high speed pull test characterization of bga solder joints

High Speed Pull Test Characterization of BGA solder joints

Anna Tiziana Valotaa, Aldo Losavioa, Loic Renarda, Antonello Vicenzoba STMicroelectronics

Via Olivetti, 220041 Agrate, Milano, Italy

b Dipartimento di Chimica, Materiali, Ingegneria Chimica "Giulio Natta", Politecnico di MilanoVia Mancinelli, 720131 Milano, Italy

Abstract effective mechanical properties of aged solder joints andmodel the solder joint mechanical behaviour under highThe changeover to lead-free solder and components sri aedfrain

metallization in conjunction with the market transition to the standate hf pportable products is expected to have a strong impact on . . .the .reliability of lead-free electronics. For handhel reliability is a board level test method, which is both timetherelalerictprodu pead-frtulatrconic hs. Forhaisheldo consuming and expensive. So far, no feasible component

sldetr fractur pinucdb dopimp. Existn tes level test method has been found for the assessment ofs joint ...drop... . . shock loading reliability of solder joints. Researchersmethods used to evaluate solder ball attachment, shear from Hitachi proposed a micro-impact test method

and pull test, thus far have not been considered suitablefor the evaluation of shock reliability, due to their i r classical Charpy impact test [3]. Basedinability to simulate the high strain rate deformation on thiswork a mcro-mpact test machne was realsedwhich characterises impact loading. Recently, pull test and applied to measure BGA ball / UBM pad bondequipment enabling high speed testing has become strength [4].commercially available, thus calling for further study to Specific test methods are used for measuring joitassess its applicability for the measurement of joint strength, i.e shear test and pull test. The later was shownstrength under dynamic load conditions. In this paper, to be superior to the ball shear test as a means to identifyboard level drop test and component level pull test results weak interface in solder joints [5]. However, shear andare reported and compared for different BGA assembly pull tests are not considered suitable for evaluation ofalloys and reflow cycle. Pull testing is performed at joint reliability under shock or drop loading [4,6], sincedifferent test speed on BGA lead-free solder joints after the applied test speed, usually lower than 1 mm s-', isreflow and after thermal ageing in order to investigate the well below the impact velocity applied to the solder jointcorrelation between failure analysis results and lead-free in a drop test. Recently, pull test equipment enabling testjoints microstructural evolution. High-speed pull testing speed up to 102 mm s-' has become commerciallyof solder joints is shown to be a promising test available, thus justifying further work on its applicabilitymethodology for the evaluation of solder joints brittle also for the evaluation of joints reliability under dynamicfracture resistance. Moreover, high speed pull test results load conditions [7].are shown to qualitatively correlate with drop test In the present work, board level drop reliability dataperformance if the failure mode is taken as criterion. for plastic BGA packages with lead-free solder balls are

reported. In addition, pull testing is performed at different1. Introduction speed in order to investigate its feasibility as a component

The microelectronics industry is facing important level test method for the assessment of joint strengthchanges in manufacturing processes and technology with under high strain rate loading. BGA joints, reflowed andalso an impact on reliability testing and metrology. This is thermally aged at different temperature, are tested andmainly the result of the trend towards steadily increasing then inspected by optical microscopy to determine failureminiaturization and portable electronics as well as the mode. Cross-sectioning and optical microscopyoutcome of legislative pressure pushing towards green examination is used to investigate microstructureelectronics manufacture and products. evolution of aged joints for failure analysis.

