Qualification, Manufacturing, and Reliability Testing
Methodologies for Deploying High-Reliability Solar Modules
by D. DeGraaff, S. Caldwell,
R. Lacerda, G. Bunea,
A. Terao and D. Rose
2010
EXECUTIVE SUMMARY
With over a decade of experience fielding and maintaining solar fields – built with a wide variety of PV
modules from more than a dozen different manufacturers – SunPower has broad experience with reliability.
Combining this field data with the manufacturing rigor inherited from roots in the semiconductor industry has
lead to the development of a robust methodology for deploying high-reliability solar modules. This paper
shares SunPower’s methodology, which spans qualification testing, supplier quality, manufacturing quality
and ongoing reliability testing. For each of these areas, the test development or process development
processes is articulated, along with the specific testing protocols used by SunPower. Also mentioned are
problems that can occur in each of these areas, with examples taken from both SunPower modules and
non-SunPower modules. The most important result of this research is that it requires considerable care and
analysis to design and manufacture high-reliability PV modules, with an emphasis on testing to determine
the physics of the failure modes. This requirement goes well beyond what is required by the UL or IEC
certification standards.
EXECUTIVE SUMMARY
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INTRODUCTION
SECTION 1
SunPower has focused for many years on the challenge of delivering high-reliability products, and has developed
the robust process illustrated in Figure 1. This paper discusses each of these boxes in turn, although it should be
noted that the process is not entirely linear, since new data causes revisions to the FMEA and thus revised test plans.
Figure 1: End-to-end process
By publishing these methodologies, along with many
examples of lessons learned, it is hoped that industry
practitioners may find aspects of this process which
they want to integrate into their own processes and
testing methods; SunPower believes a high prevalence
of robust solar modules will benefit the entire solar
industry.
INTRODUCTION
SECTION 1
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FAILURE MODES AND EFFECTS ANALYSIS
SECTION 2
Building the FMEA
The “Failure Modes and Effects Analysis” (FMEA) is
the foundation of the Design for Reliability process,
since it provides a rank-ordered list of all the known
failure modes and their causal stresses. It underlies the
qualification tests which will determine if the design of
a new product, or a product modification, meets the
functional and reliability requirements.
In order to build an FMEA, one must start with a design
concept, and a set of clearly articulated functional and
reliability specifications. The latter lists all the functions
that the product must deliver. For example, a few lines
for a PV module backsheet are:
• Must provide electrical isolation between the back
of the module and solar cells at 1000V
• Must reflect more than 80% of irradiance incident
on the cell-side surface
• Must maintain color for 30 years in the field for
each climate
• Must shrink less than 0.5% during the lamination
process along any dimension
Given the list of specifications, and a design, the
FMEA can be developed. A group of subject matter
experts brainstorm to create a prioritized list of all
the possible ways a product might fail. Note that a
specific design concept is necessary for this step, so
the requirements can be assessed against a concrete
product design – for example, a backsheet with well
specified layers and adhesives so interlayer adhesion
can be examined. This brainstorm is captured on an
FMEA chart; Table I shows a snippet example for a
new backsheet (which lists 35 different known failure
modes). Note the scoring used to create the prioritized
list: the Severity is the economic and safety impact
when the failure does occur, the Frequency is chance
that the failure actually occurs, and the Detectability
is the chance that the failure mode can be averted by
detection and prevention before it reaches the customer
(low score means high detectability). The product of
those values constitutes the Risk Priority Number which
is used to prioritize what must be tested to qualify the
product.
Reliability specifications are a subset of the functional
specifications, with detailed reliability requirements.
Continuing the backsheet example:
• Must maintain a continuous electrical barrier (no
cracks) with 99% likelihood after 25 years in each
climate with cells that are at 1000V.
• Must not bubble or delaminate with 99% likelihood
after 25 years in each climate with cells that are
at or below 110C during times of peak irradiance.
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Loss in power,
increased
Jbox detaches
leakage
from backsheet
current, safety
10
Causes / Mechanisms of
failure
RTV loses adhesion to backsheet due to surface properties
2
issue
Dielectric
degradation
Electrical isolation
Leakage
current, safety
issue
Current design:
Prevent / Detect
Test J-box and backsheet interlayer
adhesion under hot spot condition on cell on top of JBox with
and without weights; test Jbox and
backsheet adhesion in high temp
2
RPN
Effects of
failure
Detectability
Failure
Mode
Occurrence
Functional Req’t
Severity
Table I: Backsheet FMEA snippet
40
and humidity environments
10
Hot-spot creates burns on
backsheet
(insufficient temp resistance)
2
Heat soak at various
temperatures to determine
activation energy
40
Moisture degrades insulating
layer (hydrolytic embrittlement)
3
Damp heat testing; check for
cracks and increased wet
leakage current
30
Growth of pinholes or
increased conduction through
pin holes
1
Wet leakage current test and optical inspection of backsheet surface
as laminated, and post accelerated
tests (e.g. DH, HF, TC)
10
Acetic acid from EVA attacks
backsheet
1
Test for various acid resistance of
backsheet layers
10
Propagation of slit inside jbox
where ribbons exit
2
DH2000, TC200
1
20
Internal physical puncture
4
Wet leakage current test post various accelerated tests
1
40
UV attacks individual layers or
adhesives
1
Wet leakage current test post UV
exposure (equivalent dose for 25
years)
1
10
Bubbles, loss of interlayer
adhesion
5
High temperature test with and
without hanging weight
1
50
UV attacks individual layers
3
UV exposure, Combwined UV/DH
exposure, outdoor exposure
2
18
Etc.
Dielectric
separation
(backsheet
layers
separate)
Reflectivity
Color changes
Leakag
current, safety
issue
Power loss
10
3
Etc.
TC – thermal cycling (5 per day between 90C and -40C), HF – humidity-freeze cycles (1 cycle per day of 85C
with 85% relative humidity, with an hour at -40C), DH – hours of damp heat (85C with 85% relative humidity)
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To enable a thorough Qualification Test Plan, the FMEA
must have thorough coverage over the possible field
failure mechanisms, and the combinations of stresses
that cause them. The three primary approaches used by
the subject matter experts are discussed in the following
three sections: Field Experience, Highly Accelerated
Life Testing, and Theoretical Understanding.
