Reflow profile study of the Sn-Ag-Cu solder

Reflow profile study of the Sn-Ag-Cu solder

B. SalamSchool of Engineering, University of Greenwich, Kent, UKC. VirsedaEuropean Fuel CellH. DaSchool of Engineering, University of Greenwich, Kent, UKN.N. EkereSchool of Engineering, University of Greenwich, Kent, UKR. DurairajSchool of Engineering, University of Greenwich, Kent, UK

Introduction

Tin lead solders are the primary materials used forinterconnecting electronic components. However, the safeuse and disposal of lead-containing electronic products is anissue that is causing a move in the consumer electronicindustry to remove lead from products. It is widelyanticipated that the use of lead containing solders by theelectronic industry will be seriously constrained by alegislative ban on lead use in solders. An example of suchlegislation is the environmentally conscious engineering inelectronics committee in Japan, which has scheduled thatlead-free solders should be standardised from 2003.Furthermore, European legislation under the Waste fromElectrical and Electronic Equipment (WEEE) andRestriction of Hazardous Substances (RoHS) Directives isscheduled to eliminate the lead from electronic products bythe year 2006. Besides this legislation, some Europeancountries have considered imposing new regulationsrequiring the manufacturers to take full responsibility for therecycling of their products. In addition, switching to lead-free soldering can bring some advantages, in particular byimproving the reputation of companies, who areenvironmentally conscious.

The industry has embarked on a number of studies insearch of suitable lead-free alternatives, but yet there are nodrop-in solutions with respect to reflow temperature, jointreliability and assembly costs. Our survey shows that the Sn-Ag-Cu alloy is one of the most promising lead-free alloyscurrently being evaluated by the industry. However, Sn-Ag-Cu is a high melting point alloy (2178C). Furthermore, themaximum soldering temperature of electronic components/parts is often fixed. Hence, the safety window is reduced insize. Therefore, the temperature profiling for this alloybecomes more difficult and challenging than for the eutecticSn-Pb alloy.

A study of the Sn-Ag-Cu alloys was conducted by Hwang(2001). The composition of the studied Sn-Ag-Cu alloys was96.5-92.3 per cent Sn, 3-4.7 per cent Ag and 0.5-3 per centCu. The study found that the microstructure of the Sn-Ag-Cualloys consisted of Cu6Sn5 and Ag3Sn intermetallics in a Sn-matrix. These intermetallics (the Cu6Sn5 and Ag3Sn) werefound to effectively strengthen the alloy and act as a barrierto fatigue crack propagation. In addition, they also act aspartitions within the Sn-matrix grains, thereby producinga finer microstructure. The finer the Ag3Sn and Cu6Sn5intermetallics are, the more effectively they partition theSn-matrix grains, resulting in an overall finer microstructurethat can facilitate grain boundary sliding mechanisms, andwhich in turn will lengthen the life of a solder joint.The presence of these intermetallics in the bulk solder forthe traditional Sn-Pb alloy is hardly seen; hence, the

microstructure of a Sn-Ag-Cu lead-free solder joint isdifferent from that of a traditional Sn-Pb solder joint. Sincemost of the currently available studies of the reflow profileare based on the Sn-Pb solder alloy, the influence ofdifferent types of the reflow profile on the microstructure ofa Sn-Ag-Cu solder joint is unknown. Thus, this is one of theobjectives of this paper.

The reflow profile could affect the reliability of a solderjoint, because it is one of the factors that influence theformation of the intermetallic layers in a solder joint. Theintermetallic layer is in fact a crucial part of a solder joint.Although having an advantage in facilitating bondingbetween the solder and substrate, it has a disadvantage that itis generally the most brittle part of the solder joint (Frear,1991). Thus, it must be as thin as possible. Therefore, a goodreflow profile must produce solder bumps with a thinintermetallic layer.

