solder wetting

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Wetting Characteristics of Pb-free Solder Alloys and PWB Finishes

S. V. Sattiraju, B. Dang, R. W. Johnson, Y. Li, J. S. Smith and M. J. Bozack NSF Center for Advanced Vehicle Electronics (CAVE)

Auburn University162 Broun Hall

Auburn, AL 36849Email: [email protected]

Abstract

For a successful transition to Pb-free manufacturing in electronics assembly, it is critical to understand the behavior of Pb-free solders (in bulk and paste form) and their interaction with the Pb-free printed wiring board (PWB) finishes. This paper

presents the results obtained from solder paste spread tests and wetting balance experiments with several Pb-free solder alloysand Pb-free PWB finishes. The solder alloys studied were Sn3.4Ag4.8Bi, Sn4.0Ag0.5Cu, Sn3.5Ag and Sn0.7Cu. EutecticSn37Pb was used as a reference. The PWB surface finishes were Sn, NiAu, Ag and OSP. Wetting balance experiments wereconducted in air while the spread tests were performed in air and nitrogen to understand the effect of reflow atmosphere on thespreading. Surface analyses techniques such as Nomarski phase contrast microscopy, Auger Electron Spectroscopy (AES) andX-ray Photoelectron Spectroscopy (XPS) were used to characterize the as-received PWB finishes. Sequential ElectrochemicalReduction Analysis (SERA) was also performed on the as-received PWB test coupons and on the Sn test coupons after multiplereflow cycles. The effect of multiple reflow cycles on the wetting performance, spreading and the surface composition of thePWB finishes was studied.Keywords: Pb-free solder, Pb-free PWB Finish, Solderability

Introduction

The elimination of Pb in electronics assembly has been discussed since 1990. Initially, the driving force was a proposedlegislative ban in the U.S. At the time no solder alloy replacement to SnPb was identified and the legislation was dropped under strong pressure from the electronics industry. However, increasing restrictions on hazardous materials in landfills, recyclingrequirements and manufacturer responsibility for products from ‘cradle-to-grave’ have kept the topic of Pb in the mind of manufacturers. Today, with proposed legislation in Europe and global competitive market pressures, particularly in Japan, theelimination of Pb in many, if not all, electronic products appears imminent.

The successful transition to Pb-free assembly is a complex issue. One of the first challenges to the industry is the selection of a replacement solder alloy. The National Center for Manufacturing Sciences (NCMS) concluded in 1997, after a major four-year

research effort that there were no ‘drop-in’ replacements for eutectic SnPb [1]. The International Tin Research Institute (ITRI)[2] and the National Electronics Manufacturing Initiative (NEMI) [3] are both recommending the SnAgCu eutectic (or near eutectic) alloy for reflow solder applications. Momentum does appear to be building for this alloy selection. Other alloys such asSnAg for hand and repair soldering and SnCu for wave soldering applications are also being used. Additional issues aresolderability, Pb-free finishes for components and PWBs, the impact of higher reflow temperatures on PWBs and components,and reliability. Mechanical, chemical and physical properties of some bulk Pb-free solders have been well studied and reportedin the literature [4-7]. Their wetting characteristics on different finishes have not been well documented yet. In this paper,various experimental methods have been applied to study the wetting properties of Pb-free solders on several PWB finishes. Theexperimental results presented will constitute an important part of the database that is required for the implementation of thePb-free manufacturing in the electronics industry.

Pb-free assembly requires Pb-free solders as bulk solder alloys (for wave soldering applications), solder paste (for surfacemount technology (SMT)) and wire (for hand and repair soldering). In addition to the usage of Pb-free solders, the PWB finishesmust be Pb-free for total Pb-free manufacturing. Measurement of the wetting angle is the easiest way to characterize a wetting

process. Better wetting is said to have occurred when the resultant wetting angle is small. Wetting angle depends on variousfactors such as PWB finish, solder alloy, flux, soldering atmosphere, surface roughness [8], test temperature, solder – substrateinteractions, etc. For this reason, accurate determination and repeatability in measurement of the wetting angles have been anarea of debate. Very limited data has been for combinations of solder alloys, fluxes and finishes using different methods tomeasure the contact angle [9], [10]. Various test methods to determine solderability have been mentioned in the literature [11].Wetting balance and area of spread tests are two of the most popular tests that are easy to conduct and data can be obtained withreasonable repeatability. Wetting balance data provides dynamic information on the wetting process while the spread tests,much like the sessile tests, provide information after the wetting process is complete. These two methods combined providecomprehensive insight into the wetting process.



In this research, the wetting performance of several Pb-free solder pastes and alloys, obtained from a single source, onvarious PWB finishes, manufactured by a single fabricator was studied. This work involved performing wetting balanceexperiments using alloys in the bulk form and spread tests based on printing and reflow of solder pastes on the entire test matrixof finishes and preconditions. Nomarski phase contrast microscopy, Auger Electron Spectroscopy (AES), X-ray PhotoelectronSpectroscopy (XPS) and Sequential Electrochemical Reduction Analysis (SERA) were used to characterize the surface finishes.Statistical analyses using the Duncan method were performed with the data to rank the PWB finishes.

Test Matrix and Materials

Solder Alloys:Table 1 lists the Pb-free solder pastes used in the spread tests. All of the Pb-free pastes were formulated with the same

commercially available no-clean flux. This eliminated flux chemistry as a variable. Solder alloys of the same composition wereused for both solder spread and wetting balance tests, except for SnAgCu alloy. The variation in the chemical composition of this alloy is minor and is within the typical manufacturing tolerance. Solder alloys used in wetting balance studies are listed inTable 2. A commercially available no-clean flux (isopropyl alcohol and aliphatic hydrocarbon mixture) was used in the wetting

balance measurements.Table 1. Solder pastes and test temperatures.

Solder PasteComposition

MeltingPoint (°C)

ReflowTemp(°C)

∆T

(°C)

Sn37Pb 183 217 -34Sn3.4Ag4.8Bi 205-210 250 ~45Sn4.0Ag0.5Cu ~217 250 33

Sn3.5Ag 221 250 29Sn0.7Cu 227 250 23

Table 2. Bulk solders alloys and test temperatures.Solder Alloy

Melting

Point (°C)

Test Temp(°C)

∆T(°C)

Sn37Pb 183 201 18Sn3.4Ag4.8Bi 205-210 235 25

Sn-3.8Ag0.7Cu 217 235 18Sn3.5Ag 221 245 24Sn0.7Cu 227 245 18

PWB Finishes:The PWB test coupons were prepared with the following Pb-free finishes: immersion tin, electroless Ni/ immersion Au,

immersion Ag and Organic Solderability Preservative (OSP) coating. Some of the popular Pb-free solder alloys and PWBfinishes from various manufacturers have previously been evaluated in the bulk form using a wetting balance. For the same typeof PWB finish, the wetting performance varies with the board manufacturer and the type of flux [12], [13]. In this work, testcoupons for wetting balance and spread tests were obtained from test boards manufactured by a single manufacturer.

