A Nano Silver Replacement for High Lead Solders in Semiconductor Junctions

While it is now widely accepted that most electronic assembly can be reliably effected with lead-free solders, a practicable alternative to the high-lead high-melting-point solders has not been available. That reality has been acknowledged by the interim exemption from the requirements of the EU RoHS Directive granted for solders with 85% or more of lead. With no direct replacement yet found by conventional alloying of elements permitted by the RoHS Directive the search for a replacement for these high-lead solders has extended to alternative joining materials. One approach has been to take advantage of the reactivity of nano particles of silver to make a product that while ultimately having a melting point at or near the silver melting point of 961.8°C can combine to form reliable connections at temperatures much lower than that. The challenge in this approach is that the very reactivity that makes the formation of a joint possible at a relatively low temperature means that the nano silver tends to be unstable. In this paper the authors report the development of a unique nano silver material that is manufactured and stabilized in an alcohol environment to produce a material that can be used to make reliable joints between a wide range of the substrates commonly used in electronics in process conditions similar to those used with high-lead solders. This material can be used to make joints to ferrous materials (e.g. stainless steel) as well as non-ferrous materials such as copper and nickel. And most importantly for component manufacture this new material bonds strongly to semiconductor materials such as silicon. Where even longer life in thermal cycling is required the silver structure can be reinforced by the addition of other materials in the form of particles of the appropriate size. The paper will include details of mechanical and reliability testing of joints made with these materials under a range of temperature, pressure and atmosphere conditions.