What Is HDI PCB?
HDI PCB (High Density Interconnect PCB) is a circuit board with relatively high line distribution density using micro-blind and buried via technology.
This is a process that includes inner and outer layers of wire, and then uses holes and metallization in the holes to achieve the function of bonding between each inner layer.
With the development of high-density, high-precision electronic products, the requirements for circuit boards are also the same. The most effective way to increase PCB density is to reduce the number of vias and precisely place blind and buried vias to meet this requirement, resulting in HDI boards.
HDI PCB Concept
HDI: High Density Interconnect Technology. It is a multilayer board made by stacking method and micro-blind buried vias.
Microvias: In PCBs, holes with a diameter of less than 6 mils (150 μm) are called buried vias: they are buried in the inner layer of the hole and are not visible in the finished product. They are mainly used for the conduction of inner layer wires, which can reduce signal interference. probability, and maintaining continuity in the characteristic impedance of the transmission line.
Since the buried vias do not occupy the surface area of the PCB, more components can be placed on the surface of the PCB.
Blind vias: Connect surface and inner layers without passing through complete vias.
How to Set up HDI PCB Layout
If you are working with your first HDI layout, it can be difficult to see which design rules need to be set when starting the PCB layout.
Set up HDI PCB Layout
For HDI PCBs, there is very little that differentiates these products from standard PCBs except for component and routing density.
An HDI board is anything that has 10 million vias or less, 6 million traces or less, or has a pin spacing of 0.5 mm or less. Manufacturers will tell you that HDI PCBs use blind vias of about 8 mils or smaller, smaller blind vias are laser drilled.
In some ways, they’re both true because there are no specific thresholds for what constitutes an HDI PCB layout. Everyone can agree that once a design contains microwells, it’s an HDI board.
In terms of design, you need to set certain design rules before you can touch the layout. Before establishing design rules, you should gather the manufacturer’s capabilities. Once this is done, you need to set up design rules and some layout features
Trace width and via size. The width of a trace along with its impedance and trace width will determine when you enter the HDI system. Once the trace widths become small enough, the vias will also become so small that they must be fabricated as microvias.
Layer transitions. Vias need to be carefully designed based on the aspect ratio, which also depends on the desired layer thickness. Layer transitions should be defined early so they can be placed quickly during the routing process.
gap. Traces must be separated from each other and from other objects (pads, components, planes, etc.) that are not part of the net. The goal here is to ensure compliance with HDI DFM rules and prevent excessive crosstalk.
Other trace limitations, such as trace length adjustments, maximum trace lengths, and allowable impedance deviations during traces are also important, but they will apply beyond HDI boards. Here, the two most important points are via size and trace width. Clearance can be determined in a variety of ways (eg, simulation) or by following standard rules of thumb. Be careful with the latter, as this can lead to excessive interlayer crosstalk or insufficient routing density.
Stackups and Vias
HDI stacks can range from a few to dozens of layers to suit the desired routing density. Boards with high pin count fine pitch BGAs can have hundreds of connections per quadrant, so vias are required when creating layer stacks for HDI PCB layouts.
If you look at the layer stack manager in your PCB design software, it may not be possible to clearly define a specific layer transition as a microvia. That’s okay; you can still set layer transitions and then set via size limits in Design Rules.
This ability to refer to microchannels as microwells is very useful once you have set up the setup rules and created templates. To set up design rules for routing through vias, you can define the design rules to apply only to microvias. This allows you to set specific limits for the clearance by pad size and hole diameter.
Before starting to set design rules, the manufacturer should be consulted about its capabilities. The trace width then needs to be set in the design rules to ensure that the trace impedance is controlled to the desired value. In other cases, impedance control is not required and you may still want to limit the trace width on the HDI board to keep the routing density high.
Trace Width
You can determine the required trace width in several ways.
First, for impedance-controlled routing, you need one of the following tools:
Calculate the required trace size with pen and paper (the hard way)
Online calculator (quick method)
Field solver integrated into your design and layout tools (most accurate method)
The downsides of doing trace impedance calculations with trace calculators, and the same point applies when sizing traces for HDI PCB layouts.
To set the trace width, you can define it as a constraint in the design rule editor, just like with the via size. If you don’t worry about impedance control, you can set any width. Otherwise, you need to determine the impedance curve of the PCB stackup and enter this specific width as a design rule.
Since the trace width cannot be too large for the size of the via pad, you need to exercise careful balancing. If the impedance-controlled trace width is too large, the laminate thickness should be reduced, as this will force the trace width to be reduced, or the pad size can be increased. As long as the dimensions of the platform exceed the values listed in the IPC standard, it is fine from a reliability standpoint.
Gaps
After completing the two key tasks shown above, you need to determine the proper trace clearance. Unfortunately, the spacing between traces should not default to the 3W or 3H rules of thumb, as these rules are incorrectly applied to advanced boards with high speed signals. Instead, it’s a good idea to run a crosstalk simulation for the proposed trace width and check if too much crosstalk is created.
5 Design Tips for Efficient Manufacturing of HDI PCBs
HDI PCB Design Tips for Electronic Manufacturing
PCB layout design can be very complex, requiring designers to make difficult decisions about assigning the most important specifications.
The process is more complex if the design is for a critical systems industry, such as aerospace, medical device, automotive or vehicle manufacturing.
Regardless of the type of board design, designers use a combination of Design for Manufacturing (DFM) strategies for the benefits of PCB development and coordination with their CM capabilities.
DFM is not universal. This is a set of rules and guidelines for a specific manufacturing stage, such as Design for Assembly (DFA) and Design for Testability (DFT).
DFM can also specialize in specific board design types such as HDI. Let’s look at some important design tips aimed at optimizing the manufacture of HDI PCB electronics.
