Potting, by definition, entails the complete encapsulation of electronic parts or components within a durable shell intended for long-term use. This process utilizes molds for encapsulation: components are placed in the mold, filled, and then released to form a protective shell.
The primary purpose of potting is to enhance the durability and reliability of electronic assemblies, particularly in Surface Mount Technology (SMT). Potting systems offer numerous advantages, including waterproofing, dustproofing, resilience to harsh environmental conditions, protection against conductive pollutants and chemical corrosion, as well as resistance to vibration and shock. Furthermore, potting reinforces the structural integrity of components and improves their electrical insulation and heat dissipation capabilities, allowing for closer integration of high-voltage elements.
Unfortunately, potting and encapsulation are often seen as final stages in the production process, unrelated to general electrical design and involving chemical engineering. However, taking a holistic approach from the outset and collaborating closely with potting adhesive suppliers can significantly save time and costs.
The most common PCB potting compounds are based on two-component systems. These involve mixing two liquids within a specified timeframe to form a solid compound. It’s a common misconception that the ratio between these liquids is fixed; in reality, it can be adjusted to control the curing time and achieve optimal material performance. Maintaining precise ratio control is crucial to ensure proper curing. Additionally, thorough homogenous mixing of the two liquids is essential; any unmixed portions may remain liquid, leading to potential issues during the component’s lifecycle.
**Choice of Potting Material**
Firstly, consider the storage, operation, and peak temperatures of the components to determine suitable materials. Thermal performance dictates material selection based on long-term exposure:
– Polyurethane: -50°C to 140°C
– Epoxy resin: -40°C to 150°C (can reach up to 200°C with specific formulations)
– Silicone: -60°C to 250°C
These temperature ranges vary across different products and exposure durations. For instance, certain polyurethanes withstand short-term peak temperatures during reflow soldering, serving as a useful starting point for selection.
**Thermal Cycling Considerations**
During thermal cycling, factors such as hardening, expansion, and contraction are critical for electronic potting compounds:
– Polyurethanes offer customization from 3000 HS (soft gel) to 90 D (rigid glass), providing high elongation and tear resistance. Expansion and contraction characteristics vary with hardness and temperature range.
– Epoxy resins are inherently rigid (up to 90 D), offering low expansion and contraction characteristics. However, their rigidity may not match PCB solder joints, potentially leading to stress and cracking during thermal cycling.
– Silicones are softer (2000 to 90 A) with higher expansion and contraction characteristics. Their softness minimizes interaction and stress on components and solder joints. Despite this, silicone’s poor adhesion may necessitate substrate rework during repair, conflicting with the principle of effective potting.
In electronic designs prone to failure, silicone resins are favored. However, their lower adhesion may result in water ingress, compromising reliability. Many LED driver manufacturers opt for silicone to facilitate repairs, despite the risk of adhesion issues.
