Effect of Iron Plating Conditions on Reaction in Molten Lead-Free Solder

Transcription

1 Materials Transactions, Vol. 45, No. 3 (24) pp. 741 to 746 Special Issue on Lead-Free Soldering in Electronics #24 The Japan Institute of Metals Effect of Iron Plating Conditions on Reaction in Molten Lead-Free Solder Hiroshi Nishikawa 1, Tadashi Takemoto 1, Kouichi Kifune 2, Takashi Uetani 3 and Norihisa Sekimori 3 1 Collaborative Research Center for Advanced Science and Technology, Osaka University, Suita , Japan 2 Department of Environmental Sciences, Faculty of Sciences, Osaka Women s University, Sakai 59-35, Japan 3 Hakko Corporation, Osaka , Japan To demonstrate the dissolution of plated iron in molten lead-free solder and the effect of iron-plating conditions on reactions in molten lead-free solder, the reaction test between Sn-3.Ag-.5Cu (mass%) solder and plated iron was performed. An Iron-plated copper plate, which was made by electroplating iron onto an oxygen-free copper substrate, was used as the test piece. The reaction test was carried out by using an oven in normal air. The solder was placed on the test piece and it was put into an oven held at 4, 43 and 46 C. The interface between the solder and plated iron was particularly examined. It was found that the intermetallic compound of FeSn 2 was formed at the interface regardless of the plating conditions. The results showed that the grain size of plated iron decreased with the increased current density and the dissolution thickness of plated iron in molten lead-free solder increased with the increased current density in the rack plating. In the barrel plating, the grain size was rather small in all the test pieces and the dissolution thickness was rather thick. Thus it has been made clear that the dissolution of plated iron in molten lead-free solder is attributable to the grain size of plated iron. (Received September 24, 23; Accepted December 11, 23) Keywords: lead-free solder, soldering iron tip, plated iron, interfacial reaction, dissolution, grain size 1. Introduction Microsoldering is the predominant technology currently used for joining electronic devices to printed circuit boards and Sn-Pb eutectic solder has been the most popular and widely used for a long time. Sn-Pb solder has excellent wettability and an appropriate melting temperature. Due to the toxicity of lead in environmental and health concerns, 1,2) an EU directive will prohibit the use of lead after July 26. Therefore, many feasibility studies on lead-free soldering have been conducted, and many different solder alloys, such as Sn-Ag, Sn-Cu, Sn-Zn and Sn-Ag-Cu, have been proposed as the potential lead-free solder. 3,4) Among these alloys, the Sn-Ag family solders such as Sn-3.g and Sn-3.Ag-.5Cu are generally recognized as the most promising lead-free solders for general-purpose use. Various investigations have been made regarding characteristics of lead-free solder, including microstructure, mechanical properties and interfacial reactions. 5 12) Thus some defects of lead-free solder have been recently pointed out. These studies have found high reaction rates, especially the high dissolution rates of metals such as copper and iron in lead-free solder. For example, the interfacial reactions between Sn-Ag alloys and Cu substrate have been examined to obtain an understanding of their inter-reaction ) Especially the formation of intermetallic compound, the growth rate of intermetallic layer and the growth mechanism of layer have been actively investigated. Chen and Yen 13) investigated the reaction phase between Sn-Ag and Cu and concluded that Sn was the fastest moving species and Ag was the slowest among the Ag, Sn and Cu elements in the reaction systems. Bae and Kim 14) observed that the scallop-shaped Cu 6 Sn 5 intermetallic compound was formed at the newlyformed boundary between Sn-Ag-Cu alloy and Cu substrate. Zribi et al. 15) showed phase formation and growth at the interfaces between Sn-Ag-Cu solders and Cu were very similar to those commonly observed for eutectic Sn-Pb solder. But there is little information concerning the interfacial reaction between lead-free solder and iron. The dissolution of iron and stainless steel during soldering is an important issue for manufacturing facilities using equipment such as solder baths and soldering-iron tips. Concerning soldering-iron tips, plated iron usually protects the tip of a soldering iron, but the tip s lifetime is reduced by the dissolution of plated iron in contact with molten lead-free solder. In the case of lead-free solder, the soldering-iron tip reacts severely with lead-free solder and the reaction seriously damages the iron tip. 18) Therefore it is important for extending the lifetime of soldering-iron tips to inhibit the reaction between lead-free solder and plated iron. In this study, the dissolution of plated iron in molten leadfree solder has been investigated to reveal the reaction between lead-free solder and the plated iron on the solderingiron tip and to show the effect of the plating conditions on the damage to the soldering-iron tip. 2. Experimental To demonstrate the dissolution of plated iron into molten lead-free solder and the effect of iron-plating conditions on reactions in molten lead-free solder, the reaction test between Sn-3.Ag-.5Cu (mass%) solder and plated iron was performed. The solder was a commercially produced solid wire with diameter of 1. mm. Iron-plated copper plate, which was made by electroplating iron onto an oxygen-free copper substrate (23 mm 23 mm.5 mm), was used as the test piece. The thickness of plated iron was about 3 mm. Four different test pieces were prepared to study the effect of ironplating conditions, summarized in Table 1. The main change was in the current density of the plating in the rack plating. Only one condition was tried in the barrel plating. The experimental procedure is shown in Fig. 1. The test pieces were polished with sandpaper to smooth the surface and then rinsed with acetone and deionized water. A solder wire was placed on a test piece and activated flux containing ZnCl 2 was dropped on the test piece. Then the test piece was

