PV Cells and Modules Paper

1. TOPIC 5:
Operation, Performance, Reliability and Sustainability of Photovoltaics
Subtopic: 5.4 PV Cells and Modules
Compatibility of PV Ribbons and Fluxes with EVA Encapsulant Films
Narahari S Pujari1, Anna Lifton2 and Mike Murphy2
1: Alpha Cookson India Pvt. Ltd.
Vaishnavi Bhavana 89/1 Industrial Suburb II stage
Yeshvanthpur Bangalore-560094 (India)
Email: Narahari.Pujari@alphaassembly.com
2:Alpha109 Corporate Blvd.,
South Plainfield, NJ – 07080 (USA)
Email: Anna.Lifton@ alphaassembly.com
Mike.Murphy@alphaassembly.com
Executive Abstract
A number of PV (photovoltaic) ribbons and fluxes were tested for their compatibility with EVA
(ethylyne vinyl acetate) copolymer encapsulants used in crystalline-Si (silicon) PV modules. Test
matrix and design of experiments (DOE) were developed to investigate effect of individual parameter
on EVA compatibility and colorization. Matrix consisted of three types of standard PV ribbons and
Alpha’s lead-free EcoSol ribbon, two types of EVA films and Alpha’s four PV fluxes. Visual / Optical
observations of the ribbon, surface appearance, alloy composition and microstructure was done. Effect
of fluxes on gel content and yellowing of EVA was analyzed by prefluxing EVA films and curing
them. Surface contamination, ribbons thickness uniformity and edge exposure and extended exposure
of the ribbon to 85oC/ 85%RH (500 to 1000 hours) and resistance to yellowing of solder alloy itself
were evaluated. Solder joints were produced using segments of Q-cell (cells were singulated into
2.5x2.5mm squares with silver busbar in the middle) and tinned PV ribbons 2.5 cm long using various
PV fluxes. EcoSol ribbons were soldered and laminated at high temperature. Strength of the joints was
measured by Imada pull tester.
It was observed that as expected, PV ribbon coated with various fluxes but not laminated with EVA
appears to be attacked/corroded while the fluxed material which was laminated did not show corrosion,
just discoloration. EVA does protect surface of the ribbon as well as flux residue from corrosion and
oxidation. It could be easily assumed that when glass plate would be used in the whole PV module, the
degree of discoloration/corrosion would be much less (glass is a very good moisture protector). No
significant difference was observed in resistance to yellowing between ribbons coated by different
manufacturers. Fast Cure EVA15295 appears to get more yellow by itself compare to the Slow Cured
EVA A9918. It is clear that bare PV ribbon laminated using either EVA almost did not change its
surface appearance. Alpha’s PV fluxes showed similar percentage of gel formation without any
colorization indicating their compatibility with different types of EVA. No delamination was noted
even with laminating temperature of 155oC with EcoSol alloy. It was observed that solder joints were
strong and alloy does not have any effect of increased lamination temperature and is fully compatible
with standard EVA films.

2. Introduction
Purpose of the work and approach
- Currently the reliability requirements for module service life are very high (for 25-30 years of
service life).
- ‘Packaging is the predominant cause of failure in modules’.
- Also it was noted that some module manufacturers focus solely on the polymer encapsulant
selection and the lamination process. In order to improve the overall module reliability the whole
system need to be considered along with the interaction of polymer encapsulants and the ribbon
coated with PV fluxes.
- This raises concerns for dis-coloration and subsequent loss of efficiency of the module due to the
degradation in the encapsulant properties (yellowing, greening (corrosion), efficiency losses, etc.
- Considering the flux and polymer encapsulant related compatibility concerns, a detailed study was
undertaken. The goal of the DOE was to achieve reliable solder joints with reduced yellowing of
the PV module by optimizing the solder composition and flux chemistries.
- There is a trend of increasing lamination temperature in c-Si module assembly. Standard Sn-Bi
(Tin-Bismuth) alloys are susceptible for failure in joints due to their low liquidus temperature.
Similarly, there is scant amount of information available on compatibility of lead-free alloys with
EVA. To overcome above issues, Alpha’s new lead-free EcoSol alloy was soldered and laminated
at different temperatures. Its effect on EVA and other parameters like solder joint strength was
investigated.
Scientific Innovation and Relevance
DOE FACTORS:
Factor 1: PV ribbons produced by three key manufactures were selected in the study. As different
manufacturers were selected, the dimensions of the ribbons were similar and are listed below:
- Ribbon 1 (supplier1): with Sn62 Pb36 Ag2 with 1.8 mm x 0.12 mm dimensions was used.
- Ribbon 2 (supplier2): with Sn62 Pb36 Ag2 with 1.8 mm x 0.12 mm dimensions was used.
- Ribbon 3 (supplier3): with Sn62 Pb36 Ag2 with 1.8 mm x 0.12 mm dimensions was used
- Ribbon 4 (Alpha, EcoSol): Lead-free Sn-Bi ribbon with 1.8 mm x 0.12 mm dimensions was used
The same solder alloy was chosen in order to elimination the variation in melting
Factor 2: Two commonly used polymer encapsulant materials were used for the evaluation. EVAs
selected for this comparison are listed below:
- Fast Cure EVA15295 : Fast curing EVA was selected with a typical lamination time of 12 min at
160oC.
- Slow Cured EVA A9918: An EVA formulation with a typical lamination time of 25 min at 160oC
was selected.
Factor 3: Different PV fluxes with varying rosin content and solids contents (typically lower
percentage) were selected. PV-21, PV-32, PV-50 and PV-61 were Alpha’s four fluxes selected based
on the outstanding solder joint cosmetics with no clean residue:
TEST MATRIX:
• Initial PV ribbon analysis, b. Visual / Optical observations for the ribbon, c. Surface appearance, d.
Alloy composition and microstructure, e. Surface contamination, f. Thickness uniformity and edge
exposure.
• Gel content: EVA films were prefluxed with four PV fluxes and cured at different temperatures.
Gel content was measured as per ASTM D2765. Effect of flux and temperature on EVA and gelling
was analyzed.

