Lead-free Solder

1. Introduction

The motivation for developing lead-free solders is toremove Pb from the electronic manufacturing andwaste disposal processes, as its toxicity is wellestablished. It is not practical to recycle lead as isdone in other industries, because the amounts aresmall compared to the size of systems it is in. Whileshredded electronic waste is often put in landfills inthe US, this waste is incinerated in more denselypopulated areas, which puts lead vapor into the air.Moreover, in the manufacturing environment, thevapor pressure of Pb is much higher than most othermetals, posing a direct inhalation hazard to workers.In contrast, Sn has a much lower vapor pressure, andis somewhat less toxic than Pb. For this reason,European Union (EU) legislation passed the Wastefrom Electrical and Electronic Equipment (WEEE)and the Restriction of the Use of Certain HazardousSubstances in Electrical and Electronic Equipment(RoHS) directives in 2002. The four heavy metals(lead, cadmium, mercury, and hexavalent chromium)and the brominated flame retardants, polybromi-nated bypheneis (PBB) and polybrominated diphenylether (PBDE), were banned in new electronic equip-ment in the EU as of 1 July 2006 with someexemptions, for example, lead in high meltingtemperature solders (i.e., Sn–Pb solder alloys con-taining more than 85% lead, or lead in solders fornetwork infrastructure equipment for switching,signal transmission, as well as network managementfor telecommunication). However, this exemption isnot permanent and will be reviewed. Exempted usesof Pb-bearing solder must have acceptable substitutesby 2014. This multinational decision has led tovigorous development of alternative solder alloyslargely based upon Sn, and the differences inproperties affect the entire technology stream ofprinted circuit boards and packages.As tin–lead (Sn–Pb) solder has been effective for

thousands of years, there is a deep experience basewhich is lacking for Pb-free solders. Much of theexisting electronic system infrastructure is basedupon properties of Pb–Sn solder. Solders performeffectively at 80% of its melting temperature, giving ita performance capability comparable to superalloysused in jet engines, but the performance demands onsolder joints are continually increasing due toincreasing current density arising from continuingminiaturization. Sn–Pb solder has a two-phasemicrostructure with about a 2:1 volume ratio ofSn:Pb-rich phases. The very soft Pb-rich phase carriesmuch of the cyclic deformation, making the alloysoft. In contrast, Sn-based solders have about 5%

small, hard, intermetallic strengthening particles,giving it completely different metallurgical character-istics than Sn–Pb alloys, because ‘‘all’’ of the plasticdeformation takes place in the Sn phase. As Sn isharder than Pb, Sn-based solders are stronger thanSn–Pb solders, which stresses the surrounding com-ponents more than Sn–Pb solders do. This isproblematic because existing die and circuit boarddesign is based on the softer properties of Sn–Pb andits lower melting temperature.

While a drop in replacement for Sn–Pb has beensought, it has not been found. As Sn-based eutecticalloys have only a small amount of other elements, theeutectic temperatures are close to the 231 1C meltingpoint of Sn; 220 1C is a typical eutectic temperature forSn-based alloys. These differences from Sn–Pb aresignificant, making the experience base with Sn–Pbsolder nearly irrelevant; hence, coping with thesedifferences has required a number of design adjust-ments throughout the electronic manufacturinginfrastructure. Without any deep experience base,engineering design decisions require increased relianceon analytical modeling to predict product reliability.

In the following sections, the fundamental infor-mation required for analytical modeling of Sn-basedsolders is described to show what must be incorpo-rated into predictive models. In addition, challengesto solder technology that were not important whenSn–Pb was common are examined, such as highcurrent density and electromigration that comes withshrinking size scales, and problems with formation oftin whiskers, which form spontaneously and cancause shorts between circuit paths. Finally, theinfrastructural requirements for manufacturing arediscussed to illustrate how implementation of lead-free solders is accomplished.