Reflow profiling was extensively studied, for example, byLee (1999), Skidmore and Waiters (2000), Suganuna andTamanaha (2001), Suraski (2000) and Yang et al. (1995).Suraski (2000) studied the ramp-to-spike (RTS) reflowprofile. Suganuna and Tamanaha (2001) discussed theavailable reflow technology for lead-free soldering.Experiments were carried out to determine which fluxchemistries, lead-free alloys and reflow profiles had thegreatest influence on solder joint quality in terms of wettingability, solder balls, solder splashes and voids (Skidmoreand Waiters, 2000). A study of eutectic Sn-Ag reflowprofiles has also been reported by Yang et al. (1995), whostudied the effect of the soldering temperature, solderingtime and cooling rate. A study analysing the types of defectsaffected by the reflow profile has also been reported by Lee(1999). Some of these studies, such as Yang et al. (1995), areclosely related to the work reported here. However, therehave been no reports of the effect of the reflow profile on theintermetallic layer thickness and microstructure for the Sn-Ag-Cu alloys. Therefore, this paper presents a study of thereflow profile for the Sn-Ag-Cu alloys. The goal of the workwas to determine a Sn-Ag-Cu reflow profile that results in athin intermetallic compound (IMC) layer and finemicrostructure solder joint.

Experimental design

The experiment studied two types of reflow profiles. Theywere the ramp-soak-spike (RSS) and ramp-to-spike (RTS)profiles. Examples of these profiles are shown in Figure 1.Each of these profiles was investigated using a set offactorial design experiments. The design parameters for theRSS profile were the soak temperature, peak temperature

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KeywordsSoldering, Surface texture,

Surface treatment

AbstractA study of the Sn-Ag-Cu lead-free

solder reflow profile has been

conducted. The purpose of the

work was to determine the Sn-Ag-

Cu reflow profile that produced

solder bumps with a thin

intermetallic compound (IMC)

layer and fine microstructure.

Two types of reflow profiles were

studied. The results of the

experiment indicated that the

most significant factor in

achieving a joint with a thin IMC

layer and fine microstructure was

the peak temperature. The results

suggest that the peak

temperature for the Sn-Ag-Cu

lead-free solder should be 2308C.

The recommended time above

liquidus is 40 s for the RSS reflow

profile and 50-70 s for the RTS

reflow profile.

Received: 17 March 2003Revised: 6 November 2003Accepted: 6 November 2003

Soldering & Surface MountTechnology16/1 [2004] 27–34

q Emerald Group PublishingLimited[ISSN 0954-0911][DOI: 10.1108/09540910410517022]

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and cooling rate. The parameters for the RTS profile werethe peak temperature, time above liquidus and cooling rate.

The purpose of the soak temperature in the RSS profile isto provide heat to an assembly gradually and uniformly. Ingeneral applications with Sn-Pb solder, the soak temperatureis between 150 and 1708C (Skidmore and Waiters, 2000).However, the soak temperature for the high-temperaturesolder alloys is between 170 and 1908C (Lee, 1999). Thissoak temperature setting is critical because the assemblytemperature should be raised smoothly and uniformly to thepeak temperature so that the temperature differentialbetween the components is minimised. The soaktemperature was therefore investigated.

Solder paste manufacturers recommend that the peaktemperature is as high as possible to aid wetting, butcomponent manufacturers advise to keep the lowest peaktemperature possible to prevent component damage(Wickham and Hunt, 2001). A low-peak temperature is alsobelieved to create a thinner IMC layer (Yang et al., 1995).In this study, two peak temperatures were investigated:high ð250^ 58CÞ and low ð230^ 58CÞ:

Extending the time at peak temperature permits anycomponent with a large heat capacity to reach the requiredreflow temperature. The best results, in terms of goodwettability, no solder balls and no voids, have been found tooccur with a RTS profile with time above liquidus of90-120 s (Skidmore and Waiters, 2000). However, too muchheat input above the solder melting point leads to excessiveintermetallic formation. Therefore, two different timesabove liquidus for the RTS profile were investigated: short(40-60 s) and long (90-120 s). For the RSS case, the timeabove the melting point must be kept as low as possible. Inthis profile, the time above liquidus depends on the peaktemperature, e.g. for the peak temperature 2308C, the lowesttime above liquidus was 40 s, whereas for the peaktemperature 2508C, it was 60 s.

