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ISO/TS 25336:2025 — Low-Temperature Cracking Index Test for Thermosetting Resin Encapsulants
Plastics — Thermosetting resin-based materials — Embedded metal block method for LT cracking resistance
Introduction to Low-Temperature Cracking in Thermosetting Encapsulants
With the relentless advancement of large-scale integration in integrated circuits and the miniaturization of electronic components, thermosetting resin-based materials — particularly epoxy formulations — have become the cornerstone of power electronics device encapsulation. These materials provide excellent electrical insulation, thermal management, and mechanical protection. However, a critical failure mode emerges at low temperatures: brittleness-induced cracking of the encapsulant, which can compromise device reliability and lead to field failures.
ISO/TS 25336:2025 addresses this challenge by introducing a quantitative test method: the embedded metal block method. This technique simulates real encapsulation structures by pre-embedding a carefully designed stress module within a thermosetting resin specimen, then subjecting it to controlled cooling until cracks initiate at predetermined stress concentration points.
The embedded metal block method directly mimics the stress conditions found in real power modules, where a metal substrate (e.g., copper or aluminium) is encapsulated by epoxy resin. This makes the test highly representative of actual service conditions.
Apparatus, Mould Design and Stress Module
Critical Apparatus Requirements
The test requires several specialized pieces of equipment, each with specific performance thresholds:
| Equipment | Specification | Purpose |
|---|---|---|
| Vacuum drying oven | Min. pressure ≤ 100 Pa | Degassing the resin compound to eliminate气泡 |
| Vacuum batching plant | Min. pressure ≤ 100 Pa | Mixing resin, curing agent and fillers under vacuum |
| Drying oven | Must meet required cure temperature/time | Curing the resin compound |
| Temperature test chamber | Min. -70 °C, ramp rate 0.1–0.3 °C/min | Controlled cooling for crack detection |
Mould Construction
The mould is a three-piece assembly consisting of a cavity plate and two covering plates, each 10.0 ± 0.1 mm thick. The cavity plate is square (140.0 mm side) with a central cylindrical cavity 100.00 mm in diameter. A casting gate at the upper edge facilitates resin injection. The mould material — typically tool steels such as 35CrMo7 or P20 — must achieve a surface roughness of Ra 1.6 to ensure proper demoulding and consistent specimen quality.
Stress Module Design
The stress module is the key innovation in this test method. It is a quadrilateral metal plate, 10.0 mm thick, with four chamfered corners. The critical feature is the stress angle — the smallest chamfer with a radius of 2.0 ± 0.05 mm — which serves as the crack initiation site. Two to three positioning holes ensure consistent placement within the mould cavity.
The positioning of the stress module is critical: the stress angle must face away from the casting gate (inlet) to avoid premature stress concentration from the flow front during injection.
Test Procedure and Cracking Index Calculation
Specimen Preparation
The preparation process follows a rigorous sequence: preheat the mould at 100 °C for at least 2 hours, prepare the thermosetting resin compound (resin + curing agent + optional fillers/tougheners/accelerators) under vacuum stirring, cast into the preheated mould under vacuum (100–300 Pa), then cure according to the product specification. After cooling, the specimen is demoulded and inspected for visible defects.
Two-Stage Cooling Test
Rather than directly cooling to failure, the test employs a clever two-stage strategy:
Stage 1 — Preliminary test: Two specimens are cooled at 1 K/min from 30 °C. The temperature at which the first crack appears at any stress angle is recorded as T₀ (the highest cracking temperature).
Stage 2 — Cracking temperature test: The chamber starts at T₀ + 20 °C. Multiple specimens are held for 30 minutes, then cooled at 0.05–0.1 K/min — a much slower rate to accurately resolve the cracking temperature. Each specimen’s cracking temperature Tckᵢ is recorded.
Crack Resistance Index (CR)
The CR value quantifies crack resistance as:
CR = Σ(T − Tckᵢ) / (n × T)
Where T = 25 °C (laboratory room temperature), Tckᵢ is the cracking temperature of the i-th sample, and n is the number of specimens (minimum 3). A higher CR value indicates better crack resistance — the material cracks at lower temperatures, further from room temperature.
Engineering insight: The CR index enables direct quantitative comparison between different encapsulation formulations. A material with CR = 0.45 is significantly more robust than one with CR = 0.22, providing engineers with a clear selection criterion for cold-environment applications.
Engineering Design Insights
From a practical engineering standpoint, several aspects of ISO/TS 25336 deserve special attention:
1. Material formulation matters: The standard explicitly allows for fillers, tougheners, and accelerators in the compound. This means the test can be used to optimize formulations — for instance, comparing the effect of adding 5% vs. 10% silica filler on crack resistance.
2. Cooling rate sensitivity: The slow cooling rate in Stage 2 (0.05–0.1 K/min) is deliberately chosen to avoid thermal shock effects that would confound the measurement. Engineers should be aware that faster cooling in real applications may cause cracking at higher temperatures than the CR index suggests.
3. Statistical treatment: The standard notes that “the cracking temperature variability of parallel specimens is significant” due to the brittle nature of these materials. A minimum of 3 specimens is recommended, but for high-stakes applications (e.g., automotive power modules), 5–10 specimens would provide more statistically robust data.
If a specimen does not crack at the stress angle, or visible bubbles appear on the fracture surface, that data point must be discarded. This underscores the importance of proper vacuum degassing and mould preparation.
Frequently Asked Questions
Q: What types of thermosetting resins can be tested with this method?
A: The method is suitable for any thermosetting resin-based material used in power electronics encapsulation, including epoxy, silicone, and polyurethane formulations. However, it does not apply to materials that crack at 0 °C or above — those require a different test protocol.
A: The method is suitable for any thermosetting resin-based material used in power electronics encapsulation, including epoxy, silicone, and polyurethane formulations. However, it does not apply to materials that crack at 0 °C or above — those require a different test protocol.
Q: Can the CR index be used for material qualification in automotive or aerospace applications?
A: Yes, but with caution. The CR index provides comparative data under controlled laboratory conditions. For qualification, additional tests (thermal cycling, humidity exposure, mechanical shock) should complement the CR measurement.
A: Yes, but with caution. The CR index provides comparative data under controlled laboratory conditions. For qualification, additional tests (thermal cycling, humidity exposure, mechanical shock) should complement the CR measurement.
Q: Why is the stress module made of the same material as the mould?
A: To ensure consistent thermal expansion behaviour. If the stress module expanded at a different rate than the mould, the specimen geometry would be compromised during the curing process.
A: To ensure consistent thermal expansion behaviour. If the stress module expanded at a different rate than the mould, the specimen geometry would be compromised during the curing process.
Q: What is the significance of the -70 °C minimum chamber temperature?
A: This covers virtually all practical low-temperature applications, from Arctic outdoor installations to aerospace cold-soak conditions. If a material does not crack at -70 °C after 30 minutes, its Tck is recorded as -70 °C for CR calculation purposes.
A: This covers virtually all practical low-temperature applications, from Arctic outdoor installations to aerospace cold-soak conditions. If a material does not crack at -70 °C after 30 minutes, its Tck is recorded as -70 °C for CR calculation purposes.