2. Physical Properties of Tin

Tin is a group IV element that is transitional betweenthe diamond cubic structure of Si and Ge, and theface-centered cubic structure of Pb. No other metalhas a body-centered tetragonal structure crystalstructure like b Sn, which is illustrated in the insetof Fig. 1(a). The tetrahedral bonding around thecenter atom is evident, and if extended to neighboringunit cells, it becomes apparent that the crystal has asquashed diamond cubic structure. The b (white) tintransforms to a (gray) tin below 13.2 1C, which hasthe diamond cubic structure. In practice, the presenceof alloying elements stabilizes the b structure, but atlow temperatures and long times, this transformation(called ‘‘tin pest’’) is occasionally observed, which cancause internal fracture that causes a solid to become apile of powder.

Table 1 shows how Sn compares with several otherimportant elements used in electronic systems. Notethat the coefficient of thermal expansion (CTE) varies

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Figure 1(a) Anisotropic CTE and elastic modulus for tin and the body-centered tetragonal crystal structure; the magnitude isthe distance from the origin. (b) Temperature dependence of CTE and elastic modulus. From Bieler T R, Jiang H,Lehman L P, Kirkpatrick T, Cotts E J, Nandagopal B 2008 Influence of Sn grain size and orientation on thethermomechanical response and reliability of Pb-free solder joints. IEEE Trans. Compon. Packag. Technol. (CPMT)31 (2), 370–81; House D G, Vernon E V 1960 Determination of the elastic moduli of tin single crystals and theirvariation with temperature. Br. J. Appl. Phys. 11, 254–9; and Deshpande V T, Sirdeshmukh D B 1962 Thermalexpansion of tin in the beta–gamma transition region. Acta Cryst. 15, 294.

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Lead-free Solder

Table 1Comparative physical and mechanical properties of elements in electronic materials.

Property Pb Sn Ge Si Al Cu Ag Au

CTE (ppmK� 1), B300K 29.1 15–30 6.1 2.49 24 16.4 19.6 14.4Electrical resistivity (mO cm) 20.8 13–20 50 10 000 2.7 1.7 1.55 2.2Thermal conductivity (Wm� 1K� 1) 33 63.2 64 124 210 385 419 301T(melt) (1C) 327.5 232 937.4 1412 660.4 1083 962 1064Entropy of melting ( Jmol� 1K� 1) 8.3 14.0 28.7 30.0 11.2 9.6 9.2 9.7Elastic modulus (GPa) 10–40 22–69 103–155 131–196 64–76 67–192 44–120 43–116Yield strength (MPa) o5 10 15 33Ultimate tensile strength (MPa) 18 220 150 270 182 210 140 120

Where ranges are given for Sn, the first number is for the ‘‘a’’ direction, the second the ‘‘c’’ direction.

Sn

Ag3Sn

Ag3Sn

Sn

SAC305

5 μmSAC305

5 μm

150 °C/1000h

As-assembled, no aging(a)

(b)

Figure 2After assembly, Ag3Sn particles in this SAC 305(Sn–3.0Ag–0.5Cu) joint are clustered in theinterdentritic spaces (a), and after aging, they coarsen(b), making the flow stress decrease with time.

Lead-free Solder

by a factor of 2 with crystal direction (illustrated inFig. 1(a)), unlike cubic metals that have an isotropicCTE. The CTE on the (001) plane is close to that ofCu (and the FR-4 polymer matrix glass fibercomposite circuit boards). Also, the elastic modulusvaries by a factor of 3. These properties are stronglytemperature dependent, as shown in Fig. 1(b); theCTE anisotropy ratio is nearly constant at B2 withtemperature, but the elastic modulus anisotropy ratiovaries strongly with temperature. Tin has lowerelectrical resistivity and higher thermal conductivitythan Pb, which are beneficial for solder joints, andparticularly important for high-current-density ap-plications. The entropy of melting (DSm¼DHm/Tm)is between the low values common to cubic metalsand higher values for diamond cubic elements. Thisentropy change is important because much moreenergy must be released to convert amorphous liquidto an orderly crystal lattice. Consequently, a movingsolidification front will generate a heat wave as itmoves. In practice, solidification of Sn requires moreundercooling than other metals, and in the smallvolumes of solder joints, 501 of undercooling iscommonly observed. With such undercooling, acritical size nucleus will grow quickly, so joints oftenhave only one or a few crystal orientations.

