With the advancement of high integration and assembly (especially chip-scale/µ-BGA packaging) technology of electronic components (groups). Greatly it promotes the development of "light, thin, short, and small" electronic products, high-frequency/high-speed digitalization of signals, and large-capacity and multi-functionalization of electronic products. Development and progress, which requires PCB to quickly develop in the direction of very high density, high precision and multi-layer.
In the current and future periods of time, in addition to continuing to use (laser) micro-hole development, it is important to solve the "very high density" problem in PCBs. The control of fineness, position, and inter-layer alignment of wires. The traditional "photographic image transfer" technology, it is close to the "manufacturing limit" and it is difficult to meet the requirements of very high-density PCBs, and the use of laser direct imaging (LDI) is the goal to solve the problem of "very high density (referring to occasions where L/S ≤ 30 µm)" fine wires and interlayer alignment in PCBs before and in the future the main method of the problem.
1. The Challenge Of Very High-Density Graphics
The requirement of high density PCB is in essence mainly from IC and other components (components) integration and PCB manufacturing technology war.
(1) Challenge Of Integration Degree Of IC And Other Components.
We must clearly see that the fineness, position and micro-porosity of PCB wire are far behind the IC integration development requirements are shown in Table 1.
||Integrated Circuit Width /µm
||PCB Line Width /µm
||1:560 ~ 1:170
||1:200 ~ 1:500
||1:200 ~ 1:500
Note: The size of the through hole is also reduced with the fine wire, which is generally 2~3 times the width of the wire.
Current and future wire width/spacing (L/S, unit -µm)
Direction: 100/100→75/75→50/50→30/3→20/20→10/10, or less. The corresponding micropore (φ, unit µm):300→200→100→80→50→30, or smaller. As can be seen from the above, PCB high density is far behind IC integration. The biggest challenge for PCB enterprises now and in the future is how to produce "very high-density" refined guides the problems of line, position and microporosity.
(2) Challenges Of PCB Manufacturing Technology.
We should see more; Traditional PCB manufacturing technology and process can not adapt to the development of PCB "very high density".
①The graphic transfer process of traditional photographic negatives is lengthy, as shown in Table 2.
Table 2 Processes required by the two graphics conversion method
|Graphic Transfer Of Traditional Negatives
||Graphics Transfer For LDI Technology
|CAD/CAM： PCB design
||CAD/CAM： PCB design
|Vector/raster conversion, light painting machine
||Vector/raster conversion, laser machine
|Negative film for light painting imaging, light painting machine
|Negative development, developer
|Negative stabilization, temperature and humidity control
|Negative inspection, defects and dimensional checks
|Negative punching (positioning holes)
|Negative preservation, inspection (defects and dimensions)
|Photoresist (laminator or coating)
||Photoresist (laminator or coating)
|UV bright exposure (exposure machine)
||Laser scanning imaging
② The graphic transfer of traditional photographic negatives has a large deviation.
Due to the positioning deviation of the graphic transfer of the traditional photo negative, the temperature and humidity of the photo negative (storage and use) and the thickness of the photo. The size deviation caused by the "refraction" of light due to the high degree is above ± 25 µm, which determines the pattern transfer of traditional photo negatives. It is difficult to produce PCB wholesale products with L/S ≤30 µm fine wires and position, and interlayer alignment with the transfer process technology.
2 Role Of Laser Direct Imaging (LDI)
2.1 The Main Disadvantages Of Traditional PCB Manufacturing Technology
(1) The Position Deviation And Control Cannot Meet The Requirements Of Very High Density.
In the pattern transfer method using photographic film exposure, the positional deviation of the formed pattern is mainly from the photographic film. The temperature and humidity changes and alignment errors of the film. When the production, preservation and application of photographic negatives are under strict temperature and humidity control, The main size error is determined by the mechanical positioning deviation. We know that the highest precision of mechanical positioning is ±25 µm with repeatability of ±12.5 µm. If we want to produce PCB multilayer diagram with L/S=50 µm wire and φ100 µm. Obviously, it is difficult to produce products with a high pass rate only due to the dimensional deviation of mechanical positioning, let alone the existence of many other factors (photographic film thickness and temperature and humidity, substrate, lamination, resist thickness and light source characteristics and illuminance etc.) due to size deviation! More importantly, the dimensional deviation of this mechanical positioning is "uncompensable" because it is irregular.
The above shows that when the L/S of the PCB is ≤50 µm, continue to use the pattern transfer method of photographic film exposure to produce. It is unrealistic to manufacture "very high density" PCB boards because it encounters dimensional deviations such as mechanical positioning and other factors the "manufacturing limit"!
(2) The Product Processing Cycle Is Long.
Due to the pattern transfer method of photo negative exposure to manufacturing "even high density" PCB boards, the process name is long. If compared with laser direct imaging (LDI), the process is more than 60% (see Table 2).
(3) High Manufacturing Costs.
Due to the pattern transfer method of photo negative exposure, not only many processing steps and long production cycle are required, so more multi-person management and operation, but also a large number of photo negatives (silver salt film and heavy oxidation film) for collection and other auxiliary materials and chemical materials products, etc., data statistics, for medium-sized PCB companies. The photo negatives and re-exposure films consumed within one year are enough to buy LDI equipment for production or put into LDI technology production could recover the investment cost of LDI equipment within one year, and this has not been calculated by using LDI technology to provide high product quality (qualified rate) benefits!
2.2 Main advantages of Laser Direct Imaging (LDI)
Since LDI technology is a group of laser beams directly imaged on the resist, it is then developed and etched. Therefore, it has a series of advantages.
(1) The Position Degree Is Extremely High.
After the workpiece (board in the process) is fixed, laser positioning and vertical laser beam
Scanning can ensure that the graphic position (deviation) is within ±5 µm, which greatly improves the positional accuracy of the line graph, which is a traditional (photographic film) pattern transfer method can not be achieved, for high-density manufacturing (especially L/S ≤ 50µmmφ≤100 µm) PCB (especially the interlayer alignment of "very high density" multi-layer boards, etc.) It is undoubtedly important to ensure product quality and improve product qualification rates.
(2) The Processing Is Reduced And The Cycle Is Short.
The use of LDI technology can not only improve the quality of "very high density" multi-layer boards' quantity and production qualification rate, and significantly shorten the product processing process. Such as pattern transfer in manufacturing (forming inner layer wires). When on the layer that forms the resist (in-progress board), only four steps are required (CAD/CAM data transfer, laser scanning, development, and etching), while the traditional photographic film method. At least eight steps. Apparently, the machining process is at least halved!
(3) Save Manufacturing Costs.
The use of LDI technology can not only avoid the use of laser photoplotters, automatic development of photographic negatives, Fixing the machine, diazo film developing machine, punching and positioning hole machine, size and defect measuring/inspecting instrument, and storage and maintenance of a large number of photographic negatives equipment and facilities, and more importantly, avoid the use of a large number of photographic negatives, diazo films, strict temperature and humidity control the cost of materials, energy, and related management and maintenance personnel is significantly reduced.