Electronic Restoration >> Fire Damage Restoration For Electronics

Figure 11 shows a portion of the metal lead frame strip to which the individual capacitor elements will be attached in the early stages of the capacitor assembly process. The precise dimensions of the lead frame vary from manufacturer to manufacturer, but the function is the same. The leadframe provides a means to process many capacitor elements at the same time to improve Fire Damage Restoration For Electronics efficiency during mass production.

Small dollops of conductive silver adhesive have been placed on the offset metal surfaces of the lead Fire Damage Restoration For Electronics frame in preparation for insertion of the individual capacitor elements. This adhesive will subsequently be cured at elevated temperature to make a low-resistance connection between the leadframe metal and the silverpaint layer of each capacitor element. 

Uniformly-spaced guideholes appear in the leadframe. They exist toaid transport and provide precise positioning during the various manufacturing steps. In Figure 12, one sees capacitor elements electrically connected to the leadframe. The positive connection Fire Damage Restoration For Electronics is made by resistance welding of the anode wire to the leadframe. 

The negative connection is made bycuring the silver adhesive which forms a conductive layer between the leadframe and the silver paint on theelements. The silver adhesive is cured by heating until the epoxy binder cross links. During the curing process, Fire Damage Restoration For Electronics there is some shrinkage which draws the silver particles together and into contact with the surfacesof the leadframe and silver paint. 

The next step is to mold a protective epoxy case around each element. The result appears in Figure 13.The molded epoxy cases have been laser marked and the individual capacitors have been electricallyisolated from each other to facilitate electrical testing. Each capacitor can now be aged Fire Damage Restoration For Electronics and electrically tested. 

These specific capacitors were part of an experiment, Fire Damage Restoration For Electronics and evidence of multiple probe contacts is easily observed on the lower (negative) terminal.One capacitor is missing in Figure 13. This capacitor did not meet specifications and was removed fromthe leadframe strip during electrical testing. 

During high-volume production, sub-standard capacitors arephysically removed to prevent them from being shipped to a customer.After electrical testing, Fire Damage Restoration For Electronics the surviving capacitors are cut from the leadframe strip. The terminal metal thatexits from the molded case is trimmed to a suitable length and is then bent and formed around the bottomedge of the capacitor. 

This step is performed on both terminals and the capacitor is inserted Fire Damage Restoration For Electronics and sealed inside a standardized plastic carrier tape. The carrier tape is designed to feed hundreds or thousands of capacitors to automated equipment that places the capacitors on customers' circuit boards. The capacitorsappear in the carrier tape in Figure 14. 

One capacitor has been removed from the tape and inverted to showits surface mount terminals. Manufacturers of tantalum polymer capacitors often make available to their customers typical electrical performance and reliability data. Occasionally, such data can also be found in technical papers. It can be reasonably assumed that these public data are selected to show the capacitors in a generally favorable light, Fire Damage Restoration For Electronics but nevertheless they provide much useful information. 

In this section, some typical performance datafrom several manufacturers will be presented to give a basic overview of the performance characteristics oftantalum polymer capacitors. Unfortunately, Fire Damage Restoration For Electronics there is considerable variability in the test conditions as well as the format of the availabledata. Moreover, there is no direct overlap of specific part types represented (capacitance and ratedvoltage). 

Thus little competitive information can be gleaned from these data, but much general informationcan be gathered from the various part types and test conditions. In the FY06 continuation project, Fire Damage Restoration For Electronics capacitors of the same part types will be anonymously purchased from a variety of suppliers. Thesecapacitors will be subjected to uniform testing to provide a better understanding of the relative performanceand reliability of several manufacturers' product.

The presently available data will be presented in four categories. The first data category is typical electricalperformance which is intended to highlight the "as manufactured" electrical behavior of tantalum polymer capacitors. The second data category is typical dielectric robustness which Fire Damage Restoration For Electronics is intended to demonstrate thegenerally superior breakdown performance of tantalum polymer capacitors in high inrush currentapplications. 

The third data category is reliability performance which is intended to highlight differences in expected failure rates and distribution of times-to-failure between tantalum polymer capacitors and MnO2-based tantalum capacitors. The last data category is environmental stability which is intended to highlight shifts in electrical performance after initial exposure to environmental Fire Damage Restoration For Electronics and reflow conditions as well as afterlong-term exposure to heat, voltage, and humidity.

Typical Electrical PerformanceThe statistical distribution of 120Hz capacitance among 12 capacitors in a sample of 150µF, 6.3V Vishay Sprague tantalum polymer capacitors appears in Figure 15. These capacitance values are not significantly different Fire Damage Restoration For Electronics from what would be expected for tantalum capacitors manufactured with MnO2 cathode material.The median value of the distribution falls somewhat short of the nominal capacitance value, but all devicesfall within +/-10% limits.

It is not unusual for the median capacitance of tantalum capacitors of all vendors to fall somewhat short ofthe nominal capacitance value. One reason is that almost all efforts to improve device robustness result inreduced capacitance, Fire Damage Restoration For Electronics so the primary focus is often placed on simply exceeding the minimum tolerance limitwhile providing the most robust design. 

Another related reason is that there is no economic incentive to put any more expensive tantalum powder into a design than is absolutely necessary to meet specification limits. 120Hz dissipation factor (DF) data appear in Figure 16 for the same 12 tantalum polymer capacitors whose capacitance appears in Figure 15. Again the Fire Damage Restoration For Electronics data are no different than would be expected for an MnO2-based tantalum capacitor. 

At 120Hz, the dissipation factor is largely governed by dielectric loss rather thancathode conductivity, Fire Damage Restoration For Electronics and the dielectric is the same for both styles of tantalum capacitor. The DF limit forthese capacitors is 12% and it is clear that these devices comfortably comply. The DC leakage current data for the Vishay-Sprague capacitors of Figures 15 and 16 appear in Figure 17.

The leakage values are log-normally distributed and the spread of the distribution is similar to the spread ofDC leakage values for MnO2-based capacitors. However, Fire Damage Restoration For Electronics the median value of leakage is much higher forthese tantalum polymer capacitors than would be normal for MnO2-based tantalum capacitors.

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