Consumption of Soldering Iron by Pb Free Solder

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Consumption of Soldering Iron by Pb-Free Solder

*1

Hidekazu Sueyoshi

1

, Harunori Odo

1;*2

, Shinji Mizokuchi

2

, Shigeru Abe

2

and Kazuya Saikusa

2

1Graduate School of Science and Engineering, Kagoshima University, Kagoshima 890-0065, Japan 2Techno Center, JAPAN UNIX Co. Ltd., Kamimashikigun, Kumamoto 861-2202, Japan

The consumption of a soldering iron by Pb-free solder is examined and the kinetic analysis of the reaction between the solder and the soldering iron is discussed. The consumption of the soldering iron increases with prolonged contact time between the solder and the soldering iron. No thick reaction layer is detected at the solder/soldering iron interface at the position where fused solder is removed by compressed air cleaning. However, at the position where fused solder remains, a thick reaction layer is formed. In particular, a thick FeSn2reaction layer is observed when fused solder sticks on the surface of the soldering iron for a long time. On the basis of the reaction kinetics, the growth rate of the reaction layer is demonstrated to be controlled by iron diffusion through the reaction layer. The mechanism of soldering iron consumption by Pb-free solder is as that a30nm-thick layer is formed during one soldering process and then flaked off from the surface of the soldering iron during compressed air cleaning.

(Received December 19, 2005; Accepted February 27, 2006; Published April 15, 2006)

Keywords: soldering iron, lead-free solder, iron plating, diffusion, intermetallic compound

1. Introduction

Nowadays, due to the environmental concerns, conven-tional Sn-Pb soldering has been switched into Pb-free soldering in electronic packaging.1–3)

The core of a soldering iron for a soldering robot is made of copper, which has a high thermal conductivity, and its surface is covered by an Fe plating. Because the practically used Pb-free solder (Sn-Ag-Cu system) contains a large amount of Sn and has a high melting point compared with the conventional Sn-Pb eutectic solder, the use of Pb-free solder causes a significant increase in the consumption of the soldering iron,3) resulting in an increase in the number of

times which the soldering iron is changed and a reduction in production efficiency owing to production discontinuance. Thus, a reduction in soldering iron consumption is desirable. However, the consumption process has not been well understood to date. As a result, no appropriate counterplan against soldering iron consumption has been developed.

In the present study, the consumption of a soldering iron by Pb-free solder was examined, and its reaction kinetic analysis was conducted to reveal its mechanism.

2. Experimental Procedure

Pb-free solder of the Sn-3.0Ag-0.5Cu system (Senju Kinzoku Co., Ltd.: ECO SOLDER RMA02P3M705-1.0) was used. The consumption of a soldering iron was examined using a soldering robot (Japan Unix Co., Ltd.: UNIX-412). Figure 1 shows a schematic illustration of the soldering robot. The soldering iron was heated by heat conduction from a heater. Pb-free solder was supplied to the soldering iron automatically. Fused solder was removed after each solder-ing process by compressed air cleansolder-ing. In the consumption test, a soldering iron for a soldering robot (Japan Unix Co., Ltd.; C4D-R) was used.

Figure 2 shows a schematic illustration of the

longitudi-nal section of the soldering iron. The core of the soldering iron is made of copper having a high thermal conductivity. The surface of the core is covered with an Fe plating. To

Air cleaner

Solder

Soldering

iron

Heater

Fig. 1 Schematic illustration of soldering robot.

Solder Cu

Cr plating

Fe plating Cr plating

Fig. 2 Schematic illustration of the longitudinal section of soldering iron.

*1This Paper was Originally Published in Japanese in J. Jpn. Inst. Metals69 (2005) 848–854.

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prevent the spreading of fused solder, a Cr plating is applied on the upper and reverse sides of the soldering iron. The consumption of the soldering iron was measured at positions

‹and›.

The consumption test was carried out at 653 K. This temperature is practically used in the production. The surface of the as-received soldering iron is covered by Pb-free solder to prevent corrosion. Before the consumption test, the soldering iron was heated to 653 K, and then maintained until an isothermal state was achieved. We designated this holding time as the waiting period. Thereafter, Pb-free solder of 1.0 mm diameter and 10.5 mm length was supplied to the soldering iron automatically and then held for 7.25 s, followed by compressed air cleaning (air pressure; 0.49 MPa, nozzle diameter; 1.5 mm, blowing time; 0.3 s). We designated the process from solder supply to compressed air cleaning as one cycle. The consumption test was carried out for 16000 cycles. Total contact time (waiting period + consumption test time per cyclecyclic number) was regarded as the solder/soldering iron contact time.

The microstructure of the longitudinal section of the soldering iron was examined using an electron probe micro-analyzer (EPMA). The amount of soldering iron consumed was estimated from the difference between the Fe plating thickness of the as-received soldering iron and that of the soldering iron after the consumption test. The mean thickness of the Fe plating of the as-received soldering iron was 640mm.

Fused solder was held on the surface of the soldering iron at various temperatures for various times. The microstructure of the longitudinal section of the soldering iron was examined using EPMA. The influences of temperature and time on the solder/soldering iron reaction were discussed in connection with reaction kinetics.

The micro-Vickers hardness of the reaction layer formed on the surface of the soldering iron was measured under a load of 2.45 mN. To examine whether the reaction layer is blown away by compressed air cleaning, a thick reaction layer was produced by preheating at 683 K for 9 h, followed by the consumption test mentioned above.

3. Results and Discussion

Figure 3 shows SE images of the longitudinal section of the soldering iron after the consumption test. In Fig. 3(a), the solder is shown on the right-hand side of the Fe plating (the white part). As shown in Fig. 3(a), the Fe plating thickness after the consumption test is smaller than that of the as-received soldering iron. The amount of soldering iron consumed is large near the Cr plating. Under a magnification of 5000, no reaction layer was observed. A large cavity appears in Fig. 3(b). This is because the reaction product formed by solder/copper reaction flowed out together with fused solder during the compressed air cleaning. The remaining reaction product is shown at the lower part [‘‘Cu and solder’’ of Fig. 3(b)]. The lower left part (‘‘Copper’’) of Fig. 3(b) shows unreacted copper. On the lower side of the soldering iron, no reaction layer appeared under a magnifi-cation of5000. The reaction layer was observed near the Cr plating [encircled area in Fig. 3(b)].

Figure 4 shows results of the EPMA analysis of the encircled area in Fig. 3(b). The correspondence between the SE image and X-ray images indicates that the upper part of the SE image is an Fe plating and the middle part is a reaction layer [Fe and Sn are detected in the same place in Figs. 4(b) and (c)]. By qualitative analysis, Fe, Sn and O were confirmed in the reaction layer, no Ag and Cu were detected. The appearance of the reaction layer only near the Cr plating may be caused by the promotion of the Fe plating/solder reaction owing to retained solder, which cannot be blown away during compressed air cleaning.

As mentioned above, the consumption along the Fe plating plane is not uniform. Therefore, we measured the amount of soldering iron consumed at both positions ‹ (near the Cr plating) and›(center of the solder contact plane) as shown in Fig. 2. Figure 5 shows the relationship between the amount of soldering iron consumed and solder/soldering iron contact time. When the solder/soldering iron contact time is short, the amount of soldering iron consumed increases abruptly with increasing solder/soldering iron contact time. The amount of soldering iron consumed increases linearly with solder/soldering iron contact time when the latter exceeds 4 h. When the solder/soldering iron contact time exceeds 26 h, the Fe plating remains at position

› but disappears at position ‹, resulting in the loss of function of the soldering iron. As mentioned above, the amount of soldering iron consumed at position ‹is larger

Cr plating

Cr plating

(a)

(b)

Cu

Fe plating

Solder

Cu

Cu and

solder

Fe plating

Solder

Cavity

(3)

than that at position›. The consumption difference between these two positions tends to be more remarkable with extended contact time. This is because the Fe plating/solder reaction is promoted by retained solder, which cannot be blown away during compressed air cleaning.

As fused solder was held on the surface of the soldering

iron, a thick reaction layer was formed. Figure 6 shows results of the EPMA analysis of the longitudinal section of the soldering iron on which fused solder was held for 9 h at 653 K. From Fig. 6, it can be seen that the left side of the SE image is the Fe plating, the middle part is the reaction layer and the right side is the solder. The interface between the solder and the reaction layer exhibits a wavelike shape, which implies that the reaction layer grows into a fused solder matrix.

Figure 7 shows results of the EPMA analysis of the encircled area in Fig. 6(a). The small peak of C is due to a C film, which is evaporated onto the surface of the specimen. As shown in Fig. 7, Fe and Sn were confirmed in the reaction layer, but no Ag and Cu were detected. Also a small peak of O was confirmed. However, its intensity was smaller than that of O peak in the reaction layer after the consumption test (Fig. 4). In the consumption test, the fused solder and reaction layer were frequently exposed to oxygen in air during compressed air cleaning. On the other hand, under the experiment in which fused solder was held on the surface of the soldering iron, only a small amount of oxygen dissolved into the fused solder. Consequently, the difference in oxygen concentration at the reaction layer between these two cases was generated. Using quantitative EPMA analysis (point analysis), the chemical composition of the encircled area in Fig. 6 was determined as Fe 19.05 mass% (33.68 at%) and Sn 80.95 mass% (66.32 at%), which show an approximately 1:2 atomic ratio between Fe and Sn. In accordance with the Fe-Sn equilibrium phase diagram,4,5)FeSn

2is stable from room

temperature to 786 K. These results suggest that the reaction layer consists of FeSn2.

Figure 8 shows results of EPMA line analysis (along the white line in the SE image) of the longitudinal section of the soldering iron on which fused solder was held for 4 h at 683 K. In the reaction layer no change in Fe/Sn ratio was observed. Ag was absent from the reaction layer and present in the solder. This suggests that Ag is independent of the formation of the reaction layer.

Figure 9 shows the relationship between the thickness of the reaction layer and holding time. Each curve shows a

(c) (a)

(b)

Fig. 4 EPMA analysis of encircled area in Fig. 3. (a) SE image, (b) Fe K -ray image, (c) Sn K-ray image.

0 5 10 15 20 25 30

Solder/Soldering Iron Contact Time, tc / h

0 100 200 300 400 500 600 700

Amount of Soldering Ir

on Consumed,

CSI

/ m

Thickness of Fe plating

µ

(4)

parabola. This indicates that the formation of the reaction layer is related to diffusion behavior. The formation of the reaction layer was examined from the viewpoint of reaction kinetics.

The thickness of the reaction layer is given by

x2¼Kth; ð1Þ

x¼K1=2th1=2; ð2Þ

wherexis the thickness of the reaction layer,K is the rate constant andth is the holding time.K is expressed as

K¼K0expðQ=RTÞ; ð3Þ

(a)

(c) (b)

Fig. 6 EPMA analysis of the longitudinal section of soldering iron on which fused solder was held for 9 h at 653 K. (a) SE image, (b) Fe K-ray image, (c) Sn K-ray image.

Length, L /mm

Intensity,

I

(a.u.)

PET

(Intensity:3206counts) (Intensity:3910counts)

(Intensity:3629counts) (Intensity:947counts)

LDE1

LIF

TAP

Fig. 7 EPMA analysis of reaction layer shown in Fig. 6.

Ag (Intensity:300counts)

Fe (Intensity:6000counts)

Sn (Intensity:20000counts)

Length, L /mm

Intensity

,

I

(a.u.)

(5)

where K0 is a constant, Q is the activation energy for the

formation of the reaction layer,Tis the temperature andRis the gas constant. Equation (2) shows that a linear relationship exists betweenxandth1=2. LetK¼A2, whereAis the slope

of the straight line. Substituting this relation into eq. (3) and taking the logarithm of both sides yields

2 lnA¼lnK0Q=RT: ð4Þ

Figure 10 shows the relationship between 2 lnA and 1=T. The linear relationship holds approximately.

We obtained Q¼198kJ/mol from the slope of the straight line in Fig. 10. The activation energy for the self diffusion of Fe is 254 kJ/mol.6)However, no data regarding

the activation energy for Sn diffusion through Fe has been presented so far. Because the radius of a Sn atom is larger than that of an Fe atom, its value may be larger than that for Fe self-diffusion. As mentioned above, the activation energy for the formation of the reaction layer is smaller than those for the diffusion of Sn through Fe and the self diffusion of Fe. FeSn2 is an intermetallic compound that has a C16-type

space lattice (space group:I4=mcm).4,5,7)Since FeSn2 has a

tetragonal lattice consisting of Fe sublattice/Sn sublattice stacking in the [001] direction, the rate of diffusion of atoms through the sublattice seems to be different from that through a plane normal to the sublattice. However, no available data regarding the diffusion of atoms through FeSn2 have been

presented. In the FePt intermetallic compound that has an L10-type space lattice, the activation energies for Fe

diffusion are 259 kJ/mol (along the sublattice) and 309 kJ/ mol (normal to the sublattice).8)Thus, the activation energy

for Fe diffusion along the sublattice is smaller than that through a plane normal to the sublattice. From the difference in vacancy concentration between Fe and Pt sublattices, it might be reasoned that Fe atoms diffuse through the Fe sublattice.8)These results imply that the growth rate of the reaction layer is controlled by Fe atom diffusion through the Fe sublattice with vacancy and divacancies.

