How To Reduce Damage Of An Ing Iron Tip

Transcription

1 [ 溶 接 学 会 論 文 集 第 27 巻 第 2 号 p. 29s-213s(29)] Reduction of Damage of ing Iron Tip by Addition of and Ni to Sn-Ag-Cu Lead-free * by Tadashi Takemoto **, Ken-ichi Tomitsuka ***, Toshio Tooyama *** and Hiroshi Nishikawa ** To confirm the effectiveness of and Ni addition to Sn-Ag system lead-free solder on reduction of erosion of iron plating of soldering iron tip, Sn-Ag-Cu-(-Ni) system flux cored solder wires were used in operation lines under several manufacturing operation conditions. The temperatures of soldering iron were maintained at about 623~663K.The degree of damage was evaluated by cross sectional measurement of the thickness of Fe plating before and after soldering operation. The obtained data had relatively large scattering, but the and Ni addition was found to be effective to suppress erosion damage in all operation lines. Double addition of and Ni was also confirmed more effective than single addition of. Key Words: ing Iron Tip, Fe Plating, Lead-free, Flux red, Addition, Ni Addition, Erosion 1. Introduction More than two years has past after the start of RoHS 1) related to WEEE which regulated the use of toxic substances such as Pb. Accordingly, lead-free soldering has become a standard process for the recent advanced electronics assembly. The Sn-Ag-Cu system solders are the most reliable alloy under consideration of wettability and mechanical properties, however, it has some unsatisfactory characteristics such as relatively poor wettability, higher operation temperature and high dissolution rate of contacting solid metals. The last characteristic causes the severe damage of soldering equipments such as soldering iron tips and many parts of wave soldering machine 2)3). The phenomenon is called as erosion. In manual soldering, the Fe plating on soldering iron tip is damaged during manual soldering process 3). The damaged tip should be replaced with a new one that increases the cost and reduces the production efficiency. Some of the authors conducted the basic study on erosion process and found some reduction methods of erosion of Fe plating on soldering iron tip 2). One of the effective method is the addition of Fe, and Ni to lead-free solder 3)4). The laboratory test revealed the reduction of the damage of Fe plating by using the solder containing small amount of VIII group elements in the periodic table such as Fe, and Ni. All these tests were conducted under the laboratory test conditions where the temperature and solder supplying rate were constant during the test. However, the soldering iron tips are usually used under the uncertain conditions in the real operation lines with different waiting time, wipe of surface or cleaning methods and periods. Such irregular operation conducted under the judgment of an *** Received: *** Member, Joining and Welding Research Institute, Osaka University, Ibaraki, Osaka, Japan *** Sony EMCS rporation, Ichinomiya, Aichi, Japan operator may influence the damage on soldering iron tip. Accordingly, this study aimed to confirm the effectiveness of the addition of these elements on the improvement of the damage of soldering iron tip by using flux cored solder wire under the conditions in factory operation lines. 2.1 Basic experiment 2. Experimental procedure The basic experiment to investigate the effect of addition of and Ni to lead-free solder on suppression of erosion of Fe plating was conducted using Fe plated Cu substrate having size of (mm) and Fe plating with about 4 m thickness. The used solders were Sn-3.5Ag based lead-free solder containing -1. mass% and/or -.1 mass%ni. The measured erosion depth was the mean value of the difference between the initial thickness and thickness after erosion test. 2.2 Experiment in factory lines The experiments in factory lines were conducted under the conditions shown in Table 1. The diameter of flux cored solder wire, shape of soldering iron tip and the setting temperature of tip Table 1 Nominal Chemical compositions of flux cored solder wire and test conditions of erosion test. Factory FA FB Symbol Sn-3.Ag-.5Cu Ni Ni Sn-3.Ag-.5Cu Ni - Ni Diameter, Shape of mm soldering iron tip Type C Type D Temperature of soldering iron tip, K 623~ ~663

