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The difficulties of HDI PCB fabrication lie in micro-via fabrication, via metallization and fine lines.

1. Micro-via fabrication

Micro-via fabrication has always been the core problem in HDI PCB manufacturing. There are two main drilling methods:

1). As for ordinary via drilling, mechanical drilling is always the best choice for its high efficiency and low cost. With the development of mechanical processing capacity, its application in micro-via has been on the way as well.

2). There are two types of laser drilling: photothermal ablation and photochemistry ablation. The former refers to the process during which the operated material is heated to be melted after absorbing laser with high energy and is evaporated off with via formed. The latter refers to the result caused by high-energy photons in UV zone and with laser length more than 400nm.

2. Via metallization

The biggest difficulty for via metallization is it is difficult for plating to reach uniform. As for the deep-hole plating technology for micro-via, besides the usage of plating solution with high dispersing power, via plating solution on plating devices should be upgraded in time that can be accomplished by either strong mechanical stirring or vibration, ultrasonic stirring, horizontal spraying. Also, the humidity of via wall must be increased before plating.

3. Fine lines

The implementations of fine lines include traditional image transfer and laser direct imaging. Traditional image transfer has the same process with the ordinary chemical etching to form lines.

As for laser direct imaging, photographic film isn’t needed while images are formed directly on photosensory membrane through lasers. UV wave light is used to operate so that liquid anti-corrosion solution is capable of meeting demands of high resolution and simple operation.

What are the features of HDI PCB design?

An HDI board requires smaller vias to make layer transitions, particularly in fine-pitch BGA components and more traces per sq. mm. In order to accommodate fine-pitch components, you’ll find the following typical features in an HDI layout:

Smaller vias: HDI boards use microvias (mechanically or laser drilled), blind/buried, and staggered vias for layer transitions. These vias have smaller aspect ratios than typical through-hole vias. In order to use these vias with finer pitch components, their diameters are smaller, which then limits their useful depth.

Thinner traces: The thinner traces used in HDI boards are required to make connections to vias on each layer, as well as to in-pad vias. The thinner traces also allow higher trace density, thus the term HDI. Higher layer count: We’ve built non-HDI boards with high layer counts, but HDI board layer counts can easily reach 20 or more layers when working with high pin density components (e.g., FPGAs).

Lower signal levels: HDI boards are not used for high voltage or high current. This is because the high field strength between neighboring lines will cause ESD, and high currents will cause excessive temperature rise in conductors.

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