Field Experience
Field experience is a critical element for identifying real
design failure modes. Solar modules have been in the
field for many years, so most of the failure mechanisms
have been identified [1]. The pictures in Figures 2
and 3 show examples from the field which informed
the FMEA: j-box-to-backsheet adhesion failure, and
backsheet interlayer adhesion failure. Note that failure
analysis must be done on each type of field failure
to determine what combination of stresses caused the
failure, so a test can be devised to ensure the new
product does not have the same weaknesses.
Highly Accelerated Life Testing (HALT)
The second primary input for defining the FMEA is
Highly Accelerated Life Testing. This is a method of
discovering product weaknesses by applying stress
combinations which are relevant to field deployment,
but that are considerably beyond the normal operating
environment. It is not a pass/fail test, but rather a
test designed to activate the same failure modes one
would encounter after many years in the field, in just
a few days or weeks. It is difficult or impossible to
obtain information about how things fail in the absence
of actual failures, and HALT is a remarkably effective
way to reveal unanticipated failure modes, and thus to
identify and then toughen up any weak points in the
product design.
Figure 2: Front-contact-cell module J-box separation
Figure 3: Front-contact-cell module backsheet
interlayer adhesion failure
Because SunPower builds more projects than it can
supply with its own back-contact cells, modules from
more than a dozen different manufacturers are part
of SunPower projects. In addition to long-term testing,
HALT plays a significant role in qualifying these
modules, so some results of that work are presented
here to illustrate the efficacy of this methodology.
Table II summarizes the HALT protocol typically used to
find design weaknesses in modules. These tests involve
intermediate data reads to have data on relative rates
of failures, and also have some continuously monitored
1. Quintana MA, King DL, McMahon TJ, Osterwald CR., “Commonly observed degradation in field-aged photovoltaic modules,” Photovoltaic Specialists
Conference, 2002. Conference Record of the 29th IEEE, 19–24 (May 2002) 1436–1439.
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parameters (e.g. leakage current in the damp heat with
voltage bias test, and cell temperatures in the Solar
Simulator test). Note that for non-SunPower modules,
5 kWh/m2 of outdoor exposure is done before the test
commences, to accomplish light-induced degradation
(LID). SunPower back-contact cells use N-type silicon
wafers and so they do not exhibit LID.
melting of certain laminate materials, breakdown
of adhesives, corrosion, discoloration, out-gassing,
extremely hot cells, local hot spots, etc.
Table II: Module HALT
Test
Primary failure modes
5 days Solar Simulator
• short-circuited modules
• 1000 W/m2 of solar spectrum
• An additional 150 W/m2 of UV
spectrum
• 55% relative humidity and 60C
environment
Delamination along the
front interconnect ribbons
Backsheet browning and
bubbling
5-days Oven
• 130C
600 hrs damp heat with bias
• 85% relative humidity, 85C
• 1000V bias between the cells and
the grounded frame
1,000 alternating cycles of +/-2400Pa,
then 4 temp. cycles -40 to 60C
Encapsulant browning
Maximum temperatures of
reverse-bias cells
Backsheet bubbling in
certain areas (typically near
lay-up tapes or extremely
hot cells)
Series resistance increase
due to electrolytic corrosion
of metal lines or degradation of Si-metal interface
Cell cracking
The first test listed in Table II is the Solar Simulator,
which is able to concurrently apply most of the
stress factors seen by modules in the field, and with
exaggerated intensity: 1) temperature, 2) humidity,
3) visible light, 4) UV light, and 5) various electrical
load conditions. This test has proven very effective for
exposing module design weaknesses, both in power
degradation and in visually-identified defects, e.g.,
Figure 4: Comparison of efficiency degradation for
front-contact-cell modules from different manufacturers
in the Solar Simulator. Four SunPower back-contactcell modules are solid black lines (3 samples fall on
the same -0.1% line).
The HALT methodology consists of pushing the design
to the point where failures occur and weaknesses are
exposed. Completely eliminating these weaknesses is
not always necessary as it can lead to overdesign and
excessive cost. However, going beyond what is strictly
necessary often offers tremendous paybacks: the
design is more robust against contamination and other
process issues, and deviations from target processes
are more easily detected before they result in field
failure. A defect like front delamination in Figure 5
due to solder flux residue points to a manufacturing
process control problem, or a material compatibility
problem, and must be solved. Backside bubbling is
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sometimes less clear; bubbles that form under an
extremely hot cell might be deemed acceptable, as
the hottest cells in this test can exceed 170C, which far
exceeds normal field conditions.
Figure 5: Front-contact-cell modules after Solar
Simulator test: solder flux residue along the
interconnect ribbon caused delamination bubbles
(left side), and lay-up tape out-gassing cause
backsheet bubbling (right side)
The maximum cell temperature achieved during the
test is itself of significant interest, since it is predictive
of how hot reverse-bias cells can get in the field. This
temperature is determined by both cell and circuit
characteristics. PV cells are divided into two types
according to their reverse breakdown characteristics
[2]. Type A cells have a breakdown voltage that is
higher than the sum of the forward voltages of all the
other cells in the same string protected by a bypass
diode. They are said to be voltage limited. Type B
cells are said to be current limited because they can
absorb all the current from forward-biased cells without
any bypass diode protecting them. This can be either
because they have a low reverse breakdown voltage
or because they have a high leakage current due to
reduced shunt resistance.
Figure 6: Electroluminescence images of a frontcontact-cell module before testing (left side), and
EL image after 600 hours of damp heat with
+1000V bias (right side)
The Solar Simulator test can be run with and without
diodes for modules from different manufacturers to
explore worst-case reverse-bias conditions. Type A
cells with a high reverse breakdown voltage and high
shunt resistance require bypass diodes in suitable
number per module to protect against over-heating.
Cells with a high reverse breakdown voltage that are
of Type B because of a low shunt resistance are at risk
of local overheating (hot spots).
Cells of Type B that have a low reverse breakdown
voltage are intrinsically protected against over-heating
if their breakdown is controlled such that it happens
uniformly across the whole cell. That is the case for
2. UL 1703: Standard for Flat-Plate Photovoltaic Modules and Panels, 3rd Edition, March 15 (2002) ISBN 0-7629-0760-6.
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SunPower cells, so the temperature in reverse bias
does not depend on the number or even the presence
of bypass diodes.
