What is the Importance of Sequential Lamination?

High-density interconnects (HDI) PCBs are a rapidly increasing part of printed circuit boards (PCBs) and electronics industries.

To achieve higher circuit density than traditional PCBs, HDI printed circuit boards utilize a combination of advanced features and technologies such as blind/ buried vias, laser-drilled stacked micro-vias, via-in-pads techniques, among others.

One of the most crucial manufacturing techniques in the HDI PCB fabrication process is Sequential Lamination.

What is Sequential Lamination?

The manufacture of a PCB consists of layering an epoxy pre-impregnated fiberglass sheet between each copper layer and laminating together under high temperature and pressure using a hydraulic press.

The sequential lamination process involves inserting a dielectric between a layer of copper and an already laminated sub-composite.

Blind and buried vias can be built into a PCB using sequential lamination. By fabricating a layer with blind vias (as if a 2 sided PCB is being made) and sequentially laminating this layer with an inner layer results in a PCB with buried vias.

HDI boards go through this process multiple times when different combinations of layers and via structure types are needed.

To make this structure more complicated, let’s assume that the design requires additional vias connections from L1-L3 and L6-L4.
In this case, the best way to build the board is by splitting vias and making the connections as follows.

The picture above illustrates that the laser vias can be designed as stacked or staggered micro-vias. The stacked micro-vias are more space-efficient.
However, stacked micro-vias are less reliable, require complex manufacturing, resulting in a higher PCB manufacturing price.

The following constraints must be considered when designing an HDI board:

  • Four lamination cycles are the maximum for any multilayers structure. It means that the vias should not be designed in a way that requires more than 4 lamination steps
  • Material thickness for laser drilling should not exceed the size of the via. For example, if the design calls for a 4mil micro-via, the drilled thickness should be 4mil or less
  • Each lamination step will plug buried vias with epoxy.
  • Each lamination cycle will impact the registration of drilled vias
  • Min pads size for laser drilling is10 mil for 4 mil laser vias, 11mil for 5 mil laser vias
  • Pad size for mechanical drilling is 16 mil – for 8 mil vias