PCB Technical - Challenges and progress faced by HDI technology in mass production of circuit boards

With the development of products such as smart phones, tablet computers and wearable devices in the direction of miniaturization and multi-function, the technology of high-density interconnected printed circuit boards has been continuously improved, the width and spacing of PCB wires, the diameter and hole center of the micro-hole disk The distance, as well as the thickness of the conductor layer and the insulating layer are constantly decreasing, so that the number of layers of the PCB can be increased to accommodate more components without increasing the size, weight, and volume of the PCB. In addition, with the increase of wireless data transmission bandwidth and processing speed, the electrical performance of the PCB becomes extremely important.

Just as the integrated circuit industry has encountered obstacles for performance expansion and compliance with Moore's Law, the PCB industry is also facing challenges in process capability and material performance in order to continuously improve interconnect density and electrical performance. Even if the PCB adopts any-layer interconnection high-density (ALV HDI) design, there are still limitations in performance expansion and improvement, and the manufacturing cost is also increased, and there is a cost-effective problem. The PCB industry is faced with the challenge of increasing the number of layers and decreasing thickness. The thickness of the insulating layer has fallen below the critical value of 50 μm, and the dimensional stability and electrical performance of the PCB (especially signal impedance and insulation resistance) have declined.

At the same time, the density of signal traces continues to increase, and the width of the trace is less than 40 μm. It is very difficult to fabricate such a trace using the traditional subtractive method. Although the additive method technology can realize the production of more refined circuits, it has the problems of high cost and small production scale. The use of complex and automated suitable equipment increases, such as laser direct imaging (LDI) equipment and laser direct drilling (LDD) 100 μm laser hole technology can improve the above problems, but the cost will increase, and the material performance is also somewhat limited. These also mean that we need to focus on the basics to make our system more powerful and cost-effective. This article introduces the recent challenges and progress of ALV HDI technology in mass production to meet its demand for volume, reliability, and competitive price in the field of electronic packaging.

1. Overview of ALV HDI Technology With the popularity of social media, more and more communication is realized through smartphones or tablets. Social media is now an important part of any successful corporate marketing plan. It provides us with a platform to communicate with existing and potential customers, and it can also provide us with feedback and new ideas frequently. This means that the amount of data for information transmission has greatly increased in recent years, and will continue to increase. The increase in subsequent functions and the reduction in component size will be the main driving force for the development of PCB. The development speed of semiconductor technology is almost exponential, doubling every two years, and this development speed will continue in recent years. When we compare the classic rigid PCB structure used in the first generation of mobile phones with the latest PCB used in current smartphones, we can see a huge difference. It can be said that miniaturization is the main trend in recent years. Although the size of the mobile phone has not changed much, it is obvious that the components and PCB are shrinking continuously to adapt to stronger functions. In a typical smart phone or tablet computer, most of the space is occupied by the display screen and battery, and the remaining electronic devices have been reduced in size and integrated into a small area. As the component spacing decreases and the number of I/Os increases, perhaps one of the most significant changes is the thinning of the board and the increase in the number of layers. Ten years ago, the thickness of a typical rigid PCB was more than 1 mm. Now, the thickness of a typical smartphone PCB is about 0.5 to 0.7 mm. However, there is a clear trend showing that the number of layers is increasing while the thickness of the board is decreasing. According to the industry roadmap, it can be expected that PCBs less than 0.4 mm thick will appear in handheld devices in the next few years. Depending on the complexity of the product, the number of layers containing micropores will increase to 10 or even 12. Obviously this will lead to the use of thin dielectric and conductor layers. A few years ago, 0.6 mm ~ 0.8 mm pitch technology was used in handheld devices at that time. Today's smart phones, due to the number of component I/O and product miniaturization, make the PCB widely used 0.4 mm pitch technology. As expected, this trend is moving towards 0.3 mm. In fact, the development of 0.3 mm pitch technology for mobile terminals has started several years ago. At the same time, the size of the micro-hole and the diameter of the connecting plate have been reduced to 75 mm and 200 mm, respectively. The industry’s goal is to reduce the micro-holes and disks to 50 mm and 150 mm, respectively, in the next few years. Figure 2 The miniaturization of the 0.3mm pitch design specification drives the reduction of the line width, pitch and surface mount board size in the ALV HDI PCB. With the use of any layer technology, miniaturization becomes possible. Since the interconnection can be formed between any layer, this gives designers more freedom. The improvement in the capacity of the thin wire manufacturing process is obvious. And new manufacturing and processing solutions are necessary to meet the requirements of these new designs.

