How Inner Layer Residual Copper Rate Affects PCB Thickness and Quality

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.