The micro-miniaturization, intelligence and intelligent system regulations of the new generation portable electronic equipment promote the application of High-DensityAssemblyPCB board (high-density assembly board) and high-density interconnection (HDI) multilayer circuit boards. The traditional manufacturing process of laminated pcb circuit board can no longer be integrated into the application of ultra-fine particle spacing components, and then the production technology of high-precision circuit board interconnection circuit board is developed and designed. HDI board refers to the PCB whose graphic boundary/line distance is not more than 0.11mm and micro-conduction diameter is not more than 0.15mm
This commodity stipulates that the substrate material has high temperature resistance, excellent mechanical equipment performance, low thermal conductivity, low linear expansion coefficient, etc., and their laminated layer structure and manufacturing process are becoming more and more complicated.
Characteristics and regulations of basic materials for high-density assembled PCB
High-performance organic chemical compulsory PCB substrate is generally composed of dielectric layer (epoxy resin glue, glass fiber) and high-purity electrical conductor (copper foil). The key parameters to evaluate the quality of printed circuit board substrate are glass transition temperature Tg, linear expansion coefficient CTE, high temperature resistant dissolution time and temperature Td of substrate, electrical equipment performance, water absorption of pcb, electromigration CAF, etc.
The substrate of PCB is a part of the insulation layer composed of glass fiber, non-woven materials and epoxy resin, and then it is restrained into an “adhesive sheet” by epoxy resin glue and copper foil. The substrate of the board itself is made of insulating materials which are not easy to bend. It is widely used in a new generation of high-precision electronic devices and high-density interconnected multilayer circuit boards. Their regulations on the thermal performance, mechanical equipment performance and environmental protection of the substrate are becoming more and more stringent, and the structure and manufacturing process of stacked layers become complicated. However, the specification reliability, tensile strength, rigidity and ductility of PCB are limited by the characteristics of substrate materials. Professional and technical personnel should make comprehensive considerations according to the characteristics of commodities, high cost performance of materials, high assembling efficiency and credibility of panel-splicing, etc. when designing a scheme or process.
Miniaturized HDI products refer to the reduction of product specifications and net weights, such as wireless Bluetooth headsets, smart phones and portable smart home products, etc. According to the design scheme of increasing the relative density of wiring, this product is completed by applying small and medium-sized electronic devices such as CSPFC, internal interconnection of 6-layer or 8-layer boards, and adopting the buried hole process structure. The thickness of this kind of substrate is mostly less than millimeter, and it is FR4 material with high Tg(160℃).
Generally, HDI boards of high-density substrates are above four layers, and virtual beams are interconnected by buried holes or buried holes. At least two layers of HDI boards are provided with microplates, which are suitable for FC or bonded substrates. Microplate technology provides sufficient spacing for high-density flip-chip integrated ic. Microplate is used to handle PCB high-density wiring. Nowadays, the microplate has been changed from CNC to laser drill, and some CSP metallization holes are even smaller than 100μm m. Microplate technology can produce high I/O component spacing.
High-rise HDI board generally refers to the traditional pcb multilayer board with laser holes on the first to second or third floors, which adopts the sequential lamination process and carries out micropore processing on laminated glass raw materials. The technical purpose is to embed sufficient indoor space of components to ensure the specified characteristic impedance level, and it is suitable for HDI boards with high I/O number or fine spacing components.
In the traditional pcb multi-layer technology, all layers are laminated into a PCB at one time, and the whole line runs through the buried holes to carry out the virtual beam edge connection. In the HDI board technology, the electrical conductor layer and the cable sheath are laminated step by step, and the electrical conductors are connected according to the micro-buried or blind holes. With the increase of the relative density of PCB substrate interconnection, all-laminated structure or random layer interconnection has just begun to be applied. Therefore, HDI board process is generally called laminate process BUP or BUM. According to the way of micro-burying or opening and closing, it can be further subdivided into electroplating process hole lamination and conductive lamination process.