A fine microstructure solder joint could be produced bya fast cooling rate (Prasad, 1989). However, electroniccomponents are generally vulnerable to thermal shockcaused by a fast cooling rate; thus, it is necessary toinvestigate which cooling rate should be applied and itseffect towards the microstructure of the Sn-Ag-Cu lead-freesolder alloys. Therefore, two cooling rates were studied:slow, 28C/s and fast, 48C/s.

In summary, the experiment has three factors with twolevels, the details of which can be seen in Table I.

The combinations of these parameters were assigned tothe L4(2

3) orthogonal array as shown in Table II. Therefore,each type of reflow profile had four combinations of factorsto be tested, as shown in Table III.

Experimental procedure

The test vehicles used in this study were FR4 substrates withthree solder bumps on 2.5mm diameter circular Cu pads, asshown in Figure 2. The procedure to make the solder bumpsstarted from manually printing the solder paste using astencil after the bare Cu pads of the test vehicle had beenscrubbed and cleaned using iso-propanol. The solder pasteused for printing was Sn-3.8Ag-0.7Cu, with a type 3 particlesize distribution and 89 wt per cent metal content. Thestencil was 0.7mm thick and had three round apertures of2.5mm diameter. To ensure consistent printing, printingtests were conducted. The height of the solder paste depositproduced by the printing test showed good yield with 20mmdeviations between the deposits. This repeatable and goodyield manual printing was possible because the size of thestencil aperture was large and the particle size distribution ofthe solder paste was 25-45mm (type 3). The volume of thesolder bumps after reflow was estimated from the volume ofthe solder paste printed and the solder alloy volume fractionof the paste was found to be 1:66^ 0:1mm3:

The next procedure was forming the solder bumps byreflowing the solder paste deposit according to the list ofreflow profiles in Table III. A forced convection reflow oven(batch type) was used to reflow them. The first step in thiswas setting-up the reflow oven so that the test vehicle couldbe reflowed following the required temperature profiles. Thereflow profiling procedure is divided into two steps:thermocouple attachment process and oven set-up. Threethermocouples were used to measure the PCB temperatures.A thermocouple was attached on each pad of the test vehicleas shown in Figure 2. A high melting point solder (88Pb/10Sn/2Ag) was used to attach the thermocouples afterscrubbing and cleaning the pads with iso-propanol. Once thethermocouples were attached to the specimen, the reflowoven was set-up to create the required thermal profile(Table III). The oven parameters, adjusted to create thethermal profile, were: zone set point temperatures andconveyor speed. The profiling is an iterative processcomparing the test board temperature results with thedesired profile. If the results differ, the oven parameters areadjusted and the test is repeated.

Figure 1Types of reflow profiles

Table IExperimental parameters

Factor no. High level (H) Low level (L)

RSS reflow profile1 Soak temperature 175-1908C 150-1758C2 Peak temperature and time above liquidus 250 ^ 58C and 60 ^ 15 s 230 ^ 58C and 40 ^ 15 s3 Cooling rate Fast Slow

RTS reflow profile1 Peak temperature 250 ^ 58C 230 ^ 58C2 Time above liquidus 90-120 s 50-70 s3 Cooling rate Fast Slow

B. Salam, C. Virseda, H. Da,N.N. Ekere and R. DurairajReflow profile study of the Sn-Ag-Cu solder


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After reflow, the specimens were cross-sectioned to observethe microstructure and measure the intermetallic thickness.Some of the specimens were aged in a climatic chamber at1508C for 300 h to accelerate growth of the IMC layer andevolution of the microstructure. The aged specimens werealso cross-sectioned.

The IMC layer thickness was measured using a softwareattached to the microscope. On the IMC photo taken alongthe bumps at a 1,000 magnification, a poly-line was drawnalong the two borders of the IMC layer, i.e. at both the solderand the substrate side. The border at the solder side is mostlynon-planar and the border at the substrate side is oftenplanar. After the lines were drawn, the software calculatedthe maximum distance, minimum distance and averagedistance between the two lines. These steps were repeatedthree times on each bump (middle part, left side and rightside) to ensure consistency. The IMC layer thicknessesreported in this paper are the average thickness.

