Ed,
I think John put it very nicely.  I would just like to add that we would
never use a 2225.  In fact until recently we wouldn't even use a 1825 and
now only in certain specific circumstances.
regards,
Bev Christian
Nortel
> ----------
> From:         John Maxwell[SMTP:[log in to unmask]]
> Sent:         Thursday, July 09, 1998 1:25 PM
> To:   [log in to unmask]
> Subject:      Re: [TN] KEMET capacitor 2225 size --not suitable for wave
> solder  ing???
>
> >What is different about these capacitors that they cannot take the
> thermal
> >shock of wave soldering.  I have experienced over the past few years the
> >cracking of 1206 and 0805 capacitors due to various stresses, and solved
> >various stress related issues with said capacitors (thanks to all the
> >consultants who have helped me with this issue, you know who you are!!
> :-}
> >). Since I have never used this specific size capacitor, i would like to
> >know if this is a problem so it can be added to a list of do's and don'ts
> >for design guidelines.   Is the size the problem, or what?
> >
> >Thanks
> >
> >Ed Holton
> >Hella Electronics
> >
>
> Multilayer ceramic capacitors are complex structures of interleaved metal
> electrodes and ceramic dielectric surrounded by ceramic cover layers.
> These
> metal/ceramic layers are not truly bonded during the firing process
> resulting in a fragile structure that becomes increasing sensitive to
> thermal and mechanical stress as size is increased. As size and active
> area
> is increased the chance or probability of a flaw or defect between
> dielectric layers also increases making the part even more sensitive to
> mechanical and thermal stress. Once there is a crack between opposing
> electrodes failure follows. Failure is accelerated by the presence of
> humidity and DC voltage but the part will always fail when a crack between
> electrodes is present.
>
> The simplest solution is to minimize exposure of large chip capacitors to
> thermal stress by restricting their use to reflow soldered processes only.
> By large chip I am referring to 1812 and larger chips.
>
> Please contact me by direct e-mail if there are further questions.
>
> Regards,
>
>
> John Maxwell
>
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