3. Role of Alloying Elements on Microstructure

As tin has relatively low solubility for alloyingelements, the properties of the Sn-based solder matrixwill be similar to the physical properties of tin;however, the mechanical properties of a joint aregreatly affected by the spatial arrangement of second-phase precipitates that form due to the presence ofalloying elements. The most common alloyingelements are Ag and Cu. A wide variety of composi-tions near the ternary eutectic composition of Sn–3.7Ag–0.9Cu have been explored, which melts at217 1C. An alloy numbering system has been estab-lished; for example, SAC359 denotes the eutecticcomposition and SAC405 has more silver and less

copper than the eutectic, that is, 4.0Ag and 0.5Cu.The most popular alloy is SAC305, but there isgrowing interest in SAC alloys with lower Ag.

The solidification microstructure is extremelysensitive to alloy composition, volume of the solderjoint, and the heating and cooling history, whichdramatically affect the size and morphology of

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Figure 3Microstructures of Sn–1Ag, SAC305 (Sn–3.0Ag–0.5Cu), and Sn–0.1Cu solder joints illustrating a range ofmicrostructure morphologies of the tin phase. Adapted from Lehman L P, Xing Y, Bieler T R, Cotts E J Cyclic twinnucleation in tin based solder alloys. Acta Mater. (submitted).

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Figure 4Misorientation histograms of Sn–1Ag and SAC305 solder balls in Fig. 3. Pole figures and unit cell prisms illustrate‘‘beach ball’’ texture for the SAC305 obtained from the EBSP map of the SAC305 ball. Colored lines indicateboundaries and their misorientations.

Lead-free Solder

intermetallic phases as well as the microstructure ofthe b tin phase. After solidification, the mostcommon microstructure is a divorced eutectic withprimary Sn dendrites as the primary phase and aninterdendritic ternary eutectic that forms betweendendrites shown in the top image of Fig. 2.Figure 3 illustrates some extremes in observed Sn

microstructures with different amounts of Ag andCu. With no copper, the microstructure appears to bea fine-grained polycrystal, but with added copper,single crystals or tricrystals are commonly observed;sometimes, the tricrystal has the morphology of sixsegments like a beach ball. From electron backscatterpattern (EBSP) mapping, crystal orientations forboth compositions show that only three crystalorientations are present, with B601 misorientationsabout a common [100] axis. When no silver is present,

4

three plates B601 apart often form but subsequentcrystals have a variety of other orientations, andthese joints are more like a polycrystal. Figure 4shows that a misorientation histogram is rathersimilar for the Sn–1Ag and SAC305, despite the verydifferent microstructures. Both joints have a beachball texture in pole figures, such as those shown fromthe EBSP map of the SAC305 joint in Fig. 3. Suchpole figures are common regardless of whether or notthe grain morphology appears like a beach ball. Themajor difference between Sn–1Ag and SAC305 is thatthere are few low-angle boundaries in the Sn–Ag, dueto the small ‘‘grain’’ size, and many low-angleboundaries in the large grains in the SAC305. Thepreference for 601 misorientations is a signature ofsolidification twinning (two types occur, on {101}or {301} planes with 57.21 and 62.81, respectively).

10 μm

Cu6Sn5

Ag3Sn

Figure 5Morphology of intermetallic precipitates: (a) Sn etchedaway reveals shape of Ag3Sn plates and Cu6Sn5 rods.(b) Appearance of a small Ag3Sn plate within a b Sndendrite, and surrounding small particles of Ag3Sn. (a)Reproduced with permission from Lu H Y, Balkan HNg KYS 2005 Solid–liquid reactions: the effect of Cucontent on Sn–Ag–Cu interconnects. JOM 57 (6), 30–5copyright The Minerals, Metals & Materials Society(TMS). (b) Reproduced with permission from SigelkoJ, Choi S, Subramanian K N, Lucas J P, Bieler T R1999 Effect of cooling rate on microstructure andmechanical properties of eutectic Sn–Ag solder jointswith and without intentionally incorporated Cu6Sn5reinforcements. J. Electron. Mater. 28 (11), 1184–8,copyright The Minerals, Metals & Materials Society(TMS).

