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On the fatigue of solder subjected to isothermal cyclic shear
Scripta METALLURGICA et MATERIALIA
Vol. 28, pp. 349-353, 1993 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
ON THE FATIGUE OF SOLDER SUBJECTED TO ISOTHERMAL CYCLIC SHEAR Maria Rynemark, Margareta Nyltn and Bevis Hutchinson Swedish Institute for Metals Research Drotming Kristinas v~ig48, S-114 28 Stockholm, Sweden
(Received October 7, 1992) (Revised November 24, 1992) Introduction It is now well established that a major cause of unreliability in electronic circuits is due to fatigue failure of soldered joints (e.g. 1, 2, 3). In particular, the connections of surface mounted components on circuit boards are exposed to alternating shear stresses as a result of temperature cycling and the different thermal expansivities of component and substrate. For this reason a considerable effort has been applied to understanding the behaviour of solders under loading in cyclic shear (e.g. 1, 4 - 8). A peculiarity of simple shear deformation is that there is no geometrical necessity for plastic deformation to be homogeneously distributed through the material; the magnitude of the shear can vary along the normal to the shear plane without other modes of deformation occurring to maintain continuity. Work hardening will enforce a uniform distribution of plastic strain but any tendency towards work softening can lead to unhindered localisation of plasticity in a narrow band. Evidence for such a localisation of deformation in cyclically sheared solders has been reported on numerous occasions (1, 9 - 11). Metallographic sections reveal a coarsening of the eutectic structure, fragmentation of primary lead-rich dendrites and evolution of a fine-grained 'recrystallised' structure within the tin-rich matrix. Such 'coarsened zones' occur as a result of both isothermal and thermal fatigue along the edge of the solder, adjacent of the more rigid substrate. Coarsened zones are normally recognised on both flanks of a fatigue crack, showing that there is a connection between the microstructural evolution and the mechanism of crack growth. Although some of the microstructure coarsening may be associated with the propagating crack tip, there is also evidence that coarsened zones are already present after a smaller number of cycles, prior to cracking (9). On the basis that structure coarsening of this type is evidence of localisation of plasticity which then controls the fatigue process, it is important to understand how the phenomenon should be treated for predicting the life-time of joints. Two extreme conditions can be identified as shown schematically in Fig. 1. In Fig. l(a) the deformation is uniformly distributed across the joint and the nominal reversed plastic shear strain ~ = 2~/t (~ = displacement and t = thickness of the joint) should be a proper parameter on which to base a failure criterion. In Fig. 1(b) the whole plastic displacement is assumed to be localised within a narrow zone of thickness x which is determined by the material itself. In this case the failure criterion should properly be expressed in terms of the reversed plastic shear strain in the zone, ~ = 2&x, and so be independent of the thickness of the joint, as proposed by Frear et al (9). It is evidently possible to investigate what is the relevant criterion for failure in these circumstances by carrying out tests in which both the thickness of the joint and the imposed displacement axe varied independently. The present experiments were designed with this aim.
Exuerimental Ring and plug specimens of copper were used as shown in Fig. 2 where the inner diameter of the ring was varied systematically to give joint thickness of 0.1, 0.2, 0.5 and 1.0 mm. A technique was developed to very accurately centralise the plug without resort to spacers in the joint (12). Soldering was carried out inside a vacuum chamber to minimise occurrence of porosity, using a programmable heating and cooling cycle. In this way the initial microstructure of the 60/40 solder could be maintained constant for all the specimens. 349 0 9 5 6 - 7 1 6 X / 9 3 $ 6 . 0 0 + .00 C o p y r i g h t ( c ) 1993 P e r g a m o n P r e s s L t d .
350
FATIGUE
OF SOLDER
Vol.
28, No.
