SMTA International Conference Proceedings

Manufacturability Assessment Of Next-Generation PCB Surface Finishes

Authors: Sue Teng, J. Oliver and Scott Priore
Company: Cisco Systems, Inc.
Date Published: 10/13/2013   Conference: SMTA International

Abstract: Organic solder preservative (OSP) surface finishes have emerged in recent years as one of the leading low-cost, reliable printed circuit board finishes. It provides excellent solderability and solder joint strength and current formulations have introduced various improvements making it robust enough to withstand Pb-free assembly processes, multiple heating cycles and to exhibit a one-year shelf life. However, as it is not electrically conductive, solder printing of the test points on printed circuit boards using OSP surface finishes is required in order to enable electrical or functional testing. Furthermore, with the no-clean solder pastes being used for Pb-free assembly, the flux residue from the solder paste can gum up the probe tip, particularly for test vias, which results in a dimple where the flux collects. This both increases the occurrence of false fails as well as reduces the lifetime of the test probes, thereby introducing increased costs in terms of re-testing, additional cleaning materials/processes for the test probes and reduced efficiency of equipment utilization due to decreased throughput.

This investigation, therefore, is focused on evaluating alternative surface finish materials for printed circuit assemblies which are either electrically conductive or can otherwise eliminate the need to print solder on test points. The initial evaluation considers the following surface finish materials: • Pb-free Hot Air Solder Level (HASL) • Immersion Ag • Next-generation surface finish A (NG-A) • Next-generation surface finish B (NG-B)

Each of these surface finishes is evaluated with respect to Pb-free assembly feasibility, including SMT, wavesolder and press-fit processes, in addition to the material robustness, manufacturing variability and post-assembly and post-aging material integrity. The details of this study are discussed in this paper and the results will be used to provide a comparative assessment of the surface finishes.

Additionally, based on the findings from this study, a more in-depth evaluation of the material robustness, performance and reliability from a PCB fabrication, design and systemlevel perspective addressing electrical, mechanical, test and environmental concerns is discussed as future work to be pursued.

Key Words: 

surface finish, Pb-free, OSP, Pb-free HASL, Im Ag, ICT, SMT, wavesolder, press-fit

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