High-density interconnect, or HDI, circuit boards are printed circuit boards with a higher wiring density per unit area than traditional printed circuit boards (PCB). In general, HDI PCBs are defined as PCBs with one or all of the following: microvias; blind vias, buried vias or other microvia technique; built-up laminations and high signal performance considerations. Printed circuit board technology has been evolving with changing technology that calls for smaller and faster products. HDI products are more compact and have smaller vias, pads, and lines and spaces. As a result, HDIs have denser wiring, which means lighter, compact, lower layer count PCBs. Rather than using a few PCBs in a device, one HDI board can house the functionality of the previous boards used.
Microvias play a big role in the size and capabilities of HDI boards.
Smaller vias can be closer together, leading to a smaller board or freeing up space for additional components. Microvias have smaller aspect ratios than through-hole vias, and provide greater reliability for HDI boards than other vias.
As the semiconductor and electronics market called for smaller, but higher function products, HDI found its place in design engineers’ arsenal. Shortening the distance between devices, or trace space, and using a large number of transistors translated to better performance in electronics.
The compact HDI boards provide better electrical performance and lower power consumption while still maintaining close spacing between components.
Signal integrity is improved due to the shorter distance connections and lower power requirements. Other performance improvements over conventional PCBs include stable voltage rail, minimal stubs, lower RFI/EMI, and closer ground planes and distributed capacitance.
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As covered above, HDI PCBs are small, compact, and have great electrical performance. Here are additional advantages of HDI PCBs.
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When properly planned out, overall costs are reduced due to the lower number of necessary layers and smaller sizes compared to standard PCBs.
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Design efficiencies in HDI production mean faster time-to-market. Because of the easy placement of components and vias and electrical performance, it takes a shorter amount of time to go through the design and testing process for the HDIs.
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Microvias have much better reliability than typical through holes due to the use of better materials and a smaller aspect ratio. Since microvias are more dependable than through holes, the HDIs perform better with better materials and parts.
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Though HDIs are extremely useful when implemented in small devices, be aware of the electronic product’s needs. Have some information on hand before designing your HDI board.
Careful planning and consideration into the design and manufacturing process are crucial to ensuring exactly what you need is the result. It is very difficult ult to change the design and parts once the board is already being made. Having an incomplete plan for what the HDI board(s) should do and what components it needs will slow down the process. Constant changes to hardware already in production can cost more than having a properly planned out board that is made once.
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Contact your fabrication and assembly shop ahead of time for help and to see if what you are designing is within their capabilities.
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There are several types of HDI stack-ups, or classes, that you may want to use for your PCB design.
0-N-0 with laser microvias:
In this stack-up, the manufacturing steps are as follows:
- Core is laminated
- Core is mechanically drilled
- Mechanical drill is plated
- Laser drilled vias are formed
- Final through-hole via is formed
1-N-1 with laser microvia and mechanical buried core via:
In a 1-N-1 stack-up, the ‘1’ represents one sequential lamination on either side of the core. 1 sequential lamination adds two copper layers for a total of 4 layers and there are no stacked vias.
In this stack-up, the manufacturing steps are as follows:
- Core is laminated
- Core is mechanically drilled
- Mechanical drill is plated
- Inner layer is created
- Sequential lamination adds two additional layers
- Mechanical drill is now a buried via
- Laser drilled vias are formed
- Final through-hole via is formed
1-N-1 with microvia stacked on top of buried and filled core via:
In a 2-N-2 stack-up, the ‘2’ represents two sequential laminations.
As stated above, 1 sequential lamination adds two copper layers, so 2 sequential laminations adds 4 copper layers for a total of 6 layers. Again, there are no stacked vias.
2-N-2 with stacked microvias and buried core via:
This stack-up is similar to the one above, except in this case there are stacked microvias.
Because of the stacked microvias, a couple things need to happen: the microvia needs to be plated with copper and planarized flat.
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When designing HDI PCBs, you need to take the cost of multiple parts based on your needs into consideration. Different aspects and parts you want to include will change the cost of the board. This will also depend on the prices of your PCB manufacturer and assembler.
The type of via, through-hole or microvia, and the amount of these vias in your board can change costs. A smaller via will cost more than a larger via. This is because the drill needs to be increasingly precise as the via size decreases.
The cost will change depending on the height of the stack-up. The designer should be aware of what via structure the board entails, including specifications for the hole fill, and what those features require in terms of processes and cost.
Determine the material and the number of layers and how many core layers the HDI board will need. The most common and most cost-effective core material is fiberglass.
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If you’re having trouble figuring out which one is the best for you, our Materials Selector tool can help you out.
The number of layers can influence how many sequential laminations you need for your board, and the more laminations, the higher the cost.
The number of different types of via structures will be the amount laminations needed for the board. Each type of via needs a lamination.
Utilizing staggered or stacked vias will also change the price. Stacked vias require laser-drilled filled copper vias, whereas staggered microvias don’t require filling with copper, which will save time and money.
Though more laminations usually mean a higher cost, using microvias and more laminations can be cost-effective and have a higher yield than producing a board that has too many layers.
When designing a board, use the minimum trace space only when needed, such as around the fine pitch areas. When the board isn’t as dense, trace space does not need to be as conservative, and trace spaces can be increased. This will increase yield during manufacturing. The size of the pads should also be determined to minimize cost.
Decide on what HDI stack-up you will use before beginning the layout. This will make it easier to avoid including extraneous parts that have not been planned out.
Depending on what you need your board to do, you may want a specific surface finish for your board. Some materials perform better under certain conditions – high heat, extreme cold, environmental factors – in the finished electronic product will be in. Some common surface finishes include ENIG, ENEPIG, hard gold, soft gold, immersion silver, white tin, HAL, and lead-free HAL.
Turntime is a time constraint that can increase or decrease the cost of your board dramatically.
Boards that need to be finished in two days will cost more than a board that has a deadline of a couple of weeks.
See our FAQ on HDI PCBs.
