Change of substrate size in PCB fabrication

14 January 2020
New

The reason:

(1) The difference in the warp and weft direction causes the substrate size to change; in the PCB fabrication, due to the shearing, the fiber direction is not noticed, and the shear stress remains in the substrate. Once released, the shrinkage of the substrate size is directly affected.
(2) The copper foil portion of the substrate surface is etched away from the variation of the substrate, and a dimensional change occurs when the stress is removed.
(3) When the plate is brushed, the pressure is too large, causing the tensile stress to cause the deformation of the substrate.
(4) The resin in the substrate is not completely cured, resulting in dimensional change.
(5) In the manufacture of PCBs, especially before the lamination of the multilayer board, the storage conditions are poor, so that the thin substrate or the prepreg is hygroscopic, 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 variation law of the warp and weft direction to compensate for the film according to the shrinkage rate (this work is performed before the light painting). In PCB fabrication, it is processed in the fiber direction at the same time of shearing, or processed according to the character mark provided by the manufacturer on the substrate (generally, the vertical direction of the character is the longitudinal direction of the substrate).
(2) The entire board surface should be distributed as evenly as possible during PCB fabrication. If it is not possible, you must also leave a transition in the space (does not affect the circuit position). This is due to the difference in the warp and weft strength of the sheet due to the difference in the warp and weft yarn density in the glass cloth structure.
(3) The test brush should be used to make the process parameters in the best condition, and then the rigid plate is carried out. For thin substrates, chemical cleaning or electrolytic processes should be used for cleaning.
(4) In PCB manufacturing, the baking method is adopted. In particular, baking is performed before drilling, and the temperature is 120 ° C for 4 hours to ensure the curing of the resin and to reduce the deformation of the substrate due to the influence of heat and cold.
(5) The inner layer of the oxidized substrate must be baked to remove moisture. The treated substrate is stored in a vacuum drying oven to prevent moisture absorption.
(6) In the PCB fabrication, a process pressure test is required, and the process parameters are adjusted and then pressed. At the same time, according to the characteristics of the prepreg, the appropriate amount of glue can be selected.