The Impact of Inner Layer Residual Copper Rate on Board Thickness
3 min
When we were kids, we heard the children's story "The Crow Drinks Water," where a crow raises the water level by dropping stones into a fixed amount of water in a bottle. The same principle applies to multilayer PCB lamination, where during high temperature and high pressure, PP sheets flow in a liquid state to fill the gaps between the layers. This process is called "resin filling."
As seen in the diagram, when the copper coverage in the inner layer is minimal, PP sheets of the same thickness need to be evenly distributed into the gaps between the layers. This leads to a thinner overall thickness after the PP sheets cool and solidify, resulting in a thinner overall board thickness.
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.
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.
If the residual copper rate is too low (as shown below), the overall board thickness will be thinner, and the uneven copper distribution across layers can lead to board warping.
Here, a special reminder for golden finger boards: since golden finger boards need to be inserted into slots, they are more sensitive to thickness. If the board is too thin, it may result in a loose fit or poor contact when inserted into the slot.
Therefore, we strongly recommend:
1) For golden finger multilayer boards, cover the blank areas with copper, especially the inner layers at the golden finger region, to avoid issues such as boards being too thin to fit into the slot or inconsistent line widths.
2) When the residual copper rate is below 25%, to reduce the uneven electroplating that causes inconsistent line widths and excessive board thickness deviations, cover the blank areas with copper.
【Commonly Overlooked Issues in Golden 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.
Summary:
For all types of boards, whenever possible, add copper to blank areas as long as it doesn't affect the design performance. Particularly when the residual copper rate is below 25%, ensure that copper is laid. In golden finger boards, copper must be laid in the inner layer at the golden finger area, and the outer layer should have a solder mask with a solid open window at the golden finger area.
Keep Learning
PCB Material Guide : Types, Selection, and Impact on Performance
If PCB materials could talk, FR-4 would probably say, "I run 80% of the electronics industry, and still nobody appreciates me." Rogers would boast, "I'm expensive because I'm special," while Polyimide would flex literally and say, "I bend, but I don't break." The purpose of this blog is to introduce you to the types of PCB substrates available to you and how those materials affect your PCB's performance, thermal stability, signal integrity, and overall longevity. The choice of PCB material will be bas......
The Critical Role of Copper Peel Strength in Durable PCB Performance
Key Takeaways Standard Test: Measured via the IPC-TM-650 90° pull test in N/mm. Core Function: Prevents pad lifting and delamination under thermal stress and CTE mismatch. Foil Types: ED foil provides higher adhesion; RA foil yields lower signal loss. Material Benchmarks: Standard FR4 averages 1.0–1.4 N/mm, while High-Tg FR4 achieves 1.2–1.6 N/mm. Design Tips: Add teardrops, eliminate unsupported copper slivers, and perform DFM checks. Ever wonder what's making the copper stay put on your circuit boar......
Navigating Standard PCB Thickness: A Step-by-Step Guide to Selecting the Ideal Thickness for Your PCB Prototype
The thickness of a printed circuit board (PCB) may seem like a small detail, but it plays a crucial role in the performance and reliability of your electronic devices. Standard PCB thickness has become an industry norm, offering a range of benefits that impact mechanical stability, electrical performance, thermal management, and component compatibility.In this article, we will be taking a deep look at why the varying thickness of your PCB matters and provide you with a guide to choosing the right thic......
How Modular PCB Design Simplifies Complex Electronics Projects
Key Takeaway Modular PCB design simplifies complex electronics by breaking boards into independent, reusable functional blocks with clear interfaces. It boosts reusability, speeds up debugging, enhances team collaboration, and reduces errors.Shift from flat to modular design for faster development and more scalable, reliable PCBs. Ever seen a 400+ component schematic and got your head scrambled before even beginning routing? You are not alone. With the ever-increasing density of electronics combined w......
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.......
Mastering Mitered Corners for Superior High-Speed Signal Performance
Have you ever had the privilege of zooming into a PCB layout and noted those well-chamfered corners on high-speed signal traces? Mitered corners they are, and much more than a cosmetic preference. The geometry of each trace bend is directly proportional to impedance continuity, signal reflection, and the quality of your data being transmitted at the RF and high-speed digital frequencies. Having mitered corners on your PCB can be the difference between meeting your return loss target and trying to trac......