×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

PCBA contamination identification

PCBA contamination identification

PCBA contamination identification

(OP)
I have recently gotten some field returns that have high impendence shorts under some of the .1uf 1206 caps.  The assemblies pass our end of line test but fail in the field under high humidity.  I suspect contamination left over from the assembly process is the reason for failure, but need to prove it.
Does anyone know of a lab that can identify the conaminants found on a PCBA?  Plenty of places can tell me how much contamination there is, but not what it is.  If it's salt from the atmospheric conditions, that's one thing.  If it's bromine left over from the manufacturing process, that's another.

Thanks,
Scott

RE: PCBA contamination identification

How high of a short?  Could it be your caps?  1206 is a pretty big device to be shorting.  Do you have any type of coating that covers your circuit?  What are the typical atmospheric conditions that your circuit is subject too?

RE: PCBA contamination identification

I had problem similar to this about 10 years ago.  With over 4000 smt parts on a board, it was a real show stopper.

The contamination turned out to be the adhesive used to hold the parts in place before reflow.  The adhesive was being degraded by the board wash solution, but didn't show up for a while.  Changed adhesives and the problem went away.

And in my case, it was also the caps that were the problem.  At operating frequencies the contamination made them look like low value resistors.

Unfortunately, I don't know of any lab to suggest.

RE: PCBA contamination identification

Most failure analysis labs have the capability to perform various analyses such as x-ray emission or Auger.

If the boards are not coated, there may be other things such as solder whiskers and the like.

TTFN

RE: PCBA contamination identification

(OP)
The shorts range in value from 60 ohms to >200k.  The boards are not coated and have components on one side only, no adhesive.  
The failures occur after subjecting the boards to 90F, 90% RH for 24 hrs.  The boards are ramped from abient to 90, 90 in 30 minutes to avoid condensation.
In all cases removing the suspect capacitor, which reads in spec in ckt and out, eliminates the failure.  Hot air is used to remove the component.
Alpha Metals WS609 solder paste is used.

RE: PCBA contamination identification

90/90 is generally one of the promoters for solder or tin whiskers.  Maybe there's a bad batch of tin plate on the leads.

TTFN

RE: PCBA contamination identification

My client built some meters with zero problems.
You could breathe on the PCB and the meters would peg.
The problem was that water soluble flux was used and the PCB was not cleaned properly.
After proper PCB cleaning the meters were OK.
Other fluxes have the same problem, although probably not to this extent.
Heating the PCB to remove the capacitor will dry out the flux and the board will appear OK.
Robert A Pease (author of Troubleshooting Analog Circuits)has written in some of his articles about breating on PCB to increase leakage currents.
Good Luck

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources