Lead-free solder

Abstract

Lead-free solder comprising Sn, Zn and 0.001 to 0.005 wt. % Ti. The lead-free solder does not contain toxic lead, and has sufficient bonding strength to oxide materials such as glass and ceramics.

Description

FIELD OF THE INVENTION

[0002] The present invention relates to lead-free solder for soldering an oxide material, such as ceramics and glass at a low temperature.

BACKGROUND OF THE INVENTION

[0003] Electroplating or electroless plating such as gold plating, copper plating, and nickel plating is conducted on an oxide material including ceramics and glass before soldering thereof. However, another pre-plating method preceding soldering is desired because the above plating is expensive and complicated.

[0004] Solder of Pb—Sn which can be soldered directly to glass and ceramics is disclosed in Japanese patents S49-22299B and S52-21980B.

[0005] However, lead is toxic on the health, the environment, and the ecosystem, so lead-free solder is desired.

[0006] The solder of Pb—Sn—Cd—Sb disclosed in the above Japanese patent 49-22299B is possible to be soldered directly to an oxide material such as glass and ceramics. However, toxic lead eluates from the solder in quantity to cause serious problem, to the environment, when the solder contacts acid rain.

[0007] The solder disclosed in the above Japanese patent S52-21980B contains rare earth materials which are useful for joining oxide materials such as glass and ceramics. However, the solder has the same problems as above, because it comprises lead as a main component.

[0008] Lead-free solder has been investigated for mounting electronic parts. For example, solder of Sn—Ag—In is disclosed in Japanese patent H9-326554A, and solder of Sn—Zn—Bi system is disclosed in Japanese patent H8-164495A. However, their bonding strength are not enough to metal oxide materials such as glass and ceramics.

[0009] Solder of Sn—Ag—Al—Zn for soldering metal oxides is disclosed in Japanese patent S55-36032B. This solder easily separates from oxide material such as glass and ceramics, because the coefficient of thermal expansion of the solder is greatly different from that of the oxide material.

[0010] The above solders can be used as a sealing material which seals a periphery of a double-glazing unit comprising two glass plates arranged parallel with a space therebetween. When manufacturing the double-glazing unit, the solder is introduced from a solder feeding apparatus having a solder tank and a feeding line to the space between the glass plates via an introducing plate which is inserted into the space.

[0011] However, when the conventional solder is fed into the solder tank and kept for a predetermined time, a (Sn, Zn) Ti series compound is deposited on a bottom of the solder tank, whereby the compound blocks the feeding line of the solder feeding apparatus and the solder can not be fed stably.

DISCLOSURE OF THE INVENTION

[0012] It is an object of the present invention to solve problems of prior art mentioned above and to provide lead-free solder comprising no toxic lead, being capable of bonding strongly to oxide materials such as glass and ceramics and preventing producing the above (Sn, Zn), Ti series compound so that the solder can be fed stably.

[0013] The solder of the present invention comprises Sn and Zn as main components and further comprises at least one of Ti, Al and Cu.

[0014] It should be noted that a content value of each component represents a mean composition in the solder because Zn, Ti and Al in the solder are very easily oxidized, and tend to segregate at the surface of the solder.

[0015] A first aspect of the lead-free solder of the present invention comprises Sn, Zn and 0.001 to 0.005 wt. % Ti.

[0016] The lead-free solder of the present invention may further comprise 0.001 to 0.005 wt. % Al.

[0017] The lead-free solder of the present invention may further comprise 5.0 to 9.0 wt. % Cu.

[0018] The lead-free solder of the present invention preferably comprises Sn and Zn such that a ratio of Sn to Zn (Sn/Zn) is 4.0 to 19.0. Where each of “Sn” and “Zn” represents a weight percentage thereof in the solder. In case that the lead-free solder contains 91 wt. % Sn and 9 wt. % Zn for example, the ratio of Sn/Zn is 10.1 (91/9).

[0019] The lead-free solder of the present invention comprises Sn and Zn more preferably such that the ratio of Sn to Zn (Sn/Zn) is 4≦Sn/Zn<9 and 12<Sn/Zn≦19, and contains substantially no Cu. “Substantially no Cu” means that Cu content is not greater than Cu content included as impurity in usual raw material of the solder including raw metals of Sn and Zn.

[0020] The lead-free solder of the present invention may comprise one or more than two elements among Bi, Si and Sb in the range of 3.2 to 10 wt. %.

[0021] The lead-free solder of the present invention may comprise 0.001 to 1.0 wt. % Si.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] FIG. 1 is a graph showing a relationship between a temperature at which the solder is kept and an amount of a deposited compound concerning two solders having a different Ti content from each other; and,

[0023] FIG. 2 is a graph showing a relationship between a Ti content and an amount of deposited compound when a solder is kept at 270° C.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0024] The composition of the lead-free solder of the present invention is as follows. The content of components will be represented with a percentage by weight.

