1. With the continuous advancement of the electronics industry, PCB manufacturers are increasingly integrating electronic components, leading to a significant reduction in their size. BGA (Ball Grid Array) packages have become the standard choice. As a result, PCB circuits are shrinking in size, and the number of layers is increasing. To maximize the use of limited space, the line width and spacing must be reduced, while increasing the number of layers allows for more efficient use of available area. In the future, the main signal traces on circuit boards will likely be 2-3 mils, or even smaller.
2. It is widely accepted that each time the production process is upgraded to a higher grade, a substantial investment is required. This investment can be significant, as high-end circuit boards are produced using advanced equipment. However, not every company can afford such large-scale investments, and conducting trial production to collect process data after such investments can be both time-consuming and costly. A more practical approach might be to conduct experiments and trial runs based on the current capabilities of the company, then make a decision about further investment depending on the results and market conditions. This article discusses in detail the limitations of producing thin lines with standard equipment and outlines the conditions and methods for thin-line production.
3. The typical production process for PCBs can be divided into two main methods: the cover-hole acid etching method and the pattern electroplating method. Both have their respective advantages and disadvantages. The acid etching method produces very uniform circuits, which is beneficial for impedance control, and it generates less environmental pollution. However, a single break in a hole can result in scrap. On the other hand, the alkaline corrosion method offers easier process control, but the resulting circuit quality is uneven, and it also leads to higher environmental pollution.
4. The dry film plays a critical role in PCB production. Different types of dry films offer varying resolution capabilities, but typically, they can display line widths as fine as 2 mils after exposure. Standard exposure machines are capable of achieving a resolution of 2 mils. Within this range, line width and spacing issues are generally not problematic. For development machines that work with line widths of 4 mils or greater, the pressure and concentration of the developer solution are less critical. However, for line widths smaller than 3 mils, the nozzle becomes a key factor in resolution. Typically, fan-shaped nozzles are used, and the development pressure is usually around 3 BAR.
1. Although exposure energy has a significant impact on circuit quality, most of the dry films currently available on the market offer a wide exposure range. These films can be distinguished at levels 12-18 (on a 25-level exposure scale) or levels 7-9 (on a 21-level exposure scale). In general, lower exposure energy improves resolution; however, if the energy is too low, dust and impurities in the air can affect the process. This can lead to open circuits (acid corrosion) or short circuits (alkali corrosion) in subsequent steps. Therefore, actual production should consider the cleanliness of the darkroom to select the appropriate minimum line width and spacing based on the situation.
2. The effect of developing conditions on resolution becomes more pronounced as line widths decrease. For circuits above 4.0mil/4.0mil, development conditions such as speed, solution concentration, and pressure have little noticeable effect. However, when the circuit is 2.0mil/2.0mil, nozzle shape and pressure become critical factors for proper development. At this scale, development speed may significantly slow down, and the concentration of the developer can affect the final circuit appearance. The large pressure of a fan-shaped nozzle can still reach the bottom of the dry film in areas with closely spaced lines, aiding in development. In contrast, a cone-shaped nozzle has lower pressure, making it difficult to develop fine lines. Additionally, the orientation of the PCB can significantly affect both resolution and the sidewalls of the dry film.
3. Different exposure machines offer varying resolutions. One commonly used exposure machine is air-cooled with a surface light source, while another uses a water-cooled, point light source. Both types are nominally rated for 4mil resolution. However, experiments show that without special adjustments, resolutions of 3.0mil/3.0mil and even as fine as 0.2mil/0.2mil are achievable when the exposure energy is reduced. Resolutions such as 1.5mil/1.5mil can also be obtained, but caution is needed, as dust and debris can greatly impact results. Moreover, there is little to no noticeable difference in resolution between Mylar and glass surfaces in these experiments.
4. In the case of alkaline etching, the “mushroom effect” after electroplating is a common issue, although the extent of the effect varies from noticeable to subtle. For line widths larger than 4.0mil/4.0mil, the mushroom effect is typically minimal. However, when the circuit is 2.0mil/2.0mil, this effect becomes more pronounced. The overflow of lead and tin during electroplating causes the dry film to take on a mushroom shape, making it difficult to remove the film due to it being trapped inside. Solutions to mitigate this include: 1. Using pulse electroplating to ensure a more uniform coating; 2. Employing thicker dry films (typically 35-38 microns, but thicker films of 50-55 microns can also be used, albeit at higher cost), which offer better performance in acid etching; and 3. Using low-current electroplating. However, none of these methods provide a perfect solution.
5. Due to the mushroom effect, film removal for thin lines becomes quite troublesome. Since sodium hydroxide causes significant corrosion of lead and tin at 2.0mil/2.0mil, one solution is to increase the thickness of the lead and tin plating and reduce the sodium hydroxide concentration during electroplating.
6. In alkaline etching, the etching speed varies depending on both the line width and shape. If there are no specific requirements for line thickness, it is common to use a PCB with 0.25oz copper foil, or to etch a 0.5oz base and reduce the copper thickness. The electroplated copper will then be thinner, and lead-tin thickening can also affect the results. The nozzle type is important for producing thin lines in alkaline etching; fan-shaped nozzles are preferred, as conical nozzles typically achieve a minimum resolution of 4.0mil/4.0mil.
7. In acid etching, similar to alkaline etching, the etching speed varies with line width and shape. However, dry film is more prone to breaking or scratching during transfer or earlier stages of production. Therefore, caution must be exercised. Generally, acid etching yields better line definition than alkaline etching, with less side etching and no mushroom effect. In addition, fan-shaped nozzles provide noticeably better results than conical nozzles. After acid etching, the impedance of the wires changes only slightly.
8. During PCB layout design and production, factors such as film speed, temperature, cleanliness of the board surface, and the cleanliness of the diazo film have a significant impact on the yield rate. The parameters for acid etching and the flatness of the board are particularly crucial. For alkaline etching, the cleanliness of the exposure process is of utmost importance.
9. Therefore, it is possible to produce 3.0mil/3.0mil (referring to film line width and spacing) circuits with standard equipment, without the need for special adjustments. However, the yield rate is influenced by environmental factors as well as the experience and skill level of the operators. Alkaline etching is more suitable for producing circuits with line widths below 3.0mil/3.0mil, unless the base copper thickness is sufficiently reduced. In such cases, fan-shaped nozzles provide significantly better results than cone-shaped nozzles.
If your have any questions about PCB ,please contact me info@wellcircuits.com
