Martens heat resistance temperature evaluation determines the heat-distortion limit of cured underfill adhesives under a defined bending stress, providing a key indicator of thermomechanical stability for flip-chip, BGA, and CSP electronic packaging. This article explains the Martens test principle, specimen preparation, test procedure in accordance with GB/T 1035, typical result ranges for epoxy underfills, and how the data supports packaging reliability qualification in accredited third-party laboratories.
Technical Overview
Underfill adhesives are low-viscosity, typically epoxy-based encapsulants dispensed into the gap between a silicon die and its substrate in flip-chip, ball grid array (BGA), and chip-scale package (CSP) assemblies. Once cured, the underfill locks the solder joints into a rigid composite that redistributes thermomechanical stress and dramatically improves thermal-cycling and mechanical-shock reliability. Because these packages encounter elevated temperatures during second-side reflow, high-temperature storage, rework, and operation, the cured adhesive must retain adequate stiffness well above ambient conditions.
The Martens heat resistance temperature is a classical heat-distortion metric determined according to GB/T 1035. A cured specimen is subjected to a constant bending stress of 5 MPa while the surrounding temperature is raised at a uniform rate of 50 °C per hour. The temperature at which the free end of the specimen deflects by 6 mm is recorded as the Martens heat resistance temperature, marking the point at which the cured polymer network begins to soften appreciably under load.
- Martens temperature (GB/T 1035): bending stress of 5 MPa, heating rate of 50 °C/h, 6 mm deflection endpoint.
- Vicat softening temperature (ISO 306, ASTM D1525): flat-ended needle under 10 N or 50 N, 1 mm penetration endpoint.
- Heat deflection temperature (ISO 75, ASTM D648, GB/T 1634): three-point bending at 1.80 MPa, 0.45 MPa, or 8.00 MPa with standardized deflection limits.
For fully cured epoxy underfill adhesives, the Martens temperature typically lies near the lower portion of the glass transition region measured by DSC (ISO 11357-2). Evaluated together with Tg and coefficient of thermal expansion data, it gives packaging engineers a complete picture of the material's thermal-mechanical capability.
Testing Samples
Standard Cast Martens Bars
The liquid underfill is degassed and cast into polished molds to produce bars of 120 mm × 15 mm × 10 mm, the standard Martens specimen geometry. The bars are cured following the manufacturer's recommended schedule, commonly a staged bake such as a pre-gel at 100–150 °C followed by a post-cure at 150–165 °C for 0.5 to 2 hours. Specimens must be free of voids, bubbles, cracks, and edge chipping, and the cure state must represent the actual assembly process.
Machined Bars from Cured Plaques
When casting full-size bars is impractical, for example with highly filled or fast-gelling formulations, specimens may be machined from a cured plaque or puck that shares an identical cure history. Machined surfaces must be flat, smooth, and parallel, and cutting heat that could alter the cured network must be avoided.
Conditioning and Quantity
After cure, specimens are conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity for at least 16 hours. A minimum of three valid specimens is tested for each material or cure condition, and the width and thickness of every specimen are measured at three positions with precision calipers before loading.
Testing Items
The primary test item and the most common complementary thermal-mechanical characterization items are listed below:
- Martens heat resistance temperature, °C, per GB/T 1035 (primary item).
- Glass transition temperature by DSC per ISO 11357-2 (optional complementary item).
- Heat deflection temperature at 1.80 MPa per ISO 75-2 (optional complementary item).
- Vicat softening temperature per ISO 306 (optional complementary item).
- Coefficient of thermal expansion by TMA per ISO 11359-2 (optional complementary item).
Each report documents the cure schedule, conditioning state, individual specimen values, and the arithmetic mean result so that the data can be directly compared against material datasheets or customer specifications.
Testing Methods
Standard and Test Principle
The evaluation follows GB/T 1035, the plastics heat resistance test by the Martens method. The specimen is clamped vertically in a holder, and a calibrated lever-and-weight system applies a bending stress of 5 MPa to the specimen surface fibers. A deflection indicator tracks the movement of the free end while a heating cabinet raises the temperature at 50 °C per hour. The temperature read at the moment the deflection reaches 6 mm is the Martens heat resistance temperature.
Apparatus
- Martens heat resistance tester with multi-station specimen holders, calibrated lever-and-weight sets, and deflection indicators readable to 0.01 mm.
