Pb-free BGA Solder Joint Reliability Improvement with Sn3.5Ag Solder Alloy on Ni/Au Finish

Eu Poh Leng', Min Ding2, Wayne Lindsay2, Sheila Chopin2, Ibrahim Ahmad3, Azman Jalar4'Freescale Semiconductor Malaysia Sdn. Bhd.No. 2, Jalan SS 8/2 Free Industrial Zone

Sungei Way Petaling Jaya 47300Tel: 603-78734247

2Freescale Semiconductor IncCorporate Headquarters

6501 William Cannon Drive WestAtx, Texas, 78735 USA

Tel: 512-895-35173Faculty of Engineering, Department of Electrical, Electronics & System,

4Faculty of Science and TechnologyNational University of Malaysia43600 Bangi, Selangor, Malaysia

Tel: 603-89216322 Fax: 603-89216146Email: R38158@freescale.com

AbstractIn this work, Sn3.5Ag solder alloy was being studied for

the purpose of Pb-free solder joint reliability improvementover conventional Sn3.8AgO.7Cu solder balls on Ball GridArray (BGA) packages with Ni/Au pad finishing. The studywas carried out in different levels. At individual solder jointlevel, Sn3.5Ag showed no intermetallic brittle failure in coldball pull test even upto 6x multiple reflow and 168hrs hightemperature storage for TBGA & TePBGA, and 504hrs hightemperature storage for PBGA Spanish Oak. In contrast,70-100% of the failure mode of SAC387 was brittle failure.At package level, Sn3.5Ag survived 8-lOx more drop cyclesthan SAC387 in tray drop and packing drop tests. Theseresults indicate that the mechanical strength of Sn3.5Ag onNi/Au pad is considerably stronger than that of SAC387. Thedifference in mechanical strength between the two alloys wascorrelated to their microstructures. At the same time, boardlevel solder joint reliability tests such as thermal cycling andmechanical bend test were carried out. Sn3.5Ag showedbetter or similar performance as SAC387.

Introduction1.1. Background and objective ofthe study

The binary eutectic SnAg (Sn3.5Ag) and ternary neareutectic SnAgCu (Sn3.9AgO.6Cu, Sn3.OAgO.5Cu) alloys aretwo major Pb-free alloys that have been studied extensively.Both of them show lower creep rate and higher resistance tothermo-mechanical deformations compared with eutecticSnPb which lead to better thermal cycling performance [1-3].Compared to SnAg, SnAgCu is more commonly accepted bythe industry for the past few years after being recommendedby iNEMI and JEITA for BGA spheres.

However, quality and reliability concerns have been raisedrecently due to the increasing occurrence of missing ballfailures of SnAgCu on Ni/Au pad in ball grid array (BGA)assembly, test, shipping and general handling. The failure isnormally caused by brittle fracture in the solder joint throughthe intermetallic compound layer (IMC). Due to yield strength

and higher modulus, Pb-free alloys tend to induce higherstress level at the solder/intermetallic interface in highdeformation rate events such as impact or shock [4, 5].Therefore, intermetallic brittle failures are more likely tooccur. With SnAgCu, the situation becomes aggravated whenit is attached to Ni/Au finishes. The previous work shows thatthe intermetallic formed between SnAgCu alloy and Ni arehighly sensitive to the Cu content in the sphere. Dual IMClayers are formed with Cu content between (0.2-0.6%) [6].The weak interface between the two layers is the primaryfailure location. In contrast, SnAg forms only a single Sn-NiIMC layer during ball attach process and better mechanicalstrength is expected.

This work was conducted to compare the overallperformance of SnAg and SnAgCu on electrolytically platedNi/Au finish. At individual solder joint level, cold ball pull(CBP) test and the ball shear test were used to assess themechanical strength of solder joint under different stressconditions (as-assembled, multiple reflow, high temperaturestorage). At package level, the tray drop and packing drop testwere conducted to provide a close resemblance to realshipping and handling conditions to predict the missing ballrate. At the board level, air temperature cycling andmechanical bending test were carried out to evaluate thesolder joint reliability. Microstructure analysis by SEM/EDXwas performed to compare the morphology and compositionof the intermetallic layers. The relationship between the IMCmicrostructure and mechanical performance are discussed toexplain the differences between the two alloys.

