3.1

Test leg for broken interconnects and degraded solder joints

Failures of solder bonds and ribbons in the module laminate have dominated the field failure Pareto charts in a number of studies [5]. The Module Qualification Test IEC 61215 already contains 200 thermal cycles (from - 40°C to + 85°C with maximum power current flow when the modules are above room temperature) to detect such failures. Field data indicate that module types that survive this 200 thermal cycle test have still suffered broken interconnects and degraded solder bonds in the field over times that are significantly less than the warranty period. [6] Bad solder bonds (See Figure 1) can be the result of either manufacturing defects or design related for example using too few solder bonds per tabbing ribbon or designing the process to make bonds that are too small. Failure of interconnect ribbons (See Figures 2) are almost always related to poor design; for example using a ribbon material that is too stiff or too thick or not leaving enough free ribbon length between cells.

Figure 1.

Example of overheated solder bond.

Figure 2:

Picture of broken interconnect ribbon in module

Therefore this 200 thermal cycle test does not provide enough stress to validate modules for a 25-year warranty. The main questions then are how many thermal cycles should be used for the test and should the cycles be the same for the different climate zones and mounting configurations? Recent work indicates that modules susceptible to such failures begin to show significant power loss between 400 and 600 cycles [7] [8] (See Figure 3 for the thermal cycle data from Ref 7.) A preliminary level of 500 thermal cycles was proposed for all climates and mountings because this level has been utilized by a number of module manufacturers in the past [9].

Figure 3:

Power loss as a function of thermal cycles from reference 7.

We expect that there will be a significant amount of discussion about the selection of 500 thermal cycles as this test will take 65 days or more to meet the present test specification. It may be possible that the failures seen in the field are largely the result of poor quality control and that tighter controls on quality would be as effective as increasing the design requirement. There is also the issue of whether the temperature range shouldn’t be modified for roof mount in the extremely warm climate zone where modules are likely to reach temperatures higher than 85°C almost every day during the summer. Should modules designed for use in this category be cycled from -25°C to +100°C? Are the thermal cycles in this climate zone more or less stressful than those in the temperate climate?

3.2

Test leg for encapsulant delamination

Corrosion of the metallization within a PV module is another failure mode often observed in the field (See Figures 4 and 5). In most cases such a degree of corrosion is observed in conjunction with delamination of the encapsulant from either the superstrate or the cell surface. As the figures show, such corrosion happens in both humid and dry climates. In the module in Figure 4 the delamination appears to be between the encapsulant and the cell surface probably because the formulation of EVA did not contain a primer. The glass was manually primed during the module manufacturing process. The module in Figure 5 has a glass/glass construction with a non-EVA encapsulant that does not adhere well to glass. Over time thermal stresses caused delamination between the encapsulant and the glass allowing liquid water to condense inside the package during the cool night time. We need a test sequence that will duplicate each of these failure modes.

Figure 4:

Corrosion of cell metallization from Florida

Figure 5:

Corrosion of cell metallization from Arizona

We need to find an accelerated stress test or sequence of tests that can duplicate the delamination and corrosion processes observed in fielded modules. IEC 61215 contains an extended damp heat test (1000 hours at 85°C/85% relative humidity). While this test does cause failures, neither the thousand hour test from IEC 61215 nor extension of this test to longer times results in the type of delamination observed in fielded modules. Delaminations in the damp heat test tend to occur at the edges of the module not over the cells. Extending the damp heat test for long time periods causes power degradation in many types of PV modules. However, that type of degradation has not been reported in field observations [10]. Damp heat testing alone is not the answer.

IEC 61215 also contains a test sequence of UV exposure, 50 thermal cycles and 10 humidity freeze cycles. This sequence is more likely to cause delamination than damp heat alone will. The UV exposure is designed to stress the adhesion between glass and encapsulant and between the encapsulant and the cells. Once this adhesion has been weakened, the thermal cycling applies mechanical stress to adhesive bonds that may have been weakened by the UV exposure. Finally, in the humidity freeze test, humidity is forced into the module at high temperature and then when the module is rapidly cooled below freezing, any liquid water that has condensed in voids will freeze, further opening them and resulting in delaminations within the package. More delaminations are observed after this test sequence than for any of the other test legs in IEC 61215.

However, this test sequence has a number of limitations:

To address these limitations, we propose the following changes to this test sequence in the Comparative Sequence:

3.3

Test leg for encapsulant discoloration

Analysis of field data for many PV systems by Jordan, et.al [12] has shown that long term degradation in power of crystalline silicon modules correlates with loss of short circuit current. Much of the loss of short circuit current can be attributed to discoloration of the encapsulant used in the module. Jordan noted than about 2/3 of the papers on system degradation noted at least some discoloration of the encapsulant. STR has reported on the development of a UV test to evaluate whether a particular encapsulant will discolor in the field [13].

A series of UV exposure experiments are now being conducted to determine the acceleration rates and activation energies associated with UV discoloration of encapsulants [14]. The results of these experiments will be utilized to propose UV tests that should be performed on encapsulation and frontsheet materials designed for use in PV modules. These material level tests will use coupons to evaluate the ability of encapsulants and frontsheets to survive long term outdoor exposure to the sun. A UV test similar to that proposed for encapsulants in Qualification Plus [15] [16] is envisioned. In this test the materials are packaged in coupons that have similar cross sections as modules and subjected to approximately 6 months of continuous UV exposure in a test chamber at elevated temperatures to evaluate their ability to retain their optical transmission in the outdoor environment.