Ask Me Anything with Susy Webb

Use this thread to post your PCB routing, grid and BGA design questions and Susy Webb will answer on October 5th!

And for anyone interested in learning more about this topic, Susy will present a one-day seminar on PCB routing, grid and BGA design for quality boards on October 26th via Zoom.

Hi Susy! Can you share a couple of placement guidelines to set up routing?

What are the most common manufacturing issues for small trace widths and pad sizes used on BGAs?

Can you discuss the importance of thermal management in PCB routing, and what techniques are used to optimize heat dissipation in high-power components?

What are some advanced routing techniques for reducing EMI?

Hello Susy. I often have issues with crosstalk. Would you know how I could minimize or control it while routing?

Is there a good practice to determine the optimal trace width and spacing for SI and manufacturability when routing?

Hi Susy, what are your top recommendations to design PDNs? How does PDN routing affect signal integrity?

I’m designing a multi-layer board. What strategies do you recommend for effective layer stacking and signal routing between layers?

Hello Susy. As a rule of thumb, is it more beneficial to have a common ground plane below signal traces?

Well First I would look at the parts/signals that are high speed or important signals. Those might be any I/O signals, anything that needs to be in a particular position (especially to connect or interact with other parts or boards). Then I would look at anything high frequency or fast rise time. You would want to take care that they are placed well very early on so that they have the best possibility of good routing connections later on. Differential pairs would be something to consider early on as well. Hope that helps!

1 Like

Hi Mohamed, Yes, it is a necessity to have a ground plane next to every signal layer in the stack so that the signals have a low impedance path to return on. It is a law of physics that the energy must return to it’s source, so the person designing the board must provide a low impedance path for that to happen.

1 Like

Hi Lucy,

Some common manufacturing issues with small trace widths are that a designer may start with a copper that is too thick to use with the small trace widths designed and the fabricator has to etch it so long to get it down to the width desired that the traces end up too thin and/or breaking completely. Another common problem with small pads is not using or allowing tear drops to be used. If they are not allowed, and the drill happens to be off-center, it can cause a poor or no connection at the joint between the pad and the trace connected to it. So with minimum size pads and traces, always allow tear drops.

1 Like

Hello Atar,
Some of the most valuable things for reducing EMI on traces are to have the signals routed internally (where the energy is contained within the dielectric material), to have a good low impedance return path for signals (especially high speed signals), and to space the signals out as much as possible. If you have very limited area, then increase the space between signals as much as possible as often as possible.


Hi Rahuls,
Crosstalk is energy that is given from one signal to another either because one is high frequency and is changing on and off next to another signal that is slower, or the energy from the return has no good return path (ground plane) and so has to spread out in order to get back to it’s source. When the energy spreads, it can cause noise, false triggering, etc on the other signals in the area. The best way to control that is to keep the energy well contained in a small area (thickness) of the board, to take care that there is a good return path for that energy back to it’s source, and to increase the spacing between the traces.

1 Like

My preferred method of determining trace width and spacing is to work with the fabricator when building a stackup. I would decide how many routing layers I need, how many return planes are needed, choose the dielectric material, let them know where I want to signals to return (plane layer next door in the stack), whether I need interplane capacitance in the stack, and what impedance I want for each type of traces. The fabricator will generally take all that information and put it into a field solver and come up with a good stackup. I will look at it and either accept it as is, or ask for some tweaks until we both get something we are happy with. Hope that helps.

1 Like

Hi Milan,
I would look at the number of signal layers I think I need, and add a return plane (probably ground plane) next to each one so that there is a low impedance return path for each signal layer. I would also decide what the frequencies of the signals are in the board, and set up interplane capacitance in the stack. That is a power and a ground plane as close as possible to one another in the stack <10 mils apart, and it will supply the high frequency energy to the high frequency signals. I would also decide what impedance I want for the signals, because even if impedance control is not required for the circuits, it helps to have all layers be the same impedance so that there are not reflections when the signals go from one layer to another. One more thing… try to balance the construction of the board with equal amounts of copper and equal number of layers in the stack. Hope that helps.

1 Like

My top recommendation is to have interplane capacitance in the board stack. That is at least one set of power and ground planes with a minimum distance between them (<10 mils). That will form a high frequency capacitor that is very good for the higher frequency signals we work with in our boards today. If you do not get the high frequency power to the devices in a timely manner, the signals may be “warped” and not reach their full potential in the time allotted, meaning they will not have a good wave form and can have SI problems. Also it is a good idea to make the power connections with wide traces whenever possible for a low impedance path for the energy.

1 Like

Hi Steve,
You have to look at the current needed and the ability to move that much current through and around the board. It is always a good idea to have wide and possibly thick traces when delivering higher power connections. The circuits may also need multiple vias when transitioning from one layer to another. There are good calculators out there that wlll help figure out the amount of area needed. Mini-planes are often needed to carry the current. When the connections are not done correctly, there will be extra heat in that area of the board which can have a very bad effect on the materials, the solder joints and the via barrels.