PCB Density

Density is the ratio of traces, holes, and pads by area unit (mm², inch², etc…) When there are only a few traces, density is not an issue. But modern density requires modern solutions and every day, the customers and the factories are asking for more compact and more complex designs mainly due to the component technologies. The higher the density, the more complex the PCB is.

The Challenges for Production

With the advancement and the miniaturization of electronics, the demand for HDI PCBs has increased drastically but increasing density on a printed circuit board is a challenge from design to production. The spaces between the traces are down to 65µm. HDI PCBs are made with at least 4 layers, connected to each other by holes and vias to make ultra-complex design rules and production processes.

If the design is challenging for the engineers with their software, one of the main problems in production is to ensure the reliability of the plated holes. The main problem comes from the vias and holes and the plating process. To assume a good hole plating integrity, the aspect ratio is limited to 1: 0.8 for blind vias, advanced value is 1:1. Standard pre-pregs also contain fiberglass which is too thick for laser drilling. The glass contained in the pre-preg changes the laser direction and creates a mediocre or wrong shape quality of the laser via holes.

pcb density

Main Capabilities

Number of layers
HDI Builds
1+N+1, 2+N+2
3+N+3 and anylayer
FR4, ..., ETC.
See table
Copper Weight
Minimum track & gap
PCB Thickness
Surface Finishes
HASL, HASL LF, ENIG, immersion tin, OSP, immersion silver, electroplating hard gold/soft gold, gold finger, selective OSP, ENEPIG
Mechanical Drill
Laser Drill

Enhancing HDI PCB Design and Reliability

The build-up must use thinner base copper to have a good tracks definition. (many plating processes increase the total copper thickness). The base copper choice may influence the signal propagation for high-frequency applications as well. Thinner prepreg, thinner base copper, thinner PCB is the trend for HDI PCBs. Via holes are sensitives. The vias are like rivets during the assembly process. RoHS processes apply a high thermal stress on the material and furthermore on the vias. The expansion of the material in the Z axis stress the vias. The trend is to continuously reduce the via diameter as well as to increase reliability. Furthermore, the assembly process multiplies the number of thermal chocks. Due to this new situation, the only way to reduce the force applied during assembly is to use more stable FR4. Low CTE (coefficient of thermal expansion) materials are mandatory if we want to limit the breaking holes phenomena during the assembly process and in harsh environments.

Any questions?

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