ICSR (Soldering and Reliability) Conference Proceedings

How Clean Is Clean Enough to Achieve Reliable Electronic Hardware?

Authors: Mike Bixenman, DBA, David Lober, Mark McMeen and Jason Tynes
Company: Kyzen Corporation and STI Electronics, Inc.
Date Published: 5/19/2015   Conference: ICSR (Soldering and Reliability)

Abstract: Highly dense assemblies with small gap distance between adjacent leads and interconnects increases the electric field and potential for leakage currents from contamination sources. Bottom terminated components soldered onto the PCB have flux residue trapped between the component body and board. Ionics in flux residue can exacerbate contamination levels under the component. One concern is when flux residue underfills and bridges conductors. When the Z-Axis is below 50µm, flux cannot properly outgas from under the component. When this occurs, flux residues that flow away from solder pads may be unreacted. These residues are soft and may contain ionic contaminates under the component.

Pockets of contamination are influenced by numerous factors such as flux type, activation temperature, component type and placement, wash characteristics; solder paste volume, PCB cleanliness, and other sources of contamination. Conductive residues readily lead to leakage currents and electrical shorts. Ion Chromatography and Surface Insulation test methods are commonly used to assess circuit reliability. A short coming of these methods is their inability to pin point the location from which harmful contamination resides.

Surface Insulation Resistance tests the resistance between the anode and cathode. When there is ionic contamination present, resistance drops. An improvement to SIR testing was introduced at the IPC/SMTA High Performance Cleaning and Coating Conference. The method places sensors under component terminations in an effort to measure resistance of the residue at distances close to and away from the pad termination. The goal of this research is to develop a method for accurately measuring resistance of the contamination at specific areas under the component.

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