4 ms·

Funny, the simplest thing to do there, and relatively cheap, is to lacquer the whole device after it's soldered. This is done on military grade hardware - or ev

Funny, the simplest thing to do there, and relatively cheap, is to lacquer the whole device after it's soldered. This is done on military grade hardware - or even tougher conformal coating.

jacobush 7y ago
Would that help against lead, or against tin whiskers? AFAIK tin whiskers can develop through lacquer too. But probably it helps some.
johnr2 7y ago
> Funny, the simplest thing to do there, and relatively cheap, is to lacquer the whole device after it's soldered. This is done on military grade hardware - or even tougher conformal coating. That helps avoid tin whiskers, but those aren't the only problem with lead free solder. It doesn't flow as easily as tin-lead, so dry joints are more common. It's relatively brittle, so it's more susceptible to vibration and thermal fatigue. Melting point is higher so components are more likely to be damaged during soldering. Military hardware is exempt from RoHS for good reason.
robomartin 7y ago
Sadly, no, this does not avoid or stop tin whiskers at all. The general technique you are referring to us called “conformal coating”. It can help a bit but it does not stop growth. Also, at that scale the whiskers are super strong and easily puncture right through the coating. This is a deceptively complex topic (as I learned over the years). For example, a whisker that does not penetrate the coating will either buckle and curl-up under the film or grow laterally under it. With fine pitch components having pin-to-pin gaps in the range of 0.25 mm, a short between pins can happen in just a few weeks. Conformal coating is useful for other reasons, but, ultimately, it isn’t a solution for tin whiskers. Back in the day I even looked into using specially formulated epoxy encasement. What starts happening there is that the coefficient of thermal expansion differential between The epoxy, board and components can cause all kinds of failures...and it still does not stop tin whisker growth or buckling.
- Miraste 7y ago
  This is fascinating, I'd never heard of tin whiskers before today. From a short look at the literature, nickel-palladium-gold alloys seem to be an alternative lead-free finish that doesn't suffer from whiskers. Have you worked with that at all? Does it have other shortcomings?
  - robomartin 7y ago
    Well, being that most of my work during the last few years has been in aerospace we just use conventional lead-containing solder. When you can't afford for things to fall out of the sky you go with what you know works. We've known about tin whiskers since, I think, the 1940's. In fact, if I remember the history correctly, we added lead to solder precisely for this reason way back in the prehistoric era. I might be diving into some industrial work soon that does not benefit from RoHS exemptions in this domain. This is one of the important cards in my kanban board...likely a rabbit hole that will not be pleasant to navigate. I took a quick look myself. Here's a note from Maxim with interesting data: https://www.maximintegrated.com/en/design/technical-documents/tutorials/5/5250.html https://www.maximintegrated.com/en/design/technical-document... I've used pretty much all the conformal coatings they list in this article. What they don't go into is buckling under the coating, which can short adjacent pins in fine pitch chips. I truly hate this problem. I has wasted more of my time over the years than anything else. RoHS is, in my opinion, a very misguided directive. It's a perfect example of when politics gets ahead of reason and fear-mongering wins over science. If we are not careful we are on track to do something similar with climate change.

Funny, the simplest thing to do there, and relatively cheap, is to lacquer the whole device after it's soldered. This is done on military grade hardware - or even tougher conformal coating.

jacobush 7y ago

Would that help against lead, or against tin whiskers? AFAIK tin whiskers can develop through lacquer too. But probably it helps some.

johnr2 7y ago

> Funny, the simplest thing to do there, and relatively cheap, is to lacquer the whole device after it's soldered. This is done on military grade hardware - or even tougher conformal coating. That helps avoid tin whiskers, but those aren't the only problem with lead free solder. It doesn't flow as easily as tin-lead, so dry joints are more common. It's relatively brittle, so it's more susceptible to vibration and thermal fatigue. Melting point is higher so components are more likely to be damaged during soldering. Military hardware is exempt from RoHS for good reason.

robomartin 7y ago

Sadly, no, this does not avoid or stop tin whiskers at all. The general technique you are referring to us called “conformal coating”. It can help a bit but it does not stop growth. Also, at that scale the whiskers are super strong and easily puncture right through the coating. This is a deceptively complex topic (as I learned over the years). For example, a whisker that does not penetrate the coating will either buckle and curl-up under the film or grow laterally under it. With fine pitch components having pin-to-pin gaps in the range of 0.25 mm, a short between pins can happen in just a few weeks. Conformal coating is useful for other reasons, but, ultimately, it isn’t a solution for tin whiskers. Back in the day I even looked into using specially formulated epoxy encasement. What starts happening there is that the coefficient of thermal expansion differential between The epoxy, board and components can cause all kinds of failures...and it still does not stop tin whisker growth or buckling.

Miraste 7y ago
This is fascinating, I'd never heard of tin whiskers before today. From a short look at the literature, nickel-palladium-gold alloys seem to be an alternative lead-free finish that doesn't suffer from whiskers. Have you worked with that at all? Does it have other shortcomings?
- robomartin 7y ago
  Well, being that most of my work during the last few years has been in aerospace we just use conventional lead-containing solder. When you can't afford for things to fall out of the sky you go with what you know works. We've known about tin whiskers since, I think, the 1940's. In fact, if I remember the history correctly, we added lead to solder precisely for this reason way back in the prehistoric era. I might be diving into some industrial work soon that does not benefit from RoHS exemptions in this domain. This is one of the important cards in my kanban board...likely a rabbit hole that will not be pleasant to navigate. I took a quick look myself. Here's a note from Maxim with interesting data: https://www.maximintegrated.com/en/design/technical-documents/tutorials/5/5250.html https://www.maximintegrated.com/en/design/technical-document... I've used pretty much all the conformal coatings they list in this article. What they don't go into is buckling under the coating, which can short adjacent pins in fine pitch chips. I truly hate this problem. I has wasted more of my time over the years than anything else. RoHS is, in my opinion, a very misguided directive. It's a perfect example of when politics gets ahead of reason and fear-mongering wins over science. If we are not careful we are on track to do something similar with climate change.