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 University
162 Broun Hall
Auburn, AL 36849
Email: johnson@eng.auburn.edu
Abstract
For a successful transition to Pb-free manufacturing in electronics assembly, it is critical to understand the behavior of Pbfree 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 alloys and Pb-free PWB finishes. The solder alloys studied were Sn3.4Ag4.8Bi, Sn4.0Ag0.5Cu, Sn3.5Ag and Sn0.7Cu. Eutectic
Sn37Pb was used as a reference. The PWB surface finishes were Sn, NiAu, Ag and OSP. Wetting balance experiments were conducted in air while the spread tests were performed in air and nitrogen to understand the effect of reflow atmosphere on the spreading. Surface analyses techniques such as Nomarski phase contrast microscopy, Auger Electron Spectroscopy (AES) and
X-ray Photoelectron Spectroscopy (XPS) were used to characterize the as-received PWB finishes. Sequential Electrochemical
Reduction Analysis (SERA) was also performed on the as-received PWB test coupons and on the Sn test coupons after multiple reflow cycles. The effect of multiple reflow cycles on the wetting performance, spreading and the surface composition of the
PWB 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 proposed legislative 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, recycling requirements 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, the elimination 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 as
SnAg for hand and repair soldering and SnCu for wave soldering applications are also being used. Additional issues are solderability, 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 reported in 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. The experimental results presented will constitute an important part of the database that is required for the implementation of the
Pb-free manufacturing in the electronics industry.
Pb-free assembly requires Pb-free solders as bulk solder alloys (for wave soldering applications), solder paste (for surface mount technology (SMT)) and wire (for hand and repair soldering). In addition to the usage of Pb-free solders, the PWB finishes must 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 various factors such as PWB finish, solder alloy, flux, soldering atmosphere, surface roughness [8], test temperature, solder – substrate interactions, etc. For this reason, accurate determination and repeatability in measurement of the wetting angles have been an area of debate. Very limited data has been for combinations of solder alloys, fluxes and finishes using different methods to measure 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 with reasonable 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 provide comprehensive insight into the wetting process.

In this research, the wetting performance of several Pb-free solder pastes and alloys, obtained from a single source, on various PWB finishes, manufactured by a single fabricator was studied. This work involved performing wetting balance experiments using alloys in the bulk form and spread tests based on printing and reflow of solder pastes on the entire test matrix of finishes and preconditions. Nomarski phase contrast microscopy, Auger Electron Spectroscopy (AES), X-ray Photoelectron
Spectroscopy (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 were used 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 in
Table 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 Paste Melting Reflow
Composition Point ( ° C) Temp
( ° C)
(
∆
°
T
C)
Sn3.4Ag4.8Bi 205-210 250 ~45
Sn4.0Ag0.5Cu ~217 250 33
Table 2.
Bulk solders alloys and test temperatures.
Solder
Alloy
Melting
Point ( ° C)
Test Temp
( ° C) (
∆
°
T
C)
Sn3.4Ag4.8Bi 205-210 235 25
Sn-3.8Ag0.7Cu 217 235 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 PWB finishes from various manufacturers have previously been evaluated in the bulk form using a wetting balance. For the same type of PWB finish, the wetting performance varies with the board manufacturer and the type of flux [12], [13]. In this work, test coupons 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 in accordance 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 260 o C and ii) the time above 217 o C was 55 seconds. The purpose of the reflow cycles was to determine the effect of multiple reflow cycles on the solderability of the Pb-free boards. A Heller 1700 forced 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-locked
Kratos XSAM 800 surface analysis system. The base pressure of this ion and turbo-pumped system was 1 x 10 a calibrated, nude ion gauge.
-8 torr as read on
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) mode with 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 smallest allowable (5 mm 2 ) area on the specimen.
The XPS energy scale was calibrated by setting the binding energy of the Ag 3d radiation) to exactly 368.3 eV referenced to the Fermi level. The accuracy of the spectrometer ramp was verified by also measuring the Cu2p
3/2
5/2
line on clean silver (excited by Mg K α
line on clean copper. The measured energy was 932.7 eV, which is the correct recommended value. The
AES 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 source with rastering and focusing capabilities. A variable scan voltage provides a high frequency scan mode that yields a sputtered area of 1 cm 2 at the specimen position. Auger spectra were taken from a focused spot at a position near the center of the sputtered area. The angle of incidence of the ion beam with respect to the surface normal was 55 o .
