little0192
Newbie level 6
I am doing single rank ddr3 routing in a 6 layer board with the following stackup:-
Signal(L1) -> Gnd(L2) -> Signal(L3) -> Pwr(L4) -> Gnd(L5) -> Sig(L6)
(I am using Saturn PCB Toolkit for calculations)
I am confused which parameter to consider for length matching. Physical length or the Timing(step response):roll:
Lets say i have a 1.4 in trace in inner layer(L3) with two vias of lenght 12 mils from Top layer to L3.
Signal Propagation Speed
Top/Bottom 145 ps/in
Inner Layer 175 ps/in
Total Length = Trace Length + 2 * Via Length
(I am not including Fan out Stub here)
(Timing = length * propagation delay => 1.4*175 = 245)
Layer 1-3
Layer 1-6
Trace on top layer without any Via
So in all the above cases the difference in Physical Length is 10 mils only while the timing difference is 44 ps. I am curious to know which factor to take into account if going for much higher speed than the DDR3.
I already read a lot of datasheets, and i didn't find any answers that's why i post it here. Thank you.
Peace
Signal(L1) -> Gnd(L2) -> Signal(L3) -> Pwr(L4) -> Gnd(L5) -> Sig(L6)
(I am using Saturn PCB Toolkit for calculations)
Via 1 (L1-3) | Via 2(L1-6) | |
Height of Via | 12 mil | 38 mil |
Step Response | 4 ps | 15.8 ps |
I am confused which parameter to consider for length matching. Physical length or the Timing(step response):roll:
Lets say i have a 1.4 in trace in inner layer(L3) with two vias of lenght 12 mils from Top layer to L3.
Signal Propagation Speed
Top/Bottom 145 ps/in
Inner Layer 175 ps/in
Total Length = Trace Length + 2 * Via Length
(I am not including Fan out Stub here)
(Timing = length * propagation delay => 1.4*175 = 245)
Layer 1-3
Length(in) | 1.4 + 2*0.012 | 1.42 in |
Timing (ps) | 245 + 2*4 | 253 ps |
Length(in) | 1.36 + 2*0.038 | 1.43 in |
Timing (ps) | 196.48 + 2*15.8 | 228 ps |
Trace on top layer without any Via
Length(in) | 1.36 + 2*0 | 1.43 in |
Timing (ps) | 209 + 2*0 | 209 ps |
So in all the above cases the difference in Physical Length is 10 mils only while the timing difference is 44 ps. I am curious to know which factor to take into account if going for much higher speed than the DDR3.
I already read a lot of datasheets, and i didn't find any answers that's why i post it here. Thank you.
Peace