Testing was done under displacement-control using a custom-built servo-hydraulic rig at 20°C with a triangular loading ramp at rate of 0.32 lam/s. Two different displacement intervals of :LS.0 lam and +10.0 pm were tested. These displacements were chosen as being within the range of values which arise when surface mounted components on circuit boards are subjected to typical thermal cycles. In general, four specimens were tested for each set of conditions. Control and data logging was carried out by a personal computer. Figure 3 shows an example of how the peak load for each cycle varied with the number of cycles. A small initial hardening was frequently observed after which the peak load reduced monotonically. For the purpose of the present paper, the number of cycles to a load reduction of 10 % from the maximum peak value has been adopted as the life-time criterion. MetaUographic examinations of specimens after 10 % load reduction showed evidence of definite fatigue cracking. Although a criterion in terms of first crack initiation might be preferred, practical experience showed the this condition was very difficult to define. Results and discussion MetaUographic examination of sectioned specimens following fatigue testing showed that the cracks propagated close to the copper substrates but usually away from the interface where CthSn5 phase was present, in agreement with earlier findings (8, 11). In most cases there was evidence of a coarsened zone, and cracks preferentially followed grain boundaries in a recrystallised tin matrix (Fig. 4). Most commonly, the cracks propagated close to the inner (plug) copper substrate. This is to be expected since the shear stress in the solder reached its highest value here. However, cracks were not uncommon at the outer (ring) surface of the two thinnest specimens (t -- 0.1 and 0.2 mm) where the stress level was relatively constant through the joint. Cracking did not occur within the bulk of the solder except in rare cases where it usually linked together a pair of cracks which appeared to have initiated independently at the inner and outer surfaces. Examples of load-displacement loops for the same specimen following different numbers of cycles are shown in Fig. 5. The interception of the loops with the x-axis gives a measure of plastic (strictly, inelastic) swain. As cracking proceeds and the peak load falls, the total displacement remains constant (imposed) but the contribution of plastic strain increases. This effect is, however, relatively limited for small load reductions, so the plastic strain interval is approximately constant during each test. Measured values quoted here refer to plastic strain amplitudes after 10 % reduction in peak load. Figure 6 shows the maximum peak shear stress plotted as a function of the nominal plastic shear strain for the eight different conditions tested. The annular geometry of the solder joints means that the stress is not constant but is a maximum at the inner surface (plug) and minimum at the outer (ring). The range of stress is shown by a band for each testing condition. Results for the different geometries and displacements showed reasonable congruency when plotted on this basis. In Fig. 7 the number of cycles to failure (10% reduction in peak load) is shown plotted as a function of the total nominal strain amplitude. It is seen that straight line relationships exist for each set of specimens tested with the same displacement but that these lines are displaced from one another by a factor of ~ 6 in the number of cycles. The total nominal strain in Fig. 7 includes a substantial elastic contribution much of which originates in the testing machine rather than the specimen itself. Since the elastic displacements are not expected to be influential as regards fatigue life, the same data have been re-plotted in Fig. 8 in terms of the nominal plastic strain amplitude. A small discrepancy is seen to remain, depending on the magnitude of the imposed displacement, but the two lines now differ by less than a factor 2 with respect to fatigue life. The present results show a pattern of fatigue behaviour which is generally in agreement with previous work. The new findings refer principally to the results in Fig. 8 showing the relationship between plastic strain amplitude and number of cycles to failure. This permits the separate roles of joint thickness and shearing displacement to be distinguished with respect to their effects on fatigue life. If the plastic strain were uniformly distributed over the thickness of the joint as in Fig. l(a), then the nominal plastic strain should define the fatigue life and all data should lie on a single line. If, at the other extreme, a local plastic zone of constant thickness were formed in which all the displacement were accommodated, (as in Fig. l(b)), then the fatigue life should be determined solely by the plastic displacement which is virtually independent of joint thickness under present conditions. This would lead to two
3
Vol.
28, No.