Six solder bumps were formed with each reflow profileand, on each bump, there were three measurements of theIMC thickness hence there were 18 measurements (N ) foreach reflow profile. The deviation of the measured IMClayer thickness varied, especially between the as-solderedand the aged bumps. The standard deviation of the IMCthickness for the as-soldered bumps was between 0.086 and0.5mm and that of the aged bumps was between 0.128 and0.8mm. The confidence level of the data could bedetermined by substituting the highest standard deviations(s) obtained for each condition of the bumps in thefollowing equation (Ott, 1988):

za=2 ¼

ffiffiffiffiffiffiffiffiffiNE 2

s 2

rð1Þ

The interval width (E ) is chosen as 0.25 because it is areasonable value for the facts that the thinnest intermetallic

thickness is around 1mm and the highest standard deviationof the data is 0.8. Furthermore, if the interval width value istoo low, the number of samples (N) have to be increased andif the interval width value is too high, it will not bereasonable, e.g. 1^ 0:8mm ðy^ EÞ: Thus, the value of za=2could be calculated and converted to the confidence levels ofthe measured data (Ott, 1988) which, for the as-soldered andaged bumps, were 97 and 82 per cent, respectively. In otherwords, we are 97 per cent (for the as-soldered bumps’ data)and 82 per cent (for the aged bumps’ data) sure that themeasured intermetallic thicknesses will be within theinterval of^0:25mm of the values listed in Tables IV and V.

Results and discussion

The results of this study are divided into two parts: themicrostructure evaluation and the IMC layer thicknessmeasurements.

MicrostructuresMicrostructures within the Sn-Ag-Cu alloys have beenanalysed and identified earlier by Hwang (2001) and Moonet al. (2000). Hence, in this paper, the microstructures of theSn-Ag-Cu solder bumps were recognised based on theirshapes and identities as described by Moon et al. andHwang. The results of the bulk microstructure observationsare summarised in Tables IV and V. The microstructures ofthe as-soldered bumps generally showed large areas of tin(Sn) populated by small Ag3Sn and Cu6Sn5 intermetallicflakes. The finest intermetallic flakes were observed in theas-soldered bumps formed with reflow profile 4. An exampleof the microstructures can be seen in Figure 3. Themicrostructures of the aged bumps were generally similar tothe as-soldered bumps, except that the intermetallics weremore uniformly distributed compared with those in the as-soldered bumps (Figure 4). Hence, the islands of Sn in theaged bumps were reduced in size. The aged bumps formedwith reflow profile 4 still had the finest intermetallic flakes.

The presence of the very fine intermetallics in the bulksolder of the bumps formed with reflow profile 4 might becaused by the low peak temperature, short time aboveliquidus and fast cooling rate. These fine intermetallics werenot present in the bumps formed with the RTS reflow profile,even though (RTS) reflow profile 8 had parameters verysimilar to the (RSS) reflow profile 4. This might be becausethe RTS profile did not have as short a time above liquidus asthe RSS profile. In addition, the RTS profile will never havethat short time above liquidus, because it requires a longertime to achieve an even temperature distribution across theassembly.

The interface intermetallics between the bump and Cusubstrate are shown in Figures 5-12. The figures show thatprofiles 1, 2, 5 and 7 produced a very irregular IMC layermorphology and profiles 3, 4, 6 and 8 formed a moreuniform IMC layer. Frear (1991) reports that the slower thedissolution rate the more planar is the interfacialintermetallics. This indicates that those reflow profiles madethe Cu dissolve faster into the solder. From Table III, thesimilarities between those profiles could be summarized.Profiles 1 and 2 have the same soak temperature (175-1908C) as do profiles 3 and 4 (150-1708C), and profiles 5 and7 have the same time above liquidus (90-120 s) as do profiles6 and 8 (50-70 s). It seems that the soak temperature in theRSS profile and the time above liquidus in the RTSprofile are the factors most affecting the dissolution rate.Further extensive investigation is needed to confirm thisfinding.

