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A simple procedure for assessing environmental effects on fatigue failure of solder joints
Scripta METALLURGICA
Vol. 22, pp. 1543-154S, 1988 Printed in the U.S.A.
Pergamon Press plc All rights reserved
A SIMPLE PROCEDUREFOR ASSESSING ENVIRON)IENTAL EFFECTS ON FATIGUE FAILURE OF SOLDERJOINTS T.G. Lacey and D.A. Woodford Materials Engineering Department Rensselaer Polytechnic I n s t i t u t e Troy, NY 12180-3590
(Received June 17, 1988) (Revised J u l y 1, 1988) Introduction
Solder j o i n t s t y p i c a l l y operate at high fractions of t h e i r melting points (e.g. for 60-40 Sn-Pb solder, room temperature is 65% of the melting temperature). Consequently, i t is expected that the phenomenology of fracture under both sustained load and cyclic deformation should be analogous in many respects to that of high temperature materials operating at equivalent homologous temperatures. Specifically, i t is anticipated that there w i l l be strong effects of cyclic frequency and environment on fatigue fracture. Fatigue f a i l u r e of solder j o i n t s is a major cause of concern in electrical devices including surface mounted ( I ) and power devices (2), and in large scale equipment such as automobile radiators (3). One very striking observation is that an inert environment greatly improves the fatigue l i f e (2,4). Burgess et al (2) reported a major improvement in thermal fatigue resistance of solder joints in hermetic packages compared with packages in which the seals had been broken. The direct fatigue tests of Berriche et al (4) on cast specimens of 96.5Pb3.5%Sn showed a strong degrading effect of an a i r environment over a moderate vacuum (2x10-3 Pa). The key study to provide a basis for understanding oxygen effects on fracture of solder alloys is that of Snowden on pure lead (5). As the oxygen partial pressure was reduced, he found a sharp increase in fatigue l i f e at room temperature at about 10 Pa amounting to between one and two orders of magnitude as the strain amplitude was decreased from 0.14% to 0.075%. He also found that reducing the test frequency resulted in a fatigue l i f e reduction for all pressures studied (~10-4-I0 - l Pa) (6). In an elegant series of experiments, i t was established that environmental protection (fatigue lives equivalent to those in high vacuum) could be achieved in a i r tests with suitable protective coatings. Grease-coated specimens and indium plated specimens were tested. The effect of test frequency on fatigue of lead was studied in detail by Eckel (7). He developed a frequency-modified l i f e equation for intergranular fatigue in a i r which presaged the analogous studies in high temperature alloys by Coffin (8). In t h i s l a t t e r work on A286 i t was shown that testing in high vacuum (I0 -b Pa) at either room temperature or 593°C gave equivalent lives and eliminated the frequency effect apparent in a i r tests. The present work reports some i n i t i a l results of environmental effects on fatigue lives of a 60Sn-4OPb solder. Detailed analysis of frequency effects on solder layers of this alloy tested in simple shear has been reported recently by Solomon (9). In addition to using a d i f f e r e n t alloy from that in the previous environmental study (4), the procedure involves testing of actual solder j o i n t s and surface protection of the sample. I t i s , of course, not known whether this constitutes an inert environment although the results are strongly indicative that f u l l protection is achieved in the room temperature tests. Some precedence for protection against intergranular penetration of oxygen in high temperature alloys by suitable coatings is available (10). Experimental Procedure Cantilever rotating bending fatigue tests at a frequency of 1Hz were performed on a soldered j o i n t in copper wire. Two pieces of copper wire of diameter 3.2mm were inserted into a 5mm O.D. pyrex tube separated by a short slug of 60-40 f l u x solder. The tube was then heated in a Bunsen burner to melt the solder. The sample was cooled in a i r and broken out of the quartz. By f i l i n g the ends of the copper to points and-ensuring that the points
1543 0036-9748/88 $3.00
+ .00
1544
FATIGUE FAILURE OF SOLDER JOINTS
Vol.
