Balance is a very important part of our lives. Without it, the scales of justice would be hopeless, checkbook registers would be a mess, and riding a bicycle would be pointless. Even the Star Wars franchise wouldn’t have made it past the first movie if there wasn’t any way to bring balance to the force. And as everyone knows, in a crisis moment it is finding balance amidst all of the conflicts that will keep us from falling and failing.
Circuit boards can get out of whack too, and when that happens they can bend and twist in ways that are not good for the board. This effect happens when some areas of the board have more copper than others. To prevent this, it is a good idea to even out the amount of copper fill used between the different open areas of the board. Here is some more information on why this problem occurs, and what you should do to ensure that your circuit board has good PCB copper balance.
Problems That Can Happen Without Good PCB Copper Balance
Circuit board fabrication techniques have been continually improving over the years. The boards that are being produced today would have been completely impossible not too long ago. However, there is still some basic physics at work here in PCB fabrication that is dependent on how consistent the copper coverage is on the different layers in a circuit board layer stackup.
When the copper coverage between layers of a printed circuit board is not consistent, it can lead to the bow and twist of the board. PCB fabricators prefer to have the top half of the layer stackup to be a mirror of the bottom half of the stackup as much as possible. With the very high pressures and temperatures of the fabrication process, combining uneven layers together as depicted in the picture below can result in the “potato chip” effect, or warpage. The heat applied to the board during assembly for reflow or wave soldering can also exacerbate this problem.
The larger the board, and the more layers used in the board layer stackup, the more that board warpage is likely to occur. Once the board starts warping, it can put stress on the metal that makes up the connected traces on and in the board. The thin traces used for long signal paths are particularly susceptible to stress damage, and breaks like this on internal layers are difficult to find and diagnose. Another problem with warpage is the stress it can put on solder joints, resulting in intermittent problems or broken and open circuits.
Pressing two-layer structures like this together during fabrication could cause warpage
It’s All About the Stackup and How the Right Layer Configuration Will Help
As we mentioned above, PCB fabricators prefer to have the top half of the board layer stackup mirror the bottom half. This gives them the ability to balance out the board instead of trying to fabricate board layers with uneven copper. Here are some ways that you can help in your design to keep the copper on the PCB balanced:
- Layer stackup: When possible, start with the center of the board layer stackup and place the densest copper layers there. As you design the stackup outward from the center, keep the copper coverage on each layer as symmetrical as possible to its mirror on the other half of the stackup.
- Power planes: Keeping the plane layers symmetrical throughout the layer stackup is important if it can be done without affecting signal integrity and impedance. It is also important to keep the copper weights of the planes the same as well.
- Copper pour: On signal layers that don’t have a lot of metal on them, you may want to add some copper pours to those layers to beef up their density. This is often done using cross-hatched areas of metal fill.
- Prepreg and core: Don’t forget that the prepreg and core layers of the board also need to be symmetrical in their makeup and thickness.
Using a symmetrical layer stackup plan when designing a multilayer printed circuit board, and filling in empty areas with copper pour will go a long way in helping balance the copper in your design.
How Your PCB Contract Manufacturer Can Help
Thankfully you have an excellent resource at your disposal to help you with this part of your circuit board design. Your PCB contract manufacturer will have a vast amount of experience in setting up circuit board designs for effective copper balancing. They will have the expertise you need in layer stackup planning for copper pour as well as signal integrity, and you should consult with them early before you begin your design.
At VSE, we have been doing this for a long time. Our engineering staff has helped thousands of customers like you to design the right layer stack for their PCB that will satisfy the copper balancing requirements for fabrication and assembly. In addition, our team also understands the signal and power integrity needs of your board as well, and all of that will be factored into the decisions we make regarding what the best layer stackup configuration will be for your design.