IMC layer thicknessThe IMC layer thicknesses of the samples are shown inFigures 5-12 and Table IV. The measured IMC thicknessesin Table IV are approximately in agreement with thatreported by Hwang (2001). The total IMC growth kinetic forthe Sn-Ag-Cu alloy was parabolic and can be expressed bythe following equation (Hwang, 2001):

Table IIIParameter combinations of the reflow profile

RSS profileSoak temperature

(8C)Peak temperature and time above liquidus

(8C, s) Cooling rate

1 175-190 250 ^ 5 and 60 ^ 15 Fast2 175-190 230 ^ 5 and 40 ^ 15 Slow3 150-175 250 ^ 5 and 60 ^ 15 Slow4 150-175 230 ^ 5 and 40 ^ 15 Fast

RTS profile Peak temperature (8C) Time above liquidus (s)5 250 ^ 5 90-120 Fast6 250 ^ 5 50-70 Slow7 230 ^ 5 90-120 Slow8 230 ^ 5 50-70 Fast

Figure 2Test vehicle design

Table IIThe L4(23) orthogonal array

Factor numberExperiment no. 1 2 3

1 H H H2 H L L3 L H L4 L L H



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X ¼ X0 þ 1:78 £ 1022t 0:52exp257; 700

RT

� �ð2Þ

where X is the total IMC layer thickness (m) after ageing fortime t (s); X0 is the initial layer thickness at time equal tozero; R is the universal gas constant (8.314 J/molK); and T isthe temperature (K). This parabolic growth kinetic impliesthat the IMC growth in the Sn-Ag-Cu alloy is controlled bybulk diffusion of elements to the reaction interface.

From the data shown in Table V, it can be seen that thereflow profile that gave the thinnest IMC layer was reflowprofile 4 for the RSS profile and reflow profile 8 for the RTSprofile. In order to determine which factors had the biggest

effect, variance analysis was conducted, as shown inTable VI. The effect figures in Table VI are the differencebetween the average IMC thicknesses for the low and highvalue for each variable and the percentage figures are theratio of these individual differences to the sum of the effectsfor all three variables.

According to the variance analysis in Table VI, themost significant factor in the RSS profile was the peaktemperature. Before ageing, the cooling rate gave abigger contribution than the preheat temperature, butafter ageing, it was opposite. The results suggest that thepeak temperature for a thin IMC layer and finemicrostructure is 2308C for 40 s. The suggested soak

Table IVIMC layer thickness and microstructure for the RSS reflow profile

RSS profile no.IMC thickness and

standard deviation (mm) IMC shape Microstructures

Non-aged1 2.23 ^ 0.2 Very irregular Large area of Sn + a lot of long Ag3Sn + Cu6Sn5

2 1.65^0.1 Very irregular Smaller area of Sn than no. 1 + a lot of long Ag3Sn + Cu6Sn5

3 2.26^0.2 Irregular Large area of Sn + a lot of small Ag3Sn4 1.19^0.1 Irregular Large area of Sn and very fine Ag3Sn

Aged1 4.56^0.4 Smooth layer, with

several spikesLarge area of Sn + small Cu6Sn5 and Ag3Snmicrostructure

2 3.84^0.5 Smooth layer, withseveral spikes

Large area of Sn + fine Cu6Sn5 and Ag3Snmicrostructure

3 4.20^0.3 Smooth layer Large area of Sn + small Cu6Sn5 and Ag3Snmicrostructure

4 3.09^0.8 Smooth layer Large area of Sn + very fine Cu6Sn5 and Ag3Snmicrostructure

Table VIMC thickness and microstructure for the RTS reflow profile

RTS profile no.IMC thickness and

standard deviation (mm) IMC shape Microstructures

Non-aged5 2.25 ^ 0.4 Very irregular Large Cu6Sn5 and Ag3Sn6 2.49^0.5 Irregular Large Cu6Sn5 and Ag3Sn7 1.66^0.2 Very irregular Large area of Sn and small size of Ag3Sn8 1.37^0.1 Irregular Large area of Sn and small size of Ag3Sn