2. Challenges faced by ALV HDI PCB manufacturing The key production steps of ALV HDI PCB miniaturization are multilayer lamination, laser drilling, imaging, etching and electroplating processes, and how to optimize the process to meet high-volume, robust, reliable and low-cost production processes. Cost of production. 1. The evolution of micro-hole laser technology In the mid-1990s, the component pin pitch decreased. The technical difficulty lies in connecting high-I/O components with multi-layer PTH PCBs. In order to meet this challenge, the PCB industry has not only reduced the through holes of mechanical drills to less than 150 mm, but also developed micro-hole technologies, such as photoimageable dielectric layers, plasma etching holes, and laser drilling methods. However, the technology of forming holes by photoimaging requires special photosensitive materials, and plasma has no effect on FR-4. Because of its flexibility, laser drilling has now become the dominant production method. Initially, the available lasers were TEA CO2 and UV Nd: YAG. There were several shortcomings that limited their practicality and accuracy.

The TEA CO2 laser has a wavelength of 10600 nanometers, it cannot drill copper, its speed is slow, and the pulse is easy to miss. Therefore, there are certain difficulties in application. When using this kind of laser drilling machine, it is necessary to make a window (Conformal Mask) that is as large as or slightly larger than the final laser aperture on the copper surface. In addition, after this long-wavelength laser ablation, a carbonized layer will be formed in the PCB, and this carbonized layer must be removed through relatively strong dross removal parameters. The laser of the first UV laser drill launched in 1997 was Nd: YAG with a wavelength of 355 nm. The laser can be focused well through a small spot diameter, using the hole and circling method. These UV laser drills are effective when drilling copper and resin. However, there is a problem when drilling FR-4. This is because FR-4 contains glass fibers, which absorb UV light very weakly and are not easily interrupted. Therefore, PCB products using UV laser drilling need to use resin-coated copper foil (RCC) instead of FR-4 as a build-up material. The efficiency of the UV laser drilling machine is very low, and the power stability is also problematic. After the stability has improved and the rated power has increased sharply, glass fiber ablation is still a problem, and the production capacity of UV laser drilling rigs is much lower than that of carbon dioxide laser drilling rigs, so UV drilling rigs are currently only suitable for some special occasions. Later, some companies began to combine CO2 lasers with UV lasers, but this solution is only suitable for PCB prototypes and small batch production. For batch boards, this combined method is not economical and affordable.

1998 was a year when the demand for blind microplates increased substantially. Therefore, mainstream PCB manufacturers have standardized the etching + carbon dioxide laser process, and new CO2 laser drilling rigs have begun to be put on the market, which have no pulse loss and are faster. The substantial increase in production capacity of the new CO2 drilling rig will ultimately make it cost-effective in mass production. The drilling process is also very stable. By the mid-2000s, industry-leading PCB manufacturers began to develop direct drilling through copper foil. Thin the copper to 5 mm ~ 12 mm thick, and roughen and darken the copper surface before drilling. The technical advantage of this laser direct hole formation is that the step of etching the copper window is reduced, and the cost is significantly reduced. This is today the main method for the production of blind microvias for any layer interconnection. However, the disadvantage of this method is that the processing window is relatively narrow and cannot be reworked. From a quality point of view, it is a huge challenge for the stable mass production of blind microvias smaller than 100 μm. Because defects such as overhang copper in the orifice, protruding glass fiber, and resin residue will cause quality problems in the subsequent desmear and electroplating process, these micro blind holes smaller than 100 μm must be optimized to remove the overhang copper in the orifice and eliminate them. Defects such as glass fiber protrusion and resin residue. CO2 laser drilling will still dominate for some time to come. However, new picosecond and femtosecond laser drilling rigs will enter the market. These drilling rigs have advantages in processing speed, drilling quality and production efficiency. When the industry faces the challenge of small-aperture laser blind holes, these laser drilling rigs may become a development direction. Moreover, the thermal damage of these laser drills to materials is less than that of long-pulse laser drills (such as CO2 laser drills). These new laser drills can drill holes in copper foil that has not been processed in any way. 2. Electroplating and imaging process The choice of PCB electroplating process is determined by the line width/spacing, the thickness of the insulating layer, and the final copper thickness. In the 0.3 mm pitch BGA design, the diameter of the pad is 150 μm, the blind hole is 75 μm, and two 30 mm/30 mm thin lines are run between the two pads with a pitch of 0.3 mm. It is challenging to make this kind of fine circuit through the existing subtractive method. In the subtractive method, the etching ability is one of the key factors, and both the pattern transfer process and the plating uniformity need to be optimized. This is why the PCB industry uses the mSAP process to make fine lines. Compared with the subtractive method, the top width and bottom width of the fine line made by mSAP process are almost the same, that is, it is easier to control the line into a square shape. Another advantage of mSAP is that it uses standard PCB processes, such as drilling and electroplating, and other existing technologies, and the use of traditional materials can provide good adhesion between the copper and the dielectric layer to ensure the reliability of the final product. Compared with the subtractive method, the biggest advantage of the mSAP process is that the line type is easy to control, and the top width and bottom width of the entire production board are almost the same. The line thickness is reduced, the line type can be controlled, the crosstalk is low, the signal-to-noise ratio is high, and the signal integrity is improved. In fact, such thin wires and thinner dielectric layers must have characteristic impedance levels.

