交流阻抗分析

nd tro r Hu lesex has b ique nce of of the s at lo e ac im es. Th facial minat s sign test m 563� A ceived tween electrodes. The SIR technique cannot distinguish these In Fig. 1, the relationship between the real and imaginary parts of al So 3.00 © individual processes. In contrast, the ac impedance technique, using small ac voltages, has been shown to access different electrochemi- cal reactions.4,5 By scanning across a wide frequency range, differ- ent parameters associated with basic conduction processes can be quantified separately due to their different frequency dependency. 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 investigate the effect of contaminants and their concentration. Using this approach, specific features from the impedance spectrum can be attributed to electrochemical parameters, such as the double- layer 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. Theory AC impedance measurement.— AC impedance spectra made over a wide frequency range provide mechanistic information, be- cause the various conduction processes have different frequency de- pendencies. Circuit impedance, Zˆ , has a real and imaginary part and is given by Zˆ = Z� + jZ�, where Z� is the real impedance, Z� is the imaginary impedance, and j is the square root of −1. The three common conduction processes characterized by the ac impedance impedance for a parallel resistance–capacitance �RC� circuit is shown. The absolute impedance is given by �Z� = �Z�2 + Z�2, with the phase angle � = tan−1�Z�/Z��. The locus of Zˆ reflects the fre- quency dependency. At very low frequencies Zˆ has a small imagi- nary component; as the frequency increases, Zˆ includes an increas- ing imaginary component due to the capacitance, and at high frequency the capacitance acts as a short and Zˆ falls to zero. The phase angle gives the delay of impedance relative to the real part and hence the balance between capacitance and resistance. Zˆ Z’ -Z’’ Low frequency High frequency |Z| φ |Z| - Impedance φ- Phase angle Characterization of the Co Electrolyte Layers on Elec Impedance Ling Chunxian Zouz and Christophe National Physical Laboratory, Teddington, Midd The reliability impact of flux residues on electronic assemblies �SIR� measurement with dc voltages. An ac impedance techn conduction mechanisms and electrode reactions in the prese fluxes using a standard comb pattern the relative applicability assessing the reliability of electronic boards. Impedance value produce similar predictions of reliability. More importantly, th although the technique itself does not actually promote dendrit resistance of the thin water layer and impedance from inter solution resistance, between the copper-comb electrodes, do impedance from interfacial electrochemical processes become predictive capability could be developed into a nondestructive ization and indication of future reliability. © 2008 The Electrochemical Society. �DOI: 10.1149/1.3005 Manuscript submitted August 20, 2008; revised manuscript re The surface insulation resistance �SIR� technique measures the resistance of an adsorbed moisture film between two metal elec- trodes on a substrate surface using dc voltage under elevated tem- perature and humidity conditions within a chamber. Typical standard conditions are 40°C/93% relative humidity �RH� 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 com- plex. 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 resi- dues, must take place to translate electronic conduction in the elec- trodes to ionic conduction in the thin, aqueous electrolyte film be- Journal of The Electrochemic 0013-4651/2008/156�1�/C8/8/$2 C8 z E-mail: lz@npl.co.uk Downloaded 16 Aug 2010 to 121.8.210.35. Redistribution subject to EC uction Mechanisms in Adsorbed nic Boards Using AC nt TW11 0LW, United Kingdom een traditionally evaluated using surface insulation resistance has been investigated to provide detailed information on the flux residues on electronic boards. By evaluating different ac impedance and SIR techniques has been made in terms of w frequencies, �1 Hz, are close to the SIR results and hence pedance spectrum can be used to predict dendrite formation, e ac impedance method can distinguish between ionic solution electrochemical processes. At low contamination levels the es the overall impedance. At high contamination levels the ificant and increases the potential of dendrite formation. This ethod to provide a more detailed electrochemical character- ll rights reserved. September 25, 2008. Published October 31, 2008. 