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Interdiffusion and formation of intermetallic compounds in tin-copper alloy surface coatings
Surface Technology, 5 (1977) 57 - 63 © Elsevier Sequoia S.A., Lausanne -- Printed in the Netherlands
57
I N T E R D I F F U S I O N AND FO R M A T I O N OF I N T E R M E T A L L I C COMPOUNDS IN T I N - C O P P E R A L L O Y SURFACE COATINGS
L. RI~VAY Telefonaktiebolaget, L. M. Ericsson Materials Laboratory, S-126 25 Stockholm, Sweden
(Received May 4, 1976; in revised form June 28, 1976)
Summary Intermetallic layer formation at the interface between electrodeposited tin or fused tin - lead coatings on copper alloys was studied by optical microscopy. The apparent interdiffusion coefficients in the t e m p e r a t u r e range 20 to 160 °C were determined. On the basis of these values the apparent activation energy was calculated. The results show a lower activation energy for diffusion in coatings than for bulk lattice diffusion and can be used to predict the thickening rate of intermetallic growth at service temperatures.
Introduction Interdiffusion and reaction between basis metals and surface coatings o f t e n lead to intermetallic phase formation in the interracial region. For the c o p p e r - t i n system, interdiffusion plays a significant role in many commercial applications, particularly where tin or tin alloys are used as coatings in electrical c o m p o n e n t s . The f or m a t i on of intermetallic phases can cause degradation o f electrical and mechanical properties of bot h the solder joint and the co n d u cto r . Therefore, with respect to device reliability, it is important to investigate this p h e n o m e n o n . In the last decade several studies have been devoted to investigation o f interdiffusion in C u - S n systems. Starke and Wever [1] have reported the chemical bulk diffusion coefficient at high temperatures in this system. Fichter and Maverick [2] have systematically studied solder joints for several combinations. C r e ydt and Fichter [3] have also report ed diffusion p h e n o m e n a between galvanic tin or t i n- l ead coatings and copper alloys, in some cases with nickel as a barrier layer. Unsworth and Mackay [4] studied h o w substrate materials affect the growth of intermetallic layers. Recently Tu [ 5] r ep o r ted his basic study which was carried o u t with bimetallic C u - S n films; he found the f or m at i on of a CusSn ~ phase at all temperatures between --2 and 100 °C, but he did not find the Cu3Sn phase below 60 °C. L u b y o v a e t al. [6] also reported the activation energy for interdiffusion in the temperature range 200 - 450 °C.
58 The primary objective of this work was to estimate the rate of interdiffusion in coatings at m ode r at e temperatures (between 20 and 160 °C). These temperatures correspond to accelerated reliability test conditions.
Experiments and results According to our observations one or two intermetallic layers tend to persist at the interface between electrodeposited, fused tin or tin lead solders on copper substrates (R6vay, unpublished results,. 1969; [ 7] ). These intermetallic layers have been identified by X-ray diffraction and analyzed by electron microprobe. Both techniques indicated a composition of CugSns(r~') and Cu3Sn(c ). The intermetallic V' phase forms spontaneously at room temperature during a long storage time, but will thicken more rapidly at elevated temperatures. Consequently, the e x t e n t of this intermetallic formation depends upon a n u m b e r of factors, including time and temperature. Since it is difficult to measure the layer thickness at a low reaction temperature for an accurate evaluation of the diffusion rate, we have a t t e m p t e d to avoid this difficulty by extending the reaction time and have considered only those layers which were measureable with an optical microscope. After the annealing or storage procedure, sample surfaces were prepared by the usual metallographical techniques. The advantages of the metallographic approach to diffusion studies are principally its ease of operation, availability and speed, and also the opp o r t u n i t y for direct observation. Metallography does not yield concentration penetration data but can be used to calculate the diffusion coefficients. It is particularly appropriate in the case of intermediate phases where the m o r p h o l o g y of the diffusion zone is i m p o r t a n t as well as the diffusion coefficients. Microscopic identification of the Cu6Sn 5 and CuaSn layers was based on the distinct difference in colour between them. Figure 1 shows these layers formed at 160 °C after 28 days. Their width was estimated, with an optical microscope giving a magnification of 1200, by using an ocular micrometer. Table 1 shows the width of these layers, and the relative width of the CuaSn layer to the total width of the intermetallic layers (x/xt ratio). Figures 2 and 3 show the average value of the total C u - S n intermetallic width plotted against the square r o o t of time at different annealing temperatures. Only one anneal was carried out at 20 and 50 °C. The single point in the diagram is an average value of a num be r o f samples. Microhardness was estimated as 350 and 450 kg mm 2 for the CuaSn and Cu6Sn 5 phases, respectively. Figure 1 also shows the K n o o p indentor track in these phases, which indicates that a discontinuity in hardness may exist through the intermediate zone. This brittle behaviour arises from the decrease in the metallic character o f the bonding exhibited by these compounds. The volume per atom ratio in the intermediate phases is almost always less than in pure metals, which means that these phases are c o m m o n l y under
59
Fig. 1. Metallurgical cross-section of the CuSn intermediate layers. Sample was annealed at 160 "C for 28 days. stress. Bonds o f this t y p e are generally c o n s i d e r e d t o be undesirable. New evidence, however, shows t h a t f r a c t u r e in intermetallic b o n d s does n o t necessarily o c c u r within the b o d y o f the intermetallic layer. In an intermetallic b o n d i n g system the weakest b o u n d a r y is the intermetallic/substrate i n t e r f a c e [ 8 ] .
