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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