The Impact of Inner Layer Residual Copper Rate on Board Thickness

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.