Oxidation kinetics of thin copper films and wetting behaviour of copper and Organic Solderability Preservatives (OSP) with lead-free solder

Applied Surface Science 257 (2011) 6481–6488

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Oxidation kinetics of thin copper films and wetting behaviour of copper and Organic Solderability Preservatives (OSP) with lead-free solder Mauricio Ramirez a,b,∗ , Lothar Henneken a , Sannakaisa Virtanen b a b

Robert Bosch GmbH, Robert-Bosch-Strasse 2, 71701 Schwieberdingen, Germany Chair for Surface Science and Corrosion, University of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany

a r t i c l e

i n f o

Article history: Received 14 December 2010 Received in revised form 11 February 2011 Accepted 11 February 2011 Available online 17 February 2011 Keywords: Copper oxidation Soldering OSP Lead-free Final finish XPS

a b s t r a c t The oxide formation on thin copper films deposited on Si wafer was studied by XPS, SEM and Sequential Electrochemical Reduction Analysis SERA. The surfaces were oxidized in air with a reflow oven as used in electronic assembly at temperatures of 100 ◦ C, 155 ◦ C, 200 ◦ C, 230 ◦ C and 260 ◦ C. The SERA analyses detected only the formation of Cu2 O but the XPS analysis done for the calibration of the SERA equipment proved also the presence of a CuO layer smaller than 2 nm above the Cu2 O oxide. The oxide growth follows a power-law dependence on time within this temperature range and an activation energy of 33.1 kJ/mol was obtained. The wettability of these surfaces was also determined by measuring the contact angle between solder and copper substrate after the soldering process. A correlation between oxide thickness and wetting angle was established. It was found that the wetting is acceptable only when the oxide thickness is smaller than 16 nm. An activation energy of 27 kJ/mol was acquired for the spreading of lead free solder on oxidized copper surfaces. From wetting tests on copper surfaces protected by Organic Solderability Preservatives (OSP), it was possible to calculate the activation energy for the thermal decomposition of these protective layers. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The wettability problems on the final finish of the Printed Circuit Boards (PCBs) are common failures during soldering in electronic assembly. A reduction in the wetting behaviour is mainly caused by oxidation of the surface. These solderability problems are more evident when the leaded solders are replaced by lead-free solders. Due to new government legislations such as RoHS (Restriction of Hazardous Substances directive for electrical and electronic equipment), lead will not be allowed anymore for automotive applications after January 2012. [1] and this is why assemblers are now investigating the influence of lead-free final finishes on the wettability of PCBs. This paper focuses on the oxidation behaviour of copper films and the oxidation of copper surfaces coated with the final finish known as Organic Solderability Preservative (OSP). Many studies on copper oxidation kinetics have been carried out [2–13] and different measurement methods such as microbalance [3], X-ray reflectometry [4], ellipsometry [7–10], coulometric reduction and X-ray photoelectron spectroscopy [5,12] have been used for the characterization of the copper oxide layers. The data

∗ Corresponding author at: Robert Bosch GmbH, Robert-Bosch-Strasse 2, 71701 Schwieberdingen, Germany. Tel.: +49 711 811 35837; fax: +49 711 811 5183670. E-mail address: [email protected] (M. Ramirez). 0169-4332/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2011.02.048

published in the literature by Njeh [4], van Wijk [9] and Berriche [12] are closely related to the data presented in this paper. However, there is no direct correlation between the oxide thickness and the wettability of lead free solder on copper. Thus the main objective of this work is to establish a mathematical relation between the copper oxide thickness and the spreading of the solder on copper substrates. The wettability was determined by measuring the contact angle between solder and copper surface after a lead free reflow process. These angles were measured from microsections of the soldered samples. The presence of a cleaning or reducing agent is always necessary in order to remove the oxides from the copper substrate and activate the surface for the solder process. This cleaning agent is normally known as flux. The same procedure was done with copper surfaces protected by Organic Solderability Preservatives (OSP).

2. Experimental procedure 2.1. Materials Silicon wafers with a 3.5 ␮m copper layer were provided by the Robert Bosch GmbH. The structure of the copper film was polycrystalline and homogeneous. An initial cuprous oxide (Cu2 O) layer of approximately 4 nm was measured on these samples.

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Fig. 1. Standard lead-free reflow temperature profile for soldering.