The oven test in Table II is a faster and lower-cost test that
stimulates similar delamination and bubbling defects
as the Solar Simulator, although it uses only one of the
five stress factors available in the Solar Simulator. This
decouples the reverse bias characteristics of the cells
from the temperature resistance of module components.
The power degradation in Figure 7 is significantly
accelerated over damp heat testing that is done
without the electro-chemical acceleration from the
voltage bias. For standard front-contact cells, the
module leads are connected to +1000V relative to
the grounded frame (negative grounding), because
this is normal installation practice, and because these
modules degrade even faster with a -1000V bias,
as demonstrated by NREL testing [3]. The SunPower
module leads are connected to -1000V since the
installation directions require positive grounding, so
some differences in degradation modes are expected.
The third test on Table II, damp heat with 1000V
between the frame and the cells, exaggerates an
aspect of fielded modules that is frequently overlooked:
there is usually a high voltage between the cells and
the frame. Figure 6 shows electroluminescence images
confirming that cells closer to the grounded frame are
impacted more than cells near the center of the module.
Similar testing was done as part the NREL Test-toFailure Protocol, for 2000 hours of damp heat with
+/-600V bias [3]. Results from that study are plotted in
Figure 8, and show very similar trends.
Figure 7: Comparison of efficiency degradation for
modules from different manufacturers in damp heat
with bias. SunPower back-contact-cell modules are
solid black lines.
Figure 8: NREL Test-to-Failure Protocol data for
modules that have completed 2 rounds of testing
(selected from top 20 manufacturers). SunPower
back-contact-cell module is the solid black line,
and had a -600V bias. The other modules were
at +600V bias.
3. P. Hacke, K. Terwilliger, S. Glick, D. Trudell, N. Bosco, S. Johnston, S. Kurtz, “Test-to-Failure of Crystalline Silicon Modules,” 35th IEEE PVSC (2010) 0-66
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The left picture in Figure 9 shows the corrosion visible
on the metal of front-contact cells that is responsible
for the increase in series resistance and decrease
in efficiency. Data from NREL also suggest that the
degradation rate correlates to the leakage current.
This is only true, however, when comparing results for
modules with similar cells, materials, and construction.
In particular, the SunPower back-contact cells appear
significantly more resistant to electrolytic corrosion.
This is because almost the entire back surface of the
cell is plated with copper “finger” conductors (see
Figure 12), and the thickness of that plated copper is
40 microns. This provides a large safety-factor against
corrosion, with an order of magnitude more crosssectional area of copper relative to the metal fingers
on the front of a front-contact cell.
The right side of Figure 9 shows a white powder
(consisting primarily of Al2O3), which commonly forms
along the edge of the frame where it contacts the glass.
This has also been seen in the field on modules with a
high leakage current and corroding Aluminum frames.
The combination of cyclic loading and temperature
cycles in Table II is very effective for investigating
cell cracking [4], and whether the stress inherent
in the interconnect-ribbon-to-silicon wafer solder
bond is robust [5]. For front-contact cells, the copper
interconnect ribbons must be soldered onto the silicon
wafer at elevated temperatures, and because of the
coefficient of thermal expansion mismatch between
the copper and the silicon, thermal stresses are built
into the interface. If not created with well controlled
processes and materials, the solder bond can be too
strong (causing “cratering” where small cracks form in
the cell underneath the ribbon), or too weak (causing
the ribbon to tear off the cell).
Figure 10 shows typical before and after images of
front-contact polycrystalline-cell modules before and
after the dynamic load HALT stress. The dark areas show
7 cells with cracks that caused areas of these cells to
be isolated from the electrical circuit, with a consequent
2.1% drop in the measured power production.
Figure 9: Following 600 hours of damp heat with
voltage bias for front-contact-cell modules, corrosion
can be seen on the front-contact metal (left side).
The right-side picture shows the view looking down
the edge of the aluminum frame that surrounds the
laminate glass.
Figure 11 shows the same test run on SunPower backcontact cells, which generated one cracked cell near
the center of the module. The toughness of these cells is
due primarily to the thick copper plated fingers on the
back of the cell.
4. J.H. Wohlgemuth, et al. “Using Accelerated Tests and Field Data to Predict Module Reliability and Lifetime,” Proc. 23rd European PVSEC, Valencia, Spain
(2008) # 4EP1.2, 2663-2669.; 5. A. M. Gabor et al., “Soldering Induced Damage to Thin Si Solar Cells and Detection of Cracked Cells in Modules”, 21st
EPSEC, Dresden (2006) 2042–2047.
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Figure 10: Electroluminescence images of a frontcontact-cell module before (left), and after (right),
1000 cycles of +/-2400 Pa (equivalent to maximum
wind load) of alternating pressure load on the glass.
Power loss was 2.1%.
Figure 11: Electroluminescence images of a SunPower
back-contact-cell module before (left), and after (right),
1000 cycles of +/-2400 Pa (equivalent to maximum
wind load) of alternating pressure load on the glass.
No measurable power loss.
Once cracked, the mechanism for power loss between
a front and back-contact cell is different. In a frontcontact cell, areas of the wafer with cracks that cut the
front-contact metal from the interconnect ribbons isolate
areas of the cell. These areas no longer contribute
charge carriers, and show up as dark regions in the
right picture of Figure 10. For the back-contact cell,
copper fingers cover essentially the entire back of the
cell. Those fingers are ductile and do not break, so
that regions where the silicon wafer breaks remain
connected to the circuit regardless of the cracks. The
only efficiency loss is due to the recombination that
occurs at the recombination sites within the crack itself,
as shown by the black lines in the electroluminescence
image in Figure 11. This has a relatively small impact,
and in fact no power loss was measured for the
modules shown in Figure 11, which is typical.
A final example of a HALT is the “autoclave” test on
bare cells, which SunPower does for cell product
development. During the design of some of the first
solar cells made by SunPower, this test provided a
critical measure of the effectiveness of the moisturebarrier characteristics of the outermost layer of the
cells. The comparisons ruled out Titanium Oxide in
favor of Silicon Nitride, and also enabled the selection
of the optimal Silicon Nitride deposition technique.
This test is discussed more fully in Section 6.
