Temperature variations can significantly impact the operation, reliability, and quality of PCBA (Printed Circuit Board Assembly): The primary materials of the PCB have different coefficients of thermal expansion. As temperature rises, materials expand, which can induce mechanical stress, potentially causing microcracks that may not be detected during the electrical testing (open/short circuit tests) conducted at the end of production. In the best-case scenario, these microcracks are detected after soldering; in the worst-case scenario, they could lead to random failures in the finished product.

In 2002, the RoHS directive was introduced, requiring the use of lead-free alloys for soldering. Removing lead leads to an increase in the melting point temperature, requiring PCBs to withstand higher temperatures during soldering (reflow and wave soldering) processes. Depending on the chosen reflow soldering process (single-sided, double-sided, etc.), PCBs with appropriate mechanical characteristics, especially those with suitable glass transition temperatures (Tg), must be used.

What is Tg?

Tg is a mechanical property that represents the glass transition temperature, indicating the temperature at which the substrate (polymer or glass) transitions from a glassy state, solid state, or rigid state to a rubbery state. When the temperature exceeds Tg, the material does not melt but undergoes a structural change, becoming rubber-like.

Accurately measuring this temperature is difficult because many factors need to be considered, including the molecular structure of the material. Therefore, different materials have different Tg. However, in some cases, even materials with different technical characteristics may have the same Tg (for example, materials A and C in the diagram). As the material is heated, its viscosity increases, making it more prone to cracking or fracturing upon cooling.

High Tg materials have the following characteristics:

• High temperature resistance,

• Long-term resistance to delamination (material aging for safety considerations),

• Low thermal expansion.

PCB Tg

For a PCB, Tg corresponds to the temperature at which the fiberglass in the laminate becomes amorphous under high temperatures and the pressure of multilayer materials. This is not the maximum operating temperature of the PCB, but rather the temperature that the PCBA can withstand for a short period without degrading.

FR4 is the most commonly used material in PCB production, it requires the use of PREPREG layers, which only become rigid after lamination. To achieve the desired hardness of PREPREG, heating must be done without exceeding the Tg of FR4 to maintain the stability of the PCB. The Tg of standard FR4 is between 130-140°C, medium Tg is 150°C, and high Tg is greater than 170°C. Under high temperatures, high Tg FR4 offers better mechanical and chemical heat resistance and moisture resistance compared to standard FR4.

If the operating temperature of a PCB exceeds its Tg for an extended period, the PCB will transition from a glassy state to a rubbery state, affecting its performance.

Here are common types of PCBs with different Tg values as summarized by PCBWay:

1. FR-4: FR-4 is a common PCB material with a Tg value ranging from about 130°C to 180°C. It is suitable for most general electronic applications and operating temperature requirements.

2. High Tg PCB: High Tg PCBs use substrate materials with a Tg value typically exceeding 180°C. These PCBs are suitable for applications that need to operate at high temperatures for extended periods or withstand high power. They offer better thermal stability and mechanical strength.

3. Low Tg PCB: Low Tg PCBs use substrate materials with a Tg value typically below 100°C. These PCBs are suitable for low-temperature applications or scenarios with lower thermal stability requirements.

4. Mid Tg PCB: Mid Tg PCBs fall between high Tg and low Tg types, with Tg values generally between 150°C and 180°C. They provide good performance and reliability within a certain temperature range.

For some special requirements such as high-temperature environments or high-power applications, it's necessary to choose PCB materials with higher Tg values. For instance, high Tg PCBs utilize special materials with Tg values exceeding 180°C to ensure the PCB maintains good mechanical strength and electrical performance under high-temperature conditions. Selecting the appropriate Tg value is crucial for the design and reliability of the PCB and should be determined based on the actual application requirements.

Tg is one of the key characteristics to consider when specifying a PCB. It's essential to determine the PCB's exposure temperature in the early stages of design to select suitable materials, especially for PCBs exposed to high operating temperatures.

The default material of PCBWay has a TG of 150-160℃, which is higher that most companies.

In addition, to further improve product quality and meet the market's growing demands, despite a 10%-30% increase in production costs, PCBWay has decided to upgrade the 4-layer board and above from the previously used Kingboard material to Shengyi material. Moreover, this upgrade will be provided to customers free of charge. Learn More>>