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copper joints brazed with silver-base and amorphous metglas alloys
Volume 2, number 6A&B
September 1984
MATERIALS LETTERS
RESISTANCE OF COPPER/COPPER JOINTS BRAZED WITH SILVER-BASE AND AMORPHOUS METGLAS ALLOYS A. RABINKIN Allied Corporation, Metglas Products, 6 Eastmans Road, Parsippany, NJ 07054,
USA
Received 27 June 1984
Resistivities of brazed copper-to-copper joints were measured in situ using a four-probe method. Assemblies were produced by brazing with the new amorphous copper-based METGLAS@ 2000 Series alloys (2002 and 2005) and conventional silver-based filler metals BAg-1, BAg-4 and BCuP-5. The 3 mil joints brazed with 2002 and 2005 alloys have larger, i.e. better conductivity than that of widely used BCuP-5.
The electrical resistivities of brazed joints are a major concern in electrical circuitry design and performance. To minimize resistivity in these assemblies, filler metal alloys containing silver, such as the AWS BAg (Ag-Cu-Zn-Cd) or BCuP (Cu-Ag-P), are typically used. Recently, a family of rapidly solidified copper-base filler metal alloys, designated MBF-2000 Series, have been developed for electrical contact applications. The MBF-2000 Series,contain neither precious metals, such as silver, nor toxic elements, such as cadmium. Yet, their brazing characteristics are similar to those of standard silver-containing filler metal alloys. A brazed electrical contact joint typically consists of 50 to 100 micrometers of filler metal enclosed between highly conductive copper or silver parts. It therefore is difficult to measure joint resistance in situ using conventional instrumentation (e.g. double bridge). For standard silver-base alloys, which have resistivities 3-5 times that of copper, it is assumed that the joint region will have the same resistivity as the initial filler metal. Whereas this approach may be accurate for conventional materials, it cannot be applied to rapidly solidified filler metals. Rapidly solidified metals have high inherent resistivity due to their microcrystalline or amorphous structures. However, during brazing, they melt and interact with the base metals resulting in brazed joint morphologies similar to those of the crystalline base metals. 0 167-.577x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
To the best of our knowledge, there is no data available on in situ resistance of brazed joints not only for amorphous filler metals but for standard silver-base alloys as well. Therefore the present study was undertaken to obtain the data needed for use of the METGLAS@ 2000 Series alloys and to compare their properties with properties of conventional alloys. This paper presents data on electrical resistance of copper-to-copper joints brazed with standard silverbase and new amorphous alloys METGLAS@ 2002 & 2005 filler metals (table 1). Electrolytic tough pitch
Cu-simple I RST
2.5 x 10-4*
I L
I
HP 6266 DC Power Supply
Fig. 1. Schematic diagram of instrument set up for measuring brazed joint resistance.
487
Volume 2, number 6A&B Table 1 Composition, #
MATERIALS LETTERS
September 1984
melting characteristics and resistivity of brazing filler metals and copper used in the present study
AWS classification
Alloy name & source
Alloy nominal composition (wt.%)
Melting range
Electric characteristics
solidus CC)
conductivity (% IACS)
liquidus CC)
a)
resistivity (nohm m)
1
-
2002, Metglas@
Cu78NilOSn4P8
610
645
1.15
1500
2
-
2005, Metglas@
Cu77Ni6SrilOP7
590
640
0.91
1880
3
-
2002 + 1.5 Si Metglas@
Cu76.8Ni9.9Sn4P7.8Si1.5
590
650
0.82
2100
4
BAg-1
Easy Flo 45, Handy & Hartman
Ag45Cu15Zn16Cd24
605
620
24.2 (27.6) b,
71.1 (60.6) b)
5
BAg-4
C254 Engelhard
Ag40Cu30Zn25Ni5
660
780
12.3 (16.8) b)
140.3 (102.7) b)
6
BCuP-5
Met.-Braze 15 Metz Met. Corp.
