Potting is the process of filling electronics assemblies with a protective compound or resin to guard against environmental hazards like moisture, vibration, contaminants and impact. PCB potting encapsulates the board and components within a solid block that provides mechanical and environmental protection.
Specialized PCB assembly houses offer potting services to encapsulate finished circuit boards using different potting compounds and processes tailored to the required protection specifications. This article provides a detailed overview of professional PCB potting services.
Overview of Potting Process
Potting involves completely surrounding the PCB assembly in a liquid resin or polymer compound that then cures to a hardened protective block.
Some key steps in the professional PCB potting process:
- Designing potting fixtures to mask areas and hold PCBs
- Selecting the potting resin chemistry suited for needs
- Mixing, degassing and preparing potting materials
- Loading boards into fixtures and applying potting strategies
- Dispensing or injection molding the liquid potting material
- Curing the material using heat, UV exposure or chemical reaction
- Inspecting for complete fill and curing
- Removing cured potted boards from fixtures
- Testing potted assemblies for function and durability
- Applying protective labels and packaging
Potting transforms a bare PCB into a solid protected module that can withstand harsh conditions.
Benefits of Potting PCB Assemblies
Key benefits that professional PCB potting provides:
Environmental Protection – Potting seals the electronics from water, humidity, dust, chemicals and corrosive contaminants.
Mechanical Protection – The hardened potting material buffers against vibration, shock and impact damage during use.
Insulation – Potting electrically insulates the components and prevents short circuits. High voltage boards benefit.
Bonding – Potting bonds components and strengthens solder joint fatigue resistance.
Improved Thermal Performance – Potting facilitates heat transfer away from components, aiding cooling.
EMI/RFI Shielding – Some potting materials block electromagnetic and radio interference.
Dielectric Isolation – Potted modules isolate high-speed circuits and prevent signal coupling.
Added Rigidity – Potting reinforces PCBs and components to prevent flexing damage.
Aesthetic Appeal – Potting provides a uniform appearance hiding underlying PCB complexity.
For applications facing harsh environments, potting is a robust protection solution.
Industries Using PCB Potting
Many industries leverage PCB potting to protect electronics in challenging operating environments:
- Automotive – Under-hood controllers and power modules exposed to vibration, temperature cycles and moisture.
- Aerospace – Avionics control systems facing mechanical stresses, wide temperature swings and altitude pressure.
- Marine Electronics – Corrosion resistance needed on boards used in humid, salty marine environments.
- Military/Defense – Mission-critical systems requiring hardening against shock, water, EMI and contaminants.
- Medical – Protecting implanted devices from body environment and preventing electrical hazard.
- Industrial – Process control electronics seeing dust, chemicals and mechanical abuse during factory operation.
- Consumer Electronics – Devices like cameras, power tools, etc. requiring improved robustness and liquid sealing.
Any application facing contamination, condensation, vibration or impact can benefit from professional PCB potting.
Materials Used for Potting
Various potting compounds are used to encapsulate PCBs based on requirements:
Epoxy – Best overall material in terms of versatility, performance and cost. Available in many formulations. Excellent adhesion, environmental resistance and durability.
Polyurethane – Very tough protection with high impact and abrasion resistance. Typically opaque.
Silicone – Provides excellent high temperature stability up to 200°C. Maintains flexibility but can allow more moisture permeation.
Urethane – Similar protection as polyurethane but with higher clarity.
Parylene – Provides a very conformal pinhole-free thin coating applied via vapor deposition. Excels at moisture and chemical resistance.
RBCs (Rigid Black Compounds) – Opaque compounds containing epoxy and silicone providing very rigid, strong protection. High EMI/RFI shielding.
Specialized potting resins are also used for medical implantable devices, optical clarity, etc.
Potting Process Techniques
There are several process techniques used for applying potting compounds:
Dispensing – This manually applies potting material in a bead, fillet or blob using a pneumatic or electric fluid dispenser on the localized areas or zones requiring protection. Allows selective application.
Encapsulation – The entire PCB is placed in a mold cavity and liquid potting material is injected to surround the board completely.
Glob Top – A glob of potting material is dispensed over chip packages and wire bonds to protect IC components.
Underfill – Potting material is dispensed under large BGAs/components for mechanical strengthening before curing.
Brush Application – Using a brush to manually apply specialized potting compounds.
Casting – Pouring a two-part potting mixture around assembly similar to encapsulation.
Vapor Deposition – Parylene coatings are applied using a vaporization chamber to deposit polymer uniformly.
The process is dictated by the area needing protection, access needs, and production volume.
