This may sound odd but if you check out some car race shops in your local area they sometime sell a package of temperature-indicating inks in differant heat  ranges that is applied to disk rotors and other hot spots of a car that could   be silkscreened onto the pcb.

> From [log in to unmask] Wed Sep 27 09:18:16 1995
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> Date: Wed, 27 Sep 1995 08:22:35 -0700
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> From: [log in to unmask] (Glynn Shaw)
> Subject: Temperature Sensing Inks
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> I have been following the recent thread relating to temperature stickers and
> contacted the supplier of Templaq mentioned by John Crawford. Templaq is
> geared for assembly rework and is not available in a temperature setting to
> suite my requirements. 
> 
> My question is this:
> Does anyone know of a temperature-indicating ink which might be screened or
> otherwise applied to boards during manufacture and whose purpose is to
> "ballpark" the highest temperatures seen during assembly, or to indicate
> that the board has exceeded a known temperature during assembly?
> 
>