It is worth underlining that the purpose of HALT is not
to quantitatively estimate the module useful lifetime, but
to discover failure modes and causal stresses that must
be accounted for in the FMEA. It is also an effective
and fast tool for comparing different technologies or
products. Lastly, it provides some of the groundwork
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for later testing programs that will operate continuously
during manufacturing (Section 6).
Theoretical Understanding
The final means of developing a thorough FMEA is
through a theoretical understanding of the failure
mechanisms. This is especially critical for capturing
slow-moving failure modes, which are unlikely to
show up in the field for many years, and may not
even be encountered during HALT. The only way
to capture these failure modes is with lab testing to
understand the physics of the breakdown, guided
by theoretical modeling.
Good examples of critically important, but slowmoving, failure modes occur in the metal interconnect
and solder bonds that electrically connect the cells in
a module. Metal-fatigue-failure of interconnects had
been observed historically as one of the top two failure
modes in the field for traditional front-contact-cell
modules. SunPower used this information in the design
of the interconnect for back-contact cells in 2004. A
design with in-plane strain relief was chosen, with
validation including 10,000 mechanical cycles of the
interconnect itself (with no failures) and thermal cycling
of coupons and modules to 200 cycles (with a power
degradation of less than 3%).
However, subsequent testing revealed that solder
bond failure could occur in extended thermal cycling,
leading to a loss in fill-factor of ~8% at ~500 cycles.
The failure mechanism was found to be cracking as
the result of heterogeneous coarsening of the solder
microstructure [6].
As a result, this interconnect was redesigned to
reduce stress on the solder bonds by increasing the
compliance of the interconnect, and work was done to
model the failure mechanism. This effort included both
the SnPb solder which was used in 2004 and the SnAg
solder which was under development for improved
environmental stewardship. Industry literature indicated
SnAg solder should be less susceptible to solderfatigue failure, which was validated by the thermal
cycling. The combination of the higher compliance
interconnect (shown in Figure 12) and the SnAg solder
resulted in fill-factor loss of < 2% in 700 thermal cycles,
and production was switched accordingly [6].
Additional information was then sought to quantify
the acceleration factors of the solder joints to be
able to predict product life. It was found that grainboundary-creep and matrix creep are different for
the different types of solder, and that different tests
and different dwell times and temperatures in cycling
could be used to derive the strain energy behavior and
model performance [7]. Tests were done with direct
monitoring of solder bond resistance of individual
solder bonds in coupons through thousands of
temperature cycles of varying amplitudes [8], which
confirmed very high fatigue-failure resistance for the
combination of the interconnect shown in Figure 12
and SnAg solder. There were 0 failed joints in 2300
thermal cycles, significantly more than required for
25 years in a harsh climate. Based on the modeling,
extended thermal cycling with close monitoring of
fill-factor is used as a standard test for any new
product or any change which could influence solder
stress or quality.
6. A. M. Gabor et al., “Soldering Induced Damage to Thin Si Solar Cells and Detection of Cracked Cells in Modules”, 21st EPSEC, Dresden (2006) 2042–2047.
7. J. P. Clech, Proceedings SMTA International Conference (Sept 2004) 776-802. 8. Y. Meydbray, K. Wilson, E. Brambila, A. Terao, S. Daroczi, “Solder Joint
Degradation in High
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Figure 12: Strain-relieved cell interconnect between two SunPower back-contact cells. Note the three solder bonds
on each cell, combined with the plated copper connections between those bonds on the cell, create redundancy.
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QUALIFICATION TESTING
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Example qualification test: backsheet
Only after a solid FMEA is completed (and of course
it must be regularly updated as more is learned from
ongoing testing and field results), can a test plan be
developed to ensure that the product design has high
reliability. For example, the first line of the backsheet
FMEA indicates the detachment of the junction-box
from the backsheet due to a loss of adhesion of the
RTV or backsheet interlayer adhesion (Table I). Given
an RPN number high enough to make this failure
mode of concern, a test must be derived to determine
whether the product meets the specifications. To test
that particular failure mode, a series of modules and
coupons are built with junction-boxes attached to
backsheets with RTV, and put into field testing with the
cells above the j-boxes shaded to induce operating
temperatures from hot cells. Additionally, oven tests at
different temperatures up to 120C, with 1 kg weights
hanging from the junction-boxes, must not cause any
delamination in order to pass.
These qualification tests can be pass/fail if the relevant
acceleration factors are known, or they can require
an equivalent or better performance compared to a
known high-reliability baseline. In most cases, the
tests are extended to produce failures, to learn how
that happens. Table III is a summary of the testing
done to approve a new backsheet for use in the
SunPower modules – each test is motivated by one or
more failure modes in the FMEA.
Note that these tests are primarily focused on
quantifying the targeted failure modes and verifying
they will not cause a failure during the expected
lifetime of the product. It is also critical to get modules
started early in long-term testing to try to catch any
unexpected failure modes.
Long-term testing
Certification-type tests are a good place to start for
long-term testing, just by extending the tests to when
failure happens. Figure 13 shows SunPower modules
in the standard certification tests, and indicates no
run-away failure modes, only slow degradation, given
these stresses. Note that these tests are similar to
HALT, but the stresses are gentler. In the context of
qualification testing, it’s important to run the tests until
failures occur These extended tests are also repeated
periodically as part of the ongoing reliability testing
(ORT) for manufacturing.
Field testing
Field testing should always be started immediately,
and done simultaneously in different climates (hot and
humid, hot and dry, temperate, urban, rural, coastal,
etc.) on full size modules. These deployments must be
monitored and visited regularly to look for any new
developments. The problem with field testing is that
the stresses are not accelerated; the benefit is that the
tests are inherently realistic combined stresses, and
many more samples can be fielded relative to what
can be sampled with artificial stress testing. Of course
when something does go wrong in the field testing, it
means the product has serious flaws.
A good example of a field test catching a problem
happened during a failed qualification attempt for a
new encapsulant. Coupon-size prototypes had shown
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no problems in lab tests, and as soon as the supplier
had the ability to produce material that enabled
manufacturing of full size modules, the modules were
put into lab tests and deployed to the field in various
climates. The field tests were first to show delamination
at the encapsulant/glass interface – defects that did
not occur during coupon testing (although they also
did occur in damp heat testing of full size modules).
