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Why have some wide trace at the begninning, but smaller trace at the end?

This example shows traces that goes into a BGA processor with pitch 0.5mm. From the begninning it's 0.09mm in width. At the end, it's 0.12mm in width.

enter image description here

Is there any reason to do that? I mean...it's only 0.03 mm in difference. If I compute the impedenace, I can cleary see that 0.09mm is the way to go, instead of 0.12 mm.

Anyway, according to the data sheet, this track should have a specific length and it must have 55 ohm impedance because it's a high frequency trace.

enter image description here

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2 Answers 2

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It looks like the traces are thinned in that area so more of them can fit in the available space.

This is a common issue when routing high-density boards. You start the layout with the default trace width attribute set to some value. Often this is slightly wider than the minimum trace width the PC board house says they can maintain. Then you get into an area where there are 10 traces but room for only 9 with the default width, so you thin out one segment to squeeze them through.

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  • \$\begingroup\$ I understand. So increase the width is only useful if necessary space is needed :) Thank you for that explanation. In this case, I need 0.09mm and more space. I'm using 0.12 mm at the current. \$\endgroup\$
    – euraad
    Commented May 23 at 12:49
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There might be several reasons.

The first one is that there may be no reason specifically, it might be an error or just how it happened to turn out.

The second reason might be that the BGA is very hard to fanout the signals from tightly packed BGA to where they can be routed more cleanly. You might need to narrow down the signals to be able to draw them near the BGA package and maybe even betwee BGA pins.

Even if you need specific impedance and can't wire the traces, then wire the traces with wrong impedance. Depending on what the high frequency signals are for, and how much it tolerates mismatch, it may be OK to wire a few millimeters with wrong impedance.

The other option being to make the PCB more expensive to manufacture, by adding layers and using more expensive and precise PCB production line to allow for drawing the wires like they should be, to correct pins with correct impedance.

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