Pan Pacific Symposium Conference Proceedings


Authors: Kuldip Johal and Hugh Roberts et al.
Company: Atotech USA Inc.
Date Published: 2/25/2005   Conference: Pan Pacific Symposium

Abstract: Because of their numerous functional design and application possibilities, the use of flex and rigid-flex PWBs is increasing rapidly. However, this shift poses additional challenges within circuit board fabrication and assembly operations, particularly in terms of the surface finish on the PWB. In response, to improve the reliability of flex and rigid-flex circuit applications, alternative surface finish methods are increasingly being used, such as the electroless nickel/immersion gold (ENIG) process.

To avoid cracking of the nickel during bending, the use of ENIG for flex and rigid-flex circuits has typically required a relatively thin electroless nickel deposit of medium phosphorus content. In this technical paper, the reliability of ENIG with high-phosphorus electroless nickel is examined for such applications. Using electroless nickel deposits of varying thickness and phosphorus content, results of bending tests, hardness, and SEMs are compared to predict reliability of the assembled flexible circuit.

In particular, Young's Modulus (stress-to-strain ratio) is evaluated to show a direct relationship between the phosphorus content of the electroless nickel deposit and the ductility of the overall ENIG finish. The ENIG process with high-phosphorus nickel is shown to offer a more reliable surface finish for flex and rigid-flex applications.

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