Silver Migration and Reliability of Pd/Ag Conductors in Thick-Film Dielectric Crossover Structures
The relative performance characteristics of thick-film Pd/Ag conductors (0-34% Pd) for dielectric crossover structures in hybrid microelectronic circuits are described. The materials were supplied from Du Pont and Engelhard. The experiments involved waterdrop (WD) tests and temperature-humidity-bias (THB) tests at 90°C, 90% RH, and a dc bias of 5-50 V applied between the conductor electrodes for periods up to 1000 h. WD tests showed that the rate of Ag migration decreased by approximately 100 times as the Pd content in the conductor was increased from 10%-19%. However, the results of these tests were considerably scattered for electrodes of high Pd content. In addition, no conclusive results could be reached on the effect of the substrate or various encapsulants on the rate of migration. The rate of migration under THB conditions was considerably lower (10-4times) than that under the water-drop tests. Surface migration was found to be the dominant mode of failure in dielectric crossover structures. The rate of migration increased with the increasing voltage gradient. When the voltage was increased it caused an abrupt change in the capacitance and isolation resistance. The migration was more pronounced on the surface of bare alumina substrates than on substrates covered with a dielectric layer. Finally, a Du Pont 9137 glass encapsulant was found to be effective in preventing silver migration under THB conditions while an ESL 240 SB encapsulant enhanced the rate of migration.
Available from: P. Nemeth
- "The reason why silver is more susceptible to migration than other metals is that it is anodically very soluble and its precipitates have also good solubility. The anodic dissolution behavior can be reduced by alloying the silver with palladium (Pd) and platinum (Pt)    . At special ternary Pd–Pt–Ag compositions, the migration can practically be eliminated . "
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ABSTRACT: Lifetime estimation is one of today's frequently used reliability testing tool. There are standardized test methods, mathematical apparatus and failure acceleration models for predicting failure rate of electronic components, circuit modules or equipment. Do the widespread used models give a precise description for all kind of failure mechanisms? Can they define the acceleration factor of any test, and the accelerated lifetime of any test vehicles? In connection with open surface migration tests (when the test circuit samples are not covered by any protective or packaging material) two climatic test methods (Thermal Humidity Bias––THB test methods with different parameter settings) have been compared: 40 °C/95%RH suggested by “a well known IEC standard” and 85 °C/85%RH required by the newer JEDEC standard. The novelty of the paper is the comparison between the two climatic test methods. The main conclusion is a suggestion to keep on with the old method in the mentioned particular case, which may be shocking for people who prefer the new standards. All conclusions are strengthened both with theoretical and experimental test results.
Available from: tycoelectronics.info
- "A great many experimentalists have shown that the higher the polarizing dc voltage (or " bias " ), the greater will be the rate and severity of the electromigration. In addition, an inverse relationship was almost always seen between the migration rate and the width of the interconductor spacing , , , . In a related extensive series of experiments, with redundent sets of three or more different spacings on the same board (Fig. 13), we found that the filaments or dendrites always began in the narrowest spacings , and, in most cases, were never found on any of the wider spacings until bridging had first " shorted out " a narrower spacing. "
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ABSTRACT: Metallic electromigration can be defined as the movement of
metallic material, usually through or across a nonmetallic medium, under
the influence of an electric field. As such, it has assumed increasing
importance in the performance and reliability of packaging systems that
incorporate electric contacts. The characteristics of the different
electromigration processes are discussed, including descriptions of
related phenomena. The primary emphasis is on electrolytically
controlled processes that take place under low power and typical ambient
Available from: sjsu.edu
- "The Ag migration is higher on the surface of bare alumina substrates than on the ones covered with dielectric layers. The Du Pont 9137 glass encapsulant was found to be effective in inhibiting Ag migration under THB tests as reported by Naguib et al . "
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ABSTRACT: Hybrid microelectronics utilizes conductive pastes to interconnect different discrete devices on a common substrate, which is usually Alumina. Silver Palladium (Ag-Pd) alloy is widely used as the metal inclusion component in conductive pastes. Positive characteristics of Ag include low cost and low sheet resistivity. However, Ag migration in un-desired areas is its major drawback. Anodic dissolution rate of Ag can reach 10-1A/cm2 that corresponds to a catastrophic removal rate of 35.6 nm/s (5). Silver migration can be classified as electromigration and ionic migration depending upon the environmental conditions of occurrence. The second migration is the utmost failure mode in thick film systems whenever the insulator separated the conductor acquires sufficient moisture from the ambient. The mechanism of Ag migration can be viewed as three steps process: electro dissolution, ion transport and electro deposition. Detail discussion on Ag migration mechanism is addressed in this paper. Silver migration is reduced as the content of Pd in the formulation increases. Water drop test showed that the rate of Ag migration decreased by approximately 100 times as the Pd concentration in the conductors was increased from 10 to 19% (4). The formation of PdO that requires a lower potential energy anode explains for the decrease in Ag migration with Pd content. The sufficient amount of Pd to inhibit Ag migration was reported 30%. At 5-15% Pd concentration in Ag-based alloy, Ag migration was still observed in the test structures. Optical micrograph for the Ag migration indicated three forms of silver compounds- dendrite, dark cloud like and the mixed of the two found on anodes and cathodes. The conductivity of the dendrite was 6.8 x 107 sm-1 while the cloud like was varying from 2.5 x 105 to 7.6 x 106 sm-1 depending on the Pd content in alloys. Effects of electrode spacing and type of encapsulant on Ag migration are also addressed in this paper. Silver migration altered the dielectric properties, and reduced the insulation resistance. Silver migration causes short circuits and promotes devices failure especially under the high humid environment.
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