Anal and Preven. (2013)13: 194-201 DOI10.1007/sll668-013-96585 TECHNICAL ARTICLE-PEER-REVIEWED Failure Analysis of Un-Wetting for the Surface Finish on the enig Shi Yan. Fei-Jun Chen. Yue-Yue Ma Zhen-Guo Yang Submitted: 12 November 2012/in revised form: 7 January 2013/Published online: 6 February 2013 C ASM International 2013 Abstract Electroless nickel immersion gold (ENIG) together and provide the interconnection between them of the most prevailing surface finishes for printed The surface finish of PCB is one of the key factors which board. It is widely adopted by manufacturers all over the has a significant influence on its reliability performance. As world for its relative low price and compliance with the trend illustrated in Fig. l, surface is the bonding bridge of die of high density packaging. However, its reliability is always and substrate. Nowadays, electroless nickel immersion of a concern. In this paper, the poor wetting performance of gold(ENIG) is the most popular surface finish adopted by tin solder on the ENG surface, which is one of its reliability nearly all the PCB manufacturers around the world [3]. The issues was addressed. A series of modern facilities were primary process of ENIG is to deposit a layer of nickel and utilized for finding the failure mechanism and chasing down then a layer of gold onto the PCB substrate, as exhibited in the root cause. It was concluded that the inferior quality of formulae (1)and(2). According to IPC-4552 standard, the immersion gold layer during plating, to be more specifically, nickel layer should be between 3- and 6-um thick and the coarse and big grain size and the crystalline interface golden layer be 50-100-nm thick to ensure its reliability cracking were the main causes for the un-wetting failure. The specific thickness of ENIG should be determined by Last but not least, the improper choice of pure tin solder special circumstance in which the product will be applied should also contribute to the final failure The golden layer is employed for maintaining the solder ability during manufacturing process like reflow and wave Keywords ENIG. Un-wetting. Grain size soldering. In some situation, such as Golden Finger, the Failure analysis. Oxidation golden layer is utilized for providing anti-corrosion and abrasion resistance property. In another application, man- ufacturer will bond aluminum or golden wires onto the Introduction golden surface to realize the electric interconnection between the pcb and outer circuit. nickel is used as a Printed circuit board(PCB)is the foundation of all kinds of barrier preventing atoms in the substrate material from electronic devices ranging from cell phone, personal diffusing into the golden layer and forming intermetallic computer to military products. With the booming devel- compound(IMC) opment of integrated circuits (C), there is increasingly Ni2++2H,PO +2H20- Ni+2HPO3-+4H*+H2 t stringent requirements placed on reliability of PCB [1, 2] since it is the platform which connects all the chips 2Au(CN)2+Ni- 2Au+Ni++ 4CN S. Yan(), F-J. Chen.Y.Y Ma. Z.-G. Y Materials Science, Fudan University, Shanghai In this paper, a batch of PCB sample finished with ENIG Departme 200433, was found to display poor wetting performance after hot air e-mail:11210300039@fudan.edu.cn solder leveling. For the purpose of z.一G.Yang mechanism, locating the root cause and finally improving zgyang@fudan.edu.cn reliability, a series of modern analytical instruments suchTECHNICAL ARTICLE—PEER-REVIEWED Failure Analysis of Un-Wetting for the Surface Finish on the ENIG Shi Yan • Fei-Jun Chen • Yue-Yue Ma • Zhen-Guo Yang Submitted: 12 November 2012 / in revised form: 7 January 2013 / Published online: 6 February 2013 ASM International 2013 Abstract Electroless nickel immersion gold (ENIG) is one of the most prevailing surface finishes for printed circuit board. It is widely adopted by manufacturers all over the world for its relative low price and compliance with the trend of high density packaging. However, its reliability is always of a concern. In this paper, the poor wetting performance of tin solder on the ENIG surface, which is one of its reliability issues was addressed. A series of modern facilities were utilized for finding the failure mechanism and chasing down the root cause. It was concluded that the inferior quality of immersion gold layer during plating, to be more specifically, the coarse and big grain size and the crystalline interface cracking were the main causes for the un-wetting failure. Last but not least, the improper choice of pure tin solder should also contribute to the final failure. Keywords ENIG Un-wetting Grain size Failure analysis Oxidation Introduction Printed circuit board (PCB) is the foundation of all kinds of electronic devices ranging from cell phone, personal computer to military products. With the booming devel￾opment of integrated circuits (IC), there is increasingly stringent requirements placed on reliability of PCB [1, 2] since it is the platform which connects all the chips together and provide the interconnection between them. The surface finish of PCB is one of the key factors which has a significant influence on its reliability performance. As illustrated in Fig. 1, surface is the bonding bridge of die and substrate. Nowadays, electroless nickel immersion gold (ENIG) is the most popular surface finish adopted by nearly all the PCB manufacturers around the world [3]. The primary process of ENIG is to deposit a layer of nickel and then a layer of gold onto the PCB substrate, as exhibited in formulae (1) and (2). According to IPC-4552 standard, the nickel layer should be between 3- and 6-lm thick and golden layer be 50–100-nm thick to ensure its reliability. The specific thickness of ENIG should be determined by special circumstance in which the product will be applied. The golden layer is employed for maintaining the solder ability during manufacturing process like reflow and wave soldering. In some situation, such as Golden Finger, the golden layer is utilized for providing anti-corrosion and abrasion resistance property. In another application, man￾ufacturer will bond aluminum or golden wires onto the golden surface to realize the electric interconnection between the PCB and outer circuit. Nickel is used as a barrier preventing atoms in the substrate material from diffusing into the golden layer and forming intermetallic compound (IMC). Ni2þ þ 2H2PO 2 þ 2H2O ! Ni þ 2HPO2 3 þ 4Hþ þ H2 " ðEq 1Þ 2Au CN ð Þ 2 þNi ! 2Au þ Ni2þ þ 4CN ðEq 2Þ In this paper, a batch of PCB sample finished with ENIG was found to display poor wetting performance after hot air solder leveling. For the purpose of finding the failure mechanism, locating the root cause and finally improving reliability, a series of modern analytical instruments such S. Yan (&) F.-J. Chen Y.-Y. Ma Z.-G. Yang Department of Materials Science, Fudan University, Shanghai 200433, China e-mail: 11210300039@fudan.edu.cn Z.-G. Yang e-mail: zgyang@fudan.edu.cn 123 J Fail. Anal. and Preven. (2013) 13:194–201 DOI 10.1007/s11668-013-9658-5