The selection and design of surface mount components is a key part of the overall product design. Designers determine the electrical performance and functions of components during the system structure and detailed circuit design stages. During the SMT design stage, the packaging form and structure of surface mount components should be determined based on the specific conditions of the equipment and process and the overall design requirements. Surface mount solder joints are both mechanical and electrical connection points. Reasonable selection has a decisive impact on improving PCB design density, manufacturability, testability and reliability.
Surface mount components are no different from plug-in components in terms of function. The difference lies in the packaging of components. Surface mount packages must withstand high temperatures during soldering. Components and substrates must have matching thermal expansion coefficients. These factors must be considered comprehensively in product design.
The advantages of choosing a suitable package are:
1) Effectively save PCB area;
2) Provide better electrical performance;
3) Protect the internal components from moisture and other environmental influences;
4) Provide good communication links;
5) Help dissipate heat and facilitate transmission and testing [2]. Selection of surface mounted components
Surface mounted components are divided into two categories: active and passive. According to the shape of the pins, they are divided into "gull wing" type and "J" type. The following classification explains the selection of components.
Passive components
Passive components mainly include monolithic ceramic capacitors, tantalum capacitors and thick film resistors, which are rectangular or cylindrical in shape. Cylindrical passive components are called "MELF". They are prone to rolling during reflow soldering and require special pad design. They should generally be avoided. Rectangular passive components are called "CHIP" chip components. They are small in size, light in weight, have good antibiotic impact and vibration resistance, and low parasitic loss. They are widely used in various electronic products. In order to obtain good solderability, nickel-bottom barrier plating must be selected.
Active components
There are two major categories of surface mounted chip carriers: ceramic and plastic.
The advantages of ceramic chip packaging are:
1) Good airtightness, good protection of the internal structure;
2) Short signal path, significantly improved parasitic parameters, noise, and delay characteristics;
3) Reduced power consumption.
The disadvantage is that because there is no pin to absorb the stress generated when the solder paste melts, the CTE mismatch between the package and the substrate can cause cracking of the solder joints during welding. The most commonly used ceramic wafer carrier is the leadless ceramic wafer carrier LCCC.
Plastic packaging is widely used in the production of military and civilian products and has a good cost-effectiveness. Its packaging forms are divided into: small outline transistor SOT; small outline integrated circuit SOIC; plastic encapsulated leaded chip carrier PLCC; small outline J package; plastic flat package PQFP.
In order to effectively reduce the PCB area, SOIC with less than 20 pins, PLCC with 20-84 pins, and PQFP with more than 84 pins are preferred when the device function and performance are the same.