All Auger spectra were recorded at 3.0 keV beam energy and 0.7 µ A 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 a single channel multiplier and a fast response head amplifier. Detector output modes include direct pulse counting and current detection with voltage to frequency (V-F) conversion. Due to the large exciting currents used, all spectra were taken in current detection 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 PWB finishes in the as-received condition to determine the thickness of the finish on copper. This was done by chemically reducing the surface. The thickness was calculated based on the time it took to reduce the layer. In addition, SERA was performed on the
Sn finish after two and four reflow cycles to ascertain the growth of the Sn oxide layers. Also, using different chemistry, the intermetallics 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 the examination of solderability on different combinations of solder alloys, components and fluxes [9], [18-21]. Given the amount of 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 were
T a
and F max
. T a
is the time to the buoyancy corrected force line. At this time the solder contact angle to the test coupon is 90 o and the wetting forces pulling the coupon into the molten solder equals the buoyancy force pushing the less dense coupon out of the molten solder. The smaller the T is the maximum wetting force exerted by the solder on the coupon and is directly proportional to the height the solder climbs up the coupon. F force and the maximum force, F a
, the quicker the solder wets to the coupon. F max max
is the sum of the buoyancy inst
, 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 were conducted in accordance with the standard IPC J-STD 003. The solder alloys were brought to within ± 1 ° C of the specified test temperature (see Table 2). The coupon was dipped in flux for 5 seconds and the excess flux was drained off. After fluxing, the coupon 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 a constant 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 force vs. time were obtained and used to extracted values for wetting time (T a
) and maximum wetting force (F max
), measured relative to the buoyancy-corrected zero force line. Measurement with respect to the buoyancy-corrected zero force line and not to the instrument zero force line allows the data obtained to be independent of the sample size and shape.
F inst
F max
Instrument zero force line
0
Buoyancy force line; θ =90º
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 mm 3
= 50*10 -9 m 3
Buoyancy Force (F b
) = volume ( ν )*density of the solder ( ρ = 8360kg/m 3 )*acceleration due to gravity(g = 9.81m/s 2 )
= 8360*9.81*50*10 -9
F
F
= 4.1mN for the SnPb eutectic alloy b
is calculated for the solders used in this study and are presented in Table 3. These values are incorporated in the calculation of max
presented in later sections.
Table 3.
Calculated F b
Solder Alloy
values for solder alloys
Density
(gm/cm 3 )
Buoyancy Force
(F b
) (mN)
Sn37Pb 8.36
Sn3.4Ag4.8Bi 7.50
1
1
3.679
3.679
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 point of view. Two laser cut, 5mil thick stainless steel stencils were used to print solder pastes. A preliminary stencil (Stencil # 1) was designed with apertures of various sizes and shapes as shown in Figure 3.
Initial solder paste printing experiments using an automatic stencil printer (MPM-AP25) were conducted using the stencil # 1. The purpose of these experiments was to determine the size and shape of the aperture that provided the most consistency in printing. A measuring system mounted on an optical microscope 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.

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 is shown in Figure 4. The same automatic MPM-AP25 stencil printer was used to print the solder paste onto the test coupon with
Stencil # 2. The dimensions of the printed dots of the top two rows in Figure 4 were measured after printing (D
P reflow (D
R
/ D
P
) and after
R
). 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. D 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 is shown in Figure 5 and for the Sn37Pb eutectic solder paste is shown in Figure 6. The same reflow temperature profiles were used 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 photograph shows 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 the atmosphere. The sputter rate (30 Å/min) was calibrated by recording the time required to sputter through a known, standard thickness of SiO
2
. Assuming an identical sputter rate on each board finish, roughly 150 Å of each surface was removed during the 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 the surface.
Table 4.
