As a novice in PCB industry, many people will be confused whether fr4 Tg 130 is better or fr4 Tg 180 is better. Maybe you want a PCB that tends to fr4 Tg 130, because the price of Tg180 is higher than that of fr4 Tg 130. But how to choose, from the perspective of professional suppliers, which is better? You need to know the following about Tg.
Generally speaking, the higher the Tg, the more stable the material is in PCB and PCBA / SMT manufacturing process, because the board is not greatly affected by high temperature. The temperature of PCB hot lamination is about 180-200c, and in the reflow furnace of PCBA process, the temperature is 200-240c when the temperature is 10-30s. If you want to make multiple layers (more than 10 layers) with blind and buried holes, which means that you need multiple hot laminations, it is recommended to use high Tg materials.
More importantly, if the working temperature of the circuit board after assembly is about 130 ℃ or higher, 180 Tg high material must be used to prevent the circuit board from being replaced and ensure the reliability of the whole product.
Similarly, the higher the Tg, the more expensive the material.
Which one is better? The main factors to be considered here are the application and complexity of PCB. Tg 130 can be used for PCB in normal use at room temperature below 100C, and high Tg material is required for high temperature application products or multilayer PCB.
Temperature changes may have a significant impact on PCB operation, reliability and quality. The temperature rise causes the material to expand, but the main material used to make PCB has different coefficient of thermal expansion. This results in mechanical stress, which produces microcracks that may not be detected by electrical tests (open / short circuit tests) at the end of production. In the best case, these microcracks will lead to the detection of faults after brazing. In the worst case, it may cause random damage to the finished product!
In 2002, ROHS directive was introduced to require the use of lead-free alloys in welding. The removal of lead leads to an increase in the melting temperature, so the PCB is exposed to higher temperatures during soldering (reflow and wave soldering). Depending on the reflow process selected (single, double…), it is necessary to use PCB with appropriate mechanical properties, especially with proper glass transition temperature (Tg).
What is Tg?
Tg is a mechanical property that specifies the glass transition temperature, that is, the temperature at which the base material (polymer or glass) changes from glassy, solid, rigid to rubberized. When more than Tg, the material will not melt, but the structure will change and become rubber like.
Due to the consideration of many factors, including the molecular structure of the material, it is difficult to accurately measure the temperature. Therefore, different materials have different glass transition temperature. Both materials may have the same Tg even though they have different technical properties (for example, materials a and C in the figure below). When heated, the viscosity of the material increases. After cooling, they are more likely to rupture or rupture.
High Tg materials have the following characteristics:
- High temperature resistance
- Long term delamination durability (aging of materials for safety reasons),
- Low thermal expansion.
Tg of PCB
For PCB, Tg corresponds to the temperature at which glass becomes amorphous at high temperature and under pressure of different material layers. It is not the maximum working temperature of PCB, but the short-term temperature that PCBA can withstand before deterioration.
Copper clad laminate (FR4) processed by inner layer imaging is the most commonly used material in PCB production, but it needs the application of prepreg layer, and it is rigid after lamination. The heat required for prepreg stiffness must be applied without exceeding FR4 Tg to maintain PCB stability. The standard FR4 Tg is 130-140 ℃, the median Tg is 150 ℃, and the high Tg is more than 170 ℃. In hot state, high Tg FR4 will have better mechanical and chemical heat and moisture resistance than standard FR4.
If the working temperature of PCB exceeds Tg for a long time, PCB will change from glass state to rubber state, and its performance will be affected. Tg ensures the mechanical stability and normal operation of PCB during its service life.
Tg is one of the key functions to be considered when specifying PCB. In the very early stage of design, it is very important to determine the PCB exposure temperature to select the appropriate material, especially for PCB exposed to high operating temperature.