In HDI board technology, electroplating hole technology is a popular one, which basically accounts for more than 95% of HDI board sales market. It is also in the trend of sustainable development. From the initial traditional hole plating process to the hole filling plating process, the playability of HDI board design scheme has been greatly improved. The design scheme of electroplated multilayer board with holes is mainly concerned with the number of stacked layers, the structure of buried holes and micro-holes and the specifications of micro-holes. In addition, there is also a random layer gap-filling on-off interconnection technology ALIVH process, which completes the random virtual beam interconnection according to the conductive adhesive filling holes, all of which are completed by buried holes. Its main features are: firstly, the semi-dry solid sheet of non-woven aromatic fluorobenzene chemical fiber epoxy resin adhesive is used as the base material; secondly, the carbon dioxide laser is used to generate the pilot buried hole, and the pilot buried hole is filled with conductive paste, or the pilot buried hole is made of conductive paste bumps.
There are two kinds of common substrates for FPC: polyester, commonly known as PET, which is of high quality and low price. Its electrical equipment and physical properties are similar to those of poly (imino), and it can well resist wet and cold, and its thickness is 1~5mil. PET is suitable for the office environment of -40℃~55℃, but its heat resistance is weak, so it is decided that it can only be used for simple FPC, and can’t be used for FPC boards with components. Poly (imino), commonly known as PI, has excellent heat resistance, and its immersion welding resistance reaches 260℃/20s. It is basically used in all national defense hardware configurations and strictly regulated commercial machines and equipment. PI is easy to be affected with damp and expensive, and its thickness is generally 0.5~5CIL.
To sum up, FPC or Rigid-FPC substrates include injection molded copper (RA) and electrolytic nickel (ED). Injection molded copper is generally used for dynamic products with high bending frequency, while electrolytic nickel is only used for static data products with low bending frequency. Common insulation materials are poly (imino) (PI) and polyester (PET). PI is expensive and has good heat resistance, and its immersion welding resistance can reach 260℃ and 20s. However, these materials are easy to get damp and need to be baked before SMT production. However, although PET is cost-effective, it has poor heat resistance and is not suitable for SMT reflow welding.
Thermal performance regulation and selection of high-precision electronic device assembly board
When selecting PCB materials and designing schemes, the mechanical and thermal properties of materials should be suitable for the characteristics of commodities, and the problems of bending and in-situ stress caused by reflow soldering or wave soldering process should be considered. The specification reliability, tensile strength, rigidity and credibility of PCB are greatly affected by the substrate material. If the material selection is not suitable, the PCB will be deformed long before assembly, which will result in poor packaging and printing of high-precision fine-pitch components. The reasonable layout and copper coating of PCB should ensure uniform welding stress balance. For example, the point copper of PCB copper clad laminate is stronger than that of net copper.
Common pcb substrate raw materials can be divided into two types: organic chemistry and inorganic substrate. Organic materials generally include paper-based, epoxy glass cloth-based, polymer-based and kneading-based copper-coated polyimide films, while inorganic materials include porcelain substrates, metal substrates, copper-coated polyimide film glass fiber plates and BT epoxy resin for high-frequency circuits. For high-precision electronic circuit pcb, they have strict regulations on the material, structure and thermal performance of the substrate.
Optical packaging technology MPT is becoming more and more mature, especially in hand-held portable goods. The very small 0201 and 01005 passive components and ultra-narrow spacing components of micro-spacing MPT are more and more widely used. With the rapid increase of the relative density and processing speed of circuit boards, and the strict environmental protection regulations of electronic equipment, not only are the requirements for substrate materials more and more stringent. When selecting materials for high-precision PCB design, we need to investigate the thermal properties of the substrate: glass transition temperature Tg, thermal expansion index CTE, high-temperature dissolution time and dissolution temperature Td. Tg of substrate is the critical temperature that determines the performance of raw materials, and it is an important main parameter for selecting raw materials of substrate. When Tg is too low for reflow soldering, PCB tends to expand and shrink, and the I/O solder ends of PCB tend to cause the problem of coplanarity, and the in-situ stress and welding stress of mechanical equipment damage spot welding and electronic devices.
According to your understanding of the structure, thermal performance and electrical equipment performance of high-precision electronic circuit pcb, such as the material and structural characteristics of high-density assembly board, glass transition temperature Tg, linear expansion coefficient CTE, PCB dissolution temperature Td, high-temperature dissolution time, thermal conductivity, dielectric loss, electrical compressive strength, grounding resistance, electrical isolation resistance, water absorption of PCB, electromigration CAF, etc., we can better ensure our hearts when making design plans and selecting substrates.
