High precision circuit board refers to the use of fine line width/spacing, tiny 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 "thin, small, narrow, thin" will inevitably bring high precision requirements, take the line width as an example: O. 20mm line width, according to the regulations to produce O. 16 ~ 0.24mm is qualified, the error is (O.20 ± 0.04) mm; and O. For the line width of 10mm, the error is (0.10±0.02) mm. 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) Fine wire technology
The future high fine wire width/spacing will be changed from 0.20mm-O. 13mm-0.08mm-0.005mm can meet the requirements of SMT and multi-chip package (Multichip Package, MCP). Therefore, the following techniques are required.
①Using thin or ultra-thin copper foil (<18um) substrate and fine surface treatment technology.
②The use of thinner dry film and wet film process, thin and good quality dry film can reduce line width distortion and defects. Wet lamination can fill small air gaps, increase interfacial adhesion, and improve wire integrity and accuracy.
③ Using electrodeposited photoresist film (Electro-deposited Photoresist, ED). Its thickness can be controlled in the range of 5-30/um, which can produce more perfect fine wires, especially suitable for narrow ring width, no ring width and full-board electroplating. At present, there are more than ten ED production lines in the world.
④Using parallel light exposure technology. Since the parallel light exposure can overcome the influence of line width variation caused by the oblique light of the "point" light source, fine wires with precise line width dimensions and clean edges can be obtained. However, parallel exposure equipment is expensive, requires high investment, and requires work in a high-cleanliness environment.
⑤ Adopt automatic optical inspection technology (Automatic Optical Inspection, AOI). This technology has become an essential means of detection in the production of fine wires, and is being rapidly promoted, applied and developed. For example, AT&T Company has 11 AoIs, and}tadco Company has 21 AoIs specially used to detect the graphics of the inner layer.
(2) Microvia technology
The functional holes of printed boards used for surface mounting mainly play the role of electrical interconnection, which makes the application of microvia technology more important. Using conventional drill bit materials and CNC drilling machines to produce tiny holes has many failures and high costs. Therefore, the densification of printed boards is mostly due to the densification of wires and pads. Although great achievements have been made, its potential is limited. To further improve the densification (such as wires less than 0.08mm), the cost is urgent. liters, thus turning to the use of micropores to improve densification.
In recent years, breakthroughs have been made in CNC drilling machine and micro-drill technology, so micro-hole technology has developed rapidly. This is the main prominent feature in current PCB production. In the future, the technology of forming tiny holes will mainly rely on advanced CNC drilling machines and excellent tiny heads, while the holes formed by laser technology are still inferior to those formed by CNC drilling machines from the point of view of cost and hole quality.
①CNC drilling machine At present, the technology of CNC 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) by the micro-hole drilling machine is 1 times higher than that of the conventional CNC drilling machine, with fewer failures, and the rotation speed is 11-15r/min; it can drill O. 1 ~ 0.2mm micro-holes, high-quality small drill bits with high cobalt content are used, and three plates (1.6mm/block) can be stacked for drilling. 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 magazine can accommodate hundreds of pieces), and can automatically control the constant distance between the drill tip and the cover plate and the drilling depth, so blind holes can be drilled. , and will not damage the countertop. The CNC drilling machine table adopts air cushion and magnetic floating type, which moves faster, lighter and more precise, and will not scratch the table. Such drill presses are currently in short supply, such as the Mega 4600 from Prute in Italy, the ExcelIon 2000 series in the United States, and new-generation products from Switzerland and Germany.
② There are indeed many problems with laser drilling conventional CNC drilling machines and drills to drill tiny holes. It has hindered the progress of micro hole technology, so laser hole etching has been paid attention, research and application. But there is a fatal disadvantage, that is, the formation of horn holes, which is aggravated with the increase of plate thickness. In addition to the pollution of high temperature ablation (especially multi-layer boards), the life and maintenance of the light source, the repeatability of the etching hole and the cost, the promotion and application of micro holes in the production of printed boards has been limited. However, laser ablation is still used in thin and high-density microplates, especially in the high-density interconnect (HDI) technology of MCM-L, such as M. c. It has been applied in high-density interconnection combining polyester film etching in Ms and metal deposition (sputtering technique). Buried via formation in high-density interconnect multilayer boards with buried and blind via structures can also be applied. However, due to the development and technological breakthroughs of CNC drilling machines and tiny drill bits, they have been rapidly promoted and applied. Thus the laser drilled holes on the surface
Applications in mounted circuit boards cannot form dominance. But it still has a place in a certain field.
③Buried, blind and through-hole technology The combination of buried, blind and through-hole technology is also an important way to improve the high density of printed circuits. Generally, buried and blind vias are tiny holes. In addition to increasing the number of wirings on the board, buried and blind vias are interconnected between the "nearest" inner layers, which greatly reduces the number of through holes formed, and the setting of the isolation disc will also greatly reduce the number of vias. Reduced, thereby increasing the number of effective wiring and interlayer interconnections in the board, and improving the high density of interconnections. Therefore, the multi-layer board with the combination of buried, blind and through-hole is at least 3 times higher than the conventional all-through-hole board structure under the same size and number of layers. 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 mount printed boards, buried and blind via technologies are increasingly used, not only in surface mount printed boards in large computers, communication equipment, etc., but also in civil and industrial applications. It has also been widely used in the field of , and even in some thin boards, such as thin boards with more than six layers of various PCMCIA, Smart, IC cards, etc.
The printed circuit boards with buried and blind hole structures are generally completed by the "split board" production method, which means that it can only be completed after many times of pressing, drilling, hole plating, etc., so precise positioning is very important. .
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