1 30 Technical Paper Modified Semi-Additive Process Introduction Liu Binyun / Ye Shaoming Guang Dong Toneset Science&Technology Co., Ltd. Abstract: The light, small and multi-functional electronic products have led to the development of the printed circuit board on which the lines and vias depend on the high density interconnection, but it still cannot meet the requirements of the future development. With the advance of the 4G LTE and the development of the future 5G, low loss of signal transmission is required, and the shorter line is better. If the circuit board can achieve the same density as the IC board, and it is possible to load the IC chip directly on it. So the concept of the IC substrate like printed circuits board appears, but the manufacturing of this kind of board cannot be like the high cost of the IC substrate board manufacturing. Therefore, the msap new technology will be paid attention and developed. In this paper, the manufacturing process and requirements of msap are discussed, for the reference of the workers in PCB industry. resulting in PCB size shrinking, high-density interconnect (HDI) Printed Circuits Board manufacturing technology continues to increase. PCB wire width and spacing are further narrowed to accommodate more components; on the other hand, 5G's massive processing capacity requires more battery capacity, as well as more sophisticated processors and components, higher resolution displays, etc., need to reduce PCB size to the battery and the display. Therefore, the line width/spacing of the mobile PCB has dropped to 30/30µ m, and is expected to gradually [1 ] decrease to 0/0µ m. At the same time, the size of the microvia/via-pad will gradually decrease from 75/00µ m to [ ] 50/150µ m. With the increase of wireless data transmission bandwidth and processing speed, the signal trace density is continuously increasing, if the pitch of the line is less than 65ìm, it has been very difficult to produce such a line by the traditional subtractive method. Although the semi-additive process (SAP) used in the IC substrate board can realize more precise circuit Key words: Printed Circuits Board (PCB), High Density Interconnection PCB, Substrate Like PCB (SLPCB), modified Semi-Additive Process (msap). fabrication, but there is a problem that the manufacturing cost is high and the production scale is small, this resulted in the concept of a IC Substrate Like PCB (SLPCB). Therefore, it is necessary to select a suitable process for fabricating a SLPCB, this process must be able to meet the requirements for 1. Background With the advent of the 5G era, mobile terminals, especially smart phones, tablet PCs, and wearable devices, have become electroplating and filling holes, and at the same time, fine lines can be produced and improved. Modified Semi-Additive Process (msap) can achieve this goal. smaller and more versatile. Although the size of mobile phones has not changed significantly, it is clear that components and as PCBs continue to smaller; the level of integration has increased significantly to accommodate greater functionality. In a typical smart phone, most of the space is occupied by display screens and batteries, and the remaining electronic devices have been reduced in size and integrated into small areas, Apple leads the market in the direction of SLPCB. Typical products are the SLPCBs used by iphone 8 and iphone X. At present, the minimum line width/spacing manufactured by msap technology is 30/30µ m. The typical laser drilling diameter is designed to be a micro blind hole with diameter of 70µ m and thickness of dielectric layer of 50µ m. CO laser Journal of the HKPCA / 018 / Summer / Issue No. 68
2 Technical Paper 31 drilling will still dominate for a period of time in the future. Status, in the processing speed, drilling quality and production [3 ] efficiency is very advantage. The SLPCB produced by the msap manufacturing process is generally a laminated structure of micro-via holes or micro-via superimposed. For example, the process of a ten-layer board is promulgated as follows:. Modified semi-additive process introduction With the development of electronic interconnection, Integrated Circuits have been widely used to mount integrated circuit chips and other electronic components on carrier printed circuit boards (PCBs), wafers and interposers are required. The manufacturing of integrated circuit substrate (IC Substrate) requires interconnection and intercommunication between lines during assembly, and the size of the pitch varies greatly. The micro-scale dimensions and application objects are as follows: Figure 1. Typical SLPCB Process Design The SLPCB is still a kind of hard board in the PCB, but it is closer to the IC substrate. Currently, the line width/spacing of the SLPCB required is becoming smaller and smaller. This technology will reduce the occupied area of the mobile phone mainboard, thereby increasing the space of the battery and making batteries last longer, and also adapted to System in [3 ] Package (SiP) technology. Figure. Circuit width design and application Currently, circuit board manufacturing generally adopts subtractive method. Subtractive methods for producing fine lines will face restrictions on copper thickness (base copper thickness is generally 1/- oz) and copper thickness deviation Technology AP SAP msap Substrate Specific substrate ABF substrate Common substrate Base copper None None Yes Copper layer Pattern copper layer Panel plating copper + pattern Base copper layer + Electroless plating layer Copper layer (possibly with flash Plating) + pattern plating Electoless Copper 0.5µ m~6.0µ m above 1µ m 0.3µ m~1.5µ m Etching thickness None Panel plating copper layer Base copper + panel plating copper layer (possibly with flash plating) Adhesion adhesion is poor Resin surface roughening or The resin is connected to the base Molecular interface technology copper and has good adhesion to improve adhesion Reliability Explosive plate is easy to There is a risk of stratified Good reliability, can meet delamination, poor blasting and prone to poor the requirements thermal stability anodic ion migration Environmental impact No etching, less pollution Micro etching, less waste Low etching, low wastewater water discharge discharge Manufacturing cost Low High Medium Table 1. Comparison of Different Addition Methods and Their Performance
3 3 Technical Paper (uniformity of plated copper plating). Base copper, electroless copper and electroplating copper is needed to be etched away. The thickness of the etched copper is generally more than to 3µ m, so that the manufacturing cost is lower, so this article will introduce the msap process and requirements according to process 1, the flow diagram is shown As in Figure 4. 0µ m, so the line side etching is relatively large. It is very difficult to produce a line width/space below 50µ m, and the yield rate is low; to meet the needs of high-speed signal transmission and miniaturization, if the PCB line is 30µ m or less, then the IC substrate can be omitted directly. This poses a challenge to the traditional PCB manufacturing process and requires the introduction of new additions process. It can be seen that the selected semi-additive method (hereinafter referred to as msap) material is available, the process is similar to the existing PCB manufacturing, the product reliability is good, the manufacturing cost is moderate, based on the above analysis, msap method is selected for manufacturing, and the fine line SLPCB is the best choice for PCB development. Figure 4. msap process flow diagram 3. msap process requirement and control 3.1 Roughening and laser drilling The first UV laser drilling rig launched in 1997 was a 355 nm wavelength Nd:YAG. By the mid-000s, industry-leading PCB manufacturers began to develop CO laser drilling, which first reduced the thickness of copper to 5µ m to 1µ m, and use the The so-called msap has no fixed standard process, diverse process can be choosing due to different orders, equipments, and raw materials. The most typical process flow and requirements are as follows: black oxidation or brown oxide before the drilling to roughen and darken the copper surface and directly drill the copper foil with a CO laser. Due to the need to make fine PCB traces for the SLPCB, if we use 1/3-1/ oz base copper, before laser drilling, it is necessary to reduce the thickness to 3µ m, laser drilling has the requirements of the aperture and roundness, so the uniformity of copper reduction is particularly high. The technical advantage of this laser direct drilling formation is the reduction of the copper window etching step and the cost is significantly reduced, this is the main method used today for the mass production of the blind vias. However, the disadvantage of this method is that the processing window is narrow and cannot be reworked. From the quality point of view, Figure 3. Typical msap flow Because 3µ m copper foil is more expensive, and some factories directly try to use 1/3 OZ copper foil and then reduce it is a huge challenge to stabilize the production of micro blind vias with a volume of less than 70µ m. Because defects such as copper overhang, glass fiber protrusion, and resin residue will lead to quality problems in the subsequent desmearing and Journal of the HKPCA / 018 / Summer / Issue No. 68
4 Technical Paper 33 electroplating processes, these micro blind holes smaller than 100µ m must be optimized for the hole shape, remove the copper over the hole, and eliminate Glass fiber protruding and resin residues and other defects. For the current chemical copper reducing technology, it is difficult for the electrolytic copper foil to obtain a uniform thin copper and a better hole type. First of all, it is difficult to manage the uniformity of copper thickness 3.0± 0.5µ m by the normal copper reducing chemical, the best way is to use the brown oxide process to directly reduce the copper, and then Figure 5. SEM results of low etching brown oxide of msap with the uniformity design of the device, copper thickness after the reduction of copper can be controlled within ± 0.5µ m. At the same time, the conventional browning has a specific surface area of about 1.5. However, the 3µ m ultra-thin copper foil may have a specific surface area of.