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 the real impedance, Z� = R, with the current and voltage in phase. Displacement currents are defined by an imaginary impedance, Z� = j /2�fC, 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- dependent term Zˆ = ��2�f�−1/2�1 − j�, where � is the Warburg co- efficient, a constant that depends on the mobility and concentration of diffusing species.7 Interpreting impedance measurements using an equivalent circuit can be complex; however, as all circuit compo- nents have varying frequency dependencies, the main components can be extracted from a simplified equivalent circuit, and other less significant components can be eliminated. ciety, 156 �1� C8-C15 �2009� The Electrochemical Society Z’ – Real Z’’ - Imaginary Figure 1. AC impedance for a parallel RC circuit. S license or copyright; see http://www.ecsdl.org/terms_use.jsp al So Figure 2 shows the ac impedance spectra of four simple circuits. The second row gives the relationship between Zˆ and frequency, which are known as Bode plots. In all the plots where the behavior is purely resistive, an R is marked on the figure. The third row shows the phase angle as a function of frequency. The last row plots −Z� against Z�, which is known as a Nyquist plot. The arrow shows how Zˆ changes from low to high frequency. For a serial RC circuit, column three in Fig. 2, the impedance is expressed in Eq. 1 and evaluated using Eq. 2 Zˆ = R − j 1 2�fC �1� �Z� = �R2 + 1 �2�fC�2 �2� For a parallel RC circuit, the impedance is expressed in Eq. 3 and evaluated using Eq. 4 Zˆ = R 1 + �2�f�2C2R2 − j 2�fCR2 1 + �2�f�2C2R2 �3� �Z� = R �1 + �2�f�2C2R2 �4� In the Nyquist plot, the frequency dependency of Zˆ describes a semicircle for a parallel RC circuit; at a certain frequency, fz�max, at the top of the semicircle the imaginary impedance reaches a maxi- mum value, Zmax� , and is equal to the real impedance. This frequencyfz�max can be calculated using Eq. 5 or can be used to evaluate the capacitor and resistor values in the circuit fz�max = 1 2�RC �5� These equations are used later in analyzing the response from im- pedance measurements of fluxes on comb patterns. Conduction model for parallel electrode system.— An equiva- lent circuit representing each significant conduction process for a comb pattern at an elevated temperature and humidity is presented in Fig. 3, along with a simplified version for SIR. The electrochemi- cal process initiates with the adsorption of a thin water layer from R C R C R |Z| f 1/2πfC φ -Z’’ -90° f Z’ f |Z| R φ 0° f -Z’’ R Z’ |Z| f φ R -90° -Z’’ R f Z’ φ R f -90° -Z’’ R f Z’ |Z| Figure 2. AC impedance spectra and their equivalent circuits. Journal of The Electrochemic the high-humidity atmosphere, and the conduction process is sup- ported by the presence of flux residues dissociating to form ionic charge carriers. The impedance to ionic conduction is dominated by the space between the metal tracks of the comb and is represented Downloaded 16 Aug 2010 to 121.8.210.35. Redistribution subject to EC by the terms RC and CC, where RC is the ionic resistance of the thin water layer with contaminants across the interelectrode gap of the comb and CC is the capacitance of the comb pattern. The circuit Cu Cu Water with contaminants SIR RcRct Zw Electrode reaction AC impedance Rc Rct Cdl Zw Cc Electrode reaction Figure 3. Equivalent circuit for ac impedance and SIR measurement on a comb pattern. C9ciety, 156 �1� C8-C15 �2009� C9 elements in the dotted box are a representation of electrochemical processes at the interfaces of the contaminant layer and electrode; only one equivalent circuit is presented in the figure, as the two electrode interfaces are identical. These reactions represent the tran- S license or copyright; see http://www.ecsdl.org/terms_use.jsp al So sition from electronic conduction in the electrode to ionic conduc- tion in the contaminant layer. The equivalent circuit for these elec- trochemical processes includes the charge-transfer resistance RCT, associated with a faradaic reaction at the electrode surface; CDL is a double layer or blocking capacitance at the interface, which may allow ionic current to flow in the absence of a faradaic reaction;8 and ZW is the Warburg impedance, detectable when a faradaic reac- tion occurs under diffusion control near the interface. When these processes are dominant, the equivalent-circuit model in Fig. 3 can be used to evaluate the impedance parameters by measuring and ana- lyzing the impedance spectrum. In contrast, the SIR technique can only measure all three parameters in combination, and it cannot distinguish between each process. Experimental Test flux.