2 21s 研 究 論 文 Tadashi TAKEMOTO et al.:reduction of Damage of ing Iron Tip by Addition of and Ni to Sn-Ag-Cu were different according to the factory lines. The flux cored solder wires used were Sn-3.Ag-.5Cu () and -.2 and Ni. The diameters were.8 mm and 1.2 mm containing 2.6 mass% of RMA flux. Table 1 also shows the shapes of soldering iron tip and combinations of test conditions. In type C soldering iron tip flux cored solder wire of diameter with.8 mm was used at temperature of K. In type D soldering iron tip flux cored solder wire of diameter with 1.2 mm was used at temperature of K. The damage was evaluated by the residual thickness of Fe plating by measuring of the erosion depth using cross sectional micrographic observation. Residual thickness of Fe plating = (Thickness before test) (Thickness after test) In type D, the thickness is expressed by the total Fe plating thickness at both sides. 3. Experimental results 3.1 Results of basic experiment Figure 1 shows the results of basic experiment indicating the effect of and Ni addition on erosion thickness of Fe plating. It is quite clear that erosion thickness gradually reduced with increasing the additional amount of. The reducing effect is evident up to.1-.5% addition, however, the effect is only a little from.5 to 1.%. The double addition of.2 and.1ni seems to be more effective than the single addition of.3. Accordingly, under the same amount of addition the double addition of and Ni is preferable to the single addition of. The basic laboratory test had already revealed that the addition of VIII group element of Fe in the periodic table was effective for the suppression of erosion of Fe plating 2). The present study also found that the addition of VIII group elements and Ni was effective to reduce erosion of Fe plating. only Ni, +Ni 3.2 Results of Evaluation in Factory Line Table 2 shows the result of erosion tests conducted at three operation lines in factory FA using type C tip. The table summarized the life time of soldering iron tip judged by the damage of tip. There are relatively large differences in the life time of iron tip depending on each line, however, the tendency was the same in all lines, the life time of soldering iron tip depended on the composition of flux cored solder wire used. Especially the life time in line B adopted higher operation temperature the life times were shorter than in the other two lines that showed the same life time. The life time increased with the following order in every line. < - < -Ni It is quite clear that the addition of and double addition of and Ni to offered longer life time of soldering iron tip. Table 3 shows the results of erosion tests conducted at the same line in factory FA. Even in the same operation line the large scattering was observed. Within three test results, the obtained erosion depth differed more than 2 times in the results using -Ni solder, however, the reduction effect by addition of and Ni is quite obvious, the addition of these elements reduced the erosion depth about 1/3 of without addition of and Ni. The reason of these large scattering may be attributed to the following factors; 1 difference in the soldering operation time and waiting time, 2 difference in operation including the difference in amount of solder used at each soldering operation, 3 scattering of thickness of Fe plating on soldering iron tips. The above waiting time means the time of heating of soldering iron without soldering operation. Accordingly, during this time the Fe plating on soldering iron tip suffered oxidation under high temperature nearly equal to soldering operation in air atmosphere. Table 2 Evaluation of life time of soldering iron tip for Type C at different practical lines in factory FA after using of several flux cored solder wires. Life time of soldering iron tip, d (Symbol) Diameter, mm Line A 623~643K Line B 633~653K Ni Line C 623~643K Fig. 1 Effect of and Ni addition to Sn-3.5Ag solder on suppression of erosion of Fe plating. Table 3 Scattering of erosion depth data in each test, Type C in factory FA. wire Erosion depth, mm/kg diameter, First Second Third Mean (Symbol) mm test test test value Ni

3 溶 接 学 会 論 文 集 第 27 巻 (29) 第 2 号 211s Especially, large scattering of Fe plating thickness on soldering iron tips was observed, however, the mean Fe plating thickness was adopted after measuring many Fe plating of soldering iron tips in this study. In addition to the above three factors, the cleaning condition of Fe plating also gives influence on erosion. Figure 2 shows the relation between the residual Fe plating thickness and amount of solder consumption using type C soldering iron tip. As mentioned above, the obtained data had relatively large scattering, however, it is clear that the residual thickness certainly reduced with increasing of the solder consumption, that corresponding to the damage of Fe plating on soldering iron tip was increased with increase of the soldering operation time. The order of the residual thickness of Fe plating was identical with the results above mentioned. < - < -Ni The double addition of and Ni gave the best results having the minimum damage that is equal to the maximum residual thickness of Fe plating. Figure 3 shows the relation between the residual Fe plating thickness and amount of solder consumption using type D soldering iron tip. The damage rate was different in type C, however, the effectiveness of and Ni addition for suppression of erosion was evident. Figure 4 shows examples of cross section of soldering iron Residual plating thickness ( m) Initial: 54 m Type C,.8φ -Ni Amount of solder consumption (g) Fig. 2 Plots between amount of solder consumption and residual plating thickness after soldering using type C iron tip. Residual plating thickness ( m) Ni - Type D, 1.2φ Amount of solder consumption (g) Fig. 3 Plots between amount of solder consumption and residual plating thickness after soldering using type D iron tip. tips after erosion test on type C. In both factory lines the residual thickness of Fe plating in and Ni added solder is larger than in without these elements. 3.3 Effect of microstructure The -Ni solder contains Sn 2 intermetallic compound. The size and distribution of Sn 2 is affected by the history of cast and drawing of flux cored solder wire. Figure 5 shows the microstructure of flux cored SAC-Ni wire. In Fig. 5(a) relatively large Sn 2 intermetallic compounds are observed, however, such coarse compound were not observed in Fig. 5(b). To make fine microstructure, establishment of the sophisticated flux cored wire production process is important by adjusting cast temperature, solidification rate of ingot, drawing rate of wire and the other factors. Figure 5(c) shows the comparison of erosion test results using flux cored solder wires with different microstructure. The solder Factory FA Type C Line A Line B -Ni 1mm Fig. 4 Cross section of Type C soldering iron tip after using of about.5kg solder. (a) (b) (c) BSI (mpo.) BSI (mpo.) Erosion depth ( m) Type C/.5kg 1 m 5 m Fig. 5 Microstructure of flux cored solder wire with coarse Sn 2 (a), with fine Sn 2 (b) and erosion test results in Wire W and Z having coarse and fine Sn 2 respectively.

