The structure of the multilayer PCB board:
In order to control the impedance of the PCB well, we must first understand the structure of the PCB:Generally speaking, the multi-layer board is formed by laminating and pressing a core board and a prepreg. The core board is a hard, specific thickness, two-coated copper plate, which is the basic material that constitutes the printed board. . The prepreg constitutes the so-called wetting layer, which plays a role of bonding the core board. Although it also has a certain initial thickness, its thickness will change during the pressing process.
Usually the two outermost dielectric layers of the multilayer board are wetting layers, and a separate copper foil layer is used as the outer copper foil on the outside of these two layers. The original thickness specifications of the outer layer copper foil and the inner layer copper foil are generally 0.5OZ, 1OZ, 2OZ (1OZ is about 35um or 1.4mil), but after a series of surface treatments, the final thickness of the outer layer copper foil is generally Will increase by about 1OZ. The inner copper foil is the copper clad on both sides of the core board, and its final thickness is very small from the original thickness, but due to etching, it is generally reduced by a few um.The outermost layer of the multilayer board is the solder mask, which is what we often call "green oil". Of course, it can also be yellow or other colors. The thickness of the solder mask is generally not easy to determine accurately. The area without copper foil on the surface is slightly thicker than the area with copper foil, but because of the lack of copper foil thickness, the copper foil still appears more prominent. When we use You can feel it when you touch the surface of the printed board.
When making a printed board with a certain thickness, on the one hand, it is required to reasonably select the parameters of various materials; on the other hand, the final forming thickness of the prepreg will also be smaller than the initial thickness. The following is a typical 6-layer laminate structure:
PCB parameters:Different printed board factories have slight differences in PCB parameters. Through communication with Shanghai Jiajietong Circuit Board Factory technical support, some parameter data of the factory are obtained:
Surface copper foil: There are three types of surface copper foil thickness: 12um, 18um and 35um. The final thickness after processing is about 44um, 50um and 67um.Core board: Our commonly used board is S1141A, standard FR-4, two breaded copper, optional specifications can be contacted with the manufacturer to determine.
Pre-cured tablets: specifications (original thickness) are 7628 (0.185mm), 2116 (0.105mm), 1080 (0.075mm), 3313 (0.095mm). The thickness after actual pressing is usually about 10-15um smaller than the original value. Up to three prepregs can be used for the same wetting layer, and the thickness of the three prepregs cannot all be the same. At least one prepreg can be used, but some manufacturers require that at least two prepregs must be used. If the thickness of the prepreg is not enough, the copper foil on both sides of the core board can be etched away, and then the prepreg can be adhered on both sides, so that a thicker wetting layer can be achieved.
Solder mask: the thickness of the solder mask on the copper foil is C2≈8-10um, and the thickness C1 of the solder mask on the surface without copper foil varies according to the surface copper thickness. When the surface copper thickness is 45um, C1≈13-15um , When the surface copper thickness is 70um, C1≈17-18um.Conductor cross-section: I used to think that the conductor cross-section is a rectangle, but in fact it is a trapezoid. Taking the TOP layer as an example, when the thickness of the copper foil is 1 OZ, the upper bottom edge of the trapezoid is 1 MIL shorter than the lower bottom edge. For example, the line width is 5MIL, then the upper bottom edge is about 4MIL, and the lower bottom edge is 5MIL. The difference between the upper and lower bottom edges is related to the copper thickness. The following table shows the relationship between the top and bottom of the trapezoid in different situations.
Dielectric constant: The dielectric constant of the prepreg is related to the thickness. The following table shows the thickness and dielectric constant parameters of different types of prepreg:
The dielectric constant of the plate is related to the resin material used. The dielectric constant of the FR4 plate is 4.2-4.7, and it will decrease with the increase of frequency.Dielectric dissipation factor: The energy consumed by a dielectric material due to heating under the action of an alternating electric field is called dielectric loss, and is usually expressed as a dielectric loss factor tan δ. The typical value of S1141A is 0.015.
It can ensure the minimum line width and line spacing for processing: 4mil / 4mil.