22,
No.
did nottouch in the f i n a l j o i n t , i t was found that fatigue f a i l u r e occurred e n t i r e l y in the solder rather than at the j o i n t interface. This proved to be a simple, inexpensive, and also reproducible, method of making a test specimen. Fatigue tests were conducted by clamping the wire specimen in a chuck driven by an electric motor acting through a reduction gear. A small pulley attached to the free end of the wire served to support the applied load consisting of lead weights. The stress at the j o i n t could be varied by varying the load or by adjusting the distance from pulley to solder j o i n t . A microswitch connected to a counter monitored the cycles, or number of revolutions, and a control switch was wired in series with the counter and the motor to turn off the system when f a i l u r e occurred. After fatigue f a i l u r e the bending moment for each test was computed by measuring the distance between the f a i l u r e location and the line of load application. Environmental protection was established by smearing the j o i n t area with vacuum grease. Results and Discussion Figure I i l l u s t r a t e s the effect of environment on fatigue l i f e by comparing best f i t lines through the data for a i r tested specimens and surface protected specimens. I t is surmised that the aggressive species is oxygen and that the vacuum grease provides an inert condition for much o f the crack propagation l i f e . Although i t is unusual to conduct a low cycle fatigue test in cantilever rotating bending where surface stresses are high and gross p l a s t i c i t y in the wire j o i n t occurs, for comparative purposes the test is clearly successful. The ordinate scale is based on the calculated outer f i b e r stress at the start of the test using the standard formula for a circular cross-section:* 4FL where F is applied load, L is distance from the fracture plane to load l i n e , and r is the j o i n t radius. 2.0
1.9,
=
tected •
~"
1.8-
x
1.7-
O~ o 1.6,
1.5-
1,4
•
2
3
4
log C y c l e s to F a i l u r e
y = 2.3297 - 0.2112x
R = 0.95
y = 2.2558 - 0,1534x R = 0.84
Figure l
Air
Protected
Low cycle fatigue of 60Sn-4OPb solder j o i n t s with and without environmental protection.
* This equation is limited to elastic behavior; actual surface stresses are expected to be less. The ordinate thus serves as a convenient reference scale for comparison only.
9
Vol.
22, No. 9
FATIGUE FAILURE OF SOLDER JOINTS
The figure i l l u s t r a t e s a similar phenomenon to that reported by Berriche et al (4) in that the environmental s e n s i t i v i t y increases with decreasing cyclic strain range. Although the solders are different, and our tests relate to failures in joints rather than the bulk specimens, the phenomenology is remarkably similar. This further supports our belief that the technique used here produces an essentially inert environment for fatigue crack propagation. The gain in fatigue l i f e of about an order of magnitude in an inert environment for conditions that lead to a l i f e in a i r of 10,000 cycles was roughly the same in the two studies. Clearly this simple, inexpensive testing procedure may be useful as a basis for a comprehensive comparative study of chemistry, cycle frequency, hold time effects, etc. Conclusions 1) A simple technique for measuring low cycle fatigue lives of solder joints in wire specimens using a cantilever rotating bend test provides good comparative data. 2) An inert environment may be provided in room temperature tests by simply smearing vacuum grease on the specimen surface. 3) The fatigue lives of the protected specimens may exceed those of a i r tests by an order of magnitude or more depending on the cyclic strain range. Acknowledgmen~ This work was supported by Rensselear Polytechnic Institute through the Undergraduate Research Program. References I. 2. 3. 4. 5°
6. 7. 8. 9. 10.
J.H. Lau and D.W. Rice, Solid State Technology, Vol. 28, No. 10, (1985), p. 91. J.F. Burgess, R.O. Carlson, H.H. Glascock, C.A. Neugebauer and H.F. Webster, IEEE CHMT-7, No. 4, (1984), p. 405. D. Peters, International Copper Research Association, 1987, personal comunication. R. Berriche, S. Vaynman, M.E. Fine and D.A. Jeannette, "Electronic Packaging and Corrosion in Microelectronics", edt. M.E. Nicholson, AStl, (1987), p. 169, K.U. Snowden, Acta Met, Vol. 12 (1964), p. 295. K.U. Snowden, Phil Mag, Vol. 10 (1964), p. 435. J.F. Eckel, Proceedings AS~, Vol. 51, 1951, p. 721. L.F. Coffin, "Fatigue at High Temperatures", ASTM, STP 520, 1973, p. 5. H.D. Solomon, Brazing and Soldering, No. 11, Autumn 1986, p. 68. D.A. Woodford and R.H. Bricknell, "Embrittlement of Engineering Alloys", edt. C.L. Briant and S.K. Banerji, Academic Press, Vol. 25 (1983), p. 157.
