A ground with low inductance value is a crucial element during PCB designing for mitigating EMC problems. Increasing the ground area on a PCB reduces the ground inductance in the system, hence EM emission and crosstalk too. Several approaches are available when we need to connect the signals to the ground, but what is best?

The engineers at ExpressPCB have assembled a few general rules-of-thumb that can help beginners design their first circuit board. These tips are not specific to using our CAD software, but instead provide an overview to help explain how to position the components on the board and how to wire them together.

Placing Components

Generally, it is best to place parts only on the top side of the board.

• The common PCB impedance of the ground will cause a certain voltage to form between the grounding points, which will cause grounding interference. As mentioned above, the ground wire acts as a conductor and has a certain impedance. As the name suggests, the impedance is composed of two parts: resistance and inductive reactance, namely.
• Common ground loops A common type of ground loop is due to faulty interconnections between electronic components, such as laboratory or recording studio equipment, or home component audio, video, and computer systems.

When placing components, make sure that the snap-to-grid is turned on. Usually, a value of 0.050″ for the snap grid is best for this job.

First place all the components that need to be in specific locations. This includes connectors, switches, LEDs, mounting holes, heat sinks or any other item that mounts to an external location.

Give careful thought when placing component to minimize trace lengths. Put parts next to each other that connect to each other. Doing a good job here will make laying the traces much easier.

Arrange ICs in only one or two orientations: up or down, and, right or left. Align each IC so that pin one is in the same place for each orientation, usually on the top or left sides.

Position polarized parts (i.e. diodes, and electrolytic caps) with the positive leads all having the same orientation. Also use a square pad to mark the positive leads of these components.

You will save a lot of time by leaving generous space between ICs for traces. Frequently the beginner runs out of room when routing traces. Leave 0.350″ – 0.500″ between ICs, for large ICs allow even more.

Parts not found in the component library can be made by placing a series of individual pads and then grouping them together. Place one pad for each lead of the component. It is very important to measure the pin spacing and pin diameters as accurately as possible. Typically, dial or digital calipers are used for this job.

After placing all the components, print out a copy of the layout. Place each component on top of the layout. Check to insure that you have allowed enough space for every part to rest without touching each other.

Placing Power and Ground Traces

After the components are placed, the next step is to lay the power and ground traces. It is essential when working with ICs to have solid power and ground lines, using wide traces that connect to common rails for each supply. It is very important to avoid snaking or daisy chaining the power lines from part-to-part.

One common configuration is shown below. The bottom layer of the PC board includes a “filled” ground plane. Large traces feeding from a single rail are used for the positive supply.

Common Ground Dmg Button Pcb

Placing Signal Traces

When placing traces, it is always a good practice to make them as short and direct as possible.

Use vias (also called feed-through holes) to move signals from one layer to the other. A via is a pad with a plated-through hole.

Generally, the best strategy is to lay out a board with vertical traces on one side and horizontal traces on the other. Add via where needed to connect a horizontal trace to a vertical trace on the opposite side.

A good trace width for low current digital and analog signals is 0.010″.

Traces that carry significant current should be wider than signal traces. The table below gives rough guidelines of how wide to make a trace for a given amount of current.

0.010″ 0.3 Amps
0.015″ 0.4 Amps
0.020″ 0.7 Amps
0.025″ 1.0 Amps
0.050″ 2.0 Amps
0.100″ 4.0 Amps
0.150″ 6.0 Amps

When placing a trace, it is very important to think about the space between the trace and any adjacent traces or pads. You want to make sure that there is a minimum gap of 0.007″ between items, 0.010″ is better. Leaving less blank space runs the risk of a short developing in the board manufacturing process. It is also necessary to leave larger gaps when working with high voltage.

When routing traces, it is best to have the snap-to-grid turned on. Setting the snap grid spacing to 0.050″ often works well. Changing to a value of 0.025″ can be helpful when trying to work as densely as possible. Turning off the snap feature may be necessary when connecting to parts that have unusual pin spacing.

It is a common practice to restrict the direction that traces run to horizontal, vertical, or 45 degree angles.

