Overview High Density Circuit Board HDI

                               Overview High Density Circuit Board HDI

1 History of the board

The name of the printed circuit board is derived from the English PCB (Printed Circuit Board), or it is also called PWB (Printed Wiring Board) in English. It replaces some early wire assembly products, speeds up mass production replication, reduces volume, and lowers unit price. After the 1960s, various electronic and electrical products were successively manufactured using electroplated through-hole plates, and the heat-resistant and high-dimensional stability substrates developed rapidly, and they are still the main resin substrates today. As semiconductor technology moves towards a high-density structure, the one-to-one relationship of electronic assembly increases the contact density of components, which leads to the trend of high-density circuit board design.

The design concept of build-up circuit boards has existed since 1967, but it was not until IBM published SLC technology in 1990 that microvia technology was gradually put into practical use. Prior to this, the industry used full-hole boards and multi-layer laminated boards to build high wiring density. Due to the rapid progress of materials, photosensitive and thermally polymerized insulating materials have been launched one after another, micro-hole technology has become a necessary design for high-density circuit boards, and it appears in most handheld electronic products.

For connection between circuit layers, in addition to electroplating, conductive paste structure can also be used for connection. The more well-known ones are: ALIVH released by Panasonic and B2it released by Toshiba. These technologies have pushed the application of circuit boards to the era of High Density Interconnection-HDI.

2 The progress of the electronics industry

The development of electronic technology has only been about 60 years. From the traditional vacuum tube to the transistor technology, it has developed all the way and has become the largest industrial group in the world. All electronic parts must be assembled and connected to form a complete functional unit, and the work of designing and manufacturing basic components is classified as the electronic assembly industry.

In the middle of the previous century, the electronics industry had begun to take shape, and the fineness of semiconductors was quite limited. Therefore, the demand for circuit board technology is not the same as the current market demand. We do not attempt to restate the past technologies here, and the follow-up content will focus on the presentation of advanced HDI product design, manufacturing and process technology issues.

The content of this chapter focuses on introducing basic product technology thinking, and discusses the possible advantages and potential challenges and difficulties of adopting this type of HDI technology. The focus is on layout design analysis, component density and other issues. Of course, it will also involve some issues such as circuit board structure selection, possible cost and performance. As shown in Figure 1, it is the definition and concept of HDI by system designers.

Figure 1 The trend of electronic construction density, the thinking of system construction positions HDI as the interconnection technology of the entire system

The performance and density of semiconductor components continue to increase, while the requirements for the size of the package continue to shrink, all of which require an increase in the interconnection density of the circuit board. When the industry introduces ball array (BGA), chip size (CSP), layout on chip (COB), system in package (Sip), 3D and other structures in large quantities, circuit board technology must find alternatives to increase density. Subsequent content will describe some topics such as the definition of simple high-density circuit boards, related design issues, electrical performance, material selection, process technology, inspection and testing, and related product structures.

3 What is a high-density circuit board (HDI Board)

The circuit board is a structural element formed of insulating materials and conductor wiring. It will be mounted on the surface before it is made into a final product: integrated circuits, transistors, diodes, passive components (such as resistors, capacitors, connectors, etc.) Components will also be matched with peripheral functional components. Electronic signal connections and various functions can be formed by connecting wires, so the circuit board is a platform for component connections, used to undertake and contact the base of components.)

Because the circuit board is not a terminal product, the definition of the name is slightly confusing. For example, the motherboard used for personal computers is called the motherboard instead of the circuit board. Although the circuit board is used as a constituent element in the motherboard, it is not the same. Therefore, although the two industries are related, they cannot be said to be the same. Another example: if there are IC components mounted on a circuit board, the media will call it an IC board, but in essence it is not the same as a circuit board.

Electronic products are too small and multifunctional, the contact distance of IC components is shortened, the signal transmission speed is relatively high, the amount of wiring is increased, and the length of wiring is partially shortened. All these need to be matched with high-density circuit configuration and microvia technology to achieve the goal . Generally, wiring and cross-connection can be completed by double-sided boards, but it is difficult to handle complex signals and adjust electrical stability, so the circuit board will become multi-layered. And because the number of signal lines continues to increase, more power and ground planes must be added, which promotes the popularity of multilayer printed circuit boards.

