Why is conventional impedance control only 10%?

With high-speed signal transmission, high-speed PCB design has higher requirements. Impedance control is a conventional design of high-speed PCB design. There will be processing errors in more than a dozen processes of PCB processing. At present, the impedance control of conventional board factories is within 10% error. In theory, the smaller the value, the better. Why is it 10%? Why can’t the conventional control capability be further pushed to 8% or even 5%?

From the design point of view, it is mentioned in the article “Is the impedance board highly reliable? Hua Qiu has something to say”. According to the theoretical formula, impedance is related to factors such as dielectric thickness, line width, copper thickness, dielectric constant, and solder mask thickness, but the design is usually an ideal value.

From the perspective of PCB processing technology, there will be deviations in any process, that is, the so-called process tolerance. In theory, the closer the actual value is to the theoretical value, the smaller the impedance tolerance will be, the better the electrical performance of the electronic product will be, and the higher the product reliability will be.

Combining these impedance influencing factors, we can see that the impedance deviation is essentially related to the material and processing process. The deviation of the incoming board material itself, the deviation of the circuit etching, the deviation of the glue flow rate caused by lamination, the surface roughness of the copper foil, the PP glass fiber effect, the DF frequency variation effect of the medium, etc. will all have an impact on the impedance tolerance.

1.Incoming material deviation of copper clad laminate

According to IPC-4101 “Specification for Base Materials for Rigid and Multilayer Printed Boards”, the thickness deviation of copper clad laminates is divided into two types, one thickness tolerance includes the thickness of the metal foil, and the other does not include the thickness of the core board of the copper foil. The thickness tolerance of copper clad laminates (including copper foil) is divided into three levels (K, L and M), and the thickness deviation is gradually stricter from level K to level M. The thickness deviation of copper clad laminates (excluding copper foil) is divided into four levels (A, B, C, D), and the thickness deviation is gradually stricter from level A to level D. Huaqiu strictly selects Shengyi/Jiantao A-level FR4 boards to control the incoming material deviation from the source and control the impact of the board on the impedance tolerance.

2.Dielectric Deviation of Lamination Process

Lamination is one of the most important processes in the manufacture of multilayer PCBs. In short, lamination refers to combining copper foil, prepreg (PP sheet) and inner core board through “heat and pressure”. After the lamination process, the PCB needs to meet certain requirements such as board thickness, impedance control, electrical insulation, bonding between layers, dimensional stability, and flatness of the board.

Here we can see that an important factor affecting the impedance tolerance of lamination is the dielectric thickness, that is, the thickness of the prepreg (PP sheet). Generally speaking, the PP sheet purchased by the board factory has an initial thickness, which is related to the resin content (RC%) and the model of the glass cloth. There are certain differences in the glue content and glass cloth thickness of PP sheets of different brands. In terms of product stability and reliability, while meeting customer needs, Huaqiu PCB generally uses PP sheets from fixed brand manufacturers to form a relatively stable parameter relationship with the factory press, and through a large amount of process data to ensure the lamination quality of the PCB.

In the actual lamination process, the resin will flow under high temperature and high pressure, and the excess resin (RF%) will overflow from the interlayer, and the thickness of the remaining resin between the layers is the actual thickness of the resin.

The actual thickness of PP after filling is calculated as follows:
Thickness after PP lamination = theoretical thickness of a single PP sheet – filling loss

Filling loss = (1-A surface inner layer copper foil residual copper rate) x inner layer copper foil thickness + (1-B surface inner layer copper foil residual copper rate) x inner layer copper foil thickness
Inner layer residual copper rate = inner layer trace area / whole board area

Among the important factors affecting impedance control, the dielectric constant (Dk value) and dielectric thickness are determined by the PP sheet. The dielectric constant Dk value can be calculated by the following formula:

Dk = 6.01-3.34R R: resin content %
In summary, the thickness of the raw material PP sheet has an inherent thickness deviation, and the actual thickness after lamination is also due to the superposition of deviations in the lamination parameter conditions. These will affect the final impedance control, and it is difficult to control the impedance tolerance within the range of 10%.
PP sheets are distinguished by the model of their glass cloth. The most common models are 7628, 2116, and 1080. The following table is a list of various PP sheets, resin content, and thickness

3.Line deviation of etching process

PCB line process generally goes through the processes of film-exposure-development-etching-film stripping. The line etching process will affect the final line width, thereby affecting impedance control. In theory, if the quality of etching is to be accurately defined, the consistency of line width and the degree of side etching, that is, the etching factor (the ratio of side etching width to etching depth is called etching factor) must be guaranteed.

Usually, our etching accuracy tolerance is controlled by +/-1mil for line widths of 10mil and below, and +/-10% for line widths above 10mil. The smaller the line width, the more difficult it is to control the accuracy tolerance of etching. In order to control the line accuracy and line width consistency, the PCB board factory must be equipped with high-quality line exposure machines and vacuum etching machines on the one hand, and on the other hand, it is necessary to compensate the engineering film according to the etching side etching amount, photo-drawing error, and pattern transfer error to meet the line width/line thickness requirements. Huaqiu has high-precision LDI exposure machines and cosmic horizontal lines, and the line width tolerance can be controlled within ±15% (the industry standard is ±20%), the minimum line width and line spacing is 2.5/3.0 mil, the etching is uniform and high quality, and the line accuracy and consistency are more guaranteed.

4.Solder mask deviation

Solder mask, that is, a layer of solder mask (ink) is applied on the circuits and substrates on the PCB surface that do not need to be soldered, and it plays the role of solder mask insulation, oxidation prevention, and beautification of appearance.

The factors that affect the impedance of solder mask ink are mainly the dielectric constant of solder mask ink and the thickness of solder mask oil covering the impedance line. For PCB board factories, the ink models used are generally fixed, and their dielectric constants change very little. Only when the solder mask color is changed, the dielectric constant will change slightly. Compared with the dielectric constant, the thickness of solder mask ink has the greatest impact on impedance.

Generally, printing solder mask will reduce the outer layer impedance, so the influence of solder mask will be considered when controlling the impedance error. Under normal circumstances, printing one solder mask can reduce the single-ended impedance by 2Ω and the differential impedance by 8Ω; printing two times will reduce the impedance by twice as much as one time; when printing more than three times, the impedance value will no longer change.