Are Intermetallics in Solder Joints Really Brittle?

Chin C. Lee, Pin J. Wang, and Jong S. KimElectrical Engineering and Computer ScienceMaterials and Manufacturing Technology

University of CaliforniaIrvine, CA 92697-2660

cclee@uci.edu

AbstractIn nearly all soldering processes, it is the intermetallic

(IMC) layer that bonds the solder to the base metal. Thus, theIMC layer is necessary for any successful soldering operationused in the electronic industry. That is, a solder joint alwayshas at least one IMC layer. While the IMC layer is needed, itis not static. It grows in subsequent reflows and during agingwith time. Its growth without control could have adverseeffect on the reliability. This is particularly true for flip-chipsolder joints. The IMC, while necessary, seems to have alsobrought some problems. Thus, many packaging and solderexperts believe that "the intermetallics are brittle and they canoften embrittle the solder joints". On the other hand, not onereally presents quantitative analysis to illustrate that the IMCis indeed brittle. No one really interprets what it means by"brittle." In this paper, we collect and review the physicalproperties of four commonly seen intermetallics: Cu6Sn5,Cu3Sn, Ni3Sn4, and AuSn4. These properties are comparedwith that of reference materials Cu and Sn3.5Ag solder.Based on the measured properties available, we analyze andevaluate whether these IMCs are indeed brittle. Based on thefracture of flip chip solder joints reported by others, we assesswhether the fracture is caused by the "brittle" nature of theintermetallic as many believe or by something else.

1. IntroductionIn all soldering processes used in the industry, the

fundamental principle of solder being able to bond to the basemetal is the formation of intermetallic (IMC) layer. The mostwidely used soldering process is to bond Sn-based solders tocopper (Cu). In this popular process, tin (Sn) atoms formCu6Sn5 intermetallic with bare Cu. This IMC layer links theSn-based solder to the Cu. For bonding to Cu plated with Ni,Ni3Sn4 is formed. For nearly all soldering processes reportedby electronic industries, the IMC layer is necessary for anysuccessful soldering operation. That is, a solder joint alwayshas at least one IMC layer.

While the IMC layer is necessary, it is not static. It growsin subsequent reflows and during aging with time. Its growthwithout control could have adverse effect on the reliability ofthe solder joints. This is particularly true for flip-chip solderjoints [1-26]. As a result of various reliability issues related toIMCs and IMC growth, IMC seems to have brought moreproblems than solution. Thus, we often heard comments like"brittle intermetallics" [19-23] and "Au or AuSn4embrittlement" [24-26]. We probably have heard similarstatement for probably more than 3 decades. And yet, not onereally presents quantitative analysis and argument to illustratethat the IMC is indeed brittle. No one really interprets what itmeans by "brittle".

In this paper, we collect and review the physicalproperties of the important intermetallics in solders joints,Cu6Sn5, Cu3Sn, Ni3Sn4, and AuSn4 These properties arecompared with that of reference materials Cu and Sn3.5Ag tosee where these intermetallics stand in terms of brittleness.Better understanding of these properties help uncover thefundamental reason ofwhy a solder joint breaks. Four fracturecategories in flip chip solder joints are reviewed and discussedto find out whether the interfacial fracture is related to theIMC brittleness.

2. Properties of IntermetallicsTable I exhibits the important properties of four

commonly seen intermetallics in flip chip solder joints that wewere able to collect: Cu6Sn5, Cu3Sn, Ni3Sn4, and AuSn4. Cuand Sn3.5Ag solder are also included as references. As can beseen, the table is far from complete. The compound that lacksthe most measured properties is AuSn4. This is surprisingbecause "Au or AuSn4 embritlement" is a well documentedphenomenon [24-26] and yet so little was known about thiscompound.

The young's modulus of Cu, Cu3Sn, Cu6Sn5 and Sn3.5Agare: 130, 108, 86, 53 GPa, respectively. This gives thecommonly seen structure Cu/Cu3Sn/Cu6Sn5/Sn3.5Ag acontinuing decrease in stiffness and thus a decrease in elasticmismatch. The modulus of Ni3Sn4 is 133 GPa which is muchhigher than that of Sn3.5Ag solder. Thus, the Ni3Sn4/solderinterface has very high elastic mismatch. The Vickershardness of Cu6Sn5, Cu3Sn, and Ni3Sn4 are 378, 343, and 365kg/mm2. These values are extremely high comparing to 30 forCu and 100 for Ni, both in kg/mm2 (Vickers) [37]. Thehardness of these three compounds is in the range of highstrength steel. In comparison, Cu is very soft. This might havemade many solder and packaging engineers to believe that theIMC is brittle. However, many of them probably were notaware that these three IMCs were so hard before they thoughtthat IMCs were brittle. On the other hand, hard materials donot have to be brittle. The tool steel is extremely hard but it isnot brittle at all. In fact the tool steel is very tough.We were not able to find tensile strength data of the

intermetallics. Without knowing the tensile strength, thestrength of these intermetallics under tension cannot beevaluated. Keep in mind that "hardness" is measured undercompression.

