PCB manufacturing is the process of building physical PCBs from PCB design according to a set of specifications. Understanding design specifications is crucial as it affects the manufacturability, performance, and production yield of PCBs.
One of the important design specifications to follow is the “balanced copper” in PCB manufacturing. It is necessary to achieve consistent copper coverage in each layer of the PCB stack to avoid electrical and mechanical issues that may hinder circuit performance.
What does PCB balanced copper mean
Balanced copper is a method of symmetrizing copper traces in each layer of a PCB stack, which is necessary to avoid circuit board distortion, bending, or warping. Some layout engineers and manufacturers insist that the mirror stacking of the upper half layer be completely symmetrical with the lower half layer of the PCB.
PCB balanced copper effect
1. Wiring
Etching the copper layer to form wiring, the copper used for wiring transmits heat and signals throughout the circuit board. This reduces the damage caused by irregular heating of the circuit board that may cause internal track breakage.
2. Radiator
Copper is used as a heat dissipation layer in power generation circuits, avoiding the use of additional heat dissipation components and greatly reducing manufacturing costs.
3. Increase the thickness of conductors and surface pads
Copper used as a coating on PCBs increases the thickness of conductors and surface pads. In addition, solid interlayer copper connections were achieved through electroplating through holes.
4. Reduced ground impedance and voltage drop
PCB balanced copper reduces ground impedance and voltage drop, thereby reducing noise, while also improving power efficiency.
PCB balanced copper effect
In PCB manufacturing, if the copper distribution between layers is uneven, the following problems may occur:
1. Improper stacking balance
Balanced stacking houses mean having symmetrical layers in your design, with the aim of abandoning the risk areas of deformation that may occur during the stacking assembly and lamination stages.
The best method is to start the stacking design from the center of the circuit board and place the thick layer there. Usually, the strategy of PCB designers is to mirror the upper and lower halves of the stack.
2. PCB layering
The problem mainly comes from the use of thicker copper (50um or more) on the core of the copper surface imbalance, and even worse, there is almost no copper filling in the patterns there.
In this case, the copper surface needs to be supplemented with “false” areas or planes to prevent the prepreg from overflowing into the pattern and subsequent layering or interlayer short circuits.
No PCB layering: 85% copper is filled in the inner layer, so filling with prepreg is sufficient and there is no risk of layering.
No PCB layering risk
There is a risk of PCB delamination: only 45% of copper is filled, and the interlayer prepreg is not filled enough, posing a delamination risk.
3. Uneven thickness of dielectric layer
Layer stacking management is a key element in designing high-speed boards. In order to maintain the symmetry of the layout, the safest approach is to balance the dielectric layer, and the thickness of the dielectric layer should be arranged symmetrically like the roof layer.
But sometimes it is difficult to achieve uniformity of dielectric thickness. This is due to some manufacturing restrictions. In this case, the designer will have to relax the tolerance and allow for uneven thickness and a certain degree of warping.
4. Uneven cross-section of circuit board
One of the common imbalanced design problems is the improper cross-section of the circuit board. Copper deposits are larger in some layers than in others. This issue stems from the fact that the consistency of copper is not maintained on different layers. Therefore, during assembly, some layers become thicker, while other layers with low copper deposition remain thinner. When pressure is applied horizontally on the plate, it will deform. To avoid this situation, the copper cover must be symmetrical relative to the central layer.
5. Mixed (mixed material) lamination
Sometimes, designs use mixed materials in the roof. Different materials have different thermal coefficients (CTC). This type of hybrid structure increases the risk of warping during the reflow assembly process.
The impact of imbalanced copper distribution
The changes in copper deposition can cause PCB warping. Some warping and defects are mentioned below:
Warping
Warping is just a deformation of the shape of the board. During the baking and processing of the board, copper foil and substrate undergo different mechanical expansion and compression. This will cause deviations in their expansion coefficients. Subsequently, the internal stress generated on the board led to warping. According to the application, PCB material can be glass fiber or any other composite material. During the manufacturing process, the circuit board undergoes multiple heat treatments. If the heat distribution is uneven and the temperature exceeds the thermal expansion coefficient (Tg), the circuit board will warp.
