Creep Properties of Soft Solder Die Attach with Ni Balls in Power Package Applications

F. X. Che*', Dandong Ge', Yik Siong Tal, Mohamad Yazid', Swee Lee Gan' 'Infineon Technologies Asia Pacific Pte Ltd, 168, Kallang Way, Singapore 349253

2Infineon Technologies (Malaysia) Sdn Bhd (56645-D), Free Trade Zone, Batu Berendam, 75350 Melaka, Malaysia *E-mail: faxing.che@infineon.com; Tel: +65-68400612; Fax: +65-68400626

Abstract Soft solder die attach is a widely used process for power

and high-reliability automotive devices because soft solder has excellent heat-dissipation properties and outstanding robustness against delamination compared to traditional epoxy die attach. However, die tilt is usually one of the important issues and challenges during the soft solder process. The soft solder with Ni balls was introduced to reduce and control die tilt. During solder process, the Ni balls will not melt and will keep the ball shape, which would help control bond line thickness (BLT). The creep­fatigue failure is one typical failure mechanism for solder material under thermal cycling. So it is essential to investigate the effect of Ni ball on the material properties and reliability of the solder joint layer. In this work, tensile creep tests for solder wire with and without Ni ballss have been conducted at various temperatures and stress levels using Dynamic Mechanical Analyzer (DMA) to obtain the creep properties of the solders. The steady-state creep strain rates are similar for both solder typesl, which means that the Ni ball addition does not affect the creep behaviour of soft solder significantly. Creep constitutive models were developed and the material constants of the model were determined based on experimental results for both solders. The creep constitutive models can be implemented in finite element analysis (FE A) to investigate the creep behavior of solder in the power packages for evaluation of package reliability.

1. Introduction Power package is one type of electronic component

package, commonly used for high power integrated circuits. The increase of power density and performance at simultaneously decreasing cost is a constant trend in the power semiconductors world, especially with the popularity of new semiconductor materials. Solder is the most commonly used metal-metal joining material used in power semiconductors, because it provides the most reliable and effective means of conducting both electricity and heat. In addition, solder also provides enough mechanical strength and fatigue resistance to withstand the considerable mechanical and thermal stresses generated. However, die tilt is usually one of the important issues and challenges during the soft solder process because solder process is mainly determined and influenced by materials such as die backside metallization, solder material and leadframe. Pb2Sn2.5Ag solder is one of the common Iy used soft solder die attach materials and has a solidus temperature of 299°C and a liquidus temperature of 304°C. Temperature window is narrow and die tilt increases with increasing temperature. Therefore, die tilt is difficult to control in the soft solder die attach process [1].

Comparing to the traditional epoxy die attach, the bonding mechanism using soft solder is due to intermetallic compound (IMC) formation between solder and die backside metallization or solder and leadfram metal. Sufficient IMC thickness is required to achieve high reliability performance. Therefore, soft solder bond line thickness (BLT) needs to be thicker and even in order to allocate some amount of solder material for intermetallic formation with die backside metallization and leadframe [ 2 ] . Controlling die tilt to achieve even and thicker BLT becomes important for improving reliability of solder joint. In our study, the Pb2Sn2.5Ag solder with Ni balls was introduced to reduce and control the die tilt. The hard Ni balls were added and mixed with solder base material. During solder process, the Ni balls with a melting point of 1453°C will remain its ball shape at soldering temperature of 400°C, which would help achieve minimum and uniform BLT after die attach process. The selection of Ni ball size was defined by the requirement of the minimum BL T for power packages. In this study, Ni balls with diameter of 50 !tm and 0.15 wt. % were applied in the Pb2Sn2.5Ag solder. From literature review, Ni is one dominant and widely used doping material in Sn-Ag-Cu solders due to its excellent performance in improving solder microstructures, reducing IMC growth and increasing drop lifetime of electronic assembly [3-7]. However, there are some differences between Ni doping and Ni ball addition in the solders. The doped Ni particles will be mixed together with the base solder material and form the IMC particles, which improves the microstructure of the solder. There is a lack of reported research on the effect of the added Ni balls on the material properties of the soft solder. It is known that long term reliability is one of the challenges for soft solder used in power packages when subject to thermal cycling loading. The creep-fatigue failure is one typical failure mechanism for solder material under thermal cycling. Therefore, it is essential to investigate the effect of Ni balls on the material properties and long term reliability of packages with using this soft solder.

