Towards higher density and lighter
In recent years, the focus of the printed circuit board (PCB) market has shifted from computers to communications. In the past two years, it has shifted to mobile terminals such as smart phones and tablet computers. Therefore, HDI boards for mobile terminals are the main point of PCB growth. Mobile terminals represented by smartphones drive HDI boards to be higher density and thinner
PCBs are all developing towards high-density and thin-line development, and HDI boards are particularly prominent. Ten years ago, HDI board was defined as line width/line spacing of 0.1 mm/0.1 mm and below. Now the industry basically achieves 60 µm, and the advanced one is 40 µm.
PCB circuit pattern formation is traditionally a chemical etching process (subtractive method) after photoimaging on a copper foil substrate. This method has many procedures, is difficult to control, and has high cost. The current fine line production tends to be semi-additive or improved semi-processing.
The bonding force between the conductor and the insulating base material, the customary practice is to increase the surface roughness to increase the surface area and improve the bonding force, such as strengthening the decontamination treatment to roughen the surface of the resin layer, using high-profile copper foil or oxidizing the copper surface. For thin wires, this physical method is not enough to ensure the bonding force. Therefore, an electroless copper-plated high-binding copper foil on a smooth resin surface was developed. If there is a “molecular bonding technology”, the surface of the resin substrate is chemically treated to form a functional group that can be closely combined with the copper layer.
In addition, there is the transfer of dry film imaging patterns during the production of fine lines. The surface treatment of copper foil is one of the key factors for success. Use the best combination of surface cleaner and micro-etching agent to provide a clean surface with enough area to promote the adhesion of the dry film. Use chemical cleaning to remove the anti-tarnish treatment layer of the copper foil surface, and remove dirt and oxides. Choose the appropriate chemical cleaner according to the type of copper foil, followed by micro-etching the surface of the copper foil. In order to reliably combine the imaging dry film with the copper layer, the solder mask pattern and the thin circuit, a non-physical surface roughening method should also be adopted.
Semi-additive laminated base material
At present, the hot spot of the semi-additive method is the use of insulating dielectric film buildup. From the perspective of fine circuit implementation and production costs, SAP is more advantageous than MSAP. The thermosetting resin for SAP buildup is drilled by laser and electroplated copper to form via holes and circuit patterns.
At present, the international HDI laminate materials use epoxy resin with different curing agents to add inorganic powder to increase the rigidity of the material and reduce the CTE, and glass fiber cloth is also used to increase the rigidity.
Copper-plated hole filling
From the perspective of reliability, the interconnection hole adopts electroplated copper hole filling technology, including blind hole copper filling and through hole copper filling.
The ability of copper plating to fill holes is manifested in its filling: whether there are voids in the holes closed by copper; flatness: the degree of depression (Dimple) in the copper-plated hole; thickness-to-diameter ratio: the difference between the plate thickness (hole depth) and the hole diameter proportion.
Flip Chip Package IC Package Carrier Technology
Organic substrates account for more than one-third of the global semiconductor packaging market. As the production of mobile phones and tablets increased, FC-CSP and FC-PBGA increased significantly. The package carrier is replaced by an organic substrate, and the pitch of the package carrier is getting smaller and smaller, and the typical line width/line pitch is now 15 µm.
Future development trends. In BGA and CSP fine-pitch carrier boards will continue, while coreless boards and four- or more-layer carrier boards will be more used. The roadmap shows that the feature size of the carrier board is smaller, and the performance focus requires low dielectric and low Coefficient of thermal expansion and high heat resistance, pursuing low-cost substrates on the basis of meeting performance targets.
Adapt to high frequency and high speed requirements
Electronic communication technology ranges from wired to wireless, from low frequency and low speed to high frequency and high speed. The current mobile phone performance has entered 4G and will move towards 5G, that is, faster transmission speed and greater transmission capacity. The advent of the global cloud computing era has doubled the data traffic, and high-frequency and high-speed communication equipment is an inevitable trend. PCB is suitable for high-frequency and high-speed transmission. In addition to reducing signal interference and loss in circuit design, maintaining signal integrity, and maintaining PCB manufacturing to meet design requirements, it is important to have a high-performance substrate.