No changes to the design of encapsulant were made
according to the supplier, so this was due to the change
in scale of the modules, or to unknown manufacturing
volume ramp-up problems for the supplier. When
possible it is recommended to start field testing on full
size modules rather than small scale prototypes.
listed below Table V)
Sample
Tests
Moisture Vapor Transmission Rate
Partial discharge
Out-gassing post heat soak
Backsheet sample:
6”x6”
Surface morphology analysis (initial and
post HF10)
Shrinkage (at lamination conditions and
during accelerated tests)
Interlayer adhesion (as received, post
lamination and HF10)
UV exposure
Adhesion tests (to encapsulant, interlayers; J-box, labels, framing materials)
ACL168
There are also some experiments for which field
deployment is much more conducive, such as the effect
of various anti-reflective glass coatings on soiling rate.
UV
HF40
DH2000
3-cell laminated
coupons
TC200
Sequence B (UV/TC50/HF10)
HF/UV combination
Cut test (pre- and post-HF, checking for
crack formation)
T
high temperature voltage stability
field tests
chemical resistance (various chemicals
found in environment)
High temperature heat soak (check for
out-gassing due to interaction with other
module components)
Figure 13: Extended testing of SunPower backcontact-cell modules, which pass insulation
resistance testing and are less than 10% degraded
in power after more than 7x the certification
standard in DH, 11x in TC, and 26x in HF.
Table III: Backsheet qualification tests (abbreviations
J-box with weights tests for creep
Full modules
HALT (see Table II)
HF40
DH2000
TC200
field test (in various environments, and
some with additional weights on J-box)
Q U A L I F I C AT I O N T E S T I N G
SECTION 3
15
SECTION 3
Qualification Testing Conclusion
The product can spend considerable time in qualification testing, which typically involves detailed failure analysis
and then design changes and re-testing. This is time well-spent, however, as the design stage is by far the most
cost-effective time to discover any reliability weaknesses and fix them.
One might notice that the certification tests are not generally sufficient for qualifying a product design. This is
because certification tests are designed to indicate a reasonable level of initial product quality and safety, and are
not meant to indicate product lifetime [9]. It is worth noting that all commercial modules which have had a problem
in the field were of a design which had previously passed certification testing. Product qualification approaches
that are based solely on successfully completing certification tests, or simply multiples of the certification tests,
are not following good Design for Reliability methodology and are likely to miss important failure modes that are
unrelated to the particular stresses utilized by the certification tests.
Q U A L I F I C AT I O N T E S T I N G
SECTION 3
16
SUPPLIER QUALITY
SECTION 4
Four stage process
Once the design is qualified, that gives confidence
the product will be successful if it is manufactured
according to the design – which requires that the input
materials are well specified and held within those
specifications continuously. To do that, SunPower uses
a Supplier Qualification Program, and a Supplier
Continuous Monitoring Program, which is defined in
4 stages:
Stage 1 is internal to SunPower, where requirements
are clarified and a sourcing strategy is determined. Key
requirements are part of the output of the qualification
process.
Stage 2 involves contacting likely suppliers, setting
expectations and developing joint plans.
Stage 3 is where the primary suppliers go through a
“PSC audit” to evaluate how they are doing along the
three most important dimensions:
• Prevention: Extent of employee training, usage
of Statistical Process Control, FMEAs and a
formal CAPA (Corrective And Preventive Actions)
methodology, as well as their own Reliability and
Supplier Quality Programs.
• Standardized/Simplified/Scalable: Evaluation
of business processes to make sure they are of
high quality, in particular, the change notification
process.
• Customer Satisfaction: customer surveys,
responsiveness to customer issues, etc.
Stage 4 is where a strong partnership is executing to
deliver high quality products at a large scale. Regular
self-assessments with periodic reviews and validation,
as well as improvement plans, are put into place. This
rigor is absolutely necessary for ensuring a consistently
high-quality product.
As part of the periodic review the supplier also gets
a STARS score (“Supplier Total Achievement Rating
System”), and must have at least an 80% to be an
approved supplier, based on:
• Quality: customer issues, reliability, compliance
to SunPower change requests, PSC Audit
performance, problem recurrence rate.
• Cost and cost-reduction plan
• Availability: on-time delivery, lead time, etc.
• Technology: how does the product roadmap
dovetail with SunPower’s plans
Supplier materials audit
This testing is generally accomplished with a mix
of vendor tests (with shared reports), and periodic
inspections of incoming material. Table IV shows
a partial list of the incoming materials testing that
SunPower does as part of a regular sampling program.
It’s critically important to have the strong requirement
that absolutely no formulation or design changes
are to be made to the qualified components without
communication and re-qualification. This was recently
underlined when a j-box manufacturer changed the
formulation of the j-box lid to a stronger type of plastic.
Although it appeared to be a desirable change, the
SUPPLIER QUALITY
SECTION 4
17
SECTION 4
interaction between components created a new
failure mode. During damp-heat testing, significant
de-polymerization was found on the inside of the j-box
lid, as shown in Figure 14, which proved to be due to
a reaction between the new plastic and a chemical
out-gassing from the double-sided tape that is used to
hold the j-box in place while the RTV cures.
Table IV: Incoming materials audit
Material
Material
Encapsulant
Gel test, pull test, temp soak test
Backsheet
Peel test, temperature soak test
Diodes
Leakage current test
Connectors
Production test (wiring fitness, crimping,
pull test)
Frame
Mechanical load test, frame pull test
Label
Tape test, IPA test
Figure 14: De-polymerization damage on the
interior of the j-box lid due to chemical interaction
found during damp heat testing
One of the more subtle areas to pay attention
to is the qualification of the vendor’s different
manufacturing sites, even when the component being
purchased is the same part number. For example, a
particular backsheet supplier was bringing up a new
manufacturing facility, and although the pre-rampup material passed accelerated testing results that
exceeded the IEC61215 testing requirements [10],
and also passed SunPower’s internal qualification, the
fully-ramped-up material was found to have a defect.
Incoming inspection tests showed the backsheet from
the supplier’s new manufacturing facility had weaker
interlayer adhesion as compared to material received
during the qualification testing. This pointed to an
inconsistency of the material quality from the new
manufacturing facility that the supplier was then able
to correct.