Design Factors for Potted PCBs
Potting a PCB assembly requires design adaptations:
- Avoid pockets of air being trapped during filling using vent holes
- Include generous fillets and radii in potting keepout zones
- Design potting dams and containment walls to limit fill area
- Ensure components are mechanically secured before potting
- Position tall components to avoid tilting or shifting when potted
- Specify toughened solder mask materials to resist cracking
- Mark heat sinks and connectors needing to remain exposed
- Define locations needing selective dispensing or glob top application
- Allow sufficient space for potting fill ports and air evacuation
- Specify thermal vias under hot components so heat can transfer through potting material
- Define appropriate keepout regions, openings and masking
Early planning for potting improves results.
Benefits of Using a Professional Potting Service
Leveraging an experienced potting service provider offers many benefits:
-Availability of specialized potting fixtures, molds and equipment
-Expertise in selecting compatible potting materials and processes
-Precision dispensing, injection molding or encapsulation process knowledge
-Familiarity with cleaning, curing and baking procedures
-Established quality inspection practices
-Capabilities for mass production or small batch prototyping
-Testing expertise to validate potted assemblies still function as intended
-Skills performing selective potting techniques like glob tops
-Assistance designing boards for optimal potting results
-Troubleshooting issues like voids, cracks, filler errors
By outsourcing to a professional potting service provider, manufacturers can leverage established know-how and specialized equipment to receive fully tested, potted boards ready for use.
Common Potting Issues
Some common issues encountered when potting PCBs:
Voids – Air trapped during filling or insufficient filling results in voids within the potting compound where circuits are unprotected.
Cracks – Cracking occurs during curing when using inflexible potting materials with fragile components or PCBs. Thermal expansion differences also cause cracks.
Shifted components – Gravity causes tall, heavy components to tilt or fall over during filling before the compound hardens.
Wicking – Potting material adheres up component bodies away from the PCB surface leaving areas unprotected.
Outgassing – Some potting compounds release gas bubbles during curing, creating pockets.
Contamination – Particles become embedded in the potting or components react adversely with fillers.
Overheating – Excess heat during curing can damage temperature sensitive components.
Experienced potting service providers take measures to avoid these common faults.
Testing Potted Assemblies
Validating potted PCBs is critical to ensure no functional or reliability degradation:
- Visual inspection checks for complete filling, air gaps, wicking issues and cosmetic defects.
- Dimensional inspection verifies height, length, width and component placement tolerances are maintained.
- Electrical testing like ICT fixtures or flying probes perform in-circuit testing to validate all functions operate. Burn-in may also be used.
- Mechanical testing like vibration, shock and torque determine if any cracks or delamination occur indicating a potting defect.
- Hermeticity testing per IPC standards confirms a complete moisture seal.
- Thermal cycling with electrical test during temperature ramps checks for intermittencies from material stresses.
- Measurement of dielectric strength, insulation resistance and capacitance verifies electrical integrity.
Thorough testing provides confidence the potting completely protects the assembly without degrading performance.
Summary
- PCB potting encapsulates assemblies in a protective compound by dispensing, injection molding or other application techniques.
- Potting improves environmental, contamination and impact resistance for challenging operating conditions.
- Various potting compounds like epoxies, polyurethane, silicone and others provide specific benefits.
- Potting services apply specialized expertise and equipment to encapsulate PCBs using proven techniques.
- Potted assemblies require rigorous inspection and testing to validate reliable performance.
By leveraging professional potting services, manufacturers can quickly implement robust potting solutions to transform boards into durable, sealed products.
Frequently Asked Questions
What potting materials provide the best moisture and water protection?
Epoxies offer very low moisture permeability and excellent adhesion to prevent water ingress. Flexible polyurethanes and rigid black potting compounds also provide strong water resistance.
How is outgassing prevented when potting PCBs?
Outgassing can be minimized by proper mixing, filling slowly, allowing adequate venting, and ramping the cure temperature to allow gases to escape gradually from the resin. Choosing potting compounds with low volatility also helps.
What inspection techniques detect voids in potted assemblies?
X-ray imaging is ideal for detecting air gaps and voids within encapsulated assemblies. Ultrasound non-destructive testing also helps identify sub-surface voids by sensing differences in acoustic impedance.
Does potting a PCB increase or decrease thermal resistance?
Potting generally improves heat transfer away from components by providing a continuous thermal conduction path when in contact with heat sinks and metal chassis. But pockets of air trapped during filling can increase thermal resistance.
How thick should potting over components and PCBs be?
As a rule of thumb, a minimum of 3 mm potting thickness is recommended over components to provide sufficient mechanical strength. For PCBs, 2 mm thickness is typical but can vary based on environmental needs.