4 212s 研 究 論 文 Tadashi TAKEMOTO et al.:reduction of Damage of ing Iron Tip by Addition of and Ni to Sn-Ag-Cu wire having fine microstructure (Wire Z) showed lower erosion depth than coarse one (Wire X). As mentioned earlier, there is some larger scattering in the results of factory production line, the suppression effect of fine microstructure may not be reliable, however, the suppression effect can be explained in the following discussion. 4. Discussion 4.1 Mechanism of suppression of erosion by addition of and Ni to lead-free solder The effectiveness of addition is explained using the difference of Fe concentration between molten solder and saturation concentration at test temperature. The dissolution rates of solid metals in molten metals were usually expressed as follows 5)-7). dc/dt = K(A/V)(C s -C) (1) where,c is the concentration of solute in liquid after reaction time t, K is a constant, A is the interface area between the solid and liquid, V is the volume of liquid, and C s is the saturation concentration of solute in liquid. In this study, the test conditions related to the specimen size, the parameters A and V are constants. Accordingly, if (C s -C) shown in Equation (1) increases, the dissolution rate also increases. The concentration difference (C s -C) is the driving force for dissolution. Figure 6 indicates the estimated liquid phase plane of ternary Sn-Fe- system at manual soldering temperature range of 35~ 663K in this experiment. The arrow 1 corresponds to (C s -C) 623~ 663K Sn 1 2 L 623~ 663K Fe Fig. 6 Estimated ternary phase diagram at Sn rich corner of Sn--Fe at manual soldering temperature ranges showing liquid phase area. for pure Sn representing in this experiment and the arrow 2 corresponds to (C s -C) for Sn- representing - and --Ni in this experiment. Apparently, the arrow 2 is shorter than the arrow 1 indicating that the driving force of added solder is smaller than that of solder without. The suppression effect of Ni addition can be explained similarly by constructing the similar figure for Sn-Fe-Ni ternary system. rresponding to the decrease of (C s -C) that is the driving force of dissolution, and Ni added flux cored solder wire showed reduced erosion depth of Fe plating on soldering iron tips. 4.2 Effect of fine microstructure The suppression mechanism of fine microstructure can be explained as follows. The effectiveness of addition becomes effective after solution of in molten solder, because Sn has negligible small solid solubility of 8). Longer time is required for complete solution of coarse Sn 2 into molten solder than fine Sn 2. The solubility of and Ni in Sn matrix is negligible small 8)9), therefore, rapidly heated molten liquid solder contains little at initiation of melting. Erosion damage occurs at this initial stage of soldering. The finely dispersed Sn 2 dissolves rapidly into molten liquid solder, this situation suppresses dissolution of Fe by reducing the driving force of dissolution. 4. nclusions The effect of and Ni addition to flux cored solder wire on the suppression on erosion damage of Fe plating on soldering iron tips was evaluated by factory operation lines using two types of soldering iron tips. At first step the effectiveness of these elements was found in basic test using Fe plated copper plate. In factory lines the effectiveness of the addition of these elements was also confirmed even the data obtained showed relatively large scattering. The double addition of and Ni also showed better results than single addition of. The flux cored solder wire with fine Sn 2 phase seems to be more effective than the wire with coarse Sn 2. The mechanism of suppression by and Ni addition was explained by reduction of the driving force for dissolution based on dissolution kinetics. To explain this mechanism, Sn-Fe- ternary phase diagram showing liquid phase plane at soldering temperature was used. Acknowledgement The authors would like to express hearty thanks to Mr. A. Komatsu for his helpful contribution to the experimental works.

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