Test Coupons:Three different test coupons were used in this work. Coupons for surface analysis were circular in shape and were cut from

the same board as the coupons used for spread and wetting balance tests. Coupons for printing and reflow characterization tests

were large rectangles with the appropriate finish over copper. The test coupon for wetting balance studies was designed inaccordance with the IPC J STD-003 standard [14]. The coupon was 0.25 in. wide x 0.062 in. thick. The coupon was routed prior to copper plating and application of the surface finish. Thus only finished metal was exposed in the solder bath during dipping.Figure 1 shows a photograph of the wetting balance test coupons.

Figure 1. IPC Solderability Test Coupons.



Test Procedure

Preconditioning of the test coupons:The test coupons for wetting balance experiments and spread tests were studied in the as-received and after two and four

reflow cycles in air. Boards were run through the reflow oven in an air atmosphere two and four times. The salient features of this treatment profile are i) the peak temperature was 260oC and ii) the time above 217oC was 55 seconds. The purpose of thereflow cycles was to determine the effect of multiple reflow cycles on the solderability of the Pb-free boards. A Heller 1700forced convection reflow oven was used in this work. Prior to testing, all of the test coupons were stored in a positive pressure

nitrogen purged chamber.

Surface Analysis:Since solder wetting is a surface interaction process, characterization of the surface finish is important in solderability

studies. XPS and SERA were used for surface analysis. The surface analysis instrument used in this work was a load-lockedKratos XSAM 800 surface analysis system. The base pressure of this ion and turbo-pumped system was 1 x 10-8 torr as read ona calibrated, nude ion gauge.

AES and XPS spectra were collected by a 127 mm radius double focusing concentric hemispherical energy analyzer (CHA)equipped with an aberration compensated input lens (ACIL). AES spectra were recorded in the fixed retard ratio (FRR) modewith a retard ratio of 10. Such a retard ratio represents a compromise between sensitivity and resolution and is appropriate for acquisition of survey spectra. XPS spectra were recorded in the fixed analyzer transmission (FAT) mode with a pass energy of 20 eV. The magnification of the analyzer in both FAT and FRR modes was selected to collect electrons from the smallestallowable (5 mm2) area on the specimen.

The XPS energy scale was calibrated by setting the binding energy of the Ag 3d 5/2 line on clean silver (excited by Mg K α radiation) to exactly 368.3 eV referenced to the Fermi level. The accuracy of the spectrometer ramp was verified by alsomeasuring the Cu2p3/2 line on clean copper. The measured energy was 932.7 eV, which is the correct recommended value. TheAES energy axis was calibrated by setting the Cu(LMM) line to 914.4 eV referenced to the Fermi level.

Light Ar + ion sputter cleaning was accomplished by a differentially pumped Kratos Minibeam I plasma discharge ion sourcewith rastering and focusing capabilities. A variable scan voltage provides a high frequency scan mode that yields a sputteredarea of 1 cm2 at the specimen position. Auger spectra were taken from a focused spot at a position near the center of thesputtered area. The angle of incidence of the ion beam with respect to the surface normal was 55o.

All Auger spectra were recorded at 3.0 keV beam energy and 0.7 µA primary beam current, measured with applied +90 V bias. For XPS, the incident X-ray power was 300 W (15 kV @ 20 mA). The detection system of the XSAM 800 consists of asingle channel multiplier and a fast response head amplifier. Detector output modes include direct pulse counting and currentdetection with voltage to frequency (V-F) conversion. Due to the large exciting currents used, all spectra were taken in currentdetection mode.

Sequential Electrochemical Reduction Analysis (SERA) is an accurate and a non-destructive electrochemical technique for the characterization of metal oxides and organic compounds on surfaces [15]. SERA was performed on Sn, OSP and NiAu PWBfinishes in the as-received condition to determine the thickness of the finish on copper. This was done by chemically reducingthe surface. The thickness was calculated based on the time it took to reduce the layer. In addition, SERA was performed on theSn finish after two and four reflow cycles to ascertain the growth of the Sn oxide layers. Also, using different chemistry, theintermetallics were determined for the Sn finish. This is done by oxidizing the surface and determining the oxidation potential of the intermetallics.

Wetting Balance Studies:Wetting balance is an instrument that provides instantaneous quantitative information on wettability of various configurations of components [16]. The mechanics of this instrument are described in detail [17]. This instrument has been used for theexamination of solderability on different combinations of solder alloys, components and fluxes [9], [18-21]. Given the amountof data that can be generated using this instrument for a given combination of PWB finish/solder alloy/flux, it is hard to compare

the data obtained in our experiments with that obtained in other laboratories.Wetting balance experiments were conducted on all PWB finishes and evaluated in various preconditionings with all of the

alloys listed in Table 2. A characteristic wetting curve is shown in Figure 2. The key wetting balance parameters measured wereTa and Fmax. Ta is the time to the buoyancy corrected force line. At this time the solder contact angle to the test coupon is 90o andthe wetting forces pulling the coupon into the molten solder equals the buoyancy force pushing the less dense coupon out of themolten solder. The smaller the Ta, the quicker the solder wets to the coupon. Fmax is the maximum wetting force exerted by thesolder on the coupon and is directly proportional to the height the solder climbs up the coupon. Fmax is the sum of the buoyancyforce and the maximum force, Finst, recorded by the wetting balance during a particular test as illustrated in Figure 2.

A Multicore Universal Solderability Tester (MUST II) was used in the experiments. The wetting balance tests wereconducted in accordance with the standard IPC J-STD 003. The solder alloys were brought to within ±1°C of the specified testtemperature (see Table 2). The coupon was dipped in flux for 5 seconds and the excess flux was drained off. After fluxing, thecoupon was placed on a mounting clip and moved onto the wetting balance. Any dross that may have formed on the solder was



wiped away from the molten solder surface prior to coupon dipping. The coupon was then dipped into the molten solder at aconstant speed of 20 mmsec-1 to a depth of 5 mm. The total immersion time was 10 seconds. At the end of 10 sec, the solder

bath was lowered and the coupon removed from the clip. The testing was carried out in air. Seven specimens were tested for each flux/finish/solder alloy combination.

The data acquisition software collected data points every 0.001 sec for the test duration. Wetting curves of the wetting forcevs. time were obtained and used to extracted values for wetting time (Ta) and maximum wetting force (Fmax), measured relativeto the buoyancy-corrected zero force line. Measurement with respect to the buoyancy-corrected zero force line and not to theinstrument zero force line allows the data obtained to be independent of the sample size and shape.

0

Finst Fmax

Figure 2. Characteristic wetting force vs. time graph.

Calculation of the Buoyancy Force (F b):Buoyancy Force Correction depends on the shape and the dimensions of the sample. In this case, the IPC M coupon was

used and was designed per the IPC J-STD-003.The volume of the coupon that comes in contact with the molten solder = width (w)*thickness (t)*depth of immersion (d)= 6.35*1.575*5 mm3 = 50*10-9m3

Buoyancy Force (F b) = volume ( ν)*density of the solder (ρ = 8360kg/m3)*acceleration due to gravity(g = 9.81m/s2)= 8360*9.81*50*10-9 = 4.1mN for the SnPb eutectic alloy

F b is calculated for the solders used in this study and are presented in Table 3. These values are incorporated in the calculation of Fmax presented in later sections.

Table 3. Calculated F b values for solder alloysSolder Alloy Density

(gm/cm3) Buoyancy Force

(F b) (mN)Sn37Pb 8.36 1 4.1

Sn3.4Ag4.8Bi 7.50 2 3.679Sn3.8Ag0.7Cu 7.50 1 3.679

Sn3.5Ag 7.50 1 3.679Sn0.7Cu 7.30 1 3.58

1. [4]

2. estimated value

Spread Test - Printing and Reflow Characterization: Reports on the area of spread tests conducted on PWB finish/solder alloy/flux are available [10], [22], and [23]. In this work,

accurately prepared stencils were used to transfer solder volumes that are realistic from the surface mount manufacturing pointof view. Two laser cut, 5mil thick stainless steel stencils were used to print solder pastes. A preliminary stencil (Stencil # 1) wasdesigned with apertures of various sizes and shapes as shown in Figure 3. Initial solder paste printing experiments using anautomatic stencil printer (MPM-AP25) were conducted using the stencil # 1. The purpose of these experiments was to determinethe size and shape of the aperture that provided the most consistency in printing. A measuring system mounted on an opticalmicroscope was used to measure the dimensions of every printed feature on the test surface. A circular aperture of diameter 0.89mm (~35 mil) was selected based on its print consistency. A circular shaped opening was selected for ease of measuring for subsequent work.

Instrument zeroforce line

Buoyancy forceline; θ=90º



Figure 3. Design of Stencil # 1.

A second stencil (Stencil # 2) was designed with apertures of 0.89mm (~35 mil) diameter in a 7x7 array. Stencil # 2 isshown in Figure 4. The same automatic MPM-AP25 stencil printer was used to print the solder paste onto the test coupon withStencil # 2. The dimensions of the printed dots of the top two rows in Figure 4 were measured after printing (D P) and after reflow (DR ). Two coupons were used for each combination of PWB finish and solder alloy. In total, the sample size was 28 per solder alloy, finish and per precondition. DR / DP ratios, henceforth called the spread ratio, are reported in the results.

Figure 4. Design of Stencil # 2.

Spread tests were performed in air and nitrogen reflow atmospheres. The reflow profile for the Pb-free solder pastes isshown in Figure 5 and for the Sn37Pb eutectic solder paste is shown in Figure 6. The same reflow temperature profiles wereused for both nitrogen and air atmospheres.

Figure 5. Reflow Profile for Pb-free solders.

Figure 6. Reflow Profile for SnPb eutectic solder.



Results and Discussion

Each board finish was examined using Nomarski phase contrast microscopy, Auger electron spectroscopy (AES), and X-ray photoelectron spectroscopy (XPS). Figure 7 shows a representative Nomarski phase contrast photograph. This photographshows the typical surface roughness on an as-received board.

The board finishes were studied with XPS and AES techniques, as-received and after 5 minutes of Ar + sputtering.Sputtering removes surface contaminants and the adventitious C, N, and O that adsorbs on every surface exposed to theatmosphere. The sputter rate (30 Å/min) was calibrated by recording the time required to sputter through a known, standard

thickness of SiO2. Assuming an identical sputter rate on each board finish, roughly 150 Å of each surface was removed duringthe ion bombardment. XPS derived surface elemental composition of each surface is listed Table 4.

Figure 7. Nomarski phase contrast micrograph of a Ag PWB finish (20X) showing the degree of roughness of thesurface.

Table 4. XPS Surface Elemental Composition of PWB FinishesSurface Elemental Composition (atom %)

PWBPlating

Finish S n

N i / C u

P b

C l

P d

C O

A u / S i

A g / N

Sn 1 19.6 0.0 0.0 0.0 0.0 46.8 33.7 0.0 0.0

Sn 2 86.7 0.0 1.8 0.0 5.6 0.0 5.9 0.0 0.0

Ag 1 0.0 0.0 0.4 0.0 0.0 48.9 7.3 0.0 43.5

Ag 2 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0100.

0

Pd 1 0.2 0.0 0.0 0.0 16.4 83.4 7.3 0.0 0.0

Pd 2 0.0 0.0 0.0 0.0 67.7 32.3 0.0 0.0 0.0

NiAu 1 0.7 0.4 0.0 0.0 0.0 61.7 10.4 26.9 0.0

NiAu 2 0.0 0.0 0.0 0.0 0.0 0.0 0.0100.

00.0

OSP 1 0.0 /1.1 0.0 16.9 0.0 70.2 4.00.0/

0.9/6.9

OSP2

0.0 /2.6 0.0 10.0 0.0 87.4 0.0 0.0 0.0

1. As-Received

2. After Sputtering

Surface Analysis using XPS: Sn Finish: In addition to Sn, a large amount (46.8 atom %) of surface C (Figures 8(a) and 8(b)) and two surface oxides

(SnO2, SnO) were observed on the as-received surface. After 5 minutes of Ar + sputter cleaning, the C signal vanishes and theoxygen concentration decreases dramatically, indicating that the oxide layer is ≤ 150 Å. The chemical states were clearlyobserved in the high resolution spectrum over the Sn3d5/2 XPS feature shown in Figure 8(c). Deconvolution of the peak envelope yields the amount of each surface species present (see figure inset). For the as-received board finish, SnO2 is the

predominant Sn oxidation state (71%).



