Evaluation of No-Clean Fluxes for High Reliability Applications
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E. Pope, J. Watson, D. Kennish, Intel Corporation 5000 W. Chandler Blvd. Chandler AZ, 85226
ABSTRACT A series of rosin mildly activated, halide free, low residue fluxes have been evaluated for use with hot bar reflow of Tape Carrier Packages (TCP) to printed circuit boards (PCB). Materials were studied with respect to their wettability, SIR performance, water reflux extractable ionics and residue levels. Ionic levels were measured by FTIR and Ion chromatography. A flux was selected which has demonstrated acceptable performance in 156C/85%RH 1OV biased stress testing.
BACKGROUND The Tape Carrier Package (TCP) format is one way to meet the small outline high lead count interconnection needs of high performance microprocessors for size and weight sensitive components. Such components are used in portable computing and information management systems in notebook and palm top configured products. Ultra Fine Pitch Technology TCP components, on 0.25mm pitch, are shipped flat in slide carriers; the leads are designed to be formed into "gull-wing" configuration and mounted onto the PCB by localized reflow. Therefore, this localized reflow mounting process is done after standard Surface Mount Technology (SMT) processing has been completed, including cleaning. Or many PCB assemblers are now implementing total no-clean processes. Thus, no-clean flux materials are also required for TCP application. Throughout the electronics industry, alternates for traditional fluxes and cleaning methods are being evaluated and implemented in order to meet the requirements of the Montreal Protocol and the Copenhagen Amendments which mandate the elimination of ozone depleting CFCs by 1996. No clean flux materials began to receive industry-wide attention in the mid-1980's for use as replacements for CFC-based solvent cleanable rosin based mildly activated (RMA) type fluxes. [1] In addition, many proponents of no-clean materials cite decreased throughput time and reduced effluents requiring treatment. [2,3,4] Because the Tape Carrier Package is placed last, it has an additional need for a no-clean flux, based on its location in the board assembly process flow. For this application, in order to perform reliability stress testing which require in-situ bias and multiple electrical test socketing, Ultra Fine Pitch (0.25mm) surface mountable TCP components must be mounted on pin grid array "interposers". These composite parts are then subjected to accelerated stress testing. For these extremes of lead pitch, humidity and temperature, fluxes were required which have much lower levels of extractable ions than traditional materials. Fluxes which can demonstrate acceptable performance in these rigorous environments are expected to provide commensurate performance in field use. These latter two considerations formed the conceptual basis for this work. Historically, flux chemistries have ranged from aggressive inorganic acids used in heavy industry to mild rosin based materials for electronics. [5] The activity of the flux was tailored 231
Mat. Res. Soc.
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