In this newly defined context, great emphasis is placedon drop reliability of lead-free ball grid array (BGA) 2. Experimental . 3interconnect [1]. Research efforts currently focus on the The component test vehicle was a7.7x9xo1.2mmTunderstanding of the effects of thermal aging and plastic BGA package with 40 solder joints. The surfaceintermetallic compounds (IMC) formation on drop finish of the BGA copper substrate was an OSP layer. Thereliability, both at phenomenological and fundamental solder joints were composed of Sn-3.5Ag-0.75Cu (SAC)level. Modelling and simulation for board level drop test and Sn-2.5Ag-0.8Cu-0.5X (SAC-X, where X is anis as well intense; a life prediction model has been alloying element). A standard lead-free surface mountrecently proposed providing good correlations with temperature profile was used for solder reflow. For theimpact life experimental results [2]. In this frame of work, SAC solder, the peak reflow temperature was either 255a further direction of development is to establish the or 235°C (in the following also referred to as SAC and

1]-4244-0276-X/06/$20. 00®)2006 IEEE -4-

7th. Int. Conf: on Thermal, Mechanical and Multiphysics Simulation and Experiments in Micro-Electronics and Micro-Systems, EuroSimE 2006

SAC-A, respectively); for the modified SAC solder, the a failure was reported whenever the overall electricalpeak reflow temperature was 255°C. resistance of a test unit was higher than 1500 Q.

The board level drop test was performed according to Pull test measurements were performed with aan internal specification, which makes use of a test Bondtester Dage Series 4000, changing test speeds fromequipment conform to that prescribed in Jedec Standard 200 to 500, 2000 and 5000 [t s-1, i.e. over the whole rangeJESD22-B1 11 [8]. A schematic of the test apparatus is of speed allowed by the equipment. Additional testingshown in figure 1. For drop testing, a 101 x48x I mm3 test was carried out at lower (25 pi s-') or higher (105 p s-1)board was used with the layout schematically shown in test speed with a Bondtester Dage Serie 4000 HS. Afigure 2. Different component locations on the board have picture of the Bondtester head is presented in figure 2different stress conditions and will therefore fail at with an enlarged view of the tweezers. Thanks to theirdifferent time. As a rule, test components mounted on size and shape, the tweezers allow testing a single joint atlocations 5 to 8 show lower drop life compared to a time. As shown in figure 2, the tips of the tweezers arepackages at the corner locations. suitably designed, with a hemi-cylindrical slot in each

arm, in order to firmly seize the solder ball. As aconsequence, the solder ball takes the shape of a button

l est bll _ head rivet or of a mushroom as the tool is closed aroundit. For the sake of reliable testing, the tweezers close

lllll l -BasepltP earound the ball only after reaching the substrate., I I I I II I I I L:_PCB a8teft1qDrop Wble

WOO rodI SW Of

Bate plateDrop t bi

Rigid base

Figure 1. Schematicfor board level drop test.

Figure 3. The head ofthe Bondtesterforpull test4 L | [ lN lN performance and view ofthe tweezers.

Package samples with solder joints of composition Sn-3.5Ag-0.75Cu (cold reflow) and Sn-2.5Ag-0.8Cu-O.5Xwere subjected to isothermal ageing in nitrogenatmosphere at 150, 165 and 180'C for 24 and 168 hours.After ageing, the samples were subjected to pull test, in

Figure 2. Schematic ofthe drop test board layout. order to investigate joint reliability under variousaccelerated aging conditions. Pull tests were executed on

The board was fixed to the drop table at the four 320 solder joints for each lot. Joints microstructure at thecorners with mounted packages facing downward Cu pad / solder interface was examined by optical andBetween the base plate and the board a space of 10 mm is scanning electron microscopy. Samples for cross sectionleft, so that the board can freely bend. The drop table was examination were prepared by standard procedure with aallowed to fall freely along the two guiding rods from a final polishing with 0.05 ptm alumina abrasive. Theheight of 1.5 m to impact repetitively on the strike purpose of this investigation was to characterize the effectsurface. Each time a half-sine peak acceleration pulse is ofthermal ageing on joints strength.produced with peak acceleration of 1500 G and pulseduration of 1 ms. Monitoring of the impact pulse 3. Results and discussionparameters was provided by an accelerometer attached to The drop test was performed on ten boards for eachthe drop table. Test components and board were daisy- test condition (solder alloy / reflow temperature). Failurechain connected for resistance monitoring, analysis of drop test data showed that the observed

The overall electrical resistance of the solder joints Of behaviour can be represented by a 2-parameter Weibulleach package on the test board was monitored using an distribution function. The graph produced by the Weibullevent detector. The failure criterion was defined such that analysis is presented in figure 4, where data for all the

three test conditions considered are reported.