3. • Extended exposure of the ribbon to 85oC/ 85%RH (500 to 1000 hours), Resistance to yellowing of
solder alloy itself
• EcoSol soldering and lamination with and without EVA at various lamination temperatures.
Results and Discussion
Test 1
Initially, all as received tinned copper ribbon produced by different manufacturers were examined.
Surface morphology and condition can affect wetting performance of the assembly. Summary of this
study is given below: Depending on the tinning process, morphology and grain structure of the as
tinned copper ribbons appeared to be different.
- Not a lot of variation in solder thickness was observed from top to bottom of the PV ribbons for all
manufacturers
- As tinned PV ribbon appears to have a variation in solder thickness from side to side (which is
typical)
- Lead free alloy (EcoSol) has a similar appearance as alloys currently in use. Microstructure of the
alloy clearly shows homogeneous and uniform morphology (Fig. 1). This material delivers the
necessary conductivity and wetting and passes the IEC-61215 reliability tests.
Figuure1. Optical analysis and Microstructure of lead-free EcoSol alloy
Test 2 - Lamination Studies:
In order to understand flux-EVA interaction, flux residue oxidation and yellowing (cosmetic aspect) of
the residue, this study was conducted.
Overall Observations:
Figure 2: SEM/EDS of the corrosion on the surface of the tinned copper PV ribbon
- As expected, PV ribbon coated with various fluxes but not laminated with EVA appears to be
attacked/corroded while the fluxed material which was laminated did not show corrosion, just
discoloration.
C
Sn
SnPb
OC
Ag
Area which was not
corroded
C
Sn
Sn
Pb
O
C
Ag
Corroded areas
Solder
Cu

4. - EVA does protect surface of the ribbon as well as flux residue from corrosion and oxidation. It
could be easily assumed that when glass plate would be used in the whole PV module, the degree of
discoloration/corrosion would be much less (glass is a very good moisture protector).
- No significant difference was observed in resistance to yellowing between ribbons coated by
different manufacturers.
- Fast Cure EVA15295 appears to get more yellow by itself compare to the Slow Cured EVA
A9918.
- It is clear that bare PV ribbon laminated using either EVA almost did not change its surface
appearance. Alpha’s PV fluxes do not interfere curing kinetics and no yellowing was observed.
Gel content: All tested four fluxes did not have any effect on EVA and gel content was > 80 % (Fig.
3). No yellowing was noted.
Figure 3. Gel content of prefluxed EVA films.
EcoSol and EVA: PV Ribbon made up of using EcoSol alloy was soldered and laminated at different
temperatures. It was observed that due to high liquidus temperature (179o
C) of Ecosol alloy compared
to conventional Sn-Bi, ribbon could withstand high laminating temperature (155oC). No colorization
noted and solder joints were strong (>3 N/m2). In absence of EVA, pull strength deteriorated which
means EVA itself helps in maintaining good intermetallic. Flux or EcoSol do not seems to have any
adverse effect on EVA.
Pull strength: > 3 N/m2
Figure 4. Lamination and soldering with EcoSol alloy ribbon
Conclusions
• EVA protects the surface of the ribbons especially in presence of front glass would prevent whole
PV assembly from discoloration/corrosion.
• As tinned PV ribbon surface morphology does not lay a significant role in resistance to yellowing
(in case when the same alloy composition was used and surface cleanliness was acceptable).
• Various types of EVA itself exhibit different resistance to discoloration
• Alpha’s PV fluxes are compatible with EVA and show no decolorization
• Lead-free EcoSol alloy could be laminated at high temperatures. It shows good compatibility with
EVA and excellent solder joints. This data further supports transitioning to lead free solar modules.
No
EVA
With EVA