- Programmable forced-convection heating cabinet providing a uniform temperature field and a stable 50 °C/h ramp.
- Calibrated thermocouple positioned adjacent to the specimens for accurate cabinet temperature measurement, plus calipers or a micrometer for dimensional measurement.
Test Procedure
- Measure the conditioned specimen width and thickness at three points and calculate the load required to generate a 5 MPa surface bending stress.
- Clamp the specimen vertically, attach the loading lever and weights, and zero the deflection indicator.
- Place the assembly in the cabinet and start the ramp from ambient temperature or from a temperature at least 50 °C below the anticipated result.
- Monitor the deflection continuously and record the temperature at which the indicator reading reaches 6 mm.
- Terminate the test, allow the apparatus to cool, and repeat for the remaining specimens; discard results from specimens that slip, fracture, or show gross defects.
Data Expression and Judgment
Individual values and the arithmetic mean are reported to the nearest 1 °C. Fully cured epoxy underfill adhesives typically show Martens temperatures between roughly 90 °C and 135 °C, depending on resin chemistry, hardener type, filler loading, and cure completeness. Results are judged against the material specification or the customer-defined acceptance limit, which should provide a safety margin above the maximum exposure temperature of the assembly process and service environment. Excessive scatter between specimens usually indicates cure non-uniformity, internal voids, or dimensional error and warrants investigation.
Applications
Martens heat resistance temperature data supports material selection, qualification, and quality control across the electronics packaging industry:
- Reliability qualification of underfills for flip-chip, BGA, CSP, and wafer-level packages where thermomechanical stability is critical.
- Incoming material inspection and lot release of underfill adhesives for packaging and assembly lines.
- Formulation benchmarking and cure-schedule optimization during development of epoxy, anhydride-cured, and filled adhesive systems.
- Automotive electronics such as engine control units, ADAS modules, and on-board chargers that face prolonged high-temperature operation.
- Advanced packaging platforms including 2.5D interposers, fan-out packages, and stacked memory requiring robust interlayer support.
- Failure analysis, comparing the Martens temperature of degraded adhesive against virgin material to quantify thermal aging or under-cure.
Accredited third-party laboratories routinely combine the Martens evaluation with Tg, HDT, and CTE measurements to build a complete thermal-mechanical profile that supports customer-specific or IEC- and UL-based qualification programs.
FAQ
What is the difference between the Martens temperature and the glass transition temperature?
The Martens temperature is a mechanical softening point measured under a 5 MPa bending stress as temperature rises, while Tg by DSC is a calorimetric property measured without load. For a given underfill, the Martens temperature typically falls near or slightly below the DSC Tg onset, whereas the DMA tan-delta peak often appears higher. All three values describe the same softening transition from different measurement perspectives.
Why choose the Martens test instead of Vicat or HDT for underfills?
The Martens method applies a bending stress level relevant to load-bearing adhesives and is widely referenced in GB-based specifications. Vicat and HDT per ISO or ASTM standards are equally valid alternatives, and many specifications allow any of the three. Choosing the method is a matter of the governing specification, and accredited third-party laboratories can perform all of them on the same cured specimens.
Can the underfill inside an actual assembled board be tested directly?
No. The Martens method requires standard bars of approximately 120 mm × 15 mm × 10 mm that can be clamped and loaded. Thin underfill layers inside a package cannot be tested directly, so the adhesive is cast and cured into standard bars using the same cure schedule as the assembly process, ensuring the result represents the material in its true cured state.
How is a pass or fail decision made?
The measured mean Martens temperature is compared with the material datasheet limit or the customer-defined acceptance criterion. A common engineering rule is that the Martens temperature should exceed the maximum process or service exposure temperature, such as second-side reflow or high-temperature storage, with an adequate safety margin, so that the cured underfill maintains stiffness and protects the solder joints throughout the product life.
What factors influence the repeatability of the result?
Key factors include cure completeness, void content, dimensional accuracy of the bars, calibration of the lever loads and deflection indicator, control of the 50 °C/h heating rate, and specimen conditioning. Standardized preparation and calibrated apparatus in accredited third-party laboratories keep inter-specimen scatter typically within a few degrees.
How long does the evaluation take?
The overall timeline includes specimen casting and cure, at least 16 hours of conditioning, and a heating ramp of two to four hours per run. Including report preparation, a typical turnaround is three to seven working days, depending on the cure schedule and the number of test conditions requested.
