Oxidation, oxidation, oxidation!
I don't mean to oversimplify but that is predominately what is occurring.
I've seen samples where a strip of Kapton tape is placed on the board before
reflow. The section under the tape doesn't darken while the rest does.
> ----------
> From: Jana Carraway[SMTP:[log in to unmask]]
> Reply To: TechNet E-Mail Forum.;Jana Carraway
> Sent: Thursday, April 11, 2002 2:04 PM
> To: [log in to unmask]
> Subject: Re: [TN] Chemistry of discoloration?
>
> Rudy, you can even use "free radical initiation"! There must be some
> formulations folks out there who can divulge a bit more about the
> reactions
> without giving away formulation secrets! Dr. Roos, you out there???
>
> Thanks for the info Rudy,
> Jana Carraway
> MSEI
>
> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]]On Behalf Of Rudy Sedlak
> Sent: Thursday, April 11, 2002 12:11 PM
> To: [log in to unmask]
> Subject: Re: [TN] Chemistry of discoloration?
>
>
> Jana:
>
> You have asked a BIG question.
>
> There really is no known absolute answer.
>
> Organics tend to discolor, if they have any sort of reactivity left in
> them,
> and often, under the right circumstances even if they do not have any
> reactivity left in them.
>
> The general rule is they turn dark(er). The reactions are many, and
> clearly
> going to be dependent on the base material. Discoloration is driven by
> heat,
> and often exposure to sunlight.
>
> Not to get too technical, but polymers (laminate, and soldermask, for
> example) only rarely really react fully, and there is almost always some
> unreacted polymer (called monomer when it is totally unreacted, and
> oligomer
> when it is partially reacted) lurking in the matrix. When this contacts
> air,
> water, and heat and UV, all sorts of unexpected things happen. In fact,
> these very polymers often have added "preservatives" to prevent these
> post-polymerization reactions from occurring.
>
> The bottom line is (as if this really has ANY significanse in our
> industry)
> that this discoloration rarely affects the physical properties of the
> polymer. I think a great English playwright described this situation
> centuries ago..."Much ado about nothing"?
>
> Rudy Sedlak
> RD Chemical Company
>
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