Ag15Cu8OP5
640
705
11.9 (9.9) b)
150.7 (174.0) b)
Copper (Cl 1000 Electrolithic Tough Pitch)
cI099.95
_
1083
94.2 (-95-97)
7
-
a) Conductivity (%) is calculated as percentage of conductivity
18.3 (17.2-k7.4)c)
c)
of pure copper which is taken as 0.058 nohm-’
m-l.
b) According to Handy & Hartman Data Sheet. c) According to Metals Handbook, 9th Ed., Vol. 2, p. 284. Table 2 Electrical characteristics of brazed copper-to-copper Alloy name
2002
Ribbon thick. d (pm)
23 :;3
joints
Joint thick. I (pm)
Resistance increment AR, ohm m2 x 1011
Joint resistivity (nohm m)
Conductivity (% IACS)
relative d
relative I
relative d
relative 1
30.5 * 9.5 87.9 + 5
5.26 3.13
2290 453
1720 355
0.7 3.8
0.99 4.8
x 3) 2005
23 ;;3 x 3)
23 + 3.3 69.6 t 6.3
5.66 4.86
2470 820
2470 820
0.7 2.1
0.7 2.1
2002 + Si 1.5
50
42
*5
7.18
1430.0
1670.0
1.2
1.0
BAg-1
76
72
?3
0.65
85.5
90.3
20.1
19.1
BAg-4
50
?
1.58
316.0
990.0
5.4
1.7
BCuP-5
37
30.0
2.94
790.0
980.0
2.16
1.76
18.4
18.4
Copper, Cl1000
488
94.2
September 1984
MATERIALS LETTERS
Volume 2, number 6A&B
copper (99.9%) with resistivity 18.3 nohm m was employed as the base metal. Joints were prepared by butt-brazing copper rectangular bars (10 X 10 X 27.5 mm) in a nitrogen atmosphere for 16 min at temperatures 1OO’C higher than the corresponding liquidus temperature of the filler metal. After brazing, the
BAg-4,
BCuP-5,
2 mils
1.5 mils
middle parts containing the joints were machined to cylindrical shafts of 2-3 mm diameter. Control samples of the same shape were prepared from unbrazed copper bars and were subjected to heat treatments together with brazed assemblies in order to compare resistivities after identical treatments.
BAg-4
BCuP -5
Fig. 2. Copper-to-copper joints brazed with silver-base alloys (a, b, c). The cold deformed microstructure of the corresponding brazing filler ribbons is shown in (d), (e), and (f). In spite of finer crystal structure of cold deformed ribbons its conductivity is higher than conductivity of coarse crystallized structure of the corresponding joints (magnification: a,b, c X 170; d,e,f X425). 489
Volume 2, number 6A&B
MATERIALS LETTERS
A four-probe method employing a high-impedance digital microvoltmeter was applied to accurately measure such low values of sample resistance. Fig. 1 shows the schematic of instrumentation layout. Contacting electrodes of the probe for voltage measurements are made from tungsten carbide blades. The distance between the electrodes was kept constant for all samples. The increase in resistance of a sample due to joint formation (AR) was derived by subtracting the resistance of the control copper sample from the sample resis-
September 1984
tance, R.Metallographic samples were also prepared in order to examine joint microstructures and measure brazed joints thicknesses. The magnitude of resistivity of the 2000 Series alloys in the amorphous state (table 1) is similar to that of iron- and nickel-base amorphous alloys containing boron, carbon and phosphorus [ 1,2]. Small addition of silicon increases resistivity of the 2002 alloy from 1500 to 2100 nohm m. Table 2 shows electrical characteristics of copper-
Fig. 3. Copper-to-copper joints brazed with the 2000 Series alloys. Corresponding ribbons have amorphous structure which by metallography shows no structural details (magnification
X 160).
