One of the most simplistic, yet most profound, pieces of advice that anyone can be given is there is a time and place for everything. This self-evident and brief interpretation of the original statements from Ecclesiastes 3:1-8 can be applied to virtually any situation or endeavor. For example, in Linear Algebra, there is the following:
Given m linear equations and n unknowns (or variables), one of the following is true.
(1) If m > n, then there is no solution.
(2) If m < n, then there are infinite possible solutions.
(3) If m = n, then there is one solution.
Focusing on Eq. (3), we can say that for a well-defined problem, there is only one true solution or piece of advice that should be followed. The important thing is to be able to clearly define the problem, situation, or purpose.
A situation that sometimes arises when performing a particular process repeatedly is that some steps are included even when there is no clear purpose for them to be done, except that it always is. One such step is to add PCB ground pours to every design. After all, optimal board design is about knowing which PCB design steps are essential and which ones are not. With this objective in mind, let’s take a look at when and why a PCB ground pour should be used after clearly defining types of copper pours.
Copper Pours and Ground Pours
In order to accurately distinguish between copper pours and the more specific ground pours, a couple of definitions may be helpful.
Copper pour - an area of a PCB layer that is filled with copper. The layer may be the top, bottom or any internal to the PCB stackup and the pour may be used as a ground, reference or to isolate specific components or circuitry from the rest of the elements of the layer. Additionally, copper pours with vias can be used to aid in thermal dissipation.
Ground plane - an internal layer of the board stackup that is completely filled with copper and used as a signal or power ground or reference.
Ground pour - is a copper pour that is used for grounding and does not occupy an entire layer.
From these definitions, a ground pour can be viewed as performing a similar function to a ground plane but occupying less area or space. Now, let’s see why and when it can be advantageous to include PCB ground pours in your design.
When and Why to Use A PCB Ground Pour
Historically, when most boards were single-layer, PCB ground pours were efficient as they provided a single point of reference. However, today multilayer PCB designs are more common. The reasons for this are many; including the ability to accommodate circuitry with higher complexity, flexibility to be installed in smaller, more compact products and more durable and reliable construction. Along with the use of more layers came the use of ground planes and in many cases ground pours were no longer needed to provide a reference for signals. However, there are still times when ground pours can be used to your design’s advantage, as listed below.
Tips for When and Why to use a PCB Ground Pour
Tip #1: Grounding on two-layer boards
On two-layer (sometimes called double-sided) boards, both layers are used for signals and there is no ground plane. For these PCBAs, ground pours can be very helpful for efficient routing by providing a central ground.
Tip #2: EMI shielding
For good multilayer designs, it is desirable to have a ground plane between two signal layers to minimize noise. If there are internal signal layers next to the surface signal layers, ground pours can help with noise reduction by adding shielding.
Tip #3: Heat sinking
Ground pours can also be used to draw heat away from high power components. Thermal vias can then be used to remove the excess heat from the board.
Tip #4: Copper balance
PCB ground pours can also your contract manufacturer (CM) during assembly by balancing the amount of copper of both sides of the board. This reduces the possibility that warping may occur during reflow. In this case, cross-hatching may be a better alternative to solid copper ground pours.
Tip #5: High current paths
It can be advantageous to add surface ground pours to provide a short return path for high current devices; such as switching converters, instead running longer traces to a ground plane.
PCB ground pours are sometimes used, wrongly, simply to fill in the unused space on a board. Although this is not a good reason for ground pours, there are times when they should be utilized as listed above.
|Tempo's Custom PCB Manufacturing Service
At Tempo Automation, the industry’s leading manufacturer of low-volume and prototype boards, we can help you determine when it is best to employ this design technique to aid your board’s development.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on PCB ground pour and when and why to incorporate it into your design, contact us.