Reinforcement Ratio
Substrate composition ratios in PCB laminates compare the volume of glass fiber reinforcement to the total volume of the cured material. The volume glass fraction is a primary determinant of the coefficient of thermal expansion and the overall stiffness of the board. It is expressed as a percentage and is controlled by the style of the glass weave and the amount of resin applied during the prepreg manufacture.
Substrate Stiffness
A higher amount of glass makes the board more rigid and resistant to bending during the assembly process. The volume glass fraction also impacts the dielectric constant, as the glass fibers have different electrical properties than the surrounding resin matrix. Adjusting this ratio allows laminate manufacturers to create materials that are optimized for either mechanical strength or high-speed electrical performance.
Thermal Expansion
Controlling the expansion of the board in the X and Y directions is necessary to prevent stress on the solder joints of large components. Because glass expands much less than resin, a higher volume glass fraction leads to a lower coefficient of thermal expansion for the board. This property is required for boards that will be used in environments with large temperature swings or for those carrying fine-pitch ball grid arrays.
Engineers select the specific laminate grade based on the glass to resin balance needed to meet the reliability goals of the product.
