Do not compare TO-220 heat-conductive silicone caps by placing a UL 94 class beside a D5470 conductivity value. First verify the flammability specimen thickness and classification method. Then check D5470 pressure, specimen thickness, contact surfaces, and whether the report states thermal impedance or calculated conductivity. Only then relate the data to package fit, heat-sink contact, and insulation requirements.
Why flammability and thermal data need separate checks
The heat-conductive silicone cap product data lists a gray silicone cap, 0.8–1.0 W/m·K conductivity, dielectric withstand above 4 kV, a -40°C to +180°C working range, and UL 94 V-0/V-1. These values come from different tests and are not substitutes for one another. A purchasing specification should identify the relevant model, specimen conditions, and report version.
Four conditions to verify for UL 94
UL’s overview of combustion tests for plastics distinguishes horizontal and vertical methods. When checking V-0 or V-1, record the material designation, specimen thickness, conditioning, and vertical test method. The class describes specimen behavior under specified conditions; it is not a complete end-product fire certification and cannot be inferred from the word silicone alone.
Engineering equation: temperature and decomposition rate
The Arrhenius rate equation is k = A exp(-Ea/(R T)), where k is the reaction-rate constant, A is the pre-exponential factor, Ea is activation energy in J/mol, R is 8.314 J/(mol·K), and T is absolute temperature in K. Purdue University’s kinetics reference explains this temperature dependence. The model applies only when a reaction step and effectively constant parameters are reasonable approximations. Silicone burning can involve multiple reactions, oxygen supply, and heat transfer, so the equation cannot establish a UL 94 class.
D5470 datasheet comparison checklist
ASTM D5470 measures steady-state thermal impedance for thermally conductive electrical insulation. When interface resistance is significant, a value calculated from one thickness should not automatically be treated as an intrinsic material property. Compare test pressure, specimen thickness or multi-thickness extrapolation, average temperature, and surface preparation. See this D5470 test-method and limitations guide for the next technical check.
Five practical assembly and sourcing checks
- Confirm whether TO-220A/B/C or TP-3PA/B matches the actual package outline.
- Record V-0/V-1 together with specimen thickness, method version, and model identity.
- Compare conductivity together with D5470 pressure, thickness, and contact conditions.
- Validate dielectric withstand and working temperature against the end equipment’s insulation distances, environment, and safety requirements.
- Use the reference image only to confirm a gray, rounded cap profile; do not infer undisclosed fillers or internal structures.
Conclusion
The useful sourcing question is not which number is higher, but under which specimen, thickness, pressure, and method the number was obtained. After reviewing the TO-220 silicone cap specifications and size options, provide the package dimensions, operating temperature, insulation requirement, and required reports so the supplier can confirm the actual model.
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