It increases cross linking inside the epoxy of the laminate,
making is slightly more brittle and harder every time you
bake it. It alsoi ncreases the speed of tin oxide formation,
in effect it ages the board.

It has also been suggested that a high temperature bake
(> Tg) may cause warpage in a pressed board.

Alain Savard, B.Sc.
Chemical Process Analyst
CAE Electronics Ltd.
e-mail: [log in to unmask]

-----Original Message-----

Why does baking the board  reduce
the reliability and life expectancy of the board?

> -----Original Message-----
> From: Alain Savard [SMTP:[log in to unmask]]
> Sent: Tuesday, October 26, 1999 8:51 AM
> To:   [log in to unmask]
> Subject:      Re: [TN] HELP!!! - flow solder problems RT
>
> Hi Richard,
>
> Now you know why some assemblers request test coupons.
> For the age of the boards, you might have a DATE CODE on
> the board. It's usually 4 digits, 2 of them are the week number
> and the other 2 are the fabrication year. (example: 99-12 or
> 12-99 represent the 12th week of 1999).
>
> Boards older then 6 months may start to have a substantial
> amount of water absorbed into the laminate. They may also
> have a fair amount of oxidised tin forming on the surface of
> traces and inside the PTH.
>
> Do NOT bake the boards unless necessary, it slightly reduces
> the reliability and life expectancy of the board.
>
> You might have to sacrifice a board to microsection it and see
> any problems with the internal structure (mostly the plating).
>
> If you are scrapping any boards, use the area affected.
>
> If the failures are always at the same component, the batch of
> components may need to be looked at for solderability.
>
> Finally check the wave and preheat temperature. (You could do
> this first actually).
>
> Good luck,
>
> Alain Savard, B.Sc.
> Chemical Process Analyst
> CAE Electronics Ltd.
> e-mail: [log in to unmask]
>
> -----Original Message-----
>
> Hello Richard,
>
> HELLO ALAIN
>
> A few things to look at:
> - Did you perform a solderability test on the test coupons?
> NO COUPONS WERE SUPPLIED.
>
> - How old are these boards?
> NOT ENTIRELY SURE - WE GET THEM FROM AN EXTERNAL SUPPLIER.
>
> - Did you verify plating thickness and voiding in the test coupons?
> NO COUPONS SUPPLIED.
>
> - are your components pre-tinned?
> NOT BY US. FITTED AS SUPPLIED.
>
> RICHARD TILBROOK

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