Discussion Results f r o m this s t u d y can be discussed following the general p a t t e r n o f i n t e r d i f f u s i o n in bimetallic systems with phase f o r m a t i o n . L u s t m a n [9] discussed the rate o f f o r m a t i o n o f intermetallic layers and stated t h a t the simple x 2 = k t relationship and the Arrhenius e q u a t i o n o f t e n do n o t describe the b e h a v i o u r o f layer f o r m a t i o n accurately. In o r d e r t h a t these simple relations hold, the relative thickness o f a phase in relation to the total diffusion z o n e w i d t h m u s t be c o n s t a n t with time at any t e m p e r a t u r e . In o u r e x p e r i m e n t s , owing to t h e i r thinness, it was n o t possible to measure b o t h layers separately. H o w e v e r , in samples where b o t h layers existed, we f o u n d t h a t the relative width o f the Cu3Sn phase t o the total intermetallic width ( x / x t ratio in Table 1) was essentially c o n s t a n t with time at all the
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Fig. 3. Width o f the total CuSn phase after various diffusion anneals. Samples were electrodeposited tin on brass substrates.
temperatures studied. Therefore, we have used the total intermetallic width for measuring the interdiffusion coefficient (Cu6Sn5 + Cu3Sn). Samples with brass as a base material were observed to form a Cu6Sn 5 intermetallic layer only. The interdiffusion coefficient can be estimated from the thickening rate of the total intermetallic growth at different annealing temperatures using the relationsbip Dcusn
~-
X2t/t
( c m 2 S- 1 )
We calculated the apparent diffusion coefficient from the data presented in Table 1 and present them in Fig. 4 as a plot of -- log Dcusn versus 1/T. In this plot data from earlier work [1 - 6, 10] are also shown. The reported bulk diffusion coefficients which were obtained at high temperatures can be extrapolated to 20 °C, giving a value of approximately 10- 20 (cm 2 s-1 ). Obviously the extrapolated value disagrees with our experimental points as shown in Fig. 4 (the bars in the region between the broken straight lines). The extrapolation has ignored the V-V' phase transformation which occurs at the lower temperatures.
