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PCB007-Jan2020

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86 PCB007 MAGAZINE I JANUARY 2020 sults of EDS, when using EPIG surface finish, Cu 6 Sn 5 was formed as layer 1, and Cu 3 Sn was formed near Cu layer as layer 2. This solder joint is similar to immersion silver, immersion tin, and OSP solder joints, and is expected to demonstrate similar solder joint reliability. Gold Wire Bonding EPIG is a gold wire-bondable finish. Excellent bond strength and wire break location were eas- ily achieved with the "as-plated" EPIG. However, after heat treatment for 16 hours at 175°C, there was a drop in pull strength and the correspond- ing wire break location. The drop is affected di- rectly by the diffusion of copper and palladium to the bonding surface during heat treatment. The thickness of the palladium and gold layers have a direct impact on bond strength. To in- crease wire bond strength, the thickness of the EP and the IG layers have to be optimized. Cu was detected after heat treatment when EP film was thinner; however, no peak of Cu was detected when the EP thickness increased. A thicker Pd deposit was effective in prevent- ing copper diffusion to IG surface. Increasing the EP thickness to 0.15 µm will act as a diffu- sion barrier to copper from the substrate. The diffusion of Pd to the surface of the IG layer after heat treatment was not dependent on the Pd thickness and was greatly dependent on the Au thickness. A thicker IG layer will reduce the ratio of EP in the IG surface. The diffusion of EP to the top surface of the IG is mitigated by increasing the thickness of the IG to 0.2 µm. In addition, there is a marked improvement in the strength and wire break location when performing a wire pull test after plasma treat- ment. It was confirmed by AES analysis that any Cu diffused to the surface was removed after plasma treatment. Conclusion EPIG is a solderable and gold wire bondable surface finish. EPIG was introduced around 2016 and is gaining popularity as circuit min- iaturization continues with tighter spacing between the traces. The elimination of thick EN mitigates Ni corrosion, reduces processing time, and eliminates the skin effect signal loss associated with propagating high-frequency RF signals. The absence of thick nickel makes EPIG ideally suited for flex applications where bending may cause nickel fractures. PCB007 George Milad is the national accounts manager for technology at Uyemura. To read past columns or contact Milad, click here. Brook Sandy-Smith, IPC's tech- nical conference program man- ager, highlights how attendees can learn about and test drive new equipment, materials, and solutions by visiting the nearly 500 exhibiting companies and enjoy the welcome reception and ice cream social on the show floor. Click the image to watch the video. (Source: IPC) IPC APEX EXPO Show Floor Highlights

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