— Four fluxes were selected as representative of a wide range of flux chemistries, all using carboxylic acid activators, and these are listed in Table I. SB1, SB2, and WB1 are commercial fluxes, and WB2 is the flux developed for solderability tests by National Physical Laboratory.9 Fluxes were diluted using isopropyl alcohol �IPA� or DI water, depending on the flux base, to achieve SIR values that were above 106 � and did not cause dendrite for- mation. These flux concentrations are also listed in Table I. One further test was conducted using a more concentrated version of flux SB1 �80%� to examine the capability of ac impedance measurement to predict dendrite formation. Sample preparation.— An NPL-designed board consisting of four identical SIR comb patterns with Cu finish was used as shown in Fig. 4. The size of each comb pattern was 25 � 25 mm, with Table I. Test fluxes details. Flux Base Solid Acid no. Contains resin Concentration �%� SB1 IPA 1.6 15.5 No 50 SB2 IPA 6.0 41.0 Yes 30 WB1 Water 4.6 37.1 No 3 WB2 Water 4.8 41.1 Yes 3 C10 Journal of The ElectrochemicC10 Figure 4. Test board for SIR and impedance measurement. Downloaded 16 Aug 2010 to 121.8.210.35. Redistribution subject to EC 0.60 mm pitch and 0.20 mm gap. The boards were cleaned in an Ionograph 500M with 75% IPA/25% deionized water �DI� solution at 45°C. The cleaned boards were fluxed using 50 �L fluxes to cover each comb pattern. Fluxed boards were dried in a 100°C dry oven for 5 min. SIR and impedance measurement.— Five experiments �A, B, C, D, and E� were conducted for each of the four fluxes, diluted as follows; 50% SB1, 30% SB2, 3% WB1, and 3% WB2. In these experiments, A–C were impedance measurements. Sample imped- ance spectra were measured periodically during the 24 h environ- mental exposure in response to a small 50 mV peak–peak ac signal, with frequencies ranging from 10−1 to 105 Hz. Impedance spectra were determined using an ACM Instruments Gill AC. Between mea- surements the samples were conditioned with either 0 V, 5 V at 50 Hz, or 5 V dc for experiments A, B, and C, respectively. The conditioning was applied after the first measurement. Experiment D followed a continuous SIR measurement schedule, with measure- ments every 10 min using a Gen3Systems Auto-SIR. A bias voltage of 5 V dc was applied during the 24 h exposure. This instrument has a 106 � limiting resistor on each test channel to protect dendrites from burning out; hence, the minimum SIR measurement value was 106 �. For fluxes 3% WB1 and 3% WB2, experiment E, a periodic SIR measurement was performed. Here, there were four measure- ments during the 24 h exposure, and there was no conditioning bias between measurements. Concentrated SB1 �80%� was also tested to simulate conditions where dendrite formation is likely. The environ- mental condition for all experiments was 40°C/93% RH. Results and Discussion AC impedance spectra and modelling without dendrite form- ation.— The results from the three ac impedance experiments, A, B, and C, with diluted fluxes could all be interpreted using the same model equivalent circuit. In Fig. 5, results are presented as a Bode plot for the SB2 flux at 30% dilution and for increasing exposure times in the chamber. The other fluxes produced similar results. With these flux dilutions, dendrites would not occur with SIR test- ing. In Fig. 6, again for the SB2 flux, and after 3 h exposure, the data from the same experiment are presented, giving the Bode �a�, Ny- quist �b�, and phase angle �c� plots. Referring to Fig. 2 the results in Fig. 6 are clearly consistent with a simple parallel RC and CC net- work. At these flux concentrations, the impedance response has no component that can be attributed to the charge-transfer or diffusion components, RCT, CDL, or ZW, the impedance contribution expected from the electrode electrochemical processes. Figure 6 also reveals 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10 1.0E-01 1.0E+01 1.0E+03 1.0E+05 Frequency (Hz) Im pe da nc e (o hm ). @1hour @2hour @3hour @3hour @4hour @6hour @7hour @23hour @24hour Figure 5. Body plots for 30% SB2 flux with experiment A. ciety, 156 �1� C8-C15 �2009� that at high frequencies the capacitive behavior of the metal comb dominates, and impedance is independent of the nature of the sur- face contaminant layer, as can be seen by the phase angle tending to −90°. At low frequencies, impedance is more sensitive to the con- S license or copyright; see http://www.ecsdl.org/terms_use.jsp al So tamination layer, with the effect that RC is more pronounced and the phase angle tends toward 0°. So, the equivalent circuit presented in Fig. 3 can be simplified, as shown in Fig. 7, because the electrode reactions do not contribute