XPS Surface Elemental Composition of PWB Finishes
Surface Elemental Composition (atom %)
PWB
Plating
Finish
Sn 1
Sn 2
Ag 1
Ag 2
Pd 1
Pd 2
NiAu 1
NiAu 2
19.6 0.0 0.0 0.0 0.0 46.8 33.7 0.0 0.0
86.7 0.0 1.8 0.0 5.6 0.0 5.9 0.0 0.0
0.0 0.0 0.4 0.0 0.0 48.9 7.3 0.0 43.5
0.2 0.0 0.0 0.0 16.4 83.4 7.3 0.0 0.0
0.0 0.0 0.0 0.0 67.7 32.3 0.0 0.0 0.0
0.7 0.4 0.0 0.0 0.0 61.7 10.4 26.9 0.0
0.0
OSP 1
OSP 2
/6.9
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
(SnO
2
, SnO) were observed on the as-received surface. After 5 minutes of Ar + sputter cleaning, the C signal vanishes and the oxygen concentration decreases dramatically, indicating that the oxide layer is ≤ 150 Å. The chemical states were clearly observed in the high resolution spectrum over the Sn3d envelope yields the amount of each surface species present (see figure inset). For the as-received board finish, SnO predominant Sn oxidation state (71%).
5/2
XPS feature shown in Figure 8(c). Deconvolution of the peak
2
is the

S Cl
C
Sn
Sn
Sn O
100 200 30 0 400 500 600 700 800 900 1000
Kinetic Energy (eV)
8 (a)
Sn3d
O1s
600 500
C1s
400 300 200
Binding Energy (eV)
8 (b)
Sn4s
Sn4d
100 0
8(c)
Figure 8.
Surface analysis of the Sn board finish. (a) AES (b) XPS survey spectra and (c) High resolution XPS spectrum over Sn3d
5/2
. 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 surface was SnO
2
: 76%, SnO: 12%, Sn: 12%, shown in Figure 9(c). After sputter etching, these surface contaminants disappeared and the surface was 100% Au. XPS data after multiple reflow cycles shows that Au was present on the surface. Availability of Au on the surface prevents the oxidation of the underlying Ni and Cu layers thereby preserving the solderability.

Au/S
Cl Ca
N
Sn
O
Ni Ni Ni
Ni
Au
C
100 200 300 400 500 600
Kinetic Energy (eV)
9 (a)
700 800 900
Au4f
Au4p
O1s
Sn3d
Ni(L MM)
Au4d
C1s
600 500 400 300 200
Binding Energy (eV)
9 (b)
100 0
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 Sn3d
5
. 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 (Figure
10(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 Ag
2
O 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 reflow cycles.
Si
S
Ag
C+Ag
O
Ag
100 200 300 400 500 600 700 800 900 1000
Kinetic Energy (eV)
10 (a)
Ag3d
Ag3p
O1s
600 500
C1s
400 300 200
Binding Energy (eV)
10 (b)
Pb4f
Ag4s
Ag4p
100 0
Figure 10.
10 (c)
Surface analysis of the Ag board finish. (a) AES survey spectrum (b) XPS survey spectrum and (c) High resolution XPS spectrum over Ag3d
5/2
. 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%) (Figures
11(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 a trace amount of Si (0.9 atomic %). Cl and Cu were consistently found in OSP films from various manufacturers. Cu compounds increase 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-H x bonds indicated in Figure 11(c).
Si
Cu
Cl
Cu
N
O
Cu Cu
100 200
C
300 400 500 600
Kinetic Energy (eV)
11 (a)
700 800 900
C1s
600
O1s
Cu(LMM)
N1s
Cu(LMM)
500
Cl2p Cu3s
Si2s
Si2p
Cu3p
100 400 300
Binding Energy (eV)
11 (b)
200 0
Figure 11.
11 (c)
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*10 8
N*F*S*D where
M = molecular weight
I = current t = reduction time
N = number of electrons
D = density
S = surface area
T = thickness of film
F = Faraday’s constant
The calculated thickness of the uncombined Sn, Cu
3
Sn intermetallic layers are summarized in Table 5. Figure 12 shows the oxidation potentials of the Sn PWB finish after different preconditionings. Sn, Cu
3
Sn regimes are marked on the graph.
