A purchasing decision: compare three potting compound suppliers. One offers “high-temperature epoxy” at $40/lb. Another offers “automotive-qualified epoxy” at $80/lb. A third offers “aerospace-grade polyimide” at $200/lb. How do you choose?
Without clear specification of required properties, the decision defaults to price—often selecting the cheapest option that appears “high-temperature.” This approach leads to field failures, warranty costs, and project delays.
A proper buying guide focuses on performance specifications, not price, and matches potting properties to application requirements.
Feature 1: Glass Transition Temperature (Tg) and Thermal Margin
What to specify: Minimum Tg value and required operating margin.
Why it matters: Tg determines the temperature at which potting loses rigidity. Operating above Tg margin leads to plastic deformation and thermal cycling failure.
Recommended specification: For continuous operation up to 150°C, require Tg ≥220°C minimum. This provides 70°C margin above operating temperature.
Red flags:
– Vague specifications like “rated to 250°C” without clarifying whether it’s Tg, peak temperature, or decomposition temperature
– No thermal margin specified; potting Tg equals or only slightly exceeds maximum operating temperature
– Data sheets lacking thermal cycling performance data even though your application has thermal cycling
Questions to ask the supplier:
1. What is the exact Tg measured per ASTM D3418 or equivalent?
2. What is the continuous service temperature rating (Ts) at which mechanical properties are maintained indefinitely?
3. Provide thermal cycling test data (minimum 500 cycles −40°C to +150°C per ASTM D4169) showing strength retention
Feature 2: Thermal Conductivity for High-Power Applications
What to specify: Thermal conductivity in W/m·K and requirement for thermally-conductive filler loading.
Why it matters: Thermal conductivity determines how effectively potting dissipates heat from embedded components. Unfilled potting (<0.5 W/m·K) traps heat; filled potting (2–4 W/m·K) conducts heat away.
Recommended specification: For power dissipation >20W, require thermal conductivity ≥2 W/m·K. For >50W, require ≥3 W/m·K.
Red flags:
– Supplier doesn’t provide thermal conductivity data
– Thermal conductivity specification at only one temperature; potting conductivity changes with temperature
– Thermally-conductive potting that doesn’t specify filler type or loading percentage
Questions to ask:
1. What is thermal conductivity measured per ASTM E1530 or equivalent?
2. How does thermal conductivity change across the operating temperature range (−40°C to +150°C)?
3. What fillers are used, and at what loading percentage?
Feature 3: Coefficient of Thermal Expansion (CTE) and Low-Mismatch Design
What to specify: CTE value and requirement for matching copper PCB CTE.
Why it matters: CTE mismatch between potting and PCB creates thermal cycling stress that leads to delamination and solder joint failure.
Recommended specification: Require CTE 35–45 ppm/°C to match copper PCB (17–18 ppm/°C) with acceptable mismatch. Avoid standard potting with CTE 50–70 ppm/°C unless thermal cycling is minimal.
Red flags:
– No CTE specification provided
– Vague specification like “low-CTE potting” without numeric value
– Supplier unsure whether their formulation is low-CTE or standard
Questions to ask:
1. What is the exact CTE value per ASTM E228 or TMA analysis?
2. How was CTE measured (linear dimension change, TMA, or other method)?
3. Do you have adhesion test data (ASTM D4541) showing CTE mismatch impact?
Feature 4: Environmental Resistance and Corrosion Protection
What to specify: Oil, coolant, salt-fog, and moisture resistance per ASTM standards.
Why it matters: Potting must resist environmental contamination that would otherwise corrode embedded components.
Recommended specification:
– Oil immersion: <3% weight gain, >80% strength retention per ASTM D471
– Moisture absorption: <0.5% per ASTM D570
– Salt-fog: No visible corrosion after 1,000 hours per ASTM B117
– Coolant resistance: No property loss after 1,000 hours exposure
Red flags:
– No environmental test data provided
– Only generic claims like “excellent chemical resistance” without test documentation
– Different suppliers showing vastly different test results for seemingly identical potting type
Questions to ask:
1. Provide actual test data (not just spec claims) for oil, coolant, and salt-fog exposure
2. What is the moisture absorption and how does it affect electrical properties?
3. Are test reports available for third-party verification?
Feature 5: Elastomer Toughening and Vibration Damping
What to specify: Elastomer content and damping properties for vibration environments.
Why it matters: Rigid potting transmits vibration stress to solder joints; elastomer-toughened potting damps vibration and improves fatigue life by 5–10x.
Recommended specification: For vibration environments, require elastomer toughening 8–12% by weight with mechanical property data showing strain tolerance.
Red flags:
– “Toughened” potting without specification of elastomer percentage
– Supplier can’t explain the difference between rigid and toughened formulations
– Toughened potting with very high cost increase (suggests poor formulation)
Questions to ask:
1. What percentage of elastomer is in the formulation?
2. Provide vibration fatigue test data comparing toughened vs. rigid potting
3. Does elastomer toughening affect thermal conductivity or Tg?
Feature 6: Moisture Absorption and Electrical Property Retention
What to specify: Moisture absorption limits and electrical property test after moisture conditioning.
Why it matters: High moisture absorption degrades electrical insulation and enables corrosion. Potting that absorbs <0.5% moisture maintains electrical properties much better than standard potting.
Recommended specification: Maximum moisture absorption <0.5% per ASTM D570 (24 hours, 23°C/50% RH). Dielectric strength should be maintained >80% after moisture conditioning (85°C/85% RH, 1,000 hours).
