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Interfacial reactions in the liquid (Ag,Cu)Ni diffusion couples
Materials
Chemtitly
and Physics,
Interfacial Sinn-wen
reactions
271
271-276
in the liquid
(Ag,Cu)/Ni
diffusion
couples
Chen*
Department of Chemical (Taiwan, ROC)
(Received
33 (1993)
Engineering,
National
March 20, 1992; accepted
Tsing Hua
University,
No. 101, Sec. 2, Kuang
Fu Road,
30043 Hsinchu
July 29, 1992)
Abstract The reaction sequence occurring between a Ag-28wt.%Cu filler material and a Ni substrate during the brazing process has been investigated by studying liquid diffusion couples. The reaction layer structure is liquid/(Cu,Ni)/ Ni. The composition profile across the reaction layers was determined by electron probe microanalysis. The diffusion path was obtained using the phase diagram and the experimental results from this study. Based on these results, it is clear that the (Cu,Ni) layer observed in the brazed joint of the pin-grid arrays was formed as a result of liquid diffusion prior to solidification.
Introduction In electronic packaging, Ag-28wt.% Cu or Ag-lSwt.% Cu alloys are used as brazing filler materials for attaching pins to pin-grid arrays (PGAs) [l]. A SCh ematic diagram of the brazed joint is shown in Fig. 1 [l]. During the brazing process, the brazing materials melt, wet and flow. The interfacial reaction between the liquid brazing material and the Ni-plated substrate plays an
Electroplated
Au
Electroplated
Ni -ti
PI”
--!-I
Brazed pad with NI electroplated on the surface \
Ag~ZSwt%Cu
braze
important role in this phenomenon, i.e. melt, wet and flow. The melting temperature of the reacted region will be different from the starting filler and the substrate, due to either the formation of new phases or composition changes associated with diffusion. The interfacial reaction also provides the driving force for wetting and flow [2, 31. An understanding of the interfacial reaction is crucial since it is an essential component of the brazing process. Since the Ni layer formed in the standard PGA is very thin, it is difficult to quantify the interfacial reaction between the filler and the substrate. Therefore a designed study of diffusion couples between Ni foil and Ag-28wt.%Cu alloys has been carried out. The interfacial reaction can thus be investigated with a controlled geometry and thickness of the starting Ni foil. The purpose of this article is to report the results of experimental work aimed at determining the reaction rate and the reaction sequence between liquid (Ag,Cu) eutectic alloys, i.e. Ag-28wt.%Cu, and pure Ni substrate.
Experimental
Fig. 1. Schematic
section
through
a brazed joint.
*Formerly Senior Scientist, Molten Metal Processing Division, Alcoa Laboratories, 100 Technical Drive, Alcoa Center, PA 15069, USA.
0254-0584/93/$6.00
The experimental liquid diffusion couples were prepared by loading Ag-28wt.%Cu eutectic cubes and pure Ni foil inside a quartz tube, evacuating, back-filling with argon to l/3 atm, and sealing. The sample capsules were held in a furnace at 1460 “F for 10 min, 1 h, and 24 h, then removed
0 1993 - Elscvier
Sequoia.
All rights reserved
272 To Vacuum t
Bat!
Pump
Before reaction Fig.
2. A schematic
(Ag, Cu)
Filled with Argon
After reaction
diagram of the sample capsule.
(4
and air-cooled. The identification numbers of these three samples were 1, 2 and 3, respectively. The holding temperature, 1460 “F, is higher than the eutectic temperature of (Ag,Cu), 1436 “F, but lower than the melting temperature of Ni, 2647 “F. At 1460 “F, the (Ag,Cu) eutectic melts, surrounds the Ni foil and thus forms a liquid diffusion couple. A schematic diagram of the sample capsule is shown in Fig. 2. Optical microscopy and electron probe microanalysis (EPMA) were used to examine the microstructure of the (Ag,Cu)/Ni interface. Backscattered electron imaging and X-ray mapping were used to reveal the distribution of elements across the interface. The composition profile along the reaction layer for sample 3, which was reacted for 24 h, was obtained by quantitative wave-dispersive spectrometer (WDS) analysis with ZAF correction (i.e. the atomic number factor 2, the absorption factor A, and the characteristic fluorescence correction F). For the other two samples, 1 and 2, the reaction layers were too thin to obtain a reliable quantitative measurement by WDS analysis.