SubstitutingK0, which is obtained from the intercept of the

straight line in Fig. 10, into eqs. (2) and (3) yields

x¼5:03fexpð2:38104=TÞg1=2th1=2: ð5Þ

Substituting temperature (K) and holding time (s) into eq. (5), the thickness of the reaction layer is obtained. As previously mentioned, the consumption test was carried out at 653 K. Before the test, the tip temperature of the soldering iron was kept at the setup point. During the consumption test, the temperature of the soldering iron reduced owing to heat transfer into the supplied solder and compressed air cleaning. After that, it tended to return to the original temperature by heating until a subsequent solder was fed. However, the subsequent solder was supplied before temperature recovery since it took a long time to reach 653 K. As a result, the mean temperature in the steady state during the consumption test was 603 K below the setting temperature. Substituting T¼

603K and soldering/soldering iron contact timeth¼7:25s

into eq. (5), the thickness of the reaction layer formed during one soldering process was calculated to be 36.4 nm. The thickness of the reaction layer obtained from the linear relationship (position ›) shown in Fig. 5 is 29.0 nm per soldering cycle. The experimental result is in agreement with the calculation value. This indicates that the reaction layer formed during one soldering process flakes off during compressed air cleaning.

The mean micro-Vickers hardness of the Fe plating was

Hv163 (max:Hv184, min:Hv145). The mean hardness of the reaction layer was Hv378 (max:Hv418, min:Hv354). No cracks were observed around the impressions.

A thick reaction layer (20–30mmas shown in Fig. 9) was formed by holding at 683 K for 9 h, followed by the consumption test mentioned above. Figure 11 shows SE images of the longitudinal section of the soldering iron after the consumption test ((a) 0.7 h and (b) 17.5 h solder/ soldering iron contact time). In Fig. 11(a), the left-hand part is the Fe plating, the middle part is the reaction layer and the right-hand part is the solder. In Fig. 11(a), the preformed reaction layer still remains. The surface of the reaction layer is smooth. This suggests that the irregularity of the surface of the preformed reaction layer is flattened by the consumption test and compressed air cleaning. In Fig. 11(b), no reaction layer appears at the Fe plating (left-hand part)/solder (right-hand part) interface. This is because the preformed reaction

0 10 20 30 40 50 60

0 5 10 15 20 25

Holding Time, th /h

Thic

kness of Reaction La

y

e

r,

D

/

m 623 K

653 K 683 K

µ

Fig. 9 Relationship between the thickness of reaction layer,D, and holding time,th.

36

−35 −34

33

−32

31

−30

1.45 1.50 1.55 1.60 1.65

2ln(Slop,

A

/ m s

1/2

) Q =198 kJ/mol

T 1/103 K1

µ

(6)

layer disappears at a certain number of soldering cycles then the reaction layer formed during one soldering cycle flakes off during compressed air cleaning. The behavior that no reaction layer appears at the Fe plating/solder interface agrees with the result shown in Fig. 3.

In general, an intermetallic compound is very hard but has a low fracture toughness, resulting in brittle fractures. Although there has been no reports regarding the fracture toughness of FeSn2, FeSn2 may have similar property. As

shown in Figs. 6, 8 and 11, many voids exist in the reaction layer. It is considered that these voids are Kirkendall voids caused by the difference in diffusion rate between Fe and Sn atoms. A reaction layer having many voids may become brittle. The reaction layer undergoes external stresses such as the pushing pressure of the solder and the blowing pressure during compressed air cleaning, which act as cyclic stresses. Furthermore, alternate heating and cooling results in the generation of thermal stress caused by the difference in thermal expansion coefficient between the Fe plating and

reaction layer at the Fe plating/reaction layer interface and in the reaction layer. In FeSn2, the thermal expansion

coef-ficient in the direction along the sublattice differs from that in the direction normal to the sublattice. Therefore, a thermal stress caused by this difference is generated in the reaction layer. The flaking off and fracture of the reaction layer may be promoted by such thermal stresses. The consumption of the soldering iron occurs as follows: at the area without retained solder (position › in Fig. 2), the reaction layer (approximately 30 nm thickness) formed during one solder-ing cycle flakes off dursolder-ing compressed air cleansolder-ing, resultsolder-ing in the consumption of the soldering iron. On the other hand, at the area with the retained solder after compressed air cleaning (position‹in Fig. 2), a remarkable consumption of

soldering iron occurs owing to the formation of a thick FeSn2

layer.