The values for the Weibull analysis are given in table The results were collected in box-plot graphics, with1, where the correlation coefficient p gives a measure of test speed on the abscissa and pull strength on thethe reliability of the linear regression. ordinate. The graph in figure 5 presents pull strength

results obtained at changing test speed on the differentsolder joints under study. The median value of the

- 2M measured strength for each speed and for each sample~A~G a X / Jtype is reported along with the inter-quartile range. TheAC- x25390.00_-- maximum length of each whisker is 1.5 times the==_ /_ ___7_ -= = _ __ interquartile range. Values outside this range are taken as

A__S A - L___ __ - outliers. As shown by the graph in Figure 5, the general50.00 111 _ _ _ __ trend of pull strength versus test speed is to increase as

- _ _ A _ O ___ _ test speed increases, in agreement with published results._ - _ - _ 7t _ __ _ [7]. A further striking feature of these data is the

rooo /E A 4 difference in the dispersion of the measured pull strength1000- among the different lots and its change with test speed

X X = = _ _< == _ _ _ increase. The SAC solder joints (lot 1 in Figure 5) exhibit

Figure=- ____the largest range between minimum and maximum and a

_ _____ / _ _ _ _ distinct tendency to increase the dispersion range as test/ / ~~~~~~~~~speedincreases. For example, the pull strength dispersion

ln00 e X range increases from 122 at 200 ts-1 to 292 at 5000 ts-1.1 0.00 1 00.00 1 000.00

number of drops tofailure On the other hand, the pull test behaviour of cold reflowSAC-A (lot 2) as well as SAC-X (lot 3) solder joints is

..4Wilralylodotofailure of characterized by a limited dispersion range, whichSAC (circe symbol), SAC-A (triangk) and SAC-X remains almost unchanged with increasing test speed up

solderjoints (square). to 2000 ts-'. Further increasing test speed to 5000 ts-', asensible increase of pull strength dispersion is apparent

The characteristic life parameter rl is an index of also for lot 2 (from 61 at 200 ts-1 to 10 at 5000 Vts-') andimpact life. This is considerably higher for the modified lot 3 (from 47 at 200 ts-1 to 161 at 5000 ts-1).solder composition SAC-X compared to SAC solderjoints. On the other hand, lowering of the peaktemperature reflow to 235°C gives a small improvement IOLu1 Omxr "|for SAC solder. The Weibull slope or shape parameter J, lwhich is linked to the failure rate after crack initiation,shows remarkably different values. This may besuggestive of different failure mechanism between SACsolder alloy with reflow peak temperature of 255°C ascompared to both the SAC solder alloy with reflow peak I Y btemperature of 235°C and the modified SAC-X solder.

Table 1. Resultsfrom Weibull analysis ofdrop to lfailure data oflead-free solderjoints. 2 3 1 2 3 1 2 3

Solderjoints ,B C p

SAC 2.08 44.7 0.94 SAC hbt reffbw 2 SA&X 3 SAG-A - Id g1owSAC-A 3.24 84.5 0.91SAC-X 3.49 520 0.98 Figure 5. Box-plot of the pull strength versus test speed

from pull testing ofsolderjoints (1: SAC; 2: SAC-X; 3:It was also observed that failure occurred as a results SAC-A).

