Medical Electronic Hardware Cleanliness ~ How Clean is Clean Enough?Authors: Mike Bixenman, DBA
Company: KYZEN Corporation
Date Published: 9/16/2015 Conference: Medical Electronics Symposium
The medical industry is transforming itself through the use of digital technologies. The “Internet of Everything” will impact the healthcare industry through monitoring, personalized medicine and medical diagnosis. Smart machine technologies driven by sensors will allow doctors to deliver cost-effective treatments optimized for the individual patient. These technologies are made possible from advanced electronic devices that include implantable devices, scanning, monitoring, sensing and the use of bioinformatics. Each of these technologies relies on devices that are reliable within their end use environment. Highly dense assemblies with small gap distance between adjacent leads or 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 where the location from which harmful contamination resides.
This presentation talks to Surface Insulation Resistance and Ionic Species testing advancements. Surface Insulation Resistance testing can be used to test the resistance between the anode and cathode. When there is ionic contamination present, resistance drops. 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 theory behind this new test method is to accurately measure resistance of the contamination at the specific area under the component. Testing the location, flux type, quantity and mobility may provide an improved risk assessment of reliability expectations.
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