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Products # Ceramic Substrate. What is The Effect of PCB Wiring Width Change?

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In PCB wiring,such a situation often occurs:when the wiring passes through a certain area,due to the limited wiring space in this area,it is necessary to use thinner lines.After passing through this area,the lines will return to the original width.In ceramic substrate,the change of wire width will cause the change of impedance,so the reflection will occur and the signal will be affected.So under what conditions can we neglect this influence,and under what conditions do we need to consider its influence?

There are three factors related to this effect:the magnitude of the impedance change,the rise time of the signal,and the time delay of the signal on the narrow line.

This paper mainly discusses the magnitude of impedance variation.The design of many circuits requires that the reflected noise is less than 5%of the voltage swing(which is related to the noise budget on the signal).According to the reflection coefficient formula:

ρ=(z2-z1)/(z2+z1)=△Z/(△Z+2z1)≤5%

It can be calculated that the change rate of impedance is required to be△Z/Z1≤10%

As you may know,the typical indicator of impedance on the circuit board is+/-10%,which is the root cause.

If the impedance changes only once,for example,when the linewidth changes from 8mil to 6mil,the 6mil width is always maintained.In order to reach the noise budget requirement that the reflected noise of the signal at the sudden change does not exceed 5%of the voltage swing,the impedance change must be less than 10%.Sometimes it is difficult to do this.Take the situation of the microstrip line on FR4 plate as an example,let's calculate it.If the linewidth is 8 mil,the thickness between the line and the reference plane is 4 mil,and the characteristic impedance is 46.5 ohms.When the linewidth changes to 6mil,the characteristic impedance becomes 54.2 ohm,and the change rate of impedance reaches 20%.The fluctuation of reflected signal must exceed the standard.As for the influence on the signal,it is also related to the rise time of the signal and the time delay from the driving end to the reflection point.But at least that's a potential problem.Fortunately,the problem can be solved by impedance matching termination.

If the impedance changes twice,for example,the linewidth changes from 8mil to 6mil,and then changes back to 8mil after pulling out 2cm,then the reflection will occur at the two ends of 2cm long and 6mil wide line,once the impedance becomes larger,the positive reflection will occur,then the impedance becomes smaller,and the negative reflection will occur.If the interval between two reflections is short enough,the two reflections may cancel each other out and then reduce the impact.Assuming that the transmission signal is 1V,0.2V is reflected in the first positive reflection,1.2V is continuously transmitted forward,and-0.2*1.2=0.24v is reflected back in the second reflection.If the length of 6mil line is very short and two reflections happen at the same time,then the total reflection voltage is only 0.04V,less than the noise budget requirement of 5%.Therefore,whether this reflection affects the signal and how much it affects depends on the time delay at the change of impedance and the rise time of the signal.The research and experiment show that the reflection signal will not be a problem if the delay at the impedance change is less than 20%of the rise time of the signal.If the rise time of the signal is 1ns,then the time delay at the impedance change is less than 0.2ns corresponding to 1.2 inch,and the reflection will not be a problem.That is to say,in this case,the length of a 6mil wide line is only less than 3cm. When the linewidth of PCB wiring changes,it is necessary to analyze carefully according to the actual situation to see if there is any impact.There are three parameters that need to be paid attention to:how much the impedance changes,how long the signal rise time is,and how long the neck part of the linewidth changes.According to the above method,we can roughly estimate and set aside a certain margin properly.If possible,minimize the length of the neck.

It should be pointed out that in the actual PCB processing,the parameters can not be as accurate as in the theory,and the theory can provide guidance for our design,but it can not be copied or dogma,after all,this is a practical subject.For ceramic substrate,the estimated value should be revised according to the actual situation,and then applied to the design.
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