Results of Comparative Reliability Tests on Lead-free Solder Alloys

Results of Comparative Reliability Tests on Lead-free Solder Alloys Günter Grossmann, Giovanni Nicoletti, Ursin Solèr Swiss Federal Institute for Materials Testing and Research EMPA CH 8600 Dübendorf, Switzerland [email protected] [email protected] [email protected] Abstract The use of lead-free solder brings up concerns regarding the reliability of the new alloys to be used. In a European project (LEADFREE) SnAg, SnAgCu, SnAgCuSb, SnZn and SnPbAg have been tested in order to evaluate comparative data of the growth of cracks in solder joints. Reliability tests performed in other projects use accelerated tests proposed for tin-lead solders and showed a superior reliability of lead free solder over tin- lead alloys. The validity of these tests has to be questioned since they do not allow full relaxation of the stresses in solder joints. Thus each alloy is subject to another amount of strain. In LEADFREE tests are run with slow temperature ramps and long dwell times to account for this fact. As a result a faster growth of cracks has been observed in lead free solder joints compared to Sn62Pb36Ag2. Introduction The introduction of lead free solder faces two mayor obstacles: Optimised production parameters and reliability. Especially the latter gives reason for concern since neither any modelling of the deformation and of the degradation behaviour nor long term data are available up to date. The data published so far state a higher reliability of lead free solder compared to tin- lead solder. However the result have been obtained by means of heavily accelerated thermal cycling tests [1] [2]. Since these tests do not account for the creep behaviour of soft solder the results are debatable. In a previous work a deformation model for tin lead solder has been proposed stating that two deformation mechanisms occur in creep: Grain Boundary Sliding (GBS) and Dislocation Climb (DC) [3]. Which deformation mechanism is activated depends on the strain rate and on the temperature. Based on this model and on the creep data published in [2] a thermal cycle has been designed that allows for total relaxation of the shear stress occurring in each thermal cycle. Thus all solder joints see the same, maximum strain during the test unlike in the tests mentioned above where slower creeping alloys encounter less strain than faster creeping ones. SnPb36Ag2 To describe the deformation behaviour of Sn62Pb36Ag2 a Norton law with an Arrhenius extension has been used (Formula [1]).

n æ Q æJ ö C ' = Aç ÷ expç - GBS ç RT èGø è g' = Strain rate t = Stress

m ö æQ ÷ + Bæç J ö÷ expç DC ÷ ç RT èGø ø è

ö ÷ ÷ ø

[1]

A = Constant GBS = 3E15 B = Constant DC = 3.5E25 n = Stress exponent GBS = 3.3 m = Stress exponent DC = 7 G = Dynamic shear modulus = 15290- 23T [Mpa] T = Temperature [K] QGBS = Activation energy GBS = 48kJ QDC = Activation energy DC = 52kJ R = Gas constant = 8.314 J/ mol K With this model a graph can be drawn showing how much GBS is activated as a function of temperature and strain rate (Fig.1).

100% DC

100% GBS

Fig. 1: Activated fractions of GBS in SnPb36Ag2

This graph allows the design of thermal cycling tests where the temperature gradient is chosen to activate a required amount of GBS. How much of GBS and DC is activated is important since each deformation mechanism contributes its own degradation behaviour to the total damage of a material as it is stated in Miner's law (Formula [2]).

k

å i =1

ni = 1 at failure Nf

[2]

i

ni = Number of cycles with deformation mechanism i Nfi = Number of cycles to failure for deformation mechanism i Since the degradation in creep is strain driven as stated by Coffin- Manson (Formula [3]) . Nf en = C [3] Nf = Number of cycles to failure e = Plastic cyclic strain n = Fatigue exponent C = Constant Care has to be taken that all specimen in a thermal cycling test encounter the same plastic strain. The easiest approach is to choose the dwell time to be long enough to enable the full strain given by the temperature swing and the CTE mismatch of the materials involved. These dwell times too can be obtained from graphs derived from formula [1] (Fig. 2).