SUPPLIER QUALITY
SECTION 4
18
MANUFACTURING QUALITY
SECTION 5
System
Consistency in manufacture is an essential part of
achieving and maintaining the quality necessary
to meet the reliability requirements of PV cells and
modules. Statistical Process Control (SPC), Total Quality
Management (TQM), and Six Sigma are among the
general systems which can help attain and maintain
quality in manufacturing. All these methodologies
emphasize one main point: quality must be built into
the product, and cannot be audited into the product.
The semiconductor industry has been a leader in the
development of manufacturing quality procedures.
This was necessitated by manufacturing processes
that involve hundreds of steps, and 30 to 60 days of
work-in-process. Given those conditions, very rigorous
Quality and Change Control methodology were
required, since if a problem was not discovered until
the end of the line, millions of dollars of product must
be scrapped. With the 25 year warranty period of PV
modules, minimizing problems in manufacturing which
would not be discovered until later is also essential.
SunPower had the good fortune of a close relationship
with Cypress Semiconductor as a way to learn
advanced manufacturing quality techniques. Cypress
invested in SunPower in 2002, completed its acquisition
in 2004, and concluded its ownership of SunPower
shares in 2008. Cypress offered tremendous support
developing SunPower’s quality programs and discipline
during the pilot line testing and the deployment of the
first full-scale cell manufacturing facility.
The system developed by SunPower and Cypress was
built on the foundation from Cypress, and then refined
to better match PV manufacturing. Key elements of the
system include (1) systematized business processes
for all developments and changes, (2) coordinated
product, process, and equipment development
procedures with six levels of control, (3) quality and
continuous improvement mentality, and (4) variability
and waste reduction.
Process development and implementation
The development of the product and the manufacturing
processes must result in process control (including all
interactions, material variability, etc.) which is sufficient
to consistently deliver a high-quality, reliable product.
SunPower uses a six-level approach which manages
the interactions of product characteristics, process
variation, and equipment variation. The process
portion of each level is as follows:
Level 1 determines what to control. At this stage, the
key variables of product, process, and equipment
which can affect the final product are identified.
Level 2 determines how to control. Response Surface
Maps (RSMs) are used to understand the process
space, and Coefficients of Variance (COVs) are used
to determine sampling plans. Without RSMs, it is
possible that the process is statistically robust (e.g., a
Cpk > 1.3), but near a portion of the process space
that would result in significant movement out of the
process window in the event of a shift of one of the
input variables.
Level 3 establishes control. Online SPC charts, as
shown in Figures 15 and 16, are used to verify that
sufficient control has been achieved and to provide the
ability to react to a change in the process (not just to a
M A N U FA C T U R I N G Q U A L I T Y
SECTION 5
19
SECTION 5
change that results in an out-of-specification condition).
Product is also run with the extremes of process windows
of other steps in manufacturing (called “running the
corners”) to determine if the product is reliable with the
largest possible interaction between steps.
Level 4 verifies control. Implementation of out-ofcontrol action plans (OCAPs) are an important part
of this level because they ensure that manufacturing
appropriately responds to any changes.
15, the guarantee of adhesion for the material shown
is 46 N/cm (lower spec. limit), but the control limits
force action if the mean changes from the 177 N/cm
target (XBB) by more than 42 N/cm in either direction
(upper and lower spec. limits). Even an unusual shift of
the average or standard deviation with all points still
within the control limits requires determination of the
cause of the change and whether or not it can have
any product quality implications.
Level 5 establishes ownership of the process, which
includes active root-cause-corrective-action (RCCA)
and tool matching (comparing the performance and
sensitivities of similar tools) to further improve the
process.
Level 6 drives continuous improvement. Structured
business processes to manage any changes are very
important for carefully introducing improvements.
It is essential that process development be done in
coordination with product development and equipment
implementation. Interactive improvement of the product
specification is needed to both drive the process
development, and to benefit from the work done in
process development.
Figure 15: Daily gel-test measurements from the
SPC chart of one laminator. USL/LSL – upper
and lower specification limits, UCL/LCL – upper
and lower control limits, XBB – target.
Example SPC charts
Figure 15 and 16 provide 2 of the more than 100
SPC charts that SunPower uses in the manufacturing
quality program. They illustrate that control limits are
not the same as specification limits. As seen in Figure
M A N U FA C T U R I N G Q U A L I T Y
SECTION 5
20
SECTION 5
The audit of workers’ adherence to the manufacturing
specifications serves both to instill the proper mentality
on the line and to catch breeches. The mentality must
be “follow the spec, or change the spec” (with any
proposed change to the specification being done only
through the structured business process).
Figure 16: Daily pull-test measurements from the
SPC chart of one laminator
As part of the Quality Audit, SunPower regularly does
an “Out-of-box audit.” This is a random sampling
of products that have been packed for shipment, to
check the quality of the packaging, documentation,
product cleanliness and visual defects, and electrical
performance as compared to recorded performance.
Quality Audit
The Quality audit is an important part of achieving
and maintaining the consistent manufacture of a high
reliability product. This is most effective if done by
people who are outside of a manufacturing unit, such
as within an independent Quality organization, since
they are independent and more able to effectively
identify areas for improvement.
M A N U FA C T U R I N G Q U A L I T Y
SECTION 5
21
TESTING CONTINUOUS MFG RELIABILITY
SECTION 6
Two methodologies
product does not change over time. The testing plan
derives directly from the (regularly updated) FMEA,
and falls into two categories:
• Highly Accelerated Stress Audit (HASA) - to detect
any material defects or manufacturing problems
which have escaped the supplier quality and
manufacturing SPC hurdles.
• Ongoing Reliability Testing (ORT) - to continue
to validate the baseline reliability of the product.
These are longer-duration tests with milder stress
levels.
Highly Accelerated Stress Audit (HASA)
These tests must have a fast turn-around time and
must be very well structured with clear pass/fail
criteria that non-experts can interpret unambiguously.
Note that the pass/fail criteria should be set to show
problematic manufacturing variations, and must be
closely tied to Out-of-Control-Action-Plans (OCAPs) to
resolve any failed tests quickly. In many cases, there
are gradations of “fail,” ranging from a mild failure
that requires a high-priority engineering review and
failure analysis, to serious failure requiring a full
line shutdown. For HASA testing, a “fail” does not
necessarily mean a serious product problem, but
serves as a warning that something is drifting from
the target process. Unlike HASA in the electronics
industry, even passing samples used for these tests
are not sold to customers, since they are stressed to
the point where they show degradation so they must
be scrapped. This has the advantage, however, that
higher stress levels can be used to attain an improved
testing turn-around time.
