Photovoltaic (PV) module solderability testing evaluates the ability of interconnect ribbons, cell busbars, and termination surfaces to be reliably wetted by molten solder under controlled conditions, which is a decisive factor in module power output, reliability, and long-term field performance. Performed in accordance with internationally recognized procedures such as IEC 60068-2-20, IPC J-STD-002, and the IEC 61215 framework, these tests detect non-wetting, dewetting, coating degradation, and weak joints before modules are shipped. This article explains the sample types, test items, methodologies, acceptance criteria, and practical applications of solderability testing for PV modules.
Technical Overview
Solderability is defined as the ability of a metallic surface to be wetted by molten solder within a specified time, at a specified temperature, and with a specified flux, resulting in a continuous metallurgical bond through the formation of copper–tin intermetallic compounds. In crystalline silicon photovoltaic modules, this property is critical because solar cells are series-interconnected by solder-coated copper ribbon (tabbing ribbon and busbar ribbon) that is soldered to screen-printed silver busbars. Junction boxes, bypass diode leads, and output cable terminations also depend on sound soldered connections.
Poor solderability produces incomplete wetting, cold joints, voids, and excessive contact resistance. The consequences at module level include elevated series resistance and power loss, localized heating and hot spots, encapsulant delamination, and interconnect fatigue fracture during thermal cycling and humidity-freeze exposure. For this reason, solderability screening of incoming materials is a routine part of quality control in PV manufacturing, and solder joint integrity is further challenged by the thermal cycling, damp heat, and humidity–freeze tests of the IEC 61215 and IEC 61730 series.
The principal test methods derive from IEC 60068-2-20 (solder bath, solder globule, and related procedures), IEC 60068-2-58 (wetting balance method), IPC J-STD-002 (solderability of leads and terminations), MIL-STD-883 Method 2003, ASTM B678, and the regional adoptions GB/T 2423.28 and JIS C 60068-2-20. Guidance specific to solder-coated PV ribbon is provided by the IEC TS 62788 series. Testing may be carried out in-house or by accredited third-party laboratories.
Testing Samples
Solder-Coated Interconnect Ribbon
Tabbing and busbar ribbon made from electronic-grade copper, typically 0.2–0.4 mm thick and 0.8–2.0 mm wide, with an electroplated or hot-dipped tin or tin–silver–copper coating generally in the range of 5–25 µm. Specimens are cut to defined lengths, kept free of fingerprints and contamination, and tested both in the as-received condition and after simulated aging.
Crystalline Silicon Cells and Busbars
Full or half-cut cells with screen-printed silver busbars are tested to confirm that the metallization is compatible with the ribbon and solder system, particularly when paste, cell architecture, or plating processes change.
Junction Box Terminations and Lead Wires
Tinned terminals, bypass diode leads, solder tabs, and stripped output cable ends are evaluated to verify wetting behavior at the module electrical interface.
Soldered Joints and Cell-String Coupons
Coupons soldered with the production alloy, flux, and stringer process parameters are prepared for peel strength testing, contact resistance measurement, and metallographic cross-sectioning, so that joint quality is assessed in addition to surface wettability.
Testing Items
Typical test items for a PV solderability evaluation program include the following:
- Wetting solderability by the solder bath dip-and-look method or solder globule method
- Wetting balance measurement (wetting force versus time, zero-cross time)
- Solder coverage ratio of the immersed surface area
- Tin or solder coating thickness, continuity, and oxidation condition
- Peel strength of ribbon-to-busbar joints at 90° or 180°
- Contact resistance and series resistance contribution of the joint
- Metallographic cross-section analysis: intermetallic layer thickness, void content, wetting angle
- Resistance to soldering heat, i.e., the ability of components to withstand soldering temperatures without damage
- Solderability retention after aging treatments such as steam aging, dry-heat storage, or damp heat exposure
Testing Methods
Dip-and-Look (Solder Bath) Method
The specimen is cleaned, coated with a defined rosin-based flux, and immersed in a static bath of molten solder. The bath temperature is commonly 235 °C for tin–lead eutectic solder and 245–250 °C for lead-free SAC alloys, with a dwell time of typically 2–5 seconds as specified by the governing standard or customer specification. After withdrawal and cleaning, the immersed area is examined visually and under magnification. A pass result generally requires at least 95% continuous solder coverage with no evidence of non-wetting or dewetting, in line with IPC J-STD-002 and IEC 60068-2-20 criteria.
Solder Globule Method
A molten globule of solder of defined mass, typically 200 mg, at 235 °C is contacted with the wire or ribbon surface. The time required for the globule to wet and flow around the specimen is measured, providing a quantitative wettability indicator particularly suited to round leads and ribbon ends.