In this work, tensile creep tests for Pb2Sn2.5Ag solder wires with and without Ni balls have been conducted at various temperatures and stress levels using Dynamic Mechanical Analyzer (DMA) in order to obtain the creep properties of both solders. The effects of stress level and temperature on creep strain rate were investigated. The results show that the steady-state creep strain rate increases with increasing the applied stress and temperature. The steady-state creep strain rates are similar for both Pb2Sn2.5Ag solders with and without Ni balls, which means that the Ni ball addition does not affect the creep behaviour of the soft solder significantly. The Ni ball addition in the soft solder helps achieve the uniform BLT

35th International Electronic Manufacturing Technology Conference, 2012

without degrading the reliability of the soft solder die attach. The creep constitutive models such as hyperbolic sine, double power law type [3] were created and the material constants of the models have been determined based on experimental results. The stress exponent and creep activation energy have been studied and compared for both solders. The creep constitutive models can be implemented in finite element analysis (FEA) to investigate the creep behavior of solder such as creep strain energy density or creep strain in the power package for evaluation of package rei iabi Iity.

2. Creep Test The solder wire specimen with diameter of 0.76 mm

(30mil) was used in this work. The soft solder material used for die attach is commonly supplied in the form of solder wire. The specimen has a gauge length of 15 mm. The solder alloys are Pb-2.0Sn-2.5Ag without Ni ball and Pb-2.0Sn-2.5Ag with 0.15wt% 50 11m Ni ball addition. The tensile creep tests for the solder wire specimens were performed at an isothermal condition with a constant tensile load on each sample using a DMA (model: T A-Q800) tester as shown in Fig. 1. The system can provide precise control of stress (force resolution 0.00001 N). Strain is measured using optical encoder technology that provides unmatched sensitivity and resolution (strain resolution 1 nm). The testing system is equipped with a specially designed furnace combined with gas cooling accessory for efficient and precise temperature control over the entire temperature range in heating, cooling, and isothermal operation. The chamber is capable of maintaining constant temperature within ±O.I°C. The TA instrument explorer software was used to control test and record the data. The tensile creep tests were conducted at five different temperatures such as -40°C, 30°C, 80°C, 125°C and 175°C with a designed constant stress level. Table I lists all tensile creep test conditions for both solders. For testing conditions at low temperature of -40°C, liquid nitrogen was used to provide a cold isothermal condition. The tensile loading was gradually applied onto the specimen to the designed stress level. Then, this constant stress was held to continue the creep test. The creep deformation with time was recorded by software. Three samples were tested for each condition and the results were averaged. The precise DMA machine leads to desirable repeatability for creep tests at the same loading condition.

Fig. 1. DMA machine and solder wire under test. Table I. Tensile Creep Test Conditions