1.2. Key ConclusionsAt individual solder joint level, Sn3.5Ag showed no

intermetallic brittle failure in cold ball pull test under anystress condition. In contrast, 70-100% of the failure mode ofSAC387 was brittle failure. At package level, Sn3.5Agsurvived 8-1Ox more drop cycles than SAC387 in tray dropand packing drop tests. These results indicate that themechanical strength of Sn3.5Ag on Ni/Au pad is considerablystronger than that of SAC387.

243 Int'l Electronics Manufacturing Technology Symposium1-4244-1336-2/07/$25.00 (02007 IEEE

2. Study DesignThis study comprises 5 parts:

1. Assembly of samples and ball attachment with Sn3.5Agsolder alloy and conventional SAC387 solder alloy2. CBP and ball shear measurement3. Tray drop test and packing drop test4. Board Level Reliability5. Cross-sectioning, SEM and EDX

3. Subjects and Methods3. 1. SubjectsIn this study, two solder alloys were studied, namely Sn3.5Agand Sn3.8AgO.7Cu (SAC387), with both alloys supplier by thesame vendor. Table 1 lists the solder composition and testvehicle of the study.

Table 1. Solder Composition and Testvehicle.

Solder Composition 96.50 oSn3.5°/oAg

(wt%) 95.50oSn3.8%AgO.7%Cu

Networking Product:

TBGA and TePBGA;Test vehicle

Transportation Product:

PBGA Spanish Oak

Networking Product: TBGA

(1.00mm pitch and 0.66mm ball

diameter, 1.27mm pitch and

0.76mm ball diameter), andBall Pitch & Ball

TePBGA (1.00mm pitch andDiameter

0.6mm ball diameter);

Transportation Product: PBGA

Spanish Oak (1.00mm pitch and

0.6mm ball diameter)

Solder Pad finishing Electrolytic Ni-Au Plating

3.2. Methods3.2.1. Samples Preparation

4 test vehicles were selected, with 2 TBGA and 2 PBGA.For each test vehicle, 2 lots of samples were assembled,where 1 lot was attached with Sn3.5Ag solder alloy, and theother lot with SAC387. One reflow profile was obtained forTBGA, while another reflow profile was obtained for bothPBGA & TePBGA. Due to the robustness of Sn3.5Ag solderalloy despite the 40C higher melting point than SAC387(2170C MP), it was found that the same reflow profile forSAC387 with peak temperature at 2400C (235 to 2450Crange) works perfectly well for Sn3.5Ag with excellentmelting and formation of reliable solder joint system. Thereflow profile used in the study for TBGA and PBGA aredisplayed in Figure 1 and 2 respectively.

Figure 1. TBGA Reflow Profile

Z.." 81 42 43 #4 45 4 it? En,d

150.0-.

nnin n P n-. n &-nn n (IVIIVVI RT)01000 2000 3000 4000mm

Figure 2. PBGA Reflow Profile

All the samples were assembled and electrically tested perthe test flow designed for each individual product. The moststringent test flow applies to PBGA Spanish OakTransportation Product with 48hrs data retention bakingincluded in the test flow.

After electrical test, some of the samples were put throughdifferent stresses that are shown in Table 2.

Table 2. Sample Stressing

Stress Condition Description

TO As-assembled and tested

Baking at 150°CHigh Temperature

24, 48, 96, 168, 504 and 1008Storage hours

Reflow profile (Tpeak=2400C)Multiple Reflow

lx, 2x, 3x, 6x

3.2.2. CBP and Ball Shear MeasurementCold ball pull and ball shear test were performed to assess

the solder joint strength at time zero, after high temperaturestorage at 1500C, and after multiple reflow.

3.2.2. 1. CBP and Ball Shear Test ParametersDetails of the CBP test and ball shear parameters are

displayed in Table 3 and Table 4 respectively.

Table 3. CBP test speed &jaw size used.

Parameter

CBP Speed 5mm/sec

Jaw Size 0.76mm

Table 4. Ball Shear test parameters & shear tool used.

