Terry Davey asked:

>Does anyone have experience with using high value non-polarised (ceramic?)
>substitutes for SMT tantulum capacitors? Does it look like a winner or a
>loser in terms of yield/reliability problems?

Tantalum is the best in terms of stability and capacitance for size. I
believe you will find no true substitute in the higher capacitance values.
Aluminum electrolytics can provide high capacitance, but they are unstable
over long time periods and will tend to "dry out" in harsh environments.

>One problem we're looking to eliminate is the "time-bomb" effect of low ppm
>level of reversed tantulum caps (they may take months to "blow"). They are
>possibly reversed when missing autoplaced components are hand-placed in
>rework.

I had some big yellow kidney parts marked backwards once (from a major well
known supplier) which would later end up going "ka-pow" when the finished
computers were in burn-in. The result was enough ionized combustion
products to set off smoke alarms in the building, alert the fire
department, and have the alarm company call me in the middle of the night
to come open the building. I bet we had 20-30 boards do that...

Kemet makes a 3-legged tantalum through-hole part which can't be
accidentally reversed.

>On the other hand we've also had issues with a bad batch of MLC ceramics
>doing similar tricks (going low ohm s/c after shipment). Both can cause major
>problems in the field when fed from high-current power supplies.

Yes, when they crack, they may well fail short. Leaves a nasty burned spot
the on the circuit board when they are SMT parts, too. Ceramics in general
have a fraction the capacitance of similar sized tantalums.

The amount of current which a big 5V supply can provide to a board is a
hazard in many ways - which basically has no circuit protection when you
put a lot of soldered posts connecting to the VCC and ground planes. Take
40 Amps or so at 5V, you have a pretty effective welder...


regards,

Jerry Cupples
Interphase Corporation
Dallas, TX
http://www.iphase.com/