Resin Flow
Fabrication defects in multilayer boards involve the excessive flow of partially cured resin from the edges of the stack-up during lamination. Prepreg squeeze-out indicates that the pressure or temperature profile was not correctly optimized, leading to thin bond lines or contamination of internal features. This excess material can block holes or interfere with the plating of the board edges.
Bond Thickness
The amount of resin that leaves the stack-up directly affects the final thickness of the dielectric layer between the copper planes. Prepreg squeeze-out reduces the volume of resin available to fill the gaps between the copper traces which can lead to voids. If the resin flow is too high, the board may not meet the overall thickness requirements specified in the design.
Lamination Pressure
Controlling the rate of temperature rise and the timing of the pressure application is the main way to prevent this defect. A well-designed lamination cycle allows the resin to liquefy and flow just enough to bond the layers and fill all the spaces without causing prepreg squeeze-out. Fabricators measure the amount of resin loss during process qualification to set the limits for production.
Consistent monitoring of the resin flow ensures that every board in the batch has a uniform thickness and reliable internal connections.