significantly to the overall impedance. For 30% SB2 flux at 3 h, as shown in Fig. 6, RC can be evaluated using the Nyquist plot. Because at frequency fz�max, Zmax� = Zmax� , so RC = 2Zmax� = 2.0 � 107 �, then CC can be calculated using Eq. 5, CC = 1/2�fmaxRC = 7.5 � 10−11 F. This capacitance value is near the sensitivity limit for the instrument and includes stray capacitance from edge connector and leads in the experimental arrangement. The 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08 1.E+09 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Frequency (Hz) Im pe da nc e (o hm ). Measured Modelled@ 3 hour a 0.0E+00 4.0E+06 8.0E+06 1.2E+07 1.6E+07 0.0E+0 0 6.0E+0 6 1.2E+0 7 1.8E+0 7 2.4E+0 7 Z' (ohm) -Z ''( oh m ) Measured Modelled @ 3 hour b 0 30 60 90 1.0E-02 1.0E+00 1.0E+02 1.0E+04 Frequency (Hz) P ha se an gl e (° ). Measured Modelled _ _ _ @ 3 hour c Figure 6. Measuring and modelling Bode �a�, Nyquist �b�, and phase-angle �c� plots for 30% SB2 flux with experiment A at 3 h. Journal of The Electrochemic capacitance from edge connector and leads was measured, and the value was 2.9 � 10−11 F. So, the capacitance for this comb line geometry was 4.6 � 10−11 F. Based on these values, and using Eq. 3 and 4, the solid lines were plotted in Fig. 6 following an optimi- Downloaded 16 Aug 2010 to 121.8.210.35. Redistribution subject to EC zation process. There was good agreement between measured and modelled spectra, with the optimized values within 10% of the start- ing values. Comparison of SIR and impedance results.— As mentioned above, RC can be estimated using the impedance value at low fre- quency, because here the capacitive property of the comb is insig- nificant. Hence, the impedance at 0.1 Hz should be similar to the SIR value, and this is evaluated in Fig. 8-11, where these properties are plotted as a function of exposure time. The plotted data are an average from three measurements. There are some salient points to note from these figures. For two solvent-based fluxes, SB1 and SB2, the impedance responses with time were similar with different con- ditioning and were close to the SIR value as well. This probably suggests that the increase in impedance and SIR values with time was most likely caused by flux volatilization and not the electrical conditioning. For the two water-based fluxes, WB1 and WB2, how- ever, the impedance and SIR with dc conditioning were significantly higher than that with no conditioning and that with ac conditioning. RcRct Zw Electrode reaction RC CC Figure 7. Equivalent circuit for four fluxes in Table I. 1.E+06 1.E+07 1.E+08 1.E+09 1.E+10 Im pe da nc e (o hm ) . Experiment A Experiment B Experiment C Experiment D C11ciety, 156 �1� C8-C15 �2009� C11 0 10 20 30 Time (hour) Figure 8. Impedance �A, B, and C� and SIR �D� results for 50% SB1 flux. S license or copyright; see http://www.ecsdl.org/terms_use.jsp al So This is not surprising, as the dc conditioning results in nonreversible local-ion-concentration polarization1 and the formation of corrosion products �oxides or hydroxides�. This was confirmed by only apply- ing voltage periodically in the SIR test, as shown by the violet curve in Fig. 10 and 11. Here, the conditioning with dc and ac produced results that were in agreement. Dendrite formation with ac and dc conditioning.— Dendrite formation is an important failure mechanism and can lead to cata- strophic failure of electronic boards. In a further set of tests, the concentration of the SB1 flux was increased to 80% to deliberately promote dendrite formation in the SIR test, and results are presented in Fig. 12 showing two typical SIR responses from dendrite forma- tion. The rapid change on SIR in Fig. 12 indicates dendrite forma- tion on the board. This is because when metallic dendrites start to grow from the cathode, the SIR is dramatically reduced. The SIR value drops to zero if the dendrite reaches the anode and results in a short circuit. The high current due to the short circuit can burn out the dendrites, and then the SIR value rapid increases again. In Fig. 12, the SIR approaches a minimum value of 106 �. This is because, in the Auto-SIR measurement system, a 106 � resistor on each mea- surement channel is installed to preserve dendrite formed; therefore, the SIR value should be no less than 106 � even when the board is short-circuited. The ac impedance was similarly measured on boards with 80% SB1 flux after 1 and 24 h exposures. During these imped- ance tests, two conditioning modes were applied, one using 5 V dc and the second using 5 V ac at 50 Hz. These results are shown in Fig. 13, where the impedance was slightly higher wit