Aged5 4.65^0.4 Smooth layer, with

several spikesLarge area of Sn + small Cu6Sn5 and Ag3Sn


Large area of Sn + small Cu6Sn5 and Ag3Sn


Large area of Sn + fine Cu6Sn5 and Ag3Sn

8 3.55^0.5 Smooth layer Large area of Sn + very fine Cu6Sn5 and Ag3Sn

Figure 3Microstructures of non-aged solder joints for different reflow profiles



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Figure 4Microstructures of as-soldered and aged solder joints for reflow profile 1

Figure 5The IMC layer for reflow profile 1





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temperature is 150-1758C and the faster cooling rate is

recommended.

The most significant factor in the RTS reflow profile is

also the peak temperature. The second largest contributor is

the cooling rate and the reflow time is least significant. The

results suggest that the peak temperature for a thin IMC

layer and fine microstructure is 2308C. The suggested reflow

time is 50-70 s and the recommended cooling rate is fast.

The RSS reflow profile has been proven to form the

thinnest IMC layer thickness in this experiment. The

thinnest IMC formed by the RSS and RTS reflow profiles are

1.19 and 1.37mm, respectively.

Summary

A study of the Sn-Ag-Cu lead-free solder reflow profile hasbeen conducted. The main objective of this study was todetermine the most important factors in designing a reflowprofile that results in a thin IMC layer and finemicrostructure solder joints.

The observation of the bulk microstructures of the bumpsshows that reflow profile 4, which has low peak temperature,short time above liquidus and fast cooling rate, forms thefinest intermetallic (Ag3Sn and Cu6Sn5) flakes. In addition,from the IMC layer morphology observation, the soaktemperature in the RSS profile and the time above liquidus


Table VIVariance analysis of IMC thicknesses

Soaktemperature

Peak temperature and timeabove liquidus Cooling rate

RSS profile Intermetallic thickness (mm) High Low High Low High Low

Non-aged1 2.23 2.23 – 2.23 – 2.23 –2 1.65 1.65 – – 1.65 – 1.653 2.26 – 2.26 2.26 – – 2.264 1.19 – 1.19 – 1.19 1.19 –Average 1.83 1.94 1.73 2.25 1.42 1.71 1.96Effect 0.215

(17 per cent)0.825

(64 per cent)0.245

(19 per cent)Aged1 4.56 4.56 – 4.56 – 4.56 –2 3.84 3.84 – – 3.84 – 3.843 4.20 – 4.20 4.20 – – 4.204 3.09 – 3.09 – 3.09 3.09 –Average 3.92 4.20 3.65 4.37 3.46 3.82 4.02Effect 0.550

(33 per cent)0.913

(55 per cent)0.197

(12 per cent)

Peaktemperature

Time above liquidus

RTS profile High Low High LowNon-aged5 2.25 2.25 – 2.25 – 2.25 –6 2.49 2.49 – – 2.49 – 2.497 1.66 – 1.66 1.66 – – 1.668 1.37 – 1.37 – 1.37 1.37 –Average 1.94 2.37 1.52 1.96 1.93 1.81 2.07Effect 0.855

(75 per cent)0.025

(2 per cent)0.265

(23 per cent)Aged5 4.65 4.65 – 4.65 – 4.65 –6 4.34 4.34 – – 4.34 – 4.347 3.26 – 3.26 3.26 – – 3.268 3.55 – 3.55 – 3.55 3.55 –Average 3.95 4.49 3.40 3.95 3.94 4.10 3.80Effect 1.090

(78 per cent)0.010

(1 per cent)0.300

(21 per cent)



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in the RTS profile are found to be the most significant factorsin controlling the dissolution rate.

The results of the study indicate that the most significantfactor in achieving a thin IMC layer and fine microstructureis the peak temperature. The results also suggest that thepeak temperature for the Sn-Ag-Cu lead-free solder is2308C. The suggested time above liquidus is 40 s for RSSreflow profile and 50-70 s for the RTS reflow profile.

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Reflow profile study of the Sn-Ag-Cu solder

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