Common Ground Pcb Material

When placing narrow traces, 0.012″ or less, avoid sharp right angle turns. The problem here is that in the board manufacturing process, the outside corner can be etched a little more narrow. The solution is to use two 45 degree bends with a short leg in between.

Common Ground Pcb Electrical

It is a good idea to place text on the top layer of your board, such as a product or company name. Text on the top layer can be helpful to insure that there is no confusion as to which layer is which when the board is manufactured.

Checking Your Work

After all the traces are placed, it is best to double check the routing of every signal to verify that nothing is missing or incorrectly wired. Do this by running through your schematic, one wire at a time. Carefully follow the path of each trace on your PC layout to verify that it is the same as on your schematic. After each trace is confirmed, mark that signal on the schematic with a yellow highlighter.

Inspect your layout, both top and bottom, to insure that the gap between every item (pad to pad, pad to trace, trace to trace) is 0.007″ or greater. Use the Pad Information tool to determine the diameters of pads that make up a component.

Check for missing vias. ExpressPCB will automatically insert a via when changing layers as a series of traces are placed. Users often forget that via are not automatically inserted otherwise. For example, when beginning a new trace, a via is never inserted. An easy way to check for missing via is to first print the top layer, then print the bottom. Visually inspect each side for traces that don’t connect to anything. When a missing via is found, insert one. Do this by clicking on the Padin the side toolbar; select a via (0.056″ round via is often a good choice) from the drop down listbox, and click on the layout where the via is missing.

Common Ground Pcb

Check for traces that cross each other. This is easily done by inspecting a printout of each layer.

Metal components such as heat sinks, crystals, switches, batteries and connectors can cause shorts if they are placed over traces on the top layer. Inspect for these shorts by placing all the metal components on a printout of the top layer. Then look for traces that run below the metal components.

Common-impedance coupling occurs when two or more circuits share a common ground and is the result of a shared impedance in a shared ground path. The best way to explain it is by way of a common example, such as when the lights dim when an air conditioner is turned on; they probably share the same wire for the return current. The shared wire represents an impedance that is common to both the light fixture and the AC unit. The path of current through the resistors is the same path of current for the light bulb.

Anytime the AC motor’s current path and the light fixture’s current path overlap in shared components, the current multiplied by the impedance of the components (resistor) determines the value of the voltage dip that the downstream device (light bulb) experiences as a result of the upstream current draw (motor). The motor turns on, and the light dims momentarily. “Voltage droop” can be measured to show the lower than expected voltage potential for the light bulb, which causes the dimming.

Common Ground Pub

In a PCB ground plane, the effect of current flowing through the ground plane is experienced differently at different points in the ground plane. For a sensitive analog signal, the PCB ground plane is “infected” with varying currents and voltage noise, and there is no ideal ground plane in reality. It is necessary to know the actual path that a current signal takes in its return path through the ground plane if EMC noise is to be avoided. For digital designs, a .1mV noise source doesn’t register. But for very sensitive analog signals, a millivolt can be a noise problem. From the analog perspective, if there’s a lot of current flowing through one portion of a PCB ground plane, it’s prudent to move away from that area of the board to avoid noise. In other words, circuits with large differential power levels on a common PCB should avoid the same ground return path.

The problem compounds if there is a high-frequency signal involved, as in general, ground inductance becomes dominant above 1MHz in the ground plane of the PCB example in Figure 2. For high-frequency signals (generally above 100kHz), multi-point ground systems should be used to minimize the ground impedance and inductance. For current signals with frequencies below 1MHz (for audio interference), current in the reference plane will flow through the least resistive path. Above 1MHz (for radio signal interference), it tends to flow the least inductive path.

Gameboy Pocket Common Ground Pcb

Thus, current signals at lower frequencies are dominated by common-impedance coupling. Current signals at higher frequencies are dominated by common-inductive coupling. However, the size of the structure also plays a part in both cases. The “boundary frequency” where the signal becomes more influenced by shared inductance differs according to physical size. For example, with small integrated circuits, the “boundary frequency” where low frequency impedance-dominated current flow gives way to high frequency inductive-dominated current flow is around 20GHz. For larger structures like transmission towers, the boundary between low frequency resistive-dominated current flow and high frequency inductive-dominated current flow happens below 60Hz.