For the comparative thermal cycling tests LCCC's have been soldered on FR4 using various lead free alloys as well as SnPb36Ag2 as a reference (Table 2). Alloy Liquidus temperature[°C] SnPb36Ag2 179 SnZn9 196 SnAg3.8Cu0.7 217 SnAg3.5 221 SnAg2.6Cu0.8Sb0.5 226 Table 2: Alloys used in the comparative thermal cycling tests Test cycle and analysis In LEADFREE a thermal cycle has been designed that runs well in the zone of pure GBS in Fig.1 for an assumed average soldergap between the component and the pad of 30mm, as it occurs usually in reality. For the lack of data for lead free solder and because of the fact that most of the deformation takes place in the tin rich phase, we assumed that this also applies for lead free solder. The dwell times in the thermal cycling tests have been chosen to enable practically total relaxation of the stress build up during the temperature ramp. To achieve an acceleration of the test large strain per cycle has been induced into the solder joints rather than running a fast thermal cycle. For this purpose LCCC's have been soldered on FR4 because of the large CTE mismatch of the two materials and a rather wide temperature swing has been applied. On this base the following temperature profile has been defined (Fig. 3): 120

Time Temperature [min] [°C] 0 0 10 -20 40 -20 50 0 80 120 90 120 120 0

Fig. 2: Dwell times as a function of temperature necessary to permit relaxation until the strain set as parameter remains in the solder joint for a temperature ramp of 2°C/min for SnPb36Ag2. Lead free solder In [2] the creep behaviour of some lead free solder alloys has been measured. From these measurements one can estimate the creep rate at say 20MPa (Table 1).

Temperature SnPb 20°C 2E-6 s-1 75°C 8E-5s-1 125°C 1E-3 s-1 Table 1: Strain rates for shown in [2]

Alloy SnAgCu SnAg 4E-7 s-1 1E-7 s-1 3E-6 s-1 3E-6 s-1 8E-5 s-1 5E-5 s-1 three alloys read from diagrams

The measurements clearly show, that lead free solder alloys creep 10 to 100 times slower than tin-lead. As a result, solder alloys creeping slow will show better performance than fast creeping ones in accelerated thermal cycling tests with short dwell times. Specimen

Temperature (°C)

100 80 60 40 20 0 -20 -40 0

20

40

60 Time (min)

80

100

120

Fig. 3: Thermal profile used in LEADFREE for comparative fatigue tests on LCCC 20 on FR4.

The analysis was done with light microscopy on microsections of the corner joints of the components. On each specimen three LCCC's have been placed. By using all the solder joints in the corners of the LCCC's 24 joints could be used whenever a specimen was drawn for analysis. The length of the cracks has been measured with an image analysis program. Results

Two different modes of crack propagation could be observed: Along the termination or at approx. 45° through the fillet (Fig.4, Fig. 5). Combinations of both fracture modes also have been seen (Fig. 6).

However, the cracks as shown in figures 5 and 6 occurred rather rarely. Figure 4 shows the usual path of the cracks through the fillet. Thus only the cracks running 45° through the joint have been used for the evaluation of the average length of cracks. In the as soldered state the solder joints made of SnAg3.8Cu0.7, SnAg3.5 and SnAg2.6Cu0.8Sb0.5 show the well known dendritic structure (Fig. 7).

500m mm EMPA, Centre for Reliability

Figure 4: Microsection through a failed solder joint made of SnAg3.8Cu0.7 after 500 cycles. Crack grows at approx. 45° through the fillet

EMPA, Centre for Reliability

250m mm

Figure 7: SnAg3.8Cu0.7 solder joint as soldered

After thermal cycling, this structure is destroyed especially in the vicinity of the growing cracks (Fig. 8). In the other areas of the solder joints relicts of the dendrites can be seen.


500m mm

Fig. 5: Microsection through a failed solder joint made of SnAg3.5 after 500 cycles. Crack grows along the termination.


250m mm

Figure 8: SnAg3.8Cu0.7 after 600 thermal cycles.


250m mm

Fig 6: Microsection through a solder joint made of SnAg3.5 after 600 cycles. Mixed mode of crack growth.

By applying an appropriate metallographic preparation method [4] it becomes visible that the tin rich dendrites recristallized into a fine grained structure which can not be observed in the as soldered state. In figure 9 a microsection of a solder joint made of SnAg3.5 after 600 thermal cycles is shown in bright field microscopy. Figure 10 shows the same joint with polarised light where the polarisator and the analysator are crossed for approximately 87°. With this configuration the optical activity of the tetragonal tin can be used to distinguish grains with varying orientation of the crystallographic axis.