Parameter

Land speed 300um/sec

Test height 40 um

Test speed 300 um/s

Threshold/test load 1000 gOver travel 100 um

Fall back 10%

Shear Tool Face 650umWidth

3.2.2.2. CBP and Ball Shear Test Failure ModeIn both the CBP and ball shear tests done through this

study, the common and possible failure modes are listed inTable 5.

Table 5. CBP & Ball Shear Test Failure Modes.

Breaking in BulkSolder

Brittle Failure atthe IMC

Lifted SolderPad

CBP Ball Shear

T I_ _ . _ _ _ _ _

3.2.3. Tray Drop Test and Packing Drop TestIn this study, the test vehicle used was the most stringent

TBGA package with the biggest package size among all theTBGA packages assembled in Freescale KL BGA line, andyet the smallest pad to ball ratio of 0.68, and smallest outer-ball-edge-to-package-edge distance of 0.155mm. Two typesof package level drop tests, tray drop test and packing droptest, were carried out using center pedestal support trays toassess the solder joints robustness against vibration & impactshock as shown in Figure 3 and 4. Samples after electricaltesting were dropped through as many cycles as it takes untilmissing ball was found, with a maximum of 20 cycles. Aftereach cycle, the samples were inspected for dropped balls andany broken trays were replaced to prevent dropped ballscaused by chipping from the trays.

_ Test samDles for confirmatione Dummy samples to provide the weight

10 Test Ptrays t

_, l _ A.,1nt~~~1oh trav

1. Lay out 6 test samples in the 1 oth (Bottom) tray.2. Lay out dummy samples in the rest of the trays.3. Fix all the 10 trays with straps.4. Drop 10 trays from 1Ocm height from hard surface in 5 angles as shown by the arrm

Figure 3. Schematic drawing of tray drop test

Pack 2 bundlesintr) rnnim hr)y Packing Completed Drop from 10 anglE

Sample Size: 60 good units/lot(6 units/tray with one unit at 4 corners and 2 units at the centerand rest are dummy units, 5 full trays/bundle plus one empty

trav on toD. 2 bundles/box rer Dackina sDec)

Figure 4. Schematic drawing of packing drop test

3.2.2.3. CBP and Ball Shear Sample Size & Test PositionsFor CBP and ball shear test performed in this study, the

sample size used is 10 samples/cell. 8 outer corner balls were

tested per sample with 2 balls at each corner of the sample.The corner balls were perceived to have the weakest jointstrength as they have the longest distance from the packagecenter point (also called neutral point). Thus the corner ballswill experience highest stress and strain during ball attachreflow process which will result in weaker joint strength.

3.2.4. Board Level ReliabilityIn terms of board level reliability, air thermal cycling

(-40 1250C) test and monotonic bend test that complies withIPC 9702 standard were performed. The PCB pad finish isCu-OSP.3.2.5. Cross-sectioning, SEM andEDX

Microstructure and elemental analysis were conductedusing SEM and EDX to evaluate the intermetallic structureand phases that were present at the joint. The cross sectionedunits were etched for 1 minute in 10% HCI and 900O Ethanolto reveal the microstructures.

R.

I..X

4. Results and Discussion4.1. CBP and Ball Shear Result

As compared to the conventional ball shear test, CBP testhas been found to be a more reliable outgoing metrology togauge the solder joint performance [2]. This is because CBPtest produces more intermetallic brittle failures which is morestringent to differentiate the performance of different elementswithin the lead-free solder ball alloy and the solder pad. In thisstudy, both CBP and ball shear tests were performed at timezero, after high temperature storage at 1 500C, and aftermultiple reflow.

Table 6. CBP strength at Time Zero after assembly.

1.0mm TBGA 1.Omm PBGA 1.0mm TEPBGA

Sn3.5Ag SAC387 Sn3.5Ag SAC387 Sn3.5Ag SAC387

Max (g) 1994 1941 2248 2299 2291 2319

Min (g) 1743 1564 1925 1542 1875 569|

Mean (g) 8 772 2067 1 938 2103 2027

Std.Dev (g)I 5s 7 79 178 98 183

4.1.1. CBP at time zeroFigure 5 and 6 and Table 6 display the TBGA and PBGA

CBP result at time zero (TO) after assembly and after electricaltesting.

