Effect of Ground Return Path on Timing Accuracy
Effect of Ground Return Path on Timing Accuracy
Effect of Ground Return Path on Timing Accuracy
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Gert Hohenwarter<br />
GateWave Northern<br />
<str<strong>on</strong>g>Effect</str<strong>on</strong>g> <str<strong>on</strong>g>of</str<strong>on</strong>g> ground return path <strong>on</strong><br />
timing accuracy<br />
Company<br />
Logo<br />
June 8-11, 8<br />
2008<br />
San Diego, CA USA
Objective<br />
• Review problem <str<strong>on</strong>g>of</str<strong>on</strong>g> ground path<br />
• Identificati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> trouble spots<br />
• Assessment <str<strong>on</strong>g>of</str<strong>on</strong>g> individual impact<br />
• Dem<strong>on</strong>strate significance<br />
• Summary<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 2
Approach<br />
• Examine complete signal path in a probecard<br />
• Develop models (FEA, SPICE)<br />
• Use models to evaluate impact <str<strong>on</strong>g>of</str<strong>on</strong>g> changes<br />
• Provide select measurement results to<br />
corroborate models<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 3
Problem<br />
• Signal in transmissi<strong>on</strong> line needs a return path<br />
• This path must be located physically close to the signal<br />
• Disrupti<strong>on</strong>s/detours add inductance and therefore delay<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 4
Probe card signal path<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 5
Probe card signal path<br />
• The signal path in the probecard begins at the<br />
perimeter with a tester c<strong>on</strong>necti<strong>on</strong> and ends at<br />
the probe tip with a c<strong>on</strong>necti<strong>on</strong> to the DUT<br />
Al<strong>on</strong>gside the signal path a ground return must be provided<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 6
Follow the signal…..<br />
• <strong>Timing</strong> verificati<strong>on</strong> begins at the tester<br />
c<strong>on</strong>necti<strong>on</strong> with a time domain reflectometer<br />
and a test probe…..<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 7
…through the probe…..<br />
• Comm<strong>on</strong> probes <str<strong>on</strong>g>of</str<strong>on</strong>g>ten have a fixed signal<br />
c<strong>on</strong>tact and movable c<strong>on</strong>necti<strong>on</strong> at ground:<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 8
C<strong>on</strong>sequences<br />
• <str<strong>on</strong>g>Ground</str<strong>on</strong>g> path inductance changes<br />
• Signal path capacitance changes<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 9
C<strong>on</strong>sequences<br />
• <str<strong>on</strong>g>Ground</str<strong>on</strong>g> path inductance changes<br />
• Signal path capacitance changes<br />
• Delay changes<br />
FEA analysis<br />
dLprobe [nH]<br />
2<br />
1<br />
0<br />
‐1<br />
‐2<br />
‐3<br />
‐50 0 50<br />
Angle<br />
dT [ps]<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
SPICE model<br />
dT [ps]<br />
0 2 4 6<br />
Lg [nH]<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 10
Verificati<strong>on</strong><br />
• Measurement at actual probecard PCB<br />
• At first glance everything is fine….<br />
Multiple TDR<br />
Multiple TDR<br />
rho<br />
1.2<br />
1.0<br />
0.8<br />
0.6<br />
0.4<br />
0.2<br />
0.0<br />
-0.2<br />
-0.4<br />
-0.6<br />
0 1 2 3 4<br />
t [ns]<br />
rho<br />
1.000<br />
0.800<br />
0.600<br />
0.400<br />
0.200<br />
0.000<br />
3 3.5<br />
t [ns]<br />
straight<br />
l<strong>on</strong>g inline<br />
short inline<br />
ortho 30<br />
gnd2<br />
gnd3<br />
gnd4<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 11
Delay extracti<strong>on</strong><br />
• When processing the data, differences become noticeable<br />
10.0<br />
8.0<br />
Delay variati<strong>on</strong>s<br />
2.0<br />
0.0<br />
Delay variati<strong>on</strong>s<br />
6.0<br />
-2.0<br />
dt [ps]<br />
4.0<br />
2.0<br />
0.0<br />
-2.0<br />
-4.0<br />
-6.0<br />
-8.0<br />
-10.0<br />
meas 1 meas 2 meas 3 meas 4 meas 5<br />