62 6 7
o Starke,Wever (1964) * Maverick, Vichter (1967) o Fqdos,Schre]ner (19?0) Creydt,F~;ht er (19?1) • Tu (1973) A Unsv~)rt h,ivi~ckay (] 974I t, Luboyova et a((1975)
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Fig. 4. Log of the interdiffusion coefficient uersus l I T at the total width of the CuSn phase. The vertical bars represent the range of the diffusion coefficients measured in this work. A c c o r d i n g t o Tu [5] the t r a n s f o r m a t i o n has n o significant e f f e c t on the d i f f u s i o n c o e f f i c i e n t , since (Cu6Sn5) is a superlattice o f 7. T h e necessary activation energy Q for i n t e r d i f f u s i o n can be e s t i m a t e d f r o m the slope o f the line (see Fig. 4) using the f o l l o w i n g relationship: Q = 2.3 R
log D 1 -- log D 2 1/T1
(cal m o 1 - 1 )
-- 1/T2
w h e r e R is 1 . 9 8 7 cal m o l °K and 2.3 is t h e l o g a r i t h m i c base change factor. In Figure 4 our e x p e r i m e n t a l p o i n t s at l o w e r t e m p e r a t u r e s f o l l o w a line with a slope ( - 6 . 0 0 0 cal m o l - 1 ) w h i c h is smaller than that o f t h e high t e m p e r ature points. Here w e a s s u m e that this indicates a decrease in t h e apparent activation energy o f d i f f u s i o n . This b e h a v i o u r is c o m m o n for l o w t e m p e r ature d i f f u s i o n in p o l y c r y s t a l l i n e materials. T h e result also agrees w i t h the s t a t e m e n t by Balluffi [ 1 1 ] that in metallic thin films grain b o u n d a r y a n d / o r d i s l o c a t i o n short-circuit d i f f u s i o n s h o u l d b e c o m e the d o m i n a n t transport m e c h a n i s m at l o w e r temperatures. D i f f u s i o n along grain b o u n d a r i e s is a m u c h m o r e rapid process than d i f f u s i o n t h r o u g h t h e bulk since the f o r m e r n e e d s a smaller activation energy than the latter. A c c o r d i n g to O w e n [ 12] d e f e c t d i f f u s i o n also p r e d o m i n a t e s at a l o w e r t e m p e r a t u r e in p o l y c r y s t a l l i n e d e p o s i t e d metals. It is o b v i o u s that a gradual transition region m u s t exist b e t w e e n the bulk and t h e d e f e c t transport m e c h a n i s m .
63 Based o n o u r e x p e r i m e n t s , the low t e m p e r a t u r e diffusion region is e s t i m a t e d to be 20 - 120 °C. This is a significantly l o w e r t e m p e r a t u r e range t h a n t h a t investigated b y o t h e r a u t h o r s using bulk diffusion systems. The a p p a r e n t activation e n e r g y in this t e m p e r a t u r e interval is one third o f the e s t i m a t e d value o b t a i n e d f r o m higher t e m p e r a t u r e data. A c c o r d i n g to o u r o b s e r v a t i o n s the d i s c r e p a n c y c o u l d n o t be explained by a structural difference b e t w e e n the t w o t y p e s o f samples.
Conclusions T h e results o f this investigation can be s u m m a r i z e d as follows. (1} T h e e x t e n t o f the f o r m a t i o n o f CuSn intermetallic layers d e p e n d s u p o n a n u m b e r o f factors, including annealing (storage) time and t e m p e r ature. (2) The rate o f c o p p e r mass t r a n s p o r t in tin coatings at lower temperatures is c o n t r o l l e d p r e d o m i n a n t l y b y grain b o u n d a r y and d e f e c t diffusion. (3) T h e a p p a r e n t activation e n e r g y for the f o r m a t i o n o f intermetallic c o m p o u n d s in tin coatings at l o w e r t e m p e r a t u r e s is o n l y a b o u t one third o f t h a t for bulk lattice diffusion. (4) This result m a y be used to p r e d i c t the t h i c k e n i n g rate o f intermetallic g r o w t h at service t e m p e r a t u r e s .
Acknowledgements The a u t h o r w o u l d like to t h a n k Dr. King-Ning T u o f the Cavendish Lab o r a t o r y , Cambridge, for his c o m m e n t s ; t h a n k s are also due to colleagues, Miss L. Lind, Dr. U. L i n d b o r g and Mr. L.-G. Litjestrand, for stimulating discussions and t o the D e p a r t m e n t Manager, Dr. E. E d m a n , for his s u p p o r t .