6
Sn
5
and Cu
6
Sn
5
and Cu
Table 5.
SERA results for Sn PWB finish showing intermetallic layer thickness.
Preconditioning Sn ( µ m) Cu
6
Sn
5
( µ m)
Cu
3
Sn
( µ m)
As-received 0.857 0.705 0.244
After 2 reflow
Cycles
After 4 reflow
Cycles
After storage for a month
After 6 months of storage
After 9 months of storage
0.0 0.916 0.176
0.0 0.893 0.197
0.834 0.702 0.230
0.501 0.790 0.282
0.473 0.834 0.310
0.5
0.4
0.3
0X
2X
4X
0.2
0.1
0
-0.1
Cu
6
Sn
5
Cu
3
Sn
-0.2
-0.3
Sn
-0.4
-0.5
-0.6
0 100 200 300 400 500 600
Time of Measurement
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 change substantially from two to four reflow cycles. The intermetallic layers grow in thickness at the expense of the Sn. Also, there is a marked 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
Table 6. There is a significant increase in SnO
Sn helps explain the low F max
2
after two reflow cycles, but as most of the Sn has been consumed, there is very little change in the thickness between two and four reflows. SnO
2
. The effect of the multiple reflow cycles is summarized in
2
has a high reduction potential, which means that it is more difficult for the flux to reduce, thereby inhibiting the wetting process. A combination of increased SnO
2
and little uncombined
values obtained with Sn boards in the wetting balance test after multiple reflow cycles. Also, the growth 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 SnO
SnO
2
present on the surface. As shown in Table 6, SERA cannot detect the
2
in the as-received condition, although as shown in Figure 9(c), XPS does detect SnO
2
. This is a resolution issue and not a discrepancy.
Table 6.
SERA results for Sn PWB Finish showing oxide layer thickness.
Preconditioning SnO (Å) SnO
2
(Å)
Oxidized intermetallic
(Å)
As-received 31 Not detected
After 2 reflow Cycles
After 4 reflow Cycles
After storage for a month
18
15
0.28
0.34
Not detected
Not detected
After 6 months of storage
After 9 months of storage
0
-0.2
0X
2X
4X
-0.4
-0.6
-0.8
SnO
SnO
2
-1
-1.2
-1.4
Figure 13.
0 50 100 150
Time of Measurement
200 250 300
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 µ m from the oxidation of Au in SERA study. The thickness of the OSP coating was determined to be 0.992 µ m 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 F max
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 between the solder alloy and the finish under the given conditions. The magnitude of F max
indicates the height to which the solder climbs up 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 F max
value is close to zero, this means that the solder has barely climbed up the coupon and that it has barely crossed the buoyancy corrected force line. The contact angle in this case is only slightly less than 90 ° . This is shown in Figure 14 (b). Negative F
Figure 14 (c). The T a max
values indicate poor or no wetting at all. This is shown in
values for this case are equal to the duration of the test (10 seconds).
Figure 14.
a) θ << 90 ° b) θ ≤ 90 ° c) θ > 90 °
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 wetting occurred with all of the solder alloys as indicated by the high F max
values in the as-received condition. There is a significant drop in the F max
values after multiple reflow cycles. Very little wetting occurred with SnPb, SnAgCu and SnAg solder alloys and no wetting with SnAgBi and SnCu solder alloys after two reflow cycles. After four reflow cycles, there is no wetting with all of the solder alloys. As shown in Figure 16, T
High T a
values are approximately 3 seconds for coupons tested in the as-received condition. a
values can be seen for alloys with some wetting and equal to 10 seconds for alloys with no wetting after two reflow cycles. T a
values are equal to 10 seconds for all Sn coupons after four reflow cycles. In general, the Sn finish in the as-received condition had the highest F max
values of all of the alloys. However, solderability significantly degraded with multiple reflow cycles in air. The SnAg alloy wet the as-received Sn finish fast corresponding to low T a
values.
8.0
6.0
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
4.0
2.0
0.0
-2.0
-4.0
Figure 15.
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
0X 2X 4X
Condition
F max
for Sn PWB finish with various alloys
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
1.0
0.0
Figure 16.
