This paper discuses the balance between bow and adhesion in developing a BSF paste with EVA used for adhesion testing.

1. NOVEL C-SI METALLIZATION ADHESION TESTING USING MODULE ASSEMBLY MATERIALS
Rick Lathrop and Eduardo Paz
Franklin Advanced Materials
320 Circle of Progress Drive, Suite 102
Pottstown, PA 19464
ABSTRACT: In the crystalline PV module, the integrity of EVA to cell metallization adhesion is imperative for long
term reliability. In traditional module layups the c-Si wafers are encapsulated in EVA polymer for environmental
protection, optical coupling, shock absorption, and dielectric properties. In addition to glass and back sheet adhesion,
the adhesion of the EVA to BSF aluminum and front contact silver metallizations must be robust. With the
introduction of very low bow BSF formulations, the BSF to wafer adhesion may be compromised. Further
complicating this reliability issue is the lack of an industry standard to test BSF-wafer adhesion. This paper describes
and discusses several methods for testing the adhesion of these interfaces using EVA as the adhesive. Measurements
of the EVA to BSF adhesion, which can be significantly reduced by excessive micro-pilling or surface dusting, are
discussed. This paper will detail the test setups for both nail head type tensile testing and peel strength testing using
FPE backsheet for BSF adhesion. Steam aging effects are also explored and reported. Lastly adhesion-bow tradeoffs
are discused and best-of-class bow data presented.
Keywords: Back-Surface-Field, Metallization, c-Si, EVA, Bow
1 INTRODUCTION Failure mechanisms varied from epoxy-BSF surface to
The PV industry lacks established adhesion test methods wafer break. Since the BSF microstructure is fairly
for BSF metallizations, similar to front and back contact porous under normal circumstances as can be seen in
solderability adhesion testing. This presents a challenge Figure 1, the test became questionable as to whether the
for both developers and users of c-Si BSF aluminum heated epoxy was influencing the BSF adhesion by
pastes to perform their own due diligence to ensure that actually strengthening the film during cure. The Quad
the BSF has a robust and reliable bond to the wafer Group Inc. states that their epoxy coating goes from
surface. At the end of the module value chain the bond of enamel-like to water consistency just prior to
both front and back metallizations to the Ethylene Vinyl polymerizing [1]. Further work involved screening
Acetate (EVA) encapsulant is of equal importance. numerous non-heat cure adhesives to replace the epoxy
Since the sintered aluminum BSF surface is unsolderable, and small nail and stud materials. The best alternate
ribbon peel tests used to test both front and back contact adhesive found was the Loctite 454 surface insensitive
silver metallizations are not applicable. For other gel cyanoacrylate. However, there were still drawbacks
unsolderable thick films such as thick film dielectrics and with this method. The first drawback was that this
glazes the Quad Group Inc. devised a tensile pull test adhesive was sensitive to the nail or stud surface
using b-staged epoxy coated studs. The epoxy is coated metallurgy. The Loctite 454 adhesive worked well with
only on the head of a precision nail shaped stud. This zinc plated nails but not plain steel. This limited our
stud is then clamped perpendicular to the surface under choice to a nail type with an irregular shaped head, which
test and cured at 125°C for 10 minutes. When the epoxy produced non-perpendicular bonds and quite a bit of
is cooled, a very high tensile strength bond is formed to variability in the pull data. The second drawback was the
the test surface. For delicate substrates like a silicon question of applicability to steam aged BSF films.
wafer, epoxy coated alumina coupons can be adhered to Cyanoacrylate glues cure with humidity and we saw an
the back of the wafer to strengthen the assembly. The increase in adhesion after steam testing the BSF. The
stud is then pulled until the weakest bond is broken and third drawback was the question of how “real world” this
peak tensile force is recorded. This was the first test was. Although there are tensile forces on the BSF
generation adhesion test developed in-house for BSF. film in the PV module, there are no cyanoacrylates.
2 BSF MEASUREMENT
With the beginning of the in-house development of the
EVA peel test, a way of implementing EVA as the
adhesive in the pull test was the next logical move. EVA
samples were acquired from STR Inc. Solar Division.
Specifically we are using their Photocap fast cure
15295P/UF EVA formulation. EVA is one of the most
popular thermosets for encapsulating the wafer in the
module. EVA is also the “real world” material that needs
to reliably bond with both the BSF aluminum and the
front contact silver.
2.1 EVA Pull Test
The EVA is in sheet form, and a small disc would be the
obvious form factor for the pull test. To achieve this, a
Figure 1: Porous Fired BSF Microstructure
hole punch was utilized to punch a 3mm diameter disc