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Slip, balancedby recovery

0

Simple shear

Figure 6Single lap shear deformation of solder joints depends ontemperatures and strain rate. Data from Rhee H,Subramanian K N, Lee A, Lee J G 2003 Mechanicalcharacterization of Sn–3.5Ag solder joints at varioustemperatures. Soldering and Surface Mount Technology15 (3), 21–6.

Lead-free Solder

,

,

The boundaries of twin structures have a very lowinterfacial energy, which makes them highly pre-ferred. Solidification twinning is also the basis for theapparently fine-grained microstructure shown for theSn–1Ag composition, which actually has threetwinned orientations that interpenetrate each other.

The heating and cooling history also causes muchvariability in observed microstructures. If coolingrates are slow and/or undercooling is large, thenprecipitation of large plates of Ag3Sn and/or rods ofCu6Sn5 can form (e.g., Fig. 5(a)). Upon remelting(reflow), these large intermetallic phases require along time to dissolve, such that these plates becomesinks for alloying elements in solution, which locallyalters the density of small particles of the Ag3Sn thatform in the final solidification of eutectic liquid (Fig.5(b)). These large precipitates degrade mechanicalproperties. As the solidification path is complex,other alloying elements can significantly modify thesolidification path, and hence, the microstructure andproperties. Reducing the amount of undercooling canbe achieved by microalloying SAC with very smallamounts of Cr, Ti, Ni, Co, Zn, Mn, Fe, Ge, Si, Bi,and/or In.

Furthermore, Sn forms a number of other eutecticalloys with In, Bi, and Zn, which have lower meltingpoints than SAC alloys (e.g., Sn–8Zn–3Bi has 197 1Cmelting temperature). In and Bi are more expensive,and large amounts of Zn lead to corrosion problems,which make these eutectic systems less popular thanSAC-based alloys. Sn–Bi and Sn–In eutectic alloys

5

Lead-free Solder

have even lower melting points that make thesesolders valuable when building complex systems,where remelting of prior solder joints is undesirable(e.g., within packages to prevent damage fromvolume expansion from melting where joints areconstrained by a filler material).

4. Mechanical Properties of Solder Joints

Some mechanical properties of tin, in comparisonwith other materials used in packages are shown inTable 1. Clearly, Sn is much stronger than Pb,implying that lead-free solder joints are stronger,which is not always desired, as greater strengthtransfers higher loads to substrates and packages.Figure 6 illustrates how temperatures and strain

rate affect the shear stress–strain properties of SACalloys. At low temperatures, the yield strength issimilar at different strain rates, but more workhardening occurs at higher strain rates, due to thelack of time for diffusion-assisted dislocation recov-ery (annihilation) processes. When recovery is possi-ble, there is less work hardening, which reduces theflow stability, leading to heterogeneous strains andreduced strain to failure. At higher temperatures andlower rates, recovery occurs concurrently with slip,leading to almost no hardening at all.Figure 7 shows how the maximum shear strength

depends on aging time at elevated temperature forseveral alloy compositions. Aging causes coarseningof the fine-scale precipitates, which increases thedistance between particles that correspondingly

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Figure 7Effect of alloying and aging on maximum shear strength. WMedia: J. Mater. Sci.: Mater. Electron., Development of Snsolder applications, Vol. 18, 2007, pp. 55–76, Anderson I E

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decreases the stress needed to cause dislocations tobypass the particles—see Fig. 2. Alloying affects boththe as-fabricated strength and its decrease in strengthwith aging time.

As the elastic and CTE properties are strongfunctions of temperature and crystal orientation, thestress evolution in a solder joint starts with thecooling process; as the joint reaches room tempera-ture, significant internal stresses are present, whichwill relax slowly, as Sn is above half of its meltingtemperature even at � 40 1C. Hence, modeling ofstress–strain behavior requires sophisticated modelsthat can track solidification processes that define theinitial microstructure, internal stress, and theirevolution during service.