4 mm Globules of the alloy Sn95.5 Ag3.8 Cu0.7 from the company Metronelec were used for the wetting tests. A no clean flux of class REL0 was used for soldering. Test PCBs with commercial available Organic Solderability Preservatives (OSP) of last generation based on imidazole were analyzed. The thickness of this OSP coating was within the range of 0.25–0.45 ␮m.

Fig. 2. XPS depth profile of a copper surface treated with a lead free reflow process in air.

a CuO layer smaller than 2 nm were found after a lead free reflow process in air (Fig. 2). 2.4. Wetting test

2.2. Surface oxidation All the surfaces were oxidized in air with a reflow oven, as used in electronic assembly, at temperatures of 100 ◦ C, 155 ◦ C, 200 ◦ C, 230 ◦ C and 260 ◦ C. Constant temperature profiles (step profile) and different oxidation times were used in order to prepare the oxide films. The oxidation was done with the reflow oven because the heat distribution at the copper surface is very uniform and this allows the preparation of a homogeneous copper oxide layer on the test specimen. Previous experiments were done with a normal convection oven but the copper surfaces showed irregular staining after thermal treatment. This staining is due to an inhomogeneous temperature distribution in the oven. All the presented measurements were carried out on thin copper films supported on silicon wafer (homogeneous copper) and not directly on copper from the PCB. The roughness of the copper from the PCB was very high, so that the repeatability of the SERA measurements was insufficient. In order to reduce the number of experiments, an optimal Design of Experiments (DoE) was generated with the statistical software Cornerstone. This method was also applied in order to estimate without bias and with minimum variance the effects of the variables involved in this experiment.

The alloy globules were immersed in a commercial flux and then placed carefully above the oxidized surfaces. These samples were soldered subsequently with a lead-free temperature profile (Fig. 1) in a 1000 ppm oxygen atmosphere. Three samples were soldered for each oxidation stage and then microsections were prepared in order to measure the contact angle between solder and substrate. A total of six contact angles were measured for each oxidation stage. 2.5. Oxidation analysis for OSP In order to study the oxidation of the Organic Solderability Preservatives (OSP), it was necessary to analyse these surfaces with Scanning Electron Microscope (SEM) after a Focus Ion Beam (FIB) preparation with a Zeiss Auriga cross beam workstation. 3. Results and discussion 3.1. Oxide thickness Measurement on copper The XPS analysis was done to obtain information about the chemical state of the oxide layer and its thickness after a standard lead-free reflow step in air. The results indicate that there is presence of CuO on the uppermost layer followed by a 37 nm Cu2 O layer. Fig. 3 presents the Cu 2p X-ray photoelectron signal before

2.3. Determination of the copper oxide thickness The copper oxide thickness determination was carried out by Sequential Electrochemical Reduction Analysis (SERA) in borate buffer (pH 8.4) with a current density of 30 ␮A/cm2 . A QC-100 Surface Scan analyzer from the company ECI Technology Inc. was used for these measurements. Copper oxide standards (Cu2 O and CuO) are not available at the market and these standards are necessary for a proper calibration of the equipment. Thus it was necessary to produce these standards at the laboratory by treating a Cu/Siwafer with a standard lead-free temperature solder profile in air (Fig. 1). Afterwards the thickness and the type of the generated oxides were determined with X-ray Photoelectron Spectroscopy (XPS). A thermo VG scientific analyzer of type ESCALAB 250 with an Al K␣ emitting radiation source was used for this analysis. The thickness of the crater done during the depth profile was measured with a Veeco optic profilometer of type NT 3300 for the proper calibration of the height. A Cu2 O layer of approximately 37 nm and

Fig. 3. XPS-Cu 2p spectra of the uppermost layer of the surface before sputtering.

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Fig. 4. Reduction curves of cuprous oxide Cu2 O in borate buffer (pH 8.4) for a current density of 30 ␮A/cm2 . The oxides were generated at 200 ◦ C for 3, 12 and 20 min. Fig. 5. Kinetic of the copper oxide formation at 100, 155, 200, 230 and 260 ◦ C.