Creep stress Creep test temperature (MPa) -40°C 30°C 80°C 125°C 175°C

2.5 -.J

5 -.J -.J

7.5 -.J -.J -.J

10 -.J -.J -.J -.J

12.5 -.J -.J -.J

15 -.J -.J -.J -.J

17.5 -.J -.J

20 -.J -.J -.J

25 -.J -.J

30 -.J

35 -.J

3. Results and Discussion Typically, creep has three stages: namely primary,

secondary and tertiary stages as shown in Fig. 2. The primary creep stage of solder alloy is very short as compared with steady-state creep stage. After the primary creep stage of a very short time, a long steady-state creep (secondary creep) stage follows, and this occupies most of the creep lifetime. The tertiary creep stage with fast creep strain rate occurs before creep rupture. The creep strain rate in the steady-state creep can be determined for any given time by taking the derivative of the creep strain with creep time. The minimum rate was taken as the creep strain rate of the steady-state creep stage. The creep strain rate is dependent on creep stress level and test temperature. Fig. 3 shows the effect of temperature on creep curve of solder with Ni balls at a certain creep stress level of 10 MPa. The creep strain and creep strain rate increase significantly with increasing temperature at a certain stress level. The creep strain rate is much sensitive to temperature. Solder becomes very soft and much easier to creep at high temperatures. Fig. 4 shows the effect of stress on creep curve of solder with Ni balls. The creep strain rate increases sharply with increasing creep stresses. This work focuses only on steady­state creep behavior and the tensile creep tests were stopped after achieving a long term steady-state creep stage. Figs. 5 and 6 show the summary of steady-state creep strain rate at different test temperatures and creep stress levels as listed in Table I for the solder without Ni ball and the solder with Ni balls, respectively The creep strain rate is less dependent on stress in the low stress range, while it becomes more dependent on stress in the high stress range for a certain test temperature. The power law constitutive model can be used to describe the relationship between creep strain rate and stress. The stress exponent n is larger in the high stress range than that in the low stress range. This phenomenon is

35th International Electronic Manufacturing Technology Conference, 2012

referred to as power law creep behavior breakdown. In the low stress range, the creep deformation is controlled mainly by grain boundary sliding in which stress exponent is usually 2-4 [8, 9]. In the high stress range, the creep deformation is mainly attributed to dislocation climb and glide, which result in a high stress exponent value [10]. Diffusion-controlled creep like Nabarro-Herring creep is usually dominant at high temperatures. In Nabarro-Herring creep, atoms diffuse through the lattice causing grains to elongate along the stress axis.

0.5

0.4

::: 0.3 'co '-� 0.2

0.1

'''[2] .S 0 :02

.§ 0 01 til

°

L' 100 200 300

Tlme(s)

o 1000 2000 3000 4000 5000 6000 7000 8000 Time (s)

Fig. 2. Typical tensile creep curve of the solder [3].

16

14

12

� 10 = 8 'eo ... � 6

4

2

0 0 20 40

-80°C -125°C -175°C

60 Time(min)

80 100 120

Fig. 3. Effect of temperature on creep curve of the solder with Ni balls. Creep stress: 10 MPa.

30

25

20 ,-., � e..-.: 15 .. .; '"

10

5

0 0 200

-- IOMPa

--15MPa

-- 17.5MPa

-- 20MPa

--25MPa

400 Time(min)

600 800

Fig. 4. Effect of stress on creep curve of of the solder with Ni balls. Test temperature: 30°C.

� � � .. I-= . ,. .: '" Co ... ... I-U

I.E-O I

I.E-02

I.E-03

I.E-04

I.E-05

I.E-06

I.E-07

I.E-08

:l(-40°c

+30°c A 80°C • 125°C e175°C

e

Solderw/o Ni ball

e • A

e. A • A

e · A •

e • A •

•

•

•

•

10 Stress (MPa)

100

Fig. 5. Steady-state creep strain rate at various load conditions for the solder without Ni ball.

I.E-Ol

I.E-02

a I.E-03 � E I.E-04 "

� I.E-OS Q. '" '" U I.E-06

I.E-07

I.E-08

)K -40°C + 30°C A 80°C • 12Soc .175°c

•

Solder with Ni ball

•

• A • •

• A

• • A

A • :I: • • •

A :I: • • :I:

• :I:

• :I:

10 100 Stress (MPa)

Fig. 6. Steady-state creep strain rate at various load conditions for the solder with Ni balls.