References 1 2 3 4 5 6 7 8 9 10 11 12
E. Starke and H. Wever, Z. Metallkunde, 55 (3) (1964) 107. R. Fichter and L. Maverick, Schweiz. Arch., (Jan) (1967) 22. M. Creydt and R. Fichter, Metall, 25 (10) (1971) 1124. D. Unsworth and C. A. Mackay, Trans. Inst. Metal Finishing, 51 (1973) 85. K. N. Tu, Acta Met., 21 (1973) 347. Z. Lubyova, P. Fellner and K. Matiasovsky, Z. Metallkunde, 66 (3) (1975) 179. U. Lindborg, B. Asthner, L. Lind and L. R~vay, IEEE Trans. on Parts, Hybrids and Packaging, (March) (1976) 33. L. Zakaysek, Proc. l l t h Annu. Conf. on Reliability Physics, IEEE, 1973, p. 6. B. Lustman and R. F. Mehl, AIME Trans., 147 (1942) 369. H. Fidos and H. Schreiner, Z. Metallkunde, 61 (3) (1970) 225. R. W. Balluffi, Thin Solid Films, 25 (1975) 363. E. L. Owen, in R. Sard, H. Leidheiser and F. Ogburn (eds.), Properties of Electrodeposits, their Measurement and Significance, The Electrochem. Soc, Princeton, N.J., 1975, p. 80.
57
I N T E R D I F F U S I O N AND FO R M A T I O N OF I N T E R M E T A L L I C COMPOUNDS IN T I N - C O P P E R A L L O Y SURFACE COATINGS
L. RI~VAY Telefonaktiebolaget, L. M. Ericsson Materials Laboratory, S-126 25 Stockholm, Sweden
(Received May 4, 1976; in revised form June 28, 1976)
Summary Intermetallic layer formation at the interface between electrodeposited tin or fused tin - lead coatings on copper alloys was studied by optical microscopy. The apparent interdiffusion coefficients in the t e m p e r a t u r e range 20 to 160 °C were determined. On the basis of these values the apparent activation energy was calculated. The results show a lower activation energy for diffusion in coatings than for bulk lattice diffusion and can be used to predict the thickening rate of intermetallic growth at service temperatures.
Introduction Interdiffusion and reaction between basis metals and surface coatings o f t e n lead to intermetallic phase formation in the interracial region. For the c o p p e r - t i n system, interdiffusion plays a significant role in many commercial applications, particularly where tin or tin alloys are used as coatings in electrical c o m p o n e n t s . The f or m a t i on of intermetallic phases can cause degradation o f electrical and mechanical properties of bot h the solder joint and the co n d u cto r . Therefore, with respect to device reliability, it is important to investigate this p h e n o m e n o n . In the last decade several studies have been devoted to investigation o f interdiffusion in C u - S n systems. Starke and Wever [1] have reported the chemical bulk diffusion coefficient at high temperatures in this system. Fichter and Maverick [2] have systematically studied solder joints for several combinations. C r e ydt and Fichter [3] have also report ed diffusion p h e n o m e n a between galvanic tin or t i n- l ead coatings and copper alloys, in some cases with nickel as a barrier layer. Unsworth and Mackay [4] studied h o w substrate materials affect the growth of intermetallic layers. Recently Tu [ 5] r ep o r ted his basic study which was carried o u t with bimetallic C u - S n films; he found the f or m at i on of a CusSn ~ phase at all temperatures between --2 and 100 °C, but he did not find the Cu3Sn phase below 60 °C. L u b y o v a e t al. [6] also reported the activation energy for interdiffusion in the temperature range 200 - 450 °C.
58 The primary objective of this work was to estimate the rate of interdiffusion in coatings at m ode r at e temperatures (between 20 and 160 °C). These temperatures correspond to accelerated reliability test conditions.
