Reason:

(1) The difference in the direction of warp and weft causes the size of the substrate to change; due to the lack of attention to the fiber direction during shearing, the shear stress remains in the substrate. Once it is released, it will directly affect the shrinkage of the substrate size.

(2) The copper foil on the surface of the substrate is etched away, which limits the change of the substrate, and the size changes when the stress is relieved.

(3) Excessive pressure is used when brushing the PCB board, resulting in compressive and tensile stress and deformation of the substrate.

(4) The resin in the substrate is not fully cured, resulting in dimensional changes.

(5) Especially the multi-layer board before lamination, the storage condition is poor, so that the thin substrate or prepreg will absorb moisture, resulting in poor dimensional stability.

(6) When the multilayer board is pressed, excessive flow of glue causes the deformation of the glass cloth.

Solution:

(1) Determine the law of change in the latitude and longitude direction to compensate on the negative according to the shrinkage rate (this work is carried out before light painting). At the same time, the cutting is processed according to the fiber direction, or processed according to the character mark provided by the manufacturer on the substrate (usually the vertical direction of the character is the vertical direction of the substrate).

(2) When designing the circuit, try to make the entire board surface evenly distributed. If it is not possible, a transition section must be left in the space (mainly without affecting the circuit position). This is due to the difference in the warp and weft yarn density in the glass cloth structure of the board, which results in the difference in the strength of the board in the warp and weft directions.

(3) Trial brushing should be used to make the process parameters in the best state, and then rigid board. For thin substrates, chemical cleaning processes or electrolytic processes should be used for cleaning.

(4) Take baking method to solve. In particular, bake before drilling at a temperature of 120°C for 4 hours to ensure that the resin is cured and reduce the size of the substrate due to the influence of heat and cold.

(5) The inner layer of the oxidation-treated substrate must be baked to remove moisture. And store the processed substrate in a vacuum drying box to avoid moisture absorption again.

(6) Process pressure test is required, process parameters are adjusted and then pressed. At the same time, according to the characteristics of the prepreg, the appropriate amount of glue flow can be selected.