目录结构

详情信息展示

参考文献

Rare Metals2021年第1期

Aging resistance and mechanical properties of Sn3.0Ag0.5Cu solder bump joints with different bump shapes

Ping Chen Xiu-Chen Zhao Ying Liu Hong Li Yong Wang

School of Materials Science and Engineering,Beijing Institute of Technology

Center of Packaging and Testing,Beijing Microelectronics Technology Institute

作者简介：Xiu-Chen Zhao e-mail:zhaoxiuchen@bit.edu.cn;

收稿日期：4 September 2014

基金：financially supported by the National Science and Technology Major Project of China (No. 2011ZX02607);

Aging resistance and mechanical properties of Sn3.0Ag0.5Cu solder bump joints with different bump shapes

Ping Chen Xiu-Chen Zhao Ying Liu Hong Li Yong Wang

School of Materials Science and Engineering,Beijing Institute of Technology

Center of Packaging and Testing,Beijing Microelectronics Technology Institute

Abstract：

The effects of bump shape on the aging resistance and mechanical properties of Sn3.0 Ag0.5 Cu(SAC)solder bump joints were investigated by high-temperature aging test and shear test.Three different SAC solder bump joints with barrel type,cylinder type,and hourglass type,respectively,were provided by controlling solder process.The results indicate that as the bump volume decreases from barrel bump to hourglass bump in the reflow,the thickness growth rate of intermetallic compounds(IMCs)between different solder joints and Cu substrates in the isothermal aging of 150℃ decreases.And the change of shear strength of hourglass interconnection solder joint with the increase in temperature is the minimum in the three interconnection solder joints.Thus,as the solder dosage decreases properly,hourglass interconnection solder joint is obtained,which is avail to enhance the solder joint reliability at the high-temperature aging and the ability to adapt to the change of environment temperature suddenly.

Keyword：

Bump shape; Lead-free solder; Intermetallic compounds; Mechanical properties;

Received： 4 September 2014

1 Introduction

Over the past several decades,the area array packages have been one of the most promising electronic packaging technologies because of many special advantages,such as the low-cost,high-density input/output (I/O),and highperformance interconnection [ 1, 2, 3] .These area array packages require the formation of solder joints for the chip and board surfaces.The solder joints serve as electrical and thermal paths as well as structural supports,which leads to serious reliability issues.

The European Union directive on waste electrical and electronic equipment (WEEE) announced the prohibition of the use of lead in the consumer electronics market after January 2006.Recently,many different solder alloys have been proposed as potential replacements for lead-free solder.Among several candidate alloys,the Sn-Ag-Cu solder family is believed to be the most promising substitute for lead-containing solders because of its relatively low melting temperature,its superior mechanical properties,and its relatively good wettability [ 4, 5, 6] .However,many performances of the lead-free solder bump joints is reported to be inferior due to its higher melting temperature than that of Sn and is relatively active with metallization [ 7, 8] .Therefore,the reliability of lead-free solder bump joints is worth detailed study.

Numerous factors affect lead-free solder joint performance,such as joint geometry,pad shape,solder materials,substrate materials,chip size,and interface metallurgy. [ 9, 10, 11] .Of these factors,solder bump geometry plays a crucial role in the reliability of joints in electronic packaging.The solder joints processed by the conventional solder bumping technologies take on the barrel shape.Chiang and Yuan [ 12] developed algorithms for predicting the shape of solder bumps in reflow process.Liu et al. [ 13] fabricated solder bumps with the shape of hourglass and cylinder.The factors affecting the shape of solder bump were standoff height,solder volume,lower/upper contact angles of the joint,contact pad radii and pad shape,etc. [ 14] .Tian and Wang [ 15] found a principle for the forming of one kind of solder bump joint,if the standoff height and contact pad radii were kept constant,the amount of solder determined the bump shape after reflow.The less the solder amount was used,the smaller the waist radius of bump was gotten.If appropriate amount of solder was used,hourglass and barrel shape bumps would form.

Actually,previous researchers reported that the hourglass-shaped solder joints improved fatigue lifetime by about 60%over the conventional barrel-shaped solder joints by the mathematical calculations and finite element modeling [ 16] .Meanwhile,a lot of researches were extensively carried on the effects of solder bump shapes on fatigue lifetime [ 17] ,electromigration [ 18] ,and current density and temperature distributions [ 19] .However,it is not clear in the literature whether solder joint shape is the dominant factor on the aging resistance and the mechanical properties of lead-free solder bump joints.Majority of the studies on geometry influence of reliability analyzed the stress and strain distribution in different solder joint configurations and used some fatigue models to predict the lifetimes of the solder joint configurations [ 20] .

In this study,three different Sn3.0Ag0.5Cu solder bump joints with barrel type,cylinder type and hourglass type,respectively,were provided by controlling solder volume and investigated as the original motivation of this work to improve the solder joint reliability.High-temperature aging test results as well as shear test on solder joint assembly with different solder joint configurations were presented.Finally,the effects of shape on the growth of aging resistance and the mechanical properties of solder bump joints under the different temperatures were discussed.

