Manufacturability of HDI PCB Boards: PCB Materials and Specifications

Usman Zaka Monday, October 17, 2022 Content Leave a comment

The ability to manufacture your own HDI blind via printed circuit board (PCB) when you need one is often indispensable and crucial to the success of many projects and prototypes. All it takes is a bit of technical know-how and resources. There are numerous materials and tools at your disposal, but you have to select the right ones; otherwise, the whole process of creating your own HDI PCB can be challenging and even turn out to be a waste of time. This guide will discuss the manufacturability of High-Density Interconnect printed circuit boards based on the PCB material and its specifications.

If you are involved in running a successful HDI PCB manufacturing and assembly, then the following information will be highly beneficial for you. By understanding and adhering to best practices in HDI PCB manufacturing, you will likely benefit from improved results during each stage, from the initial design through to final assembly.

Manufacturability of HDI PCB Boards: PCB Materials

The application and physical properties are the essential things to consider when choosing a PCB material. Each has its pros and cons, and it is vital to understand how they will be used to make an informed decision.

There are three main types of HDI PCB materials: FR PCB, CEM-1 and CEM-3. Each has its characteristics that make it suitable for different applications.

FR-4

FR-4 is a glass epoxy laminate material – the most common material for PCBs. The name comes from the fact that it is flame resistant but not fireproof. FR-4 is highly durable and versatile, making it suitable for many industrial and consumer electronics applications. It also has excellent EMI performance, making it ideal for high-speed circuits such as those in telecommunications equipment.

CEM-1

CEM-1 is similar to FR-4 except that instead of glass fibre insulation, CEM-1 uses ceramic fibre insulation. This results in increased heat resistance and better thermal conductivity than FR-4, making it more suitable for high-power circuits such as power supplies or motors. CEM-1 boards are often used in automotive applications where high temperatures may occur regularly during operation.

CEM-3

CEM-3 is a newer type of HDI board that is less expensive than CEM-1, but has almost identical performance. It is also more flexible than CEM-1 and can be used for applications that require tight bends or high flexing. CEM-3 does not have the same thermal conductivity as CEM-1 but still performs well.

Manufacturability of HDI PCB Boards: PCB Specifications

If you want to manufacture HDI PCBs, you will need to make sure that your design meets the essential requirements specified below:

Copper Thickness

The thickness of the copper layer will determine how well your signal travels through the board. Ideally, you would also want at least 5 oz/ft2 of copper thickness for signal integrity purposes and better thermal performance. This means that most HDI boards are designed with a minimum thickness of 6 oz/ft2 or more. If you are designing an HDI board with less than 6 oz/ft2 copper thickness, then chances are it will not perform as desired.

Number of Circuit Layers

There is no hard rule when designing an HDI board with multiple circuit layers. However, plan on designing a four-layer HDI board with 2oz/ft2 copper thickness. It could potentially have issues with signal integrity and thermal performance due to a lack of thermal vias (heatsinks) on certain layers, such as top or bottom layer traces which can efficiently dissipate heat build-up during operation. Therefore, it is recommended to use as many copper layers as possible to reduce the amount of vertical crosstalk.

Trace and Space

The trace and space specifications for HDI boards are different from other PCBs because of their construction. Traces on HDI boards must be wider than traces on standard PCBs to accommodate for their thickness; however, if they are too broad, they will not fit into their slots during reflow soldering because they would not be able to bend enough due to their thickness. Therefore, you must get this right when designing your HDI boards.

Ideally, you would want a trace width of 6 mils or less for signal integrity purposes so that signals can travel through without any impedance issues or other problems. You should also ensure that the spacing between traces is at least 10 mils to ensure that there are no shorts between traces on the board.

Via Placement

Vias are used to connect different layers on an HDI board. If you use too many vias in your design, chances are it will significantly affect the board’s performance. The best practice is to use vias only when necessary and make sure that they are placed in areas where there is no trace routing, such as for grounding purposes.

Impedance Control

In HDI boards, impedance control is crucial since it provides sufficient signal integrity and prevents EMI/RFI interference. To ensure that the impedance of your HDI board is within the range specified by the standard, you will need to ensure that its characteristic impedance is controlled at around 50 Ohms. Ideally, you would want it to be approximately 50 Ohms +/- 1 Ohm. This is because if the impedance value is too high or too low, it could negatively affect the performance of your board. This is achieved by ensuring no impediments between traces of the same layer.

Impedance control also ensures that traces on different layers do not cross paths, which could potentially cause signal degradation. To achieve impedance control, you must ensure each layer has its traces and avoid crossing them with other layers as much as possible.

Conclusion

HDI technology is a versatile and practical manufacturing process for PCBs that offers excellent electrical performance, and high-density interconnects. When manufacturing HDI boards, engineers and designers need to consider many factors. There are rules for length, width, and spacing between signals, board thickness limitations, and the use of surface mount components. But with careful design and proper PCB materials, these limitations can be overcome to create high-quality HDI buried via boards. To learn more about HDI PCBs, check out Hemeixin.