the first sputtering step. A clear Cu(II) signal was obtained after the deconvolution of the Cu 2p spectra by using Scofield factors. The detected amount of Cu(II) present on the uppermost layer of the surface was 8.7 ± 0.9%. Sputtering steps of 2 nm were used for the depth profiling. After the first sputtering step, a signal of Cu(II) was not anymore detected. This means that the CuO oxide thickness must be smaller than 2 nm. Cho [5] reported also the presence of a small amount of cupric oxide (CuO) above the Cu2 O layer after thermal treatments in air at 150 ◦ C, 200 ◦ C, 300 ◦ C and 400 ◦ C. Cho [5] proposed an oxide layer structure of Cu/Cu2 O/CuO/air. In this work, the determination of the oxidation kinetics of thin copper films was done only by Sequential Electrochemical Reduction Analysis (SERA). This method is not sensitive enough to detect oxide layers under 2 nm, thus it was considered just a formation of Cu2 O for these analyses. Fig. 4 illustrates the reduction curves of the samples oxidized at 200 ◦ C. The standard reduction potential for Cu2 O is within the range between −0.3 V and −0.59 V and the standard reduction potential for CuO is within the range between −0.59 V and −0.8 V [14]. In the curves only one plateau can be observed at approximately −0.45 V which corresponds to the reduction of Cu2 O. No plateau was observed in the range between −0.59 V and −0.8 V. Berriche [12] studied the oxidation of copper at 175 ◦ C and 200 ◦ C. They measured first the copper oxide thickness with SERA and then with Secondary Ion Mass Spectroscopy (SIMS) in order to verify if the SERA method was a reliable method. This comparison was matching data from SERA method within the time range of 30–200 min quite well because all the values were approximately in the same order. They reported discrepancies between the two methods in the time interval within 0 and 30 min. The colour of the copper surface changes with temperature load from pink to orange, red and finally yellow. This indicates the formation of the Cu2 O oxide at the surface. The obtained experimental data prove that the oxide growth follows a power-law dependence on time (Fig. 5) and the following empirical formula was derived from the measured values: doxide (t) = k(T ) × t n + d0

(1)

where doxide is the thickness of the formed Cu2 O oxide as a function of time, d0 is the initial copper oxide thickness of the samples which was always approximately 4 nm, k(T) is the growth rate constant which is function of the temperature, t is the oxidation time and n is an empirical power factor. Fig. 5 shows the cuprous oxide (Cu2 O) thickness as a function of the oxidation time for different temperature loads. The values of k(T) and n were found by a least-squares fitting of the experimental data and they are presented in Table 1.

Njeh [4] studied the copper oxidation with X-ray reflectometry. They found similar mathematical relations for the oxidation of copper at 175, 200, 225 and 250 ◦ C but their n values varied between 0.82 and 0.95. The n values presented in this work vary between 0.25 and 0.71, on average 0.53. The experimental data for the oxidation at 100 ◦ C does not fit very well this model because the correlation coefficient R2 is less than 0.9 and the exponent n differ considerably from the values obtained for higher temperatures. The growth rate constant k(T) can be expressed in terms of the activation energy as follows: k(T ) = k0 exp

 −E  eff

(2)

R×T

doxide (t) = k0 exp

 −E  eff

R×T

× t n + d0

(3)

Here k0 is an initial oxide growth constant, R is the molar gas constant (8.314 × 10−3 kJ/K mol), T represents the temperature and Eeff is the oxide growth activation energy. In order to obtain only one general equation for the copper oxidation between 100 and 260 ◦ C, it is necessary to approximate the value of n to 0.5 because this corresponds to a square root time law. The mean value of the five measured n exponents is approximately 0.5. Rauh [11] proposed also the following square root time law for the oxidation of copper films in pure oxygen at temperatures within the range of 85 to 135 ◦ C: doxide = K × t 1/2

(4)

For the calculation of the activation energy, the experimental data were fitted with the next equation. doxide (t) = k0 exp

 −E  eff

R×T

× t 1/2 + d0

(5)

The growth rate constant k(T) can be calculated for each temperature from the slope of the curves presented on Fig. 6. The experimental values between 200 ◦ C and 260 ◦ C fit very well to this model because the correlation coefficients R2 are above 0.99. 2

2 (doxide (t) − d0 ) = k(T ×t )

(6)

Table 1 Experimental values of k(T) and n for the oxidation of copper in air. T (◦ C)

k(T) (Å/minn )

n

100 155 200 230 260

29.543 43.851 89.777 116.450 266.666

0.250 0.468 0.670 0.715 0.569

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M. Ramirez et al. / Applied Surface Science 257 (2011) 6481–6488 Table 2 Comparison between the oxide thicknesses reported in the literature and the calculated values from Eq. (8) for oxidation at 175 ◦ C. Time (min)

5 10 30

Fig. 6. Determination of the growth rate constants k(T) for oxidation at 100, 155, 200, 230 and 260 ◦ C.