-.4 directly treated by a low-etch LDD brown oxide process, and it also matches for subsequent etching, smaller side etching, the side wall of the circuit is very vertical, not cause under-cut, can get the good line profile. Copper foil type & B/O Etching rate of Specific Brown Oxide surface area Ordinary copper foil with 1.4~1.8µ m 1.4 ~ 1.6 normal LDD B/O 3µ m ultra-thin copper foil 0.6~0.8 m. ~.4 with LE msap B/O Table. Comparison of effects of different browning treatments Figure 6. SEM results of browning after general brown oxide treatment reduces the under-cut of the PTH layer during subsequent etching and reaches the sidewall of the circuit vertical purpose. The current blind via diameter is about 50~80µ m, and the dielectric thickness is 30~50µ m. There is enough metallization As can be seen from the figure below, the 3µ m ultra-thin copper foil with low-etch msap brown oxide is used together with the low etching, the copper surface is villous fluffy, while the copper surface treated by traditional browning solution is a honeycomb structure. ability for the current horizontal PTH, but if the via pattern of laser drilling is not good, or there is excessive biting by the next steps, the PTH is required to have sufficient coverage to prevent voids under the over-hang and crab legs, as shown in Figure 7 below. 3. Electroless Copper Making hole conductive of msap uses generally a horizontal PTH, it can deposit a thin electroless copper layer to achieve a good coverage of the insulating layer in the blind via, which However, it should be noted that high-frequency materials are more difficult to remove the drilling smear, increase the amount of bite and fear of wicking problem, which will be fatal to signal transmission, and therefore need to have a better desmearing
5 34 Technical Paper effect. On the other hand, quality of the board surface after electroless copper is the key. If the depositing copper layer is rough and there are residues or copper nodule on the surface, final copper thickness. Accordingly, the thickness of the flash copper plating layer is generally 1-3µ m or 5-7µ m or so, and can adjust according to requirements. it will directly affect the effect of electroplating, which is a fatal flaw for the fine lines. In addition, the low stress copper is generally used. Series potions to maintain a good adhesion of the copper sink layer to the substrate. 3.4 Graphic Transfer Graphics transfer includes pre-treatment, dry film image, development steps. Pre-treatment generally uses chemical micro-etching on flash plating cooper, and common acid rinse is mainly used to highbuild electroless copper. Flash plating generally uses medium roughening (flash-plated copper layer thickness is 1-3µ m) or super-roughening (the thickness of flash-plated copper is 5-7µ m). The medium roughening can use hydrogen peroxide Figure 7. Horizontal electroless copper plating coverage 3.3 Flash plating Flash Plating, also known as "Cu Strike," is mainly for the type or composite salt series of micro-etching agents, and there are also selective CuCl super-roughening agents for the thickness flash copper. The overall point of view is the control of adhesion and total copper thickness. purpose of protecting the electroless copper layer from the pattern transfer process, and can be microetched and roughened before the subsequent pattern via-filling plating to improve the copper layer inter-binding force. Fine pitches and hole rings require more rigorous control of the graphics transfer process. For fine lines, rework, repair, etc. cannot be accepted. If we want to get a higher yield, we must pay attention to the quality of the graphics tools. The use of The choice of flash plating solution and the thickness of copper plating are all important. Generally, the solution with common through holes and blind via plating capacity is selected. On the one hand, the throwing power of the blind hole is better, and the bottom of the blind via can be coated with arc-shaped laser direct imaging (LDI) to replace contact film exposure mode is increasingly introduced for the manufacture of SLPCB. Although LDI is inefficient and costly, but it has the advantage of allowing each PCB board of different increases and decreases, which will reduce scrap due to misalignment. effects. "Crab legs" (incomplete copper plating or thin copper thickness at the bottom corner of the blind via) are not allowed; at the same time, the throwing power ability of the through hole can be taken into account, in particular, the knee of the through hole cannot be cut. The development solution is nothing special, it can be prepared with Na CO. The equipment has particular attention, to achieve 3 such a fine resolution, how to avoid the "pool effect" is the key. The industry has a vertical developing machine, which has a good development effect. Copper thickness control is an important part of the msap. How much copper is plated and how much copper is etched in each stage must be accurately calculated to achieve the goal of 3.5 Pattern Plating The pattern plating of msap is a pattern via filling plating Journal of the HKPCA / 018 / Summer / Issue No. 68
