What is a Glass Transition Temperature And How Does It Affect the Thermal Resilience of PCBs?

Glass transition temperature (Tg) is the temperature at which the molecular bonds in amorphous, crystalline, or semi-crystalline polymer chains lose some of their rigidity and become more mobile. As a result, they gradually transition from a hard, glassy state to a softer, more liquid state. This chemical phenomenon is incredibly important for various industries, including PCB manufacturing.

Modern electronics and printed circuit boards are sensitive to temperature. Hence, fluctuations can impact their functionality and reliability. As temperatures in PCBs rise, the copper, resin, and glass expand and create mechanical stress, leading to micro-cracks. In most cases, these cracks aren't detectable. As a result, the end devices or equipment become more susceptible to poor performance or failure, reducing their life.

Glass transition temp plays a significant role in preventing temperature-induced stresses. Manufacturers proactively select the right Tg PCBs to optimize mechanical stability within various operating temperature ranges.

Therefore, in this post, we'll explain in detail how Tg affects the thermal resilience of printed circuit boards.

What is Glass Transition Temperature (Tg)?

The glass transition temperature can be clearly defined as the point below which polymers physically transform from hard, brittle structures to a viscous or rubbery state. As the temperature rises, the polymer chains start moving and become more flexible and malleable, making the material suitable for structural modification through stress application or shear flow.

Tg is among the key underlying factors that directly impact the design and performance of materials. It affects polymers' mechanical and thermal characteristics. For instance, materials below Tg are generally hard and brittle. In contrast, those above Tg can be deformed into different shapes. Moreover, the Tg value determines the temperatures over which a specific polymer remains stable without physical alterations. Many industries use these values to improve product shelf life and performance proactively.

Other changes glass transition temps can identify (in the context of PCBs) include:

• Volume changes (spatial efficiency)
• Melting temperature and thermal conductivity
• Dielectric constant and loss tangent
• Solder mask printability and adhesion
• Frequency performance (signal transmission)
• Lead-free soldering compatibility, etc.

How Tg Affects PCBs

Although printed circuit boards aren't made entirely of polymers, their core is fabricated using FR-4 composite material from woven fiberglass and epoxy resin. Glass fiber has a crystalline structure. In contrast, epoxy resin has a three-dimensional amorphous structure. These materials combine to form a PCB's substrate, silkscreen, and solder mask layers. They're wrapped in thin copper sheets to enable electrical conductivity and facilitate connections through drill hits and vias. 

Types of Tg PCBs Used in Modern Electronics

Below are the three most common types of TG printed circuit boards used in electronic devices today:

1. Standard Tg Printed Circuit Boards

As the term suggests, Tg PCBs are the most common and cost-effective printed circuit boards used in electronic devices. Their glass temperature transition range is between 130-140°C, making them suitable for applications that require moderate thermal stability. Common applications include:

• Smartphones, laptops, and other mobile devices
• Televisions, washing machines, and other home appliances
• General electronic items like LED lighting boards and power supplies

2. Mid-Tg Printed Circuit Boards

Mid-Tg PCBs are more reliable than standard Tg PCBs in terms of thermal performance and reliability. Their glass transition temperature typically ranges from 130-150°C. Therefore, they're commonly used in equipment, systems, or devices requiring higher heat resistance but without significantly reducing manufacturing costs. Common applications include:

• Automobile Engine Control Units (ECUs)
• Patient monitoring systems
• Advanced Driver Assistance Systems (ADAS)
• Programmable Logic Controllers (PLCs), etc.

3. High-Tg Printed Circuit Boards

High-Tg PCBs are the go-to choice for industries that require advanced machinery and equipment, including manufacturing, military, and avionics. Their glass transition temperature ranges from 150°C to 170°C, making them excellent for high-temperature applications like lead-free soldering. Moreover, electronics powered by these boards are more likely to perform optimally under increased thermal stress. Common applications include:

• Networking hardware and fiber optic communication systems
• Satellite components
• Radar systems
• Unmanned Aerial Vehicle (UAV) parts

4. Very High-Tg Printed Circuit Boards

Finally, very high-Tg PCBs are the best printed circuit boards for thermal resistance and stability under mechanical stress. These PCBs can withstand temperatures exceeding 170°C and are typically used by the automotive and aerospace industries to produce highly reliable equipment and machinery. Common examples of its use cases include:

• Spacecraft electronics
• Satellite payloads
• Oil and gas monitoring systems
• Extreme temperature monitoring sensors
• Engine powertrain control units, etc.

Choosing the Right Glass Temperature (Tg) for A Printed Circuit Board - Factors to Consider

Below are the key factors manufacturers consider when choosing the right Tg PCBs for their products or processes:

1. Application Thermal Stability Requirements

The first thing manufacturers consider is the temperature range the PCB will be exposed to during operation or manufacturing. With this, they can proactively determine the thermal stability and reliability of the system.

2. Soldering Process

The choice of PCB in terms of glass temperature transition can be determined by the soldering process used to make the board - hot bar, infrared, reflow, wave, laser, ultrasonic, plasma, etc. However, manufacturers opting for lead-free options choose higher Tg PCB variants, especially when constructing multiple circuit layers.

3. Cost

Cost is one of the most important factors manufacturers consider when choosing PCBs based on Tg. For low thermal stability applications like consumer electronics, most opt for standard Tg to reduce production costs. Some even charge a premium price for flagship products using moderate or high-Tg PCBs. However, industries with applications that demand high-temperature resistance must use higher Tg variants.

4. Environmental Conditions

Another crucial factor determining the choice of PCBs used in certain applications is environmental conditions like temperature, humidity, atmospheric pressure, etc. Generally, manufacturers use standard or moderate Tg PCBs in cool and dry environments, especially for less temperature-sensitive applications.

Conclusion

Glass transition temperature (Tg) is one of the most important properties industries use to determine and distinguish the thermal resilience and reliability of different printed circuit boards in electronic devices, equipment, or machinery.

Understanding the impact of Tg values on the physical and chemical properties of PCB materials can provide valuable insights into their potential applications. In summary, the glass temperature transition influences the choice of PCBs in various industries, from automotive to healthcare, avionics, electronics, and military.