Comparison between the solder with and without Ni ball was conducted in terms of creep behavior. At low temperature such as -40°C, the solder with Ni balls has higher creep strain rate compared to the solder without Ni ball at a certain stress level as shown in Fig. 7, which means that the Ni ball addition makes solder easier to creep at the low temperature. At room temperature and slightly higher temperature such as 80°C, both the solders with and without Ni ball show the comparable creep strain rate as shown in Figs. 8 and 9. When temperature increases to higher temperature such as 125°C, the creep strain rate of solder without Ni ball is slightly higher than that of solder with Ni balls as shown in Fig. 10. When temperature further increases to very high temperature such as 175°C, the solder with Ni balls again shows higher creep strain rate than the solder without Ni ball as shown in Fig. 11. Considering one typical temperature cycling loading condition from -40°C to 125°C, the package using solder with Ni balls should have larger creep deformation compared to the package using solder without Ni ball. The Ni ball addition reduces the creep resistance. However, the

35th International Electronic Manufacturing Technology Conference, 2012

effect of Ni ball addition on creep deformation is not significant based on the test data. Therefore, the Ni ball addition is still a desirable solution to achieve a uniform BLT without introducing much creep deformation for the power package using soft solder as die attach material.

I.E-04 .,------------------,

2, I.E-05

� E " I.E-06 � Q. '" '" ...

U I.E-07

-40°C creep test

• Solder w/o Ni ball •

• Solder with Ni ball

•

•

•

•

•

• •

•

I.E-08 +----�--�-�����.......-l 10 100

Stress (MPa)

Fig. 7. Comparison of creep strain rate at -40°C.

I.E-O I 30°C creep test

I.E-02 .Solderw/oNi ball

'" I.E-03 .Solderwith Ni ball ..... .... � t E I.E-04 .: • .. � I.E-05 • Q. • .. • .. ... I.E-06 U

I.E-07 • •

I.E-08 10 100

Stress (MPa)

Fig. 8. Comparison of creep strain rate at 30°C.

I.E-O I 80°C creep test

I.E-02 .Solderw/o Ni ball --'" I.E-03 .Solderwith Ni ball .... • �

� • .. I.E-04 ... • = .; • � I.E-05 Q. • .. .. ... I.E-06 U

I.E-07

I.E-08 10 100

Stress (MPa)

Fig. 9. Comparison of creep strain rate at 80°C.

Fig. 10. Comparison of creep strain rate at 125°C.

Fig. 11. Comparison of creep strain rate at 175°C.

From the analysis mentioned above, the stress exponent and creep activation energy in the creep models are stress level and temperature dependent. The averaged values of stress exponent and creep activation energy can be determined by fitting all the creep experimental results under different stresses and temperatures. The curve-fitting for the creep test data was conducted through iterative multi-variable nonlinear regression to develop the creep constitutive models. Commonly used creep models for solder material are double power law and hyperbolic sine type model, which are developed for both solders based on creep test data and curve-fitting method. The developed creep models were expressed below:

Double power law model for solder without Ni ball:

. -465 10-6 7.2 (5895.4) 0166 9 (12575.4) (1) &c - . x cy exp --T- + . cy exp ---T-

Hyperbolic sine model for solder without Ni ball:

i c = 6.8 x 104 [sinh(0.073CY )]605 exp( _ 8�5) Double power law model for solder with Ni balls:

(2)

35th International Electronic Manufacturing Technology Conference, 2012

&c = 1.52 x 10-7 (J 75 exp( _ 49�.2) + 1.087(J87 exp( _

125:5.2) (3)

Hyperbolic sine model for solder with Ni balls:

&c = 8.04 x 105 [sinh(0.052(J )p58 exp( _ 7�0) (4)

In above equations, Be is the steady-state creep strain

rate, (J is the applied stress, and T is the absolute temperature. In the subsequent section, the thermal cycling finite element simulation for power package was conducted by applying these creep models for solder die attach material.

