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.

High precision means that the result of “fine, small, narrow, and thin” will inevitably lead to high precision requirements. Take the line width as an example:

0.20mm line width, 0.16～0.24mm produced in accordance with regulations is qualified, and the error is (0.20±0.04) mm; while the line width of 0.10mm, the error is (0.1±0.02) mm, obviously The accuracy of the latter is increased by a factor of 1, and so on is not difficult to understand, so the high accuracy requirements will not be discussed separately. But it is a prominent problem in production technology.

Small and dense wire technology 

In the future, the high-density line width/pitch will be from 0.20mm-0.13mm-0.08mm-0.005mm to meet the requirements of SMT and multi-chip packaging (Mulitichip Package, MCP). Therefore, the following technology is required.
①Substrate

Using thin or ultra-thin copper foil (<18um) substrate and fine surface treatment technology.
②Process

Using thinner dry film and wet pasting 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.
③Electrodeposited photoresist film

Electro-deposited Photoresist (ED) is used. 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 plate electroplating. At present, there are more than ten ED production lines in the world.
④ Parallel light exposure technology

Using 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, the fine wire with precise line width size and smooth edges can be obtained. However, the parallel exposure equipment is expensive, the investment is high, and it is required to work in a highly clean environment.
⑤Automatic optical inspection technology

Using automatic optical inspection technology. This technology has become an indispensable means of detection in the production of fine wires, and is rapidly being promoted, applied and developed.

EDA365 Electronic Forum

Microporous technology

The functional holes of the printed boards used for surface mounting of the microporous technology are mainly used for electrical interconnection, which makes the application of the microporous technology more important. Using conventional drill materials and CNC drilling machines to produce tiny holes has many failures and high costs.

Therefore, the high-density of printed boards is mostly focused on the refinement of wires and pads. Although great results have been achieved, its potential is limited. To further improve the density (such as wires less than 0.08mm), the cost is soaring. , So turn to use micropores to improve densification.

In recent years, numerical control drilling machines and micro-drill technology have made breakthroughs, and thus micro-hole technology has developed rapidly. This is the main outstanding feature in current PCB production.

In the future, the micro-hole forming 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.
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) of 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 0.1-0.2mm micro-holes, using a relatively high cobalt content. The high-quality small drill bit can drill three plates (1.6mm/block) stacked on top of each other. 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 demand, such as the Mega 4600 from Prurite in Italy, the Excellon 2000 series in the United States, and new generation products from Switzerland and Germany.
②Laser 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 ablation has attracted 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 multilayer 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 high-density interconnect (HDI) technology, such as polyester film etching and metal deposition in M.C.Ms. (Sputtering technology) is used in the combined high-density interconnection.

The formation of buried vias 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 micro-drills, they were quickly promoted and applied. Therefore, the application of laser drilling in surface mount circuit boards cannot form a dominant position. But it still has a place in a certain field.

③Buried, blind, and through-hole technology

Buried, blind, and through-hole combination technology is also an important way to increase the density of printed circuits. Generally, buried and blind holes are tiny holes. In addition to increasing the number of wiring on the board, the buried and blind holes are interconnected by the "nearest" inner layer, which greatly reduces the number of through holes formed, and the isolation disk setting will also greatly Reduce, thereby increasing the number of effective wiring and inter-layer interconnection in the board, and improving the interconnection density.

Therefore, the multi-layer board with the combination of buried, blind, and through-holes has at least 3 times higher interconnection density than the conventional full-through-hole board structure under the same size and number of layers. If the buried, blind, The size of printed boards 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 have been increasingly 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 PCMCIA, Smard, IC cards and other thin six-layer boards.

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 .