For products with high frequency requirements, the circuit board must provide: characteristic impedance control, high frequency transmission, low radiation (EMI) interference and other performances. To adopt stripline (Stripline), microstrip line (Microstrip line) and other structures, multi-layer design is necessary at this time. In order to improve the quality of signal transmission, high-end products will use low dielectric coefficient (Dk) and low attenuation rate (Df) insulating materials. In order to cooperate with the miniaturization and arrayization of electronic components, the circuit board is also continuously improving the contact and winding. Density according to demand. The development of components such as BGA (Ball Grid Array), CSP (Chip Scale Package), and DCA (Direct Chip Attachment) has pushed the circuit board to an unprecedented high-density realm.

A hole with a diameter of less than 150 μm is called a microvia by the industry. A circuit board made of a microvia structure can improve the efficiency of assembly and space utilization, and is also necessary for the miniaturization of electronic products. This kind of circuit board products, the industry has a number of different titles, for example: Europe and the United States according to the production process using a sequential construction, and called the product SBU (Sequence Build Up Process), generally translated as "sequential build-up method." As for the Japanese industry, because the holes made by this product are smaller than before, the product is called MVP (Micro Via Process), which is generally translated as "micro-via process". Some people also call this type of circuit board BUM (Build Up Multilayer Board) because the traditional multilayer board is called MLB (Multilayer Board), which is generally translated as "build-up multilayer board".

The American IPC Circuit Board Association is based on avoiding confusion, and many system manufacturers also refer to various high-density construction technologies as HDI (High Density Intreconnection) technology, so they collectively refer to this type of product technology as HDI, and the direct translation becomes "high density". Internet Technology". The literature refers to this type of product as an HDI board or a full Chinese translation of "high-density interconnection technology". In order to speak smoothly, it must also look like a product, so the circuit board made of this technology is called "high-density circuit board". The follow-up content is called "high-density circuit board" in Chinese, and the commonly used "HDI" board is used in English.

4 Why do you need a high-density circuit board

Traditional circuit boards are often divided into single, double, and multi-layer boards, and multi-layer boards are divided into single-press and multiple-press structures. Of course, this design involves some electrical properties and connection density issues, but because of the rapid advancement of electronic product technology, these aggregate structures cannot meet the component mounting density and electrical requirements. In order to improve the connection density of components, from a geometric point of view, only by compressing the space of lines and connection points can more contacts be accommodated in a small space to provide connection density. Of course, multiple components can also be stacked at the same position to increase the packing density. Therefore, high-density circuit board is not only a circuit board technology, but also an issue of electronic construction and assembly.

The so-called electronic assembly (packaging) in the industry refers to the connection between the IC chip and the carrier board, while the electronic assembly is the process in which the components after the IC assembly are completed and then installed on another functional circuit board. SMT component end connection point, generally called OLB (Outer Lead Bond), refers to the external pin connection part of the component. This part of the connection is directly related to the contact density on the surface of the electronic component. When the functional integration of electronic products is high, there is a need for high-density design.

If the concept of high-density circuit board design is adopted, electronic products can obtain the following benefits:

1) The same product design can reduce the number of carrier board layers, increase density and reduce costs

2) Increase the wiring density and increase the line capacity per unit area with microporous thin lines, which can meet the assembly requirements of high-density contact components and facilitate the use of advanced structures

3) The use of micro-hole interconnection can shorten the contact distance, reduce signal reflection and crosstalk between lines, and the components can have better electrical properties and signal accuracy

4) The structure adopts a thinner dielectric thickness, and the potential inductance is relatively low

5) Microholes have a low aspect ratio, and the reliability of serial number transmission is higher than that of ordinary through holes

6) Micro-hole technology allows the carrier board design to shorten the distance between grounding and signal layers, thereby improving radio frequency/electromagnetic wave/electrostatic discharge (RFI/EMI/ESD) interference. And can increase the number of grounding wires to prevent components from being damaged by instantaneous discharge caused by static electricity accumulation

7) Microholes can improve the flexibility of line configuration and make line design easier

Modern and popular electronic products should not only be mobile and power-saving, but also easy to wear and beautiful in appearance. Of course, the most important thing is that they are affordable and can be replaced with fashion. Figure 2 shows an example of a representative mobile electronic product.