2 Experimental

2.1 Preparation of sample

The fake chips and substrates used in this study were fabricated from the FR-4 copper clad plate to reduce the impact of thermal stress mismatch.The external dimension was 34.0 mm×8.5 mm×1.6 mm on the substrate side and was 6.5 mm×8.5 mm×1.6 mm on the chip side.The solder mask opening was 640μm in both chip side and substrate side.The thickness of Cu trace was 35μm,as shown in Fig.1.The solder bumping process consisted of three processes:gluing and formation of the bump on the chip,reflowing,and assembly,as shown in Fig.2.In order to be in harmony with environment,the Sn3.0Ag0.5Cu(wt%) solder paste (M705-GRN360-K2-V,SMIC) was used to this research.At first,the solder paste was pushed on the pad of the chip by syringe which was filled with solder paste.After that,the paste was reflowed to form the bump on the chip.Finally,the substrate was coated with a certain amount of solder paste in the same way and then was contacted with the downturned chip to undergo heating at several program-controlled heating zones,where the solders melted and reacted with pads,leading to formation of IMC.After cooling down,the molten solders solidify to form the final solder joints.Figure 3 shows the schematic temperature profile that an assembly generally experiences during reflow soldering.The solder paste on the fake chip was melted into a ball at 275℃,while the final solder joint was formed at 290℃.

Fig.1 Specific dimensional sizes of different solder bump joints:a barrel-type bump,b cylinder-type bump,and c hourglass-type bump

Fig.2 Solder bumping process:a gluing and formation of bump on chip and b reflowing and assembly

Fig.3 Schematic drawing of thermal profile in reflow soldering:a initial solder joint and b assembly

Fig.4 Shear test diagram

In this research,two key parameters,the amount of paste on the chips and the amount of paste on the substrates,should be carefully regulated to achieve solder joints with different shapes.The bonding equipment (Fineplacer 145“Pico”,Finetech GmbH&Co.KG) in this study has the high precision of±5μm whose working principle is dispensing on the pad,and then solder joints were formed in the reflow.After assembly,the three-dimensional microscopic system super depth of field (VHX-2000,Keyence Co.,Japan) was utilized to examine the integrity and the shape of the joint before tests.

2.2 High-temperature aging test

High-temperature aging test is one of the most important tests used to assess the reliability of solder joint interconnection.The objective of aging test is to observe the evolution of IMC between different solder bump and Cu substrate at extreme high temperatures and then to assess the aging resistance and service life of the solder joint.Our high-temperature aging test was conducted in the draught drying cabinet in which the environment temperature kept constant.The test samples were removed from the drying cabinet and their interfaces were observed periodically.The aging condition was programmed as:environment temperature of 150℃,aging time of 48,120,and 240 h.

2.3 Shear test

The adhesion between the interface of solder joint and copper pad is critical to flip chip and ball grid array (BGA)assembly since it is one of the most vulnerable interfaces.Therefore,it is very important to test the bonding strength of the solder joints,especially at different temperatures.The mechanical property testing was performed on the bonding tester (PTR-1100).The different samples were put on the stage and then pushed by the pusher until the solder joint was finally fractured.The temperature range of the test condition was:25-50,50-90,90-150,and 150-250℃.The schematic diagram of the mechanical properties test for three different solder joints is depicted in Fig.4.

2.4 Cross-sectioning of solder joint

After the aging process,samples to be cross-sectioned were mounted in epoxy and ground with different grade abrasive sandpapers.They were polished with 1.0-and 0.5-μm diamond suspensions,and then etched with 10 vol%HCl and 90 vol%C₂H₅OH solution to reveal the interface micro structure.Energy-dispersive spectrometer (EDS,NORAN System 7,Thermo Scientific) was used to identify the composition of the interfacial reaction products.Backscatter electron (BSE,Phenom G2 Pro,FEI) photograph was used to reveal the microstructures of IMC,which was helpful to describe the evolution process of IMC of different solder bumps.

2.5 Digital measurement of IMC thickness

To make the data of the mean thickness of TMC layer reliable,five different images were taken from different areas of the solder joint per sample.Considering the irregular shape of the IMC layers,the area of the IMC layer was digitally measured using the software"Auto CAD"and then pided by the layer length to obtain the average thickness.

3 Results and discussion

3.1 Test specimen of different shape solder joints

The three actual different bump waist radii were fabricated successfully:barrel-type bump with waist radius of 400μm,cylinder-type bump with waist radius of 320μm,and hourglass-type bump with waist radius of 215μm,as shown in Fig.5.The standoff height is a constant value of about460μm.