250m mm

Fig.9 : Solder joint made of SnAg3.5 after 600 thermal cycles in brightfield. EMPA, Centre for Reliability

Fig 12 : FIB image of the wall in the cut of figure 11.

It can be clearly seen, that the small grains found on the surface of the polished surface extend into the bulk of the solder. Also it is visible that the crack is of intercristalline nature as expected if the degradation is due to grain boundary sliding. SnZn9 had a fine-grained structure already as soldered (Fig 13). No change in structure could be observed after thermal cycling.


250m mm

Fig 10 : Solder joint made of SnAg3.5 after 600 thermal cycles in polarised light. To verify that the structures found are not an artefact of the preparation a Focussed Ion Beam (FIB) preparation was done across a crack in a solder joint (Fig. 11, 12).


125m mm

Fig 13 : Solder joint made of SnZn9 as soldered in polarised light.

The same behaviour of grain refinement has already been observed with SnPb36Ag2. In the as soldered state the solder joints consist of a few large tin domains with globular finegrained lead phases (Fig. 14). After thermal cycling grain refinement of the tin phase and grain coarsening of the lead phase can be observed in the area where plastic deformation occurs and where the crack grows (Fig. 15). EMPA, Centre for Reliability

Fig 11 : SEM image of a FIB cut across a crack in a solder joint made of SnAg3.8Cu0.7. The crack is filled with resin.


Fig. 14 : Solder joint made of SnPb36Ag2 as soldered. Crack

Number of failed joints (%)

A large fraction of the joints made of SnZn9 where already completely broken after 400 cycles whereas the other lead free alloys had no failed joint. Due to this fact no investigations with higher number of cycles than 600 on SnZn9 have been done. After 500 cycles some failed joints made of lead free solder have been observed. The specimen soldered with SnPb36Ag2showed the first failed solder joints at 1000 cycles (Fig. 15). 90 80 70 60 50 40 30 20 10 0

SnZn9 SnCuAg SnAg SnAgCuSb Sn62Pb36

0

400

500

600

700

1000

Number of cycles

Fig. 17: Percentage of solder joints with cracks that occupy 100% of the length of the solder joint.


Fig. 15 : Solder joint made of SnPb36Ag2 after thermal cycling After 400 cycles all solder alloys showed cracks in the cross-sections under the device termination but not into the fillet. With increasing number of cycles the cracks propagated into the fillet. In figure 16 it is clearly visible that in slow thermal cycling with ample time for relaxation lead free solder alloys show longer cracks than SnPb36Ag2. No significant difference in the length of cracks could be observed so far between SnAg3.5, SnAg3.8Cu0.7 or SnAg2.6Cu0.8Sb0.5. SnZn9 shows the fastest degradation. Average length of cracks (um)

1600

SnZn9

1500

SnCuAg SnAg

1400

SnAgCuSb Sn62Pb36

1300 1200 1100 1000 0

200

400

600

800

1000

Number of cycles

Fig. 16 : Average length of cracks after thermal cycling tests

Discussion Since the fast degradation of the solder joints under the device termination came somewhat by surprise additional tests between 0 and 400 cycles have to be done. Additionally tests with more than 700 cycles with lead free solder alloys will be carried out. Also the question has to be raised whether the data evaluation should be extended: - Is it appropriate to use the mean value for the characterisation of the crack length or is the modal value more applicable. - Is it necessary to distinguish between the different modes of crack path we have observed. - The crack growth under the device termination has to be treated separately from the one through the fillet. For an evaluation of these questions a larger sample size than used in this evaluation will be necessary. Since the tests where intended to provide comparative reliability data they can not be used for reliability estimations on solder joints. Thus more investigations must be done: - Firstly a deformation model like for SnPb36Ag2 has to be established for lead free solder. Based on this know how the thermal cycling tests will be optimised. - The test set-up with LCCC's is somewhat critical: Whenever the joints on one side of a component are more degraded than on the opposite side the weaker joints will have to bear more strain than the other ones which results in faster degradation again. Thus a test set-up will have to be designed that avoids this problem. - The occurrence of GBS and DC has to be proofed for lead free solder alloys. - A degradation model will be set up. - Thermal cycling tests with solder joints on various components must be carried out.