Experiments and results According to our observations one or two intermetallic layers tend to persist at the interface between electrodeposited, fused tin or tin lead solders on copper substrates (R6vay, unpublished results,. 1969; [ 7] ). These intermetallic layers have been identified by X-ray diffraction and analyzed by electron microprobe. Both techniques indicated a composition of CugSns(r~') and Cu3Sn(c ). The intermetallic V' phase forms spontaneously at room temperature during a long storage time, but will thicken more rapidly at elevated temperatures. Consequently, the e x t e n t of this intermetallic formation depends upon a n u m b e r of factors, including time and temperature. Since it is difficult to measure the layer thickness at a low reaction temperature for an accurate evaluation of the diffusion rate, we have a t t e m p t e d to avoid this difficulty by extending the reaction time and have considered only those layers which were measureable with an optical microscope. After the annealing or storage procedure, sample surfaces were prepared by the usual metallographical techniques. The advantages of the metallographic approach to diffusion studies are principally its ease of operation, availability and speed, and also the opp o r t u n i t y for direct observation. Metallography does not yield concentration penetration data but can be used to calculate the diffusion coefficients. It is particularly appropriate in the case of intermediate phases where the m o r p h o l o g y of the diffusion zone is i m p o r t a n t as well as the diffusion coefficients. Microscopic identification of the Cu6Sn 5 and CuaSn layers was based on the distinct difference in colour between them. Figure 1 shows these layers formed at 160 °C after 28 days. Their width was estimated, with an optical microscope giving a magnification of 1200, by using an ocular micrometer. Table 1 shows the width of these layers, and the relative width of the CuaSn layer to the total width of the intermetallic layers (x/xt ratio). Figures 2 and 3 show the average value of the total C u - S n intermetallic width plotted against the square r o o t of time at different annealing temperatures. Only one anneal was carried out at 20 and 50 °C. The single point in the diagram is an average value of a num be r o f samples. Microhardness was estimated as 350 and 450 kg mm 2 for the CuaSn and Cu6Sn 5 phases, respectively. Figure 1 also shows the K n o o p indentor track in these phases, which indicates that a discontinuity in hardness may exist through the intermediate zone. This brittle behaviour arises from the decrease in the metallic character o f the bonding exhibited by these compounds. The volume per atom ratio in the intermediate phases is almost always less than in pure metals, which means that these phases are c o m m o n l y under
59
Fig. 1. Metallurgical cross-section of the CuSn intermediate layers. Sample was annealed at 160 "C for 28 days. stress. Bonds o f this t y p e are generally c o n s i d e r e d t o be undesirable. New evidence, however, shows t h a t f r a c t u r e in intermetallic b o n d s does n o t necessarily o c c u r within the b o d y o f the intermetallic layer. In an intermetallic b o n d i n g system the weakest b o u n d a r y is the intermetallic/substrate i n t e r f a c e [ 8 ] .
Discussion Results f r o m this s t u d y can be discussed following the general p a t t e r n o f i n t e r d i f f u s i o n in bimetallic systems with phase f o r m a t i o n . L u s t m a n [9] discussed the rate o f f o r m a t i o n o f intermetallic layers and stated t h a t the simple x 2 = k t relationship and the Arrhenius e q u a t i o n o f t e n do n o t describe the b e h a v i o u r o f layer f o r m a t i o n accurately. In o r d e r t h a t these simple relations hold, the relative thickness o f a phase in relation to the total diffusion z o n e w i d t h m u s t be c o n s t a n t with time at any t e m p e r a t u r e . In o u r e x p e r i m e n t s , owing to t h e i r thinness, it was n o t possible to measure b o t h layers separately. H o w e v e r , in samples where b o t h layers existed, we f o u n d t h a t the relative width o f the Cu3Sn phase t o the total intermetallic width ( x / x t ratio in Table 1) was essentially c o n s t a n t with time at all the
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Fig. 2. Width of the total CuSn phase after various diffusion anneals. Samples were fused t i n - l e a d on copper subs~rates.
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Fig. 3. Width o f the total CuSn phase after various diffusion anneals. Samples were electrodeposited tin on brass substrates.
temperatures studied. Therefore, we have used the total intermetallic width for measuring the interdiffusion coefficient (Cu6Sn5 + Cu3Sn). Samples with brass as a base material were observed to form a Cu6Sn 5 intermetallic layer only. The interdiffusion coefficient can be estimated from the thickening rate of the total intermetallic growth at different annealing temperatures using the relationsbip Dcusn
~-
X2t/t
( c m 2 S- 1 )
We calculated the apparent diffusion coefficient from the data presented in Table 1 and present them in Fig. 4 as a plot of -- log Dcusn versus 1/T. In this plot data from earlier work [1 - 6, 10] are also shown. The reported bulk diffusion coefficients which were obtained at high temperatures can be extrapolated to 20 °C, giving a value of approximately 10- 20 (cm 2 s-1 ). Obviously the extrapolated value disagrees with our experimental points as shown in Fig. 4 (the bars in the region between the broken straight lines). The extrapolation has ignored the V-V' phase transformation which occurs at the lower temperatures.
