Impedance constraints while manufacturing PCB

[cameo_2007]

Hi,



I have the gerbers for a single board computer,to be released for manufacturing.There are a few (20-30) system bus tracks from the processor to the RAM. Should these tracks,which i suppose are very sensitive ones to be taken care of all those impedance constraints while manufacturing?

If so how.what si that value for that.And should i instruct my manufacturer regarding that.

And what could be the possible problems which can happen if they are not manufactured accordingly?



any clues guys?



regards,

 

[yang1006]

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[cameo_2007]

what the heck is this?
moderator,

 

[JPR75]

Hi,

Depending of what RAM, speed and bus you are using, you should match impedance and tracks length carefully.

You can even cut the PCB into slices and look under a microscope the track's width and high to be sure that impedance is OK (we have done it for a 200MHz RDRAM board), if you need a high reliability.

If so how.what si that value for that.And should i instruct my manufacturer regarding that.

Depends on your chips and bus.

And what could be the possible problems which can happen if they are not manufactured accordingly?

Overshoot, undershoot ...

Regards

 

[angelote]

It depend on your singal's wavelength and your traces' length.

If traces' length is more than ,let's say, 1/10 of signals' wavelenth, your traces must be used as transmission lines, a you'll have to match impedance and traces' length.

 

[cameo_2007]

after manufacturing the board is with me now.How i can check the impedance of a particular track now?
Will a normal multi meter help?

 

[angelote]

With a multimeter you can measure trace's resistance, or impedance at 0Hz.

Impedance varies with frequency. If you don't have an impedance meter, you can use a function generator, an oscilloscope, a voltage probe and a current probe (or a voltage probe plus a 1ohm resistance to view current in oscilloscope.

You have to measure impedance at the frequency of your signals.

Chartacteristic impedance can be calculated as Z=sqrt(Zo*Zc), where Zo is the impedance mesaured with the output of the transmision line open (open circuit), and Zc whith the oputput closed (short circuit).

To measure impedance, you have to measure voltage and current amplitudes and phase deviation between voltage and current (or use an impedance meter).