of fracture at the solder to package interface. Thisobevto is of cors eseta totesoeo After testing, failure mode analysis was performed by

corariong dopfa pull testehaviour, inspection of each test site by optical microscopy. TheTheparinstud of

u

thef t of test s d area fraction of the solder remaining on the fracturesurface is used to provide an estimate of the ductility ofandc fnifiire modei wasn expec-te-d to prov1deii an altemnatiAveandfaiuremodwa exectd t prvid analtrnaive the solder joints. Accordingly. the failure mode can be

route to evaluate the mechanical behaviour of solders clsife as dutl'rbl odrfiue ie alrjoints under high strain rate loading, as also suggested in an M alr. In patclr'o h ups ftiOther work [6,7]. For each lot 320 pull tests were anIM alr.nprtca,fothpuosofhs

executed, pefrmn 10 mesrmnsfrec,pe study, the mixed mode fracture was classified as brittlevalue~~~~~~oneih difrn.akgsfo igelt whenever at least 1000O of the pad area was covered with

exposed IMG. For each test sample a fracture mode map


was prepared and the percentage ofjoints showing ductile(brittle) fracture according to the above criterion was usedto evaluate the quality of the solder joints on the package.

The results of failure analysis are graphicallysummarized in Figure 6, where the incidence of brittlefracture, as defined above, versus pull test speed is10reported. From this plot the following comments andobservations can be drawn: decreasing the reflow peaktemperature of SAC solder from 255 to 2350C causesjoints ductility to improve substantially; the fracture modeof cold reflow SAC joints changes from ductile to brittleas the test speed increases; the incidence of brittle fracture n dneriieFateI%for SAC joints increases as test speed increases; thefailure mode of SAC-X joints is ductile irrespective of the Fiue7aCaatrstclfaarmtrlro ebltest speed. A further effect of the increase of pull testing anlsso rpts aavru rtl alr niecspeed is a general decrease of the solder area fraction on i ultsig(00fs)o G onsthe pad in the mixed mode fracture, that is an increase ofbrittle fracture percentage, as most clearly shown by pule etorSCAadSC-fodrjinswsasSAC-A solder joints behaviour, peromdatruhralgin.Teefctolgigo

-9- SAC 25-C SAC 35-C SAC-Xfailure incidence as a function of ageingtiea15an

-+- SAC- 255CC -~A-~ SC - 235C SAC-X 1 80'C in the graphs of figure 8.

