The PCB board is formed by pressing the core board and the prepreg and the outer copper foil. The core board and the copper foil are deformed by heat when they are pressed. The amount of deformation depends on the coefficient of thermal expansion (CTE) of the two materials;

 

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

 

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

 

Above TG point is (250 ~ 350) X10-6, X-directional CTE is generally similar to copper foil due to glass cloth.

 

 

Notes on TG points:

 

 

When the temperature of a high Tg printed board rises to a certain area, the substrate will change from "glass state" to "rubber state". The temperature at this time is called the glass transition temperature (Tg) of the board. That is, Tg is the maximum temperature (° C) at which the base material remains rigid. In other words, ordinary PCB substrate materials not only produce softening, deformation, melting and other phenomena at high temperature, but also show a sharp decline in mechanical and electrical characteristics.

 

 

The general Tg sheet is above 130 degrees, the high Tg is generally greater than 170 degrees, and the medium Tg is approximately greater than 150 degrees.

 

 

Usually PCB printed board with Tg≥170 ℃ is called high Tg printed board.

 

 

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

 

 

High Tg refers to high heat resistance. With the rapid development of the electronics industry, especially the electronic products represented by computers, the development of high functionality and high multilayer requires the higher heat resistance of PCB substrate materials as an important guarantee. The emergence and development of high-density mounting technologies represented by SMT and CMT have made PCBs more and more inseparable from the support of high heat resistance of substrates in terms of small apertures, fine wiring, and thinness.

 

 

Therefore, the difference between the general FR-4 and the high Tg FR-4: the mechanical strength, dimensional stability, adhesion, water absorption, thermal decomposition of the material in the hot state, especially under the heat after moisture absorption , Thermal expansion, and other conditions are different, high Tg products are obviously better than ordinary PCB substrate materials.

 

 

Among them, the expansion of the core board with inner graphics is different due to the difference in the graphic distribution and the thickness or material characteristics of the core board. When the graphic distribution is different from the thickness or material characteristics of the core board, when the graphic distribution is more uniform, the material type is consistent Will deform. When the laminated structure of the PCB board is asymmetric or the pattern distribution is uneven, it will cause a large difference in CTE of different core boards, thereby causing deformation during the lamination process. The deformation mechanism can be explained by the following principles.

 

 

Suppose that there are two kinds of core plates with a large difference in CTE which are pressed together through the prepreg, in which the ATE CTE is 1.5x10-5 / ℃, and the core plate length is 1000mm. In the compressing process, the prepreg used as the adhesive sheet, then the two core boards are bonded together through the three stages of softening, flowing and filling graphics, and curing.

 

 

Under normal circumstances, the material starts to flow from around 90 ° C, and cross-linking and curing begins when it reaches the TG point. Before curing, the prepreg is in a free state. At this time, the core board and copper foil are in a free expansion state after being heated, and the amount of deformation can be Obtained through the respective CTE and temperature changes.

 

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

 

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

 

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

 

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

 

At this time, because the semi-cured is still in a free state, the two core plates have one length and one short, do not interfere with each other, and have not yet deformed.

 

 

 

When pressed, it will be kept at a high temperature for a period of time until the semi-cured is completely cured.At this time, the resin becomes a cured state and cannot flow freely. The two core plates are combined. When the temperature drops, such as without interlayer resin binding, the core The board will return to its original length, and there will be no deformation, but in fact the two core boards have been bonded by the cured resin at high temperature, and they cannot shrink at will during the cooling process.The A core board should shrink by 3.75mm. When the shrinkage is greater than 2.25mm, it will be hindered by the A core board.In order to achieve the balance of the force between the two core boards, the B core board cannot shrink to 3.75mm, and the A core board shrinkage will be greater than 2.25mm, so that the whole board toward the B core The direction of the board changes.

 

 

 

 

 

According to the above analysis, it can be seen that the laminated structure and material type of the PCB board have a uniform graphic distribution, which directly affects the CTE difference between different core boards and copper foils. The difference in expansion and contraction during the pressing process will pass through the solid piece of the prepreg The process is retained and eventually deforms the PCB.