Vol. 22, pp. 1543-154S, 1988 Printed in the U.S.A.
Pergamon Press plc All rights reserved
A SIMPLE PROCEDUREFOR ASSESSING ENVIRON)IENTAL EFFECTS ON FATIGUE FAILURE OF SOLDERJOINTS T.G. Lacey and D.A. Woodford Materials Engineering Department Rensselaer Polytechnic I n s t i t u t e Troy, NY 12180-3590
(Received June 17, 1988) (Revised J u l y 1, 1988) Introduction
Solder j o i n t s t y p i c a l l y operate at high fractions of t h e i r melting points (e.g. for 60-40 Sn-Pb solder, room temperature is 65% of the melting temperature). Consequently, i t is expected that the phenomenology of fracture under both sustained load and cyclic deformation should be analogous in many respects to that of high temperature materials operating at equivalent homologous temperatures. Specifically, i t is anticipated that there w i l l be strong effects of cyclic frequency and environment on fatigue fracture. Fatigue f a i l u r e of solder j o i n t s is a major cause of concern in electrical devices including surface mounted ( I ) and power devices (2), and in large scale equipment such as automobile radiators (3). One very striking observation is that an inert environment greatly improves the fatigue l i f e (2,4). Burgess et al (2) reported a major improvement in thermal fatigue resistance of solder joints in hermetic packages compared with packages in which the seals had been broken. The direct fatigue tests of Berriche et al (4) on cast specimens of 96.5Pb3.5%Sn showed a strong degrading effect of an a i r environment over a moderate vacuum (2x10-3 Pa). The key study to provide a basis for understanding oxygen effects on fracture of solder alloys is that of Snowden on pure lead (5). As the oxygen partial pressure was reduced, he found a sharp increase in fatigue l i f e at room temperature at about 10 Pa amounting to between one and two orders of magnitude as the strain amplitude was decreased from 0.14% to 0.075%. He also found that reducing the test frequency resulted in a fatigue l i f e reduction for all pressures studied (~10-4-I0 - l Pa) (6). In an elegant series of experiments, i t was established that environmental protection (fatigue lives equivalent to those in high vacuum) could be achieved in a i r tests with suitable protective coatings. Grease-coated specimens and indium plated specimens were tested. The effect of test frequency on fatigue of lead was studied in detail by Eckel (7). He developed a frequency-modified l i f e equation for intergranular fatigue in a i r which presaged the analogous studies in high temperature alloys by Coffin (8). In t h i s l a t t e r work on A286 i t was shown that testing in high vacuum (I0 -b Pa) at either room temperature or 593°C gave equivalent lives and eliminated the frequency effect apparent in a i r tests. The present work reports some i n i t i a l results of environmental effects on fatigue lives of a 60Sn-4OPb solder. Detailed analysis of frequency effects on solder layers of this alloy tested in simple shear has been reported recently by Solomon (9). In addition to using a d i f f e r e n t alloy from that in the previous environmental study (4), the procedure involves testing of actual solder j o i n t s and surface protection of the sample. I t i s , of course, not known whether this constitutes an inert environment although the results are strongly indicative that f u l l protection is achieved in the room temperature tests. Some precedence for protection against intergranular penetration of oxygen in high temperature alloys by suitable coatings is available (10). Experimental Procedure Cantilever rotating bending fatigue tests at a frequency of 1Hz were performed on a soldered j o i n t in copper wire. Two pieces of copper wire of diameter 3.2mm were inserted into a 5mm O.D. pyrex tube separated by a short slug of 60-40 f l u x solder. The tube was then heated in a Bunsen burner to melt the solder. The sample was cooled in a i r and broken out of the quartz. By f i l i n g the ends of the copper to points and-ensuring that the points
1543 0036-9748/88 $3.00
+ .00
1544
FATIGUE FAILURE OF SOLDER JOINTS
Vol.