100 _________________eo SAC-X ~~15000

800~~~~~~~~~~~~~~~~~~~~~~~~~4

60-~~~~~~~~~~~~~~~~~~~~~~~4260~~~~~~~~~~~~~~~~~~~~~~~2o200-

~~~~~~~ 40 ~ ~~~ ~ ~ ~ ~ ~ ~~~~~~~4

01 100 10000 1000000

Pull Test Speed pm s-1 LDiiI SAC-A 15O!QC

Figure 6. Brittlefracture percentage ofBGA solder _joints versus pull test speed.40............................

The effect of test speed on joints failure mode is of ____________________particular relevance also in connection with the drop test ____________________results. This can be appreciated considering the pull test ___________________results obtained at the highest speed value tested on allsamples, 5000 hts-'. At this test speed, SAC joints

40subjected to hot reflow show always a brittle fracturemode; fracture of SAC-X joints is almost exclusively inthe solder (200 brittle fracture incidence); cold reflow 0 2 O s. io M M G 0

SAC joints exhibit an intermediate behaviour, with a 200o AgigTmincidence of brittle fracture. Figure8 rtl rcueicdneo G odrjit

These results are consistent with the board level drop versus ageing tim t10ad10C bv A-test performance, as shown in the graph of figure 7, where jit;blwSC-jonstetped50 u.Ththe im-pact life -parameter ri (see table 1) is -plotted versus lnsaedana ud oteee

minimum brittle failures percentage observed. After and after thermal ageing at 150 and 180°C for 168 hoursageing at either 150 or 165°C (for the later T data are not are reported in figure 9. The interface microstructure atshown), a different behaviour is observed between SAC- time 0 (as-reflowed) shows the scallop-like morphologyA and SAC-X joints, while ageing at 180°C produces a of the Cu6Sn5 intermetallic layer. A very thin layer ofsimilar degradation of the solder joints ductility. The another phase could also be seen between the Cu andbrittle failure incidence of SAC-A joints does not change Cu6Sn5. This phase was later identified by cross-sectionwith ageing time at 150 and 165°C, though the average TEM analysis, not reported here, as the Cu3Sn phase. Thevalue increases from about 10% to 20%; on the contrary, thickness of the Cu3Sn was in the range of 100 and 200the brittle failure incidence of SAC-X joints increases nm for both SAC-A and SAC-X joints. The interface iswith ageing time at all temperatures. In addition, the rate much rougher for SAC-X joints as result of the formationincrease of brittle failure incidence of SAC-X joints of elongated and thinner scallops, which may explain theincreases with the ageing temperature. increased ductility of SAC-X joints compared to SAC-A.

The average pull strength was found to decrease with Upon ageing, the interfacial structure shows drasticageing time at all temperature for both SAC-A and SAC- changes following a similar evolution for both SAC-AX joints. In particular, irrespective of ageing temperature, and SAC-X solder joints. The main features of theSAC-A joints showed a higher rate decrease of the pull microstructure evolution are flattening of the Cu6Sn5strength. This behaviour can be qualitatively understood scallops, growth of the Cu6Sn5 phase and the fastin terms of the balance of different factors: softening of thickening of the Cu3Sn layer. The growth rate of IMCthe solder alloys and increasing tendency to brittle layers is graphically shown by the bar-plot in figure 10.fracture as a function of both ageing time andtemperature. This is consistent with the above observation SAC-Ah, = " SAC-Xthat the rate increase of brittle fracture of SAC-X joints = CUSNincreases with ageing temperature. In this respect, it isalso important to observe that, after ageing at 180°C for 09 lOU168 hours, the pads showing a mixed mode failure had all E 60 8D

The degradation of the Joining ductility, as 0 1 1io<demonstrated by thes ensible increase of thebrittle failure 15on 18B.Cincidence, is primarily a result of the morphologicalchange and the intermetallics growth at the solder / pad F.

failure,~~~ ~ ~ ~ ~ ~ ~~~~Iaeonlhalepacagthe /soldehadn zntefacthnfter rite

nterface. The mcrostructural evolution of the joints ratinterfac. The mcrostrutural eolutionof the oit reflow and after thermal ageing at 150 and 180 Cforinterface was examined after ageing at 150 and 180'C for r nm a n 1 n1 f168 hours. Optical micrographs of the package pad / 168hours.solder interfaces of SAC-A and SAC-X joints after reflow

SAC as-reflowed 1500C1 168 180hCC 168 h

SAC-X:as-refl 1500C X 168 h 1800C/168 h

Figure 9. Optical micrographs ofpackagepad to solderjoint interface after reflow and after thermal ageing at150 and l80°Cfor 168 hours (SAC-A, above; SAC-Xbelow).