4. Thermal Cycling Simulation To verifY the validity and consistency, the developed

creep models were implemented into FEA modeling to numerically predict the creep deformation of solder joints in power package. In a real package, the solder joints deform due to CTE mismatch among different packaging materials when subjected to thermal cycling loading. Constraints imposed on solder joints make solder joints subject to multiaxial stresses. The creep properties of solder are usually extracted from creep test with uniaxial stress condition. In FEA software, the equivalent creep strain energy and creep strain of solder joint are calculated. The creep constitutive models developed above were implemented in TO package for package level thermal cycling simulation. Temperatures ranging from -40°C to 125°C were considered. The duration of one thermal cycle is 30 min, including 10 min dwell time at both extreme low and high temperatures. The stress free temperature was selected at high dwell temperature. Due to symmetric geometry of package, a quarter FEA model was implemented and shown in Fig. 12. Package size is 10 x

9.25 x 4.4 mm. Die size is 6.7 x 4.5 x 0.25mm. Die pad size is 8.3 x 5.58 x 1.27 mm. The BLT of solder die attach is 60 /lm. The symmetric boundary condition was applied onto two cutting planes along package centerlines. One node of package bottom center was fixed to avoid rigid body movement. Three temperature cycles were simulated to obtain the converged results for creep strain and creep strain energy density accumulation per cycle.

Fig. 12. Quarter FEA model of TO package.

It was found from Fig. 13 that high creep strain locates at the corner and edge of the soft solder die attach layer, which is a potential cracking initiation site. Fig. 14 shows the comparison of the accumulated creep strain with time for the critical corner node. It can be seen that hyperbolic sine and double power law creep models lead to similar creep strain results. Thus, it verifies the consistency among the creep constitutive models. Solder with Ni balls leads to slightly higher creep strain than solder without Ni ball, which indicates that Ni ball addition increases the creep deformation of die attach layer.

.019

.02 8

.037

.0 4 6

.05 <1

.063

.072

.081

.09

.099 Fig. 13. Equivalent creep strain after 3 TC cycles.

0.1 0.09

= 0.08 � t:: 0.07 Q. '" 0.06 � " 0.05 "0 .!l 0.04 " -= E 0.03 = " 0.02 " ...:

0.01 0

0

power law -- solder wlo Ni

Hyperbolic sine - - - solder with Ni Double

power law

1000 2000 3000 4000 5000 6000 Time (s)

Fig. 14. Creep strain increasing with time for different solders using different creep models.

The volume-averaged method was used for calculating creep strain and creep strain energy density accumulation. The whole solder die attach layer was used for volume­averaging. Figs. 15 and 16 show the comparison of volume­averaged creep strain energy density and creep strain of solder die attach layer, respectively. Package using solder with Ni balls results in 7% higher creep strain energy density and creep strain accumulation per cycles than package using solder without Ni ball. Such creep strain energy density and creep strain results can be used for solder joint thermal fatigue life prediction. If considering the exponent of the life model closing to -1, then the package using solder without Ni ball addition will lead to 7% higher TC life than package using solder with Ni balls addition considering both solders follow the same fatigue life model. Such life reduction is not significant when using solder with Ni balls. Therefore, the Ni ball addition is still one desirable solution for soft solder die attach to control the minimum BL T for the power packages.

35th International Electronic Manufacturing Technology Conference, 2012

.-.. �

� -C> .;;; = ..

"Q .... � ... .. = .. = � .l: '" Q" .. .. ...

U

0.250

0.200

0.150

0.100

0.050

0.000

0.221

solder w/o Ni solder with Ni Softsolder

Fig. 15. Comparison of volume-averaged creep strain energy density accumulation per cycle.

0.016

0.014

0.012

.� 0.01

� 0.008 QI

5 0.006

0.004

0.002

o

0.0145

solder w/o Ni solder with Ni Soft solder

Fig. 16. Comparison of volume-averaged creep strain accumulation per cycle.