Results (cl Figure 3(a)-(c) shows a thin reaction layer for sample 1 after holding 10 minutes at 1460 “F. The reaction layer is indicated with an arrow in Figs. 3(a) and 3(c). Since the depicted surface is not vertical to the (Ag,Cu)/Ni interface, the apparent thickness is greater than the actual one. Assuming the foil thickness has not changed (i.e. - 100 pm) after ten minutes of holding, the reaction layer thickness is estimated to be about 3 pm. The reaction layer in sample 2 after a one-hour hold at 1460 “F is more obvious, as shown in Fig. 4(a)-(c). In this case, the interface between the reaction layer and the liquid phase (i.e. the eutectic
Fig. 3. (a) Backscattered electron image of liquid diffusion couple 1. This diffusion couple is a Ag-28wt.%Cu eutectic alloy with pure Ni foil held at 1460 “F for 10 minutes. (b) X-ray map of the distribution of Ag in sample 1. (c) X-ray map of the distribution of Cu in sample 1.
phase after solidification) is not planar. The thickness of the reaction layer is estimated to be about 12 ,um. From X-ray mapping, it is clear that the composition of this reaction layer close to the reaction layer/liquid interface is copper-rich. The intensity of the X-ray map also indicates that the copper concentration within the copper-rich re-
213
Silver -rich
Layer
(Ag,
Cu)
’ eutecti?
(4
A-A’ in Fig. 5(a) and B-B’ in Fig. 5(d) were determined by EPMA. The results are shown in Fig. 6(a) and (b) and plotted on the Ag-Cu-Ni 1460 “F isothermal section in Fig. 7. The center of the dark phase in Fig. 5(a) corresponds to the unreacted Ni foil. The reaction layer is a continuous (Cu,Ni) solid solution with compositions varying from almost pure Ni at the center toward copperrich, 69wt.%Ag-89.6wt.%Cu-3.5wt.%Ni, at the solid/liquid interface. The composition of the (Cu, Ni) phase at the solid/liquid phase was determined by EPMA as shown in Figs. 6(a) and 6(b), i.e. points 5 and 34 along line A-A’ and points 8 and 30 along line B-B’. In the two-phase region of the (Ag, Cu) eutectic, the EPMA data are scattered, depending on the probe’s location. Since the beam size of the probe is estimated to be about 5 pm, the probe usually covers two different phases in the two-phase region. The results of the composition measurement depend on the relative amount of the two different phases covered by the probe. Thus, the exact composition of the ‘eutectic’ phase is not obtained by EPMA. However, the EPMA results indicate that the Ni concentration in this two-phase region is very low and below the EPMA detecting limit, i.e. < - 0.2 wt.%. The dark region shown in Fig. 5(d), where the line B-B’ goes across, shows no unreacted Ni layer. It is likely that this dark region is a tip of the reaction layer which has grown from underneath. Therefore, the composition profile determined corresponds to only part of the diffusion path shown in Fig. 6. It is also noticed in Fig. 5(a)-(d) that the interface between the reaction layer and the unreacted Ni foil is essentially planar.
Discussion
Fig. 4. (a) Backscattered electron image of liquid diffusion couple 2. This diffusion couple is a Ag-28wt.%Cu eutectic alloy with pure Ni foil held at 1460 “F for 1 hour. (b) X-ray map of the distribution of Ag in sample 2. (c) X-ray map of the distribution of Cu in sample 2.
action layer is even higher than the eutectic composition, which is 28 wt.% Cu, while the concentration of Ag is less than that in the eutectic, which is 72 wt.% Ag. In contrast, with a 24-hour hold at 1460 “F, the Ni foil shows some discontinuities, as revealed in Fig. 5(a)-(d). The composition profiles along line
The 1460 “F isothermal section shown in Fig. 7 is constructed from: (1) three binary constituent phase diagrams [4-6], i.e. Ag-Cu, Cu-Ni, and Ag-Ni systems; (2) Chang’s assessment [7]; and (3) Murray’s calculation [8] of the Ag-Cu-Ni system. (Cu,Ni) forms a continuous solid solution with limited Ag solubility, - l-- 10 wt.%. Ag has 8 wt.% solubility for Cu but almost no solubility for Ni. The liquid (Ag,Cu) phase also has almost no solubility for Ni. For most of the liquid phase, the tie lines are in equilibrium with (Cu,Ni) solid solution. The diffusion path determined from the liquid diffusion couple 3 is plotted in Fig. 7. Its layer structure is liquid/(Cu,Ni)/Ni. Although quantitative results cannot be obtained for samples 1
274
6%
Reactit
(4
(b)
(Ag, cu)
Reaction
(c)
(d)
Fig. 5. (a) Backscattered electron image of liquid diffusion couple 3. This diffusion couple is a Ag-ZSwt.%Cu eutectic alloy with pure Ni foil held at 1460 “F for 24 hours. (b) X-ray map of the distribution of Ag in sample 3. (c) X-ray map of the distribution of Cu in sample 3. (d) Backscattered electron image of liquid diffusion coupIe sample 3.
