgap is not made out of the copper of the pcb but an extra external material that is also not soldered but welded on the pcb ?Then I don't see any problem ? You can also use a throughhole sparkgap, used those for decades without issue.

author: marc1996
author_rank: Contributor
timestamp: September 05, 2018, 09:53:21 am
content: Quote from: Kjelt on September 05, 2018, 09:23:55 amYes but the thing is that this is my university project and it has to be a printed Spark gap. Sorry for that question but soldered or welded is not the same? Basically, the machine will cover all the points that we indicate with tin so, what I will do is that the machine will put the tin in the spark gap with the shape of the model I selected, but the tin thickness is supposed to be the same as all the components that are welded in the pcb at the same time ( we are talking maybe about 35 um?).

author: tooki
author_rank: Super Contributor
timestamp: September 05, 2018, 10:49:32 am
content: I think you guys are confusing spark gapdeviceswith PCB spark gaps. The OP is clearly asking about PCB spark gaps, which Dave has done a video about...

author: tooki
author_rank: Super Contributor
timestamp: September 05, 2018, 10:55:09 am
content: Quote from: marc1996 on September 05, 2018, 09:53:21 amNo. Soldering and welding are different. Soldering (soldadura blanda) uses a low temperature filler metal to join the pieces. Welding (soldadura dura) either melts the pieces to be joined, or uses thesamemetal as filler.https://es.wikipedia.org/wiki/Soldaduradescribes the English terminology, too...

author: Kjelt
author_rank: Super Contributor
timestamp: September 05, 2018, 11:12:32 am
content: Quote from: tooki on September 05, 2018, 10:49:32 amWho is "you guys" ?I was under the impression that he meant PCB spark gaps but those do not involve welding copper parts AFAIK, so that is what confused me.

author: marc1996
author_rank: Contributor
timestamp: September 05, 2018, 11:23:48 am
content: Quote from: tooki on September 05, 2018, 10:55:09 amOkey, you caught me there. I have no idea tbh about what type but I would bet for soldering after reading the definition. Anyway I will ask to my superior later and I confirm it.edit: Does it influence in the dendritic growth the type of soldering?

author: tooki
author_rank: Super Contributor
timestamp: September 05, 2018, 11:41:29 am
content: In PCBs it’s always soldering. I’ve never heard of welding to a PCB. (Outside of linguistic confusion like here.)I don’t know that much about esd protection and spark gaps, but wouldn’t a tin whisker across a spark gap just vaporize and be eliminated? Think about this question: In what situations would a tin whisker across a spark gap cause malfunction? Would those situations cause the tin whisker to vaporize? (I don’t know the answers, these are just the questions that come to mind.)

author: Kjelt
author_rank: Super Contributor
timestamp: September 05, 2018, 12:29:23 pm
content: Quote from: tooki on September 05, 2018, 11:41:29 amA sparkgap is designed to conduct above a certain high voltage, lets say as example 6kV.It is AFAIK usually placed from the mains voltage fase/line to earth sometimes accompanied with some MOVs and acts like an open circuit untill that high voltage occurs, it then conducts that voltage to earth.So when you put it across the power lines lets say 230VAC to earth you notice nothing untill there is this extreme voltage occurring.A tin whisker will always conduct and could cause a short (blowing the fuse) or in the least case draw a significant amount of current with other risks.

author: coppercone2
author_rank: Super Contributor
timestamp: September 05, 2018, 10:00:35 pm
content: hobo pcbpuberty pcb

author: Wolfgang
author_rank: Super Contributor
timestamp: September 05, 2018, 11:19:42 pm
content: probably the wrong answer, but you could use ordinary Sn60Pb solder just to tin the spark gaps. Whiskers gone !

author: marc1996
author_rank: Contributor
timestamp: September 06, 2018, 06:41:28 am
content: Quote from: tooki on September 05, 2018, 11:41:29 amSo yes, I just searched for the word in english and that was the result haha!Quote from: Kjelt on September 05, 2018, 12:29:23 pmThat's exactly what I'm designing. After some simulations I have some models and an approximation of the gap distance needed for the jump depending on the model and the voltage(ofc is not that accurate because a lot of factors affect this value).Basically my Spark Gap is going to replace MLCC that are acting as ESD protectors in the PCB's of my companies (for example the 0603 (1.6mm*0.8mm), so it will be really small). Maybe I can't replace every MLCC for any reason, but the objective of the project is to replace as much as it's possible securing the initial functioning of the PCB and saving cost in the building of it.So, thats where my question enters, which will be the problem with dendritic growth / tin whiskers?As said before, the thickness of the electrodes will be some um I suppose of tin, but let's see how it works.btw, thanks for all your answers!

author: Kjelt
author_rank: Super Contributor
timestamp: September 06, 2018, 06:59:03 am
content: Good luck with your project, I believe NASA has done some research on tin whiskers (dendrites) but I have no idea what the state of the art research is at the moment.

author: marc1996
author_rank: Contributor
timestamp: September 06, 2018, 07:14:58 am
content: Quote from: Kjelt on September 06, 2018, 06:59:03 amYea, I read it this week but they say that dendrites and tin whiskers are not the same. Furthermore, we know how dendrites works meanwhile tin whiskers still need a lot of research in order to have more trusted information.The problem is that as I can see in Spark gaps tin whiskers could appear but holding currents of tens of Amperes would melt it but, are dendrites a problem in spark gaps?