Volume 2, number 6A&B
MATERIALS LETTERS
to-copper joints brazed with six different alloys. It is evident that conductivities of joints brazed with BAg-4 and B&P-5 are lower than those of the corresponding filler metals. In the case of BAg-1, the joint resistance is very close to that of BAg-1 ribbon. The opposite is true for joints brazed with the 2002 and 2005 alloys. Joints of 3 mils clearance and brazed with the 2000 Series alloys have practically the same or sometimes higher conductivity than those brazed with high-silver alloys. At the moment it is difficult to explain the origin of resistivity of the brazed joints or the initial ribbon samples. For example, BAg-4 has a larger concentration of high-conductivity elements than BAg-1 (see table 1). However, the conductivity of BAg-4 ribbon and joint is 2-5 times smaller than that of BAg-1. One of the possible explanations for such a relationship is the coarse microstructure of BAg- 1 joint as illustrated in figs. 2 and 3. Joints obtained from 2002,2005 and BCuP-5 have relatively finer microstructure and consequently have higher resistance. Presence of small amounts of silicon appreciably changes joint morphology, resulting in the formation of large crystals. But in this case the conductivity did not increase. The presence of silicon itself, even in small amounts, decreases inherent conductivity of the
September 1984
alloy. Since joints are usually very thin the electron dissipation at the interphase boundaries may be a substantial contribution to the joint resistance. Considering the interphase morphology (fig. 2) one can see no direct correlation between the degree of the interface “smoothness” and resistivity: The joint with BAg-1 has the highest conductivity while its interfaces are rather tortuous. In conclusion, the resistivity of the new copperbase brazing filler metal (METGLAS@ 2000-P series), in the amorphous state is in the range of 1500-2 100 nohm m, close to the resistivity of the other METGLAS@ amorphous alloys. Resistivity of copper-to-copper joints brazed with 2002 & 2005 is lower than that of BCuP-5 and BAg-4 joints but higher than that of BAg-1 joints. The conductivity of the 2002 and 2005 joints brazed in a nitrogen atmosphere at 74O’C are approximately 4 and 2% of the conductivity of pure copper, respectively.
References [l] N. Teoh, W. Teoh and S. Arajs, in: Amorphous magnetism, Vol. 2, eds. R.A. Levy and R. Hasegawa (Plenum Press, New York, 1977). [2] R. Hasegawa and J.A. Dermon, Phys. Letters 42A (1973) 407.
491
September 1984
MATERIALS LETTERS
RESISTANCE OF COPPER/COPPER JOINTS BRAZED WITH SILVER-BASE AND AMORPHOUS METGLAS ALLOYS A. RABINKIN Allied Corporation, Metglas Products, 6 Eastmans Road, Parsippany, NJ 07054,
USA
Received 27 June 1984
Resistivities of brazed copper-to-copper joints were measured in situ using a four-probe method. Assemblies were produced by brazing with the new amorphous copper-based METGLAS@ 2000 Series alloys (2002 and 2005) and conventional silver-based filler metals BAg-1, BAg-4 and BCuP-5. The 3 mil joints brazed with 2002 and 2005 alloys have larger, i.e. better conductivity than that of widely used BCuP-5.
The electrical resistivities of brazed joints are a major concern in electrical circuitry design and performance. To minimize resistivity in these assemblies, filler metal alloys containing silver, such as the AWS BAg (Ag-Cu-Zn-Cd) or BCuP (Cu-Ag-P), are typically used. Recently, a family of rapidly solidified copper-base filler metal alloys, designated MBF-2000 Series, have been developed for electrical contact applications. The MBF-2000 Series,contain neither precious metals, such as silver, nor toxic elements, such as cadmium. Yet, their brazing characteristics are similar to those of standard silver-containing filler metal alloys. A brazed electrical contact joint typically consists of 50 to 100 micrometers of filler metal enclosed between highly conductive copper or silver parts. It therefore is difficult to measure joint resistance in situ using conventional instrumentation (e.g. double bridge). For standard silver-base alloys, which have resistivities 3-5 times that of copper, it is assumed that the joint region will have the same resistivity as the initial filler metal. Whereas this approach may be accurate for conventional materials, it cannot be applied to rapidly solidified filler metals. Rapidly solidified metals have high inherent resistivity due to their microcrystalline or amorphous structures. However, during brazing, they melt and interact with the base metals resulting in brazed joint morphologies similar to those of the crystalline base metals. 0 167-.577x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
To the best of our knowledge, there is no data available on in situ resistance of brazed joints not only for amorphous filler metals but for standard silver-base alloys as well. Therefore the present study was undertaken to obtain the data needed for use of the METGLAS@ 2000 Series alloys and to compare their properties with properties of conventional alloys. This paper presents data on electrical resistance of copper-to-copper joints brazed with standard silverbase and new amorphous alloys METGLAS@ 2002 & 2005 filler metals (table 1). Electrolytic tough pitch
Cu-simple I RST
2.5 x 10-4*
I L
I
HP 6266 DC Power Supply
Fig. 1. Schematic diagram of instrument set up for measuring brazed joint resistance.
