Hot Spots and Microcracks in Solar Panels

A photovoltaic module is a series string of cells, and current through a series string is limited by the weakest cell in it. When one cell produces less than its neighbours, the others push current through it rather than around it, and that cell dissipates the difference as heat. A hot spot is the visible result: a small area running tens of degrees warmer than the glass around it. Microcracks are the usual reason a cell ends up weaker than its neighbours, and they are almost always invisible from the roof. Neither defect announces itself on a production graph until it has been there for some time.

What a hot spot is, physically

A silicon cell under load behaves roughly like a diode in forward bias, producing about half a volt. When it is shaded, soiled or mechanically damaged, it stops producing and becomes a diode in reverse bias instead, and the rest of the string drives current through it. The bypass diodes in the junction box protect groups of cells, not individual cells, so a single weak cell inside a protected group can still be forced into reverse breakdown. The energy it absorbs becomes heat.

The severity depends on how much current is forced through the weak cell and for how long. A leaf resting on one corner for a week is a different event from a cracked cell that is weak for twenty years. Sustained hot spots degrade the encapsulant and the backsheet around them, and in the worst cases char the backsheet or crack the glass. That is why hot spots are treated as a fire-risk matter rather than a cosmetic one.

Where microcracks come from

A microcrack is a fracture in the silicon wafer, often a few millimetres long, that does not separate the cell. It can be present before the module leaves the factory, introduced during handling, or created later by hail, by foot traffic during service, or by thermal cycling and wind load over years. A cell that is cracked may still conduct, but the fracture line interrupts the fine grid fingers that collect current, so the portion of the cell beyond the crack contributes less or nothing.

Microcracks are invisible to the eye and usually invisible to a visual inspection from the ground. What an owner may see instead is a hot spot forming at the crack, or a gradual fall in string output that no single obvious cause explains.

Snail trails

Snail trails are the thin, dark, curved lines that appear on some modules, usually running from a microcrack or a busbar toward the edge of a cell. They are not cracks themselves. They are discoloured paths in the encapsulant, associated with the migration of silver paste or with oxidation along a crack, and they tend to appear after some years in the field.

A snail trail is a marker, not a failure. It indicates that a crack or a busbar defect exists nearby. Many modules with snail trails continue to produce within warranty for years. The trail matters because it tells an inspector where to look with thermal imaging and electroluminescence, and because a trail that is widening or accompanied by a hot spot is a different matter from a stable line that has not changed.

How the defects are found

Two inspection methods find what the eye cannot. Both are performed by qualified inspectors, and neither requires the owner to open a combiner, probe a string or disconnect an energised connector. The DC side of an array is live whenever there is light on it and cannot be switched off at the panel; that work belongs to a licensed installer.

Thermal imaging, or infrared thermography, is performed while the array is under load, ideally on a clear day with steady irradiance. A thermal camera shows the temperature of the module surface, and a hot spot appears as a localised warm area, typically several degrees above its neighbours. Thermal imaging is fast and can be done from the ground or from a drone, but it shows the symptom, not the cause: a warm patch could be a cracked cell, a shaded corner, a failed bypass diode or a poor solder joint.

Electroluminescence inspection is the more definitive method. The module is forward-biased in the dark and photographed with a camera sensitive to near-infrared light. Intact cells glow evenly; a crack appears as a dark line, a dead area as a dark patch, and a broken grid finger as a dark streak. Electroluminescence shows the damage directly, but it requires the module to be energised in a controlled way and is usually done on a sample or on modules that thermal imaging has already flagged.

A typical inspection sequence is: thermal imaging of the whole array, then electroluminescence of the flagged modules, then a visual inspection of junction boxes and connectors. The inspector reports which findings are cosmetic, which are warranty matters, and which are safety matters.

Which defects matter

Warranty terms differ by manufacturer and by the date of manufacture, and they change. The power warranty, the product warranty and the exclusions for mechanical damage are separate documents. Read the specific terms that came with the modules rather than assuming a standard applies.

What people get wrong

A warm patch on a thermal image is read as proof of a cracked cell. It is not. Thermal imaging shows where heat is being dissipated, and the same signature can come from a shaded corner, a dirty patch, a failed bypass diode or a poor solder joint. The natural mistake is to treat the camera as a diagnostic tool rather than a screening tool. It is a screening tool. The diagnosis comes from electroluminescence or from electrical testing.

The second common error is the reverse: a snail trail is dismissed as cosmetic because the module still produces. Sometimes that is correct. But a snail trail is a marker of a crack, and a crack can be the origin of a hot spot that develops later. The trail itself is not the problem; the crack it points to is the thing to watch.

The third is to assume that any microcrack is a warranty claim. Many manufacturers exclude damage from handling, hail or foot traffic, and a microcrack with no measurable power loss may fall below the threshold for a claim. That is a contractual question, not a technical one, and it depends on the terms in force when the module was sold.

Reading an inspection report

An inspection report should distinguish findings by category rather than list temperatures. A useful report separates cosmetic findings, warranty-relevant findings and safety-relevant findings, and states the method used for each. A thermal image without a corresponding electroluminescence image or electrical measurement is a screening result, not a diagnosis.

If a report recommends work on the array, the work is on the DC side and is live whenever there is light. Roof work, module replacement, connector work and any work inside a combiner or junction box are licensed work. The owner’s role is to read the report, check the warranty terms, and hand the physical work to a licensed installer.

What changes over time

Modules degrade slowly under normal conditions, and the rate is stated in the warranty. Microcracks and hot spots are not part of that normal degradation; they are defects that either exist from manufacture or are introduced by an event. The distinction matters because a system that is producing slightly less each year is behaving as designed, while a system with a growing hot spot is not. Thermal imaging every few years, performed by a qualified inspector, is the practical way to tell the two apart. Interconnection rules, inspection requirements and warranty terms are set by the manufacturer, the utility and the jurisdiction, and they change; check the documents that apply to the specific installation.