5. Interfacial Properties and IntermetallicFormation

When shear failures are studied, the fracture surfaceis commonly entirely in the tin, even though the crackpropagates close to the interface. This is a conse-quence of the geometry of the solder joint, asmaximum stress states usually develop close to theinterface. However, at high strain rates and/or lowertemperatures, the Sn becomes stronger than theintermetallic phase boundaries, and fracture occursin the intermetallic layer that forms between thesolder and the package or circuit board. Thus, theevolution of the interfacial intermetallic phases alsoaffect the reliability of a solder joint. Figure 8 showshow aging leads to growth of the intermetallic layer

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Co

Sn−3.0Ag−0.5CuSn−3.7Ag−0.9CuSn−3.7Ag−0.7Cu−0.2Fe

at 150°C

ith kind permission from Springer Science þ Business–Ag–Cu and Sn–Ag–Cu–X alloys for Pb-free electronic.

Cu

Cu

Cu3Sn

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Cu6Sn5

Cu6Sn5

Ag3Sn (light gray)SAC305

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(b)

Figure 8(a) A joint from a ball grid array package made fromSAC30 (Sn 3.0Ag) solder with SOP surface finish (solderon pad). (b) After aging, the Cu6Sn5 intermetallic layergrew thicker with time and temperature, and a Cu3Snlayer formed between the Cu6Sn5 and copper layer.

Lead-free Solder

between solder and copper, and how a Cu3Sn layerdevelops between the Cu6Sn5 and the copper layerswith increasing time. This interface is often proble-matic, as voids often develop in the Cu3Sn layer orinterface.There are several ways that fracture can be

facilitated due to reactions in the interfacial layer.The Sn solder and the substrate metallurgy form adiffusion couple, and the reaction layer developsquickly when the solder is molten. The rate of thereaction is highly sensitive to the composition of thesolder and the substrate, which is partially dissolvedby the molten Sn, which then alters the alloycomposition near the interface. Interfacial reactionscontinue during service in the solid state, leadingto complex composition gradients and continuingreactions at rates that depend on the temperaturehistory and composition. There is much literaturethat examines the kinetics of phase evolution ofinterfacial reactions, and the outcomes are oftenspecific to the elements present and the thermo-mechanical history.

6. Shock (High Strain-rate Deformation)

For many portable electronic systems, drop testing isimportant, as the properties of solders are sensitive tostrain rate (Fig. 6). At lower strain rates, dislocationmotion and recovery (annihilation) are assisted bythermal vibration, which is a time-dependent statisticalprocess. At higher rates, motion of dislocations is notassisted by thermal vibration, so the flow stressincreases with strain rate, often leading to fracture atinterfaces rather than through the solder. Thisundesired increase in strength of the solder can bemitigated by having fewer particles that strengthen thesolder (aging coarsens the particles, but also thickensthe brittle intermetallic layer). SAC 105 has a loweryield stress, and hence a lower strength in shockconditions, that makes this solder more attractive forapplications that must survive shock (but this comes atthe cost of a 10 1C higher melting temperature).Microalloying with Ni and Cr can improve dropresistance, as they slow intermetallic layer growth,which reduces the brittleness of the intermetallic layer.Figure 9 shows how high strain-rate properties aresensitive to alloying and hence microstructure andinterfacial reactions—some alloy compositions showmuch greater variability than others.

