Growth and selection of intermetallic species in Sn-Ag-Cu No-Pb solder systems based on pad metallurgies and thermal histories (original) (raw)

2003, 53rd Electronic Components and Technology Conference, 2003. Proceedings.

The adoption of Pb-free solders will affect manufacturing processes and joint reliability for electronics packages. Since SAC solder has a higher melting temperature than eutectic Pb-Sn solder, higher processing temperatures will be required. The higher processing temperatures allow for increased solubility of some elements, such as Ni, within the solder melt. The specific selection of pad metallurgies can influence both the type of intermetallics that form at the pad interfaces, and the age related evolution of those intermetallics. In joints where both Nt and Cu pad metallurgies are present, the fast diffusing Cu species can allow the intermetallic compound, (Cu,Ni)6Sns to form at the opposite, Ni, interface. The absence of a Cu pad can starve the system of Cu and effectively stop the growth of this (Cu,Ni)& intermetallic compound. The Ag component of the SAC solder can, under proper conditions, result in the growth of AgJSn plates in the solder melt before the Sn phase nucleates. This is controlled by the specific concentration of Ag in the solder, by the coolmg rate of the solder, and by the tendency of the solder joints to undercool by a typical 30°C before the Sn phase nucleates. The critical cooling rate for the onset of this phenomenon is between 3'Cisec. and 0.3"C/sec., within the range of normal reflow practices of the electronics industry. 101 Relatively small amounts of additional elements such as Cu or Ag are added to Sn to reduce the melting point of the solder alloy, and improve its mechanical and wetting properties. These constituents dramatically affect the mechanical properties of the solder, and possibly the reliability as well. Therefore it is important to understand the evolution of the distribution of these constituents during reflow and long term aging, including changes in the morphology of the intermetallic compounds (IMC's) formed along with Sn by these elements. In SnAg -Cu alloys, Cu&n5 and Ag3Sn form in the equilibrium state, with distinct morphologies, platelets for AglSn and hollow, hexagonal rods for Cu6Sn5. [l-71 We consider the evolution of IMC's during reflow, particularly Ag3Sn, including cooling to the solid state, and during long term aging.