Defective solder contacts in junction boxes are among the technically most relevant failure modes in photovoltaic modules. Although the junction box is a key component for the electrical safety and functionality of the module, soldering defects can already occur in brand-new modules. Since the affected contact points are located inside the encapsulated junction box, such defects often remain undetected during transport, installation, and commissioning and only show their effects later during operation.

Where do defective solder contacts occur in junction boxes?

In this failure mode, defective solder contacts occur exclusively inside the junction box. The affected areas include, in particular, the solder joints between the cell string ribbons and the internal contact surfaces of the junction box, as well as the connections to bypass diodes or busbars. Especially critical are solder joints that are mechanically stressed, insufficiently soldered, or not soldered at all (“cold solder joints”), or joints where several materials meet. Since these connections are fully enclosed by the junction box and usually additionally potted, they are not visible from the outside and cannot be checked by direct visual inspection.

How can defective solder contacts in junction boxes be identified?

An external visual inspection of the junction box is generally not sufficient to identify defective solder contacts. In practice, electrical measurements provide the first indications, for example through abnormalities in the IV curve or an increased series resistance of the module. Thermographic inspections during operation are particularly informative, since defective solder joints in the junction box often lead to localized heating.

For a clear diagnosis, more advanced testing methods are used, such as partial shading experiments, electroluminescence measurements, and non-destructive X-ray inspections. X-ray images can directly reveal defects, incompletely wetted solder points, or incorrectly positioned ribbons inside the junction box without the need to open it. The Fraunhofer Center for Silicon Photovoltaics CSP in Halle offers specialized X-ray inspections for PV modules for this purpose.

What causes defective solder contacts in junction boxes?

The causes of defective solder contacts in junction boxes are mainly related to the manufacturing process. Insufficient soldering temperatures or excessively short soldering times are common factors, preventing the solder from fully wetting the contact surfaces. Ribbon misalignment, uneven solder application, or mechanical stress during junction box assembly can also result in unstable solder joints. In addition, high production cycle speeds and material variations in contact strips, solder, or diodes can contribute to these defects. Many of these issues are not detected during final inspection and only continue to develop under thermal stress in the field.

What risks do defective solder contacts in junction boxes pose?

In general, defective solder contacts in junction boxes can lead to increased contact resistance at key electrical interfaces of the module. Depending on whether all six solder joints are affected or whether only one or several connections are not properly soldered, the defect can appear in different forms. While defective solder joints at the input and output to the module cable can cause permanent heating due to increased series resistance (Rs), defects at other solder points can result in open or non-contacted bypass diode paths.

Defective solder joints at the input and output to the module cable lead to localized heating concentrated directly in the junction box. This heating causes power losses and at the same time represents a permanent safety-related risk. Over the long term, the thermal stress can accelerate the aging of the solder contact and lead to damage to the junction box, the bypass diodes, or the potting material. A characteristic feature of this failure mode is its gradual progression: the problem often only becomes clearly visible after a longer period of operation or under high thermal stress.

Particularly critical are very poor or completely missing solder contacts at the diodes. In shading conditions, this can create an extreme risk, because the affected strings may be exposed not only to sub-module voltage, but in some cases to the full system voltage. This significantly increases the risk of localized overheating and, in the worst case, can lead to severe thermal damage to the module.

How can defective solder contacts in junction boxes be prevented, and can the defect be repaired?

To prevent defective solder contacts in junction boxes, consistent quality assurance in module manufacturing is essential. This includes enhanced module testing, especially random electrical inspections. In the case of suspicious batches or damage incidents, additional X-ray analyses may be useful. Careful handling of the modules during transport and installation is also important in order to avoid additional mechanical stress on the junction box.

In practice, repairing defective solder contacts in junction boxes is generally not possible, since the boxes are potted and cannot be opened without causing damage. In most cases, the only option is therefore to replace the affected module. If photovoltaic modules need to be replaced, care must be taken to ensure that the replacement module is electrically and technically compatible with the existing system. Suitable replacement modules can be sourced via the SecondSol online marketplace.

Conclusion

Defective solder contacts in the junction boxes of photovoltaic modules represent a specific failure mode that is hardly visible from the outside. They originate during the manufacturing process, often only develop their full impact during operation, and can impair both the energy yield and the electrical safety of the modules. Targeted testing using suitable measurement and analysis methods is crucial in order to identify this failure mode at an early stage and prevent consequential damage to the PV system.

The image examples used in this article, as well as further technical insights on this failure mode, are sourced in part from Intertek CEA, Photovoltaikbuero, and Fraunhofer CSP.