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Assessment on the effects of electroless nickel plating on the reliability of solder ball attachment to the bond pads of PBGA substrate

Authors Yan, Chien Chun
Issue Date 2000
Summary The plastic ball grid array (PBGA) is an IC packaging technology developed in late 80s. This package has many advantages over conventional modules. Among them are high I/O, low profile, good thermal/electrical performance, and robust assembly process. Furthermore, PBGA packages are compatible with surface mount technology (SMT). For surface mounted components (SMCs), the solder joints not only provides the passage of electrical signal and power, but also the mechanical support to hold the module in position on the printed circuit board (PCB). Therefore, the solder joint reliability is a major concern for PBGA packages. Currently the standard package configuration of PBGA is to place eutectic Pb/Sn solder balls on the bond pads at the bottom side of BT substrate. The surface finishing metallurgy on the solder bond pads is Ni/Au deposited by electrolytic plating. However, since this process requires routing traces to connect all bond pads together, a considerable amount of space is wasted on the BT substrate (which is relatively expensive). In order to avoid this deficiency, an electroless Ni/Au plating process was recently proposed for PBGA substrates. However, some previous studies indicate that, due to the relatively high phosphorous (P) concentration (usually 9-10%) in the electroless Ni plating solution, substantial intermetallic compounds are formed. Such brittle materials could reduce the shear strength of PBGA solder balls and lead to problems with solder joint reliability. Possible remedies to this problem may be the use of low phosphorous-content (5%) solution, or electroless Nickel-Boron (Ni-B) plating solution. This thesis aims to assess the reliability of solder ball attachment on the PBGA substrates with various bond pad plating schemes, namely, electrolytic Ni/Au, electroless Ni-l0%P/Au, electroless Ni-B/Au, electroless Ni-5%P/Au and electroless Ni-l0% P/Pd/Au. At first, BT-substrates are fabricated with appropriate thickness. After the solder ball placement and normal reflow process, mechanical tests are performed to evaluate the solder ball shear strength. In order to simulate the real industrial process, multiple reflows are conducted to investigate the long-term reliability issues. The experimental results indicate that the newly proposed electroless Ni plating solution can indeed reduce the intermetallic compound and improve the solder ball shear strength.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2000
Language English
Format Thesis
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