and 2, the X-ray maps from these two diffusion couples indicate the same results. As shown in Fig. 5(c), the copper concentration is the highest at the reaction layer/liquid interface and then decreases toward the reaction layer/Ni interface. Based on the experimental results, a hypothesis for the diffusion path is given as follows: in the beginning, when the binary Ag-28wt.%Cu melt contacts pure Ni foil, Ni diffuses into the melt (dissolution), while Cu and Ag are diffusing into Ni. Since the liquid phase has an extremely low (almost none) solubility for Ni, the melt is immediately saturated with Ni. However, because this (Cu,Ni) phase at the interface is very rich in copper, the formation of the equilibrium (Cu,Ni) solid phase at the interface depends on the supply of copper by diffusion from the bulk liquid phase to the reaction interface. This is evidenced by the nonplanar morphology of the interface between the reaction layer and the liquid phase. The relationship between the interfacial morphology and the diffusion kinetics of the phases in the couple
has been investigated previously [g-11]. Figures 8(a) and 8(b) are schematic diagrams illustrating the Cu and the Ni diffusion controlled growth cases, respectively. In the case of Cu diffusion controlled growth as shown in Fig. 8(a), the Cu diffusion flux to point I is higher than that to point II. Thus, point I will grow faster than point II, so the interface morphology will be nonplanar. In the case of Ni diffusion controlled growth in Fig. 8(b), the Ni diffusion flux to point I is lower than that to point II. Point I will grow slower than point II, thus the interface will become planar. A thin Ag-rich layer was also noticed by a closer examination at the (Cu, Ni)/liquid interface of Figs. 3(a), 4(a) and 5(a). The formation of such a copperdepleted layer also confirms that the growth of the (Cu,Ni) is controlled by the diffusion of copper. Based on the results presented in this study, it is clear that the (Cu,Ni) layer found in the real brazing process [l], as shown in Fig. 1, was formed as a result of liquid diffusion prior to solidification.
275
Growth
5 ‘; : t
(A& W 60
Liquid -Cll -
40 s M 5 B
Ni (4
20
0
50 Distance
(a)
along
200
150
100 line
A-A’
Growth Front +I
(p
m)
(A& Cu) Liquid
@I -
A8
-CU -
Ni
0
50 Distance
150
100 along
line
B-B’
(p
m)
Fig. 6. (a) The composition profile determined by EPMA along line A-A’ in Fig. 5(a). (b) The composition profile determined by EPMA along line B-B’ in Fig. 5(d).
Ni
----
Diffusion Path
I
Nl
/
I
Fig. 8. (a)The interfacial morphology of the Cu diffusion controlled case. (b) The interfacial morphology of the Ni diffusion controlled case.
0
(b)
I
l
Composition within the reaction layer determined
Ni. More study is needed to quantify the reaction rate. More diffusion couples are needed in order to obtain quantitative data of growth rate. A modification of the liquid diffusion couple is also required so the vertical cross-section of the liquid/ Ni interaction interface can be easily located. Thin film diffusion couples with Ni deposited tungsten will also be examined to simulate the real brazing process. Isothermal sections of the Ag-Cu-Ni system at selected temperatures will also need to be studied for a more quantitative understanding and a prediction of the interfacial reaction.
Conclusions
(Cu.Ni)+Liquid+Ag
Ag (Cu.Ni) + Liquid
Liquid
Liquid+Ag
Fig. 7. The isothermal section of a Ag-Cu-Ni ternary system at 1460 “F with the diffusion path determined in sample 3.
