Microstrip and Stripline Skew

Two common methods to route high-speed lines are stripline, which sandwiches the signal line between ground planes, and microstrip, which lays the signal line on the board's surface. Efinix® recommends using stripline because it provides a better SNR for high-speed signals and helps to reduce skew.

Figure 1. Comparing Microstrip and Stripline Design

Skew is the timing delay difference between 2 or more signals. The following table shows skew control recommendations for double data rate and single data rate domains for various operating frequencies, as well as the recommended PCB skew for CK to DQS timing.

To help the design process, the skew (in picoseconds) is converted to an approximate physical length (in inches) for both the microstrip and stripline routing methods.

Note: The timing information provided here is for reference only. The layout, PCB, and components you use will affect the actual signal timing and skew.

Table 1. Skew Control Recommendations
Speed (Bit Rate)		400 Mhz, DDR 800	533 Mhz, DDR 1066
DQ to DQS	Skew in ps	25	18
	Skew in inches of microstrip	0.17	0.12
	Skew in inches of stripline	0.14	0.1
Address, control, and CA to CK	Skew in ps	50	38
	Skew in inches of microstrip	0.33	0.25
	Skew in inches of stripline	0.28	0.21
DQS to CK	Skew in ps	188	141
	Skew in inches of microstrip	1.25	0.93
	Skew in inches of stripline	1.04	0.78