Choose via Type to Minimize Process Complexity
Via selection is a critical decision that not only determines the equipment and manufacturing steps required, but also affects processing time and additional cost.
The use of blind or buried vias can help reduce layer count and material cost; however, the choice of whether to use in-pad, dog-bone or near-pad vias affects process complexity.
Select the Minimum Number of Components to Apply HDI
The choice of components is always important. However, component selection optimization is more important for HDI boards. HDI designed components determine the drilled and stacked trace width, location, type and size.
Obviously, performance is the primary consideration, but packaging, traceability and availability should also be considered. Having to replace components or redesign the layout balloons additional manufacturing time and material costs.
Space Components to Minimize Stress and EMI
When components are placed such that the via positions are distributed asymmetrically, uneven stress can be applied to the board, which can lead to warpage.
This can seriously affect yield, the number of boards that can be used per panel. If components are spaced apart from dense high-power components, the signal can introduce electromagnetic interference (EMI) in the traces, affecting signal quality. Additionally, parasitic capacitance and/or inductance from nearby pins or pads may affect signal quality.
Therefore, it is recommended to include EMI modeling during design to extract parasitics.
Routing Traces to Minimize Signal Integrity Issues
One of the advantages of HDI is the ability to use smaller trace widths for signal propagation. Although the trace width is reduced, it should be designed for optimum width signal integrity. This includes using the shortest trace lengths, consistent path impedance, adequate ground planes, and digital, analog, and power signal isolation.
Choice of Stacking to Minimize Material Cost
In addition to the choice of vias, the choice of PCB stack-up also has a significant impact on the manufacturing cost of HDI PCB electronics. The type of material and the number of layers directly affects the number of lamination and drilling cycles required. Cost should be one of the deciding factors when making these decisions.
Following the tips above for HDI PCB electronics manufacturing will help your CM make the process as efficient as possible. However, making HDI boards is not done once and for all. To learn how to integrate design and manufacturing, check out the following DFM for HDI case study.
The Biggest Advantage of HDI PCB
They are a great solution when you need to reduce size and weight, and still need the functionality and reliability of the product.
One of the other advantages of these boards is that they use solder-in-pad technology and blind via technology. This allows components to be placed together, reducing the length of signal paths, which helps provide faster, more reliable signals because those paths are shorter.
Even though these boards can offer many advantages, you will still find them very affordable. It’s a cost-effective solution for those looking for a reliable and durable option for their electronics.
Still, to decide if they’re right for you, it’s important to learn more about them and how to use them. You also need to understand the difference between regular printed circuit boards and HDI printed circuit boards.
Where Are HDI PCBs Used Today?
Because of the benefits they provide, you will find HDI PCBs utilized in a variety of electronic devices in many different industries. The medical industry is one of the most well-known industries. Medical devices manufactured today often need to be smaller.
Whether it’s a piece of equipment in the lab or an implant, the smaller size is often the better option, and HDI PCBs can be of great help here. Pacemakers are a good example of the type of products that use these types of PCBs.
Many types of monitoring and exploration equipment, such as endoscopes or colonoscopes, use this technique. Again, in these cases, smaller is the better choice.
In addition to the healthcare sector, the automotive industry also uses HDI PCBs. To maximize the available space in the car, they are making certain electronic components smaller. Of course, tablets and smartphones use this technology. That’s why many of these devices are getting thinner and thinner as they go.
You will also find HDI PCBs for aerospace and military applications. Their reliability and small size allow them to be used in many different applications. In the future, more and more devices from different fields may use this technology.
What Is the Main Difference between PCB and HDI PCB?
HDI PCBs have denser components per square inch
These boards end up being smaller and lighter than typical PCBs
They utilize laser direct drilling, whereas standard PCBs are usually mechanically drilled
Layers and Aspect Ratios Tend to Decrease
These are some of the differences between HDI PCB and regular PCB. However, that doesn’t mean the HDI option is always the right choice for you. Take the time to think about the project you’re working on, and then decide which option best suits your needs.
Properly Design HDI PCB
While using one of these types of PCBs may seem like the perfect solution for your needs, it’s important to know how to design them properly if you want to realize all the benefits they can convey.
For these types of designs, if possible, you will want to ensure that no more than three layers are used in the sequence. Ideally, the number of sequential laminations should be low.
Please take some time to think carefully about the design, then test it with the Gerber file viewer in the software to make sure it works. Once you’re sure it works, you might want to get a prototype to make sure it works, and then move on to full-scale manufacturing.
About Wonderful
Shenzhen Wonderful Technology Co., Ltd. is China’s top model and small batch circuit board manufacturer.
The company targets precision circuit board samples and small batches as the main target market. Since its establishment, it has adhered to the development concept of good quality and important reputation.
Invest huge sums of money to introduce advanced equipment, strictly regulate the operating standards and QC standards for incoming and outgoing materials, to ensure that the shipped products reach Seiko quality.
The self-developed ERP management system can realize the whole process of paperless operations such as online ordering, online payment, online production progress query, online logistics tracking, and online statistical report production, which greatly improves work efficiency.
It ensures that each order can be shipped quickly. After more than ten years of development, the company’s products and services have been well received by the market.
As well as the opening of an electronic component mall, integrated circuits, semiconductors, resistors, capacitors and other products are sold.
The company’s scale is developing rapidly, and the sample and small batch shipment capacity reaches 500 models/day.
The products are widely used in communications, medical equipment, industrial control products, aviation, military products, testing instruments,Automotive, computers and other peripheral products.
We will continue to pay attention to customers, with efficient delivery speed and perfect after-sales service, to become the most trusted brand supplier in the PCB industry.
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