Dear Saravanan, despite the poor quality of the photos, I suspect a
classical solder mask problem, named ´solder balling´ . The reason why it
looks like corrosion is to be found in the environment tests that you
mentioned. Elevated temperature plus voltage plus humidity starts
electrochemical processes with the following probable components: copper,
tin, lead?, potassium ions, chloride ions. If you have forgotten a
torchlight in the field for some time, and change with fresh batteries, you
may get no  light. If you look on inside of torchlight you will see
corrosion.

Instead of babbling, I will present a paper that describes in detail what I
suspect.See Dropbox/Reports../ Corrosion/Solder Mask Issue

If you search in the IPC archive you will find lots more

Inge

On 4 March 2013 16:21, SARAVANAN R <[log in to unmask]> wrote:

> Dear  Mr Inge Hernefjord,
>
>           Please see the attachments.  Picture1.jpg & Picture2.jpg  are of
> the pictures of wired board. We removed the conformal coating from the
> affected areas,  not to be confused with flux residue. The board was
> inspected and accepted. The PCB was wired, conformal coated, tested &
> thermal cycled. The PCB  was put inside a mechanical chassis.  When we
> opened the chassis for some rework, we found corrosion. Corrosion is
> noticed in the Top side only.
>
>           Picture 3 is of a bare PCB. Our Quality Control personnel
> observed the black stripes under the solder mask. We are unable to decide
> whether this lead to corrosion at later stage.
>
>         The PCBs were procured from a reputed fabricator in middle east.
> The procurement document is MIL-PRF-55110G.
>
> Kindly help me in finding the root cause problem. regards,
>
> R.Saravanan
>
>
>
>
>

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