With the values of k(T) for each temperature and with Eq. (2) is possible to determine the activation energy and the growth constant k0 by plotting the reciprocal values of the temperature against the logarithm of the growth rate constant k(T) (Fig. 7) as follows: Ln(k(T ) ) = Ln(k0 ) −

 E  eff

(7)

R×T

An activation energy of 33.1 kJ/mol and a growth constant k0 of 1/2 were obtained from these experimental data. ˚ 5.518E + 05 A/min Therefore the oxide growth in the range within 100 ◦ C and 260 ◦ C can be calculated with Eq. (8) as a function of temperature and time. doxide (t) = 5.518 × 105 exp

 −33.1  R×T

× t 1/2 + d0

(8)

A comparison between the data obtained from Eq. (8) and the data from the literature was carried out in order to compare the derived mathematical model with published data. Several literature references present data for an oxidation at 175 ◦ C. Cho [5] determined also the oxide thickness with the Coulometric reduction technique (SERA) and Berriche [12] measured the oxide thickness with SERA and SIMS (Table 2). In this work no measurements were carried out at 175 ◦ C, but only at this temperature several literature values can be found. Therefore a comparison of the data in the literature with values calculated with Eq. (8) should indicate the validity of this approach for oxidation between 100 ◦ C and 260 ◦ C.

Fig. 7. Arrhenius plot of Ln(k(T) ) versus T−1 for the determination of the activation energy Eeff .

doxide (nm)

doxide (nm)

doxide (nm)

doxide (nm)

Eq. (8)

SERA [12]

SIMS [12]

SERA [5]

17 24 42

16 34 39

8 14 28

3 10 20

From Table 2 it can be observed that the data reported by Cho [5] are inconsistent with our data, but the SERA measurements from Berriche [12] are very similar to our estimated values. The SIMS measurements also differ from our values, however, the author reported discrepancies between SERA and SIMS only in the first 30 min of oxidation. Van Wijk [9] derived activation energies for the oxidation of copper particles from Cu to Cu2 O in the range within 25 and 30 kJ/mol. Njeh [4] also reported an activation energy of 32 kJ/mol for the formation of Cu2 O. The value of the activation energy obtained from our experimental data was 33.1 kJ/mol which is very close to the data published in the literature. 3.2. Wetting test on copper surfaces The wettability of the surfaces was determined by measuring the contact angle between the solidified solder and the copper substrate from microsections (Fig. 8). To measure the real wetting angle between the molten solder and the copper surface is a difficult task since the soldering process takes place inside a reflow oven under a 1000 ppm oxygen atmosphere at 260 ◦ C. This wetting angle changes during solidification due to certain effects such as the change in the specific volume, thermal expansion and interfacial tension. The change in the wetting angle during cooling was considered in all measurements by using a systematic error. The IPC standard J-STD-003B [15] defines the contact angle as the angle of a solder fillet that is enclosed between a plane that is tangent to the solder/basis-metal surface and a plane that is tangent to the solder/air interface. In order to obtain good wetting on the oxidized copper surfaces, the oxides must be removed with the help of the flux. Only when the oxides are totally removed, the solder can react with the copper to form an intermetallic layer, allowing the solder to spread [16]. Smaller contact angles are caused by better spreading, i.e., higher wettability.

Fig. 8. Measurement of the wetting angles with microsections after a heat treatment of 3 min at 230 ◦ C.

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Fig. 10. Correlations between oxide thickness and contact angle for temperatures at 200, 230 and 260 ◦ C.

Fig. 9. Contact angles between solder and copper substrate after oxidation at 200, 230 and 260 ◦ C.

