Hacker News Reader

Comment by Slansitartop | Hacker News Reader

Can't they coat the PCBs to mitigate/elminiate the problem?

I remember watching a teardown of a spare-no-expense embedded military computer, and the guy couldn't stop talking about how much "conformal coating" the board had.

Edit: I think this is the video: https://www.youtube.com/watch?v=55z_0BYb5is

View on HN

Typically, the coating is Parylene, which is a room temp vapor phase conformal coating (e.g. roughly uniform thickness independent of the contact angle of the materials). It's used for corrosion, moisture, vibrational. wear, and reduced breakdown voltage, etc.

It is not used for Sn (tin) whiskers at all, since it wouldn't penetrate the pads/package of the IC and wouldn't stop such a high modulus process in any case. Whiskers are most problematic with modern Pb-free solders and fine pitch SMT practices (QFNs & BGAs), but it's worth noting that other materials (like Zn) also have significant issues.

http://www.paryleneengineering.com/why_use_parylene.htm

From that page:

> The coating completely penetrates spaces as narrow 0.01mm.

I would have thought that would make it very effective in protecting against whiskers in QFPs that might have a lead pitch of 0.4mm or larger. There's not a lot you can do under the bodies of BGAs and QFNs, but if you're worried about whiskers you're probably using QFPs instead.

This app note suggests that Parylene and a few other dip-type conformal coatings do slow down tin whiskers, but don't stop their spread:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/52...

I dig the guy much better than David Jones ( EEVBlog ).

Here is Mike's BBC micro : https://www.youtube.com/watch?v=la-sGpTpkxE

Great stuff.

From what I understand, conformal coating is intended to protect against external agents (moisture, etc), and it does the job well, but it's not equally effective when the problem is between adjacent conductors of the same pcb; for that purpose the substance should be able to penetrate the tiny space between chip pins. It would probably help to some extent though.

Depends. If its dipped (or just sprayed on) it won't get inbetween fine pitches, but application in a vacuum will force it into every tiny space. It's essentially the same as vacuum-potted transformers.

I’ve read about tin whiskers extensively, and the consensus seems to be that while conformal coatings will stop tin whiskers from falling accross two leads, they won’t stop a whisker that’s growing from one lead toward another - they will just pierce right through it.

Conformal coating is standard for PCBs going into space. Lead solder (not RoHS) is used in assembly. For commercial BGA packages, companies specialize in de-balling the RoHS balls and replace them with lead balls.

Reply on news.ycombinator.com