PCB substrate
PCB substrate for many species, divided into two categories: organic and inorganic substrate materials, substrate materials. Organic substrates, also known as CCL (Copper Clad Laminates, CCL) is the main PCB manufacturing materials. The inorganic ceramic substrate mainly enamel coated steel plate and the substrate. For example, we use a large number of FR-4 is a double-pane glass fabric CCL.
Select the substrate should consider the following factors: the choice should be based on PCB substrate conditions and the use of mechanical, electrical performance requirements; structure determination based on PCB copper clad substrate layers (single, double or multilayer); under the PCB components determine the size and weight of the substrate thickness; substrate parameters Tg, CTE and the smoothness and other compliance requirements; price factors. PCB substrate for the SMT process has the following requirements:
(1) glass transition temperature (Tg) higher. Glass transition temperature (glass transition temperature, Tg) is a unique performance of polymer is to determine the material properties of the critical temperature, is to choose one of the key parameters of the substrate. Tg epoxy resin around the 125 ~ 140 ℃, reflow temperature 245 ℃, far higher than the PCB substrate Tg, high-temperature thermal deformation of PCB board likely to cause, serious damage to components. Therefore, the choice of base material chosen higher Tg substrate, it is recommended above Tg at 140 ℃.
(2) coefficient of thermal expansion (CTE) is low. For the multilayer structure of the PCB, because the X, Y direction (ie length, width direction) and Z direction (ie the thickness direction) of the thermal expansion coefficient is inconsistent, making it easy PCB deformation will cause severe breakage and damage to the holes components. Shown in Figure 3.2.
(3) high heat resistance. PCB is usually two to go through the return of the second patch in order to ensure the reliability of a PCB must be requested after high temperature deformation is smaller. T260's recommendation is 30 minutes or more, T288 value is greater than the recommended five minutes.
(4) good flatness. General printed circuit board warpage allowed rate of 0.75%, for the PCB substrate thickness of 1.6mm, the warping ≤ 0.5mm, under the warping ≤ 1.2mm.
(5) good electrical properties. Due to the high frequency of communication technology development, PCB's requirements also will improve high frequency characteristics. Will cause increased frequency dielectric PCB substrate (ε) increases, resulting in decreased speed of signal transmission circuit. There are other electrical performance dielectric loss tangent, dielectric strength, dielectric strength, arc resistance strength.