Causes and Improvement of PCB Deformation 3

- May 22, 2017-

ring press material, structure, graphics on the plate deformation analysis

    The PCB board is made up of the core plate and the prepreg and the outer copper foil. The core plate and the copper foil are heated and deformed when they are pressed. The deformation depends on the thermal expansion coefficient (CTE) of the two materials,

    Copper foil thermal expansion coefficient (CTE) is about 17X10-6;

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

    TG point above (250 ~ 350) X10-6, X to CTE due to the existence of glass cloth, and copper foil is generally similar.

    Notes on the TG point:

    High-Tg Printed Board When the temperature rises to a certain area, the substrate will be changed from "glassy" to "rubber", where the temperature is called the glass transition temperature (Tg) of the plate. That is, Tg is the highest temperature (° C) at which the substrate remains rigid. In other words, ordinary PCB substrate material at high temperatures, not only produce softening, deformation, melting and other phenomena, but also in the mechanical, electrical characteristics of the sharp decline.

    Generally, the Tg sheet is 130 degrees or more, the high Tg is generally greater than 170 degrees, and the medium Tg is greater than about 150 degrees.

    A PCB printed circuit board, typically Tg ≥ 170 ° C, is called a high-Tg printed circuit board.

    The Tg of the substrate is improved, and the characteristics such as heat resistance, moisture resistance, chemical resistance, and stability of the printed board are 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 in the computer as the representative of the electronic products, toward the high functionality, high multi-layer development, PCB substrate material needs higher heat resistance as an important guarantee. To SMT, CMT as the representative of the emergence and development of high-density installation technology, the PCB in the small aperture, fine line, thin, more and more inseparable from the substrate high heat resistance support.

  Therefore, the difference between FR-4 and FR-4 in high Tg is the mechanical strength, dimensional stability, adhesion, water absorption, thermal decomposition of the material under hot conditions, especially after heat absorption , Thermal expansion and other circumstances there are differences, high Tg products significantly better than ordinary PCB substrate materials.

    The expansion of the core plate is better than the thickness of the core or the material properties. When the pattern distribution is different from the thickness of the core plate or the material properties, when the pattern distribution is uniform and the material type is consistent, Will produce deformation. When the PCB board laminated structure is asymmetric or uneven distribution of graphics will lead to different core CTE difference between the larger, resulting in deformation during the compression process. The deformation mechanism can be explained by the following principles.

Assuming that there are two kinds of CTE large difference between the core board through the semi-cured sheet together, in which A core CTE is 1.5x10-5 / ℃, core length are 1000mm. The prepreg is used as a prepreg for the adhesive sheet, and the two sheets are bonded together by softening, flowing and filling the pattern and curing the three stages.

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