3
FATIGUE OF SOLDER
351
separated vertical lines in the plot shown in Fig. 8. A small but probably statistically significant separation exists between the lines in Fig. 8. This suggests that some tendency towards concentration of deformation in local plastic zones exists, but the overall behaviour appears to be dominated by homogeneous deformation so far as the fatigue life is concerned. To a good approximation the life-time can be estimated on the basis that an imposed displacement is accommodated over the full thickness of the solder. The observation that fatigue life of solder in shear could be well approximated by a model involving homogeneous deformation was rather surprising in view of recent findings in the literature. In particular, the work of Frear et al (9) gave rise to a quite different, albeit tentative, conclusion that joint thickness did not significantly affect fatigue life. These differences may reflect the significantly different test conditions since Frear et ars work was not isothermal but thermal fatigue with a fairly wide temperature range from -55 C to +125°C. Also, the plastic strain range estimated for these conditions was higher, generally - 20 %, as opposed to our values in the range 0.2 to 10 %, and failure was judged on a qualitative basis rather than with a quantitative criterion. Although it is still not certain what failure criterion is most relevant for these types of simulation tests, the 10% load reduction values do have the virtues of being objective and corresponding to fairly realistic cracking conditions. Further work is evidently necessary, however. The above findings have important implications for the design of solder joints for surface mounted electronic components. A thicker joint, (a greater stand-off) can be employed to improve the reliability of joints under conditions where they are susceptible to fatigue. From Fig. 8 it can be estimated that doubling the thickness of a solder joint should increase its fatigue life approximately three-fold, all other factors being equal. Such an effect is at least in qualitative agreement with practical experience in the electronics industry (13). Conclusions Local zones having coarsened structures susceptible to fatigue cracking are a common feature of solder which is exposed to cyclic shearing. Despite this evidence for some localisation of deformation, the fatigue life of the solder can be best rationalised on the basis that plastic deformation is distributed almost homogeneously through the thickness of the joint. Increasing the thickness of a solder joint can usefully improve its life under conditions of fatigue loading in shear. Acknowledgements The present work was carried out within the general research programme of the Swedish Institute for Metals Research with support from the Swedish National Board for Technical Development together with industrial companies in the Nordic area. The authors thank Ulla Gustavsson, Rolf Sandstr6m, Peter Bate and Lars Lind6 for help and advice, and the Industrial Advisory Group on Electronic Joining Technology for their guidance. References 1.
2. 3. 4. 5. 6. 7.
8. 9. 10.
R.N. Wild, Welding Research Suppl., 51,521, (1972). W. Engelmeier, Electronic Packaging and Production, 23, No. 4, 58, (1983). H.D. Solomon, Electronic packaging Materials and Processes, p. 29, Ed. J.A. SorteU, ASM, (1985). S. Vaynman and A. Zubelewicz, Welding Journal, 69, No. 10, 395, (1990). T.S.E. Summers and J.W. Morris, J. of Electronic Packaging, 112, 94, (1990). S. Vaynman, M.E. Fine and D.A. Jeannotte, Solder Mechanics-A State of the Art Assessment, p. 155, Santa F6, TMS, (1990). M.E. Warwick, Brazing and Soldering, No. 8, 20, (1985). G. Engberg, L-E. Larsson, M. Nyl6n and H. Steen, Brazing and Soldering, No. 11, 62, (1986). D.R. Frear, D. Grivas and J. W. Morris, J. Electronic Materials, 18, 671, (1989). A.C. Chilton and M.A. Whitmore, Solder and Surface Mount Techn., No. 3, 21 (1990).
352
FATIGUE
11.
OF SOLDER
Vol.
28, No.
J.W. Morris, D. Grivas, D. Tribula, T. Summers and D. Frear, Solder and Surface Mount Techn., 3, 4, (1989). M. Rynemark, M. Nyl6n and W.B. Hutchinson, Swedish Institute for Metals, Research Report IM-2864, (1992). Private communication within the Swedish Institute of Metals Industrial Advisory Group on Electronic Joining Technology.
12. 13.
ii
,////////////////////,
__~__[~/thickness
////////////////////Z
,,,2mm
iiiiiiiiiiiiiiiiiiiiiii l t
~
"////////////////////Z
x
"////////////~/////////
laf
Fig. 1 Showing how a simple shear displacement in the solder joint may be accommodated, (a) homogeneously, Co) in a localised zone.