The result showed that Sn3.5Ag alloy has zero CBP brittlefailure rate in both the TBGA and TePBGA NetworkingProducts, as well as the PBGA Spanish Oak TransportationProduct. In terms of CBP strength, Sn3.5Ag demonstratedsignificantly higher strength than SAC3 87, thus suggestedhigher mechanical robustness for Sn3.5Ag solder alloy.

4.1.2. CBP after high temperature storage at 1500CCBP result after high temperature storage at 1 500C is

shown in Figure 7 and 8.Sn3.5Ag solder alloy has zero CBP brittle failure rate upto

168hrs HTS for TBGA, and upto 504hrs HTS for PBGA. Thisresult has confirmed that Sn3.5Ag has higher mechanicalrobustness as compared to SAC387 solder alloy.

Figure 7. TBGA CBP result after high temperature storage at 1500C.

Figure 5. TBGA CBP result at TO after assembly and after test.

Figure 8. PBGA Spanish Oak and TePBGA CBP result at TOafter high temperature storage at 1500C.

Figure 6. PBGA Spanish Oak and TePBGA CBP result at TOafter assembly and after test.

4.1.3. CBP after multiple reflowCBP result after multiple reflow is shown in Figure 9.

Sn3.5Ag solder alloy demonstrated zero brittle failure upto 6xmultiple reflow. This result has again shown the mechanicalrobustness of Sn3.5Ag solder alloy as a more reliable Pb-freesolder joint system.

Table 9. Tray Drop Test: Drop Till Fail Result

Figure 9. TBGA CBP result after multiple reflow.

4.1.4. Ball Shear ResultBall shear test has been found to be less stringent in solder

joint testing, as the bond will gain support from the side wallsof the cavity, and cause a bad bond to pass. This is evident inthis study, where zero brittle failure was found in bothSAC387 and Sn3.5Ag solder alloy.

Table 7. Ball Shear brittle failure rate.

1.27mm TBGA 1.00mm TBGA 1.00mm PBGA 1.00mm TePBGAPad-to-ball ratio 0.84 0.68 0.84 0.84

Brittle Failure (Mode 2) in SAC S SAC SAC S

Ball Shear 387 SnAg 387 SnAg 387 SnAg SAC 387 SnAg

TO 0% 0% 0% 0% 0% 0% 0% 0%

Reflow 0 0% 0% 0% 0% 0% 0% 0%

MuItiple Reflow 0 0% 0% 0% 0% 0% 0% 0%Reflow 3x

Ar Reflow 0% 0% 0% 0% 0% 0% 0% 0%Atr Reflow ______

Test 0 0% 0% 0% 0%0%0%0% |%0% 0% o% 0%24 0% 0% 0% 0% 0% 0% 0% 0%48 0% 0% 0% 0% 0% 0% 0% 0%

H1T50C 496 0% 0% 0% 0% 0% 0% 0% 0%

168 0% 0% 0% 0% 0% 0% 0% 0%504 0% 0% 0% 0% 0% 0% 0% 0%1008 0% 0% 0% 0% 0% 0% 0% 0%

4.2. Tray Drop Test and Packing Drop TestTable 8 and 9 showed the Drop Till Fail result for Tray

Drop Test and Packing Drop Test using center pedestalsupport trays. The result showed that Sn3.5Ag solder alloysurvived 8-1Ox more drop cycles in package drop tests whencompared to SAC3 87. From Packing drop test result,Sn3.5Ag has > 6x higher drop-till-fail cycles than SAC387.As for Tray drop test, Sn3.5Ag has > 8x higher drop-till-failcycles than SAC387. This result suggested that Sn3.5Ag hasmuch better mechanical robustness and has at least > 6xhigher resistance against handling and shipping impact shock.