[0025] Sn (tin) is not toxic and gives good wetting property against materials to be joined, and Sn is an indispensable constituent for the solder. Zn (Zinc) is comprised in the solder in order to improve adhesion strength thereof to oxide materials such as glass and ceramics.

[0026] Ti (titanium) is extremely easy to be oxidized, but it bonds the solder to an oxide material strongly. When Ti content is less than 0.001 wt. %, the solder does not bond to an oxide material firmly enough. When Ti content is more than 0.005%, heat cycle resistance decreases due to increase of hardness of the solder, and further the solder becomes too hard to use due to a rise of a melting point thereof. Accordingly, Ti content is preferably 0.001 to 0.005%.

[0027] Al (aluminum) is also extremely easy to be oxidized, and Al makes the solder to bond to an oxide material firmly. When Al content is less than 0.001 wt. %, the solder does not bond the oxide firmly. When Al content is more than 0.005%, heat cycle resistance of the solder decreases due to increase of hardness of the solder, the solder has an increased melting point, and the solder loses workability. Accordingly, Al content is preferably 0.001 to 0.005%.

[0028] Cu (copper) has a good effect on mechanical strength of the solder. This effect is insufficient when Cu is less than 5.0%. When Cu is more than 9.0%, the melting point of the solder increases, the mechanical strength decreases and a lot of Cu—Sn intermetalic compounds grow. Accordingly, Cu content is preferably 5.0 to 9.0%, more preferably 0.01 to 3.0%.

[0029] In the lead-free solder of the present invention, the ratio of Sn to Zn (Sn/Zn), where Sn and Zn are expressed with a percentage by weight, is preferably 4.0 to 19.0. Sn and Zn make eutectic reaction in an alloy, and eutectic structures therein consist of a minute mixture of fine Sn phases and Zn phases. The eutectic structures have high flexibility, so that stress applied to the solder is easily dispersed in the eutectic phases. The stress does not concentrate on the interface of the solder and oxide material such as glass, so that the solder hardly separates from the oxide material.

[0030] When the lead-free solder of the present invention has the Sn/Zn ratio between 9.0 and 12.0 where Sn and Zn are percentages thereof by weight, the solder preferably contains substantially no copper. When the solder has the Sn/Zn ratio of 9.0 to 12.0, the composition of the solder is near or equal to the ratio of the eutectic composition (Sn/Zn=10.1), and proeutectoids of Sn or Zn hardly grow large so that the eutectic structure of the solder becomes fine. A solder containing a lot of eutectic structures has high flexibility so that it is suitable for soldering glass and the like as mentioned above. However, grains of the eutectic phases grow large when the solder contains a lot of seed compounds. Cu and Zn in the solder tend to react with each other to form CuZn intermetalic compounds. The compounds work as seeds for of the proeutectoids, so that the eutectic grains grow large. When the grains grow large in the solder, the stress applied thereto concentrates on the grain boundary and causes fracture. Accordingly the solder is preferable to contain substantially no Cu.

[0031] The lead-free solder of the present invention may contain one or more elements among Bi, Si and Sb in a range 3.2 to 10%. Bi and Si improve wettability of the solder. Sb improves an appearance of the soldered solder and increases creep resistance of the solder. The solder may contain further another element such as Cr, Be, Fe, Ni, Co, Ga, Ge and P in a small amount in order to improve wettability and mechanical strength of the solder.

[0032] When Si is less than 0.001%, above effects are achieved insufficiently. Si of more than 1.0% raises the melting point of the solder so that workability of soldering is lowered. Accordingly Si content is preferably 0.001 to 1.0%, more preferably 0.01 to 0.1%.

[0033] The lead-free solder of the present invention may contain In (indium). The In decreases the melting point of the solder, improves wetting property of the solder, and improves flexibility of the solder, so that the stress applied to the interface of the soldered solder and oxide material is relaxed.

[0034] The lead-free solder of the present invention directly solders to not only an oxide such as glass and ceramics but also metal such as Al, Ti, and Zr which is hard to be soldered due to a metal oxide film thereon.

[0035] It is preferable to use an ultrasonic equipment which gives ultrasonic vibration to the solder during soldering in case of hardsoldering materials. It is also preferable to use an equipment having a member which transmits a physical stimulus to the interface of the solder and the hardsoldering material to promote bonding to each other. The member may have a shape of a plate or a rod. The member may be rotated or vibrated.

[0036] Hereinafter, the present invention will be described referring to examples.

EXAMPLES 1 To 24

[0037] A soda-lime glass plate (50×50×3 mm) was used as a material to be adhered with lead-free solder. The lead-free solder has a composition shown in Tables 1, 2 and 3. The solder was soldered to the glass using an ultrasonic soldering iron having a tip which vibrates at 60 kHz. The compositions in the tables are represented with a percentage by weight.