3.2 High-temperature aging resistance

Figure 6 shows BSE images of SAC solder/Cu interface with different aging time (reflow,48,120,240 h) at150℃.On the whole,the morphology of the IMC layer changes from an uneven sc allop-type to a layer-type with the increase in aging time,and a darker strip of IMC layer appears close to the copper pad in all samples after aging48 h.The composition of the IMCs layers is identified by the previous research results.The IMC layers contain the Cu₆Sn₅ (light-colored and scallop-type) and CU₃Sn (darkcolored and layer-type).What's more,the CU₃Sn will shorten the mechanical property of the solder bump if its thickness is a little big [ 21] .As a result,it can be seen that a discontinuous sc allop-type IMC layer of Cu₆Sn₅ is formed after reflow,while Cu₃Sn is not noticeable between Cu pad and Cu₆Sn₅ scallops in all samples.After aging at 150℃for 48 h,Cu₆Sn₅ layer is gradually flattened and its thickness increases,while the Cu₃Sn phase starts to emerge near the Cu pad obviously.

The histograms of the IMC growth rate of different solder bump joints which include the overall IMC and Cu₃Sn layer are revealed in Fig.7.With the increase in aging time,the thickness of the overall IMC and Cu₃Sn layer increases,while the growth rate of IMC layer presents an obviously decreasing trend.However,the growth rate of the overall IMC decreases at first from 0.012 to0.002μm·h^-1 and the CU₃Sn layer decreases at first from0.0100 to 0.0004μm·h^-1 in the barrel-shaped joints.The above value of the cylinder-shaped joints is,respectively,from 0.007 to 0.002μm·h^-1 and from 0.010 to0.009μm·h^-1,lower than those of barrel-shaped solder joints.And the final growth rate of the hourglass-shaped joints is,respectively,0.00200 and 0.00002μm·h^-1,lower than those of barrel-shaped and cylinder-shaped solder joints.In a word,the growth rate of IMC layer of the hourglass-shaped solder joints decreases in a larger scope than the other two shapes solder joints.Therefore,the hourglass-shaped solder joints will make the IMC thickness keep constant,and it will not greatly increase when its service time is longer and longer.And the aging resistance of hourglass-shaped solder joints is much better than those of the other two solder joints.

Fig.5 Actual solder joints with different bump shapes:a barrel-type bump,b cylinder-type bump,and c hourglass-type bump

Fig.6 BSE images of solder joints at 150℃with different aging time:a barrel type,b cylinder type,and c hourglass type

Fig.7 Growth rate of different solder bumps:a overall IMC and b Cu₃Sn

3.3 Mechanical properties

Figure 8 shows the changing curves of shear strength of three kinds of solder joints with different temperatures.It can be clearly seen that the curves can be pided into two periods and the cutoff point was 90℃.The first period reveals that the shear strength decreases dramatically with temperature increasing in all samples,while the shear strength value of barrel-shaped solder joints is the best,then cylinder shape,and finally hourglass shape.The second period reveals that the slope of three curves is smaller than the first period,and the shear strength value of barrelshaped solder joints is the best,then hourglass shape,and finally cylinder shape.The reason that the shear strength value of barrel-shaped solder joints is higher than those of cylinder-shaped and hourglass-shaped solder joints is that the solder dosage of barrel shape is the most,so that its mechanical properties are the best under a certain temperature.However,when the temperature is changed,the previous results will change.

Fig.8 Shear strength of three kinds of solder joints with different temperatures

Table 1 summarizes the change rate of shear strength of three kinds of solder joints under the different temperature ranges.Although all the trends decrease,it is obvious that the average change rate of hourglass-shaped solder joints is lower than those of the other two kinds solder joints.The temperature range are successively 25-50,50-90,90-150,and 150-250℃.The change rate of the barrel shape joints decreases at first from 0.0469 to 0.0378 MPa·℃^-1 and then changes to 0.0073 and 0.0038 MPa·℃^-1 with the temperature range changing.The change rates of the cylinder-shaped joints are 0.0501,0.0137,0.0243 and0.0019 MPa·℃^-1,respectively,which are sometimes lower than those of barrel-shaped solder joints and sometimes higher than those of barrel-shaped joints.And the change rates of the shear strength of hourglass-shaped joints are 0.0305,0.0164,0.0128,and 0.0052 MPa-℃^-1,respectively,which are lower than those of barrel-shaped solder joints and cylinder-shaped solder joints at a holistic level.Therefore,the hourglass-shaped solder joints have the best ability to adapt to the change of environment temperature suddenly.

下载原图

Table 1 Change rate (MPa.℃^-1) of shear strength of solder joints under different temperature ranges (ΔT)

4 Conclusion

The reliability of fabricated solder joints is in connection with the shape of the solder bump.As the solder dosage changes properly with constant standoff height,hourglassshaped,cylinder-shaped,and barrel-shaped solder joints can be obtained.Under the condition of high-temperature aging test,hourglass-shaped solder joints have the lowest growth rate of interfacial IMC layer with the increase in service time.Compared with the barrel-shaped solder joints and cylinder-shaped solder joints,the shear strength change of hourglass-shaped solder joints under different temperatures is the smallest.Therefore,the hourglass-shaped solder joints have the much higher resistance to the change of environment temperature suddenly.