Overview of high precision technology for printed circuit boards
The high precision of printed circuits refers to the use of fine line width/pitch, micro holes, narrow ring width (or no ring width), and buried, blind holes and other technologies to achieve high density. The high precision means that the result of "fine, small, narrow, thin" will inevitably bring high precision. Take the line width as an example: O.20mm line width, and O.16-0.24mm is qualified according to the regulations. The error is (O.20 soil 0.04) mm; and the line width of O.10mm, the same error is (0.10±O.02) mm, obviously the accuracy of the latter is doubled, and so on is not difficult to understand. Therefore, the high precision requirements are not discussed separately. But it is a prominent problem in production technology.
(1) Fine wire technology The future high-density line width/pitch will be from 0.20mm-O.13mm-0.08mm-0.005mm to meet SMT and Multichip Package (MCP) requirements. Therefore, the following technology is required.
1. Use thin or ultra-thin copper foil (<18um) substrate and fine surface treatment technology. < p="">
2. Using a thinner dry film and wet filming process, a thin and good quality dry film can reduce line width distortion and defects. The wet film fills up small air gaps, increases interface adhesion, and improves wire integrity and accuracy.
3. 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 ring width, ringless width and full plate plating. At present, there are more than ten ED production lines in the world.
4. Using parallel light exposure technology. Since the parallel light exposure can overcome the influence of the line width variation and the like of the oblique light of the "dot" light source, a fine wire with a precise line width and a smooth edge can be obtained. However, parallel exposure equipment is expensive, has a high investment, and requires work in a high cleanliness environment.
5. Using Automatic Optic Inspection (AOI). This technology has become an indispensable means of detection in the production of fine wires, and is rapidly being promoted and applied. For example, AT&T has 11 AoI, and }tADCo has 21 AoIs dedicated to detecting the inner layer of graphics.
(2) Microporous technology The functional holes of the printed circuit board for surface mounting are mainly used for electrical interconnection, thus making the application of microporous technology more important. The use of conventional drill materials and CNC drilling machines to produce micro-holes has many failures and high costs. Therefore, the high density of printed boards is mostly done in the thinning of wires and pads. Although great achievements have been made, their potential is limited. To further improve the densification (such as wires less than 0.08 mm), the cost is urgent. Lit, thus turning to micropores to improve the densification.
In recent years, breakthroughs have been made in the development of CNC drilling machines and micro drill bits, and the micro-hole technology has developed rapidly. This is the main outstanding feature in the current production of printed boards. In the future, the micro-pore forming technology mainly relies on advanced CNC drilling machines and excellent micro-heads, and the small holes formed by laser technology are still inferior to the small holes formed by the CNC drilling machine from the viewpoints of cost and hole quality.
1. CNC 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 featuring drilling micro holes. Micro-hole drilling machine drilling hole (less than 0.50mm) is 1 times more efficient than conventional CNC drilling machine, less failure, speed is 11-15r/min; can drill O.1-0.2mm micro-hole, with high cobalt content The high-quality small drill 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 change the drill bit and check the diameter (the tool magazine can accommodate hundreds of pieces), can automatically control the constant distance between the drill tip and the cover plate and the drilling depth, so the blind hole can be drilled. It will not break the countertop. The numerical control drilling machine table adopts air cushion and magnetic floating type, which is faster, lighter and more precise, and will not scratch the table. Such drilling machines are currently very tight, such as the Italian Pyruette Mega 4600, the American ExcelIon 2000 series, and a new generation of products such as Switzerland and Germany.
2. Laser drilling of conventional CNC drilling machines
There are many problems with drill bits to drill tiny holes. It has hindered the progress of micro-hole technology, so laser etch holes have been valued, researched and applied. However, there is a fatal disadvantage in that the horn holes are formed and become severe as the thickness of the plate increases. In addition to the high-temperature ablation pollution (especially multi-layer board), the life and maintenance of the light source, the repeatability of the hole and the cost, the promotion and application of the production of micro-holes in printed boards is limited. However, laser etch holes are still used in thin high-density microplates, especially in MCM-L high-density interconnect (HDI) technology, such as polyester film etch holes and metal deposition (sputtering) in MCMs. Technology) is used in a combination of high-density interconnects. Buried hole formation in high-density interconnected multilayer boards with buried, blind via structures can also be applied. However, due to the development and technical breakthrough of CNC drilling machines and micro drill bits, it has been rapidly promoted and applied. Therefore, the application of laser drilling in surface mounted printed boards cannot be dominant. But still have a place in a certain field.
3. Buried, blind, through-hole technology buried, blind, through-hole bonding technology is also an important way to improve the high density of printed circuits. Generally, buried and blind holes are micro holes. In addition to increasing the number of wires on the board surface, the buried and blind holes are interconnected by "nearest" inner layers, which greatly reduces the number of through holes formed. The reduction increases the number of effective wiring and interlayer interconnections in the board, and increases the density of the interconnection. Therefore, the buried, blind, and through-hole combined multi-layer board has a higher interconnect density of at least 3 times in the same size and number of layers than the conventional all-through-hole structure. If under the same technical specifications, buried, blind, The printed board combined with the through holes will be greatly reduced in size or the number of layers will be significantly reduced. Therefore, in high-density surface-mounted printed boards, buried and blind-hole technologies are increasingly used, not only in surface-mounted printed boards in large computers, communication equipment, etc., but also in civil and industrial applications. The field has also been widely used, and even in some thin plates, such as various PCMCIA, Smard, IC card and other thin six-layer boards.
The printed circuit board with buried and blind hole structure is generally completed by the “sub-board” production method, which means that it can be completed after multiple presses, drilling, and hole plating, so precise positioning is very important.