PCB board is formed by pressing core board, semi solidified sheet and outer copper foil. The core board and copper foil are subject to thermal deformation when pressing, and the deformation amount depends on the thermal expansion coefficient (CTE) of the two materials;

The coefficient of thermal expansion (CTE) of copper foil is about 17X10-6;

While the CTE of common FR-4 substrate in Z-direction at Tg point is (50~70) X10-6;

Above TG point is (250~350) X10-6. Due to the existence of glass cloth, CTE in X direction is generally similar to copper foil.

Notes on TG point:

When the temperature of a high Tg printed circuit board rises to a certain area, the substrate will change from a "glass state" to a "rubber state". The temperature at this time is called the glass transition temperature (Tg) of the board. That is, Tg is the highest temperature (℃) at which the substrate can maintain its rigidity. That is to say, under high temperatures, ordinary PCB based materials not only produce softening, deformation, melting and other phenomena, but also show a sharp decline in mechanical and electrical properties.

Generally, the Tg of the plate is more than 130 degrees, the high Tg is more than 170 degrees, and the medium Tg is more than 150 degrees.

Generally, PCB printed boards with Tg ≥ 170 ℃ are called high Tg printed boards.

The Tg of the substrate is increased, and the heat resistance, moisture resistance, chemical resistance, stability resistance and other characteristics of the printed board will be improved. The higher the TG value is, the better the temperature resistance of the plate is. Especially in the lead-free process, high Tg is used more frequently.

High Tg refers to high heat resistance. With the rapid development of electronic industry, especially the electronic products represented by computers, which are developing towards high functionality and high multilayer, higher heat resistance of PCB substrate materials is required as an important guarantee. With the emergence and development of high-density installation technology represented by SMT and CMT, PCB is increasingly dependent on the support of high heat resistance of the substrate in terms of small aperture, fine wiring and thinness.

Therefore, the difference between the general FR-4 and the high Tg FR-4 is that the mechanical strength, dimensional stability, adhesiveness, water absorption, thermal decomposition, thermal expansion and other conditions of the materials are different under the hot state, especially under the heat after moisture absorption. The high Tg products are obviously better than the common PCB substrate materials.

Among them, the expansion of the core plate with good inner layer pattern is different because the pattern distribution is different from the core plate thickness or material characteristics. When the pattern distribution is different from the core plate thickness or material characteristics, when the pattern distribution is uniform and the material type is consistent, no deformation will occur. When the laminated structure of PCB board is asymmetric or the pattern distribution is uneven, the CTE of different core boards will vary greatly, which will cause deformation in the pressing process. The deformation mechanism can be explained by the following principles.

Suppose that two kinds of core plates with large difference in CTE are pressed together through the semi curing sheet, wherein the A core plate CTE is 1.5x10-5/℃, and the core plate length is 1000mm. In the pressing process, the two core plates are bonded together through three stages: softening, flowing, filling and curing.

Generally, the material starts to flow at about 90 ℃, and starts to cross link and cure above the TG point. Before curing, the semi cured sheet is in a free state. At this time, the core plate and copper foil are in a free expansion state after being heated, and their deformation can be obtained through their CTE and temperature change values.

Simulate the pressing conditions, and the temperature rises from 30 ℃ to 180 ℃,

At this time, the deformation of the two core plates are

△LA=(180℃~30℃)x1.5x10-5m/℃X1000mm=2.25mm

△LB=(180℃~30℃)X2.5X10-5M/℃X1000mm=3.75mm

At this time, as the semi curing is still in a free state, the two core plates are long and short, do not interfere with each other, and have not been deformed.

During pressing, it will be kept under high temperature for a period of time until the semi curing is completely cured. At this time, the resin becomes cured and cannot flow freely. The two core plates are combined When the temperature drops, if there is no inter layer resin binding, the core board will return to its original length without deformation. However, in fact, the two core boards have been bonded by the cured resin at high temperature, and cannot shrink at will during the cooling process. The core board A should shrink by 3.75 mm. In fact, when the shrinkage is greater than 2.25 mm, it will be blocked by the core board A. In order to achieve the stress balance between the two core boards, the core board B cannot shrink to 3.75 mm, while the core board A will shrink by more than 2.25 mm, Thus, the whole board will be bent to the direction of core board B.

It can be seen from the above analysis that whether the laminated structure and material type of PCB board are uniformly distributed has a direct impact on the CTE difference between different core boards and copper foils. The difference in inflation and shrinkage during the pressing process will be retained through the fixing process of the semi cured sheet and finally form the deformation of PCB board.