Photovoltaic panel PCB temperature rise testing measures how much the printed circuit boards, bypass diodes, and interconnects inside solar modules and junction boxes heat up under continuous rated and overload currents. As a core safety and reliability assessment required by IEC 61215, IEC 61730, UL 61730, and related standards, it verifies that critical components remain within safe thermal limits throughout a service life that often exceeds 25 years. This article explains the sample types, test items, measurement methods, and typical applications of temperature rise testing for photovoltaic PCBs.
Technical Overview
Printed circuit boards are embedded in nearly every layer of a photovoltaic power system. Inside the module junction box, a small PCB carries the bypass diodes that protect shaded or damaged cells; downstream, dedicated boards perform maximum power point tracking, power conversion, monitoring, and protection in power optimizers, microinverters, string inverters, charge controllers, and combiner boxes. Because these boards conduct substantial DC currents, sometimes at full module short-circuit current during shading or fault events, their thermal behavior directly determines product safety and service life.
Temperature rise testing quantifies the difference between the temperature of critical components and the ambient temperature when the PCB assembly is loaded with a specified current until thermal equilibrium is reached. The measured rise, combined with worst-case installation ambient temperatures, allows engineers to verify that diode junctions, solder joints, copper traces, connector terminals, and laminate materials all remain below their rated limits. Excessive temperature rise accelerates insulation aging, degrades solder joints, increases contact resistance, and in extreme cases can trigger bypass diode thermal runaway or junction box fires.
The governing requirements are defined in international and national standards, including IEC 61215-2 (bypass diode thermal test, MQT 18), IEC 61730-1/-2 and UL 61730 (module safety qualification), IEC 62979 (bypass diode thermal runaway test method), IEC 62109-1 and UL 1741 (safety of power converters), UL 796 (printed wiring boards), IPC-2152 and IPC-2221 (trace current-carrying capacity), together with regional equivalents such as EN 61215, EN 61730, GB/T 9535, and GB/T 20047. Accredited third-party laboratories perform these tests as part of type approval and market access programs.
Testing Samples
Test samples range from complete module assemblies to individual coupons, depending on the standard invoked and the engineering objective. The principal sample categories are described below.
Module Junction Box Assemblies
The most frequently tested sample is a complete photovoltaic module fitted with its production junction box, including the internal PCB, bypass diodes, potting compound or encapsulant, cables, and connectors. This configuration reproduces the true thermal interfaces between the diode, PCB, box housing, and module backsheet, all of which strongly influence the measured temperature rise under IEC 61215-2 MQT 18.
Bypass Diode Sub-Assemblies
For design development and supplier qualification, the diode carrier board, ribbon, or PCB with mounted bypass diodes can be tested in a representative fixture. This approach accelerates iteration on diode selection, copper weight, thermal via design, and encapsulation materials before full module testing is undertaken.
Power Optimizer and Microinverter Boards
Power stage and control PCBs from module-level power electronics are evaluated under normal and abnormal operating conditions per IEC 62109-1 and UL 1741. Key monitored points include MOSFETs, inductors, current shunts, transformer windings, and electrolytic capacitors operating at rated load and at specified elevated ambient temperatures.
Charge Controller, Combiner Box, and Monitoring PCBs
Balance-of-system boards, including solar charge controllers, rapid shutdown devices, arc-fault detection modules, and string monitoring boards, are tested at their rated continuous current to verify that connectors, relay contacts, and shunt resistors remain within the limits of their insulation systems.
Current-Carrying Capacity Coupons
Custom test coupons containing defined trace widths, thicknesses, and copper weights, together with representative via and copper plane structures, are fabricated to validate trace sizing against IPC-2152 design curves or project-specific derating rules before production artwork release.
Testing Items
Typical test items assessed during a photovoltaic PCB temperature rise program include:
- Steady-state temperature rise of PCB traces, planes, and laminates at rated continuous current
- Bypass diode junction temperature under 1.25 times module short-circuit current (IEC 61215-2 MQT 18.1)
- Reverse current overload withstand and post-test diode functionality (IEC 61215-2 MQT 18.2)
- Bypass diode thermal runaway susceptibility per IEC 62979
- Trace and plane current-carrying capacity up to a specified rise limit or to failure
- Terminal, connector, and crimp contact temperature rise at rated and overload current
- Infrared hot-spot survey of the assembled board under load
- Post-test dielectric withstand and insulation resistance verification
- Solder joint, laminate, and conformal coating integrity inspection after thermal stress
Testing Methods
Instrumentation and Thermal Measurement
Fine-gauge type K or type T thermocouples, typically 0.25 mm to 0.5 mm in diameter, are bonded directly to diode leads, solder fillets, high-current traces, and terminals using thermally conductive adhesive or high-temperature solder. A multi-channel data acquisition system scans all channels at fixed intervals, and the test continues until thermal equilibrium is achieved, commonly defined as a change of less than 1 K over 10 minutes. Infrared thermography with emissivity correction, usually applied through matte black reference coatings at an emissivity of approximately 0.95, is employed first to locate hot spots and confirm optimal thermocouple placement. Where direct junction access is impossible, junction temperature is calculated from the measured lead or case temperature plus the dissipated power multiplied by the manufacturer's junction-to-lead thermal resistance, or alternatively by using the forward voltage temperature coefficient method.
