How Inner Layer Residual Copper Rate Affects PCB Thickness and Quality
4 min
In printed circuit board (PCB) manufacturing, precision is crucial for maintaining quality and performance. One key factor that significantly influences PCB quality is the residual copper rate in the inner layers. This concept becomes particularly critical in multilayer PCBs, where the balance of copper distribution impacts the final board thickness. This article explores how the inner layer residual copper rate affects board thickness and the importance of optimizing this rate to ensure robust and reliable PCBs.
How Inner Layer Copper Affects Board Thickness
As shown in the diagram, when the copper coverage in the inner layer is minimal, the (PP prepreg) sheets, regardless of their thickness, must spread evenly to fill the gaps between layers. Once the PP sheets cool and solidify, the reduced resin volume leads to a thinner overall board thickness.
Importance of Residual Copper Rate
So, how much copper should be laid in the inner layer to ensure that the board thickness doesn’t fall below tolerance limits? This is where the "residual copper rate" comes in. The residual copper rate refers to the percentage of the inner layer's copper circuit patterns relative to the entire surface area of the board.
Residual copper rate = the area of copper in the current layer / total area of the board.
The Role of PP Sheets in Multilayer Lamination
In multilayer board lamination, PP sheets are cut into pieces and placed between the inner core board and another core board, or between the core board and copper foil. The resin on the PP melts under high temperature and pressure, filling the copper-free areas on the core board. After cooling, the resin solidifies, bonding the core board and copper foil together.
Consequences of Low Residual Copper Rate
If the residual copper rate is too low, the overall board thickness decreases, and uneven copper distribution across layers can lead to board warping.
This is especially critical for boards with gold fingers, as their thickness must be precise to ensure a proper fit in slots. A thinner board may result in a loose fit or poor contact when inserted into the slot.
JLCPCB Recommendations
JLCPCB enginners strongly recommend :
1. For Gold Finger Multilayer Boards
Cover the blank areas with copper, especially in the inner layers near the gold finger region. This prevents issues such as the board being too thin to fit into the slot or variations in line widths.
2. For Residual Copper Rates Below 25%
To minimize uneven electroplating, which can cause inconsistent line widths and excessive board thickness deviations, add copper to the blank areas.
Common Issues in Gold Finger Design
For the golden finger areas in both the inner and outer layers, ensure that there is an open window (i.e., no solder mask bridge between each golden finger pad) to prevent frequent insertion and removal from causing ink to fall into the golden finger slot, which can result in poor contact and other functional issues.
For all types of PCBs, add copper to blank areas whenever possible, as long as it does not affect the board's performance. For boards with a residual copper rate below 25%, ensure that copper is added. In gold finger boards, copper should be applied to the inner layer near the gold finger area, and the outer layer should have a solder mask with a properly open window in the gold finger area.
Common Issues in PCB Gold Finger Design
For the golden finger areas in both the inner and outer layers, ensure that there is an open window (i.e., no solder mask bridge between each golden finger pad) to prevent frequent insertion and removal from causing ink to fall into the golden finger slot, which can result in poor contact and other functional issues.
For all types of PCBs, add copper to blank areas whenever possible, as long as it does not affect the board's performance. For boards with a residual copper rate below 25%, ensure that copper is added. In gold finger boards, copper should be applied to the inner layer near the gold finger area, and the outer layer should have a solder mask with a properly open window in the gold finger area.
Keep Learning
PCB Copper Pour Basics
What is Copper Pour in PCB Design? Copper pour refers to the technique of filling unused areas of a PCB's copper layers with solid copper planes. These planes are connected to power or ground nets, creating a continuous conductive path. Copper pour is typically used in the power and ground planes, as well as in signal layers for specific purposes. Purpose and Benefits of Copper Pour: Copper pour is primarily used to fill unused areas on PCB copper layers with solid (or hatched) copper connected to pow......
How to Choose the Thickness of PCB
First, in the world of electronic products, the PCB is often referred to as the "heart", it connects everything together making the board thickness an important parameter for this crucial component. Whether the PCB thickness is appropriately selected directly affects the performance, stability, and reliability of the final electronic products. The process of selecting PCB thickness is influenced by various factors, such as product application scenarios, board material, and number of copper layers. The......
Metal Core PCB Materials: Thermal Truth & Design Rules
Metal-core PCBs (MCPCBs) are speciality boards where a metal substrate replaces the standard FR-4. This metal core acts as a built-in heatsink, improving heat dissipation in high-power electronics. The basic stackup is simple: A copper conductor layer on top. A thin dielectric insulator in the middle. A thick metal base at the bottom. This structure provides excellent thermal spreading and a convenient ground plane. But it comes at the cost of a much heavier and more expensive board than typical FR-4.......
FR4 PCB Deep Guide: Material Truth, Real Specs & When to Use (or Avoid) It
FR-4 isn't a secret code, it literally means Flame Retardant (grade 4). In PCB terms, FR-4 is a NEMA (National Electrical Manufacturers Association) grade designation for a glass-reinforced epoxy laminate. We can say it is a composite of woven fiberglass cloth bonded with an epoxy resin that contains flame-retardant additives. The “FR” stands for flame retardant, but note this doesn’t automatically mean UL94 V-0 certified. It just indicates the resin is formulated to self-extinguish if it catches fire......
OPAMP 101: Basics of Operational Amplifiers Every Engineer Should Know
Analog mathematics? Yes, it is what we are going to learn in this series of OPAMP 101. An operational amplifier is the most common and most widely used type of component in an analog circuit. We can not imagine an integrated circuit without amplifiers. It is common, but students, on the other hand, often have a love-hate relationship: “How can something that looks so simple (just a triangle!) cause so much confusion?” An operational amplifier can perform a lot of mathematical operations; we will see t......
What is the Standard Thickness of a PCB?
A Printed Circuit Board (PCB) is the foundation of electronic devices, which serves as a substrate to support electrical components. In PCB design, thickness is a crucial parameter as it influences several key processes. It influences not only the mechanical performance of the board but also the electrical properties, processability, and cost. The prevailing PCB thickness is 1.6 mm, but there will be many further options suitable for various purposes. Knowing the rule and when it’s acceptable to break ......