The HASA tests are built to exercise the important
degradation mechanisms determined from the earlier
steps in the product design process (HALT, historical
data, theoretical understanding, and FMEAs from
the product design and the process designs). The
list of degradation mechanisms is a guide for the
stresses required to degrade the product which in
turn helps to define the tests. It is also important to
determine the appropriate pass/fail metric, which
in most cases comes from a performance test (IV
curve trace with the associated efficiency, fill-factor,
Imp, Vmp, etc.). For some degradation mechanisms,
however, performance testing is not the appropriate
metric, e.g., if the failure mode is delamination of
the encapsulant, then it makes more sense to do an
adhesion measurement. Table V lists the failure modes
and degradation mechanisms captured by SunPower’s
current HASA tests. Note that the HASA test battery
is not necessarily comprehensive, but is designed to
complement the incoming materials inspections and
manufacturing SPC.
SunPower does HASA testing on bare cells, 3-cell
coupons, and full sized modules, depending on the
T – temperature, UV – ultraviolet light, TC – thermal
cycling, RH – relative humidity, ACL168 – autoclave
(168 hours in 100% relative humidity, 121C at 2
atmospheres of pressure), HF – humidity-freeze cycles
degradation mechanism being tested. For instance,
to measure the sensitivity of the cells to moisture
and temperature degradation, there is no need to
encapsulate the cell in a module. Bare cells can be
placed in a high temperature (121C) and high humidity
(100% relative humidity) environment, and the rate of
oxidation of the Silicon Nitride moisture-barrier layer
can be inferred from the Vmp drop after 7 days (168
TESTING CONTINUOUS MFG RELIABILITY
SECTION 6
22
SECTION 6
hours) in this harsh environment – this is the “autoclave” test, called “ACL168” in the top row of Table V.
The efficacy of this approach was demonstrated during the ramp-up of SunPower’s second back-contact cell
fabrication facility. Some of the first-runs of cells were found to have an increased oxidation rate after the
autoclave test, suggesting they were not as robust as normal cells, even though the modules were passing the
standard ORT tests. Failure analysis revealed a potassium surface contamination, and this was traced to a design
problem in the rinsing process on a new manufacturing tool. This was corrected, and performance of the cells in
the HASA autoclave test returned to normal. 6.3 Ongoing Reliability Testing (ORT)
Table V: HASA test matrix
Failure Mode
Stress
Test
Measurement
T, RH
Cells - ACL168
Performance
UV
Cells – UV test
Performance
Voltage Bias
Coupons – Oven with
voltage bias
Performance
Metal-to-contact misalignment,
residual oxide, contamination
TC
Cells – ACL168
Performance
Corrosion
Contamination
T, RH
Cells – ACL168
Performance,
Metal pull test
Cell cracking
Overly-thick metalization,
surface irregularities
Mechanical
loading, TC
Coupons – HF10
Performance,
EL imaging
Cell Vmp
degradation
Back-surface
passivation
degradation
Surface contamination,
non-uniformities or pinholes in
moisture barrier
T, RH
Cells - ACL168
Performance
Module
delamination
Reduction in
surface bonds
Contamination on cell or
glass, over curing, EVA voids
or bubbles, shift in lamination
process
T, RH, UV, Bias
Modules – DH with
voltage bias
Visual Inspection
Performance
Backsheet pull
High pot. test
EVA
transmission loss
EVA Browning
Incoming material variation
(variation in UV absorber)
R, RH, UV
Coupons – UV test
Visual inspection
Performance
Backsheet
degradation
Cracking, interlayer adhesion
Backsheet wrinkling, incoming
failure, Insulamaterial variation
tion de-polymerization
T, RH
Backsheet sample –
ACL168
Visual inspection
Cell Imp
degradation (or
both Imp and
Vmp)
Degradation
Possible Defects
Front-surface
passivation
degradation
Surface contamination,
non-uniformities or pinholes in
moisture barrier
Metal
delamination
TESTING CONTINUOUS MFG RELIABILITY
SECTION 6
23
SECTION 6
These tests are longer with lower stresses, smaller
sampling sizes, and provide a general indication that
the baseline reliability of the product is not changing.
1000 hours of damp heat (85C and 85% relative
humidity), 200 thermal cycles (5 per day between
90C and -40C), and 40 cycles of humidity-freeze (1
cycle per day of 85C with 85% relative humidity, with
an hour at -40C), are good ongoing reliability tests
because they are the standard tests for qualification
testing.
For all ORT tests, the performance of the module is
monitored using mid-reads at predetermined intervals
to provide an early indication if there is a problem.
In addition to the performance measurements, the
samples undergo visual inspection, insulation resistance
tests and measurement of electroluminescence during
the mid-reads to look for defects that do not clearly
impact the performance measurements in the short
term. As part of the ORT testing, it’s also worthwhile
to periodically do very long-term tests-to-failure with
new product samples, similar to what is shown in
Figure 13. This improves the chances that a failure
mode that is not activated by the standard ORT tests
has not been accidentally introduced. This is done
periodically as an extension of the standard ORT tests.
Table VI list the SunPower ORT tests as well as
the applicable failure modes and degradation
mechanisms. For these tests, the measurements taken
are all approximately the same: visual inspection
(discoloration, delamination, etc.), flash test (efficiency,
Imp, Vmp, fill-factor, etc.), insulation resistance test
(leakage current), electro-luminescence imaging
(relative recombination), and J-box pull tests.
Table VI: ORT test matrix
Test
DH1000
TC200
HF40
Failure Mode
Degradation Mech.
Glass AR-coating deg.
