Analysis of Differences in Inductance of Ni-Cu-Zn Ferrite Chip Inductors during the Plating Process

Transcription

1 J. Chem. Chem. Eng. 8 (2014) doi: / / D DAVID PUBLISHING Analysis of Differences in Inductance of Ni-Cu-Zn Ferrite Chip Inductors during the Plating Process Yin-Lai Chai 1, Shih-Feng Chien 2*, Wen-Hsi Lee 2, Chin-Pei Lin 3, Wen-Yu Lin 3 and Pei-Yi Wei 3 1. Department of Jewelry Technology, Dahan Institute of Technology, Hualien County 97145, Taiwan (R.O.C.) 2. Department of Electrical Engineering, Cheng Kung University, Tainan City 701, Taiwan (R.O.C.) 3. Cyntec Co., LTD., Hsinchu County 30076, Taiwan (R.O.C.) Abstract: Although plating is a necessary process for SMT components, it alters the magnetic characteristics and inductance level of Ni-Cu-Zn ferrite components. The results of this work show that the following three factors in plating affect these components, and the effects are different for Ni- and Sn-plating: (1) Plating layers exert stresses and react with the residual stress of components to change the inductance level, and the effect of the tin layer is greater than that of the nickel one; (2) The plating current induces a magnetic field inside the components directly and indirectly, and this remains as remanence inside the components and reduces the inductance level, and the effect level of Ni-plating is greater than that of Sn-plating; (3) The plating solution corrodes the interface of the termination and ferrite core of the components to release the residual stress, and causes an increase in inductance, and the effect of Sn-plating is greater than that of Ni-plating. In addition, the inductance level is the result of the net effect of these three factors, and if the sintering temperature is increased to change in the type of residual stress, the net effect will be changed. Key words: Plating, ferrite, stress, inductors. 1. Introduction The stability of the inductance of power inductors has a very close relation with the performance of electronic circuits. In our previous study (not yet been published), we found that both stress and temperature affect the stability of inductance, and the source of stress and temperature characteristics of chip inductors come from their manufacturing processes. According to the results of our previous work, every process will contribute their own stress that reacts with the original stress inside the components. The reaction includes two situations, which are accumulation and counteraction, and these depend on the original type of stress inside the components. The results of our previous work show that the plating process has the most complicated effects during the manufacturing of SMT components. The purpose of the plating process is to make a tin layer for soldering the components to the circuit boards. * Corresponding author: Shih-Feng Chien, Ph.D. Candidate, research fields: magnetic material and multilayer processes. eddie.chien@cyntec.com. However, due to poor adhesion between the tin layer and silver termination, a nickel layer has to be plated before tin plating to achieve this. Both Ni-plating and Sn-plating need a low ph solution and DC current to promote the reaction rate of the dissolve-reduction of Ni or Sn materials. Therefore, there are four potential effects during plating that can alter the performance of components, as follows: (1) The plating layers formed in the plating process will exert mechanical stress on the body of components [1, 2], and this situation is similar to the dipping and curing processes, and also it depends on the type of original stress inside the components, whether they accumulate or counteract, with the former one reducing the inductance and the latter increasing it; (2) The plating current might create a magnetic field inside the components and affect the status of the magnetic domain, resulting in a change in inductance. There are two possible mechanisms that might cause this. The first is that a remanence might remain after plating, resulting in lower inductance. The second is

