Compare three potting suppliers: one offers “high-temperature epoxy” at $40/lb, another “automotive-qualified epoxy” at $80/lb, a third “aerospace-grade polyimide” at $200/lb. Without a clear specification of required properties, the decision defaults to price — usually the cheapest option that merely sounds high-temperature. That path leads to field failures, warranty costs, and project delays.
A proper buying process focuses on performance specifications tied to your actual application, not marketing language. Here are the ten features worth pinning down before you sign a purchase order.
1. Glass Transition Temperature and Thermal Margin
Tg determines the temperature at which potting loses rigidity, and operating above your required margin invites plastic deformation and cycling failure. For continuous operation to 150°C, require Tg ≥220°C — a 70°C margin. Watch for vague claims like “rated to 250°C” that don’t clarify whether that’s Tg, peak temperature, or decomposition temperature, and for data sheets missing cycling performance data even when your application involves cycling. Ask specifically for Tg measured per ASTM D3418 or equivalent, the continuous service temperature (Ts) at which properties hold indefinitely, and cycling test data (500+ cycles, −40°C to +150°C) showing strength retention.
2. Thermal Conductivity for Power Dissipation
Conductivity determines how effectively potting moves heat away from embedded components — unfilled potting (under 0.5 W/m·K) traps it, filled potting (2–4 W/m·K) conducts it away. Require ≥2 W/m·K above 20W of dissipation and ≥3 W/m·K above 50W. A supplier who can’t provide conductivity data, or only offers a single-temperature figure when conductivity actually shifts with temperature, hasn’t fully characterized their product. Ask how conductivity changes across your operating range and what filler type and loading percentage is used.
3. Coefficient of Thermal Expansion and PCB Matching
CTE mismatch between potting and PCB drives cycling stress that leads to delamination and solder failure — a mechanism we cover in depth in why potting compound delaminates after thermal cycling. Require CTE 35–45 ppm/°C to reasonably match copper PCB (17–18 ppm/°C); avoid standard 50–70 ppm/°C potting unless cycling is minimal. A supplier unsure whether their own formulation is low-CTE or standard, or unable to produce adhesion data per ASTM D4541 showing the real-world impact of any mismatch, isn’t ready for this application.
4. Environmental and Corrosion Resistance
Potting must resist oil, coolant, salt-fog, and moisture exposure that would otherwise corrode embedded components. Look for oil immersion under 3% weight gain with 80%+ strength retention per ASTM D471, moisture absorption under 0.5% per ASTM D570, no visible salt-fog corrosion after 1,000 hours per ASTM B117, and no coolant-related property loss after 1,000 hours of exposure. Generic claims like “excellent chemical resistance” without actual test documentation — or wildly different results between suppliers claiming the same potting type — are signs to dig deeper before committing.
5. Elastomer Toughening and Vibration Damping
Rigid potting transmits vibration stress straight to solder joints; elastomer-toughened potting damps it and improves fatigue life 5–10x, as detailed in our piece on preventing vibration damage. For vibration-duty applications, require 8–12% elastomer toughening by weight with supporting strain-tolerance data. If a supplier can’t state the elastomer percentage, explain the difference between rigid and toughened formulations, or justify a large cost jump for “toughened” material, treat that as a gap in their own understanding of the product.
6. Moisture Absorption and Electrical Retention
High moisture absorption degrades electrical insulation and enables corrosion; potting under 0.5% absorption (per ASTM D570, 24 hours at 23°C/50% RH) retains electrical properties far better than standard formulations. Dielectric strength should stay above 80% of initial value after 1,000 hours of 85°C/85% RH conditioning. Treat “moisture-resistant” claims paired with absorption over 1.0%, or no moisture data at all, as unverified marketing rather than a real spec.
7. Adhesion to PCB Substrates
Poor adhesion leads to delamination under cycling, exposing the PCB to moisture and defeating the entire point of potting. Require pull-strength above 1.0 MPa per ASTM D4541 with 80%+ retention after 500 cycles (−40°C to +150°C), and ask how adhesion varies with surface preparation and what prep the supplier recommends. Adhesion data taken only at time zero, with nothing after cycling, tells you little about real service life.
8. Cure Profile and Pot Life
Pot life sets how long you have to work with the material after mixing; gel time sets when the assembly must be immobilized. For medium pours (500ml–1L), require 90–120 minutes of pot life at 77°F, with an extended-pot-life option (150–180 minutes) available for larger pours. A pot life under 30 minutes forces rushed assembly and raises mistake risk; ask for exact pot life and gel time at both 70°F and 80°F, and whether heat-accelerated cure is available and by how much.
9. Flammability and Safety Compliance
Some applications require UL-94 V-0 self-extinguishing performance; aerospace applications add smoke-generation limits. See UL’s combustion testing methodology for how these classifications are actually determined. Require V-1 minimum for general use and V-0 for indoor or confined-space placement, with smoke generation under 3% for aerospace and under 5% for automotive per ASTM D2843. A flammability rating claimed without a test report behind it isn’t a rating — it’s a guess.
10. Documentation and Technical Support
Potting selection often runs into design changes or troubleshooting mid-project, and a supplier with weak technical support multiplies that risk. Require complete data sheets (including cycling, environmental, and electrical test data), safety data sheets, and a named technical support contact. Ask for two or three customer references in similar applications and call them directly — a supplier who can’t produce references, or takes days to answer a technical question, is telling you what post-sale support will look like.
Turning This Into a Purchase Order
Specify your requirements in writing using the categories above rather than vague language like “high-temperature” or “reliable”: continuous operating temperature and required Tg margin, power dissipation and required conductivity, vibration environment and required elastomer content, environmental exposures (humidity, oil, salt, UV), electrical requirements (voltage, dielectric strength), and any certifications needed (UL, aerospace, automotive).
Request full technical data sheets rather than abbreviated spec sheets, call the references you’re given, and prototype before committing to production volume — a higher unit cost with genuinely superior cycling performance routinely costs less per year of actual service than a cheaper compound that fails early. For a deeper look at how these ten features apply specifically to power electronics, see our companion piece on potting for power supplies and industrial electronics.
Email Us with your draft specification and we’ll flag any gaps before you send it out for quotes.
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