The contact angle must be smaller than 90◦ for wetting to occur, if the contact angle is larger than 90◦ a dewetting takes place [16]. The French standard NF 89400 [17] defines that the wettability is excellent when the contact angle is smaller than 30◦ . When the contact angle is smaller than 40◦ , the wettability is classified as good. The wettability is acceptable according to this standard only when the contact angle is smaller or equal to 55◦ . All the measured contact angles for oxidation temperatures at 100, 155, 200, 230 and 260 ◦ C are presented in Fig. 9. A considerable change in the contact angles was not observed for an oxidation between 9 and 20 min at 100 ◦ C because the oxide growth was very slow in this time interval (about 1 nm). The measured values indicate that the flux could not remove all the oxides when they were too thick. The wettability was always insufficient after a certain oxide thickness since the contact angles were above 55◦ . The experimental data show a significant change in the contact angle for an annealing at 155 ◦ C only after 15 min because the angle grows just above the limit value for an acceptable wetting. The measured oxide thickness after 20 min at 155 ◦ C was 18 nm. Heat treatment for 3 min at 230 ◦ C leads already to massive surface oxidation with a wetting angle above the limit. It can be concluded that the oxide thickness must be smaller than 16 nm in order to obtain an acceptable wettability according to standard [17]. As expected, the contact angle increases by elevated oxidation temperatures because the oxide growth is much faster at higher temperatures and the used flux is not able to remove all the oxides when they are too thick. It is also necessary to investigate the oxidation at 100 and 150 ◦ C for periods longer than 20 min in order to generate thicker oxides. The values for oxidation between 100 ◦ C and 155 ◦ C were not taken into account for the deviation of a correlation between oxide thickness and contact angle because the oxidation in this range was not strong enough. The data of the contact angles versus the oxide thickness and the derived correlations are presented on Fig. 10. From Fig. 10, it can be derived the following equations which were obtained from a least-squares fitting of the experimental data: At 200 ◦ C :

doxide () = 61.342 exp(0.021 )

(9)

At 230 ◦ C :

doxide () = 43.569 exp(0.027 )

(10)

At 260 ◦ C :

doxide () = 7.071 exp(0.047 )

(11)

where d0 is the initial copper oxide thickness,  is the contact angle and kS(T) is a first order rate constant which is function of the temperature and can be determined assuming Arrhenius behaviour as follows: kS(T ) = kS0 exp

 −E  a R×T

(13)

Ea is the activation energy for the spreading of lead free solder on oxidized copper surfaces; R is the molar gas constant and kS0 is the pre-exponential factor. The activation energy can be calculated by plotting the logarithm of the kS(T) values obtained from Fig. 10 versus the reciprocal values of the temperatures. An activation energy Ea of 27 kJ/mol and a kS0 value of 1/0.05◦ were obtained from Fig. 11. 3.3. Oxide thickness measurement on OSP The measurements of the oxide thickness between Cu/OSP were not possible with the sequential electrochemical reduction analysis SERA because a very thin layer of organics remains on the surface after thermal treatment. The literature [19,20] also states that most of the imidazole/copper complex remains stable on the copper surface when it is subjected to high temperatures. Some type of reaction took place between the borate buffer and the imidazole which generated a compound of dark colour at the measuring area. Normally the borate buffer solution is able to dissolve all the oxides when a cathodic current is applied to the system, but no characteristic plateaus for Cu2 O or CuO were detected when the organic layer was present. In order to measure the oxides with the SERA method, it was tried to dissolve only the organic layer

These equations can be expressed as a general form of the exponential growth [18]. doxide () = d0 exp(kS(T ) × )

(12)

Fig. 11. Arrhenius plot of Ln(kS(T) ) versus T−1 for the determination of the activation energy of the spreading of solder on oxidized surfaces.

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Fig. 13. XPS depth profile of a commercial available Organic Solderability Preservative (OSP) of last generation as delivered.

Paw [21] has also proposed oxide growth under the OSP layer after several reflow processes in air, but no evidence was presented. Wetting test were also done on PCBs coated with Organic Solderability Preservatives. All the measured angles on OSP surfaces are presented in Fig. 15. The experiments were carried out under the same oxidation conditions used for bare copper.

Fig. 12. FIB-SEM Images of printed circuit boards with OSP: (a) as delivered; (b) after 2 h at 155 ◦ C.

by using different organic solvents such as acetone, isopropanol, methanol and dichloromethane. However, it was not possible to remove the organic layer with these solvents, and no signal for Cu2 O or CuO could be detected by SERA. The organic layer could be dissolved only by a mixture of methanol with hydrochloric acid, but this solution also dissolved the whole oxides. With the intention to understand how the oxidation of surfaces coated with Organic Solderability Preservatives (OSP) is affected, it was necessary to make some measurements with Focus Ion Beam (FIB) and Scanning Electron Microscope (SEM). From Fig. 12 it can be observed that the oxidation takes place at the interface between copper and OSP and not at the uppermost layer of the surface. It can be noticed also that the OSP material of last generation is not released after the thermal treatment. XPS depth profiles done on OSP surfaces at delivery condition do not show almost presence of oxygen inside the organic layer (Fig. 13), thus the oxidation at the interface between copper and OSP must take place due to oxygen diffusion through the organic layer. Fig. 14 exhibits the diffusion of oxygen through the organic layer. An oxide formation was not observed after a standard leadfree process (Fig. 1) under an oxygen concentration of 1000 ppm. The formation of an oxide layer at the interface between copper substrate and OSP was observed only when the standard lead-free reflow process was carried out under an air atmosphere, thus the oxygen must diffuse from the outside through the organic layer. From these figures it should be noted that the OSP material of last generation is not released after the standard lead-free reflow step.

