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In this paper, ac impedance is measured on different flux types and compared with SIR results. Using standard circuit analysis, equivalent inductance-capacitance-resistance circuits are constructed to invesnts and their concentration. Using this approach, specific features from the impedance spectrum can be attributed to electrochemical parameters, such as the doublelayer capacitance and the charge-transfer resistance. The dominant conduction processes can be estimated by ac impedance spectrum analysis, and it is shown that dendrite formation can be predicted.
Ling Chunxian Zouz and Christopher Hunt
National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom
The reliability impact of flux residues on electronic assemblies has been traditionally evaluated using surface insulation resistance ͑SIR͒ measurement with dc voltages. An ac impedance technique has been investigated to provide detailed information on the conduction mechanisms and electrode reactions in the presence of flux residues on electronic boards. By evaluating different fluxes using a standard comb pattern the relative applicability of the ac impedance and SIR techniques has been made in terms of assessing the reliability of electronic boards. Impedance values at low frequencies, Ͻ1 Hz, are close to the SIR results and hence produce similar predictions of reliability. More importantly, the ac impedance spectrum can be used to predict dendrite formation, although the technique itself does not actually promote dendrites. The ac impedance method can distinguish between ionic solution resistance of the thin water layer and impedance from interfacial electrochemical processes. At low contamination levels the solution resistance, between the copper-comb electrodes, dominates the overall impedance. At high contamination levels the impedance from interfacial electrochemical processes becomes significant and increases the potential of dendrite formation. This predictive capability could be developed into a nondestructive test method to provide a more detailed electrochemical characterization and indication of future reliability. © 2008 The Electrochemical Society. ͓DOI: 10.1149/1.3005563͔ All rights reserved.
is given by Zˆ = ZЈ + jZЉ, where ZЈ is the real impedance, ZЉ is the
imaginary impedance, and j is the square root of −1. The three common conduction processes characterized by the ac impedance
Theory
AC impedance measurement.— AC impedance spectra made over a wide frequency range provide mechanistic information, because the various conduction processes have different frequency dependencies. Circuit impedance, Zˆ , has a real and imaginary part and
Manuscript submitted August 20, 2008; revised manuscript received September 25, 2008. Published October 31, 2008.
The surface insulation resistance ͑SIR͒ technique measures the resistance of an adsorbed moisture film between two metal electrodes on a substrate surface using dc voltage under elevated temperature and humidity conditions within a chamber. Typical standard conditions are 40°C/93% relative humidity ͑RH͒ or 85°C/85% RH, which results in a moisture layer of approximately 100 nm. This technique is used to qualify flux residues, e.g., ISO 9455 part 12, or to assess the effect of contaminants on the assembly reliability, IEC 61189 part 10.2 and Ref. 1-3. The SIR measurement technique itself is uncomplicated, but the science behind the measurement is complex. Although the SIR technique entails a straightforward ohmic measurement, albeit in the nanoamp range, the intertrack region does not behave as a simple ohmic element, as there are a number of conduction processes involved. Electrochemical processes at the metal-track/electrolyte interfaces, in the presence of the flux residues, must take place to translate electronic conduction in the electrodes to ionic conduction in the thin, aqueous electrolyte film between electrodes. The SIR technique cannot distinguish these individual processes. In contrast, the ac impedance technique, using small ac voltages, has been shown to access different electrochemical reactions.4,5 By scanning across a wide frequency range, different parameters associated with basic conduction processes can be quantified separately due to their different frequency dependency.