In order to solve the increase in speed and signal integrity of the PCB, it is mainly aimed at the loss of electrical signals. The key factors of substrate selection are dielectric constant (Dk) and dielectric loss (Df). When Dk is less than 4 and Df 0.010 or less, it is a medium Dk/Df laminate, and when Dk is less than 3.7 and Df 0.005 or less, it is a low Dk/ Df grade laminate.
The surface roughness (profile) of the conductor copper in high-speed PCBs is also an important factor affecting signal transmission loss, especially for signals in the range above 10 GHz. At 10 GHz, the copper foil roughness needs to be less than 1 µm, and it is better to use super-planar copper foil (surface roughness 0.04 µm).
Improve heat and heat dissipation
With the miniaturization, high functionality, and high heat generation of electronic equipment, the thermal management requirements of electronic equipment continue to increase. One solution chosen is the development of thermally conductive printed circuit boards. PCBs are required to have high thermal conductivity and heat resistance, and efforts have been made to this end for the past ten years. There are high heat dissipation PCBs such as flat thick copper substrate PCB, aluminum metal base PCB, aluminum metal core double-sided PCB, copper base flat PCB, aluminum base cavity PCB, embedded metal block PCB, flexible aluminum base PCB, etc. .
The metal substrate (IMS) or metal core printed circuit board is used to dissipate the heat of the heating component, which reduces the volume and cost compared with the traditional radiator and fan cooling. At present, most metal substrates or metal cores are metal aluminum. The advantages of aluminum-based circuit boards are simple and economical, reliable electronic connections, high thermal conductivity and strength, solder-free and lead-free environmental protection, etc., and can be designed and applied from consumer products to automobiles, military products and aerospace.
New trends in flexible and rigid-flex board technology
The miniaturization and thinning of electronic equipment will inevitably use a large number of flexible printed circuit boards (FPCB) and rigid-flex printed circuit boards (R-FPCB).
With the expansion of applications, in addition to the increase in number, there will be many new performance requirements. Polyimide films are available in colorless, transparent, white, black, and yellow, with high heat resistance and low CTE properties, which are suitable for different occasions. Cost-effective polyester film substrates are also available in the market. New performance challenges include high elasticity, dimensional stability, film surface quality, and film photoelectric coupling and environmental resistance to meet the changing requirements of end users.
FPCB and rigid HDI boards must meet the requirements of high-speed and high-frequency signal transmission. The dielectric constant and dielectric loss of flexible substrates must be paid attention to. Flexible circuits can be constructed using polytetrafluoroethylene and advanced polyimide substrates. . Adding inorganic powder and carbon fiber filler to the polyimide resin can produce a three-layer structure flexible thermally conductive substrate. The inorganic fillers used are aluminum nitride (AlN), aluminum oxide (Al 2O3) and hexagonal boron nitride (HBN).
In terms of FPCB manufacturing technology, the technology of direct metallization on polyimide (PI) film to manufacture double-sided FPCB has been developing. There is a new technology of molecular bonding agent aqueous solution, which does not change the surface roughness of PI film but can increase chemical precipitation. Bonding strength of copper layer. The PI film is used for molecular bonding and then directly chemically plated with copper. The double-sided flexible printed circuit board is produced through a semi-additive process, which simplifies the process and is environmentally friendly, and meets the requirements for bonding force, flexibility and reliability.
There is also the use of printed autocatalytic electronic circuit technology, in roll-to-roll production (R2R), the first printing and coating of autocatalytic ink on the PET film, and then into the electroless copper plating tank, because the ink has autocatalytic ability on the ink Deposit a copper layer to form a copper conductor pattern, and complete the production of thin metal lines on the PET film.