[0038] Adhering property of the solder to the glass was estimated by knifing the solder on the glass with a knife. In Tables 1, 2 and 3, a circle mark (◯) of the adhering property shows that more than half of the solder remains on the glass, and a cross mark (X) shows that the solder peels off in its entirety.

TABLE 1
Compositions of the solder (wt %)
	Example
	1	2	3	4	5	6	7	8	9	10
Sn	90.85	89.8	91.5	91.2	92.5	88.6	91.2	92.2	86.5	93.2
Zn	8.99	9.9	8.35	7.6	7.1	11.1	8.39	7.41	12.4	6.2
Ti	0.16	0.2	0.15	1	0.4	0.3	0.08	0.05	0.1	0.15
Al	0	0.1	0	0.2	0	0	0	0	0	0.1
Cu	0	0	0	0	0	0	0.33	0.34	1	0.35
Sum	100	100	100	100	100	100	100	100	100	100
adhering	◯	◯	◯	◯	◯	◯	◯	◯	◯	◯
property

[0039]

TABLE 2
Compositions of the solder (wt %)
	Example
	11	12	13	14	15	16	17	18	19	20
Sn	90.5	80	89.9	95	80.7	70	50	40	90.05	60
Zn	9.0	19.85	9.0	3.7	9.8	29.98	49.99	56.5	9.9	39.99
Ti	0.15	0.03	0.5	1	2	0.007	0.005	1.5	0.05	0.003
Al	0	0.07	0.5	0.1	2.5	0.005	0.002	1.5	0	0.003
Cu	0.35	0.05	0.1	0.2	5	0.008	0.003	0.5	0	0.004
Sum	100	100	100	100	100	100	100	100	100	100
adhering	◯	◯	◯	◯	◯	◯	◯	◯	◯	◯
property

[0040]

TABLE 3
Compositions of the solder (wt %)
	Example	Comparative Example
	21	22	23	24	1	2	3
Sn	75	65	87	45	93.7	89	70
Zn	24.8	32	10.5	49.5	0	0	0
Ti	0.07	0.3	0.7	2.5	3.5	0	0
Al	0.05	0.7	0.3	2	2.8	1	30
Cu	0.08	2	1.5	1	0	10	0
Sum	100	100	100	100	100	100	100
adhering	◯	◯	◯	◯	X	X	X
property

[0041] As shown from Tables 1, 2 and 3, each of the solder of the present invention adheres to the glass firmly, since it contains at least one of Ti, Al, Cu, Sn and Zn. The solder solders glasses each other firmly, since it has high mechanical strength, and relaxes the stress applied to the interface between the glass and the solder during the solder is cooled. The solder of the present invention does not peel off when impact is applied thereto after it is soldered.

COMPARATIVE EXAMPLES 1-3

[0042] Table 3 shows compositions and adhesive properties of comparative examples. The compositions are represented with a percentage by weight.

[0043] In the comparative examples 1 to 3, contents of Zn and Ti are out of the scope of the present invention. In the comparative example 2, a content of Cu is out of the scope of the present invention. In the comparative example 3, a content of Al is out of the scope of the present invention. The adhesive property between the lead-free solder of the comparative examples and the glass is inferior, so that all the solder separates or peels off completely from the glass.

EXAMPLES 25-34

[0044] A soda-lime glass plate (50×50×3 mm) was used as a material to be soldered. Lead-free solder shown in Table 4 was used. The solder was soldered to the glass plate using the ultrasonic soldering iron having the tip which vibrates at 60 kHz. Compositions shown in Table 4 are represented with a percentage by weight.

[0045] The adhesive property between the glass and the lead-free solder was estimated by knifing the solder adhered on the glass in the same way as in the Examples 1-24. In the adhesive property shown in Table 4, a circle mark (◯) shows that more than half of the solder does not separate but remains on the glass, and a cross mark (X) shows that the solder separates from the glass in its entirety.

TABLE 4
Compositions of the solder (wt %)
	Example
	25	26	27	28	29	30	31	32	33	34
Sn	83	84	86.8	76.8	81.3	90.4	89.99	90.5	90.45	90.49
Zn	9	8.5	8.7	7.7	8.1	9.0	9.0	8.5	9.0	9.0
Ti	0.15	0.05	0.15	0.15	0.15	0.08	0.105	0.15	0.15	0.159
Al	0	0	0	0	0	0	0	0	0	0
Cu	0.35	0.35	0.10	0.30	0.35	0.35	0.35	0.35	0.35	0.35
Sb	3	2	1	5	0	0	0	0	0	0
Si	0.5	0.1	0.2	0	0.05	0.02	0.005	0	0	0
Bi	1	3	2	5	0	0	0	0	0	0
In	3	2	1	5	10	0.1	0.5	0	0	0
Cr	0	0	0.05	0	0	0	0	0	0	0
Be	0	0	0	0.05	0	0	0	0	0	0
Fe	0	0	0	0	0.05	0	0	0	0	0
Ni	0	0	0	0	0	0.05	0	0	0	0
Co	0	0	0	0	0	0	0.05	0	0	0
Ga	0	0	0	0	0	0	0	0.5	0	0
Ge	0	0	0	0	0	0	0	0	0.05	0
P	0	0	0	0	0	0	0	0	0	0.001
Sum	100	100	100	100	100	100	100	100	100	100
adhering	◯	◯	◯	◯	◯	◯	◯	◯	◯	◯
property