At present, the circuit of PCB products is getting thinner and thinner, and the thickness of the dielectric layer is continuously reduced. Therefore, it is necessary to choose a suitable PCB manufacturing process. This process must be able to meet the requirements of electroplating and filling holes, and at the same time be able to produce fine lines. Finer lines, smaller pitches, and ring holes require stricter control over the pattern transfer process. For fine lines, methods such as repair, rework or repair cannot be used. If you want to get a higher pass rate, you must pay attention to the quality of the graphics production tools, the parameters of the laminated prepreg and the parameters of the graphics transfer. For this technology, the use of laser direct imaging (LDI) instead of contact exposure seems increasingly attractive. However, LDI has low production efficiency and high cost, so more than 90% of PCB products use contact exposure for graphic transfer. Only when LDI can greatly improve the yield rate, it is more cost-effective to use LDI. Now, the PCB yield improvement of complex arbitrary layer interconnection is very important, therefore, we tend to use LDI. Without LDI, it would be impossible to produce PCBs for high-end smartphones. The advantage of LDI is that it allows each PCB board to use different expansion and contraction, which will reduce the scrap due to inaccurate alignment. In order to give full play to the superiority of LDI, dry film or wet film needs to be matched with the graphics transfer technology to obtain the best production capacity. Recently, the process capability and production capacity of dry/wet film have been greatly improved. This may help you buy LDI to make graphics transfers. Because when you are faced with some other choices, you always want to use tried-and-tested technology. In addition, there is also a DI machine that can also be used in PCB production. About 25% of the newly sold DI machines are used for the production of solder mask patterns. The use of DI in the solder mask process may greatly increase the yield, but the disadvantage is that its production capacity is too low.

3. ALV HDI technology summary This article mainly introduces the key manufacturing process of any layer interconnection PCB board in the production process and its impact on cost. When choosing a process, it should be considered that this technology must meet the current and future needs of electronic packaging products. The challenges faced by HDI PCB are: the increase in PCB functions and the reduction in size, as well as the ultra-thin structure that frequently appears in recent terminal products. In order to prepare materials and production methods in a timely manner, it is necessary to effectively manage the supply chain, shorten the prototype production cycle, and bring their products to the market faster. Subtractive methods (copper foil or electroplating) to make fine lines will face the limitations of copper thickness and copper thickness deviation, which are sensitive to wire spacing, thickness deviation and base copper roughness. The additive method has a higher resolution, and the line type is good when making fine lines, but for engineers, the control is more complicated and may require a lot of investment. The fine lines of the mSAP process have straighter sidewalls, so the transmission loss and crosstalk are relatively low, and the PCB signal integrity is improved. There is no simple answer to the choice of the PCB production process, because the choice of the PCB production process mainly depends on the characteristics of the product design. If the engineer is involved in the product design process early, it will help to find the most economical solution.