HDI technology is in accordance with the laminate -> multilayer board -> incremental multilayer board -> high-density circuit board Roadmap technology path development.
PCB boards, which started around the 1960s, are usually plated through-hole boards produced from resin substrates. Because of its fast production replication, small size and low unit cost replaced the early wire assembly products.
Gradually, with the miniaturization of electronic products and multi-function, the distance to the IC component contacts, and signal transmission speed requirements are increasingly high, double-sided panels have been unable to cope with the complex signal and stability requirements, so the circuit board gradually developed multilayer circuit boards. The use of a multilayer circuit board laminated to meet the performance requirements of the board is called a multilayer board.
The advent of the 4G, and 5G eras, so that the current electronic equipment must have high-frequency demand, and the corresponding circuit boards need to have the characteristic impedance control, high-frequency transmission, low radiation interference, and other performance, which requires the use of low dielectric coefficient and low attenuation rate of insulation materials.
In addition, with the miniaturization of electronic equipment, components are also compact and miniaturized, followed by the development of BGA, CSP, DCA, and other parts, which makes the board pushed to an unprecedented high-density situation. And in order to meet the high-density requirements, the use of micro via structure (less than 150um diameter holes for microtia) to improve assembly and space utilization. This type of circuit board, is called an incremental multilayer board.
When the communication frequency increases again, the multi-layer type multilayer board cannot meet the demand. In order to increase the link density of components, geometrically compressed lines, and connection point space, or stacking multiple components in the same location in order to improve the density of the assembly. This technology is HDI technology, i.e. high-density circuit board technology.
At present, the main application scenarios of HDI technology include: carrier and intermediary boards, modules, portable products, high-performance demand products.
(1) carrier board and intermediary board carrier board and intermediary board technology is used in the application of cladding or tying, micro-hole design allows very high-density cladding area to build out an array configuration of contacts and winding.
HDI technology can be used to build modules that allow ICs to be wire bonded, laminate assembled or TAB connected on small carrier boards, or it can be used to create detailed CSPs.
3）Portable products and miniaturized consumer products using HDI technology have a smaller profile and more detailed feature size.
4）High performance demand products
HDI boards are used in conjunction with microvia structures for the manufacture of high level generics, high I/O, and small spacing components, especially high density components.
The introduction of HDI technology brings many benefits in line configuration, component arrangement, material selection, product design and manufacturing procedures, all of which are indicators to assess whether the product is intended. Specific benefits are seen in.
1) Improved performance The typical performance improvements are mainly
(a) present a relatively low conductive hole parasitic electrical noise.
(b) to minimize the connection holes and line branching structure.
(c) have a stable voltage path.
(d) can remove unnecessary decoupling capacitance.
(e) relatively low talk type and general noise.
(f) RFI/EMI interference is much lower.
(g) relatively close to the ground plane, relatively close to the distribution of electrical capacity.
(h) surface ground plane with holes in the pad structure, can block the radiation effect.
2）Importing advanced components
HDI has more obvious advantages than laminate when using 0.8mm spacing components after IC assembly. For example, for FPGA high pin products, the through-hole technology may require 20 layers on the structure, while only about 60% of the layers are required after using HDI technology. In addition, HDI technology blind hole also saves the inner layer with the hole pad space, also can do hole in the pad design.
3) Product time to market is accelerated
HDI technology uses a blind hole and hole-in-pad structure, which facilitates the configuration of electronic components and also allows products to enter the market in a shorter time. At the same time, such a design takes up less space, increases the efficiency of product design space, improves the performance of BGA component applications, increases the flexibility of winding, and makes the line design more concise.
In addition, the use of blind and buried hole design and let the electrical properties enhance, can significantly shorten the system design adjustment time, reduce the signal integration and noise reduction work, reduce the opportunity to redesign.
Compared with traditional through-hole, the aspect ratio of blind holes is mostly less than 1:1, while traditional through-hole is in the range of 4:1 to 20:1. This makes the blind hole/microvia structure have a higher reliability of serial number transmission.
In addition, the thin structure and low Z-axis expansion coefficient material of HDI board make the HDI board have low potential inductance as well as better thermal conductivity.
5) Lower cost
The use of HDI technology can reduce the number of board design layers, improve component density, while improving system speed and adjusting impedance performance. Therefore, in combination, HDI multilayer structure is less costly than traditional through-hole.
HDI technology brings many benefits, but there are also many problems at the level of concrete implementation.
Since HDI technology is less used, there is not enough experience accumulated in the current PCB design to predict the stacking condition, number of holes, and price point of HDI at the beginning of the project design. This is contrary to the current quotation process and needs to rely on the subsequent accumulation of experience.
