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Pcb design tips

Why the lines on the PCB board are not straight, but various turns?

  • 2019-08-13 10:20:00

The lines are commonly showed on our conventional pcb boards ,which are dominated by straight lines. But why are some lines on the PCB board not straight, sometimes are with various turns? This blog will explain the specific reasons through example.

multilayer pcb layout

The pic shown above is the DDR memory module which belongs to high-speed PCB. The design of high-speed PCB must ensure the integrity of the signal in order to make the chip work normally. General speaking, these design rules are used to make the diagram eye opened during data communication. If the routing rules are not strictly, the eye diagram will be closed, the judgment of the data will be wrong, and the communication will fail if it is serious, so that the machine cannot be opened normally.

multilayer pcb layout

Let's take a look at the signal lines of the 32-bit DDR2.

multilayer pcb design

These consist of 4 sets of data lines DQ (8 bits are a group), 4 sets of data select DQS (differential pair), 4 sets of data mask DM, a set of clock CLK (differential pair), and command address line CA. These signals need to be specially processed on the PCB and strictly controlled.The single-ended trace (DQ\DM\CA) requires 50 ohm impedance control and length equal length control. The differential trace (DQS\CLK) requires 100 ohms and length equal length control. From the PCB in the figure, there is no copper foil covering next to the surface trace. The signal line is referenced to the second layer for impedance control. Various curved bends (serpentine lines) on the signal line are mainly used to make equal length designs of the same set of data lines. The specific design rules are as follows:

multilayer pcb design

All in all, the traces in the figure are not straight lines but some curved lines (snake lines), mainly to make various signal lines meet the requirements of equal length, to ensure the diagram eye opening. that is, signal integrity.

Clink here to know more about the High speed board pcb layout design.

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