487
Volume 2, number 6A&B Table 1 Composition, #
MATERIALS LETTERS
September 1984
melting characteristics and resistivity of brazing filler metals and copper used in the present study
AWS classification
Alloy name & source
Alloy nominal composition (wt.%)
Melting range
Electric characteristics
solidus CC)
conductivity (% IACS)
liquidus CC)
a)
resistivity (nohm m)
1
-
2002, Metglas@
Cu78NilOSn4P8
610
645
1.15
1500
2
-
2005, Metglas@
Cu77Ni6SrilOP7
590
640
0.91
1880
3
-
2002 + 1.5 Si Metglas@
Cu76.8Ni9.9Sn4P7.8Si1.5
590
650
0.82
2100
4
BAg-1
Easy Flo 45, Handy & Hartman
Ag45Cu15Zn16Cd24
605
620
24.2 (27.6) b,
71.1 (60.6) b)
5
BAg-4
C254 Engelhard
Ag40Cu30Zn25Ni5
660
780
12.3 (16.8) b)
140.3 (102.7) b)
6
BCuP-5
Met.-Braze 15 Metz Met. Corp.
Ag15Cu8OP5
640
705
11.9 (9.9) b)
150.7 (174.0) b)
Copper (Cl 1000 Electrolithic Tough Pitch)
cI099.95
_
1083
94.2 (-95-97)
7
-
a) Conductivity (%) is calculated as percentage of conductivity
18.3 (17.2-k7.4)c)
c)
of pure copper which is taken as 0.058 nohm-’
m-l.
b) According to Handy & Hartman Data Sheet. c) According to Metals Handbook, 9th Ed., Vol. 2, p. 284. Table 2 Electrical characteristics of brazed copper-to-copper Alloy name
2002
Ribbon thick. d (pm)
23 :;3
joints
Joint thick. I (pm)
Resistance increment AR, ohm m2 x 1011
Joint resistivity (nohm m)
Conductivity (% IACS)
relative d
relative I
relative d
relative 1
30.5 * 9.5 87.9 + 5
5.26 3.13
2290 453
1720 355
0.7 3.8
0.99 4.8
x 3) 2005
23 ;;3 x 3)
23 + 3.3 69.6 t 6.3
5.66 4.86
2470 820
2470 820
0.7 2.1
0.7 2.1
2002 + Si 1.5
50
42
*5
7.18
1430.0
1670.0
1.2
1.0
BAg-1
76
72
?3
0.65
85.5
90.3
20.1
19.1
BAg-4
50
?
1.58
316.0
990.0
5.4
1.7
BCuP-5
37
30.0
2.94
790.0
980.0
2.16
1.76
18.4
18.4
Copper, Cl1000
488
94.2
September 1984
MATERIALS LETTERS
Volume 2, number 6A&B
copper (99.9%) with resistivity 18.3 nohm m was employed as the base metal. Joints were prepared by butt-brazing copper rectangular bars (10 X 10 X 27.5 mm) in a nitrogen atmosphere for 16 min at temperatures 1OO’C higher than the corresponding liquidus temperature of the filler metal. After brazing, the
BAg-4,
BCuP-5,
2 mils
1.5 mils
middle parts containing the joints were machined to cylindrical shafts of 2-3 mm diameter. Control samples of the same shape were prepared from unbrazed copper bars and were subjected to heat treatments together with brazed assemblies in order to compare resistivities after identical treatments.