_L,
t=0.1, 0.Z, 03,1.0 mm
S.,m l , \ ,Oom I
t Fig. 2 Design of the ring and plug specimens used in the present investigation.
2.0 |
"-i6;/Z-tg~-;
Z
~
~, 1.1
N~
10
.
.
.
.
.
'~'6o
.
.
.
.
.
.
.
,,
~6oo '
.
.
.
.
.
10 000
Number of cyet.es
Fig. 3 Example of results of cyclic shear testing showing the reduction in peak load/cycle with increasing number of cycles. The failure criterion is defined as the number of cycles to a 10 % reduction from maximum peak load.
Fig. 4 Optical micrograph showing a typical coarsened zone and fatigue crack. Section perpendicular to specimen axis.
3
Vol.
28, No.
3
FATIGUE
2.0- Lood (kN) 1.5- ~ " ~ f=,,1.0. . . . . . .
~1 °'°'.... ..-.'7//
~
N=16
~
/N=1006 ,'~,,:,'~,,~,~,, ZN = 2001
"
~.~1:~ ~ "
/_.~¢"~
353
30 25-
MQX
/.:.."
-05'
...........
OF S O L D E R
_
J
20-
Disptocement
I~l
.x 15-
lI
t~
- 1.5
.....
~• 10"
12
Ptostic stroin, % Fig. 6 Variation of peak slaear stress with plastic
Fig. 5 Load-displacement loops recorded on the same specimen after different numbers of cycles.
shear strain for different test conditions. Max and min stresses apply at inner and outer diameters of the solder joint respectively.
100" t
~95 % confidenceintervot
,95 % confidence intervot
100-
c_.~10. c
~ 10-
._u
t4
-6
0.1
o~ 0
lb
Ido
ldoo
1oooo
Number of cyctes Fig. 7 Fatigue life as a function of the nominal total strain/cycle for joints of different thickness with two different displacement ranges +5 lam and +10 pm.
0
10
160
Number. of cycles
10'00
10000
Fig. 8 Fatigue life as a function of the nominal plastic strain/cycle for joints of different thickness with two different displacement ranges +5 pm and +10 pm.
Vol. 28, pp. 349-353, 1993 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
ON THE FATIGUE OF SOLDER SUBJECTED TO ISOTHERMAL CYCLIC SHEAR Maria Rynemark, Margareta Nyltn and Bevis Hutchinson Swedish Institute for Metals Research Drotming Kristinas v~ig48, S-114 28 Stockholm, Sweden
(Received October 7, 1992) (Revised November 24, 1992) Introduction It is now well established that a major cause of unreliability in electronic circuits is due to fatigue failure of soldered joints (e.g. 1, 2, 3). In particular, the connections of surface mounted components on circuit boards are exposed to alternating shear stresses as a result of temperature cycling and the different thermal expansivities of component and substrate. For this reason a considerable effort has been applied to understanding the behaviour of solders under loading in cyclic shear (e.g. 1, 4 - 8). A peculiarity of simple shear deformation is that there is no geometrical necessity for plastic deformation to be homogeneously distributed through the material; the magnitude of the shear can vary along the normal to the shear plane without other modes of deformation occurring to maintain continuity. Work hardening will enforce a uniform distribution of plastic strain but any tendency towards work softening can lead to unhindered localisation of plasticity in a narrow band. Evidence for such a localisation of deformation in cyclically sheared solders has been reported on numerous occasions (1, 9 - 11). Metallographic sections reveal a coarsening of the eutectic structure, fragmentation of primary lead-rich dendrites and evolution of a fine-grained 'recrystallised' structure within the tin-rich matrix. Such 'coarsened zones' occur as a result of both isothermal and thermal fatigue along the edge of the solder, adjacent of the more rigid substrate. Coarsened zones are normally recognised on both flanks of a fatigue crack, showing that there is a connection between the microstructural evolution and the mechanism of crack growth. Although some of the microstructure coarsening may be associated with the propagating crack tip, there is also evidence that coarsened zones are already present after a smaller number of cycles, prior to cracking (9). On the basis that structure coarsening of this type is evidence of localisation of plasticity which then controls the fatigue process, it is important to understand how the phenomenon should be treated for predicting the life-time of joints. Two extreme conditions can be identified as shown schematically in Fig. 1. In Fig. l(a) the deformation is uniformly distributed across the joint and the nominal reversed plastic shear strain ~ = 2~/t (~ = displacement and t = thickness of the joint) should be a proper parameter on which to base a failure criterion. In Fig. 1(b) the whole plastic displacement is assumed to be localised within a narrow zone of thickness x which is determined by the material itself. In this case the failure criterion should properly be expressed in terms of the reversed plastic shear strain in the zone, ~ = 2&x, and so be independent of the thickness of the joint, as proposed by Frear et al (9). It is evidently possible to investigate what is the relevant criterion for failure in these circumstances by carrying out tests in which both the thickness of the joint and the imposed displacement axe varied independently. The present experiments were designed with this aim.
