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Pure Palladium and Palladium Phosphorus Depositions used in ENEPIG and ENEP Surface Finishes - Comparison of Physical Properties and Their Influence on Soldering and Au Wire Bonding
Keywords: ENEPIG, wire bonding, solder joint integrity
As a surface finish, electroless nickel / electroless palladium / immersion gold (ENEPIG) is receiving increasing attention for both Packaging/IC-Substrates and the PWB applications. With lower gold thickness than ENIG, the ENEPIG finish offers the potential for higher reliability, better performance and reduced cost. One subtle difference in the ENEPIG processes available in the market pertains to the deposition of electroless palladium. The electroless palladium layer in ENEPIG can be deposited either as a palladium phosphorous alloy (PdP) or as pure palladium. The deposition mechanism may be similar, because both can be deposited in an electroless manner (autocatalytically), but the physical properties are quite different. As part of these investigations, the physical properties that were investigated included hardness, solder wetting properties and crystal structure. The differences in the physical layer properties also have an influence on assembly-related matters, such as solder spread characteristics, solder joint integrity, intermetallic compound (IMC) properties, as well as wire bonding performance. This paper will discuss and compare the properties and performance of the two ENEPIG processes (with pure Pd and Pd-P) regarding both soldering and wire Bonding. The paper summarizes an in-depth study of results from wire bonding tests and micro-section analyses. The results are evaluated to allow the comparison of the pure palladium and PdP deposits in ENEPIG layers with respect to their physical properties, as well as their performance in Au wire bonding. The results of investigations include: (1) wire bonding test results with both pull forces and fracture modes. (2) micro-section and FIB cuts through ENEPIG layers, (3) comparison of the crystal structure, (4) comparison of the different Pd deposit topographies, (5) cold ball pull testing to evaluate solder joint integrity, (6) IMC examinations to quantify nickel thickness degradation after multiple solder reflow cycles and (7) surface wetting through solder spread examinations.
Mustafa Oezkoek, Product Manager Surface Finishing
Atotech Deutschland GmbH
Berlin 10553,
Germany


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