100 200 300 400 500 600 700 800 900 1000

S Cl

C

Sn

Sn

Sn

O

E

d N ( E ) / d E

Kinetic Energy (eV) 8 (a)

600 500 400 300 200 100 0

Sn4d

Sn4s

C1s

O1s

Sn3d

I n t e n s i t y ( c

o u n t s )

Binding Energy (eV) 8 (b)

8(c)Figure 8. Surface analysis of the Sn board finish. (a) AES (b) XPS survey spectra and (c) High resolution XPS

spectrum over Sn3d5/2. Spectra are of the as-received surface.

NiAu Finish – In addition to Au, a large amount (61.7 %) of C, a small amount of Sn (0.7%), Ni (0.4%), and O (10.4% )(Figures 9(a) and 9(b)) were observed on the as-received surface. The chemical state for the small amount of Sn at the surfacewas SnO2: 76%, SnO: 12%, Sn: 12%, shown in Figure 9(c). After sputter etching, these surface contaminants disappeared andthe surface was 100% Au. XPS data after multiple reflow cycles shows that Au was present on the surface. Availability of Auon the surface prevents the oxidation of the underlying Ni and Cu layers thereby preserving the solderability.



100 200 300 400 500 600 700 800 900

Sn Ni Ni Ni

O

NCa

C

Cl Au/S

Au

Ni

E

d N ( E ) / d E


600 500 400 300 200 100 0

Sn3d

Au4pO1s

Ni(L MM) Au4d

C1s

Au4f

I n t e n s i t y ( c o u n t s )


9 (c)Figure 9. Surface analysis of the NiAu board finish. (a) AES survey spectrum (b) XPS survey spectrum and (c) High

resolution XPS spectra over Sn3d5. Spectra are of the as-received surface.

Ag Finish: In addition to Ag, a large amount (48.9 atom %) of C (Figure 10(a)), a small amount (0.4 atom %) of Pb (Figure10(b)), and 7.3% O was observed on the as-received surface. After 5 minutes of Ar sputter cleaning, the surface was 100% Ag.It was not possible to unambiguously determine the chemical state of the native Ag oxide because the XPS binding energies for elemental Ag, AgO, and Ag2O are nearly identical, as shown in Figure 10(c). Ag was observed on the surface even after



multiple excursions in the reflow oven. This suggests that a solderable surface was available depsite the abuse of the reflowcycles.

100 200 300 400 500 600 700 800 900 1000

C+Ag

Ag

Ag

SiS O

E

d N ( E ) / d E


600 500 400 300 200 100 0

Pb4f

Ag4s

Ag4p

C1s

Ag3d

O1s

Ag3p


Binding Energy (eV)

10 (b)

10 (c)Figure 10. Surface analysis of the Ag board finish. (a) AES survey spectrum (b) XPS survey spectrum and (c) High

resolution XPS spectrum over Ag3d5/2. Spectra are of the as-received surface.



OSP – The predominant elements observed on the as-received surface were C (70.2%), Cu (1.1%), and Cl (16.9%) (Figures11(a) and 11(b)). A small amount of O (4.0%) and N(6.9%) due to atmospheric exposure were also observed, in addition to atrace amount of Si (0.9 atomic %). Cl and Cu were consistently found in OSP films from various manufacturers. Cu compoundsincrease the rate at which OSP is formed on a Cu surface, especially cupric and cuprous chloride. The C in the film consists of C-organic, C-C, and C-Hx bonds indicated in Figure 11(c).

100 200 300 400 500 600 700 800 900

Cu Cu

Cu

Cu

SiN

O

Cl

C

E d N

( E ) / d E


600 500 400 300 200 100 0

O1s

Cu(LMM)

N1s

Cu(LMM)

Cu3p

Si2pCu3s

Si2s

Cl2p

C1s



11 (c)Figure 11. Surface analysis of the OSP finish. (a) AES survey spectrum (b) XPS survey spectrum and (c) High

resolution scan over C1s peak . Spectra are of the as-received surface.



Surface Analysis using SERA:

Sn Finish – The intermetallic thicknesses were calculated with the following equation derived in [15].T = M*I*t*108

N*F*S*Dwhere

M = molecular weightI = current

t = reduction time N = number of electronsD = densityS = surface areaT = thickness of filmF = Faraday’s constant

The calculated thickness of the uncombined Sn, Cu6Sn5 and Cu3Sn intermetallic layers are summarized in Table 5. Figure 12shows the oxidation potentials of the Sn PWB finish after different preconditionings. Sn, Cu6Sn5 and Cu3Sn regimes are markedon the graph.

Table 5. SERA results for Sn PWB finish showing intermetallic layer thickness.

Preconditioning Sn (µm) Cu6Sn5 (µm)Cu3Sn

(µm)As-received 0.857 0.705 0.244

After 2 reflowCycles

0.0 0.916 0.176

After 4 reflowCycles

0.0 0.893 0.197

After storage for a month

0.834 0.702 0.230

After 6 monthsof storage

0.501 0.790 0.282

After 9 monthsof storage

0.473 0.834 0.310

-0.6

-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 100 200 300 400 500 600

Time of Measurement

V o l t a g e ( V )

0X

2X

4X

Sn

Cu6Sn5

Cu3Sn

Figure 12. SERA Analysis of Sn PWB Finish after different preconditioning showing the presence of intermetallics.

After two and four reflow cycles, there is no Sn that was available for soldering. The intermetallic thicknesses do not changesubstantially from two to four reflow cycles. The intermetallic layers grow in thickness at the expense of the Sn. Also, there is amarked difference in the thicknesses of these layers after storage in dry nitrogen atmosphere as shown in Table 5.

Figure 13 shows the reduction potentials of the SnO and SnO 2. The effect of the multiple reflow cycles is summarized inTable 6. There is a significant increase in SnO2 after two reflow cycles, but as most of the Sn has been consumed, there is verylittle change in the thickness between two and four reflows. SnO 2 has a high reduction potential, which means that it is moredifficult for the flux to reduce, thereby inhibiting the wetting process. A combination of increased SnO2 and little uncombinedSn helps explain the low Fmax values obtained with Sn boards in the wetting balance test after multiple reflow cycles. Also, thegrowth of intermetallics contributes to the decrease in wettability. XPS analyzes a surface only a few atoms deep. Due to its



high resolution, it can detect even trace amounts of SnO2 present on the surface. As shown in Table 6, SERA cannot detect theSnO2 in the as-received condition, although as shown in Figure 9(c), XPS does detect SnO2. This is a resolution issue and not adiscrepancy.

Table 6. SERA results for Sn PWB Finish showing oxide layer thickness.

