Many circuit board manufacturers, in the process of production, will strictly control the quality of the products to avoid the uneven thickness of the PCB circuit board. In the end, what are the harmful effects of the uneven thickness of the PCB circuit board? The manufacturer will tell you in detail. Vacuum laminator, reduce the pressure to reduce the flow of glue, try to maintain a larger amount of resin, because the resin affects εr, the resin is stored more, εr will be lower. Control laminate thickness tolerances. Because the thickness of the PCB board is not uniform, it indicates that the thickness of the medium changes, which will affect Z0.
Strictly according to the customer's requirements for the PCB circuit board type, the model is wrong, εr is wrong, the board thickness is wrong, the PCB manufacturing process is all right, the same is scrapped. Because Z0 is greatly affected by εr, the finished multi-layer board should avoid water absorption as much as possible, because the water εr=75, the line pair Z0 will bring great drop and instability effect.
The solder mask of the PCB board surface will reduce the Z0 value of the signal line by 1 to 3 Ω. In theory, the thickness of the solder mask should not be too thick, and the effect is not very large. The surface of the copper wire is in contact with air (εr = 1), so the Z0 value is measured to be high. However, after the solder mask, the Z0 value will drop by 1 to 3 Ω, because the εr of the solder mask is 4.0, which is much higher than the air.
The inner layer must find the wire gap and the notch. For the 2GHZ high-speed signal, even the 0.05mm gap must be scrapped; controlling the inner line width and defects is the key.