Lead Free BGAs Soldered with SnPb36Ag2 Solder

Günter Grossmann+ and Giovanni Nicoletti

Laboratory for Reliability Science and Technology, EMPA Dübendorf, 8600 Dübendorf, Switzerland

Electronic components shrink in their dimensions due to space restrictions on electronic printed circuit boards (PCB). This is only possibleon a limited scale for integrated circuits. Their number of connectors increases due to the growing complexity of the semiconductors. Howeverthe miniaturisation of the connectors is limited because of handling constrains. To overcome this dilemma the connectors of the ICs have beenmoved from the sides to the bottom where a matrix of solder balls forms the connection to the PCB, so called Ball Grid Arrays (BGA). Due tolegislative reasons the balls of these components are made of lead free solder. However, there are still applications where tin lead solder ismandatory. It was the goal of the investigation to work out the process parameters to solder lead free BGAs with tin lead solder and to evaluatethe degradation of this mixed technology. [doi:10.2320/matertrans.MI201409]

(Received December 16, 2014; Accepted January 16, 2015; Published June 25, 2015)

Keywords: lead free soldering, mixed technology, degradation of solder joints

1. Introduction

Production restrictions limit the miniaturisation of contactleads of integrated circuits. For the production where thesolder is printed with a stencil a pitch of 0.5mm is the lowerlimit for an acceptable yield. In rare cases ICs with a pitch of0.3mm are used. On the other hand the complexity of thesemiconductors is constantly growing which means that morefunctions can be integrated per square millimetre. This hasthe effect that the packages of the components are growingwhile the size of the chip inside is shrinking. To overcomethis dilemma the connectors on the sides of the ICs have beenmoved to the bottom where a matrix of solder balls connectsthe component with the PCB (Fig. 1). These BGAs offer theadvantage that they accommodate more I/Os with the samepackage size compared to leaded components or that for thesame amount of I/Os a more relaxed pitch of the connectorscan be used which simplifies the production.

Due to the RoHS regulation which came in force by 2004lead containing solder is banned. Thus the connectors of theBGAs have to be made out of lead free solder. However thereare still applications, namely in the aerospace industry, thatneed to be produced with tin-lead solder. Because alsotemperature sensitive components are often placed on thePCB the maximum peak temperature in the solder process islimited. Thus the production parameters need to be optimisedto solder lead free components with SnPb36Ag2 togetherwith the sensitive components.

2. Test Procedure

BGAs with a pitch of 0.8mm with 15 © 15 balls made oftin-lead and ones with lead free balls have been soldered withSnPb36Ag2 solder on a 1.6mm thick PCB made of standardFR4 in a convection oven with varying process parametersand analysed with micro sections.

To evaluate the degradation of the solder joints thespecimens where subjected to passive thermal cycles in aconvection chamber. Since the dominant failure mechanismin solder joints is low cycle fatigue which is strain driven a

slow thermal cycle has been applied to allow for stressrelaxation by creep. The temperature extremes of the testwhere ¹20°C and 120°C. The dwell times at the extremeswhere 60min at ¹20°C and 30min at 120°C and thetemperature ramp was 2°C/min. After 2000, (3500) 4000 and6000 cycles one specimen of each technology has beenremoved from the test chamber and investigated withmicrosections.

Microsections where made through the rows of balls at theedge of the BGAs as well as along the edge of the chip whichoccupied an area of 7 © 7 balls. The microsections whereanalysed by optical microscopy and SEM/EDX.

3. Process Window

In electronics industry the PCBs are soldered in hot airconvection ovens. The solder is applied by stencil printingwith a solder paste that contains the solder alloy together withthe flux. Up to 2004 SnPb36Ag2 with a liquidus of 183°Cwas state of the art. With an upper limit of 245°C for somecomponents the resulting process window of approx. 60°Coffered ample space for measuring inaccuracies, temperaturegradients in the convection oven and variations of the thermalmass on the PCB. Changing to lead free solder altered thissituation considerably. The ternary eutectic SnAg3.8Cu0.7has a liquidus of 217°C which means that the processwindow shrinks to roughly 30°C. Temperature inaccuracies

(a) (b)

Fig. 1 Leaded component (a) and BGA (b) with the same amount of I/Os.