Poor electroplating of conductive patterns
In order to set up the electroplating process correctly, the copper balance on the conductive layer is very important. If copper is imbalanced at the top and bottom, and even at each individual layer, excessive electroplating can occur, leading to insufficient connection trajectory or etching. Especially, this involves differential pairs with measured impedance values. Setting up the correct electroplating process is complex and sometimes even impossible. Therefore, it is important to supplement copper balance with “fake” patches or all copper.
Arcuate
If the copper coating is unbalanced, the PCB layer will exhibit cylindrical or spherical curvature. In simple language, you can say that the four corners of a table are fixed, and the top of the table rises above it. It is called a bow and is the result of a technical malfunction. The bow produces tension on the surface in the same direction as the curve. In addition, it can cause random current to flow through the circuit board.
Bow effect
1) Twisting is influenced by factors such as circuit board material and thickness. When any corner of the circuit board is not symmetrically aligned with the other corners, distortion occurs. A specific surface rises diagonally, and then other corners twist. It is very similar to pulling up a cushion from one corner of the table while the other corner is twisted.
2) The resin voids are simply the result of improper copper plating. During assembly pressure, the pressure is applied to the plate in an asymmetric manner. Due to the lateral force of pressure, surfaces with thin copper deposits will exude resin. This will create a gap at that location.
3) According to IPC-6012, the maximum allowable value for bending and twisting on circuit boards with SMT components is 0.75%, while for other circuit boards, it is 1.5%. Based on this standard, we can also calculate the bending and twisting of specific PCB sizes. Bow allowance=plate length or width × The percentage of bow allowance/100 twist measurement involves the diagonal length of the circuit board. Considering that the board is constrained by one of the corners and twisted in both directions, factor 2 is included. Maximum allowable twist=2 × Plate diagonal length × Here, you can see examples of plates with a length and width of 4 inches and 3 inches respectively, with a diagonal length of 5 inches.
Measure the bending allowance of the entire length with bow torsion=4 × 0.75/100=0.03 inch width bending allowance=3 × 0.75/100=0.0225 inches maximum allowable twist=2 × five × 0.75/100=0.075 inches
Design specification for PCB balanced copper
- During stack design, it is recommended to set the center layer to the maximum copper thickness and further balance the remaining layers to match their mirrored opposite layers. This suggestion is important for avoiding the potato chip effect discussed earlier.
- In areas with wide copper areas on a PCB, it is wise to design them as grids rather than solid planes to avoid mismatched copper density in that layer. This largely avoids bow and twist issues.
- In the stack, the power layers should be symmetrically placed, and the weight of copper used in each power layer should be the same.
- Copper balance is not only necessary in the signal or power layer, but also in the core layer and prepreg layer of the PCB. Ensuring a uniform proportion of copper in these layers is a good way to maintain overall copper balance in the PCB.
If there are excess copper areas in a specific layer, the symmetrical opposite layer should be filled with a small copper grid to balance. These tiny copper grids will not be connected to any network and will not interfere with functionality. However, it is necessary to ensure that this copper balancing technology does not affect Signal integrity or circuit board impedance.
Techniques for Balancing Copper Distribution
1) Fill pattern
Cross hatching is a process in which certain copper layers are grid like. It actually involves regular openings, almost resembling a large sieve. This process creates small openings on the copper plane. The resin will be firmly bonded to the laminate through copper. This will result in stronger adhesion and better copper distribution, thereby reducing the risk of deformation.
The following are some advantages of shaded copper planes relative to solid casting for filling patterns:
- Controlled impedance routing in high-speed circuit boards.
- Allowing wider dimensions without affecting circuit assembly flexibility.
- Increase the amount of copper under the transmission line and increase the impedance.
- Provide mechanical support for dynamic or static flexible plates.
