If you set your mind on using a large dense printed circuit board, it is time to talk about the specifics of its layer stack design.
In this article, we will focus on layer stackup for HDI PCB. You will read everything you should know about different standards and design preferences. That will ensure a flawless PCB design that will perfectly fit your preferred application.
So, don’t waste any time, and begin reading to learn more about the topic!
1、HDI PCB--What Is a Large Dense PCB?
Technology is making huge progress every day, and the printed circuit board industry is no exception. One of the biggest leaps happened about two decades ago when the High-Density Interconnection (HDI) technology was implemented.
It is that technology that made large dense PCBs possible. Unlike traditional boards which utilize mechanical drilling, HDI boards take advantage of the laser drilling technology. They were the obvious next step for the industry, and their popularity is rising even today.
You will find large dense PCBs across many industries, including but not limited to consumer electronics and handheld products.
Image 1: Printing circuit board manufacturing - SMT line
2、HDI PCB--Advantages of Using a Large Dense PCB
First, let’s make an overview of some benefits that come from using a large dense PCB for your application:
• You can populate the board on both sides, which allows you to fit more components while keeping a small board size.
• The board will probably be considerably lighter than the traditional PCB, although that depends on design and manufacturing.
• HDI boards are capable of delivering improved signal quality and fast transmission.
• Long-lasting and reliable boards that use a modest amount of power.
These specifics make large dense boards extremely versatile. It doesn’t come as a surprise that users throughout different industries are choosing them over traditional boards. Many users emphasize that the reliability in even the most demanding settings is why they stick to using HDI PCBs.
Image 2: 3D illustration of an electronic PCB
3、Why Is the Design of a Layer Stackup for HDI PCBs Important?
Image 3: PCB design concept
If you have any experience with ordering printed circuit boards, you know that the design process is equally important as the fabrication itself. Both methods need to keep in mind the final application, usability, and reliability of the board.
However, it is also vital to note that the designer should note manufacturing capabilities. In other words, the design shouldn’t only be perfect for the application, but feasible to manufacture.
If you keep in mind the standards set in the industry, most HDI PCBs use a Type III stackup design. That approach allows utilization of multiple micro-via layers on board’s sides. You do not have to place them on both sides of the PCB, but it is an option that is perfectly suitable for dense boards.
3.1. Designer Tips for HDI PCBs
Here are some tips that you can use when designing a large dense PCB:
• Many designers decide to stack or stagger micro-vias by positioning them optimally compared to one another but also compared to other vias in the board.
• The layers on the outside can serve as GND planes as that can enhance requirements related to EMC/EMI. The inner layers can then serve for micro-vias and power planes that would be used for signal routing. It is an approach that works particularly well if your board has eight or more layers.
• When it comes to routing a board that has multiple layers, stacking vias may be more costly, but it will ensure better efficiency and flexibility for routing the PCB.
OurPCB is a company with over a decade of experience in manufacturing different types of printed circuit boards. Their exceptional manufacturing capabilities are the reason why they constantly exceed the expectations of customers throughout the world. If you have any question or need to assemble a PCB quickly, don’t hesitate to get in touch with this company.
4、HDI PCB--How Can Stackup Design Affect Signal Integrity?
Image 4: A woman holding a printed circuit board
You don’t have to be an expert to understand that a layer stack for HDI PCB can affect the signal integrity of the board. It is the reason why stackup design needs to be a primary consideration during the entire design process. That is something the designers understand, and they are ready to help the clients find the optimal solution for their desired application.
The crucial thing to keep in mind is that the design process needs to match the fabrication possibilities. In some cases, the options for fabrication may not respond to design requests in terms of selected material, copper thickness, or trace width. The fabrication possibilities of a manufacturer may not be able to respond to all those requests. That's why you need to make some changes.
In those situations, a reliable company will go the extra mile to secure an optimal replacement for your board. That means, for example, finding the right material that will have the same properties as the one initially chosen.
During the entire fabrication process, the manufacturer may need to make other adjustments, too. That is why it is crucial to pick a reliable company with a skilled team of experts. They will be able to calculate all tolerances so that the adjustments do not affect the suitability of the board for your application.
5、HDI PCB--Stackup Design Standard
If we are talking about design standards, we should mention that there are three approaches the manufacturers commonly use. The first one involves utilizing through vias for standard lamination. It is a solution that used to be popular due to its reduced cost and improved reliability.
Although most manufacturers are well familiar with the process, they do not utilize this approach often these days. That is because it has a restricted layer count, and many clients are demanding a higher number of layers than they can add with this approach.
One of the alternatives in stackup design is to utilize buried and blind vias and sequential lamination. That approach allows to reduce sizes of the holes and works well for routing capability of the board. However, trace widths remain the problem, which is why this design has never become famous.
5.1. HDI PCB Stackup Design
HDI PCB technology utilizes micro vias. In this approach, the manufacturers utilize laser beams, which result in vias that have a tiny diameter. It is a revolutionary technique that guarantees an extremely compact size of both traces and vias. Thanks to that, you can decrease the number of layers in a board while securing impressive routing density.
The manufacturers choose to stagger or stack these tiny vias. It is this method that makes it possible to design while using several big Ball Grid Arrays (BGA) that have a pitch of no more than 0.8mm.
You shouldn’t worry about the cost of the HDI PCB approach because the industry experts have done everything in their power to optimize it. Apart from excellent value for money, you can also benefit from enhanced power and signal integrity.
Image 5: Printed circuit board design
6、How to Improve Routing in Layer Stackup for HDI PCB
Image 6: A fragment of the printed circuit board
For a designer, the primary consideration when optimizing routing ability should be the placement of vias in comparison to the BGA pads. A smart placement can be significant for enhanced routing since HDI PCB utilize BGAs of the fine pitch.
Some designers decide to go with a dog-bone pattern, which involves placing vias adjacent to the pads. If a designer wants to pursue the biggest density possible, it would be wise to use the via-in-pad approach. This design can be partial and offset, but the latter secures improved routing density.
The reason why this is important is that it will subsequently help to decrease the overall layer number, which will decrease the total manufacturing price.
Additionally, the location of the ground and power planes may determine the integrity of both signal and power. The usual practice is to place the GND place to the outside layer because it optimizes EMI shield. Depending on the requirements, the designer can also identify adjacent layers to that one, and add some VCC to them.
It is a tip that can also bring a capacity coupling of Power and GND layers to the next level, which will minimize bypass capacitors that a BGA require. In this case, you can also utilize bypass capacitors and embedded pull-up resistors to open up extra routing room on layers used for signal purposes. These are only some of the tricks that can optimize return paths and minimize crosstalk.
If you need to distribute power to a big BGA, it may demand more than a single voltage supply. That is when you should consider using either a layer dedicated to voltage or split planes.
The article only confirmed what we already knew – the design of a layer stackup for HDI PCB is extremely important. The designers should keep different factors in mind, but as long as everything is done right, you will get a reliable, high-performing, and compact-sized printed circuit board. Apart from the design, it is also crucial to find the right manufacturer to turn that design into reality while maintaining PCB reliability. The only thing you should do is to register on the OurPCB website and place an order. The desired boards will arrive at your doorstep in the shortest possible time-frame!