2 1126 Analysis of Differences in Inductance of Ni-Cu-Zn Ferrite Chip Inductors during the Plating Process that demagnetization might occur after plating, resulting in increased inductance; (3) The plating solution corrodes the surface of components [3], especially when the surface has some defects such as cracks or low densification, and this will enhance the process of corrosion or penetration of the solution, and might alter the inductance level of the components; (4) The water in the solution will be ionized by the current to produce H + ions, and these will reduce the Fe 3+ of ferrite to Fe 2+ partially and produce free electrons, the electrons will jump between the crystal lattice of Fe 3+ and Fe 2+, and this mechanism will lower the resistivity of the surface of components [4-7]. While plating is a necessary process for SMT (surface-mount technology) components, few articles study the relationship between magnetic characteristics and the various factors involved in plating. Therefore, in this work, we focus on the plating process and try to explain the causes and effects of the changes in inductance that can occur after this. With regard to the electrolysis of water during the plating process, as mentioned in (4) above, for Ni-Cu-Zn ferrite materials, this can be eliminated easily by drying after plating, and so this is not investigated in this study. According to previous research (not yet been published), the increase in inductance after heat treatment is based on two factors, which are the releasing of stress and changing the magnetic domain status. Even though the latter leads to a greater increase in inductance than the former one, because most of the magnetic status is readily affected by the magnetic field, the inductance will drop significantly after applying a current to the components. The reflow treatment induces both stress release and a change in magnetic status, whereas the TC (thermal cycling) treatment only induces the former. To distinguish the magnetic and stress effects after the plating process, we can treat the samples under reflow and TC conditions, respectively, and then, calculate the difference in the increased inductance between these treatments, thus finding the effect of the magnetic field. 2. Experiments 2.1 Sample Preparation Chip Samples A powder with the composition shown in Table 1 was used to prepare the slip and foils, and 8.5 turns of silver paste were printed as the coil on the foil, then the foils were stacked, laminated and cut into the rectangle samples with the dimensions of 2.5 (l) 2.0 (w) 1.0 (t) mm. The samples were fired at, 885, and 905 o C for 1 h, then the termination was dipped and cured at 700 o C 30 min, and then plating completed the sample preparation process Circular Samples The foils produced in Section were stacked, laminated (under 2,000 psi for 1 h) and molded to prepare the circular type samples with a core size of 1.0 mm and 7/17 mm for the thickness and inner/outer diameter, and these samples were then fired at 885 o C for 1 h. We wound the copper wire at primary and secondary sides for equal numbers of turns (0, 14 and 24), and then used the 14-turn samples for another Sn-plating experiment with twice the plating current. 2.2 Measurement and Observation (1) The chip samples produced in Section were used to measure the inductance before/after reflow treatment (270 o C for 30 min) and TC treatment ( o C for 30 min at each peak temperature, for a total of 10 cycles) after each process, including sintering, curing and plating; (2) The circular samples produced in Section were used to measure the inductance before/after TC treatment after each process, including Ni-plating and Sn-plating; (3) The chip samples without plating from Section Table 1 The composition of Ni-Cu-Zn ferrite. Composition Fe 2 O 3 NiO CuO ZnO Moi %

3 Analysis of Differences in Inductance of Ni-Cu-Zn Ferrite Chip Inductors during the Plating Process 1127 Table 2 The conditions of the plating experiments. Sintering temperature ( o C) Ni-plating conditions Sn-plating conditions Ni thickness (μm) 16 A 90 min 9 A 140 min 10 A 145 min 7 A 175 min 16 A 90 min 9 A 140 min 10 A 145 min 7 A 175 min Sn thickness (μm) Origial Ls (μh) After TC After reflow Reflow-TC were used for the plating experiments, with the conditions listed in Table 2, and the inductance was measured of before/after reflow and TC treatment; (4) The chip samples fired at and 885 o C from Section were soaked in the nickel and tin solutions, and the inductance was measured of before/after soaking; (5) The chip samples fired at 885 o C in Section were soaked in tin solution with different ph values (3.3, 4.5, 5.5), and then the 885 o C-fired sample was soaked in the ph 4.5 tin solution under rolling conditions, and the inductance of these samples was measured before/after soaking; (6) The chip samples fired at 885 o C in Section were soaked in ph 4.5 of tin solution without rolling conditions for 0, 12 and 24 h, and the interface of the termination and ferrite body was observed by SEM; (7) The chip samples fired at and 905 o C in Section 2.1.1, and their wetting angles of the surfaces were measured and observed by SEM. 3. Results and Discussion We measured the inductance of the samples before/after TC treatment and reflow treatment of each process, including sintering, curing and plating, and calculated the difference in inductance after the reflow and TC treatment. This difference could be used to eliminate the effects of stress and show only the pure magnetic ones. As the result show in Table 3, the plating had the highest change in inductance due to the magnetic effect, which also means that the magnetic effect started during Ni-plating. To uncover the contributions of each factor in the plating process to the stress and magnetic responses of chip samples, we performed various experiments with different combinations of conditions, and measured the inductance before/after reflow treatment and TC treatment, and the results are shown in Table 4. For the o C-fired samples after TC treatment, the rate of change in the inductance of the samples with a thicker tin layer was greater than that of samples with thicker nickel layer, and this reveals that the tin layer exerted a relatively large stress on the chip samples, and this Table 3 The effect of the magnetic field in raising the inductance increment of the chip samples after each process. Reflow-TC Sintering -1.61% Curing -1.11% Ni plating 1.47% Sn plating 4.86%