7. High-current-density Issues

With decreasing sizes of electronic packages andsystems, the current density increases in solder joints.Moreover, the under bump metallization (UBM) andsolder bump contact areas become smaller; flip-chip solder bumps are in the range of 50–100 mm,and as solder balls need to carry currents of about0.2A, the current density reaches 104A cm� 2. Thus,electromigration is an increasingly important issue, ashigh DC currents provide an electron wind thatmoves Sn atoms preferentially in the direction ofelectron flow. Increased current density causes jouleheating that increases the temperature and softens thesolder. Voids commonly develop on the side fromwhich electrons enter, and hillocks develop whereelectrons depart. However, the imposition of theelectric field puts a bias on the direction of probablevacancy (atom) jumps, which alters rate processesthat depend on diffusion, such as creep deformation.Electric currents can cause changes in grain orienta-tion to orient the c-axis with the direction of currentflow; this driving force arises from the fact that the c/a ratio of Sn self-diffusion and resistance is 1.53 and0.38, respectively (resistance is higher and diffusionis slower in the c direction). Hence, grains with thec-axis aligned with the current flow will experience adriving force to rotate. Interstitial diffusivity is alsohighly anisotropic; Cu interstitial diffusion is 500times faster along the c-axis than the a-axis (and Nidiffuses 70 000 times faster along the c-axis). These

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Figure 9Impact strength of several solder alloys. With kind permission from Springer Science þ Business Media: J. Mater.Sci.: Mater. Electron., Development of Sn–Ag–Cu and Sn–Ag–Cu–X alloys for Pb-free electronic solder applications,Vol. 18, 2007, pp. 55–76, Anderson I E.

Lead-free Solder

effects make tin grains with the c-axis aligned withcurrent flow particularly prone to early damage bytransport of underbump metal atoms in the directionof electron flow. Current crowding occurs in loca-tions where the path length of mobile electrons isconcentrated, leading to pileups of Sn or alloy atomswhose transport is blocked by the intermetallicboundary. This leads to hillock or whisker formationon the side where electrons leave the joint. High-frequency RF currents cause electron flow to be onthe surfaces due to the skin effect, so the magnitudeof damage is smaller.

8. Tin Whisker Formation and Mitigation

Tin whiskers commonly form on substrates coatedwith tin. These are problematic, as their growthcan cause shorts between current carrying lines. Thisproblem is a consequence of Sn being in a state ofcompression, which results from copper diffusioninto Sn, which causes a volume increase from theformation of intermetallic compounds. Grain bound-aries provide fast paths that allow Sn atom flux tolocations where the surface oxide is broken, so thatan emerging whisker can relieve the pressure imposedby a growing intermetallic. The problem can bereduced or eliminated by preventing Cu diffusion intoSn, and managing the internal stress state so that thesurface of the Sn remains in tension rather thancompression.

9. Manufacturing and Technology Issues

As shown in Fig. 10, Pb-free solder materials are usedin various parts of interconnections in electronic

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packages. The first-level interconnectis between thechip and the package substrate and lead-free solderbump material are used in the case of a Flip ChipBGA (FCBGA). Because under fill material is used inthis level, thermal fatigue and the mechanical shock isa lesser concern, but electromigration is an importantissue because of the higher function temperature atthe chip and higher current density.

The second-level interconnect is the solder jointbetween the package substrate and the printed circuitboard (PCB) or printed wire board (PWB). Based onthe packaging structure and form factors there areseveral families of packages, each of which haveparticular challenges with lead-free solder. Forexample, chip scale packages (CSP) and wafer levelpackages (WLP) have relatively small body size buthave a high CTE mismatch, plastic ball grid array(PBGA) packages are one of the most popularpackage types which have a plastic overmold overthe wire bonded chip, and there are FCBGApackages which have relatively large body size andusually have a heat spreader on top of the package.To assure their reliability, where reliability of thesolder joint is the ability to function during serviceconditions for a specific period of time withoutexceeding acceptable failure levels, several drivingforces which alter the structural stability of thepackage during time must be considered. One of thedriving forces is the thermal expansion differencebetween the package and the PCB. Figure 11 showsorientation maps of three joints that illustrate theeffect of CTE on internal stress history. As solderjoints tend to have a random set of orientations, theaverage CTE defines the amount of displacement ofthe package from the board. As the red orientationhas the c-axis nearly parallel to the substrate, its low

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W2 RRX−SRM MEDF = 50 / S = 0

Early failures

Figure 12Weibull plot shows statistics of failure; points far to the left of the linear regression are identified early failures.The cause of early failures is not yet identified. Reproduced with permission from Hillman D, Wilcoxon R 2006JCAA/JG-PP lead free solder testing for high reliability applications. As originally published in the 2006 SMTAInternational Conference Proceedings. Surface Mount Technology Association, Edina, MN, pp. 836–60.

