Electronic devices can be complicated because different parts need to interact perfectly with each other to function correctly. There are many things that can cause design issues — some are very obvious, but there are other ones hidden inside your components and PCB. If you are not experienced enough, attempting to complete a high-density circuit like HDI PCB as your first project can be even more daunting.
HDI PCB refers to the process by which multiple fine layers of copper traces are snapped together on both sides of the board without glue, tape, or solder. These PCBs are commonly found in aerospace and defense industries because they are more robust than traditional fiberglass boards and can be stacked efficiently in complex systems. HDI PCBs also provide better electrical insulation, lower inductance, and withstand electromagnetic interference (EMI) from outside sources.
For you to resolve any complications — it is vital that you fully understand the implications of an HDI PCB. Here, we have compiled all the best practices for constructing and designing a high-density board into this single, easy-to-follow post. This blog post will teach you everything you need to know about HDI PCBs.
A Handy Guide to Achieve a Perfect HDI PCB
Adopt a DFM Strategy
Design for manufacturing (DFM) refers to designing electronic products with an understanding of their intended manufacturing process. This helps reduce unnecessary costs and time spent on production, resulting in more efficient production. The importance of DFM is especially apparent in HDI PCBs. If you want to ensure that your board manufacturing process is 100% efficient and effective, then you must follow a DFM strategy. This will help you eliminate all potential issues and increase the quality of your product.
Choose the Type of Via
The choice of vias plays an essential role in determining how well an HDI board performs. The most common types of vias are solid vias, blind vias, and buried vias. Solid vias connect two layers of metal on a single layer board. Blind vias connect two layers of metal on different sheets or plies within a multilayer board. Buried vias connect two layers of metal on separate sheets or plies within a multilayer board.
In general, solid vias are preferred over buried vias because they offer better mechanical strength than buried vias, and they are cheaper than blind via types due to their simplicity and ease of manufacturing process.
Do Not Overpopulate Components
HDI PCBs are highly effective in reducing the electronics’ size and improving the devices’ performance. This is because they are made up of fewer layers, making them more compact compared to traditional PCBs. However, designers need to keep a few things in mind while designing their HDI circuit boards.
The first thing that you need to do is make sure that you do not overpopulate your design with components. This is because it will increase the parasitic inductance and capacitance, which in turn will decrease the bandwidth and cause signal integrity issues on your board. You should also avoid placing any high-frequency switching circuits near low-frequency analog circuitry. This is because it can lead to crosstalk between signals on different layers of your board, affecting its performance significantly.
Choose a Stackup Design
The stackup design of a high-density interconnects (HDI) printed circuit board is essential in determining whether the board will meet its performance requirements. The stackup design is the arrangement of layers in a multilayer PCB, which defines the electrical properties of each layer and how they interact with one another.
For example, if you have multiple ground planes on a board and connect them, this can cause crosstalk between signal traces and cause signal integrity issues. An adequately designed HDI stackup can help avoid these issues before they become problems.
There are two approaches to designing a stackup: top-down and bottom-up. In a top-down approach, the designer selects the number of layers and then chooses the thicknesses of each layer. In a bottom-up approach, the designer makes decisions about thickness based on placement requirements for components.
Pay Attention to Board Thickness
The selection of a proper PCB thickness is one of the most critical factors in achieving a successful HDI PCB. It can determine whether the single layer flex pcb design can provide adequate mechanical and thermal performance.
A good starting point is to select a board thickness equal to or greater than the minimum trace/space requirement for high current applications. If you need to reduce your PCB cost, you should consider reducing the trace/space requirements as much as possible within reason. However, there are situations where reducing the trace/space requirements may not be possible due to circuit complexity or other factors. Increasing the board thickness may be required to meet your design criteria in these cases.
Use Pre-Release Testing
Before releasing any product into production, it is essential to perform tests on prototypes or small batches of boards to ensure they meet all requirements specified by the customer. This helps reduce the risk of manufacturing defects or failures in the field after production.
Eliminate Unnecessary Strain Reliefs
Strain reliefs are generally required to protect wires from being damaged during installation or removal from their connectors on printed circuit boards PCBs. However, these strain reliefs add extra steps during manufacturing which can increase costs and lead to defects if not done correctly. So try eliminating them whenever possible.
Specify Your Copper Weight
The first step to achieving a perfect HDI PCB is specifying your copper weight. The choice of copper can be made based on a number of factors, including the thickness of the laminate and the amount of current being passed through the board. For example, if you are designing a high-speed application where current carrying capacity is critical, it would be wise to choose a heavier copper layer. Heavy copper layers can also support thicker laminates and withstand more thermal stress and strain than lighter layers.
Generally, the higher the copper weight, the more conductive the board will be. So if you have many high-speed signals on your board, you may want to use a higher copper weight for increased signal integrity. The copper weight can be specified as either ounce per square foot (oz/ft²) or mils (1000 micrometers). The standard copper weight for HDI boards is 2 oz/ft² or 1.6 mils.
Get the Reference Plane Right
The reference plane is the imaginary surface that all components lie on top of. It is essential because it gives you a measuring point for all your component sizes and locations. If you have a terrible reference plane, your parts will be off by some amount, which can lead to problems with assembly or operation later. Moreover, if you get the reference plane wrong at any point in your design process, it can be difficult or impossible to fix later. Reference planes should always be chosen based on your application requirements and not by default based on what manufacturers recommend or what you have seen others use in their designs.
The impedance of the HDI layer has a direct impact on the signal integrity of the board. The impedance of the HDI layer should be controlled to within 1% at all frequencies relevant to the application. It is also important that the impedance remains constant with temperature.
The impedance of an HDI layer can be controlled by adding a metallic foil or printed circuit board material to the surface of the dielectric layer. An appropriate combination of material thickness and conductivity must be chosen to achieve a desired level of control.
The thickness of a metallic foil can be used to control impedance in two ways:
This method involves printing multiple layers of material on top of each other with different thicknesses, thereby controlling the overall impedance. This approach works well for low frequencies up to 10 MHz but becomes less effective at higher frequencies due to parasitic capacitance between layers.
Adding conductive foils enables us to control impedance at any frequency as long as we ensure that each foil does not overlap itself or other foils in its vicinity.
see also What Is Rogers PCB and Rogers Material Selection Guide
In the end, figure out what you all want the target audience to experience. Try to map that out as best you can to have an effective HDI strategy. Addressing this aspect of the design process early on will make it easier for you to create a unique emotional impact on your user base. From there, it is just a matter of applying a few methods that best fit your project’s needs until you achieve a perfect flex PCB. Hope this article has been helpful in some way.
Finally, the most important thing you can do while looking for a partner to manufacture your product is to have a clear head. If you are burnt out and stressed out from the planning phase, your product may not end up as strong as it could be. If you have thoroughly thought out the engineering and design of your product, and taken specific steps to simulate customer needs and expectations, then feel confident about going into production with