Micro-voiding formation could not be unambiguously can be used as an indicator of board level drop testdetected by optical microscopy, most likely as a performance. At the current stage of this study, thisconsequence of screening by smeared out material during correlation cannot be considered but tentative;metallographic preparation. Micro-voiding was more nonetheless it is a promising result for furthereasily detected by scanning electron microscopy development.examination of the joint interface structure. An example isshown in figure 10, where a SEM micrograph of the A nedgments

co of a SAC-X/Cu joint after ageing at 150'C The authors would like to thank Xavier Baraton, Luccross-section Of a Petit/wAonle g1ga Ofor 168 hours is reported. A chain of small voids can be t, rmelle Kapferer, Julen Fortel for their assistanceseen mostly along the interface between the Cu3Sn layer with microstructural analysis.and the Cu pad, as expected with copper being the Referencesdominant diffusing species. As also recently pointed out 1. Tee, T.Y., Ng, H.S., Lim, C.T., Pek, E., Zhong, Z.,both for SAC [6] and SnPb [9] solder joints on bare Cu, "Impact life prediction modeling of TFBGA packagesthe formation of micro-voiding appears to relate to brittle under board level drop test", Microelectronicsfracture of solder joints. Reliability, Vol. 44 (2004), pp. 1131-1142.

2. Yi-Shao, L., Ping-Feng, Y., Chang-Lin, Y.,"Experimental studies of board-level reliability of

SAC-X ci-scale packages subjected to JEDEC drop testcondition" Microelectronics Reliability, Vol. 46(2006), pp. 645-650.

3. Date, M., Shoji, T., Fujiyoshi, M., Sato, K., and Tu,~~~~ ~ ~ ~ ~ ~ ~ K N,"Ductile-to-brittle transition in Sn-Zn solder

joints measured by impact test," Scripta Materialia,Vol. 51 (2004), pp. 641-647.

........4. Shengquan, 0., Yuhuan, X., and Tu, K N., Alam,M.O., and Chan, Y.C., "A Stud of Impact ReliabIlity

...........1 |||||||||--of Lead-free BGA Balls on Au/Electrolytic Ni/CuBond Pad", Mater. Res. C. ymp.,Proc. Vol.MRS 2005, Paper B 10.5.1-6.

5. Coyle, J.R., Seraf no, A.J., Soan, P.P., "Ball S earversus Ball Pull Test Methods for EvaluatingInterfacial Failures in Area Array Packages", Proc2soler/C innual IEESM Internaional ElectrnicsManfacturing Techno Symposium, IEMT 2002

hours(standsforCu6Sn5mforCu3Sn).San Jose, C, July, 2002, pp. 200-205.in ~~~ ~ ~ ~ ~ ~ ~~~~6.Ci,T.C., Zeng, K., Stierman R., and Edwards, D.,

Figue 1. Coss ecton icrorap thou htheSAGAno, K., "Effect of Thermal Aging on Board Level

Figre 1.ros setio mirogaphthrughtheSAC Drop Reliability for Pb-free BGA Packages", Proc.Xsolder/Cu interface after ageing at l5O0Cfor 168 54th Electronic Components and Technology

hours ( i7 standsfor Cu6Sn5; efor Cu3Sn). Conference, Las Vegas, June 1-4, 2004, pp. 1256-1262.

4. Conclusions 7. Newman K., "BGA Brittle Fracture - AlternativeThe main conclusions from pull testing ofBGA solder Solder Joint Integrity Test Methods", Proceedings

joints at changing test speed is that the incidence of brittle 2005 Electronic Components and Technologyfailure (i.e., according to the present study, fracture mode Conference" pp. 194-1201, 2005.with at least 10% area fraction of IMC on the pad 8. JEDEC Solid State Technology Association, JESD22-surface), as well as the IMC exposed area in mixed mode B 1: "Board level drop test method of componentfracture, increases as test speed increases. Therefore, for handheld electronics products", 2003.these findings confirm that pull test is a viable 9. Zeng, K., Stierman R., Chiu, T.C., and Edwards, D.,methodology to evaluate brittle fracture resistance of Ano, K., Tu, K.N., "Kirkendall void formation insolder joints. eutectic SnPb solder joints on bare Cu and its effect

As to the correlation between board level drop test on joint reliability", J. Appl. Phys., Vol. 97 (2005), pp.performance and component level pull test behaviour, it is 24508/1-8.found that the impact life parameter from Weibullanalysis of drop test results is anti-correlated with theincidence of brittle fracture in pull testing of BGA solderjoints. In other words, as already pointed out by others[6], the non-ductile fracture mode incidence in pull testing

-6-7th. Int. Conf: on Thermal, Mechanical and Multiphysics Simulation and Experiments in Micro-Electronics and Micro-Systems, EuroSimE 2006

high speed pull test characterization of bga solder joints

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