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c-axisdirection

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Figure 11Three orientation maps showing orientation of c-axes in three joints, one of which cracked after 2500 thermal cycles.The plot shows how stress state varies for three single crystals with different crystal orientations (Bieler et al. 2008).

Lead-free Solder

CTE perpendicular to the board will cause the jointto be in tension at high temperature and compressionat low temperature. The purple orientation, whichhas the c-axis perpendicular to the board, will have

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the opposite stress–strain history. In joints withthe green orientation, or in joints with multipleorientations, the CTE will more likely be closer tothe average value, and hence have smaller stresses

Lead-free Solder

perpendicular to the board. The red orientation hasbeen found to be more likely to crack during thermalcycling, but the blue orientations are more likely tofail under high current conditions. Thus, it isdesirable to avoid these two extreme orientations,but to date, there is no obvious way to control thecrystal orientations during solidification.The rate of cycling is important, as accelerated

thermal cycles (or thermal shock tests) do not allowstress relaxation to occur, and hence, the magnitude ofplastic cyclic strain is lower. It is well established thatthe number of cycles to failure decreases as hold timesfor a thermal cycle are increased, implying thatdamage develops during hold times. Mechanical shockand vibration are especially important for Pb-freesolder materials. These loading conditions affect thestability of the solder joint individually or as a system,where degradation and ultimately failure is acceler-ated. To estimate the overall lifetime of the solderjoint, there are several statistical approaches. Failuresin solder joints, in particular due to constant degrada-tion do not happen simultaneously and are usuallywell-distributed over time. One of the commonmethods of estimating the expected lifetime of solderjoints statistically uses the Weibull distribution. Twoparameters are used in this method, the slope (ameasure of the degree of spread of the distribution)and the other is some intercept value (typically N(63.2%))—the characteristic life of the Weibull dis-tribution. On Weibull distribution graphs, using thetwo defining parameters, measured data will ideallyplot as a straight line, frequently simplifying dataanalysis. Figure 12 shows an example of such ananalysis for a particular kind of package, where ‘‘earlyfailures’’ are suggested (points outside the range of thelinear data). This example also shows that the data arenot linear, implying that there are different failuremodes for early failure and later failure. There isconsensus that Pb-free solder joints have a longerthermal cycling lifetime than Sn–Pb solder joints.However, the factors of grain orientation, the complexnature of Sn as the base material, and the combinationof thermal and mechanical cyclic deformation makeassessment and prediction challenging.For a product having Pb-free material as an

interconnect, it is recommended to follow the Designfor Reliability (DfR) guideline which applies tocertain products to achieve a good, in other words,an expected lifetime at the end-use condition. Eventhough there are time-dependent factors such as creepduring function, mechanical fatigue, and Sn grainorientation effects, accelerated tests are available togive some indication of the relative lifetimes of thesolder joint or the product by using carefully selectedacceleration factors. The standardized test methodsare listed in several documents, such as the IPC-JEDEC9701A ‘‘Performance test methods and qua-lification requirements for surface mount solderattachments’’, and for mechanical testing such as

monotonic bend testing, IPC-JEDEC9702 is a goodreference.

10. Implications of Pb-free Technology on Die andBoard Design

The anisotropic properties of Sn along with differentchemical behaviors will provide engineering anddesign challenges for the foreseeable future inelectronic systems. As the industry copes with theanisotropic properties of Sn-based solders, next-generation board and die fabrication design decisionswill be affected, so that they can accommodate thestronger properties of Sn-based solders.

See also: Electrical and Electronic Connectors: Ma-terials and Technology; Electronic Packages: Qualityand Reliability; Electronic Packaging Materials:Properties and Selection; Electronic Packaging:Thermal, Mechanical, and Environmental Durabil-ity; Optoelectronic Packaging: Solder Assembly.

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T. R. Bieler and Tae-kyu Lee

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