Silver-palladium fillers

Silver-palladium powders which are now used in conductive products are of two different types. One type is co-precipitated powders, wherein silver and palladium are both precipitated from solution to form a powder in which the two metals are intimately associated but do not form a true alloy. Another, more expensive filler, involves first forming a true silver-palladium alloy, and then pulverizing this to a fine powder. 

Metallic fillers can impart special properties to an epoxide formulation, such as giving response to magnetic fields and providing a radiation shielding capability. They are also used to enhance properties such as thermal conductivity and electrical conductivity and it is this last feature that makes them so useful in the electronics industry, where they find wide use in die attach materials. The principal metals used for enhancing electrical conductivity are silver, copper and gold, although various metal alloys such as silver/palladium have also been utilised. 

Filler Metals. Braze filler metals initially used for brazing titanium and its alloys were silver with additions of lithium, copper, aluminum, or tin. Most of these brazed filler metals were used in low-temperature applications (540 to 600 °C, or 1000 to 1100 °F). Commercial braze filler metals (see Table 6), including silver-palladium, titanium-nickel, tita-nium-nickel-copper, and titanium-zirco-nium-beiyUium, are now available that can be used in the 870 to 925 °C (1600 and... 

A new area of concern for electrical stability arises because of the increasing use of conductive adhesives as replacements for solder. Some conductive adhesives show unstable electrical-contact resistance when used on non-noble metal surfaces such as copper or tin-lead solder. Although stable on gold, palladium, platinum, and silver surfaces, the same adhesives were found to be unstable on tin, tin-lead, copper, and nickel surfaces.The unstable resistance and increase in resistance in temperature-humidity exposures have been attributed to the growth of an oxide layer separating the filler particles from the substrate at the interface, a mechanism similar to that for the loss of backside contact in die-attach materials. 

Reliability. Silver-filled ICAs have a potential for silver migration, which cause electrical shorts especially in fine pitch applications. A palladium-treated silver filler exhibited a much improved antimigration characteristics compared to standard silver-filled ICAs. However, the preparation of the Pd-coated silver particles is expensive. Some low cost approaches must be developed. More comprehensive understanding of the fatigue resistance of ICA joints is required. Activities in this area have been limited and nonconclusive. 

There is a whole range of silver-nickel and palladium-based braze fillers of high oxidation and corrosion resistance that have been developed for joining the nickel-rich alloys however, the presence of sulfur, lead, or phosphorus in the base-metal surface or in the filler can be harmful, since quite small amounts can lead to interface embrittlement. 

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