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Journal of ELECTRONIC MATERIALS, Vol. 42, No. 8, 2013
DOI: 10.1007/s11664-013-2609-9
2013 TMS
Effect of Thickness and Phosphorus Content on Au/Pd/Ni(P) Metal Finish of Printed Circuit Board
CHIH-KAI HUANG,1 KEH-WEN LIN,2 YU-MING HUANG,2ALVIN R. CAPARANGA,1,3 RHODA B. LERON,1 and MENG-HUI LI1,4
1.Department of Chemical Engineering and R&D Center for Membrane Technology, Chung Yuan Christian University, Chung Li City 32023, Taiwan, ROC. 2.OMG (Asia) Electronics Company Limited, Chung Li City 32053, Taiwan, ROC. 3.School of Chemical Engineering and Chemistry, Mapa Institute of Technology, Intramuros, 1002 Manila, Philippines. 4.e-mail: [email protected]
Electroless nickel/electroless palladium/immersion gold [Au/Pd/Ni(P) or ENEPIG] pads consisting of layers of Ni(P) (200 lin), pure palladium (Pd)
or palladium phosphorus (PdP) (2 lin, 4 lin or 6 lin), and gold (Au) (2 lin or 4 lin) were prepared using two different processes (wire bonding and lead-free soldering). Each of these processes was done with zero- or two-time reow. Different tests on solderability, wettability, wire-bonding capacity, and corrosion resistance were performed on different combinations of ENEPIG pads formed using different combinations of processes and conditions. Scanning electron microscopy was also performed to examine the surface characteristics of the pads. It was found that the ENEPIG pad sample with the 4-lin-thick Au and 4-lin-thick PdP layers possessed stable wire-bonding capacity and excellent lead-free solder reliability. In addition, the ENEPIGPdP systems showed better corrosion resistance, which is attributed to the presence of the amorphous PdP layer protecting the nickel layer.
Key words: ENEPIG, PCB, corrosion, wire bonding, lead-free soldering
INTRODUCTION
Of the ve types of metal nish used on printed circuit board (PCB) in the electronics industryhotair solder level, immersion tin, immersion silver, organic solderability preservatives, and electroless nickel/immersion gold [Au/Ni(P) or ENIG]the last one (ENIG) has been the most widely used. However, the immersion gold process has been reported to cause corrosion on the nickel layer (i.e., oxidation of Ni0 by Au2+), resulting in the formation of so-called black pads.15 The black-pad defect is problematic because it can result in solder joint failure and low shear strength after assembly. Nonetheless, after understanding the black-pad phenomenon and the virtual elimination of black-pad defects, the ENIG process still persisted.6
Recently, however, the electroless nickel/electroless palladium/immersion gold [Au/Pd/Ni(P) or ENEPIG] process has provided an alternative to ENIG. This can improve...