However, since the deposited Ni layer is very thin, the whole Ni layer might be consumed completely during the brazing process. If this situation occurs, the phenomenon of the real process is similar to a thin film diffusion couple with limited supply of
(1) The reaction layer structure between a liquid Ag-28wt.%Cu alloy and a Ni foil is liquid/(Cu, Ni)/Ni. (2) The growth of the (Cu,Ni) layer of the diffusion couples in this study is controlled by the diffusion of Cu. (3) The (Cu,Ni) layer observed in the brazed joint of the PGAs was formed as a result of liquid diffusion prior to solidification.
Acknowledgements
The author wishes to acknowledge Dr J. L. Murray for providing calculated phase diagrams, Dr D. A. Weirauch for tutoring the packaging
276
process, Dr J.-C. Lin for many valuable discussions, and Dr D. A. Granger for reviewing the manuscript.
References 1 D. A. Weirauch, unpublished work, Aluminum Company of America, 1991. 2 F. G. Yost, TMS Fall Meeting, Cincinnati, OH, 1991. 3 A. D. Romig, Jr., and Y. A. Chang, TMS Fall Meeting, Cincinnati, OH, 1991. 4 J. L. Murray, Metall. Trans. A, 15 (1984) 261.
5 D. J. Chakrabarti, D. E. Laughlin, S.-W. Chen and Y. A. Chang, in T. B. Massalski, H. Okamoto, P. R. Subramanian and L. Kacprzak (eds.), Binary Alloy Phase Diagrams, Vol. 2, ASM, Materials Park, Ohio, 2nd edn., 1990, pp. 1442-1446; Bull. Alloy Phase Diagr., in press. 6 M. Singleton and P. Nash, BUN.AIloy Phase Diagr., 8(2) (1987) 119. 7 Y. A. Chang, D. Goldberg and J. P. Neumann,1 Phys. Chem. Ref: Data, 6(3) (1977) 621. 8 J. L. Murray, Alcoa Internal Commun., 1991-04-30. 9 C. Wagner, Z. Anorg. Allgem. Chem., 236 (1938) 320. 10 C. Wagner, J. Electrochem. Sot., 103 (1956) 571. 11 J.-C. Lin, J. Schultz, K.-C. Hsieh and Y. A. Chang, .I. Electrochem. Sot., 136 (1989) 3006.
Chemtitly
and Physics,
Interfacial Sinn-wen
reactions
271
271-276
in the liquid
(Ag,Cu)/Ni
diffusion
couples
Chen*
Department of Chemical (Taiwan, ROC)
(Received
33 (1993)
Engineering,
National
March 20, 1992; accepted
Tsing Hua
University,
No. 101, Sec. 2, Kuang
Fu Road,
30043 Hsinchu
July 29, 1992)
Abstract The reaction sequence occurring between a Ag-28wt.%Cu filler material and a Ni substrate during the brazing process has been investigated by studying liquid diffusion couples. The reaction layer structure is liquid/(Cu,Ni)/ Ni. The composition profile across the reaction layers was determined by electron probe microanalysis. The diffusion path was obtained using the phase diagram and the experimental results from this study. Based on these results, it is clear that the (Cu,Ni) layer observed in the brazed joint of the pin-grid arrays was formed as a result of liquid diffusion prior to solidification.
Introduction In electronic packaging, Ag-28wt.% Cu or Ag-lSwt.% Cu alloys are used as brazing filler materials for attaching pins to pin-grid arrays (PGAs) [l]. A SCh ematic diagram of the brazed joint is shown in Fig. 1 [l]. During the brazing process, the brazing materials melt, wet and flow. The interfacial reaction between the liquid brazing material and the Ni-plated substrate plays an
Electroplated
Au
Electroplated
Ni -ti
PI”
--!-I
Brazed pad with NI electroplated on the surface \
Ag~ZSwt%Cu
braze
important role in this phenomenon, i.e. melt, wet and flow. The melting temperature of the reacted region will be different from the starting filler and the substrate, due to either the formation of new phases or composition changes associated with diffusion. The interfacial reaction also provides the driving force for wetting and flow [2, 31. An understanding of the interfacial reaction is crucial since it is an essential component of the brazing process. Since the Ni layer formed in the standard PGA is very thin, it is difficult to quantify the interfacial reaction between the filler and the substrate. Therefore a designed study of diffusion couples between Ni foil and Ag-28wt.%Cu alloys has been carried out. The interfacial reaction can thus be investigated with a controlled geometry and thickness of the starting Ni foil. The purpose of this article is to report the results of experimental work aimed at determining the reaction rate and the reaction sequence between liquid (Ag,Cu) eutectic alloys, i.e. Ag-28wt.%Cu, and pure Ni substrate.
