Sounds like you need to do a HALT test to the extents of the product experiment,
then find out the weakest failures first and improve their design, (ie.
materials, fr4 or polyimide, hole size or aspect ratio etc) this will help with
how rebust the product is. After you get the optum design and find out the max
profiles the product can withstand then the production tests are derived from
that as a baseline. The production test might be only 60-80% of the temp to
failure and should only consume about 5% of the product life. Good luck!







Terry Exell <[log in to unmask]> on 07/16/2002 01:45:47 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond to
      Terry Exell <[log in to unmask]>








 To:       [log in to unmask]

 cc:       (bcc: Warren Crow/US/I-O INC)



 Subject:  [TN] Stress screening of multi layer production
           boards for optimum         reliability








Dear All,

I am experiencing occasional open circuits on 16 layer boards during assembly
and
during in-service use.  I have started thermal stressing all production boards
with
the aim of screening out any boards with latent defects and this is identifying
and
removing some problem boards.  However, an equal number of problem boards still
fail during assembly.

Does anyone use similar thermal stressing of production boards to help improve
assembly yields and product reliability?  If so, do you find it successful?

My thermal stressing uses the following parameters:
10 cycles of hot / cold exposure in a HALT chamber,
-60 deg C to +120 deg C temperature excursions,
40 deg C per minute temperature ramp rate.
The boards are electrically tested after thermal stressing.

I calculate the screening is using approximately 20% of the expected life of the
assembled boards, so I am reluctant to increase the stress levels.

My board manufacturer has made several process improvements over the last year
but
I have not seen a significant improvement in my assembly yield.

Best regards

Terry Exell
PCB Assembly Engineer
BAE SYSTEMS
Plymouth, UK



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