Terry,
The question should be is why are you recieving boards with intermittent
opens from your board manufacturer.
It sounds like there is a serious design or manufacturing
flaw with this particular board. Are the opens in same location of the board
or are they random throughout the board?

Tony Steinke
AIT-Atlanta Inc.

----- Original Message -----
From: "Terry Exell" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Monday, July 15, 2002 11:45 PM
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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