How to select SMT components?
Release time：2025-12-27
Click count： 989
The selection and design of surface mounted components are a key part of the overall product design. Designers determine the electrical performance and functionality of components during the system structure and detailed circuit design stages. In the SMT design stage, the packaging form and structure of surface mounted components should be determined based on the specific equipment and process conditions and overall design requirements. Surface mounted solder joints are both mechanical and electrical connection points, and reasonable selection has a decisive impact on improving PCB design density, manufacturability, testability, and reliability. Surface mounted components have no difference in functionality from plug-in components, but the difference lies in the packaging of the components. Surface mounted packaging must withstand high temperatures during soldering, and its components and substrate must have a matching coefficient of thermal expansion. These factors must be fully considered in product design. The advantages of choosing a suitable packaging are mainly: 1) Effectively saving PCB area; 2）. Provide better electrical performance; 3）. Protect the internal components from environmental influences such as humidity; 4）. Provide good communication connections; 5）. Assist in heat dissipation and provide convenience for transmission and testing. Surface mount components are divided into two categories: active and passive. Divided into gull wing shape and "J" shape according to pin shape. Below, we will explain the selection of components based on this classification. Passive components mainly include single-chip ceramic capacitors, tantalum capacitors, and thick film resistors, which have a rectangular or cylindrical shape. The cylindrical passive device is called "MELF", which is prone to rolling when using reflow soldering and requires special pad design. Generally, it should be avoided from use. Rectangular passive components, known as "CHIP" chip components, are widely used in various electronic products due to their small size, light weight, good resistance to antibiotic impact and shock, and low parasitic losses. In order to achieve good weldability, it is necessary to choose electroplating with a nickel based barrier layer. There are two main types of surface mount chip carriers for active devices: ceramics and plastics. The advantages of ceramic chip packaging are: 1) good airtightness, providing good protection for internal structures; 2) shorter signal paths, significantly improving parasitic parameters, noise, and delay characteristics; 3) reduced power consumption. The disadvantage is that the lack of pins can absorb the stress generated by the melting of solder paste, and the mismatch of CTE between the package and the substrate can lead to cracking of the solder joints during soldering. The most commonly used ceramic wafer carrier is the leadless ceramic wafer carrier LCCC. Plastic packaging is widely used in military and civilian production, with good cost-effectiveness. Its packaging form is divided into: small shape transistor SOT; Small form integrated circuit SOIC; Encapsulated with lead chip carrier PLCC; Small size J package; Plastic flat packaging PQFP. In order to effectively reduce the PCB area, SOICs with less than 20 pins, PLCCs with 20-84 pins, and PQFPs with more than 84 pins are preferred when the device functionality and performance are the same.
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