What Is HDI PCB?
High Density Interconnect (HDI) PCBs represent one of the fastest growing technologies in PCBA processing.
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: 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 (150μm) 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 Via: Connects the surface layer and the inner layer without passing through a complete through hole.
process
High-density interconnect technology can be divided into
First order process: 1 + N + 1;
Second order process: 2 + N + 2;
Third-order process: 3 + N + 3.
How to Set Up HDI PCB Layout
HDI PCB layout can be very cramped, but the right set of design rules will help you design successfully.
More advanced PCBs pack more functionality into less space, often using custom ICs/SoCs, higher layer counts, and smaller traces.
Properly laying out these designs requires a powerful set of rule-driven design tools that can check routing and placement against design rules as the PCB is created. 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.
I’ve seen designers point out that an HDI board means anything with 10 million vias or less, 6 million traces or less, or anything with 0.5 mm or less pin spacing. Your manufacturer 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 the trace 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 decrease, 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.
Gap
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.
Automotive HDI PCB Classification and Application
The rapid development of the electronics industry has promoted the rapid development of many industries. In recent years, electronic products have become more and more widely used in the automotive industry.
The traditional auto industry has more efforts in mechanics, power, hydraulics and transmission. However, the modern automotive industry relies more on electronic applications that play an increasingly important and potential role in automobiles. Automatic electrification is all about processing, sensing, information transfer and recording, something that would never be possible with a PCB.
Due to the needs of automobile modernization and digitization, as well as people’s requirements for automobile safety, comfort, simple operation and digitization, PCBs have been widely used in the automotive industry, from ordinary single-layer PCBs, double-layer PCBs to complex multi-layer PCBs. PCB or High Density Interconnect (HDI) PCB, possibly with cross-layer blind vias or dual build layers.
To achieve high reliability and safety of automotive HDI PCBs, strict policies and measures must be followed by HDI PCB manufacturers, which will be the focus of this article.
How to Avoid Problems in HDI PCB Design?
Higher densities mean that HDI designers must always keep factors such as layout and spacing in mind. Here are four tips to avoid common HDI PCB design mistakes
Put Space First
Considering space during the HDI-PCB design process goes beyond compressing as many components as possible. First, the designer must ensure that the board will be properly maintained in the future.
Determining the amount of space between specific parts and choosing additional space is a requirement. This can also make things simpler in the PCB manufacturing process. Via diameter, pad diameter, and trace width should be considered before starting implementation.
Otherwise, the PCB may have to be completely redesigned, which requires more time, money and effort.
Understand the Meaning of Blind Vias and Buried Vias
To make full use of the limited space, HDI-PCB integrates blind and buried vias. Blind vias connect the outer and inner layers but do not pass through the entire board. Buried vias connect multiple inner layers but do not pass through the outer layers.
Blind vias and buried vias are different from traditional vias in terms of practicality and practicality, and have a great impact on the overall design of the PCB. Designers must keep vias in mind because they not only affect how the board is constructed, but can also affect signal performance depending on where and where it is located.
Avoid Solder Related Problems
Solder can cause multiple HDI-PCB problems if the designer is not careful. For example, vias in pads can draw solder away from certain components, resulting in poor quality connections. One solution to this problem is to avoid using vias in the pads. Unfortunately, depending on the design of the HDI PCB and its purpose, this is sometimes not possible.
In this case, covering the pads with vias and solder mask eliminates the problem. However, using a solder mask can lead to its own complications. Solder masks with large openings can allow a lot of solder to reach the board, causing logical deletions or broken pins. Before the design begins, it is critical for the designer to monitor for potential solder problems.
Include in Design Review
This applies to all PCB design projects, but deserves special attention in HDI. It can be difficult for designers to check their work and spot small mistakes. This becomes doubly difficult when designers use higher density components.
Combining advice and perspectives from other designers, technologists, etc. is a strategic way to correct small mistakes before they become bigger, more costly problems. Skipping design reviews to meet tight deadlines can save some immediate time, but in the end, reviews are the ideal way to save time and money.
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.
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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.
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