Bond testing of solar cell interconnections

Bond testing applies a load to an interconnect, measuring the force necessary to destroy the mechanical connection. In solar cells, the most important interconnects are the aluminum or copper ribbons used to carry current out of the cells, and the most pertinent bond test is a 90° peel test. Important equipment features for reliable results are axes coordination, sufficient clamping, and analysis of the ribbon’s breakage pattern.

Bond testing applies a load to an interconnect, measuring the force necessary to destroy the mechanical connection. In solar cells, the most important interconnects are the aluminum or copper ribbons used to carry current out of the cells, and the most pertinent bond test is a 90° peel test. Important equipment features for reliable results are axes coordination, sufficient clamping, and analysis of the ribbon’s breakage pattern.

Figure 1. Tweezers gripping aluminum ribbon during peel test.
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Those familiar with the construction and operation of solar cells realize that electronic interconnections are vital to their function. The essential interconnects include the conduits for current output, and the ancillary microelectronics that manage the output or control the tilting of more sophisticated solar panel arrays. Solar cells are subject to harsh environments, and the reliability and integrity of the electronic interconnects is paramount, both electrically and mechanically.

Bondtesting Methodology
Using highly accurate load cartridges and a variety of load tools, bondtesting applies a load to an interconnection to the point of destruction. The principal output is the force measured at the point of failure. Non-destructive testing can also be performed.

 In solar cells, the most important interconnects are the aluminum or copper ribbons used to carry current out of the cells. The most relevant bond test is the 90° peel test. To perform this test correctly requires coordinated movement of the z-axis in conjunction with either the x- or y-axis. It is vital to use bond testing equipment capable of this movement. A load cartridge is mounted on the z-axis and applies a vertical load to the ribbon gripped by tweezers (Figure 1).

Clamping of the solar cell is highly important, as an inadequate fixture will result in failure of the silicon wafer rather than the ribbon. The load at failure force is the most significant measurement, but in a peel test it is also important to evaluate the evenness of adhesion along the length of the ribbon. This is revealed in a force versus displacement graph (Figure 2).

Figure 2. Force displacement graph for a ribbon peel test denoting variations in adhesion strength .
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There are international standards applicable to certain types of bondtesting — notably gold ball bonds and gold wires in semiconductor and microelectronics, but none yet exist specifically for peel testing of ribbons in solar cells. Bond tester suppliers provide all the equipment and fixtures appropriate for performing these tests, which should facilitate sharing of data and the creation of standards.

Conclusion
In addition to ribbons, numerous other types of interconnections exist within the ancillary microelectronics of most solar cells. These typically are tested for their mechanical reliability using conventional bond testing methods of wire pull and ball and die shear, before and after accelerated life testing. Back-contact solar cells, which are analogous to flip chip devices, are an emerging technology within the solar industry and bondtesting, especially peel methods, will play an important role in evaluating their mechanical integrity.

Stephen Clark, PhD, bond tester product manager, Dage Precision Industries, a Nordson Company, may be contacted at s.clark@dage-group.com.

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