2. from the EVA film. Figure 2 illustrates this pre-cure the wafer width with the backsheet strip placed directly
assembly. The clip not only provides positive alignment on top of the EVA, justified to one end of the wafer. A
of the nail and the EVA disc but also puts pressure on the semi-rigid piece of Teflon sheet is placed on top of the
EVA, similar to a laminator. The assembly is placed in an backsheet followed by a section of stainless steel U-
oven and allowed to reach 150°C for 10 minutes. After channel to provide compression during the cure, similar
cooling, the clip is removed and the coupon is clamped to to a laminator. The tray with this assembly is placed in a
box oven and allowed to reach 150°C for 10 minutes to
cure the EVA. The actual time that this assembly is in the
oven is about 30 minutes. After cooling, the wafer with
the laminated EVA-backsheet strip is placed in the peel
test fixture. This fixture is mounted on two linear
bearings to ensure a true 180 peel test. The backsheet is
peeled at 200mm/min until the end of travel on the test
EVA stand is achieved as can be seen in Figure 4. The entire
peel test is recorded and plotted (force vs position). The
failure mechanisms are recorded.
Figure 2: Nail-EVA-Wafer Coupon Assembly
the upper surface of a pair of linear bearings mounted to
the test stand stage. These bearings ensure a pure tensile
pull with no shear components to the test. A flat washer
with a 4mm center hole is placed on the upper surface to
limit the pull forces to the immediate area surrounding
the nail head as shown in Figure 3. Use of this washer
elliminates the need to strenghten the wafer prior to test.
The pull test is performed at a pull rate of 5mm/min until
a peak force is recorded. Failure mechanisms are also
noted. This method has been used to test front contact
and both pre and post steam aged BSF adhesion. Backsheet
Wafer
Figure 4: 180 Degree EVA Peel Test
2.3 BSF Tape Peel Test
The third BSF adhesion test is a scotch tape test that is
performed similarly to the EVA peel test. Industrial clear
3M adhesive tape (#600) is cut to about 2 1/2 times the
wafer width to enable a 180 degree peel. This test
Figure 3: EVA Pull Test Fixturing
originated from the printed circuit board industry and can
show gross failures of the sintered aluminum BSF
2.2 EVA BSF Peel Test coating. The tape width is 0.75”. The entire length of the
The EVA peel test has been fashioned after our in-house wafer is covered with this tape. The tape is pressed on
front and back contact silver solder adhesion peel test [2]. with finger pressure, folded back on itself, placed in the
Since the cured EVA is highly elastic, it needed to be peel test fixture and peeled at 180 degrees at a rate of
strengthened with a more rigid material. To accomplish 500m/min until the end of travel on the test stand is
this, Dun-Solar FPE backsheet was acquired from the achieved. The section of the tape that was peeled from
Dunmore Corporation. A 4mm wide strip of backsheet the BSF surface is placed on white paper and examined
and EVA are cut with the outer Fluoro layer of the for any lifted aluminum.
backsheet marked to indicate that the PE inner layer on The tape test can be considered, for the most part,
the other side needs to be mated with the EVA prior to redundant to the EVA peel test but is prefered by some to
cure. The length of the backsheet strip is about 2 1/2 be a simple go no-go test. The EVA peel test is a much
times the wafer width to enable a 180 degree peel, while more severe adhesion test in that peel force per mm has
the EVA strip length is cut about 1cm short of the wafer been found to be more than 20 times that of the tape test.
width to avoid any EVA flowing onto the tray during the
cure process. The wafer under test is placed on a metal 2.4 Bow Measurement
tray with the BSF side up; the EVA strip is centered on The aluminum BSF backplane dominates the finished cell