2) Large copper area in grid form
If possible, the large copper area on your circuit board should always be a grid. This can usually be set in the layout program. For example, the Eagle program refers to the grid area as “hatching”. Of course, this is only possible when there are no sensitive high-frequency conductor traces. Grid “helps to avoid” distortion “and” bow “effects, especially for circuit boards with only one layer.
3) Fill copper free areas with (grid) copper. Copper free areas should be filled with (grid) copper. Advantages:
- Better uniformity of the plating through hole wall has been achieved.
- Prevent circuit board twisting and bending.
4) Ensure copper symmetry
Ensure that the copper symmetry and large copper area should be balanced with the opposite “copper filling”. Also, try to distribute the conductor traces as evenly as possible throughout the entire circuit board. For multi-layer circuit boards, match the symmetrical relative layers with “copper filling”.
5) Symmetrical Copper Distribution in Layered Stacks
The thickness of copper foil in the circuit board layer should always be symmetrically distributed. Asymmetric layer stacking can be created, but we strongly recommend not doing so as it may deform.
Use thick copper plate
If the design allows, choose thicker copper plates instead of thinner ones. When you use thin plates, the chance factors for bows and twists will increase. This is because there is not enough material to maintain the stiffness of the board. Some standard thicknesses are 1mm, 1.6mm, and 1.8mm. The risk of warping for thicknesses below 1 millimeter is twice that of thick plates.
Uniform trace
The conductor wiring should be evenly distributed on the circuit board. Try to avoid copper pits as much as possible. The wiring should be symmetrically distributed on each layer.
Stealing copper
You can see that the current accumulates more in the area with isolation traces. Due to this fact, you cannot obtain smooth square edges. Copper theft is the process of adding small circles, squares, or even solid copper planes to the large blank space on a circuit board. Stealing copper distributes copper evenly throughout the entire circuit board. Other advantages are:
A uniform electroplating current results in the same etching amount for all traces.
Adjust the thickness of the dielectric layer.
Reduced demand for excessive etching, thereby reducing costs.
Copper filling
If a larger copper area is required, the open area is filled with copper to maintain balance with the symmetrical relative layer.
Power plane symmetry
It is important to maintain the copper thickness in each signal or power plane. The power layer should be symmetrical. The simplest form is to place the power layer and strata in the middle. If you can bring the power supply and ground closer, the loop inductance will be much smaller, so the propagation inductance will also be smaller
Prepreg and core symmetry
Maintaining the symmetry of the power plane alone is not enough to achieve a uniform copper cladding. Matching prepreg and core materials in terms of layering and thickness is also important.
Copper weight
Fundamentally speaking, the weight of copper is a measure of the thickness of copper on a board. A specific weight of copper rolls over a one square foot area on each layer of the board. The standard copper weight we use is 1 ounce or 1.37 mils. For example, if you use 1 ounce of copper on an area of 1 square foot, the thickness of copper is 1 ounce.
The weight of copper is the determining factor of the current carrying capacity of a circuit board. If your design has high voltage, current, resistance, or impedance requirements, you can modify the copper thickness.
Heavy copper
There is no universal definition for heavy copper. We do use 1 ounce as the standard copper weight. However, if the design requirement exceeds 3 ounces, it is defined as heavy copper. The higher the weight of copper, the higher the current carrying capacity of the wiring. The thermal and mechanical stability of the circuit board has also been improved. It is now more resistant to high current exposure, high temperatures, and frequent thermal cycles. All of these will weaken conventional circuit board design. Other advantages are:
High power density
Greater ability to accommodate multiple copper weights on the same layer
Light copper
Sometimes, you need to reduce the weight of copper to achieve a specific impedance, and it is not always possible to adjust the wire length and width, so achieving a lower copper thickness is one of the feasible methods. You can use a wiring width calculator to design the correct wiring for your circuit board. When using thick copper cladding, the spacing between the wires needs to be adjusted. Different designers have different specifications for this. The following is an example of the minimum space requirement for copper weight.