Bypass Diode Thermal Test
Following IEC 61215-2 MQT 18.1, the module or assembly is placed in still air at a controlled ambient temperature, and a DC current equal to 1.25 times the module short-circuit current at STC is passed through each diode for at least one hour or until equilibrium. The resulting junction temperature must not exceed the diode manufacturer's rated maximum, commonly 150 °C for standard rectifiers, adjusted for modules intended for high-ambient installations. After the test, each diode must remain fully functional and continue to meet specified forward and reverse electrical characteristics. Sub-assemblies tested for engineering purposes reference the same 1.25 multiplier and equilibrium criteria so that results remain comparable.
Reverse Current Overload and Thermal Runaway Testing
MQT 18.2 of IEC 61215-2 applies an elevated reverse current to the diode for a defined duration, after which the sample is inspected for cracking, burning, or loss of blocking capability, and diode functionality is re-verified. IEC 62979 provides a dedicated thermal runaway method for crystalline silicon modules: the diode is heated by an applied current, and the current is then stepped down in a controlled sequence while the case temperature is monitored. If the temperature falls toward a stable operating point at each current level, the design passes; if the temperature continues to climb as the current decreases, the design exhibits thermal runaway susceptibility and must be revised.
Current-Carrying Capacity Evaluation
Trace heating is evaluated on coupons or production boards by driving calibrated DC currents through individual nets while measuring trace mid-span temperature. Results are compared against IPC-2152 design curves or the project's allowable rise, frequently 20 K for external conductors and 10 K for internal conductors in higher-reliability designs, while ensuring the laminate never exceeds its UL 796 rated maximum operating temperature or glass transition range. Overload endurance runs at 1.5 to 2 times rated current document failure modes and design margins.
Ambient and Mounting Conditions
Because convection dominates PCB cooling, tests are performed in still air inside enclosures or chambers free from drafts and direct solar gain. Reference ambient temperature is measured by thermocouples positioned near, but not thermally influenced by, the sample. For rooftop, desert, or enclosed installations, the entire test is repeated inside an environmental chamber at the elevated ambient the product will experience, for example 40 °C to 70 °C, and the results are extrapolated to worst-case installation temperatures.
Applications
Temperature rise test data supports every stage of the photovoltaic product lifecycle:
- Certification and market access under IEC 61215, IEC 61730, UL 61730, EN, and GB schemes through accredited third-party laboratories
- Thermal design validation of junction box PCBs, diode selection, copper weights, thermal vias, and encapsulation materials during product development
- Supplier qualification and incoming quality control for junction box and bypass diode vendors
- Failure analysis of melted junction boxes, discolored laminates, or failed diodes in field returns
- Derating studies and inputs for accelerated aging models and long-term insulation life prediction
- Qualification of products for high-ambient installations such as rooftops, desert sites, and enclosed combiner locations
FAQ
What is photovoltaic panel PCB temperature rise testing?
It is a safety and reliability test that measures how much the temperatures of PCB traces, bypass diodes, solder joints, and terminals increase above ambient when the assembly carries rated or specified overload currents until thermal equilibrium. The results verify that every component stays below its maximum rated temperature during both normal and fault operation.
Which standards apply to this testing?
The primary references are IEC 61215-2 MQT 18 for bypass diode thermal and reverse current tests, IEC 61730-2 and UL 61730 for module safety, IEC 62979 for thermal runaway, IEC 62109-1 and UL 1741 for power converter boards, UL 796 for laminate temperature ratings, and IPC-2152 for trace current-carrying capacity. Regional equivalents include EN 61215, EN 61730, GB/T 9535, and GB/T 20047.
What test current is applied and for how long?
For the bypass diode thermal test, a current of 1.25 times the module short-circuit current at STC is passed through each diode for at least one hour, or until temperatures stabilize, typically defined as less than 1 K of change over 10 minutes. Development and overload studies may use higher multipliers defined by the applicable product standard.
How is junction temperature measured if the diode is encapsulated?
When direct access is impossible, junction temperature is derived from the measured lead or case temperature plus the product of dissipated power and the manufacturer's junction-to-lead thermal resistance. The forward voltage temperature coefficient method, in which the diode's own voltage drop serves as a temperature sensor, offers a non-invasive alternative for research programs.
What are the typical pass criteria?
The diode junction temperature must remain below its rated maximum, commonly 150 °C, adjusted for the intended installation ambient. Traces and laminate must stay within the UL 796 rated operating temperature and the glass transition limits of the material system, contact points must not exceed connector ratings, and the diode must retain full blocking capability and functionality after overload exposure.
Can the test be performed on sub-assemblies instead of complete modules?
Yes. Junction box PCB sub-assemblies, diode carriers, and trace coupons are widely tested for design validation and supplier qualification, provided the fixture reproduces the thermal interfaces and still-air conditions of the final product. Certification testing, however, is generally conducted on complete modules as required by the IEC 61215 and IEC 61730 test sequences.
How many samples are required?
Certification programs draw sample quantities from the tables in the IEC 61215-1 and IEC 61730-2 test sequences, which typically require complete modules for the bypass diode tests. For engineering evaluation, three to five sub-assembly units provide reasonable statistical confidence in measured temperature rise and failure margins.