2) Design model
For simple HDI structures, the layout of small areas is simple and the design is less difficult; while for complex structures, effective tools are needed to model the winding model, component data, geometric relationships and board dimensions for import, and use them to simulate the performance of the final product. And currently only a few manufacturers have this ability.
3) Signal Integration
The use of HDI structure is a prerequisite for understanding the electrical improvements it can bring, but this is a difficult conversion for the current preference for through-hole design of traditional circuit board design.
4) The application of new materials
For HDI board, the current import of many new materials, such as resin coating copper skin, vacuum lamination dielectric layer. These new materials of all kinds of parameters on the characteristics of the board has a significant impact, need to continue to practice and figure out. Among the various parameters, low loss substrate, low dielectric parameters and high heat resistance are key.
5) Assembly issues
Full fill-in-the-blank structure can increase the inductance of the line, but will increase the board manufacturing costs by more than 10%. Therefore, how to reasonably choose these structures, related to the balance between performance and cost.
In the initial design of the HDI board needs to be considered with the test design, the need for test engineers and board designers to jointly plan, which is conducive to later prediction of possible failure conditions, planning test strategies, understanding the scope of failure. This is quite important for mass production, involving product testing costs.
Reasonable test design can be reasonably expected for each node, components, board signals of possible failure types and provide a reasonable test program.
7) Cost estimation
How to establish the Design Sheet in the design, how to choose the best design parameters? Minimum hole diameter, hole circle, line width, etc., have a significant impact on the output yield performance, while material thickness, stacking structure, number of line holes, hole density, etc., have a greater impact on the cost side.
In addition, the final metal surface treatment, void filling, allowable tolerances, etc. all have an impact on the cost end. Many parameters have a significant impact on both cost and performance. The trade-offs depend on experience and modeling of cost performance.
8) Design Tools
For HDI boards, the traditional through-hole automation design tools are no longer applicable. Instead, current design tools are relatively expensive, but have perfect features in blind microvia design, stacked structures, wirewrap, etc.
9) Electrical performance and signal integration
The electrical impact of holes in high-speed networks should not be ignored. Through-hole has high capacitance, inductance and other parasitic noise, and the structure around the through-hole, brings more than ten times the amount of parasitic noise, which will become obvious signal performance interference. And there are more through-hole blind holes in HDI, so how to combine structural design and signal impact for design is one of the important design points to consider.
The HDI design process is divided into six parts.
1) System segmentation
At the beginning of a new product design, the entire product will be broken down into component levels for design and manufacturing. Proper system partitioning helps clarify system performance, speed up the design process, and ensure the success of the product by mastering the trade-offs between structural configuration, components, risk control and manufacturability at an early stage of design.
2) Product design
Product design includes logic design, line simulation, component simulation, custom IC, mechanical design, etc. HDI boards have a greater advantage in electrical and thermal management, to improve the electrical and thermal characteristics of the product.
3) Board design and layout
Different from the traditional board structure, because there are blind holes, buried holes, HDI board design layout affects the performance and manufacturing of HDI boards. Therefore, you need to do a good job of pre-research to fully understand the impact of various types of design on performance and manufacturing.
4) Board manufacturing
Circuit board manufacturing has a large contribution to the impact of the finished product. One of the more influential processes include: alignment, fine line imaging technology, metallization treatment, plating, etc..
How to do a good job on the HDI board these processes and testing, which is the biggest requirement for each manufacturer.
(5) board assembly of different HDI board components require different solder-back operation curve and repair technology. And because of the greater density of components, HDI board is thin, in the back-soldering process is prone to different failures and risks.
For more problems in the assembly, need to be thoroughly evaluated and tested in the specific manufacturing process.
6) Assembly testing
The final result step of HDI technology is the assembly test.
Properly arranged test pads can reduce the complexity and cost of testing and reduce unnecessary parasitic noise. Therefore, the assembly test should be considered at the beginning of the design.
Generally speaking, HDI design can be examined by relevant performance indicators.
1) Assembly complexity Component density (Cd) and assembly density (Ad) are two important metrics. Component density is the number of components per unit area, and assembly density is the number of contacts per unit area. The higher the value of both, the higher the complexity.
2) IC assembly
Component complexity is also one of the important indicators, characterized by the average number of pins per component (I/O). Symbiotic indicators for the pin spacing size.
3) Board density
One is the length of the line per unit area expressed, usually expressed in terms of the average length of lines configured per square inch of area, including all signal layers.
The second is the expression of the number of lines configured per unit length, usually the number of lines configured per centimeter. In addition to these three indicators, there are several indicators to weigh the advantages and disadvantages of HDI board layout and feasibility.
1) Layout efficiency
Evaluation of the percentage of the maximum configurable line volume within the layer versus the actual configuration volume.
2) Predicted windable line capacity
Determining the ease of configuring lines in the available space.
3）Line winding density
The length of lines per unit area within the layer.