Exuerimental Ring and plug specimens of copper were used as shown in Fig. 2 where the inner diameter of the ring was varied systematically to give joint thickness of 0.1, 0.2, 0.5 and 1.0 mm. A technique was developed to very accurately centralise the plug without resort to spacers in the joint (12). Soldering was carried out inside a vacuum chamber to minimise occurrence of porosity, using a programmable heating and cooling cycle. In this way the initial microstructure of the 60/40 solder could be maintained constant for all the specimens. 349 0 9 5 6 - 7 1 6 X / 9 3 $ 6 . 0 0 + .00 C o p y r i g h t ( c ) 1993 P e r g a m o n P r e s s L t d .
350
FATIGUE
OF SOLDER
Vol.
28, No.
Testing was done under displacement-control using a custom-built servo-hydraulic rig at 20°C with a triangular loading ramp at rate of 0.32 lam/s. Two different displacement intervals of :LS.0 lam and +10.0 pm were tested. These displacements were chosen as being within the range of values which arise when surface mounted components on circuit boards are subjected to typical thermal cycles. In general, four specimens were tested for each set of conditions. Control and data logging was carried out by a personal computer. Figure 3 shows an example of how the peak load for each cycle varied with the number of cycles. A small initial hardening was frequently observed after which the peak load reduced monotonically. For the purpose of the present paper, the number of cycles to a load reduction of 10 % from the maximum peak value has been adopted as the life-time criterion. MetaUographic examinations of specimens after 10 % load reduction showed evidence of definite fatigue cracking. Although a criterion in terms of first crack initiation might be preferred, practical experience showed the this condition was very difficult to define. Results and discussion MetaUographic examination of sectioned specimens following fatigue testing showed that the cracks propagated close to the copper substrates but usually away from the interface where CthSn5 phase was present, in agreement with earlier findings (8, 11). In most cases there was evidence of a coarsened zone, and cracks preferentially followed grain boundaries in a recrystallised tin matrix (Fig. 4). Most commonly, the cracks propagated close to the inner (plug) copper substrate. This is to be expected since the shear stress in the solder reached its highest value here. However, cracks were not uncommon at the outer (ring) surface of the two thinnest specimens (t -- 0.1 and 0.2 mm) where the stress level was relatively constant through the joint. Cracking did not occur within the bulk of the solder except in rare cases where it usually linked together a pair of cracks which appeared to have initiated independently at the inner and outer surfaces. Examples of load-displacement loops for the same specimen following different numbers of cycles are shown in Fig. 5. The interception of the loops with the x-axis gives a measure of plastic (strictly, inelastic) swain. As cracking proceeds and the peak load falls, the total displacement remains constant (imposed) but the contribution of plastic strain increases. This effect is, however, relatively limited for small load reductions, so the plastic strain interval is approximately constant during each test. Measured values quoted here refer to plastic strain amplitudes after 10 % reduction in peak load. Figure 6 shows the maximum peak shear stress plotted as a function of the nominal plastic shear strain for the eight different conditions tested. The annular geometry of the solder joints means that the stress is not constant but is a maximum at the inner surface (plug) and minimum at the outer (ring). The range of stress is shown by a band for each testing condition. Results for the different geometries and displacements showed reasonable congruency when plotted on this basis. In Fig. 7 the number of cycles to failure (10% reduction in peak load) is shown plotted as a function of the total nominal strain amplitude. It is seen that straight line relationships exist for each set of specimens tested with the same displacement but that these lines are displaced from one another by a factor of ~ 6 in the number of cycles. The total nominal strain in Fig. 7 includes a substantial elastic contribution much of which originates in the testing machine rather than the specimen itself. Since the elastic