dt [ps]<br />
-4.0<br />
-6.0<br />
-8.0<br />
-10.0<br />
-12.0<br />
-14.0<br />
l<strong>on</strong>g inline<br />
short inline<br />
ortho 30<br />
gnd2<br />
C<strong>on</strong>diti<strong>on</strong><br />
gnd3<br />
gnd4<br />
least favorable<br />
Measurement to measurement<br />
For different probe positi<strong>on</strong>s<br />
(probe straight)<br />
dT = 5 ps<br />
dT tot= 5 ps<br />
dT counter: -><br />
(average, not max. dT)<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 12
Follow the signal…..<br />
• <str<strong>on</strong>g>Ground</str<strong>on</strong>g> via selecti<strong>on</strong> (<strong>on</strong>ly <strong>on</strong>e ground via present for each case):<br />
dL [nH]<br />
dL for 0.5 mm pitch<br />
0.4<br />
0.3<br />
0.2<br />
0.1<br />
0<br />
near side side far<br />
Inductance difference for 0.5 mm pitch<br />
dT = 3 ps<br />
dT tot= 8 ps<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 13
Follow the signal…..<br />
• The signal travels into the PCB through a<br />
c<strong>on</strong>nector and returns through an array <str<strong>on</strong>g>of</str<strong>on</strong>g> vias:<br />
Disorganized<br />
Organized<br />
dT = 10 ps<br />
dT tot= 18 ps<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 14
Follow the signal…..<br />
• PCB via positi<strong>on</strong> (vias must be provided near level shifts):<br />
dT [ps]<br />
dT [ps]<br />
12<br />
10<br />
8<br />
6<br />
4<br />
2<br />
0<br />
0 50 100<br />
dy [mils]<br />
Inductance change as a functi<strong>on</strong><br />
<str<strong>on</strong>g>of</str<strong>on</strong>g> lateral via positi<strong>on</strong><br />
dT = 5 ps<br />
dT tot= 23 ps<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 15
Follow the signal…..<br />
• <str<strong>on</strong>g>Ground</str<strong>on</strong>g> planes are disrupted in LGA/interposer area<br />
Inductance per unit length changes<br />
If traces get close to via antipads,<br />
capacitance per unit length changes,<br />
too<br />
Example (inductance <strong>on</strong>ly):<br />
Uniform array <str<strong>on</strong>g>of</str<strong>on</strong>g> 1mm pitch vias<br />
<str<strong>on</strong>g>Ground</str<strong>on</strong>g> path inductance changes by<br />
12% for a 40 mm l<strong>on</strong>g path<br />
dT = 16 ps<br />
dT tot= 39 ps<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 16
Follow the signal…..<br />
• <str<strong>on</strong>g>Ground</str<strong>on</strong>g> via positi<strong>on</strong>s in the interposer c<strong>on</strong>tact array<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
Inductance [nH]<br />
1 2 3 4<br />
# gnds<br />
Inductance as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> via distance from signal and number <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
ground vias<br />
dT = 10 ps<br />
dT tot= 49 ps<br />
0.5<br />
1<br />
1.5<br />
2<br />
mm<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 17
Follow the signal…..<br />
Delay change as a result <str<strong>on</strong>g>of</str<strong>on</strong>g> branch imbalance in ground path<br />
150<br />
dT [ps]<br />
100<br />
50<br />
0<br />
-50<br />
-100<br />
dT top 10k<br />
dT bot 10k<br />
dT top 50<br />
dT bot 50<br />
-150<br />
0 0.5 1 1.5 2<br />
Lg [nH]<br />
Different load c<strong>on</strong>diti<strong>on</strong>s result in<br />
different delay changes<br />
dT = 30 ps<br />
dT tot= 79 ps<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 18
….to the DUT<br />
Delay change as a result <str<strong>on</strong>g>of</str<strong>on</strong>g> grounding c<strong>on</strong>figurati<strong>on</strong> at the DUT<br />
Dense ground<br />
With typical dimensi<strong>on</strong>s assumed the<br />
inductance difference for the two<br />
different c<strong>on</strong>figurati<strong>on</strong>s is 0.88 nH<br />
Sparse ground<br />
dT = 5 ps<br />
dT tot= 84 ps<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 19
Overall changes<br />
Graphic representati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> relative change as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g><br />
where change occurs<br />
90<br />
80<br />
70<br />