+Corresponding author, E-mail: guenter.grossmann@empa.ch

Materials Transactions, Vol. 56, No. 7 (2015) pp. 988 to 991Special Issue on Nanojoining and Microjoining II©2015 The Japan Institute of Metals and Materials

in a convection oven are around 5°C and variation of thelayout of PCBs might well result in temperature variations ofanother 10°C. Additionally one has to take into account that15°C above liquidus are necessary to enable a diffusion fastenough for mass production. Thus one can easily see thatthere is not much left for variations in the process parameters.

4. Soldering Lead Free Balls with Tin-Lead Solder

When lead free balls are soldered with SnPb36Ag2 solderbelow the liquidus of the balls the balls are partially dissolvedby the liquid solder until the liquidus of the resulting alloy isabove the solder temperature or until the solder process iscompleted (Fig. 2). As soon as the solder temperature isabove the liquidus of the alloy of the balls both alloys mix(Fig. 3).

To characterise a solder process incorporating temperatureand time, Harrison and Vincent1) proposed to use thetemperature® time integral above liquidus of the solder

profile which they called QT (Fig. 4). By varying the peaktemperature and the conveyor speed of the convection ovenand thus the time where the PCBs are in the peak zone of theoven QT can be varied. In this evaluation various settingswhere used (Table 1) were QT is referred to the liquidus ofSnPb36Ag2 (183°C):

The higher the temperature or the longer the time aboveliquidus (and thus QT), the finer is the resulting structurewhich can be seen on the size of the lead phases (Fig. 5).

This means, that the size of the lead rich phases can beused to characterise the structure of the solder after thesoldering process. If the average size of the lead rich phasesis plotted as a function of QT the refinement of the lead richphases is clearly visible. To show the refinement of thecoarse lead rich phases only the areas >10 µm2 have beenconsidered (Fig. 6). The plot shows that after QT is in therange of 2000°Cs, referred to the liquidus of SnPb36Ag2,the distribution of the lead in the solder matrix comes intoequilibrium and doesn’t change considerably with further

Fig. 2 Lead free ball soldered with SnPb36Ag2 at 210°C.

Fig. 3 Lead free ball soldered with SnPb36Ag2 at 226°C.

Fig. 4 Definition of QT with respect to SnPb36Ag2.

Table 1 Settings of the production parameters used in the investigations.

Peak temp[°C]

Conveyor speed[mm/min]

QT[°Cs]

217 900 1400

218 700 1880

223 900 1960

233 900 2480

226 700 2790

246 700 4700

(a) (b) (c)

Fig. 5 Lead free ball soldered with SnPb36Ag2 at 210°C (a), 218°C (b) and 226°C (c).

Lead Free BGAs Soldered with SnPb36Ag2 Solder 989

increasing QT. This means that it is possible to process BGAswith SnPb solder at fairly low temperature. However, to openthe process window it is desirable to solder lead free BGAsbelow the liquidus temperature of the balls with theconsequence that the balls are only partially dissolved inthe solder.

5. Degradation

One of the main concerns in soldering lead free BGAs withSnPb36Ag2 solder below the liquidus of the balls is thedegradation under thermomechanical load. Especially theinterface of the lead rich zone and the original lead free alloyraised concerns regarding the reliability of the solderconnection. Metallographic investigations on this zone, alsowith EDX, showed that both alloys merge seamlessly intoeach other. The tin dendrites of the lead free balls continueuninterrupted into the tin matrix of the solder and the silverand copper rich eutectic between the dendrites dissolves intothe bulk of the solder joint forming precipitations along thegrain boundaries of the tin rich phases together with the lead2)

(Fig. 7).