3. bow behavior. This is largely due to the glass frit binder
used in BSF pastes [3]. This glass frit is necessary to An interesting comparison can be seen in Figure 6. This
provide adhesion to the wafer surface. The thinner the comparison shows a slightly higher median adhesion of
wafer the more pronounced the bow behavior is. The the LunAl 988-F BSF paste over the bare wafer back
nominal wafer thickness is 180 microns. Severe bow surface and a significantly higher median adhesion of a
causes handling issues at both cell manufacture and large pad of front contact formulation SunAg 898-L2
ribbon attachment during module assembly. over the bare wafer front surface. The bare wafer front
The in-house procedure developed for measuring bow is surface adhesion is higher than the back presumably due
a relatively simple non-contact method. First, a finished to texturing of the front surface. When testing a 1.8mm
cell is placed with the front contact side up on a granite wide front contact buss bar, the adhesion is similar to the
slab and a laser triangulation sensor is zeroed to the bare wafer surface. This is likely due to the overhang of
center of the relaxed cell. Second a large “C” washer is the nail head and EVA onto the bare wafer. Equal or
placed around the center to compress the bow as in better EVA adhesion to the wafer surfaces of these
Figure 5. The weight of this washer is 105 grams. The metallization films is considered excellent.
distance reading on the sensor is the bow in millimeters.
3.2 EVA and Tape Peel Results
The plot below (Figure 7) shows a consistent peel in the
25 to 35 Newton range with no removed BSF in the
picture overlay. The strip of backsheet is approximately
4mm wide. The tape test (Figure 8) shows a very
consistent peel in the 10 Newton range. The tape is
19mm wide.
Laser Spot
“C” Washer
Figure 5: Cell Bow Measurements
3 RESULTS
The results reported in this section are in box plot form Figure 7: BSF EVA Peel Test
for pull tests and force-displacement curves for peel tests.
BSF data is from the LunAl aluminum paste series and
front contact data from the SunAg paste series. Wafers
were 156mm multicrystalline with a nominal thickness of
180 microns. Peak firing temperatures were in the 775°C-
790°C range.
3.1 EVA Pull Results
Figure 8: BSF Tape Peel Test
3.3 Steam Aging Effects
Although cells are encapsulated with EVA, moisture
intrusion into the module and its negative effects on
reliability are of concern. The porous nature of the BSF
microstructure, and thus the potential vulnerability to
moisture, has given rise to various methods of accelerating
the long-term effects of moisture. Typically the accelerant
is temperature in the form of damp heat [4], steam or
boiling DI water. As mentioned earlier, the cyanoacrylate
adhesive was discounted for steam aged BSF adhesion
testing due to its curing method (humidity) and the trend of
increasing BSF adhesion after steam testing. Figure 9
shows that the EVA adhesion testing also increases in both
steam and boiling DI water. The strengthening of the BSF
Figure 6: EVA Pull Adhesion film seems to be independent of both the adhesive used and
the accelerant method.

4. small sections of BSF ripped off the wafer when peel
forces exceeded the materials adhesion to the wafer.
Once a section was removed, peel forces were relieved
then would build up to a failure level and a new section
would be removed. This cycle continued throughout the
test as can be seen in Figure 11. The glass containing
paste retained all of the material on the wafer as can be
seen at the bottom in Figure 12.
Figure 11: BSF Adhesion Failure
Figure 9: Effects of Moisture on BSF Adhesion
4 BALANCING BOW AND ADHESION
Common to all thick film formulations is balancing key
properties to optimize the overall performance of the
product. Key properties of BSF aluminum paste are wafer
bow and adhesion. Figure 10 illustrates this point well. Figure 12: Glass-Free Formula on Top
Glass A is driving adhesion up at the expense of bow.
Glass B is driving bow down at the expense of adhesion.
A blend of these glasses provides optimization of both of 4.2 Low Bow Formulation
these key properties. By moving to an optimized blend of glass from a single
glass formula, and careful selection of the aluminum
source and powder specifications the current generation
II BSF metallization paste (LunAl 988-F) demonstrates
best-in-class low bow. Figure 13 shows this performance
against generation I and a leading competitor at three
different firing temperatures.
Figure 10: BSF Glass Formulation Optimization
4.1 Enough Adhesion
Since there is a tradeoff between adhesion and bow and
there are no industry standards for adhesion, how much
adhesion is enough?
Two experimental BSF pastes were created, one with no
glass and one with glass in the normal range of 2-5%.
The tape test was performed with little contrast between
the two formulas. The peeled tape was slightly darker
with the glass-free formula but too similar to the glass
containing formula to be photographed for this paper. Figure 13: Best-in-Class Bow
The EVA peel test, however, showed a significant
contrast in failure modes. With the glass-free material,

5. 6 CONCLUSIONS
• EVA is an ideal real world adhesive for both
tensile pull and peel testing. For peel testing it
must be strengthened with backsheet.
• Steam or boiling water conditioning increases
BSF adhesion using either cyanoacrylate or
EVA adhesives.
• Glass frit is required to give adequate adhesion
but can have a negative impact on wafer bow.
• Careful selection and characterization of the
glass or glass blends can optimize bow and
adhesion BSF properties.
• Best-in-class sub half millimeter total bow can
be achieved on 125 monocrystalline wafers
with diligent formulation and material
selection.
7 REFERENCES
[1] Quad Group Inc. website, “Stud Pull Tests”,
http://www.quadgroupinc.com/studpull.html
[2] Lathrop et al: “Novel Approaches to Benchmarking
Solar Cell Tabbing Solderability”, Proceedings 26th
EU PVSEC, 2011 Hamburg
[3] Carroll et al: “Advances in PV Metallisation
Technology”, Proceedings 20th EU PVSEC, 2005
Barcelona
[4] Ketola et al: “Degradation Mechanism Investigation
of Extended Damp Heat Aged PV Modules”,
Proceedings 26th EU PVSEC, 2011 Hamburg