Experimental
Fig. 1. Schematic
section
through
a brazed joint.
*Formerly Senior Scientist, Molten Metal Processing Division, Alcoa Laboratories, 100 Technical Drive, Alcoa Center, PA 15069, USA.
0254-0584/93/$6.00
The experimental liquid diffusion couples were prepared by loading Ag-28wt.%Cu eutectic cubes and pure Ni foil inside a quartz tube, evacuating, back-filling with argon to l/3 atm, and sealing. The sample capsules were held in a furnace at 1460 “F for 10 min, 1 h, and 24 h, then removed
0 1993 - Elscvier
Sequoia.
All rights reserved
272 To Vacuum t
Bat!
Pump
Before reaction Fig.
2. A schematic
(Ag, Cu)
Filled with Argon
After reaction
diagram of the sample capsule.
(4
and air-cooled. The identification numbers of these three samples were 1, 2 and 3, respectively. The holding temperature, 1460 “F, is higher than the eutectic temperature of (Ag,Cu), 1436 “F, but lower than the melting temperature of Ni, 2647 “F. At 1460 “F, the (Ag,Cu) eutectic melts, surrounds the Ni foil and thus forms a liquid diffusion couple. A schematic diagram of the sample capsule is shown in Fig. 2. Optical microscopy and electron probe microanalysis (EPMA) were used to examine the microstructure of the (Ag,Cu)/Ni interface. Backscattered electron imaging and X-ray mapping were used to reveal the distribution of elements across the interface. The composition profile along the reaction layer for sample 3, which was reacted for 24 h, was obtained by quantitative wave-dispersive spectrometer (WDS) analysis with ZAF correction (i.e. the atomic number factor 2, the absorption factor A, and the characteristic fluorescence correction F). For the other two samples, 1 and 2, the reaction layers were too thin to obtain a reliable quantitative measurement by WDS analysis.
Results (cl Figure 3(a)-(c) shows a thin reaction layer for sample 1 after holding 10 minutes at 1460 “F. The reaction layer is indicated with an arrow in Figs. 3(a) and 3(c). Since the depicted surface is not vertical to the (Ag,Cu)/Ni interface, the apparent thickness is greater than the actual one. Assuming the foil thickness has not changed (i.e. - 100 pm) after ten minutes of holding, the reaction layer thickness is estimated to be about 3 pm. The reaction layer in sample 2 after a one-hour hold at 1460 “F is more obvious, as shown in Fig. 4(a)-(c). In this case, the interface between the reaction layer and the liquid phase (i.e. the eutectic
Fig. 3. (a) Backscattered electron image of liquid diffusion couple 1. This diffusion couple is a Ag-28wt.%Cu eutectic alloy with pure Ni foil held at 1460 “F for 10 minutes. (b) X-ray map of the distribution of Ag in sample 1. (c) X-ray map of the distribution of Cu in sample 1.
phase after solidification) is not planar. The thickness of the reaction layer is estimated to be about 12 ,um. From X-ray mapping, it is clear that the composition of this reaction layer close to the reaction layer/liquid interface is copper-rich. The intensity of the X-ray map also indicates that the copper concentration within the copper-rich re-
213
Silver -rich
Layer
(Ag,
Cu)
’ eutecti?
