High-precision PCB refers to the adoption of technologies such as 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 and thin” will inevitably bring high precision requirements. Take the line width as an example:
0.20 mm line width, 0.16 ~ 0.24 mm produced by the regulation is qualified, and its error is (0.20 soil 0.04) mm; While the line width of 0.10mm, by the same token, has an error of (0.1 0.02) mm, obviously the accuracy of the latter is doubled, and so on is not difficult to understand, so the requirement of high accuracy is not discussed separately. But it is a prominent problem in production technology.
Fine traverse technology
In the future, the high fine line width/pitch will be 0.20mm-0.13mm-0.08mm-0.005mm to meet the requirements of SMT and Mulitichip Package,MCP). Therefore, the following technologies are required.
① substrate
Adopt thin or ultra-thin copper foil (< ＜18um) substrate and fine surface treatment technology.
② Process
Using thin dry film and wet film process, thin and good quality dry film can reduce line width distortion and defects. Wet film pasting can fill small air gaps, increase interface adhesion, and improve conductor integrity and accuracy.
③ Electrodeposition of photoresist film
Electro-deposited photoresist (ed) is used. Its thickness can be controlled in the range of 5 ~ 30/um, which can produce more perfect fine wires. It is especially suitable for narrow loop width, loop-free width and full-plate electroplating. At present, there are more than ten ED production lines all over the world.
④ Parallel light exposure technology.
Parallel light exposure technology is adopted. Because parallel light exposure can overcome the influence of oblique light rays of "point" light source on line width variation, fine wires with accurate line width and smooth edges can be obtained. However, parallel exposure equipment is expensive and requires high investment, and it needs to work in a clean environment.
⑤ Automatic optical detection technology
Automatic optical detection technology is adopted. This technology has become a necessary means of detection in the production of fine wires, and is being rapidly popularized, applied and developed.
Microporous technology
The functional holes of printed boards for surface mounting of micropores are mainly used for electrical interconnection, thus making the application of micropores more important. Using conventional drill bit materials and numerical control drilling machine to produce tiny holes has many faults and high cost.
Therefore, the high density of printed circuit boards mostly focuses on the densification of wires and pads. Although great achievements have been made, its potential is limited. To further improve the densification (such as wires smaller than 0.08 mm), the cost rises sharply, so we turn to micropores to improve the densification.
In recent years, numerical control drilling machine and micro-bit technology have made breakthrough progress, so micro-hole technology has developed rapidly. This is the main outstanding feature in current PCB production.
In the future, the micro-hole forming technology mainly depends on the advanced numerical control drilling machine and excellent micro-head, while the micro-hole formed by laser technology is still inferior to that formed by numerical control drilling machine in terms of cost and hole quality.
① CNC drilling machine
At present, the technology of NC drilling machine has made new breakthrough and progress. And formed a new generation of numerical control drilling machine characterized by drilling tiny holes.
The efficiency of drilling small holes (less than 0.50mm) by micro-hole drilling machine is twice as high as that by conventional numerical control drilling machine, with fewer failures and a rotating speed of 11 ~ 15 R/min. 0.1 ~ 0.2 mm micropores can be drilled, and high-quality small drill bits with high cobalt content can be used. Three plates (1.6 mm/block) can be stacked to drill holes. When the drill bit is broken, it can automatically stop and report the position, automatically change the drill bit and check the diameter (the tool library can hold hundreds of pieces), and automatically control the constant distance between the drill tip and the cover plate and the drilling depth, so blind holes can be drilled without damaging the table top. CNC drilling machine table adopts air cushion and magnetic levitation, which can move faster, lighter and more accurately, and won't scratch the table.
At present, such drilling machines are in short supply, such as Mega 4600 from Prurite, Italy, Excellon 2000 series in the United States, and new generation products from Switzerland and Germany.
② Laser drilling
There are indeed many problems with conventional CNC drilling machines and drill bits to drill tiny holes. It has hindered the progress of micro-hole technology, so laser hole etching has been paid attention to, studied and applied.
However, there is a fatal disadvantage, that is, the horn hole is formed, which becomes more serious with the increase of plate thickness. In addition, the pollution caused by high-temperature ablation (especially multilayer boards), the life and maintenance of light source, the repeated accuracy of etched holes and the cost are some problems, so the popularization and application of micro-holes in printed boards are limited. However, laser etching is still used in thin and high-density microplates, especially in the high-density interconnection (HDI) technology of MCM-L, such as the combination of polyester film etching and metal deposition (sputtering technology) in M.C.MS.
The formation of buried holes in high-density interconnected multilayer boards with buried and blind hole structures can also be applied. However, due to the development and technical breakthrough of numerical control drilling machine and micro-drill, they are rapidly popularized and applied. Therefore, the application of laser drilling in surface mounted circuit boards can't form a dominant position. But it still has a place in a certain field.
③ Burying, blind and through hole technology
The combination of buried, blind and through-hole technology is also an important way to improve the density of printed circuits. Generally, the buried and blind holes are tiny holes. Besides increasing the number of wiring on the board, the buried and blind holes are interconnected by "nearest" inner layers, which greatly reduces the number of through holes and the number of isolation disks, thus increasing the number of effective wiring and inter-layer interconnection in the board and improving the high density of interconnection.
Therefore, the interconnection density of the multilayer board with buried, blind and through holes is increased by at least 3 times under the same size and number of layers compared with the conventional all-through hole board structure. If the printed board with buried, blind and through holes is combined under the same technical index, its size will be greatly reduced or the number of layers will be obviously reduced.
Therefore, in high-density surface-mounted printed boards, buried and blind hole technology has been applied more and more, not only in surface-mounted printed boards of large computers and communication equipment, but also in civil and industrial fields, and even in some thin boards, such as thin boards with more than six layers of PCMCIA, Smard and IC cards.
Printed circuit boards with buried and blind hole structures are generally completed by "dividing boards", which means that the printed circuit boards can only be completed after many times of pressing, drilling, hole plating, etc., so precise positioning is very important.
