HDI PCBs – Opportunities and limitations in design and manufacturing

HDI-Leiterplatte KSG

The most important questions and answers

They are used in smartphones, aircraft and medical devices, enabling high technical performance in the smallest of spaces: HDI printed circuit boards have become integral to the electronics industry. However, as miniaturisation progresses and circuits become increasingly complex, the demands placed on the design and manufacture of multilayer printed circuit boards are increasing.

What are the characteristics of an HDI PCB?

HDI PCBs are compact printed circuit boards with four of more layers, usually manufactured sequentially in several pressing cycles using sequential build-up (SBU) technology. HDI stands for “high-density interconnect” and means a high circuit density. This can be achieved with a multilayer structure of the printed circuit board, line structures and spacings of less than 150 µm and vias measuring less than 150 µm (often called “microvias”).

Overview technology & design to cost

What are the requirements placed on PCB manufacturers?

For one thing, PCB manufacturers must make sure the base materials are suitable. The most important factors here are the lowest possible laminate and copper thicknesses and a low thermal-expansivity coefficient. For another, ever smaller line-space structures go hand in hand with higher requirements in terms of systems technology. Here, direct-image (DI) exposure is becoming increasingly important, which, in addition to a high layout resolution, also offers better positioning accuracy. DI exposure is also on the rise in the solder-mask sector. The direct exposure of the resist can reduce the solder-mask dam and requires less solder-mask clearance around pads. The HDI sector also places high demands on the etching process, in which an even etching rate of top and bottom must be ensured.

What needs to be considered in the PCB layout of HDI circuit boards?

It is always advisable to contact the PCB manufacturer as early as possible in order to find out what their possibilities are. After all, the layout process should result in a layout that the manufacturer can produce. For developers and layout designers, it is first necessary to describe what the future printed circuit board should look like. Among others, the following questions have to be answered:

• Which base material is to be used?

• How many layers are needed? 

• Where are the holes in the stack?

• What base copper strengths are needed?

• Are components with a high connection density (e.g., BGAs) being used?

This often results in the unbundling strategy for the entire layout, making it possible to define the layering structure and the constraints for all parameters, such as conductor track widths, spacings and sizes for blind or buried vias.

Example: BGA unbundling
BGA Entflechtung Lage Top
Top position
1te Innenlage BGA Entflechtung KSG Group
First inside layer
2te Innenlage BGA Entflechtung KSG Group
Second inside layer

What role do closure technologies play in HDI printed circuit boards?

Even if they are associated with higher process costs, plugging technologies such as microvia copper filling or plugging in accordance with IPC-4761 Type VII offer a range of benefits. These include, in particular, greater possibilities for design and unbundling, as well as space saving due to an almost-planar surface topography. In addition, air and fluid entrapment can be prevented, and assemblies protected with potting lacquer. If microvias are also used for heat dissipation, the thermal performance of HDI PCBs can also be increased.

Where is HDI headed – what else is possible in the future?

In the medium and long term, it will be possible to use even thinner materials with better properties and to increase the number of pressings. There will also be further progress in miniaturisation, so that even smaller structures can be realised both in the conductor layout and in the solder mask, and even smaller holes can be reliably processed.

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