(4
A-A’ in Fig. 5(a) and B-B’ in Fig. 5(d) were determined by EPMA. The results are shown in Fig. 6(a) and (b) and plotted on the Ag-Cu-Ni 1460 “F isothermal section in Fig. 7. The center of the dark phase in Fig. 5(a) corresponds to the unreacted Ni foil. The reaction layer is a continuous (Cu,Ni) solid solution with compositions varying from almost pure Ni at the center toward copperrich, 69wt.%Ag-89.6wt.%Cu-3.5wt.%Ni, at the solid/liquid interface. The composition of the (Cu, Ni) phase at the solid/liquid phase was determined by EPMA as shown in Figs. 6(a) and 6(b), i.e. points 5 and 34 along line A-A’ and points 8 and 30 along line B-B’. In the two-phase region of the (Ag, Cu) eutectic, the EPMA data are scattered, depending on the probe’s location. Since the beam size of the probe is estimated to be about 5 pm, the probe usually covers two different phases in the two-phase region. The results of the composition measurement depend on the relative amount of the two different phases covered by the probe. Thus, the exact composition of the ‘eutectic’ phase is not obtained by EPMA. However, the EPMA results indicate that the Ni concentration in this two-phase region is very low and below the EPMA detecting limit, i.e. < - 0.2 wt.%. The dark region shown in Fig. 5(d), where the line B-B’ goes across, shows no unreacted Ni layer. It is likely that this dark region is a tip of the reaction layer which has grown from underneath. Therefore, the composition profile determined corresponds to only part of the diffusion path shown in Fig. 6. It is also noticed in Fig. 5(a)-(d) that the interface between the reaction layer and the unreacted Ni foil is essentially planar.
Discussion
Fig. 4. (a) Backscattered electron image of liquid diffusion couple 2. This diffusion couple is a Ag-28wt.%Cu eutectic alloy with pure Ni foil held at 1460 “F for 1 hour. (b) X-ray map of the distribution of Ag in sample 2. (c) X-ray map of the distribution of Cu in sample 2.
action layer is even higher than the eutectic composition, which is 28 wt.% Cu, while the concentration of Ag is less than that in the eutectic, which is 72 wt.% Ag. In contrast, with a 24-hour hold at 1460 “F, the Ni foil shows some discontinuities, as revealed in Fig. 5(a)-(d). The composition profiles along line
The 1460 “F isothermal section shown in Fig. 7 is constructed from: (1) three binary constituent phase diagrams [4-6], i.e. Ag-Cu, Cu-Ni, and Ag-Ni systems; (2) Chang’s assessment [7]; and (3) Murray’s calculation [8] of the Ag-Cu-Ni system. (Cu,Ni) forms a continuous solid solution with limited Ag solubility, - l-- 10 wt.%. Ag has 8 wt.% solubility for Cu but almost no solubility for Ni. The liquid (Ag,Cu) phase also has almost no solubility for Ni. For most of the liquid phase, the tie lines are in equilibrium with (Cu,Ni) solid solution. The diffusion path determined from the liquid diffusion couple 3 is plotted in Fig. 7. Its layer structure is liquid/(Cu,Ni)/Ni. Although quantitative results cannot be obtained for samples 1
274
6%
Reactit
(4
(b)
(Ag, cu)
Reaction
(c)
(d)
Fig. 5. (a) Backscattered electron image of liquid diffusion couple 3. This diffusion couple is a Ag-ZSwt.%Cu eutectic alloy with pure Ni foil held at 1460 “F for 24 hours. (b) X-ray map of the distribution of Ag in sample 3. (c) X-ray map of the distribution of Cu in sample 3. (d) Backscattered electron image of liquid diffusion coupIe sample 3.
and 2, the X-ray maps from these two diffusion couples indicate the same results. As shown in Fig. 5(c), the copper concentration is the highest at the reaction layer/liquid interface and then decreases toward the reaction layer/Ni interface. Based on the experimental results, a hypothesis for the diffusion path is given as follows: in the beginning, when the binary Ag-28wt.%Cu melt contacts pure Ni foil, Ni diffuses into the melt (dissolution), while Cu and Ag are diffusing into Ni. Since the liquid phase has an extremely low (almost none) solubility for Ni, the melt is immediately saturated with Ni. However, because this (Cu,Ni) phase at the interface is very rich in copper, the formation of the equilibrium (Cu,Ni) solid phase at the interface depends on the supply of copper by diffusion from the bulk liquid phase to the reaction interface. This is evidenced by the nonplanar morphology of the interface between the reaction layer and the liquid phase. The relationship between the interfacial morphology and the diffusion kinetics of the phases in the couple
has been investigated previously [g-11]. Figures 8(a) and 8(b) are schematic diagrams illustrating the Cu and the Ni diffusion controlled growth cases, respectively. In the case of Cu diffusion controlled growth as shown in Fig. 8(a), the Cu diffusion flux to point I is higher than that to point II. Thus, point I will grow faster than point II, so the interface morphology will be nonplanar. In the case of Ni diffusion controlled growth in Fig. 8(b), the Ni diffusion flux to point I is lower than that to point II. Point I will grow slower than point II, thus the interface will become planar. A thin Ag-rich layer was also noticed by a closer examination at the (Cu, Ni)/liquid interface of Figs. 3(a), 4(a) and 5(a). The formation of such a copperdepleted layer also confirms that the growth of the (Cu,Ni) is controlled by the diffusion of copper. Based on the results presented in this study, it is clear that the (Cu,Ni) layer found in the real brazing process [l], as shown in Fig. 1, was formed as a result of liquid diffusion prior to solidification.