60<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
ps<br />
Delay as a functi<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> locati<strong>on</strong><br />
meas<br />
via pos<br />
TR vias<br />
Sum<br />
Individual<br />
trace vias<br />
interposer vias<br />
interposer gp holes<br />
Space TR - SDR<br />
probes/DUT<br />
Comment:<br />
Line length matching to<br />
within 20 mils results in<br />
an inaccuracy <str<strong>on</strong>g>of</str<strong>on</strong>g> 3.5 ps<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 20
The impact <str<strong>on</strong>g>of</str<strong>on</strong>g> locati<strong>on</strong><br />
Delay change as a result <str<strong>on</strong>g>of</str<strong>on</strong>g> inductance change at<br />
different locati<strong>on</strong>s al<strong>on</strong>g the signal path<br />
Approximate equivalent circuit <str<strong>on</strong>g>of</str<strong>on</strong>g> signal path<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 21
The impact <str<strong>on</strong>g>of</str<strong>on</strong>g> locati<strong>on</strong><br />
Delay change as a result <str<strong>on</strong>g>of</str<strong>on</strong>g> inductance change at<br />
different locati<strong>on</strong>s al<strong>on</strong>g the signal path<br />
dt [ps]<br />
dT at different positi<strong>on</strong>s (50%)<br />
50<br />
40<br />
30<br />
20<br />
10<br />
0<br />
1n<br />
2n<br />
3n<br />
1 2 Pos # 3 4<br />
Signal at DUT (SPICE simulati<strong>on</strong>)<br />
Delay change<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 22
Waveform change<br />
Wave form changes as a result <str<strong>on</strong>g>of</str<strong>on</strong>g> inductance change<br />
dT at different positi<strong>on</strong>s (85%)<br />
50<br />
0<br />
dt [ps]<br />
-50<br />
-100<br />
-150<br />
1n<br />
2n<br />
3n<br />
1 2 Pos # 3 4<br />
Signal at DUT (SPICE simulati<strong>on</strong>)<br />
Delay change<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 23
Different loads<br />
Delay change for 3 different cases (10kOhm 2pF, 150 Ohm 2pF, 50 Ohm)<br />
50.0<br />
dT all loads (50%)<br />
40.0<br />
dt [ps]<br />
30.0<br />
20.0<br />
10.0<br />
0.0<br />
1 2<br />
Pos #<br />
3 4<br />
Load has additi<strong>on</strong>al impact <strong>on</strong> skew<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 24
Shared resources<br />
Delay change as a result <str<strong>on</strong>g>of</str<strong>on</strong>g> inductance change in<br />
case <str<strong>on</strong>g>of</str<strong>on</strong>g> split lines/ shared drivers<br />
Simplified equivalent circuit<br />
Signal change<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 25
Shared resources<br />
Delay change as a result <str<strong>on</strong>g>of</str<strong>on</strong>g> inductance change in<br />
case <str<strong>on</strong>g>of</str<strong>on</strong>g> split lines/ shared drivers<br />
Delay change at 85% level<br />
Delay change at 85% level<br />
(10kOhm/2pF load)<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 26
Summary<br />
• Overview to problem <str<strong>on</strong>g>of</str<strong>on</strong>g> ground path provided<br />
• <strong>Timing</strong> accuracy is sensitive to locati<strong>on</strong> <str<strong>on</strong>g>of</str<strong>on</strong>g> ground disc<strong>on</strong>tinuity<br />
• <strong>Timing</strong> accuracy depends <strong>on</strong> architecture <str<strong>on</strong>g>of</str<strong>on</strong>g> signal delivery system<br />
• Budgeting should be performed<br />
• Compensati<strong>on</strong> seems possible, if trends are known<br />
• Other c<strong>on</strong>tributi<strong>on</strong>s to timing arise from line length matching, fabricati<strong>on</strong><br />
variances such as dielectric c<strong>on</strong>stant changes, coupled secti<strong>on</strong>s,<br />
transiti<strong>on</strong>s and other disc<strong>on</strong>tinuities <strong>on</strong> the signal itself and must be<br />
accounted for separately. These c<strong>on</strong>tributi<strong>on</strong>s have NOT been taken<br />
into account here, yet a significant skew has already accumulated …..<br />
Thank you!<br />
June 8 to 11, 2008<br />
IEEE SW Test Workshop 27