Journal of SMT Article

Pb-Free Flux: A Chemical View of Reliability

Authors: Phil Isaacs, Eddie Kobeda and Jing Zhang
Company: IBM Corporation
Date Published: 1/31/2015   Volume: 28-1

Abstract: While the initial transformation to Pb-free solder and the challenges that it presents have in general been overcome, there remains additional concerns within the electronics industry as to what the future will hold for materials. Sustainability and the preservation of our environment will further restrict the use of certain substances, and likely move our current accepted practices into new processes that are yet unknown. This warrants further understanding of the role of materials in the assembly of electronic systems. The use of these materials is pervasive across the electronics industry, and as we learned with previous solder technologies, future products will undergo additional thermal challenges that require, a better understanding of how these changes affect reliability.

In light of these developments, we studied the board assembly process for high end server technology applications in detail, and focused on the increasingly important role that flux plays during the reflow of solder. As it relates to electronic assembly flux is simply defined as a reducing agent which prevents oxide formation on the surface of a molten metal (viz. solder). Recognizing that both 50 plus years of electronics research and development, and the use of eutectic SnPb solder will become obsolete as current exemptions to Restriction of Hazardous Substances, RoHS, and other legislations expire, we focused our efforts on Pb-free solders which are subject to increased temperatures and smaller process windows. The chemistry of flux and its affect on the flow of solder during joint formation will be evaluated using several analytical techniques including Differential Scanning Calorimetry, DSC, Thermogravimetric Analysis, TGA, and Surface Insulation Resistance, SIR, as we seek further understanding on how flux can sufficiently remove oxide formation during solder reflow. We will also examine what occurs after the redox reaction to render no-clean, NC, flux reaction residue acceptable and of no consequence to reliability.

Keywords: 

Pb-free, No clean flux, flux activation and flux residue



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