Surface contamination,
non-uniformities or pinholes in
moisture barrier
Adhesion loss
Metal-to-contact misalignment,
residual oxide, contamination
EVA trans. loss
Contamination
Backsheet
degradation
Overly-thick metalization,
surface irregularities
Solder bond
failure
Surface contamination,
non-uniformities or pinholes in
moisture barrier
Delamination
Contamination on cell or glass,
over curing, EVA voids or bubbles, shift in lamination process
J-box failure
Embrittlement, internal metal corrosion, attachment to backsheet
Connector
degradation
Embrittlement
Sample-size determination
Determining the best sample size for HALT and
ORT is a challenging balance of resources and test
coverage. In general, the stakes of having a problem
released into the field are high due to the high cost
of replacement, and so a significant sampling rate is
required to minimize the risk. Increasing the sample
size decreases the expected amount of time it takes
to detect a problem, since larger sample sets means
it is more likely that at least one of the test samples
has the defect and will fail the test. Thus, increasing
the sample size will decrease the number of defective
products that escape manufacturing. On the other
hand, increasing the sample size requires testing
personnel and equipment, and reduces output.
TESTING CONTINUOUS MFG RELIABILITY
SECTION 6
24
SECTION 6
The sampling rates also depend on the maturity of the manufacturing line, and its historical performance.
SunPower has taken a conservative approach; for example, mature manufacturing lines have a sampling rate of
approximately 1 cell per 20,000 for the HASA ACL168 test (Table V), and 1 module per 5,000 for the ORT tests
(Table VI). Each company must make its own determination of the right balance.
hfghfhj
TESTING CONTINUOUS MFG RELIABILITY
SECTION 6
25
CLOSED-LOOP LEARNING FROM THE FIELD
SECTION 7
From the perspective of reliability, field failures present a tremendously valuable opportunity for closed-loop
learning about real product degradation and the causal stresses. Every new failure should have a thorough and
thoughtful failure analysis done, and then an update to the FMEA and associated testing. Below is a selection
of examples of failure modes that were learned from fielding modules, which now advise many of the test plans
currently in use.
Glass-surface contamination
This is a good example of a seemingly insignificant
change in manufacturing process having a surprising
effect. For a short period in 2007, a modification was
made to the cooling racks for SunPower modules
coming out of the laminator that used a different
insulating cloth to protect the hot modules from being
scratched while they cooled. This new cloth, however,
turned out to contain a silicone oil which caused the
glass to have a different contact angle with water.
This was invisible in the factory, but creating uneven
soiling when deployed to the field.
using water for washing, whereas previously these
tests in the factory were conducted with isopropyl
alcohol. Additionally, all manufacturing accessories
that have any physical contact with modules are now
included in the list of materials to be controlled as part
of production.
Although this problem was discovered fairly quickly, it
still required significant investment in a 9-step cerium
oxide cleaning process to bring all the customer
installations up to the normal look of new modules.
This episode shows the value of fully vetting every
process change. SunPower now uses an outgoing
inspection for non-washable stains on glass by using
water spray. This matches the field methodology of
Figure 17: Fielded modules with silicone
contamination on glass surface show 4 stripes.
Loose Junction-box terminals
CLOSED-LOOP LEARNING FROM THE FIELD
SECTION 7
26
SECTION 7
During a short period in 2006, SunPower determined
that some screws securing output wires into an
old-style j-box were insufficiently tightened during
production. The effects of this issue manifested as
a high-resistance connection in the j-box, which in
turn caused low power modules and heating of the
terminal blocks.
like the use of torque wrenches to assemble frames
or connect wires. To avoid this failure mode entirely,
SunPower eliminated screw connections inside j-boxes.
Connectorized cable insulation resistance
This example did not quite make it to the field, but it
does illustrate the same process of closing the loop
from final testing to new standard test plans and
updated FMEAs. In 2009, the final qualification test
for a new wire/connector pair was humidity-freeze,
and a wet insulation resistance failure occurred. This
was traced to a sizing mismatch between the cable
and connector that resulted in a broken sealing ring
in the connector.
Continued investigation revealed that even dimensionally
compatible cables also sometimes had failures, which
lead to a new qualification test to compare lifetimes of
a population of cable/connector pairs with cycles of
humidity-freeze.
Figure 18: J-box showing a loose screw connector
(right side screw shows thermal damage)
The root cause was calibration drift for the torque
wrenches used during assembly. SunPower reworked
all the deployed modules in situ, and implemented
a manufacturing process requiring validation of the
torque wrench calibration at every shift to address the
root cause.
This issue is a good reminder that production facilities
must closely monitor and control use of every tool used
in module production – not only the cell manufacturing
processes, but also the more mundane processes,
y
t
i
l
i
b
a
i
l
e
R
1.0
0.8
0.6
0.4
0.2
0.0
0
20
40
HF cycles
60
80
100
Figure 19: Wet-insulation resistance reliability vs.
number of Humidity-Freeze cycles for 4 different
cable/connector pairs.
CLOSED-LOOP LEARNING FROM THE FIELD
SECTION 7
27
SECTION 7
From the perspective of reliability, field failures present a tremendously valuable opportunity for closed-loop
learning about real product degradation and the causal stresses. Every new failure should have a thorough and
thoughtful failure analysis done, and then an update to the FMEA and associated testing. Below is a selection
of examples of failure modes that were learned from fielding modules, which now advise many of the test plans
currently in use.
CLOSED-LOOP LEARNING FROM THE FIELD
SECTION 7
28
CONCLUSIONS
SECTION 8
A complete design-for-reliability process has been presented to illustrate a methodology for deploying highreliability modules, even in the face of the rapid materials and process changes required to decrease product
costs while still delivering on the industry-standard 25 year warranty.
This starts with a thorough FMEA based on field experience, HALT, and theoretical understanding – the output
of which determines the qualification testing needed to deliver a high-reliability product design, as well as an
understanding of the product’s failure modes and causal stresses. Supplier quality is maintained with a supplier
qualification program, with continuing audits of incoming materials. Manufacturing builds quality into the product
through Statistical Process Control of all the important variables, and ongoing sample testing of the final products;
HASA and ORT assures the continued high-reliability of the product.
It is important that many of these tests are “tests to
failure” since the goal is to understand and then
eliminate failure modes in the product design and
manufacturing processes. This is a broader goal
than the standard certification pass/fail tests, since
those are designed as a baseline assurance of initial
product quality and safety, and are not meant to be
indicative of product lifetime.
This material is based in part upon work supported
by the Department of Energy under Award Number
DE-FC36-07GO17043.
Figure 20: The primary methodologies discussed
in each process area
CONCLUSIONS
SECTION 8
29
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SUNPOWER and the SUNPOWER logo are trademarks or registered trademarks of SunPower Corporation.
© February 2011 SunPower Corporation. All rights reserved.
30