To verify the stability of the mixed technology, PCBs withlead free BGAs and BGAs with tin lead balls, both solderedwith SnPb36Ag2 at 210°C, have been tested with a passivethermal cycling test.

After 2000 cycles the specimens with lead free ballsshowed no signs of fatigue, while in the solder joints of thespecimens with tin lead balls crack initiation in some balls onthe component side have been observed (Fig. 8). Whether ornot a ball showed a crack depended on the location relative tothe chip. Those balls located at the edge of the chip showedthe early signs of fatigue while the balls at the periphery ofthe BGA where still intact.

After 4000 cycles the lead free balls at the periphery of theBGA showed crack initiation together with clear damage ofthe PCB material while the tin lead balls at the periphery ofthe BGA where still intact after 3500 cycles and the PCBshowed no sign of fatigue. In the environment of the chipapprox. half of the lead free balls as well as of the tin leadballs where damaged (Fig. 9).

After 6000 cycles all balls in the environment of the chip,lead free ones as well as tin lead balls, where completelybroken. At the periphery of the BGA most balls wherecompletely broken. What’s more in both cases the PCBmaterial below all balls was severely damaged in theenvironment of the chip as well as at the periphery of theBGAs (Fig. 10).

The average length of cracks in the environment of thechip was very similar in both kinds of solder balls as well asthe percentage of solder joints with cracks longer than 50%of the wetted length of the pad (Fig. 11).

6. Conclusion

The concept of the QT is well suited to characterise asoldering process. The evaluation of the soldering productionof lead free BGAs with SnPb36Ag2 solder with the help of

Fig. 6 Average size of lead rich phases greater than 10 µm2 as a function ofQTSnPb36Ag2.

SEM Sn Pb Ag Cu

Fig. 7 EDX mapping of the interphase of the lead free ball and SnPb36Ag2 solder.

(a) (b)

Fig. 8 Lead free balls (a) and tin lead balls (b) at the edge of the chip after 2000 thermal cycles.

G. Grossmann and G. Nicoletti990

QT showed no metallurgical problem. The material of theballs dissolves partially in the solder until an alloy is formedwhose liquidus is above the peak temperature of the processor until the soldering process is completed. If a total solutionof the ball in the solder is required the liquidus of the alloy ofthe ball must be exceeded but a QTSnPb36Ag2 of 2000°Cs issufficient.

The transition zone of the lead free balls to the SnPb solderis not a weak point in the solder joint. The degradation occursalong the interphase of the pads to the solder. It seems as ifin lead free balls soldered with SnPb36Ag2 cracks initiatesomewhat later than in tin lead balls. However, thedegradation catches up and after 6000 cycles all balls at theedge of the chip are broken in both cases while at theperiphery of the BGA the balls are not completely broken but

the PCB degrades. However, a considerable spread in theextent of the damage has been observed. Only partiallydamaged solder joints have where found right beside totallyruptured ones. Thus, no higher risk of failing solder joints oflead free BGAs with SnPb36Ag2 compared to BGAs with tinlead balls soldered with SnPb36Ag2 could be proved in thetests especially when considering the wide variations indamage observed.

REFERENCES

1) M. R. Harrison and J. H. Vincent: Improved design life and environ-mentally aware manufacturing of electronic assemblies by lead-freemanufacturing, www.leadfreeassembly.com (1999).

2) G. Grossmann, J. Tharian, P. Jud and U. Sennhauser: Sold. Surf. MountTechnol. 17 (2005) 10­21.

(a) (b)

Fig. 9 Lead free balls (a) and tin lead balls (b) at the periphery of the BGA after 4000 (a) and 3500 (b) thermal cycles.

(a) (b)

Fig. 10 Lead free balls (a) and tin lead balls (b) at the periphery of the BGA after 6000 thermal cycles.

Fig. 11 Average length of cracks in % of the wetted length of the pad and number of balls with cracks longer than 50%.

Lead Free BGAs Soldered with SnPb36Ag2 Solder 991