Preconditioning SnO (Å) SnO2 (Å)Oxidized

intermetallic(Å)

As-received 31 Not detected Not detectedAfter 2 reflow Cycles 18 0.28 Not detectedAfter 4 reflow Cycles 15 0.34 Not detected

After storage for amonth

33 Not detected Not detected

After 6 months of storage


After 9 months of storage


-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

0 50 100 150 200 250 300

Time of Measurement

V o l t a g e ( V )

0X

2X

4X

SnO

SnO2

Figure 13. SERA Analysis of Sn PWB Finish after different preconditioning showing the presence of oxides.

NiAu and OSP Finishes –The thickness of the gold coating was 0.115 µm from the oxidation of Au in SERA study. Thethickness of the OSP coating was determined to be 0.992 µm from the oxidation potential of the OSP coating.

Wetting Balance Tests: In order to qualitatively describe the data obtained, the following criterion was used. A positive Fmax value indicates that in a

dip test, the solder has climbed up the test coupon beyond the immersion depth. This is due to the positive interaction betweenthe solder alloy and the finish under the given conditions. The magnitude of Fmax indicates the height to which the solder climbsup the coupon above the mean solder level (MSL) in the solder bath. The greater the height, the lower the contact angle and the

better the wetting. This is shown in Figure 14 (a). When the Fmax value is close to zero, this means that the solder has barelyclimbed up the coupon and that it has barely crossed the buoyancy corrected force line. The contact angle in this case is onlyslightly less than 90°. This is shown in Figure 14 (b). Negative Fmax values indicate poor or no wetting at all. This is shown inFigure 14 (c). The Ta values for this case are equal to the duration of the test (10 seconds).

b) θ ≤ 90°a) θ << 90° c) θ > 90°

MSLh

b) θ ≤ 90°a) θ << 90° c) θ > 90°

MSLh

Figure 14. Meniscus shapes for different degrees of wetting a) Positive meniscus b) Flat meniscus and c) Negative

meniscus



Sn Finish – The results for the Sn PWB finish are shown in Figures 15 and 16. As shown in Figure 15, good wettingoccurred with all of the solder alloys as indicated by the high Fmax values in the as-received condition. There is a significant dropin the Fmax values after multiple reflow cycles. Very little wetting occurred with SnPb, SnAgCu and SnAg solder alloys and nowetting with SnAgBi and SnCu solder alloys after two reflow cycles. After four reflow cycles, there is no wetting with all of thesolder alloys. As shown in Figure 16, Ta values are approximately 3 seconds for coupons tested in the as-received condition.High Ta values can be seen for alloys with some wetting and equal to 10 seconds for alloys with no wetting after two reflowcycles. Ta values are equal to 10 seconds for all Sn coupons after four reflow cycles. In general, the Sn finish in the as-receivedcondition had the highest Fmax values of all of the alloys. However, solderability significantly degraded with multiple reflowcycles in air. The SnAg alloy wet the as-received Sn finish fast corresponding to low Ta values.

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

0X 2X 4X

Condition

F m a x ( m N )

63Sn37Pb at 201 C

Sn3.4Ag4.8Bi at 235 C

Sn3.8Ag0.7Cu at 235 C

Sn 3.5Ag at 245 C

Sn 0.7Cu at 245 C

Figure 15. Fmax for Sn PWB finish with various alloys

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0X 2X 4X

Condition

T a ( s e c )

63Sn37Pb at 201 C



Sn 3.5Ag at 245 C

Sn 0.7Cu at 245 C

Figure 16. Ta for Sn PWB finish with various alloys

NiAu Finish – As shown in Figure 17, good wetting occurred with the NiAu PWB finish corresponding to high Fmax values.After two and four reflow cycles, Fmax values are still reasonably high with the exception of the SnAgBi alloy. This indicatesthat the NiAu PWB finish survived the multiple reflow cycles. There is minimum wetting after two reflow cycles and negativewetting after four with the SnAgBi alloy. Corresponding Ta values are shown in Figure 18. It can be seen that the Ta for SnAgBialloy is about 9 seconds after two reflow cycles and at the maximum limit of T a after four reflow cycles, indicating no wetting.The SnAg and SnCu alloys wet fast on the NiAu boards. Fmax values decrease and Ta values increase with multiple reflowcycles.

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

0X 2X 4X

Condition

F m a x ( m N )

63Sn37Pb at 201 C



Sn 3.5Ag at 245 C

Sn 0.7Cu at 245 C

Figure 17. Fmax for NiAu PWB finish with various alloys



0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0X 2X 4X

Condition

T a ( s e c )

63Sn37Pb at 201 C



Sn 3.5Ag at 245 C

Sn 0.7Cu at 245 C

Figure 18. Ta for NiAu PWB finish with various alloys

Ag Finish – As shown in Figures 19 and 20, the Ag board finish recorded positive Fmax and Ta values lower than four seconds in all of the three preconditions with all of the solder alloys. The Fmax decreases with the number of reflow cycles and Ta increases, in general. The Fmax values of Ag finish are greater than NiAu, in general, for all alloys and in all preconditioningssuggesting that the deleterious effect of multiple reflow cycles is less pronounced for Ag PWB finishes.

-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

0X 2X 4X

Condition

F m a x ( m N )

63Sn37Pb at 201 C



Sn 3.5Ag at 245 C

Sn 0.7Cu at 245 C

Figure 19. Fmax for Ag PWB finish with various alloys

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0X 2X 4X

Condition

T a ( s e c )

63Sn37Pb at 201 C



Sn 3.5Ag at 245 C

Sn 0.7Cu at 245 C

Figure 20. Ta for Ag PWB finish with various alloys

OSP Finish – As shown in Figure 21, good wetting occurred with SnPb, SnAgCu and SnAg alloys with the OSP coated testcoupons in the as-received test condition. Fmax values are the lowest in all preconditions with all the solder alloys for the OSPcoating. There is a significant drop in the Fmax values after two reflow cycles with the SnAgBi and SnCu alloys, going fromslightly positive to negative Fmax. There is no evidence of any wetting after four reflow cycles with any of the alloys.Corresponding Ta values are shown in Figure 22.