4. Conclusions

The consumption of a soldering iron was examined using Pb-free solder and the solder/soldering iron reaction was discussed in connection with its kinetics. The results obtained are as follows.

(1) The amount of the soldering iron consumed increases with increasing solder/soldering iron contact time. When fused solder is removed by compressed air cleaning, no thick reaction layer is formed at the Fe plating/solder interface. When fused solder remains on the surface of the soldering iron after such cleaning, a thick reaction layer is formed at the Fe plating/solder interface.

(2) As fused solder is held for a long time on the surface of the soldering iron, a thick FeSn2 reaction layer is

formed. The rate of formation of the reaction layer is controlled by the diffusion of Fe atoms through the Fe sublattice with vacancies and divacancies.

(3) The reaction layer is relatively hard (Hv378) and has many voids. During compressed air cleaning, the reaction layer flakes off together with fused solder. (4) On the basis of these results, it is clear that the soldering

iron consumption by Pb-free solder occurs according to the mechanism of the formation of a reaction layer of approximately 30 nm thickness through one soldering process, followed by the flaking of this layer during compressed air cleaning.

REFERENCES

1) K. Suganuma: Materia Japan38(1999) 919. 2) T. Suga: Materia Japan38(1999) 920–922. 3) T. Nishimura: Materia Japan43(2004) 651–654.

4) ASM International:ASM Handbook Vol. 3 Alloy Phase Diagrams(ASM International, Ohio, 1992) p. 2203.

5) P. M. Hansen:Constitution of Binary Alloys(McGraw-Hill, New York, 1958) pp. 718–720.

6) Japan Inst. Metals:A Data Book on Metals(Maruzen, Tokyo, 1974) p. 25.

7) R. Kiriyama and H. Kiriyama:Kouzoumukikagaku I(Kyouritu Press, Tokyo, 1971) p. 234.

8) Y. Nose, T. Ikeda and H. Nakajima: Kinzoku74(2004) 1003–1008.

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Figure

Fig. 1Schematic illustration of soldering robot.
Fig. 1Schematic illustration of soldering robot. p.1
Fig. 3SE images of the longitudinal section of soldering irons afterconsumption tests
Fig. 3SE images of the longitudinal section of soldering irons afterconsumption tests p.2
Fig. 5Relationship between the amount of soldering iron consumed, CSI,and solder/soldering iron contact time, tC.
Fig. 5Relationship between the amount of soldering iron consumed, CSI,and solder/soldering iron contact time, tC. p.3
Figure 8 shows results of EPMA line analysis (along the

Figure 8

shows results of EPMA line analysis (along the p.3
Fig. 7EPMA analysis of reaction layer shown in Fig. 6.
Fig. 7EPMA analysis of reaction layer shown in Fig. 6. p.4
Fig. 8EPMA line analysis of the longitudinal section of soldering iron onwhich fused solder was held for 4 h at 683 K.
Fig. 8EPMA line analysis of the longitudinal section of soldering iron onwhich fused solder was held for 4 h at 683 K. p.4
Fig. 6EPMA analysis of the longitudinal section of soldering iron onwhich fused solder was held for 9 h at 653 K
Fig. 6EPMA analysis of the longitudinal section of soldering iron onwhich fused solder was held for 9 h at 653 K p.4
Fig. 9Relationship between the thickness of reaction layer, D, and holdingtime, th.
Fig. 9Relationship between the thickness of reaction layer, D, and holdingtime, th. p.5
Fig. 10Relationship between 2 ln A and 1=T.
Fig. 10Relationship between 2 ln A and 1=T. p.5
Fig. 11SE images of the longitudinal section of soldering irons afterconsumption tests
Fig. 11SE images of the longitudinal section of soldering irons afterconsumption tests p.6

References