2022-11-08 10:10:53 HDI Board 434 Viewed
Production Process of HDI Board
General HDI board uses metallized blind holes to connect the various circuit layers that need to be connected. The HDI board manufacturing process includes: after laminating a certain layer of copper foil, an etching process is used to process blind holes through the copper foil layer, blind holes. The diameter of the hole is generally not greater than 0.2, and then the dielectric layer under the blind hole is removed by laser ablation process to form a blind hole reaching the upper copper foil layer.
The HDI board metallizes the blind holes to realize the interconnection of the two copper foil layers; then, the build-up layers can be continued to be laminated, and the metallized blind holes can be made in the same way after the build-up layers, and the metallization can be made in the same way after the build-up layers. Blind vias can be used to realize interconnection between other layers, the high density of HDI board is mainly reflected in the points of hole, circuit, pad density and interlayer thickness.
●Micro-via. The HDI board contains micro-via designs such as blind holes, which are mainly reflected in the high requirements of the micro-hole forming technology with the diameter of the HDI board less than 150um and the cost, production efficiency and hole precision control. In traditional multilayer circuit boards, there are only through holes and no tiny buried blind holes.
● Refinement of line width and line spacing. It is mainly manifested in the increasingly stringent requirements for wire defects and wire surface roughness. Generally, the line width and line spacing of HDI board do not exceed 76.2um.
●The pad density of the HDI board is high. The density of welded joints is greater than 50 per square centimeter.
●Reduction of the thickness of the medium. It is mainly manifested in the trend that the thickness of the dielectric between the HDI board is developing to 80um and below. The HDI board has more and more stringent requirements for thickness uniformity, especially for high-density boards and packaging substrates with characteristic impedance control.
Difference Between HDI Boards and Ordinary PCB
Compared with ordinary pcb, the most notable feature of HDI board is the high wiring density. The difference between the two is mainly reflected in the following aspects.
1. The HDI Board Is Smaller and Lighter.
The HDI board is based on the traditional double-sided board as the core board, which is made by continuous lamination. This circuit board made by continuous lamination is also called a build-up multilayer (BUM). Compared with traditional circuit boards, HDI board has the advantages of "light, thin, short and small".
The electrical interconnection between the layers of the HDI board is realized through conductive through holes, buried holes and blind vias. Its structure is different from ordinary multi-layer circuit boards. Micro-buried blind holes are widely used in HDI boards. HDI board uses laser direct drilling, while standard PCBs are usually mechanically drilled, so the number of layers and aspect ratios tend to be reduced.
2. The Electrical Performance of the HDI Board Is Better.
HDI board can not only enable more miniaturization of end product designs, but also meet higher standards of electronic performance and efficiency at the same time.
The increased interconnect density of HDI board allows for enhanced signal strength and improved reliability. In addition, HDI board has better improvement for radio frequency interference, electromagnetic wave interference, electrostatic discharge, heat conduction, etc. The HDI board also adopts full digital signal process control (DSP) technology and a number of patented technologies and has a full range of adaptable load capacity and strong short-time overload capacity.
3. HDI Board Has Very High Requirements for Buried Holes and Plug Holes.
As can be seen from the above, whether it is the volume of the board or the electrical performance, HDI board is better than ordinary PCB. Every coin has two sides. The other side of HDI board is that it is manufactured as a high-end PCB. Its manufacturing threshold and process difficulty are much higher than ordinary PCBs, and there are many problems to be paid attention to during production-especially buried holes and plug holes.
At present, the core pain point and difficulty of HDI board manufacturing is the buried hole plug hole. If the HDI board buried hole plugging is not done well, there will be major quality problems, including uneven board edges, uneven dielectric thickness, and potholes on the pads.
The HDI board surface is not flat, and the line is not straight, causing the beach phenomenon in the depression, which will cause defects such as line gap and disconnection;
●The characteristic impedance will also fluctuate due to the uneven thickness of the dielectric, causing the signal to be unstable;
●The unevenness of the pad makes the subsequent packaging quality poor, resulting in the associated loss of components.
Advantages of HDI Board
Advances in HDI board technology have given engineers unprecedented design freedom and flexibility to place more components on both sides of the original PCB as needed, while allowing smaller components to be placed together. This means that HDI board ultimately results in faster signal transmission as well as enhanced signal quality.
HDI board relies on lasers rather than mechanical drilling. In turn, HDI board designs allow for smaller apertures and smaller pad sizes. Reducing the aperture allows the design team to increase the layout of the HDI board area. Shortening electrical paths and enabling denser trace routing improves the signal integrity of the design and speeds up signal processing.
HDI Board Design
HDI board is designed to maximize surface component density, providing a breakout solution for ICs with a large number of closely spaced pins or pads, and boards that propagate high frequency signals. The aim is to provide greater functionality in a smaller package.
Instead of using through holes, HDI board design uses blind and buried vias. Staggered and accurate placement of buried and blind vias reduces mechanical stress on the board and prevents any chance of warping.
Smart Component Selection
Typically, HDI board contains SMD and BGA (≤0.65 mm) components. The spacing/spacing between pins should be chosen wisely as this helps define trace width, via type and PCB stack-up.
Use of Micropores
Using micro-hole or continuous stacking techniques (<0.15mm), it can help save more area during the hdi board design process. The low inductance of the microvia makes it suitable for high-speed applications, decoupling capacitors to connect power planes, and where noise reduction is required.
Material selection is important for every PCB design. But it must be said here that it is more important for HDI board. The designer's goal is to select materials suitable for fabrication while meeting temperature and electrical requirements. The physical thickness of the material is important when considering the aspect ratio of the microvia to be plated.
Different HDI board layer materials have different CTE values and moisture absorption rates, which leads layered PCB designers to avoid this by using the same material for each layer, or a material with the same CTE value.
Functional or JTAG test methods are used for HDI board designs, not ICT. ICT is decisive, but requires a comprehensive nodal analysis.
For better design management, please refer to IPC-2226, which covers thermal issues. Because of the high component density of HDI board circuits, designers need to pay more attention to thermal issues. Thinner dielectrics work with microvias to help dissipate heat. To maximize heat dissipation, consider adding thermal vias.
Wiring Requirements and Substrate Capacity
Routing requirements are the total length of connections required to connect all components in a hdi board circuit. Baseboard capacity is the available wiring length to connect all components. The substrate capacity should be greater than the routing requirements so that there is enough capacity for routing to complete the design with minimal cost.
Hdi Board Density
Calculate the hdi board density of the design to measure the complexity of the design. HDI board (Wd) density, measured as the average trace length per square inch (including all signal layers).