[SOLVED] Altium Line impedance calculator

[DavidZlavan]

Hello,

Due to my RF/MW background I am used to calculate with Linecalc tool (ADS) the necessary with of a transmission line in order to be 50ohms.

The problem is that ECAD department works with Altium. I ordered a PCB routing with lines at 50ohms for 200MHz.

[FR4, epsilon=4.2, h=1.4mm, t=0.35um, G=0.15mm]

My W calculated via Linecalc is 0.8mm and Altium impedance calculator gives a W of 0.6mm.

I am not an expert with Altium and need to know why is this discrepance due to.

Thank you very much in advance.

 

[SLK001]

Your thickness is wrong. I assume you mean 35µm. Also, I am assuming that you are using a Grounded Coplanar Waveguide (since you gave a gap value). With these values, I get 51Ω with 0.8mm trace widths. Change the gap to 0.14mm and the W to 0.82mm to get 50Ω.

I have no idea why the two programs do not concur. Maybe incorrect values were entered. I used TXLINE 2003 by AWR to get my values. I have used it for years. IIRC, I had problems with the ADS program in the past. There should be some documentation from HP/Keysight as to the formula used in their calcs.

 

[DavidZlavan]

Thank you @SLK001 for your answer. Yes it is CPWG line sorry.

I finally found the error.

First of all is not Altium impedance calculator what they use. It is polar si9000.
And the error was that the calculus they made is done having in account the soldering mask. Is a more complete calculus than linecalc.
We have tried to remove the mask and linecalc and polar gave the same result :0.8mm. With soldering mask this value is reduced to 0.6mm.

Here I found a more profund explanation:

http://www.sigcon.com/Pubs/edn/PassivationandSolderMask.htm

BR

 

[FvM]

Solder mask sounds like a plausible explanation. Problem of the design is however that the impedance depends mostly on the small gap also on the exact trace profile. Respectively it's quite sensitive to fabrication parameters.

 

[Mattylad]

Does Altium also take into account the shape of the final track? as it is not the rectangle that the CAD package sees it as but more trapezoid after etching.

Often the CAD package calculators are different to the si9000 results.

 

[DavidZlavan]

Does Altium also take into account the shape of the final track? as it is not the rectangle that the CAD package sees it as but more trapezoid after etching.

Often the CAD package calculators are different to the si9000 results.

Hello @Mattylad,

polarsi 9000 does take into account the trapezoid shape. Although CAD people told me it is negligible. If I had to calculate this effect I think I would have to know the fabrication process (quimics, time of etching, etc.), right?.
I think it is negligible in front of the mask problem which I understand is alterating the dielectric constant, and even more if we are talking about CPW where waves propagate in a greater percentage on the dielectric surface.

BR and thank you for your answer

 

[FvM]

Trapezoidal shape matters even more for CPW than for MS, particularly if the gap is so small as in your design.

Assuming something around 60 - 65 degree edge will be hardly completely wrong. In other words, top trace width = bottom width - trace height (W = W1 - T).