Research on Creep Properties of SnAgCuRE Lead-Free Soldered Joints

Keke Zhang, Jie Yang, Yaoli Wang, Yanli Fan, Xin Zhang, Yanfu YanMaterials Science & Engineering College, Henan University of Science & Technology

No.48 Xiyuan Road, Luoyang 471003,P.R.Chinazhkeke@mail.haust.edu.cn Tel: +86-379-64276880

AbstractThe creep properties and its rupture life of

Sn2.5AgO.7CuXRE lead-free soldered joints were separatelyinvestigated and forecasted under constant temperature by themeans of single shear lap creep specimens and finite elementmethod. The experiments show that the creep rupture life ofSn2.5AgO.7CuXRE lead-free soldered joints can obviouslyincrease with adding O.lwt % RE, which is 8.4 times longerthan that of Sn2.5AgO.7Cu solder and is apparently higherthan that of present commercial employed Sn3.8AgO.7Cusolder. And the actual testing results of creep rupture life ofSn2.5AgO.7CuXRE lead-free soldered joints are better inaccord with the predicted results by finite element method.Key words: lead-free solder; finite element method; creepproperties

IntroductionWith the development of electronic products to

miniaturization, lightweight and multifunction and thereinforcement of people's environmental protection, it hascaused people's more attention that the creep properties oflead-free soldered joints for surface mount technology (SMT)as one of the most important aspects of reliability areconsidered[1]. The Ref.[1,2,9] researched the relationshipbetween creep properties, microstructure of SnAg, SnCu andSnAgCu lead-free soldered joints and its rupture life. TheRef.[3] reported the creep resistance of solder alloy withadding Cu particles can be obviously increased. It has beenproved feasible and useful to add tiny rare earth (RE) in thelead-free solder to improve or increase the creep properties ofsolder joints[4-7]. Especially RE is rich in our country, thisinvestigation is very significant. But there are very fewreports about the study of creep properties and reliability oflow Ag content SnAgCuRE soldered joints[8]. Withreference to the commercial employed Sn3.8AgO.7Cu solder,the creep rupture life of low Ag content Sn2.5AgO.7CuXRElead-free soldered joints were investigated by single shear lapand predicted by the means of finite element method (FEM)in this paper. The results are expected to be helpful for thepractical application of lead-free solder.

1. Experimental Material and Procedure

1.1 Experimental MaterialThe raw materials are Sn, Ag, Cu (purity 99.9wt%) and

mixture RE. First the interalloy is fabricated in non-consumable melting furnace (type ZHW-600A) at thevacuum 5 x 10-3Pa. Then considering the melting loss of REand Cu, the interalloy and the suitable quantity Sn, Ag andCu are mixed together and Sn2.5AgO.7CuRE solder alloy ismade under the same conditions. During the smelting

process, the solder alloy should be upturned several times toobtain the symmetrical chemical composition. IRIS IntrepidInductively Coupled Plasma-atomic Emission Spectrometryis used to measure RE remainders in the solders in theexperiments. And the commercial employed Sn3.8AgO.7Culead-free soldered is considered as the reference system.

0.2 9t.

0, 1

Fig.1 Specimen size of creep rupture life test

1.2 Measuring of Creep Properties of Lead-Free SolderJoints

Based on the dimension, the actual stress andenvironmental condition of SMT soldered joint, the specimensize of creep rupture life test is shown as Fig. 1. The basematerial of subminiature single shear lap specimen is 0.2mmthickness pure Cu tinsel. Using the lead-free solder as thematerials of soldering seam, the soldering area of single lapsoldered joints is 1mm2 and the soldering seam is O.1mm.The special aluminium alloy mould was designed to obtainthe coherence of subminiature single shear lap joints for thecreep rupture life test.

1. Calculagraph 2. Specimen 3. Dead loadFig.2 Principle diagram for creep test

As is shown in Fig.2, the creep tests were made under thecondition of dead load single shear lap in the self-made creeptester with ±1 C temperature error. Based on the Ref.[6-7,9-11], the creep stress ranges of SMT soldered joints areusually 5-20MPa and the operating temperature ranges ofelectronic products are -40--150C. So the creep rupture lifetests were performed in this paper at the temperature of

1-4244-0620-X/06/$20.00 C 2006 IEEE. 2006 7th International Conference on Electronics Packaging Technology

25°C, 45C, 65C, 85C and 105°C with 16.7MPa deadload. The average value, acquired from 8-12 creep samplesunder the same temperature, can be considered as the creeprupture life of this solder joints at this condition.

1.3 Predicting the Creep Life of Lead-Free Solder JointsOn the basis of analysis of force, used the recessive creep

analytical module of commercial ANSYS finite elementanalysis software, the nonlinear finite element analysis modelof SnAgCuRE lead-free solder single shear lap joint wasfounded using two-dimensional 8-noed quadrangularelement[7], and the model was meshed as shown in Fig.3.According to creep curve and corresponding creep rateobtained from FEM, then the creep rupture life is predictedby means of Monkman-Grant equation[12]. Young smodulus (E) and coefficient of thermal expansion (CTE) ofSnAgCuRE lead-free solder were obtained through the actualmeasure and linear insert, the E and CTE in predicting thecreep life of joint in this paper are respectively 6.8-10.2GPaand 20.3-21.5x10-6/ C.

