Server: Netscape-Commerce/1.12 Date: Friday, 21-Nov-97 04:10:42 GMT Last-modified: Tuesday, 15-Jul-97 20:38:31 GMT Content-length: 6686 Content-type: text/html CONTROVERSY: TANTALUM VS. ALUMINUM CAPACITORS

CONTROVERSY: TANTALUM VS. ALUMINUM CAPACITORS

Recently there have been numerous articles in the news media concerning the substitution of aluminum capacitors for tantalum capacitors such as;

Al Cap Use In PC Motherboards Decried

April 28, 1997 - Electronic News

Time-bomb ticks in Pentium Motherboards
April 28, 1997 - EETimes

Are power supply caps a weak link in some PCs?
May 5, 1997 - Electronic Buyers News

Controversy Boils On Cap Quality
May 5, 1997 - Electronic News

This "controversy" was brought to light by a "warning" issued by Linear Technology Corporation about several mother-board manufacturers which had supply voltages out-of-spec from the use of aluminum capacitors in place of tantalum capacitors.

Although both tantalum oxide and aluminum oxide are excellent dielectrics, they differ greatly in the type of electrolyte used to make contact with the oxide. Almost all aluminum capacitors are manufactured with a "wet" electrolyte that greatly affects their performance with temperature and time. At temperatures below room temperature, the "wet" electrolyte becomes more resistive causing a high rise in ESR. With high temperatures and time, it is possible to cause the electrolyte to dry out. Tantalums, on the other hand, are "solid" electrolyte capacitors and the electrolyte is considerably less affected by temperature and time. Tantalums maintain their low ESR's over a wide temperature range and do not change with time.

However, there is another important difference that should not be overlooked. This is the parasite inductance. Aluminums are wound capacitors, that is, foil is wound with a porous insulator creating a fairly high inductor, around 25nH for small devices and much higher for larger parts. Small tantalums, on the other hand, run around 2nH to 5nH inductance. This parasitic inductance plays a major role in the decoupling of high speed microprocessors where it is extremely important to keep the inductance low. The general approach is to distribute ceramic capacitors close to the IC packages where they can supply immediate current requirements. These distributed capacitors must be recharged often and many at one time which requires large current to come from the mother-board power supply. Even though this is at a slower rate, msec vs. nsec, the large current still causes large voltage variation at the power source. The larger the inductance of the bulk capacitor, the greater the voltage variation. The five to ten times greater inductance of aluminum over the tantalum can cause the supply voltage to go out-of-spec.

The attached Figure 1 shows ESR versus frequency for a tantalum 10/16 and an aluminum 100/10. As is clear the tantalum is better for frequency behavior. Analysis of motherboards and power supplies has shown that because of the improved inductance and ESR a 100µF tantalum performs as well as a 1000µF aluminum for both ripple (Figure 2) and transient load change (Figure 3). While aluminums are cheaper than the same CV tantalum, they are less reliable, have worse temperature performance, worse frequency behavior and are larger in size. On top of all this there is now evidence of a 10:1 replacement rule which makes the cost differential much lower and in some cases non existent, see Figure 4 and Figure 5.

Most boards in the United States use tantalum capacitors, but in the Far East where aluminum capacitors are used for cost savings, out of tolerance designs are more prominent. At Pentium® switching speeds of +200MHz, the parasitic inductance of capacitors plus power supply plane and trace inductance becomes a far more significant contributor to power supply noise than the ESR of the caps. Board level inductance is a problem and should be kept as low as possible by using very low inductance capacitors as close to the processor as possible.




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