Table 8. Packing Drop Test: Drop Till Fail Result

Figure 10. 1.27mm Pitch TBGA: Weibull plot ofATC test

Figure 11. 1.00mm Pitch TBGA: Weibull plot ofATC test

Figure 12. 1.00mm Pitch PBGA: Weibull plot ofATC test

Solder No of No of Cycle No of units No of ballsComposition Cycle when ball drop with ball drop drop

tested happenedSn3.5Ag 20 No ball drop 0 0

seen

ISAC 3871

3 3l 31

5

4.3. Board Level Reliability TestAir thermal cycling (ATC) data was gathered at -40 to

1250C to demonstrate the 2nd level solder joint reliability ofthe two alloys in terms of thermal fatigue. Statistical analysis(Figure 10 to 13) showed that both SAC387 and Sn3.5Ag haslonger ATC lifetime than SnPb, with Sn3.5Ag having similarfirst failure cycles, but larger mean-cycle-to-failure and widerdistribution.

structures. Compared to the polygonal structures, the finedendrites are more likely to create stress concentration whichresults in crack initiation and intermetallic brittle failure.

(a)

Figure 13. 1.00mm Pitch TePBGA: Weibull plot ofATC test

Monotonic bend test based on IPC 9702 was carried out tosimulate the stress condition that the solder joint willexperience on PCB board during board level electrical test.According to the Weibull plot, SAC387, Sn3.5Ag and SnPbperformed equally well in terms of failure strain.

(b)

Figure 15. IMC structure at TO (a) SAC387; (b) Sn3.5Ag

Further EDX analysis shows the existence of ternary Cu-Ni-Sn IMC at the interface of SAC387 solder and the padmetallization (Figure 16(a)). For Sn3.5Ag, no copper wasdetected to in the IMC, as shown in Figure 16(b). Theelemental constitution of IMC for both alloys did not changesignificantly even after baked at 150°C for 168 hours. The re-deposition of Au into the interfacial IMC layers after hightemperature storage was not observed for both SAC387 andSn3.5Ag which is different from SnPb solder [7]. This is dueto the grain boundaries in Pb-free alloy that does notencourage Au diffusion during thermal aging.

Figure 14. Weibull plot of Monotonic Bend Test

4.4. Cross-sectioning, SEM and EDXfor Intermetallic Study4.4.1. Package Level

Microstructure analysis was conducted using cross-sectioning, SEM and EDX to study the intermetallicmorphology and composition.

Figure 11 shows the SEM images of intermetallic formedbetween the Ni/Au finish and the two alloys, (a) SAC387 and(b) Sn3.5Ag, respectively at TO. Microstructure ofintermetallic from SAC387 samples has dendrite or needlelike structures while that from Sn3.5Ag has polygonal shaped

Varbb lp

SnPbAgSAC387SnA g

Tabh fS tatistksS ha p,. S ah C -, F C

17.8148 5850.06 0.920 3 01 1.7 1 1 25 9 0 4.4 2 0.9 6 9 5 02 2. 1 6 5 35 4 3 9. 5 2 0.8 9 7 5 0

* SAC 387*mSn3.5Ag

0 24 48 96 168

Hiqh Temperature Storaqe (HTS)

Fiauurp 17 1 97mm Pitch TR(CA- Tnt-rmi-tnlic thicknPQQ nfter

200-

1001-

Pm

(a) (b)

Figure 16. Elemental analysis of IMC at TO(a) SAC387; (b) Sn3.5Ag

A lot of effort has been done to examine the Cu-Ni-Snternary phase diagram and explain the differences in themicrostructure between Sn3.5Ag and SAC387 on Ni/Aufinish [6, 8-11]. It is found that the reactions between Ni andthe solders with compositions near the Sn-Ag-Cu ternaryeutectic point depend strongly on the Cu concentration andreflow process [6, 12]. When the Cu concentration is low(<0.2 wt%) and reflow time is short, only one layer of (Ni1,CU,)3Sn4 type of IMC would form at the interface. In extremecase like Sn3.5Ag, x equals 0 and the formed IMC is Ni3Sn4.At concentration between (0.2-0.6wt%), bi-layer IMCstructure that is consisted of (Ni1l-Cu,)3Sn4 and (Cu1l-Ni,)6Sn5will form. And when Cu concentration is high (0.6-3.Owt%),only a continuous layer of (Cu1l-Ni,)6Sn5 will be observed.The current SnAgCu alloy normally has Cu concentrationbetween 0.5-0.9wt%, which has high possibility of formingthe bi-layer IMC structure. Reports have pointed out that thebi-layer structure are prone to interfacial fracture due to brittlecracking in the Cu-rich IMC or within the interfacial area

between the two IMC layers [13, 14]. Therefore, themechanical strength of the SnAgCu alloy on Ni/Au pad issensitive to Cu concentration and process which makes it a

less robust alloy system. This does not happen in Sn3.5Agsolder system as Cu6Sn5 IMC does not exist and there is onlyone IMC layer which is Ni3Sn4.

