?2005 kemet electronics corp. may 2005 product update ? t498 hi gh temperature (+150c) tantalum smt capacitors the tantalum smt capacitor in its solid-state structure is typically rated as capable of 125c applications. the t498 tantalum surface mount capacitor has a maximum temperature rating of 150c. the difference between the t498 and the standard tantalum capacitors lies within its material set and design. the materials changed include the carbon, the silver epoxy, the silver paint, the plastic molded encapsulant, and the leadframe. these materials still contain no lead and are fully rohs 1 compliant. the standard finish on the leadframe is a matte tin plating over nickel, and the device is capable of the 260c reflow pro- files as defined in j-std-020c 2 . a gold or tin-lead finish is also available. figure 1. structure of ne w t498 tantalum capacitor. the appearance of this devi ce is radically different from the previous smt tantalum capacitors from kemet, in that the color is no longer gold with brown lettering. the gold material has a heat activation of color change from yellowish gold to brown created in the presence of a controlled surface temperature rise. a laser flash through a mask is used to create the polarity and identifying marks on the gold plastic. using this material and process in the presence of higher heats created an effect where the entire surface of the package was turning brown, and the mark- ings became indiscernible. the black compound has no discernable change in col- oration when exposed to extended temperatures above 125c. the marking on the package for the polarity stripe, capacitance code, voltage rating, the kemet logo, and the print week code (pwc) are still created with a laser in a dot matrix pattern. the effect does not create a coloration change, but creates a surface abrasion as the laser removes the reflectivity of the plastic surface in the desired pattern. figure 2. photograph of ?d ? case, t498 smt capacitors. through oblique lighting and vision systems, the mark- ings can be quite revealing as shown in figure 2. an ex- planation for the pattern with the markings is shown in figure 3. figure 3. component marki ng diagram for t498 capacitor. kemet electronics corp. p. o. box 5928 greenville, sc 29606 (864) 963-6300 www.kemet.com
?2005 kemet electronics corp. may 2005 the carbon, conductive epoxy, and the silver paint ma- terials were chosen for the best offerings that would allow the device to exist in the 150c environment without deg- radation. the tantalum anode structure, the tantalum- pentoxide, and the mno 2 cathode system have been proven to withstand this temperature exposure without degradation. voltage rating the voltage rating of a component is fixed so as to cre- ate an acceptable failure rate at accelerated life conditions. inability of a device to meet that criteria may cause the voltage rating of the component to be reduced, or the com- ponent to be redesigned (thicker dielectric) to allow for that failure rate to be achieved. accelerated life testing of this component at 150c and with 2/3 nameplate voltage applied has shown the failure rate to be less than 0.5% per thousand-piece-hours. fail- ures were designated by positional fuse failure or paramet- ric shifts beyond the initial limits. the temperature-voltage derating of this device is slightly different from the standard tantalum smt capaci- tor. these devices were created using thicker dielectrics for the voltage ratings than is required for the standard product line. figure 4. voltage-temperature derating for standard and t498 ca- pacitors. for both the t498 and the standard tantalum capaci- tors, the voltage rating up through 85c is the same as the nameplate voltage for the capacitor. for the standard ca- pacitors above 85c, the voltage rating is linearly reduced from 100% of nameplate voltage at 85c, down to 2/3 rd (67%) of nameplate voltage at 125c. for the t498 ca- pacitors, the voltage rating is linearly reduced from 100% of nameplate voltage at 85c, down to 2/3 rd (67%) of nameplate voltage at 150c. this allows the 125c rating for the t498 to be about 80% of nameplate voltage. application derating we will use the guidelines established with the long history of the commercial tantalum product to fix the rec- ommended application at no more than 50% of the rated voltage. for a t498 rated at 50 vdc, this would create a recommended application of 25 vdc. this application would then apply up to 85c, at which point the tempera- ture-voltage derating requir ements effectively lower the voltage rating of the part. at 150c, this 50 vdc has a temperature-voltage derated rating down to 33 vdc, and