4 1128 Analysis of Differences in Inductance of Ni-Cu-Zn Ferrite Chip Inductors during the Plating Process might because the thickness of this layer was greater than that of the nickel one. The column reflow-tc shows the contribution of the magnetic effects to the change in inductance. It is clear that when the plating thickness was increased, both nickel and tin layers reduced the contribution of the magnetic effect to the change in inductance, and that the tin layer had a greater effect in this respect than the nickel layer did. Because this result was opposite to that found for stress, it can be concluded that the thicker plating layer exerted higher stress, and might inhibit the magnetic moment rotation and magnetic domain wall motion, as mentioned in other studies (not yet been published), resulting in a lower contribution from the magnetic effect. The results for the o C-fired samples show that the initial inductance of these samples was smaller than that of o C-fired ones, and it can also be seen that these samples underwent a huge increase after the TC treatment comparing to the o C-fired ones. It is known that a higher sintering temperature will further reduce the porosity and increase the grain size in the microstructure of such samples, and both of these conditions will increase the permeability of the materials, and thus the o C-fired samples should have a higher initial inductance. However, when the sintering temperature is increased, this will increase the residual stress level inside the components, and counteract the increase in permeability due to the changes in microstructure. This mechanism caused the lower inductance level of the o C-fired samples, which was even lower than that of the o C-fired samples, as shown in Table 4. In the meantime, both the thicker nickel and tin layers increased the stress in the chip samples due to the higher sintering temperature, although the difference in stress between them was reduced, because the total level of residual stress in the samples increased. Compared to the o C-fired samples, the o C-fired ones had a greater magnetic effect, which suggests that the increase in magnetic domain motion caused by the higher sintering temperature not only contributes to greater permeability, but also the higher magnetic sensitivity of the chip samples. In addition, the magnetic effect of the thicker tin layer was greater than that of thicker nickel layer, different to the results found for the o C-fired samples. In another of our studies (not yet been published) about the relationship between stress and sintering temperature, the results show that in the specific silver and ferrite materials of the chip samples, their TMA curves cross at 885 o C. That is, the Table 4 The results of the plating experiments. Sintering Ni-plating conditions Ni thickness Sn thickness Origial Ls temperature ( o C) Sn-plating conditions (μm) (μm) (μh) After TC After reflow Reflow-TC % 10.02% 9.06% 16 A 90 min % 9.45% 8.51% 9 A 140 min % 8.75% 6.46% 10 A 145 min % 9.12% 8.85% 7 A 175 min % 8.13% 6.93% % 19.54% 8.86% 16 A 90 min % 25.14% 13.81% 9 A 140 min % 26.10% 15.10% 10 A 145 min % 22.55% 11.80% 7 A 175 min % 23.16% 12.41%

5 Analysis of Differences in Inductance of Ni-Cu-Zn Ferrite Chip Inductors during the Plating Process 1129 shrinkage rate of silver is greater than that of ferrite at o C, whereas the reverse is true at o C. This result shows that the chip samples had a change in the shrinkage rate of these two materials as the temperature rose from o C and o C, and that the stress changed not only its level, but also its type, shifting from compressive to tension stress. The type of stress is related to the level of magnetostriction, with compressive stress increasing this and the tension stress decreasing it, and so this change in stress also will be affected by the magnetic characteristics of the inductance of the chip samples. As noted above, we think the difference in the results was caused by the change in the type of residual stress in the chip samples fired at o C and o C. In addition, while the thickness of the plating layers was increased by increasing the current and prolonging the duration, neither of these conditions changed the results significantly, showing their weak relation to the stress and magnetic effects. To further understand the interaction between the magnetic effect and plating current, we had to eliminate the interference of the residual stress and stress from outside the sample. Because the residual stress and foreign stress come from the cofiring of coil/core and termination plating, we used ferrite foil to prepare circular type samples and wound the copper wire around them after sintering, and then carried out experiments with various combinations of conditions. First, as can be seen in Fig. 1, the increase in inductance of the circular samples before and after TC treatment was almost constant, and this shows that there was no stress present inside the circular samples. Fig. 1 also shows that the inductance changed from 6.1 μh to 5.4 μh after Ni-plating, whereas the inductance change was much less after Sn-plating. The significant inductance change after Ni-plating could be attributed to the remanence that was built up by the plating current. However, this remanence cannot exceed a certain amount, and so the plating current could not further affect the magnetic status inside the circular samples Inductance (µh) Fig. 1 The inductance level of the circular and chip samples after each stage in the plating process. Inductance change % -5% -10% -15% -20% -25% Original Af-Ni Af-TC Af-Sn Af-TC Processes Turn numbers Fig. 2 The change in inductance of circular samples with different numbers of coil turns after the plating process. during Sn-plating. As shown in Fig. 2, the fall in inductance was directly proportional to the number of turns of the coil, however, there was 6% fall in inductance even for the circular samples without any coil, and this suggests that a magnetic field could be directly induced in the ferrite core by the plating current. However, when the wire was involved, the magnetic effect caused by the plating current could make the inductance drop even further. As shown in Fig. 3, because the strength of the magnetic field was directly proportional to the plating current, when the plating current increased, the fall in inductance drop also increased after the components had undergone plating.