0X
T a
for Sn
2X
Condition
4X
PWB finish with various alloys
NiAu Finish – As shown in Figure 17, good wetting occurred with the NiAu PWB finish corresponding to high F max
values.
After two and four reflow cycles, F max
values are still reasonably high with the exception of the SnAgBi alloy. This indicates that the NiAu PWB finish survived the multiple reflow cycles. There is minimum wetting after two reflow cycles and negative wetting after four with the SnAgBi alloy. Corresponding T a cycles.
values are shown in Figure 18. It can be seen that the T a alloy is about 9 seconds after two reflow cycles and at the maximum limit of T a
The SnAg and SnCu alloys wet fast on the NiAu boards. F max
for SnAgBi
after four reflow cycles, indicating no wetting.
values decrease and T a
values increase with multiple reflow
8.0
6.0
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
4.0
2.0
0.0
-2.0
-4.0
Figure 17.
0X 2X 4X
Condition
F max
for NiAu PWB finish with various alloys

6.0
5.0
4.0
3.0
2.0
10.0
9.0
8.0
7.0
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
1.0
0.0
0X 2X 4X
Figure 18.
Condition
T a
for NiAu PWB finish with various alloys
Ag Finish – As shown in Figures 19 and 20, the Ag board finish recorded positive F seconds in all of the three preconditions with all of the solder alloys. The F max increases, in general. The F max max
and T a suggesting that the deleterious effect of multiple reflow cycles is less pronounced for Ag PWB finishes.
values lower than four
decreases with the number of reflow cycles and T a
values of Ag finish are greater than NiAu, in general, for all alloys and in all preconditionings
8.0
6.0
4.0
2.0
0.0
-2.0
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
-4.0
Figure 19.
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0X 2X 4X
Condition
F max
for Ag PWB finish with various alloys
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
0.0
0X 2X 4X
Figure 20.
Condition
T a
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 test coupons in the as-received test condition. F max
values are the lowest in all preconditions with all the solder alloys for the OSP coating. There is a significant drop in the F slightly positive to negative F
Corresponding T a max
values after two reflow cycles with the SnAgBi and SnCu alloys, going from max
. There is no evidence of any wetting after four reflow cycles with any of the alloys.
values are shown in Figure 22.

8.0
6.0
4.0
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
2.0
0.0
-2.0
-4.0
Figure 21.
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
0X 2X 4X
Condition
F max
for OSP PWB finish with various alloys
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
2.0
1.0
0.0
0X 2X 4X
Figure 22.
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 Sn finish board in air and nitrogen atmospheres. For the SnPb solder paste, the spread ratio was almost 2.0 in air. This means that the SnPb paste spread to twice its original printed diameter. For Pb-free solder pastes, the spread ratio was slightly above 1.0 in the 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.
5.0
4.5
4.0
Sn37Pb
Sn3.4Ag4.8Bi
Sn4.0Ag0.5Cu
Sn3.5Ag
Sn0.7Cu
3.5
Condition
T a
for OSP PWB finish with various alloys
3.0
2.5
2.0
1.5
1.0
0.5
0.0
As Recd:Air As Recd:N2 2X:Air 2X:N2 4X:Air 4X:N2
Condition
Figure 23.
NiAu Finish – The spread ratios are noticeably higher in nitrogen than in an air atmosphere. Also, the difference in the spread ratio for the SnPb solder paste is significant in the as-received condition as well as after multiple reflow cycles. The spread ratios for the Pb-free solder pastes are very close to each other. There is a slight decrease in the spread ratios with the multiple reflow cycles indicating a slight drop in the solderability. A halo effect on the NiAu boards was noticed which will be explained in detail in the following sections. The results obtained on NiAu boards are shown in Figure 24.
D
R
/D
P
ratio for Sn PWB Finish.

3.5
3.0
2.5
2.0
1.5
1.0
5.0
4.5
4.0
Sn37Pb
Sn3.4Ag4.8Bi
Sn4.0Ag0.5Cu
Sn3.5Ag
Sn0.7Cu
0.5
0.0
As Recd:Air As Recd:N2 2X:Air 2X:N2 4X:Air 4X:N2
Condition
Figure 24.
