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Good PCB Substrate Selection Makes Your PCB Design Half-successful

by: May 21,2020 2682 Views 0 Comments Posted in PCB Basic Information

PCB Design PCB Manufacturing CCL PCB Substrate

Many characteristics of PCB are determined by the substrate,improper selection of the substrate will not only affect some performance of the product. Such as heat resistance, product operating temperature, insulation resistance, dielectric loss, etc. It will even affect the cost of the manufacturing process, so understanding the characteristics of the PCB substrate, selecting the substrate, and choosing the substrate reasonably is one of the important contents of PCB design.

However, many PCB designers are specialized in circuit design, and some PCB designers believe that the choice of substrate is a matter of PCB manufacturing process, ignoring an important point that the design specifications should be selected according to the product assembly process environment, and the result is the design products cannot meet the technological requirements of mass production. The factory’s staff sometimes cause improper selection of the corresponding welding process in production due to insufficient knowledge of PCB materials, causing scrapping of large quantities of products or important welding quality problems.

There are many types of substrates used for PCBs, and the CCL, the Copper Clad Laminate, is the main material for manufacturing PCBs. According to the variety of reinforcing materials used, it is mainly divided into paper base, glass fiber base, composite base, and metal base. Among the electronic products we use every day, paper base and glass fiber base are the most commonly used.

The paper substrate is a copper-clad plate reinforced with fiber paper impregnated with phenolic or epoxy resin. In paper-based CCL, common brands are XPC, XXXPC, FR-1, FR-2, FR-3, and other varieties.

Due to the looseness of paper, it can only punch holes during processing, and cannot drill holes. Besides, the water absorption is also high, so the general paper substrate is only suitable for making single panels. Due to the large Z-coefficient of thermal expansion of the paper-based substrate, it will lead to metallization holes and the solder joints will be broken when the temperature changes. It is generally not recommended to use metalized vias when selecting this type of substrate design.

In general, the main disadvantages of paper-based materials are their poor physical strength, poor impact resistance, poor material dimensional stability, unstable dielectric properties, and easy to be damaged. Its advantages are its punch ability and low cost, and its electrical performance can meet the design requirements of general non-critical electronic products. Among them, the punching processability makes the mass-produced paper-based products more economical in terms of processing.

In terms of heat resistance, the heat resistance of all paper substrate dip soldering surfaces with a thickness of 1.6mm or above should meet the process requirements of no abnormality within five seconds at 260 degrees Celsius, and the heat resistance in the air should meet the test condition of no bubble layering within 30 minutes at 120 degrees. Based on the above characteristics, such substrates can generally meet the welding conditions of wave soldering, dispensing, and curing. Due to the high temperature and duration of lead-free reflow soldering, such substrates are generally not suitable for reflow soldering.

Glass fiber base CCL is a copper-clad plate with glass fiber impregnated with resin as the reinforcing material. Common standard grades are G10, G11, FR4, FR5. The standard FR4 epoxy glass fiberboard (FR4 epoxy / E glass fiber) has been used for many years and proved to be the most successful material widely used in the circuit board industry. For some products in the high-frequency field, when the ordinary FR4 epoxy glass fiber can no longer meet the performance requirements of the product, multi-functional high-temperature epoxy resin and polyimide glass fiber have been developed and applied. Other resin materials include Bismaleimide modified triazine resin (BT), diphenylene ether resin (PPO), maleic anhydride imide-styrene resin (MS), polyisocyanate resin, polyolefin resin, etc. In terms of reinforcement materials, the materials that meet the requirements of high performance are S-glass fibers and D-glass fibers. For electronic products with no special requirements in general, FR4 materials with a glass transition temperature of 130 to 140 degrees can be used. The dielectric constant is generally between 4.5 and 5.4. For rigid substrates, the lower the resin content, the higher the dielectric constant. When the circuit board has high heat resistance requirements, it is necessary to select epoxy materials from 170 to 180 degrees Celsius. This kind of base material has better dimensional stability, so it is mainly used in BGA, TAB and the design that requires multiple high-temperature welding in assembly, such as double-sided surface mount.

Generally speaking, the rigid FR4 epoxy glass fiber board has high mechanical strength, excellent workability, and excellent drillability, coupled with its good dimensional stability, low water absorption, and good flame retardancy, making it becomes the most widely used PCB substrate. This kind of material has a high-cost performance because it has been used for a long time in the industry, and its processing process has been regarded as a standard process in the circuit board industry.

In terms of heat resistance, its 260-degree dip soldering resistance is maintained at more than 120 seconds. Such substrates can generally meet the process requirements of reflow soldering, double-sided reflow soldering, wave soldering, and dispensing curing.

In summary, different substrates have different properties and parameters, and they adapt to different process environments. In addition to meeting the electrical performance, designers also need to adopt different design specifications according to the PCB board processing and assembly processes to meet the different requirements in PCB production and assembly. At the same time, the staff who are responsible for assembly should fully understand the performance and assembly requirements of PCB materials when formulating process documents, to ensure that qualified products that meet design performance requirements and quality requirements are produced.

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