How to achieve high precision PCB circuit board

How to achieve high precision PCB circuit board

The high-precision circuit board refers to the use of fine line width/spacing, micro holes, narrow ring width (or no ring width) and buried and blind holes to achieve high density. And high precision means that the result of "fine, small, narrow, thin" will inevitably lead to high precision requirements. Take line width as an example: O. 20mm line width, produced according to regulations. 16～0.24mm is qualified, and its error is (O.20±0.04)mm; For a line width of 10mm, the error is (0.10±0.02)mm in the same way. Obviously the accuracy of the latter is doubled, and so on is not difficult to understand, so the high precision requirements will not be discussed separately. But it is a prominent problem in production technology.

(1) In the future, the fine wire technology will have a high fine line width/spacing from 0.20mm-O. Only 13mm-0.08mm-0.005mm can meet the requirements of SMT and multi-chip packaging (MultichipPackage, MCP). Therefore, the following technology is required.

①Using thin or ultra-thin copper foil (<18um) substrate and fine surface treatment technology.

②Using thinner dry film and wet filming process, thin and good quality dry film can reduce line width distortion and defects. Wet film can fill small air gaps, increase interface adhesion, and improve wire integrity and accuracy.

③Using parallel light exposure technology. Since the parallel light exposure can overcome the influence of the line width variation caused by the oblique rays of the "point" light source, it is possible to obtain fine wires with precise line width dimensions and smooth edges. However, the parallel exposure equipment is expensive, the investment is high, and it is required to work in a high cleanliness environment.

④Using electrodeposited photoresist film (Electro-deposited Photorest, ED). Its thickness can be controlled in the range of 5-30/um, and it can produce more perfect fine wires. It is especially suitable for narrow ring width, no ring width and full-board plating. At present, there are more than ten ED production lines in the world.

⑤Using automatic optical inspection technology (AutomaticOpticInspection, Aoi). This technology has become an indispensable means of detection in the production of fine wires, and is rapidly being promoted, applied and developed. For example, the AT&T company has 11 AoIs, and the {tadco company has 21 AoIs dedicated to detecting the inner layer graphics.

(2) Microporous technology. The functional holes of printed circuit boards for surface mounting are mainly used for electrical interconnection, which makes the application of microporous technology more important. Using conventional drill materials and CNC drilling machines to produce tiny holes has many problems and high costs. Therefore, the high-density of printed boards is mostly focused on the refinement of wires and pads. Although great achievements have been made, its potential is limited. To further improve the density (such as wires less than 0.8 mm), the cost is urgent. Therefore, it turned to use micropores to improve the densification.

In recent years, numerical control drilling machines and micro-drill technology have made breakthroughs, so micro-hole technology has developed rapidly. This is the main outstanding feature in current PCB production. In the future, the micro-hole formation technology will mainly rely on advanced CNC drilling machines and excellent micro-heads, and the small holes formed by laser technology are still inferior to those formed by CNC drilling machines from the viewpoint of cost and hole quality.

①There are indeed many problems with laser drilling conventional CNC drilling machines and drill bits to drill tiny holes. It has hindered the progress of micro-hole technology, so laser ablation has received attention, research and application. But there is a fatal shortcoming, that is, the formation of a horn hole, which becomes more serious as the plate thickness increases. Coupled with high temperature ablation pollution (especially multi-layer boards), the life and maintenance of the light source, the repeatability of the corrosion holes, and the cost, the promotion and application of micro-holes in the production of printed boards has been restricted. However, laser ablation is still used in thin and high-density microporous plates, especially in MCM-L's high-density interconnect (HDI) technology, such as M． C. The polyester film etch hole and metal deposition (sputtering technology) in Ms combined high-density interconnection are applied. The formation of buried vias in high-density interconnection multilayer circuit boards with buried and blind via structures can also be applied. However, due to the development and technological breakthroughs of CNC drilling machines and micro-drills, they were quickly promoted and applied. So laser drilling on the surface

The application in the mounting circuit board cannot form a dominant position. But it still has a place in a certain field.

② Numerical control drilling machine The current technology of numerical control drilling machine has made new breakthroughs and progress. And formed a new generation of CNC drilling machine characterized by drilling tiny holes. The efficiency of drilling small holes (less than 0.50mm) of the micro-hole drilling machine is 1 times higher than that of the conventional CNC drilling machine, with fewer failures, and the speed is 11-15r/min; it can drill 0. 1~0.2mm micro-holes, using high-quality small drills with high cobalt content, can be drilled by stacking three plates (1.6mm/block). When the drill bit is broken, it can automatically stop and report the position, automatically replace the drill bit and check the diameter (the tool library can hold hundreds of pieces), and can automatically control the constant distance between the drill tip and the cover and the drilling depth, so blind holes can be drilled , It will not damage the countertop. The table top of the CNC drilling machine adopts air cushion and magnetic levitation type, which can move faster, lighter and more precise without scratching the table. Such drilling machines are currently in short supply, such as Mega 4600 from Italy's Purite, ExcelIon 2000 series from the United States, and new generation products from Switzerland and Germany.

③Buried, blind, and through-hole technology The combination of buried, blind, and through-hole technology is also an important way to increase the density of printed circuits. Generally, the buried and blind holes are all tiny holes. In addition to increasing the number of wiring on the board, the buried and blind holes are all interconnected with the "nearest" inner layer, which greatly reduces the number of through holes formed, and the setting of the isolation disk will also be greatly reduced. Reduction, thereby increasing the number of effective wiring and inter-layer interconnection in the board, and improving the interconnection density. Therefore, the multi-layer circuit board with the combination of buried, blind, and through-holes has at least three times higher interconnection density than the conventional full-through-hole board structure under the same size and number of layers. If the buried and blind are under the same technical indicators, The size of the printed board combined with through holes will be greatly reduced or the number of layers will be significantly reduced. Therefore, in high-density surface-mounted printed boards, buried and blind hole technologies are increasingly being used, not only in surface-mounted printed boards in large computers, communication equipment, etc., but also in civil and industrial applications. It has also been widely used in the field, even in some thin boards, such as all kinds of PCMCIA, SMard, IC cards and other thin six-layer boards or more.

printed circuit boards with buried and blind hole structures are generally completed by "sub-board" production methods, which means that they must be completed through multiple pressing, drilling, and hole plating, so precise positioning is very important .