HDI Boards – High-Density Interconnect- HDI PCB Fabrication Process
High-Density Interconnect PCBs keep up with the current technology trend of packing more capabilities into smaller devices. HDI PCB manufacturers use different fabrication methods to make this possible and create a circuit board that uses finer lines and spacing to add more components to a smaller area.
The design and manufacturing process of an HDI PCB can be complicated. You can face multiple challenges throughout the process, such as longer routes leading to longer flight times, limited circuit board space, etc. Thus, you need professional HDI PCB manufacturers to get the job done. With the right layout, design process, and tools, you can add unlimited features to the HDI PCB.
Let’s take a detailed look at the design and manufacturing process for HDI PCBs to have a better idea.
Difference Between HDI PCB Design and Fabrication
Since the board area is smaller, manufacturers face many limitations in the design process, and thus its manufacturing is also different from traditional PCBs. The use of vias, thinner traces, and smaller components can be challenging for routing the circuit board.
The main differences in the HDI PCB manufacturing process are the materials and their compatibility. For example, what is the board material's dielectric constant and dielectric loss? Will there be adequate spacing between the layers based on the material you choose? Can it handle extremely high temperatures? Etc.
The efficiency of your HDI printed circuit board is also dependent on the design guidelines followed, the stackup type used, and the architect or manufacturing method applied. Thus, before manufacturing, you need to determine the layout of your HDI PCB board, the design limitations you will face, and more to analyze which features can be added or removed.
HDI PCB Design Considerations
When it comes to the design guidelines for an HDI PCB, a few requirements need to be met, such as a board material with controlled impedance, trace widths and spacing limits, aspect ratio limits, and stackup type. Since an HDI PCB layout packs more components with thinner traces and micro vias, the right material and component selection is crucial. Let’s see how we can do that
HDI PCBs are designed to optimize and increase routing to improve signal integrity. In an HDI PCB, the trace width is smaller to occupy less space. The reduced space affects signal integrity if requirements are not met; thus, keep a close eye on it.
When designing an HDI PCB, the next most important decision is the number of layers and the via type being used. There is 8 total via types for HDI PCBs, each differing how they are drilled, electrocuted, and stacked. The most popular choices are blind, buried, through-hole, and micro vias. Micro vias can be either used as a blind or buried via, depending on how they are drilled.
The type of via used also determines the manufacturing method used further on.
The stackup type is also crucial depending on your project requirements. There are 6 stackup types for HDI PCBs, out of which types I, II, and III are most commonly used.
• 1+N+1: It is the simplest and cheapest stackup type. The micro vias are placed on a single side on the HDI PCB and need only one laser drilling.
• 2+N+2: In this stackup type, the micro vias are placed on one or both sides of the circuit board and need laser drilling twice. The manufacturers can place micro vias either in a staggered or stacked way, depending on the need and complexity of the project.
• 3+N+3: In this stackup type, the micro vias are placed on both sides and need laser drilling thrice. This is expensive as it requires high accuracy and expertise.
The other 3 stackup types have expensive manufacturing costs and are unsuitable for HDI PCBs.
The component type, such as the surface mount devices, has higher pinpoints and pads. The bigger the component, the more board space you need. The components also define stackup type, trace widths, and more.
Wiring demand refers to the total connection length required to connect all parts. The wiring length should always be less than the substrate capacity.
The complexity of the manufacturing process determines the cost of the HDI printed circuit board. We have listed the manufacturing considerations below:
Laser or Mechanical Drilling
These are the two methods used for drilling holes in the PCBs. The drill is made of micro-grain carbide that can be sharpened and reused many times when it comes to mechanical drilling. With mechanical drilling, you can create multiple consistent holes on a single layer without any error, and it is faster than any other method used. However, each hole created needs to be deburred, as the elevated ends of copper affect the surface level.
Laser drilling is a new and improved method used in HDI PCBs. It is beneficial as it can cut through a wide range of materials differing in diameter and widths, which is impossible with mechanical drilling. It is also a no-contact process and requires less handling.
You need to use via types that are small and have an aspect ratio of less than 1. Thus, micro vias make a perfect choice. Moreover, a smaller aspect ratio means easy drilling, plating, and thermal compatibility.
Each stackup type needs to be laminated before being drilled and electrocuted. There are two HDI PCB structures: build-up and any-layer. In the build-up structure, the layers are stacked and then laminated together. In contrast, in any-layer structures, the layers are laminated separately and then stacked. The higher is the number of laminations, the higher will be the cost.
Other manufacturing considerations include surface finish, laser technology type, copper compatibility, etc. As the manufacturing process is complex, it’s best to consult professional HDI PCB manufacturers, like Hemeixin, for a quote and design guide. We hope this article helped!