[0046] As shown in Table 4, each solder of the examples contains the components of the invention and components such as Cr, Be, Fe, Ni, Co, Ga, Ge, and P. The solder solders glasses each other firmly, since it has high mechanical strength, and relaxes the stress applied to the interface between the glass and the solder during the solder is cooled. The solder does not peel off when impact is applied thereto after it is soldered.

[0047] The inventors have found that a Ti content of the solder has an influence on the deposition of the (Sn, Zn) Ti series compound deposited on the bottom of the solder tank.

[0048] The inventors conducted experiments for the Ti content which prevents deposition of the compound and enables to feed the solder stably.

[0049] The inventors, at first, fed a Ti containing solder having a composition of (Sn—Zn eutectic with Ti) into a solder tank, and melted the solder at a certain temperature to obtain a melt of the solder. The melt was kept at a predetermined temperature for 10 minutes, and then the melt was flown out of the tank and the deposition on the bottom of the tank was recovered to measure the weight thereof. An amount of the deposition (%) was calculated according to the following equation:

[amount of the deposited compound (%)]

=[amount of the recovered compound (g)/amount of the melt flown out of

the tank (g)]×100.

[0050] The results were shown in FIG. 1 wherein two kinds of solders having different Ti contents were described.

[0051] FIG. 1 shows that the amount of the deposited compound becomes less as the Ti content decreases.

[0052] The inventors, referring above, conducted experiments for a Ti content of the solder which can be fed stably at 270° C. It should be noted that a solder is usually fed at 270° C. at a usual plant.

[0053] The results were shown in FIG. 2 where a relationship between the Ti content of the solder and the amount of the compound deposited therefrom at 270° C., at which the solder was kept.

[0054] As shown from FIG. 2, the solder having the Ti content of equal to or less than 0.007 wt. % preferably equal to or less than 0.005 wt. % has the deposition amount of the compound of not more than 2.4%. The solder of the deposition amount of the compound of not more than 2.4% can be fed stably without a practical problem at a usual plant.

[0055] Industrial Capability

[0056] As mentioned above, the lead-free solder of the present invention does not contain toxic lead, and contains components according to the invention, and a small amount of Cr, Be, Fe, Ni, Co, Ga, Ge and P. The solder solders glasses each other firmly, since it has high mechanical strength, and relaxes the stress applied to the interface between the glass and the solder during the solder is cooled. The solder does not peel off when impact is applied thereto after it is soldered.

Claims

1. Lead-free solder comprising Sn, Zn and 0.001 to 0.005 wt. % of Ti.

2. Lead-free solder as claimed in claim 1, wherein a ratio of Sn to Zn (Sn/Zn) is 4.0 to 19.0.

3. Lead-free solder as claimed in claim 1, wherein the ratio of Sn to Zn (Sn/Zn) is 9.0 to 12.0, and said solder contains substantially no Cu.

4. Lead-free solder consisting essentially of Sn, Zn, 0.001 to 0.005 wt. % of Ti, and 0.001 to 0.005 wt. % of Al, wherein a ratio of Sn to Zn (Sn/Zn) is 4.0 to 19.0.

5. Lead-free solder consisting essentially of Sn, Zn, 0.001 to 0.005 wt. % of Ti, and 0.001 to 9.0 wt. % of Cu wherein a ratio of Sn to Zn (Sn/Zn) is 4≦Sn/Zn<9 and 12<Sn/Zn≦19.

6. Lead-free solder consisting essentially of Sn, Zn, 0.001 to 0.005 wt. % of Ti, 0.001 to 0.005 wt. % of Al, and 5.0 to 9.0 wt. % of Cu, wherein a ratio of Sn to Zn (Sn/Zn) is 4≦Sn/Zn<9 and 12<Sn/Zn≦19.

7. Lead-free solder consisting essentially of Sn, Zn, 0.001 to 0.005 wt. % of Ti, and at least one element selected from a group consisting of Bi, Si and Sb in a range of 3.2 to 10 wt. %, wherein a ratio of Sn to Zn (Sn/Zn) is 4.0 to 19.0.