FPCB application markets such as smart phones, wearable devices, medical equipment, robots, etc., put forward new requirements for FPCB performance structure, and developed new FPCB products. Such as ultra-thin flexible multilayer board, four-layer FPCB is reduced from the conventional 0.4 mm to about 0.2 mm; high-speed transmission flexible board, using low Dk and low Df polyimide substrate, to reach the 5 Gbps transmission speed requirements;
High-power flexible boards use conductors with a thickness of 100 µm or more to meet the needs of high-power and high-current circuits; high-heat-dissipation metal-based flexible boards are R-FPCBs that use metal plate substrates locally; tactile-sensitive flexible boards are pressure-sensitive The sensing film and the electrode are sandwiched between two polyimide films to form a flexible tactile sensor; a stretchable flexible board or a rigid-flex board, the flexible substrate is an elastomer, and the shape of the metal wire pattern is improved adjustable.
Printed electronics has a very early history, but it has gained momentum in recent years. Printed electronics technology is applied to the printed circuit industry and is a part of printed circuit technology.
The continuous development of printed electronics shows that the prospects for commercial applications are very broad. Now PCB manufacturers have invested in printed electronics. They started with flexible boards and replaced printed circuit boards (PCB) with printed electronic circuits (PEC). Printed electronic technology is the closest to FPCB. At present, there are many substrates and ink materials. Once there is a breakthrough in performance and cost, it will be used in large quantities. Lowering costs will open up a larger market.
The hybrid system of organic and printed electronics contributes to the growth of the industry. The traditional hybrid system combining silicon and printed electronic components may open up a new PCB industry. These hybrid technologies include large-area lithography, screen printing or inkjet printing, and flexible PCB technology.
An important aspect of printed electronics technology is materials, including substrates and functional inks. Flexible substrates are not only suitable for existing FPCBs, but also higher performance substrates. At present, there are high-dielectric substrate materials composed of a mixture of ceramics and polymer resins, as well as high-temperature substrates, low-temperature substrates and colorless transparent substrates. , Yellow substrate, etc.
In addition to using some polymer materials, printed electronics also requires functional ink materials, mainly conductive inks, which are constantly developing towards improved conductivity, printing adaptability, and low cost. Currently, conductive inks are available for printed electronic products. There are many types. There are also piezoelectric, thermoelectric, and ferroelectric materials, which can be used in combination in printed electronics to achieve versatility.
Another important aspect of printed electronics technology is the printing process and corresponding printing equipment, which is an innovative development of traditional printing technology. Printed electronics can use different printing methods, such as gravure printing, letterpress printing, screen printing and inkjet printing. Screen printing has been applied in PCB manufacturing with mature technology and low cost. It is currently developing towards automation and high precision.
The scope of application of inkjet printing in PCB manufacturing is expanding, from marking symbols, solder resist to resist patterns, and further direct printing of conductive patterns; at the same time, inkjet printing is developing towards high-definition and rapid graphics. Nu Skin’s aerosol jet technology is significantly better than piezoelectric jet printing, and the formation of wires meets the requirements of fineness and three-dimensionality. Electronic circuits and components can be directly printed on flat or three-dimensional components.
There is also a method of inkjet printing that uses laser irradiation to instantaneously cure the ink. The ratio of the thickness to the width of the conductive circuit is more than 1.0, such as the line width of 10 µm and the line height of 10 µm. Examples include the production of a line width of 30 µm and a line thickness of 20 µm on PI FPCB.
The current key application of printed electronics is the manufacture of low-cost radio frequency identification (RFID) tags, which can be printed in rolls. The potential is in the areas of printed displays, lighting and organic photovoltaics. The wearable technology market is currently an emerging favorable market.
Various products of wearable technology, such as smart clothing and smart sports glasses, activity monitors, sleep sensors, smart watches, enhanced realistic headsets, navigation compasses, etc. Flexible electronic circuits are indispensable for wearable technology devices, which will drive the development of flexible printed electronic circuits.