22,
No.
did nottouch in the f i n a l j o i n t , i t was found that fatigue f a i l u r e occurred e n t i r e l y in the solder rather than at the j o i n t interface. This proved to be a simple, inexpensive, and also reproducible, method of making a test specimen. Fatigue tests were conducted by clamping the wire specimen in a chuck driven by an electric motor acting through a reduction gear. A small pulley attached to the free end of the wire served to support the applied load consisting of lead weights. The stress at the j o i n t could be varied by varying the load or by adjusting the distance from pulley to solder j o i n t . A microswitch connected to a counter monitored the cycles, or number of revolutions, and a control switch was wired in series with the counter and the motor to turn off the system when f a i l u r e occurred. After fatigue f a i l u r e the bending moment for each test was computed by measuring the distance between the f a i l u r e location and the line of load application. Environmental protection was established by smearing the j o i n t area with vacuum grease. Results and Discussion Figure I i l l u s t r a t e s the effect of environment on fatigue l i f e by comparing best f i t lines through the data for a i r tested specimens and surface protected specimens. I t is surmised that the aggressive species is oxygen and that the vacuum grease provides an inert condition for much o f the crack propagation l i f e . Although i t is unusual to conduct a low cycle fatigue test in cantilever rotating bending where surface stresses are high and gross p l a s t i c i t y in the wire j o i n t occurs, for comparative purposes the test is clearly successful. The ordinate scale is based on the calculated outer f i b e r stress at the start of the test using the standard formula for a circular cross-section:* 4FL where F is applied load, L is distance from the fracture plane to load l i n e , and r is the j o i n t radius. 2.0
1.9,
=
tected •
~"
1.8-
x
1.7-
O~ o 1.6,
1.5-
1,4
•
2
3
4
log C y c l e s to F a i l u r e
y = 2.3297 - 0.2112x
R = 0.95
y = 2.2558 - 0,1534x R = 0.84
Figure l
Air
Protected
Low cycle fatigue of 60Sn-4OPb solder j o i n t s with and without environmental protection.
* This equation is limited to elastic behavior; actual surface stresses are expected to be less. The ordinate thus serves as a convenient reference scale for comparison only.
9
Vol.
22, No. 9
FATIGUE FAILURE OF SOLDER JOINTS
The figure i l l u s t r a t e s a similar phenomenon to that reported by Berriche et al (4) in that the environmental s e n s i t i v i t y increases with decreasing cyclic strain range. Although the solders are different, and our tests relate to failures in joints rather than the bulk specimens, the phenomenology is remarkably similar. This further supports our belief that the technique used here produces an essentially inert environment for fatigue crack propagation. The gain in fatigue l i f e of about an order of magnitude in an inert environment for conditions that lead to a l i f e in a i r of 10,000 cycles was roughly the same in the two studies. Clearly this simple, inexpensive testing procedure may be useful as a basis for a comprehensive comparative study of chemistry, cycle frequency, hold time effects, etc. Conclusions 1) A simple technique for measuring low cycle fatigue lives of solder joints in wire specimens using a cantilever rotating bend test provides good comparative data. 2) An inert environment may be provided in room temperature tests by simply smearing vacuum grease on the specimen surface. 3) The fatigue lives of the protected specimens may exceed those of a i r tests by an order of magnitude or more depending on the cyclic strain range. Acknowledgmen~ This work was supported by Rensselear Polytechnic Institute through the Undergraduate Research Program. References I. 2. 3. 4. 5°
6. 7. 8. 9. 10.
J.H. Lau and D.W. Rice, Solid State Technology, Vol. 28, No. 10, (1985), p. 91. J.F. Burgess, R.O. Carlson, H.H. Glascock, C.A. Neugebauer and H.F. Webster, IEEE CHMT-7, No. 4, (1984), p. 405. D. Peters, International Copper Research Association, 1987, personal comunication. R. Berriche, S. Vaynman, M.E. Fine and D.A. Jeannette, "Electronic Packaging and Corrosion in Microelectronics", edt. M.E. Nicholson, AStl, (1987), p. 169, K.U. Snowden, Acta Met, Vol. 12 (1964), p. 295. K.U. Snowden, Phil Mag, Vol. 10 (1964), p. 435. J.F. Eckel, Proceedings AS~, Vol. 51, 1951, p. 721. L.F. Coffin, "Fatigue at High Temperatures", ASTM, STP 520, 1973, p. 5. H.D. Solomon, Brazing and Soldering, No. 11, Autumn 1986, p. 68. D.A. Woodford and R.H. Bricknell, "Embrittlement of Engineering Alloys", edt. C.L. Briant and S.K. Banerji, Academic Press, Vol. 25 (1983), p. 157.