THE CASE FOR ELECTROLESS NICKEL/ELECTROLESS PALLADIUM/IMMERSION GOLD (ENEPIG) AS A SURFACE FINISH

摘要

The electronics industry has had interest in new and improved surface finishes for substrates, components and packages, because they function not only in the formation of the solder joint by wetting solder, but have an influence on the long term reliability of the solder joint. The requisite higher soldering temperatures of lead free alloys have renewed the interest in many companies to study surface finishes and to make changes. Requirements continue to change with more complex assembly processes, which now can include wave solder, surface mount and wire bonding on a single product. This paper will present the case for the use of deposits of electroless palladium (0.1 um) and immersion gold (0.03 um) over an electroless nickel underlayer (5 um), abbreviated as the acronym ENEPIG. One appeal of ENEPIG is that it has been shown to meet the demand for a surface finish offering both excellent solderability for circuit boards, and wire bonding capabilities. Since palladium is plated from an electroless bath containing a reducing agent, the surface composition of the underlying electroless nickel deposit is preserved. Costs of ENEPIG coatings compare favorably with those of other surface finishes capable of meeting the demand for both solderability and wire bonding, such as electrolytic nickel/electrolytic gold or electroless nickel-electroless gold. For circuit applications, material costs are similar to ENIG. The paper will describe the plating process and deposit properties, all of which contribute to the excellent characteristics of the composite coating as a surface finish. Measurements of solder spread, ball pull and shear strength after artificial aging conditions are presented to demonstrate the capabilities of ENEPIG for solderability and solder joint strength.