Red flags:
– No moisture data provided
– Moisture absorption not tested per standard
– High moisture absorption (>1.0%) with claims of “moisture-resistant”
Questions to ask:
1. What is the exact moisture absorption measured per ASTM D570?
2. How does potting electrical property (dielectric strength, resistivity) change with moisture absorption?
3. Can you provide test data showing property retention after moisture conditioning?
Feature 7: Adhesion to PCB Substrates
What to specify: Adhesion strength per ASTM D4541 and adhesion retention after thermal cycling.
Why it matters: Poor adhesion leads to delamination under thermal cycling, exposing PCB to moisture and defeating potting protection.
Recommended specification: Minimum adhesive pull-strength >1.0 MPa per ASTM D4541, with >80% strength retention after thermal cycling (500 cycles −40°C to +150°C).
Red flags:
– No adhesion data provided
– Adhesion tested only initially; no thermal cycling adhesion data
– Weak adhesion data (<0.5 MPa) on low-CTE potting formulations
Questions to ask:
1. What is the measured adhesive pull-strength per ASTM D4541 on copper PCB?
2. Provide adhesion test data after thermal cycling
3. Does adhesion vary with surface preparation? What is the recommended prep?
Feature 8: Cure Profile and Pot Life
What to specify: Pot life, gel time, and full-cure time at room temperature and elevated temperature options.
Why it matters: Pot life determines how long you have to work with the potting after mixing. Gel time affects when the assembly must be immobilized.
Recommended specification: For medium-size pours (500ml–1L), require pot life 90–120 minutes at 77°F. For large pours, require extended pot life (150–180 minutes) option.
Red flags:
– Very short pot life (<30 minutes) forcing rapid assembly and increasing mistake risk
– No gel time specification
– Different pot times at different temperatures (suggest incomplete characterization)
Questions to ask:
1. What are the exact pot life and gel times at 70°F and at 80°F?
2. Is there an extended-pot-life formulation if we need to slow the cure?
3. Can cure time be accelerated by heating, and by how much?
Feature 9: Flammability and Safety Compliance
What to specify: Flammability per UL-94, smoke generation, and any aerospace/automotive certifications required.
Why it matters: Some applications require UL-94 V-0 (self-extinguishing) certification. Aerospace applications require minimal smoke generation.
Recommended specification:
– Minimum UL-94 V-1 for general use; V-0 for indoor or confined-space applications
– Smoke generation <3% for aerospace applications; <5% for automotive
Red flags:
– No flammability rating provided or “not tested”
– Aerospace-rated material sold as “V-0” without showing test reports
– High smoke generation (>10%) for aerospace application
Questions to ask:
1. What is the UL-94 classification with test report?
2. What is the smoke generation per ASTM D2843?
3. Are there any aerospace or automotive certifications (e.g., FAR Part 25 for aircraft)?
Feature 10: Supplier Documentation and Technical Support
What to specify: Complete technical documentation, test reports, and post-sale technical support.
Why it matters: Potting compound selection often involves design changes or troubleshooting. Suppliers with strong technical support reduce risk of specification errors.
Recommended specification: Require complete data sheets (including thermal cycling, environmental, and electrical test data), safety data sheets (SDS), and direct access to technical support via email or phone.
Red flags:
– Limited documentation; vague data sheets with no test data
– No technical support contact or slow response times
– Supplier unable to answer detailed questions about their product
Questions to ask:
1. What documentation is available (full data sheet with test data, SDS, processing guides)?
2. Is there a technical support contact available for post-sale questions?
3. Can you provide references from other customers using this potting in similar applications?
4. What is your typical turnaround time for technical questions?
Complete Specification Template
When specifying high-temperature potting, use this template:
Thermal requirements:
– Continuous operating temperature: [specify]
– Maximum component temperature: [specify]
– Required Tg minimum: [specify, typically operating temp + 70°C]
– Thermal cycling requirement: [specify cycles and temperature range]
Mechanical requirements:
– Power dissipation: [specify watts]
– Required thermal conductivity: [specify W/m·K]
– Vibration environment: [specify G and frequency]
– Required elastomer toughening: [specify % or “light/moderate/heavy”]
Environmental requirements:
– Humidity exposure: [specify if yes, conditions]
– Oil/coolant exposure: [specify types]
– Salt-spray environment: [specify if yes]
– Outdoor/UV exposure: [specify if yes]
Electrical requirements:
– High-voltage exposure: [specify voltage]
– EMI shielding required: [specify if yes]
– Dielectric strength requirement: [specify kV/mm if applicable]
Regulatory:
– Certifications required: [specify UL, aerospace, automotive]
– Outgassing limits (if applicable): [specify ASTM E595 limits]
Performance testing:
– Thermal cycling test data required: [specify cycles and temperature range]
– Environmental test data required: [specify which tests]
– Field-proven applications: [request references]
Buying Decision Process
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Specify requirements clearly. Use the template above; avoid vague language like “high-temperature” or “reliable.”
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Obtain full technical data. Request complete data sheets with test reports, not abbreviated “spec sheets.”
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Request field references. Ask for 2–3 customer references using the potting in similar applications. Call them directly.
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Evaluate supplier support. Assess technical support quality and responsiveness to your questions.
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Prototype and validate. Purchase sample quantities and test prototypes through your actual duty cycle (thermal cycling, environmental exposure).
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Compare cost per year of service. Higher-cost potting with superior thermal cycling performance may cost less per year of actual service life.
A higher material cost upfront is justified by reduced field failures, warranty costs, and long-term reliability.
Incure technical specialists are available to help you specify potting compounds matching your exact requirements and provide comprehensive technical support throughout your project lifecycle.
Email Us to discuss your potting requirements and receive a detailed specification recommendation based on your application’s thermal, mechanical, and environmental demands.
Visit www.incurelab.com for more information.