The selection and design of surface mounted components is a key part of the overall product design. The designer shall determine the electrical performance and function of components in the system structure and detailed circuit design stage. In the SMT design stage, the packaging form and structure of surface mounted components shall be determined according to the specific conditions of equipment and process and the overall design requirements. Surface mounted solder joints are both mechanical and electrical connections. Reasonable selection has a decisive impact on improving PCB design density, producibility, testability and reliability.
There is no difference between surface mounted components and plug-in components in function. The difference lies in the packaging of components. Surface mounted packages must withstand high temperature during welding, and their components and substrates must have matching coefficient of thermal expansion. These factors must be taken into account in product design.
The advantages of choosing appropriate packaging are as follows: 1) effectively saving PCB area; 2) . provide better electrical performance; 3) Protect the interior of components from humidity and other environmental effects; 4) Provide good communication; 5) . help dissipate heat and facilitate transmission and testing.
Selection of surface mounted components
Surface mounted components are divided into two categories: active and passive. According to the pin shape, it is divided into gull airfoil and "J" type. The selection of components is described below.
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", which are easy to roll when reflow welding is adopted, so special pad design is required, which should be avoided in general. Rectangular passive components are called "chip" chip components. They are widely used in all kinds of electronic products because of their small volume, light weight, good impact and shock resistance of antibiotics and small parasitic loss. In order to obtain good weldability, the electroplating of nickel base barrier layer must be selected.
There are two types of surface mount chip carriers: ceramic and plastic.
The advantages of ceramic chip packaging are: 1) good air tightness and good protection for internal structure; 2) short signal path, significantly improved parasitic parameters, noise and delay characteristics; 3) reduced power consumption. The disadvantage is that there is no pin to absorb the stress generated when the solder paste dissolves, and the CTE mismatch between the package and the substrate can lead to solder joint cracking during welding. The most commonly used ceramic wafer carrier is leadless ceramic wafer carrier LCCC.
Plastic packaging is widely used in military and civil products production, with good cost performance. Its packaging forms are divided into: small shape transistor sot; Small shape integrated circuit SOIC; Plastic encapsulated lead chip carrier PLCC; Small outline J package; Plastic flatpack PQFP.
In order to effectively reduce 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.