2 742 H. Nishikawa, T. Takemoto, K. Kifune, T. Uetani and N. Sekimori Table 1 Plating conditions. Plating method Plating bath Current density Thickness (A/dm 2 ) (mm) Substrate Rack FeCl 2 5, 1, 2 3 Cu FeCl 2 3 Cu Cu plate Solder wire ( 225mg ) Fig. 1 Electroplating of Fe Polishing surface Sheet specimen Holding at constant temperature Flux (.2ml ) Experimental procedure for reaction test. put into the oven in normal air. The holding temperature was 4, 43 and 46 C and the holding time was 3.6, 1.8 and 32.4 ks. The mentioned temperatures are the real temperature of specimens measured by a thermocouple attached with the specimen. The test was carried out to examine the reaction between the solder and the plated iron. The test piece was removed from the oven and cooled in normal air after the specified reaction times. Then the test pieces were cut and cross-sections were polished and etched for microstructure observation. Optical microscopy and scanning electron microscopy (SEM) were used to examine the interface between the solder and the plated iron and the surface of the plated iron. Energy dispersive X-ray analysis (EDX) and X-ray diffraction analysis (XRD) were applied to determine the composition and metallographic structure of the reaction phases. The dissolution thickness of these phases was measured. Thickness data reported in this study are an average of the measurements of three different areas for four test pieces. 3. Results and Discussion Figure 2 shows SEM micrographs of the solder/plated Fe interface reacted at 46 C for 32.4 ks. Figure 2(a) shows rack plating, with current density at 5 A/dm 2, and Fig. 2(b) shows (a) Solder (b) Reaction phase Plated Fe 1µm Fig. 2 Scanning electron micrographs of solder/plated Fe interface. (Holding temperature: 46 C, holding time: 32.4 ks) (a) Rack plating (Current density: 5 A/dm 2 ), (b) plating. SEM Image Sn Fe 1µm Fig. 3 SEM image and EDX analysis of solder/plated Fe interface. (Holding temperature: 46 C, holding time: 32.4 ks).

3 Effect of Iron Plating Conditions on Reaction in Molten Lead-Free Solder 743 barrel plating. In these figures, the upper, bottom and middle materials are respectively solder, plated Fe, and reaction phase. Regardless of the plating method, formation of reaction phase with scallop morphology was observed and only one reaction phase can be confirmed at the interface. Similar results were found for tests on all the other samples of Sn-3.Ag-.5Cu/plated Fe. The size of scallop like compounds is larger in rack plating with current density of 5 A/ dm 2 than in barrel plating. The size of scallops seems to be affected by microstructure of underneath plated Fe such as grain size. The effect of plating conditions on the grain size of plated Fe will be demonstrated later. In order to determine the compositions of the reaction phase, EDX analysis and XRD analysis were performed. Figure 3 shows the tin distribution and the iron distribution on the interface of Sn-3.Ag-.5Cu/plated Fe as analyzed by EDX. It is clear that the reaction phase is composed of tin and iron suggesting that the phase is intermetallic. Furthermore, a typical XRD pattern taken from the reaction phase between Sn-3.Ag-.5Cu solder and plated Fe is shown in Fig. 4. The reaction phase shown in Fig. 2 is identified by the positions of the diffraction peaks. As seen in the figure, FeSn 2 is detected, as well as Sn, which is solder material. Thus it was found that the reaction phase for Sn- 3.Ag-.5Cu / plated Fe was the intermetallic compound of FeSn 2. To estimate the reaction between Sn-3.Ag-.5Cu solder and plated Fe, the dissolution thickness is defined as shown in Fig. 5. The surface of the unsoldered plated iron was a Intensity FeSn2 Sn θ Fig. 4 X-ray diffraction pattern of reaction phase between solder and plated Fe. Reaction phase Fig. 5 Plated Fe Solder Cu plate Definition of dissolution thickness. Dissolution thickness reference line and the thickness of the reaction phase below the original plated surface was defined as the dissolution thickness. Figure 6 shows the average dissolution thickness for each plated Fe after reacting at 4, 43 and 46 C. Although the dissolution thicknesses measured under the same condition vary widely, they tend to increase evidently with higher holding temperature and longer holding time. There was a significant difference of dissolution thickness among the different plating conditions, particularly when the holding temperature was 46 C. In the rack plating, the more the plating current density increased, the more the dissolution thickness increased. The dissolution thickness of the barrel m µ Dissolution thickness,x / m µ Dissolution thickness,x / m µ Dissolution thickness,x / Time, t 1/2 / s 1/ Time, t 1/2 / s 1/ (a) 46 C (b) 43 C (c) 4 C Time, t 1/2 / s 1/2 Fig. 6 Effect of plating condition on dissolution thickness. Holding temperature (a) 46 C, (b) 43 C, (c) 4 C.

