cancel
Showing results for 
Show  only  | Search instead for 
Did you mean: 
johnvivm
Voyager
Voyager
2,841 Views
Registered: ‎08-16-2018

Package delays for DDR3 length matching

Jump to solution

 

Can I assume an average propagation speed for all signals in a BGA package from the die to the balls?

The reason I'm asking is I would like to convert the package delays (in time) to some "trace length" then add that to my CAD software (Altium) to length-match those signals.

Any better way to do that is welcome, as well.

0 Kudos
1 Solution

Accepted Solutions
vortex1601
Explorer
Explorer
2,724 Views
Registered: ‎12-11-2017

If you have the package delays, then converting this to length using your own PCB trace velocity parameters would yield a 'length' value as a stand-in for package delay, which you'd add to the trace length for matching purposes.

Your PCB vendor should be able to tell you your velocity for strip and microstrip. This is stackup- and material-dependent.

In general however it's probably not worth it - you're looking at tens of ps that you can squeeze out.

Can you convince Altium to use delays based on your PCB parameters instead? That's my experience with Allegro anyway.

View solution in original post

4 Replies
watari
Professor
Professor
2,766 Views
Registered: ‎06-16-2013

Hi @johnvivm 

 

I suggest you to a formula.

Here is the formula to calculate delay time.

Would you use it ?

 

# micro strip line

delay time [ns/m] = (epsilon + 1)/ + (epsilon -1) /2 * ( 1 + 10 * h / w) ^ -0.5

 

# strip line

delay time [ns/m] = (epsiton) ^ 0.5 / (3 x 10^8)

 

[note]

h => height of insulating layer

w => wiring width

 

Best regards,

 

0 Kudos
johnvivm
Voyager
Voyager
2,730 Views
Registered: ‎08-16-2018

@watari 

That's fine for a PCB where I know all the details about thicknesses and dielectrics, but I don't know what's in between the BGA balls and the die pads

0 Kudos
vortex1601
Explorer
Explorer
2,725 Views
Registered: ‎12-11-2017

If you have the package delays, then converting this to length using your own PCB trace velocity parameters would yield a 'length' value as a stand-in for package delay, which you'd add to the trace length for matching purposes.

Your PCB vendor should be able to tell you your velocity for strip and microstrip. This is stackup- and material-dependent.

In general however it's probably not worth it - you're looking at tens of ps that you can squeeze out.

Can you convince Altium to use delays based on your PCB parameters instead? That's my experience with Allegro anyway.

View solution in original post

johnvivm
Voyager
Voyager
2,711 Views
Registered: ‎08-16-2018

@vortex1601 

That's the conclusion I ended up with in the interim. The real speed and therefore the real lengths within the package don't matter. What matters is the delay, so if those delays are converted to PCB lengths that the CAD package will use with the PCB speed then the delays are reconstructed as to match the original. A bit weird for not using actual values but I believe it works.

0 Kudos