PCB Technical - Description and Development of Printed Circuit Boards

PCB is mainly used to connect and fix Integrated circuits and other electronic components so that signals can be circulated among different electronic components, so it is known as "the mother of electronic system products". In the early days, printed circuit boards could not be mass-produced because the materials and processing methods were immature. Until the transistors began to come out in the early 1950s, circuit boards were also widely used. However, nowadays, with the advancement of software and hardware equipment and the vigorous development of fifth-generation mobile communication, Artificial Intelligence (AI), loop, Networkingcomputaion, and smart city technologies, the increase in frequency and transmission speed has made printed circuit board performance requirements are increasing.

The printed circuit board production process can be divided into five major processes:

Material, Stack-Up, Interconnection of connection holes, Surface finishing, and others, mainly introduces the first three items: For the material part, the materials used in the manufacture of printed circuit boards today are mostly composite materials, which will be added later. Some reinforcing materials Reinforcement and Fillers improve mechanical properties. It is easy to divide materials into conductors and non-conductors, and the materials used will affect the dielectric constant (DK) and the dissipation factor DissipationFactor (Df), which will have a certain effect on the speed and quality of the transmission signal. Impact. The second is the stacking structure, that is, the stacking method between layers. The traditional method is to press the core with Prepreg, press it according to the number of layers required by the user, and then use drilling to make vias for each layer. The connection between them; and the modern approach is to build-Up with film on the inner layer of the core, and the number of layers can also be superimposed according to the needs of users. After the stacking is completed, it is then possible to enter into the form of determining the connection holes.

Traditionally, when stacking, each layer needs to be connected through holes. However, too many holes will become a source of interference in high-speed PCB products, so they must be drilled. The tolerance of the machine, the rotational speed of the mechanical drill, the speed of the drilling feed, etc. must be paid attention to; however, today's High-Density Inverter (HDI PCB) and each layer interconnect technology Every Layer Inter Connect (ELIC) use laser drilling to connect each layer. Afterward, electroplating is performed in the connection form of a variety of different holes, so that the electronic signal moves between different layers, and the common hole types include through holes, stepped holes, blind holes, and buried holes. In order to meet the current requirements for high-speed and high-frequency, such as low dielectric constant and ultra-low dissipation coefficient, impedance, loss, etc.

The circuit board has begun to move towards more advanced technology, which can also be called an advanced high-frequency circuit board and high-speed circuit board. Among them, the main judgment indicators covered are the number of layers, line width, and line spacing, copper thickness, and the state and alignment ability of vias.

Generally speaking, because the number of layers of the printed circuit board represents the number of independent wiring layers, the more layers, the better the technology, but the greater the impact on the yield; the line width and line spacing are also important indicators, usually The printed circuit board can fall on the micron level; in the circuit board, if the copper thickness below is uneven, the dielectric will also become uneven, so special attention should be paid to the deviation and tolerance of copper thickness; in the part of the via hole, The current electroplating technology is to fill all the holes with copper plating, and the standard for evaluating the ability of the holes is based on the aspect ratio, which represents the ratio of the thickness of the circuit board to the aperture. If the ratio of the aspect ratio is larger, it represents the board. Thicker, smaller holes contribute a lot to overall routing density, but present a challenge for electroplating; the last evaluation metric focuses on layer-to-layer alignment.

Register action Layer (RDL) and Flip Chip Ball Brid Array (FCBGA) technology are the latest technology, the number of layers can reach 8 to 20 layers, the thickness of the medium is about 6 to 10 microns, and the line width and line spacing are about 12-30 microns. Pore size is about 15 microns. The advantages of the panel are low cost, low vertical height, no unit limit, high density of fine bump pitch, and can reach 5 microns according to today's alignment capability; and for the part of the flip-chip ball gate array package carrier board, can be stacked up to 20 layers, because the material used is ABF resin, which is a non-glass fiber material, so the via hole can be made smaller. In addition, in the technology of FCBGA, the technology of Modified Semi-Additive Process (mSAP) is also combined, which can control the line width more accurately. mSAP uses thin copper foil electroplating for circuit construction. The detailed process steps are laser etching - copper plating - adding photoresist - exposure - circuit board development - secondary plating - photoresist removal - flash etching. In the future, we hope to be able to Make the lines 2 to 3 microns grade.

In the selection of printed circuit board materials, thermal properties, mechanical properties, and physical properties will be affected, which should be taken into consideration when selecting materials. In addition, the dielectric constant and dissipation coefficient will be affected by temperature, humidity, and frequency are affected, and the material selected should not make the variation of the two coefficients too large. In the dielectric part, if different materials are selected, special attention should be paid to the alignment ability of positive and negative signals. If the alignment ability is too low, the delay will occur. The most common way to improve is to use glass fiber cloth with a small resin opening as the material selection. In terms of process, the shape of the via hole should be designed according to the needs of the user, and the copper surface treatment on the electroplating should reduce the roughness without weakening the bonding force. It can be used with an adhesion promoter The material selection of the metal will affect the mechanical properties according to the degree of etching. In the surface treatment part, it is necessary to pay attention to the skin effect. The higher the frequency of the current, the skin effect is likely to occur, so that the current in the wire is concentrated on the surface of the wire, rather than evenly dispersed in the wire, this effect will cause material loss. degree increase. At this time, nickel can be added for processing, because nickel has high conductivity, so the thickness of a nickel is usually reduced through nickel-gold, or the nickel is directly removed so that the signal can still smoothly reach the bottom layer of copper. Finally, it is necessary to pay attention to thermal resistance, which should be minimized within the reachable range. Common methods include reducing the thickness of copper, increasing the heat dissipation area, and placing copper blocks.

Emerging technologies born with the advancement of the times have brought many challenges to printed circuit boards, including production material selection, process selection, and product management, product design simulation, reliability and test requirements, and thermal resistance issues. In this market, it is necessary to improve adaptability at any time, enhance advanced technology, and actively recruit talents to facilitate the development of iPCB in the future.