On Wed, 14 Aug 2002, Bob Barr wrote:

>On Tue, 13 Aug 2002 09:57:45 -0700, Barry Gershenfeld wrote:
>
>
>>I could believe that, because they are older, and they have the
>>chip-on-board (under the expoxy blob) so reliability probably
>>wasn't at the top of the design list.
>
>I'm confused by that statement.
>
>My (admittedly vague) understanding is that chip-on-board's biggest
>advantage is cost savings for large-volume manufacturing. I also
>thought, though, that long-term reliability for COB was quite good as
>well. Am I missing part of the picture?

Low cost cob can use lower grade resins and materials and this can lead to
creep or weight loss or dimensional instability over time. The result is a
cracked chip or bonding wires ripped off after a minor shock or all by
itself. The long term reliability should be good if the mounting materials
are compatible (thermal expansion coeff., low dimensioanl creep) and of
compatible modulus (chip on glass lasts almost forever afaik). I suppose
that the fact that no plastic buffer layer is used on low cost cob is
known ...

Peter

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