author: tooki
author_rank: Super Contributor
timestamp: September 06, 2018, 04:18:23 pm
content: Dendrites only grow in liquids or in the presence of substantial moisture. This is something you can control. So in electronics, tin whiskers are the issue that's much more challenging (and still hasn't proven to be a huuuuge issue).But they'll never, ever handle "tens of amperes" of current. We're talking about crystals of tin much thinner than hairs. The current to blow one open is, from what I've read, usually under 50mA.This is why I asked above whether they're really an issue in a spark gap, where current would only be expected to flow when there's substantial voltage and current anyway.

author: marc1996
author_rank: Contributor
timestamp: September 07, 2018, 06:50:47 am
content: Quote from: tooki on September 06, 2018, 04:18:23 pmOkay, I got what you mean but, if there is a tin whisker connecting both electrodes, will it do that the arc doesn't jump? Because in case that the arc will occur anyway, I think that the whisker will be melt, but then, the problem is that the tin will go onto the pcb surface and maybe can short something. Or atleast that's what I think

author: Kleinstein
author_rank: Super Contributor
timestamp: September 07, 2018, 07:53:55 am
content: Tin whiskers can be a problem with lead free solder. Normally the PCB spark gaps are not covered with solder, as the extra solder would effect the shape as well. This can be a problem if wave soldering is used. However for small parts like 0603 SMD wave soldering is no that suitable anyway and with reflow soldering the spark gaps could be kept clear of solder.

author: marc1996
author_rank: Contributor
timestamp: September 07, 2018, 08:29:14 am
content: Quote from: Kleinstein on September 07, 2018, 07:53:55 amWe use tin with silver in the soldering phase. The gap between electrodes will and has to be free of solder mask. As far as I know the flux applied is not a problem in this situation

author: helius
author_rank: Super Contributor
timestamp: September 07, 2018, 08:36:34 am
content: The "dendrites" you are worried about are technically called CAF, Conductive Anodic Filaments. You can search for technical reports on the subject by using that term.It has nothing to do with flux or tin, but is a metal migration process that happens inside the PCB laminate as copper ions are dissolved and redeposited in the electric field. To prevent it, you must visualize the electric field gradient and keep vias and tracks well away from it.By making the spark gap sharply pointed, you can minimize the area of copper exposed to the field, which will restrict the amount of ions that can migrate. Thin copper is less subject to CAF than thick copper.

author: tooki
author_rank: Super Contributor
timestamp: September 07, 2018, 09:21:11 am
content: Quote from: marc1996 on September 07, 2018, 06:50:47 amJust how much tin do you think it is? Remember, we're talking about crystals that are so thin they're practically invisible to the naked eye.Or do as others have said and don't tin it at all, leave the spark gap as bare copper.

author: marc1996
author_rank: Contributor
timestamp: September 07, 2018, 10:07:45 am
content: Quote from: helius on September 07, 2018, 08:36:34 amI will search info of CAF. Thank you for your info.About the sharp points, thats true, the copper area that will be affected by the field will be less but then, the electrode will be damaged faster and the vertex will be destroyed faster. It's like a balance.The info that I have recollected before do not recommend at all to do the shape very sharply because of that. So maybe I should search an equilibrium between them...Quote from: tooki on September 07, 2018, 09:21:11 amI thought that before about leaving the spark gap as copper but I do not know at all how this works and furthermore, the 80% of Printed spark gaps that I saw in the net are made of tin, that's why I thought that maybe it could be better..

author: eb4fbz
author_rank: Regular Contributor
timestamp: September 08, 2018, 04:17:08 pm
content: I would not tin the spark gap pads. Thickness will not be tightly controlled, not even using HASL process. There will be some risk of bridging, etc.Flux residue will also affect breakdown voltage. Some residues are hygroscopic and would absorb moisture with time, with even worse consecuences.Better use ENIG over them.

author: marc1996
author_rank: Contributor
timestamp: September 10, 2018, 07:30:11 am
content: Quote from: eb4fbz on September 08, 2018, 04:17:08 pm.

author: eb4fbz
author_rank: Regular Contributor
timestamp: September 10, 2018, 09:57:39 am
content: If ENIG is not available, check if any other chemical finishing is. Chemical finishing processes are much more controlled than mechanical ones.It depends on the enviroment it will be used at, but i would not leave bare copper exposed.

author: Wolfgang
author_rank: Super Contributor
timestamp: September 10, 2018, 10:22:28 am
content: Just to play the devils advocate: Can PCB spark gaps be really worthwile investing such effort ? I mean, their primary deficiencies remain even after careful manufacturing:- dependency on PCB material, moisture, atmopsheric pressure, dust, aging, ...- dimensions must be very carefully controlledIf you need something reliable, who not resort to dedicated flashover components ?

author: Kjelt
author_rank: Super Contributor
timestamp: September 10, 2018, 10:50:43 am
content: Quote from: Wolfgang on September 10, 2018, 10:22:28 amI am still not sure what the application is, if it is lighting protection than I would definitely recommend a dedicated sparkgap component. If like the first post states to prevent possible ESD problems, than it might be used on all I/O pins and than components would be quite a pricy solution. But I thought they normally used TVS diodes for that not sparkgaps ?

author: marc1996
author_rank: Contributor
timestamp: September 10, 2018, 11:30:16 am
content: .