62 6 7
o Starke,Wever (1964) * Maverick, Vichter (1967) o Fqdos,Schre]ner (19?0) Creydt,F~;ht er (19?1) • Tu (1973) A Unsv~)rt h,ivi~ckay (] 974I t, Luboyova et a((1975)
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Fig. 4. Log of the interdiffusion coefficient uersus l I T at the total width of the CuSn phase. The vertical bars represent the range of the diffusion coefficients measured in this work. A c c o r d i n g t o Tu [5] the t r a n s f o r m a t i o n has n o significant e f f e c t on the d i f f u s i o n c o e f f i c i e n t , since (Cu6Sn5) is a superlattice o f 7. T h e necessary activation energy Q for i n t e r d i f f u s i o n can be e s t i m a t e d f r o m the slope o f the line (see Fig. 4) using the f o l l o w i n g relationship: Q = 2.3 R
log D 1 -- log D 2 1/T1
(cal m o 1 - 1 )
-- 1/T2
w h e r e R is 1 . 9 8 7 cal m o l °K and 2.3 is t h e l o g a r i t h m i c base change factor. In Figure 4 our e x p e r i m e n t a l p o i n t s at l o w e r t e m p e r a t u r e s f o l l o w a line with a slope ( - 6 . 0 0 0 cal m o l - 1 ) w h i c h is smaller than that o f t h e high t e m p e r ature points. Here w e a s s u m e that this indicates a decrease in t h e apparent activation energy o f d i f f u s i o n . This b e h a v i o u r is c o m m o n for l o w t e m p e r ature d i f f u s i o n in p o l y c r y s t a l l i n e materials. T h e result also agrees w i t h the s t a t e m e n t by Balluffi [ 1 1 ] that in metallic thin films grain b o u n d a r y a n d / o r d i s l o c a t i o n short-circuit d i f f u s i o n s h o u l d b e c o m e the d o m i n a n t transport m e c h a n i s m at l o w e r temperatures. D i f f u s i o n along grain b o u n d a r i e s is a m u c h m o r e rapid process than d i f f u s i o n t h r o u g h t h e bulk since the f o r m e r n e e d s a smaller activation energy than the latter. A c c o r d i n g to O w e n [ 12] d e f e c t d i f f u s i o n also p r e d o m i n a t e s at a l o w e r t e m p e r a t u r e in p o l y c r y s t a l l i n e d e p o s i t e d metals. It is o b v i o u s that a gradual transition region m u s t exist b e t w e e n the bulk and t h e d e f e c t transport m e c h a n i s m .
63 Based o n o u r e x p e r i m e n t s , the low t e m p e r a t u r e diffusion region is e s t i m a t e d to be 20 - 120 °C. This is a significantly l o w e r t e m p e r a t u r e range t h a n t h a t investigated b y o t h e r a u t h o r s using bulk diffusion systems. The a p p a r e n t activation e n e r g y in this t e m p e r a t u r e interval is one third o f the e s t i m a t e d value o b t a i n e d f r o m higher t e m p e r a t u r e data. A c c o r d i n g to o u r o b s e r v a t i o n s the d i s c r e p a n c y c o u l d n o t be explained by a structural difference b e t w e e n the t w o t y p e s o f samples.
Conclusions T h e results o f this investigation can be s u m m a r i z e d as follows. (1} T h e e x t e n t o f the f o r m a t i o n o f CuSn intermetallic layers d e p e n d s u p o n a n u m b e r o f factors, including annealing (storage) time and t e m p e r ature. (2) The rate o f c o p p e r mass t r a n s p o r t in tin coatings at lower temperatures is c o n t r o l l e d p r e d o m i n a n t l y b y grain b o u n d a r y and d e f e c t diffusion. (3) T h e a p p a r e n t activation e n e r g y for the f o r m a t i o n o f intermetallic c o m p o u n d s in tin coatings at l o w e r t e m p e r a t u r e s is o n l y a b o u t one third o f t h a t for bulk lattice diffusion. (4) This result m a y be used to p r e d i c t the t h i c k e n i n g rate o f intermetallic g r o w t h at service t e m p e r a t u r e s .
Acknowledgements The a u t h o r w o u l d like to t h a n k Dr. King-Ning T u o f the Cavendish Lab o r a t o r y , Cambridge, for his c o m m e n t s ; t h a n k s are also due to colleagues, Miss L. Lind, Dr. U. L i n d b o r g and Mr. L.-G. Litjestrand, for stimulating discussions and t o the D e p a r t m e n t Manager, Dr. E. E d m a n , for his s u p p o r t .
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