6 Technical Paper 35 technology. This via filling process is a high-tech process, it breaks the regularity of the distribution of the traditional electroplating current field, and changes the difference in current distribution between the plate surface and the hole, the copper plating speed in the hole is fast. On the surface of the board, the recessed holes are filled. For blind vias with a diameter of about 50~80µ m and a thickness of 30~50µ m, microcavity-free in the filled via,the dimples and protrusions of the filling holes should be less than 5µ m, and it must control the copper thickness after filling, the scope of the requirements, rather than the thinner the better. On the other hand, the inhomogeneity of the graphics will 3.6 Film stripping The traditional NaOH cannot meet the requirements of fine line film stripping. The need to remove the dry film between fine lines requires the use of a special stripper, usually an organic amine or solvent type, which can effectively strip the 0 to 30µ m pitch dry film. There must be no residue of dry film debris on the board after stripping. In addition, there is a higher requirement for film stripping equipment and filtration effect, and prevent the film from sticking to the surface of the board. Figure 9 shows the effect of removing the film with organic film stripper: result in inconsistent distribution of the current density in the plane direction, to make the surface copper plating uniform, the requirements for equipment and chemical are even higher. The thickness difference between the dense circuit area and the sparse area of the SLPCB is guaranteed within 5µ m, and there is no phenomenon of film clamping. Moreover, the surface of the circuit needs to be flat and straight, and cannot assume an arc shape of the trace line surface. The arc rate of the line surface requires 0%. Figure 9. The effect of removing the film of fine line 3.7 Quick etching The quick etching is also called "Flash Etching", the etching adopted by the msap process is completely different from the traditional one. It does not have dry film or tin plating as the resist layer. The whole board of the SLPCB after the film Figure 8. msap via filled by pattern plating In addition, the quality of this copper platting is strict, because the line width is only about 30µ m, plating can not have any pitting & dent, wrinkle and rough, even a pinhole on the circuit will cause scrap. Special attention is paid to the fact that the microcavity caused by subsequent etching is not allowed, maybe the plating is too large crystal. stripping is only copper, but the height difference is not the same, the copper thickness of the circuit surface is more than is more than 5µ m and the copper thickness between the lines is only a few microns, so the differential etching method is used to quickly etch away the copper layer between the lines, as the same time, the copper on the line surface also erodes the same thickness copper, it just comes back need to control the finish line copper thickness.
7 36 Technical Paper Hydrogen peroxide-type etching solution are also commonly used in msap, and also known as "flash" agents. The main products and pave the way for the early realization of new technologies such as Internet of Things and 5G. reason is that the vertical downward etching rate is controllable, and the line side etching is smaller, and the ratio of down etching to side etching is larger than 1:0.6. Strictly control the line type, the circuit etched out must be straight, the top line width and bottom width of the entire production board is almost the same, to ensure the required signal impedance and insulation resistance, so that the transmission loss and crosstalk are relatively low, the signal to noise ratio is high, the signal integrity is good. References  Meny Gantz, msap: An Indispensable New PCB Manufacturing Technology for 5G Smart Phones [J]. Hong Kong Circuit Board Directory, 017/Winter, (66):  Erkko Helminen, Tatja Rapala-Virtanen, Miniaturization Influence on Electrical Functionality of High Density Printed Circuit Board in press ECWC13, 014  Wu Jinhua. The Challenge of the Next Generation of Ultrathin HDI Printed Circuit Boards [J]. Printed Circuit Information, 015, (6):  PCB Info, an article to understand PCB technology changes and market trends [EB/OL]. Figure 10. The geometry of the conductors after quick etching  PCBTech.Net, a paper that reads the status quo of major 4. Conclusions and Prospects With the rapid development of mobile communications, the development of compact and multifunctional portable and PCB countries in the world [EB/OL] wearable devices is still the mainstream, resulting in the development of traditional high-density interconnect (HDI) boards toward the Substrate Like PCB (SLPCB). The result is msap technology application and development. This article through the typical SLPCB manufacturing process elaboration, msap process and requirements have a clear outline, but after all, different SLPCB have different designs and requirements, msap process will also be different; msap also due to the current technology base of different companies, from resources and with the continuous development and progress of various processes, will continue to improve and improve in the mass production. It is hoped that the majority of workers in PCB industry will continue to work hard to improve the msap process capability, to improve the quality and yield of SLPCB Journal of the HKPCA / 018 / Summer / Issue No. 68