HDI PCBs – Technical Specification
High-Density Interconnect PCB is a type of printed circuit board that allows for more space on the circuit board so you can add extra components and functionality and increase data transmission speed and efficiency. It is a highly preferred PCB design for companies that want to expand their project’s functionality without compromising on the size.
The growing popularity of HDI PCBs is due to the freedom provided to the manufacturers. Designers can work with more space and add smaller components together and on both sides of the PCBs, resulting in a PCB design with high signal integrity and faster and more capable than any other device. PCBs are used in multiple devices for various purposes, such as medical equipment, camera, network communications, etc.
But to understand how HDI PCBs are limitless in their application, you need to look at their technical specifications. Let’s get started!
HDI PCB Construction
The construction specs are the first specifications that you need to provide an HDI PCB manufacturer when crafting the right HDI design for your project or device. For example, the panel sizes, the thickness, and more. It is done so you can have a proper base for your project. Ideally, you can determine these specs depending on your requirements and the number of layers or components you add to the PCB. We have listed some of the common specs:
• Panel size: could be anywhere between 30 to 74 cm.
• Minimum core thickness: 0.51mm
• Minimum to maximum board thickness: 0.178 to 6.4mm
• Thickness tolerance: less than 10%
• Bow and twist tolerance: less than 7%
• Minimum dielectric: 0.051mm
HDI PCB Board Material
Choosing the right board material for your HDI PCB design is crucial for strong signal integrity and fast speed. The most commonly used PCB material is the FR-4 family. Its examples are Ventec VT47, Isola 370 HR, etc. It is suitable for analog and digital applications and balances the speed and loss as it has high dielectric loss and flat frequency response.
Other board materials for HDI PCBs include Teflon, ceramic, flex, rigid, and polyimide. Each has its pros and cons. For example, the Isola I-Speed provides high speed and low loss. The choice depends on your requirements and the type of device you create. If you want better signal transmission, then you need a material with lower dissipation and dielectric constant.
Your HDI PCB material also depends on:
• It should be stable with high soldering heat.
• No adhesive requirement.
• Small dielectric constant and loss.
• Should be compatible with CTA (coefficient of thermal expansion)
HDI PCB Via Type
Depending on the number of layers in your HDI PCB, you add vias or holes in the printed circuit board to connect the different layers and connect to external components, such as crews and connectors. There are many different types of via used in HDI PCBs; we will discuss the most common ones.
• Buried Via: As the name suggests, buried via is drilled on the internal layers of the HDI PCB and is used to connect 2 or more inner layers. A disadvantage of buried vias is that they cannot be used to connect more than 3 layers; manufacturers would have to drill and pile the vias by overlapping or staggering them.
• Through-hole: For HDI circuit boards that connect from top to bottom layer, through-hole via is a perfect choice. It is drilled from top to bottom layer and is the most affordable. But, it does take up more space and has limited application. There are two types of through-hole vias, PTH and NPTH; the former is used for electrical connection, while the latter is issued for mechanical connection.
• Blind Via: Blind via is drilled from the top layer to an inner layer of the printed circuit board. Its manufacturing is difficult as you need to be very accurate; thus, designers drill holes in each layer separately and then stack them up.
• Micro Via: Micro via is a type of blind or buried via and is more commonly used nowadays. It can be converted to each type depending on how it is drilled and electroplated. For example, if drilled from top to inner layer, it forms a blind via, and if drilled internally, it forms buried via. The only difference is that it has a maximum diameter of 0.15mm and a maximum depth of 0.25mm.
Other types include vias in pad, staggered via, stacked via, and skip vias. The choice between these depends on your project requirements.
For better signal and data transmission, you need to use a conductor to tolerate the high soldering mask and perform well under extreme temperatures. The most commonly used conductor in HDI PCBs is copper. The thickness can be anywhere from 5 microns to >3oz. Remember, the thickness includes the starting foil plus the panel plates.
Solder masks protect the copper traces and interfaces between the PCBs. They also prevent the risk of short circuits by adding a conductive solder between the components. There are many types of HDI PCB solder masks used, out of which two are most common.
• Dry Film Photoimageable: this solder mask is applied using vacuum lamination. Once developed, you can create the openings and solder the parts on the copper pads. The copper is layered in the internal layers through electrochemical processing, and once the dry film is removed, the exposed copper is etched.
• Liquid photoimageable (LPI): LPI is a type of electronic material used in HDI PCBs to protect copper traces. It is a dual-cured system and is used in almost all electronic products. It is either silkscreened or sprayed, and an opening is created according to the pattern on the circuit boards.
Tests and Reports
Before you conclude and finalize the HDI PCB design, there are several tests that you need to run to check its compatibility and performance, such as electrical tests, solderability, ionic contamination, micro section, x-ray fluorescence, etc.
There are many technicalities involved when manufacturing HDI PCBs; thus, it's best to consult HDI manufacturing professionals, such as Hemeixin, to get the job done. They have years of experience in PCB assembly, PCB technology, thermal and signal integrity management solutions, and more. We hope this article helped!