D
R
/D
P
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 PWB finishes. There is little difference in the data for air and nitrogen atmospheres suggesting that wetting on Ag boards is independent of atmosphere. There is very little or no effect of multiple reflow cycles on the solderability of Ag boards. The results obtained on Ag boards are shown in Figure 25.
5.0
4.5
4.0
Sn37Pb
Sn3.4Ag4.8Bi
Sn4.0Ag0.5Cu
Sn3.5Ag
Sn0.7Cu
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
As Recd:Air As Recd:N2 2X:Air 2X:N2 4X:Air 4X:N2
Figure 25.
Condition
D
R
/D
P
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 spreads the 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.
5.0
4.5
4.0
Sn37Pb
Sn3.4Ag4.8Bi
Sn4.0Ag0.5Cu
Sn3.5Ag
Sn0.7Cu
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
As Recd:Air
Figure 26.
As Recd:N2 2X:Air 2X:N2 4X:Air 4X:N2
Condition
D
R
/D
P
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 PWB finishes were observed. An scanning electron microscope (SEM) micrograph illustrating this phenomenon is shown in Figure
27 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 D
R
, the diameter of the solder dot after reflow. When the diameter of the halo was considered in the measurements of wetting and spreading, the NiAu PWB finish appeared to be significantly 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., not including 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 D
R
/D
P
, D
R
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 observe and analyze the morphology and composition of the solder dot and halo. A cross-sectional slice through the halo and alloy was also 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. EDS was 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 the image 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 the region identified as the solder dot used to measure D
R
. 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 SERA experiments. The electron beam used in the EDS analysis analyzes material to a depth of approximately 1-2µm below the surface. As there is a very thin layer of solder mixed with Au in the halo region, the electron beam penetrated this layer and detected 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, a sudden 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” are shown in Figures 30 and 31. The grainy nature of this region can be clearly seen from Figure 31. A line scan was performed along 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 and
Au 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 spot marked X in Figure 31 are shown in Figure 33. Quantitative analysis at this spot and at different spots in the same region reveals a stoichiometric ratio of Sn/Au ~ 2/1, suggesting possible formation of the AuSn intermetallic. It is not possible to determine
2 the crystallographic structure of the Au-Sn compound from mere EDS data. Other analytical tools such as X-ray diffraction are necessary 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 micrographs showing 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 the halo 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 the solder dot region and is present in low quantities compared to the solder volume. Substantial amount of Au is present in the halo region (Sn/Au ratio is 2 to 1 as determined by the quantitative anlayis). Au distribution in the halo region is shown in Figure 36.
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 a continuous, smooth interface with no grainy appearance as shown for a spread test coupon. In other words, halo formation did not 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 dipped into 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 continuously changes as gold dissolves into the solder. As the solder alloy, consisting of (predominantly) Sn, melts, wets, and spreads, it dissolves Au from the board finish.
Strictly speaking, a ternary phase diagram for Sn-Cu-Au should be considered in the following explanation. But since a ternary 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
AuSn
4
-Sn hypereutectic region, as shown in the Sn-rich portion of the Au-Sn phase diagram (Figure 38). As shown in the phase diagram, a Au-Sn eutectic exists at 217 ° C and 93.7% (at.) Sn. In comparison, the SnAg eutectic melts at 221 ° C and SnCu eutectic melts at 227 ° C. Sn is the reactive species in these solder alloys. As the solder alloy melts, Sn immediately reacts with
Au 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 its occurrence. As more Au is dissolved from the surrounding area, a solder front emantes and travels in a radial direction away from the solder dot. This phenomenon is illustrated in Figure 39.

Figure 38.
