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Analysis of the impact of pressing materials, structures and graphics on plate deformation
2017-11-20 16:24
The PCB board is formed by pressing the core board, the prepreg and the outer copper foil, wherein the core board and the copper foil are deformed by heat during pressing, and the deformation depends on the coefficient of thermal expansion (CTE) of the two materials;
The coefficient of thermal expansion (CTE) of the copper foil is about 17X10-6;
The Z-direction CTE of the common FR-4 substrate at the Tg point is (50~70)X10-6;
TG point above (250~350)X10-6,X CTE due to the existence of glass cloth, generally similar to copper foil.
Note on TG points:
When the temperature of the high Tg printed board rises to a certain area, the substrate will change from "glass state" to "rubber state", and 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 substrate maintains rigidity. That is to say, the ordinary PCB substrate material at high temperature, not only produce softening, deformation, melting and other phenomena, but also in the mechanical and electrical characteristics of the sharp decline.
Generally, the Tg of the plate is above 130 degrees, the high Tg is generally greater than 170 degrees, and the medium Tg is about greater than 150 degrees.
Usually Tg> 170 ℃ PCB printed board, called high Tg printed board.
The Tg of the substrate is increased, and the characteristics of the printed board such as heat resistance, moisture resistance, chemical resistance, and stability resistance are improved. The higher the TG value, the better the temperature resistance of the plate, especially in the lead-free process, the high Tg application is more.
High Tg means 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 multilayering requires higher heat resistance of PCB substrate materials as an important guarantee. With the emergence and development of high-density mounting technology represented by SMT and CMT, PCB is increasingly inseparable from the support of high heat resistance of the substrate in terms of small aperture, fine wiring and thinning.
Therefore, the difference between general FR-4 and high Tg FR-4 is that in the hot state, especially after moisture absorption, the mechanical strength, dimensional stability, adhesion, water absorption, thermal decomposition, thermal expansion and other conditions of the material are different. High Tg products are obviously better than ordinary PCB substrate materials.
The expansion of the core plate with the inner layer pattern is different due to the difference between the pattern distribution and the thickness or material characteristics of the core plate. When the pattern distribution is different from the thickness or material characteristics of the core plate, when the pattern distribution is more uniform, the material type is consistent, and no deformation occurs. When the laminated structure of the PCB board is asymmetrical or the pattern distribution is uneven, the CTE of different core boards will be greatly different, thereby causing deformation during the pressing process. The deformation mechanism can be explained by the following principle.
Assuming that there are two kinds of core plates with large difference in CTE are pressed together by prepreg, wherein the CTE of core plate A is 1.5x 10-5/℃, and the length of core plates is 1000mm. In the pressing process as a bonding sheet prepreg, the two core sheets are bonded together through three stages of softening, flowing and filling the pattern and curing.
In general, the material starts to flow from 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 heating, and their deformation can be obtained through their respective CTE and temperature change values.
Simulated compression conditions, temperature from 30 ℃ to 180 ℃,
At this time, the deformation of the two core plates is respectively
△LA=(180℃~30℃)x1.5x10-5m/℃X1000mm=2.25mm
△LB=(180℃~30℃)X2.5X10-5M/℃X1000mm=3.75mm
At this time, because the semi-curing is still in the free state, the two core plates, one long and one short, do not interfere with each other, and have not yet been deformed.
When pressing, it will be kept at high temperature for a period of time until the semi-curing is completely cured. At this time, the resin becomes a cured state and cannot flow freely. The two core plates are combined together. When the temperature drops, if there is no interlayer resin binding, the core plate will return to the initial length without deformation. However, in fact, the two core plates have been bonded by cured resin at high temperature and cannot shrink at will during the cooling process. Among them, the core plate should shrink by 3.75mm. In fact, when the shrinkage is greater than 2.25mm, it will be hindered by the core plate. In order to achieve the force balance between the two core plates, the B- core board cannot shrink to 3.75mm, while the-core board shrinks more than 2.25mm, thus bending the whole board to the B- core board.
According to the above analysis, it can be seen that the laminated structure and material type of the PCB board are evenly distributed, which directly affects the CTE difference between different core boards and copper foil. The expansion and contraction difference in the pressing process will be retained through the fixing process of the semi-cured sheet and finally form the deformation of the PCB board.
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