z E-mail: lz@
measurement are ohmic conduction, represented by resistance R,
dielectric displacement, represented by capacitance C, and diffusion of electroactive species, represented by the Warburg impedance.6,7 Ohmic conduction can be either electronic or ionic and is defined as
C8
Journal of The Electrochemical Society, 156 ͑1͒ C8-C15 ͑2009͒
0013-4651/2008/156͑1͒/C8/8/$23.00 © The Electrochemical Society
Characterization of the Conduction Mechanisms in Adsorbed Electrolyte Layers on Electronic Boards Using AC Impedance
the real impedance, ZЈ = R, with the current and voltage in phase. Displacement currents are defined by an imaginary impedance, ZЉ
= j/2fC, where f is the frequency of the applied voltage. The current leads the voltage by a phase angle of 90°. The Warburg impedance is complex but can be described by a frequency-
Ling Chunxian Zouz and Christopher Hunt
National Physical Laboratory, Teddington, Middlesex TW11 0LW, United Kingdom
The reliability impact of flux residues on electronic assemblies has been traditionally evaluated using surface insulation resistance ͑SIR͒ measurement with dc voltages. An ac impedance technique has been investigated to provide detailed information on the conduction mechanisms and electrode reactions in the presence of flux residues on electronic boards. By evaluating different fluxes using a standard comb pattern the relative applicability of the ac impedance and SIR techniques has been made in terms of assessing the reliability of electronic boards. Impedance values at low frequencies, Ͻ1 Hz, are close to the SIR results and hence produce similar predictions of reliability. More importantly, the ac impedance spectrum can be used to predict dendrite formation, although the technique itself does not actually promote dendrites. The ac impedance method can distinguish between ionic solution resistance of the thin water layer and impedance from interfacial electrochemical processes. At low contamination levels the solution resistance, between the copper-comb electrodes, dominates the overall impedance. At high contamination levels the impedance from interfacial electrochemical processes becomes significant and increases the potential of dendrite formation. This predictive capability could be developed into a nondestructive test method to provide a more detailed electrochemical characterization and indication of future reliability. © 2008 The Electrochemical Society. ͓DOI: 10.1149/1.3005563͔ All rights reserved.
is given by Zˆ = ZЈ + jZЉ, where ZЈ is the real impedance, ZЉ is the
imaginary impedance, and j is the square root of −1. The three common conduction processes characterized by the ac impedance
Theory
AC impedance measurement.— AC impedance spectra made over a wide frequency range provide mechanistic information, because the various conduction processes have different frequency dependencies. Circuit impedance, Zˆ , has a real and imaginary part and
Manuscript submitted August 20, 2008; revised manuscript received September 25, 2008. Published October 31, 2008.
The surface insulation resistance ͑SIR͒ technique measures the resistance of an adsorbed moisture film between two metal electrodes on a substrate surface using dc voltage under elevated temperature and humidity conditions within a chamber. Typical standard conditions are 40°C/93% relative humidity ͑RH͒ or 85°C/85% RH, which results in a moisture layer of approximately 100 nm. This technique is used to qualify flux residues, e.g., ISO 9455 part 12, or to assess the effect of contaminants on the assembly reliability, IEC 61189 part 10.2 and Ref. 1-3. The SIR measurement technique itself is uncomplicated, but the science behind the measurement is complex. Although the SIR technique entails a straightforward ohmic measurement, albeit in the nanoamp range, the intertrack region does not behave as a simple ohmic element, as there are a number of conduction processes involved. Electrochemical processes at the metal-track/electrolyte interfaces, in the presence of the flux residues, must take place to translate electronic conduction in the electrodes to ionic conduction in the thin, aqueous electrolyte film between electrodes. The SIR technique cannot distinguish these individual processes. In contrast, the ac impedance technique, using small ac voltages, has been shown to access different electrochemical reactions.4,5 By scanning across a wide frequency range, different parameters associated with basic conduction processes can be quantified separately due to their different frequency dependency.
z E-mail: lz@
measurement are ohmic conduction, represented by resistance R,
dielectric displacement, represented by capacitance C, and diffusion of electroactive species, represented by the Warburg impedance.6,7 Ohmic conduction can be either electronic or ionic and is defined as
C8
Journal of The Electrochemical Society, 156 ͑1͒ C8-C15 ͑2009͒
0013-4651/2008/156͑1͒/C8/8/$23.00 © The Electrochemical Society
Characterization of the Conduction Mechanisms in Adsorbed Electrolyte Layers on Electronic Boards Using AC Impedance
the real impedance, ZЈ = R, with the current and voltage in phase. Displacement currents are defined by an imaginary impedance, ZЉ
= j/2fC, where f is the frequency of the applied voltage. The current leads the voltage by a phase angle of 90°. The Warburg impedance is complex but can be described by a frequency-