Fig.3 FEM model of single shear lap joint

2. Results and Discussion

2.1 Creep Properties of Sn2.5AgO.7CuXRE Lead-FreeSolder Joints

Effect of Adding Tiny RE on Creep PropertiesThe effect of adding tiny RE on creep rupture life of lead-

free soldered joints is shown as Fig.4. From the Fig.4, itappears that the creep rupture life of lead-free soldered jointsis quite sensitive to the RE adding content. When the REadding content of Sn2.5AgO.7CuXRE solder alloy is 0.1wt%, the creep rupture life of soldered joints is the longestabout 5760 min., which is 8.4 times longer than that ofSn2.5AgO.7Cu solder and is apparently higher than that ofpresent commercial employed Sn3.8AgO.7Cu solder about

5000 L...

- 40004--

a) 3000:z

; 2000

a1a) 1 000cs

Creep stress:16. 7MPaTemperature: 65°C

1116 min.. But the creep rupture life of lead-free solderedjoints is sharply decreasing with the increment of RE addingcontent. When the adding RE content of the solder alloy is 0.5wt%, the creep rupture life of Sn2.5AgO.7CuO.5RE lead-freesoldered joints is the same as that of Sn2.5AgO.7Cu solder. Sothe adding RE content in the lead-free solder alloy isexistence an optimal value, i.e, O.lwt%.

Fig.5 Interfacial layer of SnAgCuRE soldered joint

The intermetallic compound (IMC) interfacial layer ofsolder joints between Sn2.5AgO.7CuXRE lead-free solderand copper substrate usually includes two parts: o-Cu3Sn andT-Gu6SnA4,6], as is shown in Fig.5.The size andconfiguration of the interfacial layer have an importantinfluence on the reliability of this lead-free soldered joint[6].The investigation indicates that adding tiny RE inSn2.5AgO.7Cu lead-free solder alloy can effectually affectthe size and configuration of the interfacial layer, whichdetermine the creep rupture life of the soldered joints at quiteextent. For Sn2.5AgO.7CuXRE lead-free solder, in theoptical O.lwt%RE adding content, the IMC interfacial layeris thinner about 10gm and its thickness is homogeneous.

Effect of Temperature on Creep PropertiesThe effect of temperature on creep rupture life of

Sn2.5AgO.7CuO.lRE lead-free soldered joints is shown asFig.6. From the Fig.6, it shows that the creep rupture life ofsoldered joints sharply decreases with the increasing oftemperature. Due to the increment of temperature, thestrength of grainboundaries and intercrystalls decrease,then the creep activation energy decreases and the strain rateincreases rapidly, so this may lead to the creep rupture life ofSnAgCuRE lead-free soldered joints decreasing sharply. Thehigher the temperature, the shorter the creep rupture life of

40000

a;4 30000

a;4 20000

a 10000a~)

Creep stress:16. 7MPa

20 40 60 80 100

Temperature/°c

Fig.6 Temperature vs creep rupture life

0.0 0.1 0.2 0.3 0.4 0.5

RE adding content/%Fig.4 RE adding content vs creep rupture life

v0

r-

lead-free solder joints.2.2 Predicting the Creep Life of Lead-Free Alloy JointsFrom the Fig.3, the creep curve and corresponding creep

rate of Sn2.5AgO.7CuXRE lead-free solder alloy are obtainedunder the creep condition of 65°C and 16.7MPa by means offinite element analysis. The creep rupture life ofSn2.5AgO.7CuXRE soldered joints is predicted byMonkman-Grant formula (1) [12].

tr = k4s (1)

where t1 is the creep rupture life, 4Lis the creep rate, non-

dimension constant p equals to 1, the constant k ofSnAgCuRE lead-free solder alloy approximately is 0.529under 65°C.

C 4000

w 3000

: 2000

C) 1000

L

creep stress:16. 7MPatemperature: 65°C-.- predictingo-1- testing

0.0 0.1 0.2 0.3 0.4 0.5

RE adding content/%Fig.7 RE adding content vs creep rupture life

The creep rupture life of Sn2.5AgO.7CuXRE lead-freesoldered joints predicted by FEM and formula (1) is shown inFig.7. As shown in Fig.7, the creep rupture life predicted byFEM is well in accord with that of actual testing results. Withadding O.lwt % RE, the most error is 7% and the creeprupture life obtained from FEM is also higher than that ofcommercial employed Sn3.8AgO.7Cu lead-free solder. Itindicates that the experimental design of creep rupture life isfeasible and the result obtained is true.

3. ConclusionThe creep rupture life of Sn2.5AgO.7CuXRE lead-free

soldered joints can obviously increase with adding O. lwt %RE, which is 8.4 times longer than that of Sn2.5AgO.7Cusolder and is apparently higher than that of presentcommercial employed Sn3.8AgO.7Cu solder. The creeprupture life of Sn2.5AgO.7CuXRE lead-free soldered jointspredicted by FEM is well in accord with that of actual testingresults. The creep rupture life of Sn2.5AgO.7CuXRE lead-free soldered joints decreases sharply with the increasing oftemperature. Above results are of importance to design thereliability of SnAgCuXRE system lead-free soldered jointsfor SMT.

Talent (2004KYCX020), Program for Youth SkeletonTeacher in University ofHenan Province ([2002] 114), China.

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AcknowledgementsThis work was supported by the Innovative Talents

Foundation in University of Henan Province (2004-294),Henan Innovation Project For University Prominent Research

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