Wetting Balance (Meniscograph) Method
The fluxed specimen is clamped to a sensitive force transducer and immersed a few millimeters into the molten solder at a controlled speed. The instrument records the wetting force as a function of time; evaluation parameters include the zero-cross time, typically required to be no more than about 2 seconds, and the wetting force reached at defined time points. This method, covered by IEC 60068-2-58 and IPC J-STD-002, is the most quantitative approach and is well suited to comparing ribbon lots, coating types, and flux activities.
Peel Strength Testing of Soldered Joints
Ribbon is soldered to a cell busbar or to a metallized coupon using production materials and parameters. The joint is then peeled at 90° or 180° at a constant crosshead speed while the force is recorded. Acceptance thresholds are usually defined by the module manufacturer and are commonly expressed in newtons per millimeter of ribbon width. The failure mode—cohesive within the solder, adhesive at the interface, or in the silver metallization—is documented as part of the assessment.
Metallographic Cross-Section Analysis
Selected joints are mounted, polished, and etched for optical or scanning electron microscopy. Key characteristics include the thickness and uniformity of the Cu6Sn5 and Cu3Sn intermetallic layers (excessively thick intermetallics indicate over-soldering and embrittlement), void content within the joint, and the solder wetting angle. Cross-sections are also used to verify coating thickness on the as-received ribbon.
Aging and Conditioning
To simulate storage degradation, specimens may be subjected to steam aging, typically 1 hour or 8 hours, or to dry-heat conditioning before solderability testing. Comparing results before and after aging reveals whether a coating or flux system will tolerate realistic warehouse shelf life.
Applications
Solderability testing supports PV quality and engineering activities at multiple stages of the product life cycle:
- Incoming inspection of interconnect ribbon, busbar paste lots, flux, and solder alloys to verify wetting performance against specification limits
- Qualification of new solder alloys, flux chemistries, ribbon geometries, and low-temperature interconnection processes for multi-busbar, half-cut, shingled, and back-contact cell designs
- Process window validation for tabber–stringer equipment, including head temperature, dwell time, and flux deposition parameters
- Supplier change management and periodic lot surveillance to detect coating drift or plating process instability
- Root-cause analysis of field or production failures such as hot spots, series resistance increases, and interconnect fracture after thermal cycling
- Support for type testing under the IEC 61215 and IEC 61730 frameworks, where interconnect robustness is demonstrated by accredited third-party laboratories through damp heat, thermal cycling, and humidity–freeze sequences
FAQ
Which standards govern PV module solderability testing?
The core methods come from IEC 60068-2-20 and IEC 60068-2-58, supplemented by IPC J-STD-002 for terminations, MIL-STD-883 Method 2003, ASTM B678, and the regional equivalents GB/T 2423.28 and JIS C 60068-2-20. Module-level reliability requirements are established by the IEC 61215 and IEC 61730 series, while ribbon-specific measurement guidance is given in the IEC TS 62788 series.
What test temperature and dwell time are used?
For tin–lead eutectic systems, the solder bath is typically held at 235 °C; lead-free SAC systems are normally evaluated at 245–250 °C. Immersion times of 2–5 seconds are typical, with the exact values defined by the applied standard or the manufacturer's internal specification.
What is the acceptance criterion for a pass result?
In dip-and-look testing, at least 95% of the immersed surface must show continuous solder coverage with no non-wetted or dewetted areas. In wetting balance testing, the zero-cross time must fall within the specified limit, commonly around 2 seconds, and the wetting force must meet the defined minimum. Peel strength and cross-section criteria are set by the module manufacturer, typically including a minimum force per ribbon width and limits on void content and intermetallic thickness.
How does solderability differ from resistance to soldering heat?
Solderability evaluates how easily a surface is wetted by molten solder. Resistance to soldering heat evaluates whether the component or material survives the soldering thermal exposure without degradation, such as coating damage, warpage, or deterioration of the cell metallization. Both properties are relevant to reliable interconnection and are covered by the IEC 60068-2 test family.
How should samples be prepared and stored before testing?
Specimens must be handled with clean gloves or tweezers, kept in original packaging away from humidity and contamination, and tested within a defined time after receipt. Unless explicitly required by the specification, samples should not be chemically cleaned before testing, because the test is intended to reflect the surface condition as delivered.
How often should solderability testing be performed?
Best practice is to test every incoming lot of ribbon and each new lot of flux or solder alloy, to retest after any change in supplier, plating line, or storage condition exceeding the defined shelf life, and to include aged specimens periodically to monitor solderability retention over time.