displacements are not expected to be influential as regards fatigue life, the same data have been re-plotted in Fig. 8 in terms of the nominal plastic strain amplitude. A small discrepancy is seen to remain, depending on the magnitude of the imposed displacement, but the two lines now differ by less than a factor 2 with respect to fatigue life. The present results show a pattern of fatigue behaviour which is generally in agreement with previous work. The new findings refer principally to the results in Fig. 8 showing the relationship between plastic strain amplitude and number of cycles to failure. This permits the separate roles of joint thickness and shearing displacement to be distinguished with respect to their effects on fatigue life. If the plastic strain were uniformly distributed over the thickness of the joint as in Fig. l(a), then the nominal plastic strain should define the fatigue life and all data should lie on a single line. If, at the other extreme, a local plastic zone of constant thickness were formed in which all the displacement were accommodated, (as in Fig. l(b)), then the fatigue life should be determined solely by the plastic displacement which is virtually independent of joint thickness under present conditions. This would lead to two
3
Vol.
28, No.
3
FATIGUE OF SOLDER
351
separated vertical lines in the plot shown in Fig. 8. A small but probably statistically significant separation exists between the lines in Fig. 8. This suggests that some tendency towards concentration of deformation in local plastic zones exists, but the overall behaviour appears to be dominated by homogeneous deformation so far as the fatigue life is concerned. To a good approximation the life-time can be estimated on the basis that an imposed displacement is accommodated over the full thickness of the solder. The observation that fatigue life of solder in shear could be well approximated by a model involving homogeneous deformation was rather surprising in view of recent findings in the literature. In particular, the work of Frear et al (9) gave rise to a quite different, albeit tentative, conclusion that joint thickness did not significantly affect fatigue life. These differences may reflect the significantly different test conditions since Frear et ars work was not isothermal but thermal fatigue with a fairly wide temperature range from -55 C to +125°C. Also, the plastic strain range estimated for these conditions was higher, generally - 20 %, as opposed to our values in the range 0.2 to 10 %, and failure was judged on a qualitative basis rather than with a quantitative criterion. Although it is still not certain what failure criterion is most relevant for these types of simulation tests, the 10% load reduction values do have the virtues of being objective and corresponding to fairly realistic cracking conditions. Further work is evidently necessary, however. The above findings have important implications for the design of solder joints for surface mounted electronic components. A thicker joint, (a greater stand-off) can be employed to improve the reliability of joints under conditions where they are susceptible to fatigue. From Fig. 8 it can be estimated that doubling the thickness of a solder joint should increase its fatigue life approximately three-fold, all other factors being equal. Such an effect is at least in qualitative agreement with practical experience in the electronics industry (13). Conclusions Local zones having coarsened structures susceptible to fatigue cracking are a common feature of solder which is exposed to cyclic shearing. Despite this evidence for some localisation of deformation, the fatigue life of the solder can be best rationalised on the basis that plastic deformation is distributed almost homogeneously through the thickness of the joint. Increasing the thickness of a solder joint can usefully improve its life under conditions of fatigue loading in shear. Acknowledgements The present work was carried out within the general research programme of the Swedish Institute for Metals Research with support from the Swedish National Board for Technical Development together with industrial companies in the Nordic area. The authors thank Ulla Gustavsson, Rolf Sandstr6m, Peter Bate and Lars Lind6 for help and advice, and the Industrial Advisory Group on Electronic Joining Technology for their guidance. References 1.
2. 3. 4. 5. 6. 7.
8. 9. 10.
