Reworkability of PCB finishes

[mtwieg]

For an upcoming PCB I'm having fabbed, I want better planarity than the HASL finish provides. But this is for research, not a product, so I expect I will need to do a lot of repetitive rework on the boards. Another catch is that I can't tolerate nickel in the board (has to be nonmagnetic) so that eliminates ENIG and other gold finishes as an option. I think that only leaves silver and tin immersion as candidates, and I've gotten conflicting info on whether such finishes are suitable at all for rework. Can anyone more experienced clarify this for me, or maybe give some other options?

 

[marce]

What is the board for that you cant tolerate any ferromagnetism? The only jobs where I know this is not allowed are MRI scanners and some boards for physics research, sourcing non magnetic components is a pain!

Once a devices has been soldered on a pad the surface finish is gone, so it is not a problem. Both silver and tin finishes have a limited shelf life, but if you apply solder paste to all pads your surface finish will be gone.

 

[mtwieg]

The only jobs where I know this is not allowed are MRI scanners and some boards for physics research, sourcing non magnetic components is a pain!

Yes and yes. A lot of the time I have no choice but to accept some nickel plating, but I avoid it when possible.

Once a devices has been soldered on a pad the surface finish is gone, so it is not a problem. Both silver and tin finishes have a limited shelf life, but if you apply solder paste to all pads your surface finish will be gone.

Interesting. So my SMD pads will be fine so long as they're soldered before corrosion sets in. But what about things like exposed vias and other plated holes? Should I just coat them with solder manually?

 

[marce]

Nope, if you are concerned conformal coating is the best method of protection.

 

[mtwieg]

For mass production, sure. But these will be research prototypes, so coating isn't an option.

 

[FvM]

Under the given conditions chemical tin or chemical silver should be the best solution.

 

[marce]

Possibly one of the newer organic finishes, I have heard claims that one has a 12 month shelf life, I will try and find which one.


Try This:
http://www.enthone.com/Industries/E...chnology_Selector/Products/ENTEK_PLUS_HT.aspx

 

[Mattylad]

Why no nickel?

ISTR HASL makes the best surface for experimentation as it can be reworked more than the other finishes.

 

[marce]

read post 2! Mattylady:-D