Embedded component printed circuit technology
The embedded component printed circuit board (EDPCB) is a product that realizes high-density electronic interconnection. The embedded component technology has great potential in PCB. Embedded component PCB manufacturing technology has improved the function and value of PCB. In addition to applications in communication products, it also provides opportunities for automotive, medical and industrial applications.
The development of EDPCB, from printed resistors made of carbon paste and thin-film resistors made of nickel-phosphorus alloy foils, and planar capacitors composed of high-dielectric constant substrates, to form embedded passive component printed boards, to embedded ICs Chips and embedded SMD components form printed boards with embedded active and passive components. The current issues are the complexity of embedded components and the thinning of EDPCB, as well as heat dissipation and thermal deformation control, and final inspection technology.
Component embedding technology has now been applied in portable terminal equipment such as mobile phones. The EDPCB manufacturing process has entered the practical use of the B2it method, which can achieve high reliability and low cost; there is the PALAP method, which achieves high-level number and low power consumption, and is used in automotive electronics; there is a communication module with embedded wafer-level packaging chips, Reflecting good high-frequency characteristics, eWLB with embedded BGA chips will appear in the future . With the establishment of EDPCB design rules, such products will develop rapidly.
Surface finishing technology
The copper layer on the PCB surface needs to be protected in order to prevent oxidation and deterioration of the copper, and to provide a reliable surface during assembly. Some commonly used surface coatings in PCB manufacturing include lead-containing or lead-free hot air leveling solder, immersion tin, organic solderability protective film, electroless nickel/gold plating, and nickel/gold electroplating.
The surface finishes of HDI boards and IC package carrier boards have now evolved from electroless nickel/gold (ENIG) plating to electroless nickel/palladium/gold (ENEPIG) plating, which helps prevent black disks from appearing after component mounting and affecting reliability.
The palladium layer in the ENEPIG coating has been analyzed. The palladium layer structure has pure palladium and palladium-phosphorus alloys, which have different hardnesses. Therefore, different palladium layers must be selected for wire bonding and welding.
After reliability evaluation, the presence of a small amount of palladium will increase the thickness of copper-tin growth; and too much palladium will produce brittle palladium-tin alloys, which will reduce the strength of solder joints, so appropriate palladium thickness is required.
From the perspective of PCB fine circuit, surface treatment application of electroless palladium/immersion gold (EPIG) is better than electroless nickel/palladium/immersion gold (ENEPIG), reducing the impact on fine pattern line width/line spacing. EPIG coating is thinner and will not cause circuit deformation; EPIG can meet the requirements through soldering test and wire bonding test.
There are also new direct electroless palladium (EP) or direct immersion gold (DIG) coatings on copper, or electroless palladium and autocatalytic gold (EPAG) coatings on copper, which have the advantage of being suitable for pressure bonding of gold or copper wires. Because there is no nickel layer, it has better high frequency characteristics, the coating is thin and it is more suitable for fine line patterns, and reduces the process and cost.
The final coating layer of PCB is improved. In addition, there is the introduction of electroless nickel immersion silver (NiAg) coating. Silver has good conductivity and solderability, and nickel has corrosion resistance. The organic coating OSP improves performance to improve heat resistance and solderability. There is also an organic-metal composite (OM) coating, which is cost-effective to coat the OM coating on the copper surface of the PCB.
“Green” and “environmentally friendly” are now important signs of technological progress in PCB manufacturing. In addition to trying to adopt revolutionary cleaner production technologies such as printed electronics and 3D printing, the existing PCB manufacturing technology is constantly improving to cleaner production. Such as looking for materials to replace toxic and hazardous substances, reducing processing steps, and reducing the consumption of chemicals, as well as reducing the use of water and energy, and the recyclability of materials.
Specifically, there are halogen-free substrates that use non-toxic inorganic materials as flame retardants and also improve electrical properties, thermal conductivity, and thermal expansion coefficient; use laser direct imaging to reduce