R.N. Wild, Welding Research Suppl., 51,521, (1972). W. Engelmeier, Electronic Packaging and Production, 23, No. 4, 58, (1983). H.D. Solomon, Electronic packaging Materials and Processes, p. 29, Ed. J.A. SorteU, ASM, (1985). S. Vaynman and A. Zubelewicz, Welding Journal, 69, No. 10, 395, (1990). T.S.E. Summers and J.W. Morris, J. of Electronic Packaging, 112, 94, (1990). S. Vaynman, M.E. Fine and D.A. Jeannotte, Solder Mechanics-A State of the Art Assessment, p. 155, Santa F6, TMS, (1990). M.E. Warwick, Brazing and Soldering, No. 8, 20, (1985). G. Engberg, L-E. Larsson, M. Nyl6n and H. Steen, Brazing and Soldering, No. 11, 62, (1986). D.R. Frear, D. Grivas and J. W. Morris, J. Electronic Materials, 18, 671, (1989). A.C. Chilton and M.A. Whitmore, Solder and Surface Mount Techn., No. 3, 21 (1990).
352
FATIGUE
11.
OF SOLDER
Vol.
28, No.
J.W. Morris, D. Grivas, D. Tribula, T. Summers and D. Frear, Solder and Surface Mount Techn., 3, 4, (1989). M. Rynemark, M. Nyl6n and W.B. Hutchinson, Swedish Institute for Metals, Research Report IM-2864, (1992). Private communication within the Swedish Institute of Metals Industrial Advisory Group on Electronic Joining Technology.
12. 13.
ii
,////////////////////,
__~__[~/thickness
////////////////////Z
,,,2mm
iiiiiiiiiiiiiiiiiiiiiii l t
~
"////////////////////Z
x
"////////////~/////////
laf
Fig. 1 Showing how a simple shear displacement in the solder joint may be accommodated, (a) homogeneously, Co) in a localised zone.
_L,
t=0.1, 0.Z, 03,1.0 mm
S.,m l , \ ,Oom I
t Fig. 2 Design of the ring and plug specimens used in the present investigation.
2.0 |
"-i6;/Z-tg~-;
Z
~
~, 1.1
N~
10
.
.
.
.
.
'~'6o
.
.
.
.
.
.
.
,,
~6oo '
.
.
.
.
.
10 000
Number of cyet.es
Fig. 3 Example of results of cyclic shear testing showing the reduction in peak load/cycle with increasing number of cycles. The failure criterion is defined as the number of cycles to a 10 % reduction from maximum peak load.
Fig. 4 Optical micrograph showing a typical coarsened zone and fatigue crack. Section perpendicular to specimen axis.
3
Vol.
28, No.
3
FATIGUE
2.0- Lood (kN) 1.5- ~ " ~ f=,,1.0. . . . . . .
~1 °'°'.... ..-.'7//
~
N=16
~
/N=1006 ,'~,,:,'~,,~,~,, ZN = 2001
"
~.~1:~ ~ "
/_.~¢"~
353
30 25-
MQX
/.:.."
-05'
...........
OF S O L D E R
_
J
20-
Disptocement
I~l
.x 15-
lI
t~
- 1.5
.....
~• 10"
12
Ptostic stroin, % Fig. 6 Variation of peak slaear stress with plastic
Fig. 5 Load-displacement loops recorded on the same specimen after different numbers of cycles.
shear strain for different test conditions. Max and min stresses apply at inner and outer diameters of the solder joint respectively.
100" t
~95 % confidenceintervot
,95 % confidence intervot
100-
c_.~10. c
~ 10-
._u
t4
-6
0.1
o~ 0
lb
Ido
ldoo
1oooo
Number of cyctes Fig. 7 Fatigue life as a function of the nominal total strain/cycle for joints of different thickness with two different displacement ranges +5 lam and +10 pm.
0
10
160
Number. of cycles
10'00
10000
Fig. 8 Fatigue life as a function of the nominal plastic strain/cycle for joints of different thickness with two different displacement ranges +5 pm and +10 pm.