-4.0

-2.0

0.0

2.0

4.0

6.0

8.0

0X 2X 4X

Condition

F m a x ( m N )

63Sn37Pb at 201 C



Sn 3.5Ag at 245 C

Sn 0.7Cu at 245 C

Figure 21. Fmax for OSP PWB finish with various alloys

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

0X 2X 4X

Condition

T a ( s e c )

63Sn37Pb at 201 C



Sn 3.5Ag at 245 C

Sn 0.7Cu at 245 C

Figure 22. Ta for OSP PWB finish with various alloys

Spread Tests:Sn Finish – Higher spread ratios, meaning more spreading and more wetting, were observed for samples reflowed in a

nitrogen atmosphere. The greatest spreading occurred with the SnPb alloy and least with the SnCu alloy on the as-received Snfinish board in air and nitrogen atmospheres. For the SnPb solder paste, the spread ratio was almost 2.0 in air. This means thatthe SnPb paste spread to twice its original printed diameter. For Pb-free solder pastes, the spread ratio was slightly above 1.0 inthe as-received condition (higher in nitrogen) and close to 1.0 after the boards has been subjected to multiple reflow cycles. This

indicates very little spreading of the Pb-free pastes on the Sn boards. The results obtained on Sn boards are shown in Figure 23.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

As Recd:Air As Recd:N2 2X:Air 2X:N2 4X:Air 4X:N2

Condition

R a t i o

Sn37Pb

Sn3.4Ag4.8Bi

Sn4.0Ag0.5Cu

Sn3.5Ag

Sn0.7Cu

Figure 23. DR /DP ratio for Sn PWB Finish.

NiAu Finish – The spread ratios are noticeably higher in nitrogen than in an air atmosphere. Also, the difference in thespread ratio for the SnPb solder paste is significant in the as-received condition as well as after multiple reflow cycles. Thespread ratios for the Pb-free solder pastes are very close to each other. There is a slight decrease in the spread ratios with themultiple reflow cycles indicating a slight drop in the solderability. A halo effect on the NiAu boards was noticed which will beexplained in detail in the following sections. The results obtained on NiAu boards are shown in Figure 24.



0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


Condition

R a t i o

Sn37Pb

Sn3.4Ag4.8Bi

Sn4.0Ag0.5Cu

Sn3.5Ag

Sn0.7Cu

Figure 24. DR /DP ratio for NiAu PWB Finish.Ag Finish – Three differences are noticeable immediately when examining the data for the Ag finish. The difference in

spread ratio for SnPb solder is not significantly higher than the Pb-free when compared with the data obtained on the other PWBfinishes. There is little difference in the data for air and nitrogen atmospheres suggesting that wetting on Ag boards isindependent of atmosphere. There is very little or no effect of multiple reflow cycles on the solderability of Ag boards. Theresults obtained on Ag boards are shown in Figure 25.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


Condition

R a t i o

Sn37Pb

Sn3.4Ag4.8Bi

Sn4.0Ag0.5Cu

Sn3.5Ag

Sn0.7Cu

Figure 25. DR /DP ratio for Ag PWB Finish.

OSP Finish – The data for SnPb paste is significantly higher in nitrogen than in air. In air, although the SnPb paste spreadsthe most, the difference is not very dramatic compared to the Pb-free pastes. The spread ratios are close to 1.0 for almost all of the Pb-free pastes in all conditions indicating minimal spreading. The results obtained on OSP boards are shown in Figure 26.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0


Condition

R a t i o

Sn37Pb

Sn3.4Ag4.8Bi

Sn4.0Ag0.5Cu

Sn3.5Ag

Sn0.7Cu

Figure 26. DR /DP ratio for OSP coating.

“Halo” on the NiAu PWB finish

Several instances of "halo" formation around wetted droplets of SnAg and SnCu solders when wetted to the NiAu PWBfinishes were observed. An scanning electron microscope (SEM) micrograph illustrating this phenomenon is shown in Figure27 after the residues from the no clean flux were cleaned.



Figure 27. A macrograph of an unpolished sample showing the spread of Sn0.7Cu solder alloy on a NiAu PWB finish.

The presence of halo initially caused confusion when measuring DR , the diameter of the solder dot after reflow. When thediameter of the halo was considered in the measurements of wetting and spreading, the NiAu PWB finish appeared to besignificantly better than all the other PWB finishes. When wetting balance data was considered, however, wetting to the Ag

board finish was, in general, better than wetting to NiAu. When the diameter of the solder dot only was considered (i.e., notincluding the halo border), NiAu boards did not appear to wet dramatically better than the rest of the board finishes. Thus in the

previous plots of DR /DP, DR was measured excluding the halo.In order to explain the halo formation, top-view and cross-sectional view energy dispersive spectroscopy (EDS)

measurements of the uniformly solidified solder and halo region were conducted. Top-view analysis was performed to observeand analyze the morphology and composition of the solder dot and halo. A cross-sectional slice through the halo and alloy wasalso examined by SEM and elemental mapping was performed to determine the relative distribution of the elements.

Experimental Work and Discussion:

A JEOL JSM-840 SEM, operated at an accelerating voltage of 20 kV was used for the line scans and the EDS analysis. EDSwas performed using an Oxford Instruments ISIS energy dispersive x-ray spectrometer, equipped with an ultrathin window(UTW) detector. Standard, top-view EDS line scan analyses of the wetted alloy droplet and the "halo" region were performed.The location of this line scan is shown, marked as LS1 in Figure 28.

Figure 28. SEM micrograph of Sn0.7Cu on NiAu PWB finish showing the location of the line scan. The dark areas in theimage are no clean flux residues.

In top-view and at a lower magnification, the line scan (Figure 29) reveals a relatively constant Sn concentration through theregion identified as the solder dot used to measure DR . In the halo region, there is a steady decrease in Sn concentration and a



corresponding increase in the Au concentration. The thickness of Au on the Ni is 0.115µm, as measured from the SERAexperiments. The electron beam used in the EDS analysis analyzes material to a depth of approximately 1-2µm below thesurface. As there is a very thin layer of solder mixed with Au in the halo region, the electron beam penetrated this layer anddetected the signal from the underlying Ni layer. In the solder dot region, the solder layer is thick enough to mask the Ni signal.In the “bare board finish area”, a strong signal from the Au atoms and from the Ni atoms underneath can be observed. Also, asudden drop in the Sn signal can be observed at the edge of the halo where the Ni and Au signal increases.

Figure 29. EDS line scan along the line (LS1) shown in Figure 28, showing the relative distribution of the elements Au,Sn, and Ni as the line is scanned from bottom to top.

A “white band” was observed at the edge of the halo as shown in Figure 28. Magnified images of the “white band” areshown in Figures 30 and 31. The grainy nature of this region can be clearly seen from Figure 31. A line scan was performedalong the line marked as LS2 in Figure 31. The results from this line scan are as shown in Figure 32. The periodic profile in this

plot is the result of the grainy nature of the surface. It can be observed from Figure 32 that the mean concentrations of Sn andAu remain relatively constant in the region marked as “white band region”. Also, the Ni signal obtained in this region is weak and is obtained from the penetration of the electron beam used in the line scan.

Figure 30. SEM micrograph of the “white band” region around a specimen of Sn0.7Cu on NiAu PWB finish



Figure 31. Magnified image of the "white band" showing its grainy surface morphology.