6 1130 Analysis of Differences in Inductance of Ni-Cu-Zn Ferrite Chip Inductors during the Plating Process Inductance change 1% -4% -9%- -14% Current of plating (times) Fig. 3 The change in inductance of the circular samples produced under different plating current settings after the plating process. By comparing the results for the chip and circular samples, we can conclude the following: (1) The inductance level of the circular samples fell significantly after Ni-plating, while that of the chip samples increased. The explanation for this is that the stress did not exist in these samples but remanence did, due to the plating current, and this caused the inductance of the circular samples to fall after Ni-plating. However, both stress and remanence existed inside the chip samples, and since the latter reduced the inductance, so the increased inductance of the chip samples came from the stress from the nickel layer, which counteracted the original stress inside the samples; (2) Both circular samples and chip samples had a smaller increase in inductance after Sn-plating. As mentioned earlier, after the remanence in both circular and chip samples were caused by Ni-plating, the current from Sn-plating no longer has any effect. Furthermore, one more factor in the inductance of the chip samples was the counteraction of the remanence and stress induced by the tin layer; (3) Therefore, the difference in inductance levels after plating for the circular and chip samples is caused by the fact that the former were affected only by the plating current, whereas the latter were affected by both the plating current and plating layers, which created mechanical stress that counteracted the residual stress in the chip samples. Furthermore, this net stress would react with the magnetic effect caused by the plating current, and thus the stress inhibited the motion of the magnetic domain. The results were much more complicated when the sintering temperature of the chip samples was increased to a level high enough to change the type of residual stress remaining inside the chip samples; (4) It is known that a leakage flux exists in magnetic components when a current is applied, and thus if the coverage or thickness of the metal, such as the tin layer on the surface, is increased, the leakage flux is reduced and the effect of the magnetic field on the components will be increased. On the other hand, the nickel layer is kind of magnetic material, and its shielding effect is relatively low compared to that of other metals, and it also emits a leakage flux outside the component. As a result, the leakage flux increased, and the effects of the magnetic field on the inductance were lowered. Therefore, this is one of the causes of the change in inductance of the components at each stage of the plating process; (5) So far, we can induce two factors that cause inductance in the plating process, which are stress and magnetic force, as well as their interaction. The former is a net effect of the plating layers and the residual stress inside components. The latter includes the remanence and flux leakage, both of which are induced by the plating current. The inductance trend can be explained completely by these factors. The third factor in the plating process is the solution, as an acid solution could corrode the surface of a component. To investigate this factor and eliminate the effects of the plating layer and current, we soaked the chip samples in the plating solution without applying a current. As shown in Table 5, all the inductance levels of the soaked samples increased, and the increase of the samples soaked in tin solution was greater than that of the samples soaked in nickel solution. The ph value of nickel solution was 3.8, less than that of tin solution,