4 744 H. Nishikawa, T. Takemoto, K. Kifune, T. Uetani and N. Sekimori plating was as great as that of the rack plating when current density was 2 A/dm 2. As shown in Fig. 6, there is a linear relationship between the dissolution thickness and the square root of holding time. The relationship can be expressed as the following equation: p x ¼ k ffiffiffiffiffi Dt ð1þ where x is the dissolution thickness, k is the growth rate constant, D is the diffusion coefficient and t is holding time. This equation assumes that the controlling p process of the dissolution is diffusion. The value k ffiffiffiffi D can be determined from the slope in Fig. 6. The activation energy of intermetallic growth for dissolution can then be determined from the Arrhenius equation D ¼ D exp Q ð2þ RT where D is the frequency factor, R is the gas constant, T is the temperature, and Q is the activation energy. Figure 7 shows the Arrhenius plot for the growth rate of the dissolution thickness. Although two cases show the poor linearity, we calculated the apparent activation energies under the assumption of linear relationships shown in Fig. 7. Table 2 summarized the obtained activation energies according to the plating conditions. The activation energies ranged from 9 kj/mol to 14 kj/mol. The activation energies estimated in this study are one-third to two-thirds when compared with the activation energies of the self-diffusion of Sn in sold-state Fe, which is 239 kj/mol. Therefore, it can be assumed that the reaction, which is the dissolution of plated iron in molten lead-free solder, is controlled by the grain boundary diffusion of tin in solid-state iron. The surface of plated Fe was analyzed by using XRD and microscopy observation to determine the reason the dissolution thickness of plated Fe was so clearly affected by the plating conditions as shown in Fig. 6. Figure 8 indicates the X-ray diffractogram of the plated surfaces for all plating conditions. Diffractograms of all test pieces gave one clear peak corresponding to (11), which is the ideal peak of iron, and there were no differences among them, proving that the orientation of the Fe plated surface is the same regardless of the plating conditions. Figure 9 shows the optical microscope images of the Fe plated surfaces before reacting with solder. The grain can be observed on the plated surface and the measured average grain diameter for each plating condition is indicated in Fig. 9. At the plating current density of 5 A/dm 2, the largest average grain size of all the test pieces was observed, while the average size was about 1.5 mm. In the rack plating, the ln kd 1/2 / µ ms -1/ T / 1-3 K -1 Fig. 7 Table 2 Arrhenius plot of dissolution thickness. Activation energy for dissolution. Specimen Activation energy (kj/mol) barrel Intensity Fe_cal (Cu Kα) bcc Im3m a = θ Fig. 8 Effect of plating condition on XRD pattern.

5 Effect of Iron Plating Conditions on Reaction in Molten Lead-Free Solder 745 (a) (b) (Average grain size: 1.5µm) (c) (Average grain size: 6.7µm) (d) 2µm (Average grain size: 3.9µm) (Average grain size: 2.8µm) Fig. 9 Microstructure of plated surface. Plating condition (a) 5 A/dm 2, (b) 1 A/dm 2, (c) 2 A/dm 2, (d). more the plating current density increased, the less the grain size decreased. In the barrel plating, the average grain size was about 2.8 mm and it was rather small in all the test pieces. Figure 1 shows the effect of the grain size on the reaction rate between the solder and the plated Fe. The reaction temperatures are 4, 43 and 46 C. The reaction rate was calculated from the slope of the graph as shown in Fig. 6. Regardless of the holding temperature, the reaction rates tended to decrease with the increase of average grain size. The grain size seems to affect on reaction rate at every temperature. It has been made clear that the reaction rate, which corresponds to the dissolution thickness, is attributable mainly to the grain size of the plated surface. As mentioned above, the calculated activation energies of reaction between molten solder and plated Fe are assumed to be controlled by the grain boundary diffusion, and the result obtained in Fig. 1 showed good agreement with this assumption. Therefore, it is very effective for inhibiting the dissolution of the soldering-iron tip in molten lead-free solder to make the specific iron plating with the larger grain size on the soldering-iron tip. 4. Conclusion The dissolution of plated Fe in molten solder was investigated to reveal the reaction between lead-free solder and plated iron on soldering-iron tips. The results obtained are summarized as follows. (1) The intermetallic compounds of FeSn 2 were formed at the interface between Sn-3.Ag-.5Cu solder and plated Fe regardless of the plating condition. Reaction rate, v / 1-8 m s -1/ C 43 C 46 C Rack 5 1 Grain size, d / µ m Fig. 1 Effect of grain size of plated iron on reaction rate. (2) Although the dissolution thicknesses measured at the same condition vary widely, the dissolution thickness of the plated Fe is considered that there is a linear relationship between the dissolution thickness and the square root of holding time. (3) The dissolution thickness of the plated Fe increased with higher holding temperature and longer holding time. Under the same experimental conditions, the dissolution thickness increased with the increase of the plating current density in the rack plating. (4) It has been made clear that the dissolution of plated iron into molten lead-free solder is largely attributable to the grain size of the plated surface. The rate decreased with the increase in the grain size.

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