So far, we still could not find a scientific and quantitativedefinition of "brittle" or "brittleness." "Brittleness" appears tobe a subjective impression rather than a quantitativeevaluation. We could not find any established method thatmeasures "brittleness." If brittleness cannot be measured, howcan anyone make a comment of "intermetallics are brittle."?

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Table I - Key properties of Cu, Sn3.5Ag solder and four commonly seen intermetallics in flip chip solder joints.

Properties Copper 1271 96.5Sn3.5Ag 1271 Cu6Sn5 1281 Cu3Sn 1281 Ni3Sn4 1281 AuSn4

Melting Point (C) 1,083 221 415 676 796 252

Density (g/cc) 8.94 7.4 8.28 8.90 8.65

Thermal Conductivity 3.862 0.78 0.341 0.704 0.196(watt/cm-K)

Electrical Conductivity 5.88xl 05 0.812xl 05 0.57xl 05 1.12xl 05 0.35xl 05(/Qcm)

Thermal Expansion 16.42x1 0-6 22.2x10-6 [35] 16.3x1 0-6 19.OxlO-6 13.7xl 0-6 19.3x1 0-6CoeffJ (/0C) [29]

Yield Strength (psi) 10,000 3,600Ultimate Tensile 32,000 5,000-7,000Strength (psi)

Fracture Toughness 2.80 [30] 5.72[30] 4.22[30] 2.50[30](MPa/m112)

Young's modulus (GPa) 129.8 [32] 52.73 85.56 [33] 108.3 133.3 71 [30]

Poisson's ratio 0.339 0.36 0.309 0.299 0.330 0.31 [33]

Hardness** (Vickers) 37 (Brinell) 14.8 (Brinell) 378 [34] 343 [34] 365 [34] 59.2 [30]

**Note: For conversion between Brinell hardness and Vickers hardness, see [36]

3. Are Intermetallics in solder joints really brittle?In situations like this, we check the dictionary to find the

meaning of "brittle' that is understood by ordinary people.The dictionary defines "brittle" as "easily broken." [38].Condition under which an object is "easily broken" is notspecified. Thus, the condition should be that of which anobject is used or being used. So, the complete question to askshould read "Are intermetallics in solder joints easilybroken?" The answer can be either "yes" or "no" dependingyour impression of "how easy is easily." With the "easily"understood by ordinary people, we would tend to believe thatthe answer is no. That is, intermetallics in solder joints are noteasily broken. If the IMC were easily broken or brittle, all theelectronic products with solder joints would have reliabilityissues.Among the measurable properties, the one most closely

related to "brittleness" is probably fracture toughness.Fracture toughness is measured during hardness test usingindentation. The hardness (Vickers) is defined as the load onthe indenter divided by the surface area of the indentationobserved, expressed in Kg/mm2. If cracking shows up fromthe corners of the indentation, fracture toughness can bedetermined. It is given by [39],

Fracture toughness =0.016 (modulus/hardness)1/2 [load/(crack length)3/21

Here, the crack length is measured from the crack tip to thecenter of the indentation. If cracking is not observed on theindentation with the load applied, fracture toughness cannotbe determined. The fracture toughness of the four IMCsshown in Table is 2 to 3 times of typical glass. Recent nano-indentation study on Cu6Sn5 and CuNiSn compounds does notshow cracking on the indentation marks [40].

We do not believe that fracture toughness data in Table Ican be applied to IMCs in solder joints without consideringthe setting difference of the IMC in solder joint versus theIMC in indentation test. In the indentation test, the IMC issubjected to localized compression load produced by adiamond tip. This condition does not exist for IMCs in solderjoints. The materials adjacent to IMC layer in solder joints aresolder, Cu, or Ni. Under compression, these materials are verysoft comparing to the IMC. The IMC layer is also quite thinwith large aspect ratio. Under compression, we do not believethat the IMC will fracture.

If fracture within an IMC layer occurs, it most likelyhappens under tension rather than under compression. Sincetensile strength of IMCs is still not available, there is notsufficient scientific data to support the statement of "IMC iseasily broken under tension." Thus, there is no sufficientscientific information to support the comment of "IMCs arebrittle."