following the 50% application guides, the recommended maximum application is the 17 vdc. it is very important to consider the failure rate at rated voltage and 150c is listed as 0.5% per thousand-piece- hours; but that application derating will allow for an ap- preciably reduced failure rate. using the voltage factor calculations from mil-hdbk-217f 3 , at rated voltage the multiplying factor for the failure rate is 5,909. compared to ?50% of rated? factor of 1.045, then the improvement in failure rate at 150c would be down from the 0.5% level to 884 parts per billion-piece-hours. remember though, the rated voltage is changing with temperature as shown in figure 4. at temperatures up through 85c, the recom- mended application voltage is 50% of nameplate voltage. above 85c, the recommended application voltage is 50% of the temperature-voltage derated level. for a standard tantalum at 125c, the recommended application voltage is 50% of 67%, or 33% of the nameplate voltage. for a t498 at 125c and 150c, the application becomes 40% and 33%, respectively, of nameplate voltage (figure 5). figure 5. recommended application voltages versus temperature. power rating the power rating for capacitors is reflective of the al- lowable heat generated in the device and there is a direct correlation between these two. without a standard tem- perature rise defined, the majority of manufacturers use the
?2005 kemet electronics corp. may 2005 +20c internal rise and an arbitrary figure in defining the power capability for these devices. this arbitrary rise added to the ambient temperature creates the absolute in- ternal temperature of the component. since capacitors are life tested under dc or static stress, there is no temperature rise at the maximum rated temperature of the device. only by using the positive tolerance of +2c at this temperature, can we define a ?tested capability? at this temperature ex- treme. we then need to look at the difference between the temperature extreme and the assi gned or arbitrary rise of +20c, to calculate the point at which a power derating is applied. for 150c, and an allowable rise of +20c, the power derating must begin at 130c. the power capabil- ity (allowing a +20c rise) for this device is the same from -55c through 130c. if the case power is defined as 150mw, then the power capability is defined as 150mw for this temperature range. th ere is a linear reduction in that power capability then applied from 100% at 130c, down to 15 mw (10% or 2c/20c) at 150c. figure 6 shows this delineation. figure 6. power derating to maximum temperatures. the allowable temperature rise is arbitrary and two considerations must be weighed when choosing this figure. first, the internal temperature rise plus the ambient must never exceed the maximum temperature plus 2c. to do so would create an environment in which there is no reli- ability data to justify this application. second, the rise must be considered as a potential thermal shock condition when the device is at ambient temperature and immedi- ately after power is applied. deltas in excess of +50c may lead to thermal gradients that could induce stresses high enough to cause an internal fracture and failure. it is evident from the plot of figure 6 that the differ- ence in these two types of cap acitors creates entirely dif- ferent power capabilities between 105c and 125c. for example, the power dissipation for the standard tantalum at 125c is down to 10% of the case defined power capa- bility, while the t498 shows a capability at this tempera- ture of 100% of the case defined power. consider that these are two ?d? case units and the actual power capabil- ity here is 15 mw for the standard and 150 mw for the t498. for devices of equal capacitance and esr, the rip- ple capability for the t498 would increase by a factor of 3.16 (square root of 10). application areas the ideal applications for these components begin where the standard products? end. at temperatures be- tween 115c and 140c, these applications would still al- low a 10c margin or better, between the rating and the application. new under hood or in system applications may be considered with the t498 that were previously thought to be too precarious for the standard tantalum. 1 rohs ??r estriction o n the use of certain h azardous s ubstances in electrical and electronic equipment? (european union directive 2002 / 95 / ec) 2 j-std-020c ? ipc/jedec joint industry standard ? moisture/reflow sensitivity classification 3 mil-hdbk-217f ? notice 2, reliability prediction of electronic equipment, department of defense, december 2, 1991, washington, dc.
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