275
Growth
5 ‘; : t
(A& W 60
Liquid -Cll -
40 s M 5 B
Ni (4
20
0
50 Distance
(a)
along
200
150
100 line
A-A’
Growth Front +I
(p
m)
(A& Cu) Liquid
@I -
A8
-CU -
Ni
0
50 Distance
150
100 along
line
B-B’
(p
m)
Fig. 6. (a) The composition profile determined by EPMA along line A-A’ in Fig. 5(a). (b) The composition profile determined by EPMA along line B-B’ in Fig. 5(d).
Ni
----
Diffusion Path
I
Nl
/
I
Fig. 8. (a)The interfacial morphology of the Cu diffusion controlled case. (b) The interfacial morphology of the Ni diffusion controlled case.
0
(b)
I
l
Composition within the reaction layer determined
Ni. More study is needed to quantify the reaction rate. More diffusion couples are needed in order to obtain quantitative data of growth rate. A modification of the liquid diffusion couple is also required so the vertical cross-section of the liquid/ Ni interaction interface can be easily located. Thin film diffusion couples with Ni deposited tungsten will also be examined to simulate the real brazing process. Isothermal sections of the Ag-Cu-Ni system at selected temperatures will also need to be studied for a more quantitative understanding and a prediction of the interfacial reaction.
Conclusions
(Cu.Ni)+Liquid+Ag
Ag (Cu.Ni) + Liquid
Liquid
Liquid+Ag
Fig. 7. The isothermal section of a Ag-Cu-Ni ternary system at 1460 “F with the diffusion path determined in sample 3.
However, since the deposited Ni layer is very thin, the whole Ni layer might be consumed completely during the brazing process. If this situation occurs, the phenomenon of the real process is similar to a thin film diffusion couple with limited supply of
(1) The reaction layer structure between a liquid Ag-28wt.%Cu alloy and a Ni foil is liquid/(Cu, Ni)/Ni. (2) The growth of the (Cu,Ni) layer of the diffusion couples in this study is controlled by the diffusion of Cu. (3) The (Cu,Ni) layer observed in the brazed joint of the PGAs was formed as a result of liquid diffusion prior to solidification.
Acknowledgements
The author wishes to acknowledge Dr J. L. Murray for providing calculated phase diagrams, Dr D. A. Weirauch for tutoring the packaging
276
process, Dr J.-C. Lin for many valuable discussions, and Dr D. A. Granger for reviewing the manuscript.
References 1 D. A. Weirauch, unpublished work, Aluminum Company of America, 1991. 2 F. G. Yost, TMS Fall Meeting, Cincinnati, OH, 1991. 3 A. D. Romig, Jr., and Y. A. Chang, TMS Fall Meeting, Cincinnati, OH, 1991. 4 J. L. Murray, Metall. Trans. A, 15 (1984) 261.
5 D. J. Chakrabarti, D. E. Laughlin, S.-W. Chen and Y. A. Chang, in T. B. Massalski, H. Okamoto, P. R. Subramanian and L. Kacprzak (eds.), Binary Alloy Phase Diagrams, Vol. 2, ASM, Materials Park, Ohio, 2nd edn., 1990, pp. 1442-1446; Bull. Alloy Phase Diagr., in press. 6 M. Singleton and P. Nash, BUN.AIloy Phase Diagr., 8(2) (1987) 119. 7 Y. A. Chang, D. Goldberg and J. P. Neumann,1 Phys. Chem. Ref: Data, 6(3) (1977) 621. 8 J. L. Murray, Alcoa Internal Commun., 1991-04-30. 9 C. Wagner, Z. Anorg. Allgem. Chem., 236 (1938) 320. 10 C. Wagner, J. Electrochem. Sot., 103 (1956) 571. 11 J.-C. Lin, J. Schultz, K.-C. Hsieh and Y. A. Chang, .I. Electrochem. Sot., 136 (1989) 3006.