Figure 17 and 18 show IMC thickness changes at variousstress conditions. Sn3.5Ag has lower intermetallic thicknessesfor all the conditions of HTS and multiple reflow compared toSAC387. This is expected since the reaction rate of Ni and Snis much slower than that of Cu and Sn [8].

(I)

(I)

C

.I-

C.)

.2

43.53

_ 2.52

1.51

0.50

iX 2X 3X

Multiple Reflow

* SAC 387*m Sn3.5AQ

6X

Figure 18. 1.27mm Pitch TBGA: Intermetallic thickness aftermultiple reflow

4.4.2. Board LevelSEM analysis was also performed after the samples were

mounted onto the PCB boards. Figure 19 shows the IMCstructure at the package side for (a) SAC3 87 and (b)Sn3.5Ag, respectively. It is noticed that the microstructure ofIMC of Sn3.5Ag becomes similar to that of SAC387 with Cuincorporated into the originally Ni-Sn IMC layers.

(a) (b)Figure 19. IMC at package side after PCB board mounting:

(a) SAC387; (b) Sn3.5Ag

Cu atoms in the IMC layers of the Sn3.5Ag solder couldcome from two sources as illustrated in Figure 20. The firstsource is the PCB solder pad which is Cu with OSP layer. Thesecond source is the Pb-free solder paste which is normallySnAgCu based. During the board mounting process, Cu atomsdiffuses from the above two sources into the melted solder.This increases the Cu concentration in the Sn3.5Ag solderball and makes it a SnAgCu alloy compositionally. With the

en 6_l 50s 4

=3

X 2E 140a).Ir_

rising Cu level, (Cu1l-Ni,)6Sn5 starts to form at the packageside and changes the IMC structure. Therefore, after the boardmounting process, the microstructure of Sn3.5Ag becomesmore like that of SAC387. Based on this, the board levelmechanical performances, especially the ones that aresensitive to interfacial strength, of Sn3.5Ag on PCB boardwith Cu pad are expected to be at least similar to those ofSAC387, if not better.

Package Side Packaqe Side Package Side

Ni-Sn IMC

Cu atoms

PCB Side

Figure 20. Illustration of microstructure change in Sn3.5Ag during boardmounting to PCB board

ConclusionsFrom the result of the study, it can be concluded that at

package level, the solder joint formed by Sn3.5Ag showedsignificantly lower intermetallic brittle failure rate in CBP testand survived 8-1Ox more drop cycles in package drop testswhen compared to SAC387. This suggests that Sn3.5Ag on

Ni/Au pad is mechanically much more robust than SAC387.The major cause of SAC387's weak strength was the bi-layerintermetallic structure formed when it was jointed to Ni/Aupad. Sn3.5Ag's better performance was related to its singlelayer intermetallic structure. At board level, Sn3.5Agexhibited better or similar ATC and monotonic bend testperformance as SAC387. It was concluded that the additionalCu introduced into the Sn3.5Ag solder joint during the boardmounting reflow from the PCB side solder pad finish andsolder paste changed the microstructure of Sn3.5Agconsiderably and made it became more like that of SAC387.Even so, Sn3.5Ag demonstrated similar first failure cycles,but larger mean-cycle-to-failure and wider distribution.

Based on the evaluation in this work, Sn3.5Ag showsgreat potential of having considerably reduced missing balloccurrence rate than SAC387 while maintaining similar levelof board level solder joint reliability.

AcknowledgmentsWe would like to acknowledge the great support of

Freescale management and MOSTI for sponsoring the fundfor this project. Our gratitude is also extended to Prof DrBurhanuddin Yeop Majlis as the Chairman for this Sector ofUKM. We would also like to acknowledge and thank JimHowell and Paul Galles for conducting all the board levelreliability tests.

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IPCUB Side PCB Side