4. The fracture of flip chip solder jointsFlip chip solder joints may fracture during various

reliability tests such as isothermal aging, thermal cycling,thermal shock, mechanical loading, bending test, impact test,and drop test. Fracture surfaces or interfaces vary and oftendepend on test methods. They can be roughly divided intofour categories below:

1. Within solder,2. Solder/IMC interface,3. Within IMC region,4. IMC/copper interface,

It seems that the key parameter that dictates the fracturemechanism or fracture interface is the strain rate. For low

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strain rate such as in isothermal aging, thermal cycling, andslow mechanical loading, fracture tends to occur inside thesolder due to fatigue. In this case, the solder has enough timeto go through the complete plastic stain variation cycle andbecomes fatigued after certain number of testing cycles. Forhigh strain rate such as in the case of impact test, drop test,rapid bending test, and thermal shock test, the strain rate is sohigh that the solder material does not have time to incurplastic deformation before the momentum transmits throughthe entire solder joint structure. The momentum experiencedby the solder joint is proportional to the strain rate. Fracturewill occur along the weakest interface of the solder jointstructure. At high strain rates, the solder material, with notime to deform in-elastically, appears stronger in terms oftensile strength. This is referred to as strain-rate hardening.Thus, at high strain rates, the chance of fracture inside thesolder material decreases.

Fig. 1- Cracks are inside the solder after thermal cyclingat 0°C-1000C [2]

Fig. 2-Crack propagates along the interface of the bulksolder/IMC layer after shear cycling test. The frequency ofthe tests is 0.43 Hz. The shear displacement amplitudes areset to values between 0.18 and 0.28 mm [41]

Fig. 3-Crack propagates through the (Cu, Ni)6Sn5 IMC afterthe drop test according to the JESD22-B1 11 standard [17]

Fig. 4-Voids form between Cu3Sn and Cu substrate after7600-hour high temperature storage at 175 °C [42]

Among the four fracture categories, three are related tointermetallic region (Figs. 2, 3, and 4). For fracture withinIMC region, it is usually caused by having more than one IMCin the region. Fracture tends to go through the boundarybetween two different IMCs [8]. Fracture seldom cuts cross asingle IMC phase. Table II exhibits the crystal structure andlattice constants of Cu, Sn, and the four intermetallicsreviewed. For interfaces along Sn/IMC, IMCI/IMC2, andIMC/Cu in a solder joint structure, the crystal structure andlattice constants of materials in both sides of the interface arevery different. Thus, it is not realistic to expect that theseinterfacial boundaries would always give a strong bond. Whenfracture occurs along one of these interfaces, we really do notsee how this is related to the "brittle nature of IMCs." Twomaterials put together do not always produce a strong bond onthe interface if they cannot share electrons effectively. Planarboundary is easier to break comparing to a weaving boundarybecause the materials in both sides of a planar boundary donot have gripping or locking mechanism to increase thestrength. Fracture along interfacial boundaries can beaccelerated by interfacial stresses produced by difference inphysical properties such as thermal expansion coefficients andmass density.

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Table II - Crystal structures and lattice constants of Cu, Sn and four intermetallics

Crystal structure Lattice constantCu Face-centered cubic [43-44] a =3.61509 A [43-44]Sn oc phase: Face-centered cubic [45-46] oc phase: a= 6.4912 A [45-46]

D phase: Body-centered tetragonal [47-49] D phase: a = 5.8314 A, c = 3.1815 A [47-49]Ni3Sn4 Monoclinic [50] a=12.472A, b=4.069 A, c=5.293 A, f=101051'[50]Cu6Sn5 Monoclinic [51] a=11.022A, b= 7.282A, c= 9.827A, f=98.84°[51]Cu3Sn Hexagonal [50] a=2.753 A, c=4.385 A [50]AuSn4 Orthorhombic [52] a = 6.45 A, b = 6.49 A, c = 11.60 A [52]

5. SummaryWe collected and reviewed the properties of Cu6Sn5,

Cu3Sn, Ni3Sn4, AuSn4, Cu and Sn3.5Ag to first understandwhether IMCs are brittle as many believe. Our conclusionseems to indicate otherwise. There is no sufficient scientificdata to support the statement of "IMCs are brittle." Fracturealong interfaces such as solder/IMC, IMCG/IMC2, andIMC/Cu is not related to whether the IMC is brittle or not.Rather, this interfacial fracture is caused by insufficient bondstrength along the boundary. To blame brittle fracture in flipchip solder joints to IMCs being brittle probably needsreconsideration and further assessment.

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