7 Analysis of Differences in Inductance of Ni-Cu-Zn Ferrite Chip Inductors during the Plating Process 1131 Table 5 The change in inductance of the chip samples after soaking in plating solutions. Initial Ni-soaking Sn-soaking o C +16.2% +3.73% 885 o C +0.95% +3.01% which was 4.5. We checked the appearance of the chip samples soaked in the solutions, and those samples soaked in tin solution had more serious corrosion on their surfaces. The same result was found in one of our earlier works (not yet been published), which demonstrated the different damage modes of chip samples that underwent nickel and tin solution soaking, with the former due to the exchange of matter, whereas the latter was due to damage to the structure of the samples. Therefore, even though the ph value of tin solution was greater than that of nickel solution, the former still had a higher level of corrosion. Based on this result, we soaked the samples in tin solution with a lower ph value, and kept this value constant for other chip samples with the solution in a rolling status, with the results shown in Figs. 4 and 5, respectively. As the ph was lowered, the change in inductance increased, and, if the chip samples were in the rolling status during soaking, the change in inductance was reduced. Fig. 6 shows the SEM for different durations of soaking in tin solution, and it demonstrates that the solution corroded the interface of the termination and ferrite core, separating them. The adhesion of the termination and ferrite core comes from the melting of the glass frits in the termination paste, and this then penetrates into the open pores of ceramic body during curing [8-11], and this is known as the arching effect. Furthermore, as the material in the termination densifies and shrinks, the termination exerts stress on the components that becomes part of the residual stress, reducing the inductance. Therefore, the plating solution corrodes the glass frits located at the interface of the termination and ferrite core, damaging the arching status, thus releasing stress and increasing the inductance. As the result, the solution affected the inductance of the component at the interface of the termination and Change rate of inductance 20% 15% 10% 5% 0% ph value of Sn solution Fig. 4 The change in inductance of the circular samples after soaking in different ph values of Sn plating solution. Change of inductance 20% 15% 10% 5% 0% Static Rolling State of samples Fig. 5 The change in inductance change of the circular samples after soaking in Sn plating solution under static and rolling status. ferrite core, rather than the surface of the ferrite core. Therefore, the tin solution damaged the interface status and changed the inductance indirectly. The results also show that as the plating proceeded, the corrosion also continued, and stress was released by this mechanism, and thus the lower ph of the tin solution lead to a higher level of corrosion. Consequently, the inductance of the samples soaked in low ph tin solution was greater, and the change of inductance after TC treatment was smaller. In addition, when the chip samples were in the rolling status during Sn-plating, the change of inductance of the components after TC treatment was lower, and this was because the rolling status hindered the process of corrosion. Figs. 7 and 8 show the wetting angles of the chip

8 1132 Analysis of Differences in Inductance of Ni-Cu-Zn Ferrite Chip Inductors during the Plating Process samples fired at different sintering temperatures, and as this temperature increased, the wetting angle also rose, suggesting that firing the components at a higher temperature would increase the wetting ability of plating solution to the surface of chip samples, leading to more corrosion of the ferrite core. However, when the sintering temperature was increased, this lead to a denser surface and less area of grain boundary as well, as shown in Fig. 9, and the corrosion effect was thus reduced. Therefore, the effect of the plating solution on (a) (b) (c) Fig. 6 The SEM of the interface of the termination and ferrite core after the chip samples were soaked in Sn plating solution for different times: (a) 0 h; (b) 12 h and (c) 24 h. (a) (b) (c) (d) Fig. 7 Photographs of the wetting angle for the chip samples fired at different temperatures: (a) oc; (b) 885 oc; (c) oc and (d) 905 oc.

9 Analysis of Differences in Inductance of Ni-Cu-Zn Ferrite Chip Inductors during the Plating Process 1133 Wetting angle ( o ) Sintering temperature ( o C) Fig. 8 The trend of wetting angle of chip samples with different sintering temperatures. (a) (b) Fig. 9 SEM of the surface of chip samples with different sintering temperatures: (a) o C and (b) o C. the components is a net effect from the grain boundary area and wetting ability on the surface of the components. 4. Conclusions In the results of this work, they show that the relation between the plating process and chip samples does not only affect the solderability, but also the inductance level, and the sensitivity of inductance to temperature, stress and magnetic field. The following factors of plating process are related to inductance: (1) After plating, nickel and tin layers exerts stress on the chip samples, which, in general, lowered the inductance, although this effect also depends on the type of residual stress and whether it accumulates or counteracts, reducing or increasing the inductance. In addition, the stress level of the tin layer is greater than that of nickel layer; (2) During plating, the plating current induces a magnetic field through two mechanisms, the first one is that the magnetic field remains as remanence inside the chip samples to lower the inductance, and effect of the nickel plating current is greater than that of tin plating current. We also found that the magnetic effect increases when the plating current and number of coil turns increases, even though the coil was not needed to create the magnetic field inside the samples. The second mechanism involves the plating layer, as the nickel layer would emit the flux and tin layer would shield it, resulting in reducing the magnetic and increasing magnetic effect to the chip samples, respectively; (3) The plating solution corrodes the interface between the termination and ferrite core during plating, and this would release the residual stress and increase the inductance level. In addition, the effect of the tin solution was greater than that of nickel solution in this respect, and the effect of a low ph tin solution is greater than that of a high ph one, and the effect of a rolling solution was greater than that of a static one. Another factor related to the corrosion was the surface status of components, as the corrosion was based on the net effect of the wetting ability of the solution on the surface and grain boundary area.

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