Conclusions The test show that it is very important to take the deformation behaviour of the materials involved into account when tests are designed for reliability estimations on solder joints. If this is not done misleading results might be obtained that lead to debatable statements. Based on the know how for SnPb36Ag2 and on data published so far a thermal cycling test has been defined that activates pure GBS in SnPb36Ag2 due to the low temperature ramp applied. The dwell times are chosen to enable relaxation of the stresses that build up during the test to a large extend also in solder alloys that creep considerably slower than SnPb36Ag2. This means that essentially the entire strain that occurs due to the temperature swing and the CTE mismatch of component and PCB will be induced. Thus, all alloys encounter the same strain, which is important, since low cycle fatigue is strain driven. The structure of SnAg3.5, SnAg3.8Cu0.7 and SnAg2.6Cu0.8Sb0.5 changed considerably due to the thermal cycling test. As soldered the joints consisted of tin rich dendrites that broke off into small globular units. Since the same grain refinement of the tin rich phase due bto strain has also been observed earlier in solder joints made of SnPb36Ag2 in the regions where large plastic strain occurred it can be assumed that the same holds for lead free solder. This means due to plastic strain the tin rich phase recristallises while the other phases coarsen due to diffusion. The measurement of the crack length shows that lead free solder alloys degrade faster that SnPb36Ag2. After 700 cycles the average crack length of SnAg3.5, SnAg3.8Cu0.7 and SnAg2.6Cu0.8Sb0.5 was approx. 15% bigger than the one of SnPb36Ag2. No singificant difference in the degradation could be observed between SnAg3.5, SnAg3.8Cu0.7 and SnAg2.6Cu0.8Sb0.5. SnZn9 showed the fastest degradation. When looking at the number of failed solder joints the spread of a statistical evaluation with microsections through solder joints becomes evident. Because of the varying geometry quite big variations are to be expected. Thus it is questionable whether the differences of the lead free alloys between 500 cycles and 700 cycles are significant. A larges sample size could ease the problem. The early failures of SnZn9 are partially because of the faster crack growth partially because of the filet formation of the solder. SnZn9 solder joints show a quite narrow shape compared to SnAg3.5 or SnAg3.8Cu0.7 (Fig. 18) which might well be due to different printing properties of the SnZn9 solder paste. This gave the crack less material to grow through in SnZn9.

Fig. 18 : Solder joints made of SnAg3.8Cu0.7 (left) and SnZn9 (right).

Even if the tests show that lead free solder degrade faster than SnPb36Ag2 does not mean that lead free solder, including SnZn9, are not suitable for use in electronic equipment. The important point is to find out how fast the cracks in these materials evolve in order to estimate the reliability risk arising out of their use. Acknowledgements The authors want to thank the Swiss Commission for Technology and Innovation (KTI) as well as Ascom, Siemens Switzerland, Siemens Cerberus, Schindler, Elcoteq and Contraves for their support of LEADFREE. Additionally, G. Grossmann wants to thank his little daughter Tanja (10) for her invaluable help on weekends in sample preparation. References 1. M.H. Biglari, M. Oddi, M.A. Oud, P. Davis, E.E. de Kluizenaar, P. Langeveld, D. Schwarzbach “Pb- Free Solders Based on SnAgCu, SnAgBi SnAg and SnCu for wave soldering of Electronic Assemblies,” Proc. Electronic Goes Green 2000, Berlin 2000, pp. 73-82. 2. National Center for Manufacturing sciences, Lead-Free Solder Project, ncms CD, 1999. 3. G. Grossmann, “The deformation behaviour of Sn62Pb36Ag2 and its Implications on the Design of Thermal Cycling Tests for Electronic Assemblies,” IEEE Trans-CPMT, Vol. 22, No. 1 (1999), pp. 71-79. 4. G. Grossmann, G. Nicoletti, “Preparation of soft solder joints,” Materials Characterization, Vol. 36, No. 4/5 (1996), pp. 235-242.