Sn-rich portion of the Au-Sn phase diagram. [24]
Direction of the moving solder front
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 liquid solidifies into different structures as predicted by the phase diagram. As the Au-Sn liquid passes through the two phase region, liquid+AuSn
4
, between 217 ° C and 252 ° C isotherms, some of the Sn is consumed in the formation of AuSn
4
. In this region, Sn concentration 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 AuSn
4
-Sn hypoeutectic 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+AuSn
2 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 AuSn compositions of Au and Sn match the stoichiometric ratio in AuSn
2
2
intermetallic, provided the chemical
. Line scans in this region reveal relatively constant Sn and
Au 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 AuSn
4
intermetallic compound and AuSn
2
is present in the “white” band. The halo formation on NiAu PWB finish has practical implications. When NiAu boards are used in conjunction with the SnCu or SnAg solder alloys, a loss of adhesion between the solder mask layer and the underlying Au may occur due to this halo formation.

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 in
Table 7, in general, OSP had the lowest rank. This means that the OSP coatings had the least F max
and T a
was the highest in the as-received condition. In the as-received condition, the selection of the best finish for a particular alloy depends on whether F a
are used as the criterion. In a wave solder application, a large T max a
value would indicate the need for a longer contact time and T between the board and the solder wave. It is worthwhile to note that the Sn PWB finish, in the as-received condition, recorded the highest F max
values with SnPb, SnAgBi and SnCu solder alloys followed by Ag or NiAu. However, there is a dramatic drop in 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 SnAgBi alloy) 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 condition
Solder Alloy F max
(Best>Worst)
T a
(Best>Worst)
Sn3.4Ag4.8Bi Sn>Ag>NiAu>OSP Sn=Ag=NiAu>OSP
Sn3.8Ag0.7Cu Ag>NiAu>Sn>OSP Sn>NiAu>Ag>OSP
Table 8.
Summary of the wetting balance results with test coupons after two reflow cycles
Solder Alloy F max
T a
(Best>Worst) (Best>Worst)
Sn37Pb Ag>NiAu>OSP>Sn Ag>NiAu=OSP=Sn
Sn3.4Ag4.8Bi Ag>NiAu>Sn=OSP Ag>NiAu>Sn=OSP
Sn3.8Ag0.7Cu NiAu>Ag>Sn>OSP Ag=NiAu>OSP>Sn
Table 9.
Summary of the wetting balance results with test coupons after four reflow cycles
Solder Alloy F max
T a
(Best>Worst) (Best>Worst)
Sn3.4Ag4.8Bi Ag>NiAu>Sn=OSP Ag>NiAu=Sn=OSP
Sn3.8Ag0.7Cu 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 asreceived 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 D
R
/D
P
ratio with all of the Pb-free alloys reflowed in 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, D
R
/D
P, in general for OSP, was equal to 1 indicating no spread at all. For the Sn37Pb alloy, the spread ratio depends 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 D
R
/D
P
for Reflow in
Air
D
R
/D
P
for Reflow in Nitrogen
Sn3.4Ag4.8Bi Ag=OSP=Sn=NiAu NiAu>Sn>Ag>OSP
Sn4.0Ag0.5Cu NiAu>Sn>Ag>OSP NiAu>Sn>Ag>OSP
Table 11.
Ranking of PWB finishes after two reflow cycles
Solder Alloy D
R
/D
P
for Reflow in
Air
D
R
/D
P
for Reflow in Nitrogen
Sn37Pb NiAu>OSP=Sn=Ag NiAu>OSP>Sn>Ag
Sn3.4Ag4.8Bi NiAu>Ag>Sn=OSP NiAu>Ag>Sn>OSP
Sn4.0Ag0.5Cu NiAu>Ag>Sn>OSP NiAu>Ag>Sn>OSP
Table 12.
Ranking of PWB finishes after four reflow cycles
Solder Alloy D
R
/D
P
for Reflow in
Air
D
R
/D
P
for Reflow in Nitrogen
Sn3.4Ag4.8Bi NiAu>Ag>OSP=Sn NiAu>Ag>Sn>OSP
Sn4.0Ag0.5Cu 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 the dynamic 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 applications where the assembly process involves multiple reflow cycles. Multiple reflow cycles for Sn PWB finish decreased the solderability 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 (T a wetting balance tests in all preconditionings
) and slowest T a
was recorded with SnAgBi alloy in the
Acknowledgments
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 the
Center for Advanced Vehicle Electronics for their support of this project. Finally the authors would like to acknowledge the support of Heller Industries and MPM Corporation by providing equipment used in these experiments.
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