Fig. 14. FIB-SEM Images of printed circuit boards with OSP after a standard leadfree reflow process: (a) under a 1000 ppm oxygen concentration; (b) under an air atmosphere.

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A k␪0 value of 108.5 min−1 and an activation energy for the OSP thermal decomposition of 32.6 kJ/mol were obtained from the curve.

4. Conclusions

Fig. 15. Contact angles between solder and PCB’s with OSP as final finish. Oxidation at 100, 155, 200, 230, and 260 ◦ C.

The wetting angles on OSP surfaces were much smaller than the wetting angles on bare copper, which means that the OSP coating really protects the surface from oxidation during thermal processes, leading to a much better solder spreading. Fig. 12b illustrates that the oxides generated at the interface between OSP and copper form an inhomogeneous layer of non-uniform thickness which seems to be porous. Therefore the flux is able to penetrate these oxides in a much easier way, generating a better wettability. In order to calculate the kinetics of the thermal decomposition of OSP, It is necessary to express the equations presented on Fig. 15 on the general form of exponential growth. (t) = 0 exp(k(T ) × t)

(14)

where  0 is the contact angle when the OSP surface has not been oxidized (initial contact angle), t is the exposure time to temperature and k(T) is a first order rate constant which is function of the temperature and can be determined assuming Arrhenius behaviour as follows: k(T ) = k0 exp

 −E  d R×T

(15)

Ed is the activation energy for the thermal decomposition of OSP; R is the molar gas constant and k␪0 is the pre-exponential factor. The activation energy can be calculated by plotting the logarithm of the k(T) values obtained from Fig. 15 versus the reciprocal values of the temperatures (Fig. 16).

Fig. 16. Arrhenius plot of Ln(k(T) ) versus T−1 for the determination of the activation energy for the thermal decomposition of OSP.

An oxidation method was developed in order to produce homogeneous copper oxide layers. A copper substrate with a very flat surface (Cu/Si-wafer) was necessary to obtain a good repeatability of the SERA measurements. Only Cu2 O was detected with the Coulometric Reduction Technique (SERA) but XPS analysis done for the calibration of the SERA equipment proved also the presence of a CuO layer smaller than 2 nm above the Cu2 O oxide after an oxidation at 260 ◦ C. The oxidation kinetics of thin copper films follows a power-law dependence. An activation energy of 33.1 kJ/mol was obtained for the oxidation from Cu to Cu2 O. It was possible to establish a correlation between copper oxide thickness and wetting of copper surfaces by measuring the contact angles between solder and copper substrate after soldering. It was found that the wetting is acceptable only when the oxide thickness is smaller than 16 nm. An activation energy of 27 kJ/mol was obtained for the spreading of lead free solder on oxidized copper surfaces. FIB-SEM images illustrate that the oxidation of surfaces coated with OSP takes place at the interface between copper substrate and organic layer and not on the uppermost layer of the organic coating. The oxidation takes place there due to the oxygen diffusion through the organic layer. The SERA method was not appropriate for measuring the oxides when the organic layer was present. Other methods such as ellipsometry are not suitable because the oxide layer is under the organic coating. The only appropriate method to measure directly the oxidation of copper surfaces coated with OSP seems to be FIBSEM analysis. The wetting tests done on PCBs coated with Organic Solderability Preservatives were always better than the wetting test on bare copper. Thus the flux is able to remove the oxides generated under the OSP layer much better than the compact oxides generated on bare copper. An activation energy for the OSP thermal decomposition of 32.6 kJ/mol was found.

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[15] Industrial Printed Circuits IPC, Standard IPC J-STD-003B, Solderability test for printed boards, USA (2007). [16] Gen 3 Systems Ltd., MUST system, solderability testing using the wetting balance and micro-wetting balance methods, Hampshire, GU14 0NR, UK (2002). [17] French standard NF A 89400, Brasage tendre, Mesure de brasabilité au méniscographe, France (1991).

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