Hi Stella,

        I'm no soldering expert, but I Know a little bit. The general problem
with mutiple thermal cycles is this: The tin forms a Tin/Copper intermetallic
with the Copper upon reflow. The first thermal cycle (reflow) usually is not
too bad, but subsequent thermal cycles increase the thickness of the
intermetallic. This is sometimes referred to as a "Weak Knee" syndrome, as it
seems to be worse at the Knee or edge of the Hole where the solder is usually
thinner after reflow. The Intermetallic does not wet well, thus presenting a
surface that results in poor solderability.

I hope this is of some help,

Regards,

Les Connally

[log in to unmask]

>  From: [log in to unmask], on 8/13/97 12:46 PM:
>  Dear fellows:
>  Can somebody explain me a theory(chemistry)  of the soldrablility reduction
>  during PCB baking?
>  We are baking the PCB assy prior to the wave soldering process.  The board
>  laminate is special nylon material and has gold over the bare copper pads.
>  The time between baking and soldering is 48 hours. ( 2 hours, 135 degree
>  C).   When production can not keep up, they rebake the PCB assy again for
>  the second time.
>  I have read, that each temperature exposure reduces the solderabilty of the
>  PCB.
>  Is it correct?  Why?
>  Thank you,
>  Sincerely
>  Stella Neyman
>
>