Surface Mount International Conference Proceedings


Author: Zequn Mei
Company: Hewlett Packard Company
Date Published: 8/23/1998   Conference: Surface Mount International

Abstract: Multi-layer surface finish, from the bottom to top, of Electroless Ni, electroless Pd, and immersion Au (Ni/Pd/Au) is being introduced in printed circuit board (PCB) industry recently. This paper reports an evaluation of this surface finish from the perspective of solder joint attachment reliability, especially to see if the Ni/Pd/Au could be immune from the brittle interracial fracture of PBGA on electroless Ni / immersion Au, recently observed and reported by us. PCBS with Ni/Pd/Au finishes, made from two vendors (A and L) with varied Pd layer thickness were attached with PBGA packages, and tested in four point bending. The results were compared with other metal finishes for reference, organic coated Cu (OCC), hot air level (HAL) Sn-Pb on Cu, electroless Ni / immersion Au, and electrolytic Ni / electrolytic Au. When joint strength is strong, bending tests resulted in peeling off the PCB pads; otherwise, brittle fractures occurred at the interface between solder balls and PCB pads. Solder joints on the Ni/Pd/Au made by vendor A, like electroless Ni / immersion Au, failed by a brittle fracture at the interface between Ni-Sn intermetallic compound and Ni-P alloy. Solder joints on the Ni/Pd/Au made by vendor L are strong, like OCC, HAL, and electrolytic Ni / Au, unless the Pd layer was too thick, e.g. > 30 micro-inches. The formation of a thick and dense PdSm layer caused a brittle fracture at the interface between the PdS~ layer and Pd-Ni alloy. After aging, e.g. 150”C for a few days, solder joints on all Ni/Pd/Au and reference metal finishes, failed by the same brittle fracture at the interface between Ni-Sn and Au-Sri intermetallic compounds. During aging, the Au-Sri intermetallic compound, once resided inside the solder joints, deposited at the interface between Sn-Ni intermetallic compound and solder, making the interface brittle. It is concluded that the interracial fracture was controlled by something else other than the Pd, and the existence of the Pd did not prevent the interracial fracture. Also, the presence of Pd could not prevent the Au migration and subsequent fracture.

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