The selection and design of SMT surface-mounted components is a key part of the overall design of the product. The designer determines the electrical performance and function of the components in the system structure and detailed circuit design stage, and determines the packaging form and structure of the surface assembled components in the SMT design stage according to the specific conditions of the equipment and process and the overall design requirements. Surface mounted solder joints are both mechanical connection points and electrical connection points, reasonable selection of PCB design density, productivity, testability and reliability have a decisive impact.
First, SMT patch processing selection of appropriate packaging, mainly has the following advantages:
1. Effectively save PCB area and provide better electrical performance.
2. Provide good communication links to help dissipate heat and facilitate transmission and testing.
3. Protect the interior of the components from humidity and other environmental influences.
2. Selection method of SMT surface mounting components:
SMT surface-mounted components are classified into active and passive types, and can be divided into gull wing type and "J" type according to the pin shape.
1. Passive devices
Passive devices mainly include monolithic ceramic capacitors, tantalum capacitors and thick film resistors with rectangular or cylindrical shapes. The cylindrical passive device is called "MELF". It is easy to roll when adopting reflow welding, so special pad design is required, which should be avoided in general. Rectangular passive device known as "CHIP" chip components, it is small size, light weight, antibacterial impact and shock resistance good, small parasitic loss, is widely used in all kinds of electronic products. In order to obtain good weldability, nickel base barrier plating must be selected.
2. Active devices
Surface-mounted chip carriers come in two broad categories: ceramic and plastic.
The advantages of ceramic chip package are: 1) good air tightness, good protection to the internal structure 2) short signal path, parasitic parameters, noise, delay characteristics are significantly improved 3) reduce power consumption. The disadvantage is that there is no pin to absorb the stress generated when the solder paste melts. The CTE mismatch between the package and the substrate can lead to cracking of the solder joint during welding. The most commonly used ceramic cake carrier is the leadless ceramic patch carrier LCCC.
Plastic packaging is widely used in military and civilian production, with good cost performance. Its packaging form is divided into: small shape transistor SOT; Small shape integrated circuit SOIC; Plastic-sealed leaded chip carrier PLCC; Small shape J package; Plastic flat package PQFP.
How to judge SMT processing from solder joints and appearance
With the development of science and technology, many electronic products have entered our life and become an indispensable part of our life. Especially in recent years, mobile phones and tablets are being replaced more frequently. Light and portable has become the development trend. In order to adapt to the market, the electronic components used for SMT processing in Shenzhen are also getting smaller. How to guarantee the quality of solder joint has become an important issue in high precision patch. As we all know, solder joint as a bridge of welding, its quality determines the quality of electronic products. So how to judge the quality of a solder joint?
A good solder joint should work properly throughout the service life of the equipment, so that its mechanical and electrical properties cannot fail. The appearance of a good solder joint is:
(1) The surface is complete, smooth and bright.
(2) The component height is moderate, the appropriate amount of solder and solder completely cover the welding parts of the pad and lead.
(3) Good wettability: the edge of the welding point should be thin, and the wetting Angle of the solder and the pad surface should be less than 300, and the maximum is not more than 600.
SMT appearance inspection
(1) Check whether the components are missing
(2) Check whether the components are pasted incorrectly
(3) Check whether there is short circuit
(4) Check whether there is virtual welding
The appearance check, of course, is a simple judgment. The quality of a product can not be checked by the naked eye. Hundred thousand into has advanced production equipment, strict management system. The company has always been adhering to the "quality first, customer first" business philosophy, advocating dedication, integrity, team, innovation of the spirit of enterprise. The quality policy of deepening quality service, continuous improvement and customer satisfaction. People-oriented, integrity management, and strive to achieve economic benefits and social benefits of synchronous growth.