Figure 32. Periodic appearance of the line scan in the “white band” region revealing the grainy nature of the surface.

EDS single spot analyses were conducted at different locations in the “white band” region. Results from a particluar spotmarked X in Figure 31 are shown in Figure 33. Quantitative analysis at this spot and at different spots in the same region revealsa stoichiometric ratio of Sn/Au ~ 2/1, suggesting possible formation of the AuSn 2 intermetallic. It is not possible to determinethe crystallographic structure of the Au-Sn compound from mere EDS data. Other analytical tools such as X-ray diffraction arenecessary to conclusively determine the existence of a true Au-Sn intermetallic.



Figure 33. EDS single-point analysis from the “white band” region, marked as X in Figure 31.

A cross-section taken through the middle of the alloy droplet was analyzed. A montage of SEM cross-sectional micrographsshowing the Sn0.7Cu solder after spreading over the NiAu board is shown in Figure 34. Two distinct regions can be seen,

corresponding to the solid solder dot that has solidified uniformly and a grainy and fragmented region, which corresponds to thehalo region. Elemental mapping was performed to visualize the concentration of various elements present. Results obtained for the distribution of Au are shown in Figures 35 and 36.. It can be observed from Figure 35 that Au is distributed uniformly in thesolder dot region and is present in low quantities compared to the solder volume. Substantial amount of Au is present in the haloregion (Sn/Au ratio is 2 to 1 as determined by the quantitative anlayis). Au distribution in the halo region is shown in Figure 36.

Halo Region DR /2Halo Region DR /2

Figure 34. Montage of several pictures taken on a cross section of a sample with halo.

Figure 35. Element mapping showing the distribution of Au in the solder dot.



Figure 36. Element mapping showing the distribution of Au in the halo region.

In contrast, a similar cross section through a specimen of Sn0.7Cu wetted to NiAu in the wetting balance reveals acontinuous, smooth interface with no grainy appearance as shown for a spread test coupon. In other words, halo formation didnot take place in wetting balance experiments.

It is believed that the presence and/or absence of an alloy halo is due to the difference in the test configuration. In a wetting balance experiment, a coupon is dipped at a controlled rate into a large reservoir of solder. When the NiAu coupon is dippedinto the solder, gold dissolves into the solder bulk. Since the proportion of Au in relation to the solder bath is negligible in the

wetting balance, a large reservoir of fresh solder is always available to the board at any given time during a test.In a spread test, however, there is a limited amount of solder available and the solder alloy composition continuouslychanges as gold dissolves into the solder. As the solder alloy, consisting of (predominantly) Sn, melts, wets, and spreads, itdissolves Au from the board finish.

Strictly speaking, a ternary phase diagram for Sn-Cu-Au should be considered in the following explanation. But since aternary phase diagram does not exist for Sn, Au and Cu and given that the Cu content in the solder is very low, Au-Sn binary

phase diagram is used as shown in Figure 37.

Figure 37. Au-Sn binary phase diagram [24].

The liquidus temperature of the Au-Sn alloy continuously drops as depicted by the negative slope of the liquidus line in the

AuSn4-Sn hypereutectic region, as shown in the Sn-rich portion of the Au-Sn phase diagram (Figure 38). As shown in the phasediagram, a Au-Sn eutectic exists at 217°C and 93.7% (at.) Sn. In comparison, the SnAg eutectic melts at 221°C and SnCueutectic melts at 227°C. Sn is the reactive species in these solder alloys. As the solder alloy melts, Sn immediately reacts withAu from the PWB finish lying under the solder dot. This reaction occurs in accordance with the Au-Sn binary phase diagram.This reaction continues while the combination of the prevailing temperature and compositional ratios are favourable for itsoccurrence. As more Au is dissolved from the surrounding area, a solder front emantes and travels in a radial direction awayfrom the solder dot. This phenomenon is illustrated in Figure 39.



Figure 38. Sn-rich portion of the Au-Sn phase diagram. [24]

Figure 39. Radially outward movement of the solder front due to dissolution of the Au by Sn

The solder front consists of Au and Sn (neglecting Cu). Depending on the composition and temperature, Au-Sn liquidsolidifies into different structures as predicted by the phase diagram. As the Au-Sn liquid passes through the two phase region,liquid+AuSn4, between 217°C and 252°C isotherms, some of the Sn is consumed in the formation of AuSn 4. In this region, Snconcentration drops gradually and Au concentration increases as shown in the line scans. This region solidifies as the “halo”region. The remaining liquid mixture is rich in Au. As the moving solder front continues to dissolve more Au from the finish,the liquidus temperature of the mixture increases as shown by the positive slope of the liquidus line in the AuSn4-Snhypoeutectic region of the phase diagram. The Au-Sn reaction comes to a halt when the peak temperature of the reflow profile is

reached, at about 250°C, as shown in Figure 2. Further, based on phase diagram, there is another two phase region liquid+AuSn2 at 252°C, which is at about the peak temperature of the reflow cycle. This region solidifies as a “white band” around the halo.From the phase diagram, this temperature is suitable for the formation of AuSn2 intermetallic, provided the chemicalcompositions of Au and Sn match the stoichiometric ratio in AuSn2. Line scans in this region reveal relatively constant Sn andAu concentrations. Also the quantitative analysis in this region shows that the ratio of Sn to Au is approximately 2 to 1.

It can be inferred from the above discussion that “halo” region consists of AuSn4 intermetallic compound and AuSn2 is present in the “white” band. The halo formation on NiAu PWB finish has practical implications. When NiAu boards are used inconjunction with the SnCu or SnAg solder alloys, a loss of adhesion between the solder mask layer and the underlying Au mayoccur due to this halo formation.

Direction of themoving solder front



Statistical Analysis

Duncan analysis was performed on the data obtained from the wetting balance and spreading experiments in order to rank the PWB finishes. Tables 7 – 9 summarize the ranking of the PWB finishes in the as-received condition, after two and four reflow cycles respectively for all of the solder alloys based on the results obtained from the wetting balance tests. As shown inTable 7, in general, OSP had the lowest rank. This means that the OSP coatings had the least Fmax and Ta was the highest in theas-received condition. In the as-received condition, the selection of the best finish for a particular alloy depends on whether Fmax and Ta are used as the criterion. In a wave solder application, a large Ta value would indicate the need for a longer contact time

between the board and the solder wave. It is worthwhile to note that the Sn PWB finish, in the as-received condition, recordedthe highest Fmax values with SnPb, SnAgBi and SnCu solder alloys followed by Ag or NiAu. However, there is a dramatic dropin wettability of the Sn PWB finish after two reflow cycles and no wetting at all after four reflow cycles with all the solder alloys. OSP coatings also offered no wetting after multiple reflow cycles. All solder alloys wet to Ag and NiAu (except SnAgBialloy) PWB finishes after multiple reflow cycles. The data presented in Tables 8 – 9 suggest that the Ag PWB finish performed

better than the NiAu PWB finish. However, the quantitative difference between Ag and NiAu is small.