BAg-4
BCuP -5
Fig. 2. Copper-to-copper joints brazed with silver-base alloys (a, b, c). The cold deformed microstructure of the corresponding brazing filler ribbons is shown in (d), (e), and (f). In spite of finer crystal structure of cold deformed ribbons its conductivity is higher than conductivity of coarse crystallized structure of the corresponding joints (magnification: a,b, c X 170; d,e,f X425). 489
Volume 2, number 6A&B
MATERIALS LETTERS
A four-probe method employing a high-impedance digital microvoltmeter was applied to accurately measure such low values of sample resistance. Fig. 1 shows the schematic of instrumentation layout. Contacting electrodes of the probe for voltage measurements are made from tungsten carbide blades. The distance between the electrodes was kept constant for all samples. The increase in resistance of a sample due to joint formation (AR) was derived by subtracting the resistance of the control copper sample from the sample resis-
September 1984
tance, R.Metallographic samples were also prepared in order to examine joint microstructures and measure brazed joints thicknesses. The magnitude of resistivity of the 2000 Series alloys in the amorphous state (table 1) is similar to that of iron- and nickel-base amorphous alloys containing boron, carbon and phosphorus [ 1,2]. Small addition of silicon increases resistivity of the 2002 alloy from 1500 to 2100 nohm m. Table 2 shows electrical characteristics of copper-
Fig. 3. Copper-to-copper joints brazed with the 2000 Series alloys. Corresponding ribbons have amorphous structure which by metallography shows no structural details (magnification
X 160).
Volume 2, number 6A&B
MATERIALS LETTERS
to-copper joints brazed with six different alloys. It is evident that conductivities of joints brazed with BAg-4 and B&P-5 are lower than those of the corresponding filler metals. In the case of BAg-1, the joint resistance is very close to that of BAg-1 ribbon. The opposite is true for joints brazed with the 2002 and 2005 alloys. Joints of 3 mils clearance and brazed with the 2000 Series alloys have practically the same or sometimes higher conductivity than those brazed with high-silver alloys. At the moment it is difficult to explain the origin of resistivity of the brazed joints or the initial ribbon samples. For example, BAg-4 has a larger concentration of high-conductivity elements than BAg-1 (see table 1). However, the conductivity of BAg-4 ribbon and joint is 2-5 times smaller than that of BAg-1. One of the possible explanations for such a relationship is the coarse microstructure of BAg- 1 joint as illustrated in figs. 2 and 3. Joints obtained from 2002,2005 and BCuP-5 have relatively finer microstructure and consequently have higher resistance. Presence of small amounts of silicon appreciably changes joint morphology, resulting in the formation of large crystals. But in this case the conductivity did not increase. The presence of silicon itself, even in small amounts, decreases inherent conductivity of the
September 1984
alloy. Since joints are usually very thin the electron dissipation at the interphase boundaries may be a substantial contribution to the joint resistance. Considering the interphase morphology (fig. 2) one can see no direct correlation between the degree of the interface “smoothness” and resistivity: The joint with BAg-1 has the highest conductivity while its interfaces are rather tortuous. In conclusion, the resistivity of the new copperbase brazing filler metal (METGLAS@ 2000-P series), in the amorphous state is in the range of 1500-2 100 nohm m, close to the resistivity of the other METGLAS@ amorphous alloys. Resistivity of copper-to-copper joints brazed with 2002 & 2005 is lower than that of BCuP-5 and BAg-4 joints but higher than that of BAg-1 joints. The conductivity of the 2002 and 2005 joints brazed in a nitrogen atmosphere at 74O’C are approximately 4 and 2% of the conductivity of pure copper, respectively.
References [l] N. Teoh, W. Teoh and S. Arajs, in: Amorphous magnetism, Vol. 2, eds. R.A. Levy and R. Hasegawa (Plenum Press, New York, 1977). [2] R. Hasegawa and J.A. Dermon, Phys. Letters 42A (1973) 407.
491