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|Development of High Temperature Tantalum Polymer Capacitors|
|Keywords: Conducting polymers, Tantalum polymer capacitor, High temperature stability|
|Tantalum Polymer capacitors are increasingly being used for applications demanding high reliability in automotive, medical, military and space systems. In this paper, polymer tantalum capacitors with conducting polymers as cathode materials were evaluated for high temperature capability. This paper focuses on the development of new 150°C capable surface mount polymer tantalum capacitors and the enabling technologies. The conductivity stability of the conducting polymers at high temperatures as well as the equivalent series resistance (ESR) stability of the polymer tantalum capacitors at these temperatures were investigated in this study. Poly(3,4- ethylenedioxythiophene) (PEDOT) polymers with polymeric dopants such as high molecular weight polystyrene sulphonic acid (PSSA) has shown higher temperature stability compared to conducting polymers with low molecular weight dopants such as tosylate (Tos) anions.1 X-ray Photoelectron Spectroscopy studies showed loss of dopants for the PEDOT: TSA in-situ films on exposure to an 85°C environment. The enhanced thermal stability of PEDOT: PSSA slurry film is attributed to the protective effect of the PSSA shell around PEDOT. However, PEDOT: PSS based polymer films showed a decrease in conductivity under extended exposure at 150°C. A decrease in the protective effect and enhanced oxidation rate of the polymer at higher temperature contributes to this decrease in conductivity. This degradation in conductivity leads to ESR increase in polymer tantalum capacitors in high temperature environments. Advances in PEDOT materials and advances in packaging enabled 125°C capability 2 but more technological innovations were needed for enabling the capability to 150°C. Polymer oxidation occurs when oxygen enters the capacitors and finds its way into the conducting polymer through the mold epoxy and conducting layers such as carbon and silver. We started our developmental studies by analyzing the pathways for oxygen into the conducting polymer layers in the capacitor. Oxygen permeates into the conducting polymer at different rates in each capacitor depending on the materials used and the processing history. High temperature ESR shift variation in a typical sample of the capacitors is due to these differences in oxygen permeation rate. There are several primary and secondary pathways for oxygen permeation into the capacitor and into the conducting polymer. Primary pathways are the pathways through which oxygen from environment enters the capacitor through mold epoxy. Secondary pathways are the pathways through which oxygen in the encapsulant or mold epoxy find its way into the conducting polymer. There are three primary pathways for oxygen permeation into the package. The first one is the gap between lead frame egress and the mold epoxy. The second pathway is through the defects such as pin holes, show throughs, and cracks in mold epoxy. The third primary pathway is the oxygen diffusion through the bulk of the mold epoxy. Oxygen permeation through the egress and through defects occurs at a higher rate than through the bulk of the epoxy since oxygen encounters no tortuous path. Depending on the adhesion of the mold epoxy to the lead frame, these egresses can be larger or smaller. Secondary pathways of oxygen include oxygen permeation into interfacial gap in the cathode layers, cracks in cathode layers, and any exposed polymer layers. Oxygen permeation through cathode interfaces are high due to the gap between the interfaces. These gaps were generated by poor interfacial adhesion between the cathode layers or through delamination caused during reflow. Conducting polymers have a relatively higher coefficient of thermal expansion (CTE) than the tantalum or carbon filled coating and so these interfaces are prone to delamination due to CTE mismatch induced thermomechanical stresses. These interfaces can delaminate if the adhesion force between the layers is lower than the induced stresses. Permeation of oxygen through this delaminated interface can reach the conducting polymer at a faster rate and can cause oxidation at a faster rate. Residual volatiles in the capacitors can diffuse into cathode layers or diffuse out of cathode layers and these diffusion under thermomechanical stresses can cause cracks in the cathode layers. We have examined some of 150°C ESR failure parts from our developmental studies to determine the oxygen permeation paths in these parts. SEM cross sections of the high ESR parts showed cracks in the cathode layers. Oxygen permeation through the cracks in the cathode layers and subsequent polymer oxidation is responsible for the high ESR shift in these parts. Our analysis of the oxygen permeation pathways suggests that improvements in cathode layer material and interfaces can inhibit oxygen permeation through these pathways. These analyses led to the development of new material technology. 3 This technology involves new cathode and protective coating materials which significantly enhance interlayer crosslinking of cathode layers and the bond between the lead frame and mold epoxy. Interlayer crosslinking can be done with any reactive functional groups. Some examples of reactive functional groups are hydroxy and epoxy functional groups. These reactions result in formation of covalent bonds between cathode layers and mold epoxy and reduces the oxygen permeation through these interfaces. Interlayer crosslinks formed by the covalent bond between polymer chains on adjacent layers provide a tortuous path for oxygen permeation. These interlayer crosslinking results in decrease in oxygen permeation through several of the primary and secondary oxygen permeation pathways. The reduced interfacial gap and higher crosslink density between the cathode layers significantly reduces the oxygen permeation rate. High temperature storage and life test (bias) performance of these improved capacitors demonstrated excellent ESR stability of these capacitors. This new technology enables 150°C 2000-hour storage (unbiased) and life (biased) test capability and these new products are currently being commercialized|
|Antony P. Chacko, Technical Director
KEMET Electronics Corporation.