Table 7. Summary of the wetting balance results with test coupons in the as-received conditionSolder Alloy Fmax

(Best>Worst)Ta

(Best>Worst)Sn37Pb Sn>Ag=OSP>NiAu Sn=Ag=OSP=NiAu

Sn3.4Ag4.8Bi Sn>Ag>NiAu>OSP Sn=Ag=NiAu>OSPSn3.8Ag0.7Cu Ag>NiAu>Sn>OSP Sn>NiAu>Ag>OSP

Sn3.5Ag Ag>NiAu>Sn>OSP Ag=NiAu=Sn>OSP

Sn0.7Cu Sn>NiAu>Ag>OSP NiAu>Sn=Ag>OSP

Table 8. Summary of the wetting balance results with test coupons after two reflow cyclesSolder Alloy Fmax

(Best>Worst)Ta

(Best>Worst)Sn37Pb Ag>NiAu>OSP>Sn Ag>NiAu=OSP=Sn

Sn3.4Ag4.8Bi Ag>NiAu>Sn=OSP Ag>NiAu>Sn=OSPSn3.8Ag0.7Cu NiAu>Ag>Sn>OSP Ag=NiAu>OSP>Sn

Sn3.5Ag Ag>NiAu>Sn>OSP Ag>NiAu>Sn=OSPSn0.7Cu Ag>NiAu>Sn=OSP Ag>NiAu>Sn=OSP

Table 9. Summary of the wetting balance results with test coupons after four reflow cycles

Solder Alloy Fmax (Best>Worst)

Ta (Best>Worst)

Sn37Pb Ag>NiAu>Sn=OSP Ag=NiAu>Sn=OSPSn3.4Ag4.8Bi Ag>NiAu>Sn=OSP Ag>NiAu=Sn=OSPSn3.8Ag0.7Cu Ag>NiAu>Sn=OSP Ag=NiAu>Sn=OSP

Sn3.5Ag Ag>NiAu>Sn=OSP Ag>NiAu>Sn=OSPSn0.7Cu Ag>NiAu>Sn=OSP Ag>NiAu>Sn=OSP

Results from the spread tests are presented in Tables 10 – 12. Table 10 lists the ranking of the PWB finishes in the as-received condition for all the solder pastes considered in this work. Analysis of the results after two and four reflow cycles is

presented in Tables 11 and 12 respectively. In general, NiAu has the highest DR /DP ratio with all of the Pb-free alloys reflowedin nitrogen and air. Spreading on the OSP was the least for all of the Pb-free alloys reflowed in nitrogen and air. In fact, as

shown in Figure 24, DR /DP, in general for OSP, was equal to 1 indicating no spread at all. For the Sn37Pb alloy, the spread ratiodepends on the reflow atmosphere. In general, the order of the finishes starting from best to worst is NiAu>Ag>Sn>OSP in all preconditions of the samples.



Table 10. Ranking of PWB finishes in the as-received condition.

Solder Alloy DR /DP for Reflow inAir

DR /DP for Reflowin Nitrogen

Sn37Pb NiAu>Sn>OSP, Ag NiAu>Sn>OSP>AgSn3.4Ag4.8Bi Ag=OSP=Sn=NiAu NiAu>Sn>Ag>OSPSn4.0Ag0.5Cu NiAu>Sn>Ag>OSP NiAu>Sn>Ag>OSP

Sn3.5Ag NiAu>Sn>Ag>OSP NiAu>Sn>Ag>OSPSn0.7Cu NiAu>Ag>Sn>OSP NiAu>Sn>Ag>OSP

Table 11. Ranking of PWB finishes after two reflow cycles



Sn37Pb NiAu>OSP=Sn=Ag NiAu>OSP>Sn>AgSn3.4Ag4.8Bi NiAu>Ag>Sn=OSP NiAu>Ag>Sn>OSPSn4.0Ag0.5Cu NiAu>Ag>Sn>OSP NiAu>Ag>Sn>OSP

Sn3.5Ag NiAu>Ag>Sn>OSP NiAu>Ag>Sn>OSPSn0.7Cu NiAu>Ag>OSP=Sn NiAu>Ag>Sn>OSP

Table 12. Ranking of PWB finishes after four reflow cycles



Sn37Pb NiAu>OSP=Ag>Sn NiAu>OSP>Ag=SnSn3.4Ag4.8Bi NiAu>Ag>OSP=Sn NiAu>Ag>Sn>OSPSn4.0Ag0.5Cu NiAu>Ag>OSP>Sn NiAu>Ag>Sn>OSP

Sn3.5Ag NiAu>Ag>OSP>Sn NiAu>Ag>Sn>OSPSn0.7Cu NiAu>Ag>OSP=Sn NiAu>Ag>Sn>OSP

The Ag finish is statistically better than the NiAu from the wetting balance experiments and vice versa from the spread tests.This difference can be attributed to a composition difference in the no clean flux used in these respective experiments and thedynamic differences in the test methods.

Conclusions

1. Sn is the best finish when used in the as-received condition. Sn is not suitable choice for a PWB finish in applicationswhere the assembly process involves multiple reflow cycles. Multiple reflow cycles for Sn PWB finish decreased thesolderability significantly.

2. NiAu and Ag PWB finishes are the best in a process that involves multiple reflow cycles.3. OSP did not fare well with any of the Pb-free alloys in all preconditions.4. Better spreading was observed when a nitrogen atmosphere was used.5. A halo ring concentric with the solder dot was observed on the NiAu PWB finish using SnAg and SnCu solder alloys.

This is due to the possible formation of lower melting Au-Sn compositions.6. SnPb solder paste, in general, spread the most and SnCu solder paste, with exceptions, spread the least in spread tests in

all preconditionings and in air and nitrogen atmospheres.7. In general, SnAg solder alloy had the fastest wetting times (Ta) and slowest Ta was recorded with SnAgBi alloy in the

wetting balance tests in all preconditioningsAcknowledgments

The authors would like to acknowledge Alpha Metals Inc., NJ for providing the alloys and fluxes and Photocircuits, NY for providing the test coupons used in this research. The authors would also like to acknowledge the industrial members of theCenter for Advanced Vehicle Electronics for their support of this project. Finally the authors would like to acknowledge thesupport of Heller Industries and MPM Corporation by providing equipment used in these experiments.

References

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