Journal of SMT Article

NEW GENERATION METALLIC SOLDERABILITY PRESERVATIVES

Author: Donald P. Cullen
Company: MacDermid
Date Published: 10/1/1999   Volume: 12-4

Abstract: An impressive amount of R&D has been devoted to circuit board surface finishes during the past 3-5 years. The interest in new PCB surface finishes is fueled by two drivers, demands for increased functionality/ yield at electronic assembly, and demands for process ease/ productivity at PCB fabrication.

Original equipment manufacturers and the contract assemblers that supply them are interested in replacing HASL.[1] Flatness and functionality demands from the end-users have resulted in increased use of Electroless Nickel Immersion Gold. However, this process can be expensive and difficult at fabrication. Assemblers are also returning to their long-term preference for the Copper-Tin solderjoint over the Nickel-Tin solderjoint.

PCB fabricators are under ever-increasing pressure to reduce cost. They are able to achieve this from the surface finish process in several ways. Lower total chemical cost, increased first-pass yield, increased productivity per unit area of floorspace, and lowered engineering/ laboratory support costs. Productivity and cost demands from fabricators have resulted in increased usage of Organic Solderability Preservatives. However, OSP’s have disadvantages in solderability after multiple reflow operations with weak fluxes. In addition, more PCB designs are calling for surface conductivity for touchpads, switches, and electrical testing. OSP’s cannot deliver this functionality.

New finishes now available on the market can meet the demands from both the end-user and the fabricator. In particular, the Immersion Silver surface finish is easy to apply at a low cost and solders easily to achieve a reliable Copper-Tin intermetallic. Details of the joint strength, intermetallic formation, electrical characteristics, and solder wettability (as well as other topics) are discussed in the paper.



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