A comprehensive introduction to the HDI board
With modern-day technology advancements, the need for compact, lightweight, and reliable electronic devices has evolved. This gave rise to what is called HDI board design and manufacturing. In this article, we will have a comprehensive introduction to HDI boards and we will cover the following topics:
What does HDI board stand for in PCB?
PCB or Printed Circuit Board is a mechanical structure that supports or holds various electronic components together. At the same time, providing connectivity between these various components. It has become part of our daily lives; They are in every single electronic device used daily at home and work. Including but not limited to TVs, laptops, smartwatches, and home appliances. The PCB industry has been evolving rapidly to get along with new technologies.
HDI is the abbreviation for High-Density Interconnect. In the context of Printed Circuit Boards (PCB), it refers to printed circuit boards with a high number of interconnects per unit area; to accommodate the need for packing more electronic components in a tighter space. It has grown popular with the advance in technology and the rising need to add more features in smaller areas.
What is HDI in semiconductor?
In the world of semiconductors and integrated circuits, a die or a chip is a block of a semiconductor circuit that performs a certain function. They are often produced in large batches on a silicon wafer or other material like gallium arsenide and then cut to individual circuits resulting in what is called a die or a chip. The resulting die is packaged in many forms to facilitate its assembly on PCB boards.
In fast-growing silicon technology, chips with smaller transistors, and an increased number of inputs/outputs (I/O) in the smaller areas gave rise to the use of smaller packages like Flip chip packages, direct chip attachments, and chip-scale packages. As component manufacturers look for ways to add more functions to a single chip it is possible to achieve high-density interconnects on the package substrate level.
Why was HDI created?
The fast-growing HDI board industry was initiated by the need for complex and dense interconnects. In recent years, this need is driven by fast-growing industries like telecommunications, and portable electronics all of which strive to produce lightweight, reliable, and compact products. These devices have increased features and improved performance.
HDI board design and fabrication started when researchers found different methods to reduce the size of vias in PCB in the 1980s.The first production sequential build-up (SBU) PCBs appeared in the 1980s. since then, component manufacturers have always looked for ways to add more functions to a single chip.
Also, PCB board designers have been investigating ways to add more functions to a single board. Smaller vias technology, reduced trace width, and clearances made it possible to achieve high-density interconnect on the package substrate and PCB board level.
What are HDI boards used for?HDI boards are used in many industries where there is a need to pack more components into a limited area. These industries include
- Portable devices and smart wearables
- Military and Aerospace
- Healthcare and medical equipment
- Automotive industry
What is HDI PCB material?
The PCB material selection is a critical aspect of the HDI board design. It shouldn’t be overlooked by HDI board designers, to ensure the reliability of the HDI PCB design. It is a good practice to reach out to the fabricator at the stack-up selection phase of the design, and ask them for the datasheets to narrow the stack-up options beforehand.
The most common HDI board materials used are:
- Laser-Drillable Prepregs
- Conventional Prepregs
Prepreg or copper clad laminates are semi-curable resins -which are also called B-stage resins(mostly epoxy)- around the woven fiber-glass matrix, which are stacked in between copper layers to form the copper clad laminate.
Choosing the right type of prepreg is a critical aspect of PCB design in general and for HDI boards. Tightly woven glass fibers are used for high-frequency applications, while higher resin content of prepregs is better at handling high voltages. The manufacturer tested controlled impedances on their different laminates to make sure that they operate accordingly to specification.
Another important substrate material -especially for HDI boards- is resin-coated copper(RCC), which is composed of uncured B-stage resin on a layer of copper foil. RCC became popular as it addresses the shortcomings of fiberglass dielectrics, like the thickness of woven fiberglass. It also solves its shortcomings with laser drilling.
To overcome the larger thickness of woven fiberglass, copper foil is used as a carrier for the dielectric. Nowadays, Dielectric layers reaching down to 25 microns are available allowing for extremely thin HDI boards. However, it usually costs more than prepreg copper-clad laminates. However, when we consider laser-drilling time as the number of vias and board size increases, RCC foil can be a better option as the cost becomes close.
What are the 4 indicators of HDI board?
To sum up the indicators of an HDI board, It can be said that there are main features that characterize it from a traditional PCB:
- Small PCB area.
- The use of small PCB component footprints packed in that small area like Ball grid arrays (BGA) packages with a high number of I/O (fanouts).
- Very thin traces with tight clearances. These routing for them can be longer as they are routed between many layers and hence signals have longer flight times.
- The use of microvias with a low aspect ratio( that is diameter to via length), Microvias have a less than or equal to 1 aspect ratio, and they allow the designer to escape-route from the high-density flip chips.
What are the advantages of HDI board?
We can now conclude the advantages of HDI boards:-
- Efficient use of PCB area:
This is the most significant advantage, that is because:
HDI board designers use smaller component packages like BGAs(Ball Grid Arrays), by using the underside of the package instead of pins around the edge, this also comes with many other added advantages.
The use of suitable combination blind, buried, and microvias makes the board compact by reducing board area requirements, in contrast to using only through hole vias in the case of traditional PCB.
- Better electrical performance for high-speed designs:
With the very short distance between the PCB and package leads, the inductance of the leads is very low, especially in the case of BGA packages.
HDI Boards use blind microvias, and via in pad under BGA components. That can lead to shorter signal path lengths, as components can now be placed closer.
- Better heat conduction:
Better thermal conductance as heat can flow more easily from the package to the PCB preventing the IC from overheating.
- HDI boards are cost-effective:
The functionality of 8 layers of traditional through-hole PCB can be made into a 6-layer HDI PCB with the same quality.
What is HDI board stackup?Manufacturers use a process called sequential lamination process/ sequential build-up (SBU) to build up the HDI boards. In this process, a layer-by-layer fashion construction is performed in multiple steps, in which very thin dielectrics are formed in layers one-by-one around the thick core. The manufacturing steps are：
- Photoresist deposition and exposure:
- Etching and cleaning:
- Via formation and drilling:
- Via metalization:
- Stack build-up:
The fast-growing HDI board industry was initiated by the need for complex and dense interconnects.
Manufacturers use a process called sequential lamination process/ sequential build-up (SBU) to build up the HDI boards.
In the world of semiconductors and integrated circuits, a die or a chip is a block of a semiconductor circuit that performs a certain function.
Compared to a traditional PCB, HDI boards are characterized by using smaller vias, smaller trace width, smaller clearances between traces, and higher layer count to route a larger number of signals. They also use component packages like BGAs (Ball Arid Array) for their small footprint areas and compact thickness besides the high number of fan-outs.