5. Conclusions Tensile creep tests for soft solders with and without Ni

ball addition have been conducted at various temperatures and stress levels. The main conclusions are summarized as below: 1. The creep constitutive models have been developed and

validated for the soft solders. Such creep models can be implemented in the FEA simulation. The creep models for soft solder will provide useful material property data in design and optimization of soft solder die attach reliability for the power package assemblies.

2. The steady-state creep strain rate of soft solder is less dependent on stress in the low stress range, while it becomes more dependent on stress in the high stress range. The steady-state creep strain rate increases with increasing the applied stress and temperature.

3. A small amount of Ni ball addition in soft solder slightly reduces the creep resistance at extreme low and high temperatures. The creep behavior is comparable at the medium temperatures for both solders with and without Ni ball addition.

4. The developed creep models are consistent when applying them in the thermal cycling FEA simulation . Solder with Ni balls results in slightly higher creep strain and creep strain energy density than solder without Ni ball. Therefore, package using soft solder with Ni balls as a die attach material will lead to lower thermal fatigue life, but not significantly .

References 1. Joho F., "Impact of Material and Process Parameters in

Soft Solder Die Bonding," SEMICON Singapore 2006, pp. 1-3 .

2. Amiruddin, M. S., Tan, C. E., and Dhanapalan, P., "Soft Solder Die Attach in Small Outline Discrete Packages for Both Reliability and Cost Breakthrough," Proc 111h Electronics Packaging Technology Con/, 2009, pp. 758-761.

3. Che, F. X., Zhu, W. H, Poh, S. W., Zhang, X. R., Zhang, X. W., Chai, T. c., and Gao. S., "Creep Properties of Sn-1.0Ag-0.5Cu Lead-Free Solder with the Addition of Ni," 1. Electron. Mater. Vol. 40, No. 3 (2011), pp. 344-354.

4. Laurila, T., Hurtig, J., Vuorinen, V. and Kivilahti, J. K., "Effect of Ag, Fe, Au and Ni on the growth kinetics of Sn-Cu intermetallic compound layers," Microelectron. Reliab. 49, (2009), pp. 242-247.

5. Liu, P., Yao, P., and Liu, J., "Evolutions of the interface and shear strength between SnAgCu-xNi solder and Cu substrate during isothermal aging at 150°C," 1. Alloys Compd. 486, (2009), pp. 474-479.

6. Ventura, T., Gourlay, C. M., Nogita, K., Nishimura, T., Rappaz, M., and Dahle, A. K., "The Influence of 0-0.1 wt.% Ni on the Microstructure and Fluidity Length of Sn-0.7Cu- x Ni," 1. Electron. Mater. 37, (2008), pp. 32-39.

7. Che, F. X., Zhu, W. H., Poh, S. W., Zhang, X. W., Zhang, X. R., "The Study of Mechanical Properties of Sn-Ag-Cu Lead-Free Solders with Different Ag Contents and Ni Doping under Different Strain Rates and Temperatures," 1. Alloys Compd. 507, (2010), PP. 215-224.

8. Shin, S. W., and Yu, J., "Creep deformation of Sn-3.5Ag-xCu and Sn-3.5Ag-xBi solder joints," 1. Electron. Mater. 34, (2005), pp. 188-195.

9. Zhang, G. S., Jing, H .Y., Xu, L. Y., Wei, J., and Han, Y. D., "Creep behavior of eutectic 80Au/20Sn solder alloy," 1. Alloys Compd. 476, (2009), pp. 138-141.

10. He, M., Ekpenuma, S. N., and Acoff, V. L., "Microstructure and Creep Deformation of Sn-Ag-Cu­BilCu Solder Joints," J. Electron. Mater. 37, (2008), pp. 300-306.

35th International Electronic Manufacturing Technology Conference, 2012