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How to choose components in SMT?


The selection and design of surface mount 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. In the SMT design phase, it should be based on the specific equipment and process. The situation and overall design requirements determine the package form and structure of the surface mount components. Surface mount solder joints are both mechanical joints and electrical joints. Reasonable choices have a decisive influence on improving PCB design density, manufacturability, testability and reliability.

Surface mounted components are functionally different from plug-in components, and the difference is in the package of the components. Surface mount packages must withstand the high temperatures of the milk during soldering. The components and substrates must have a matching coefficient of thermal expansion. These factors must be considered in the product design.

Select the right package, the main advantages are: 1). Effectively save PCB area; 2) Provide better electrical performance; 3) Protect the internal parts of components from moisture and other environmental impacts; 4). Provide good communication links; 5) Help heat dissipation and facilitate transmission and testing.

Surface mounted component selection

Surface mounted components are divided into active and passive categories. It is divided into gull wing type and "J" type according to the shape of the pin. The following is a breakdown of the selection of components.

passive devices

Passive components mainly include monolithic ceramic capacitors, tantalum capacitors and thick film resistors. They are rectangular or cylindrical. The cylindrical passive device is called “MELF”. It is easy to roll when reflow soldering. It needs special pad design and should be avoided. Rectangular passive components are called "CHIP" chip components. They are small in size, light in weight, good in anti-impact and shock-resistant, and have low parasitic losses. They are widely used in various electronic products. In order to obtain good solderability, it is necessary to select a plating of a nickel bottom barrier layer.

Active Devices

Side mounted chip carriers come in two broad categories: ceramics and plastics.

The advantages of ceramic chip package are: 1) good air tightness and good protection for internal structure 2) short signal path, significant improvement of parasitic parameters, noise and delay characteristics 3) reduced power consumption. The disadvantage is that the CTE mismatch between the package and the substrate can cause cracking of the solder joint during soldering because of the stress generated by the lead-free absorption solder paste. The most commonly used ceramic chip carrier is the leadless ceramic wafer carrier LCCC.

Plastic packaging is widely used in military and civilian production, with good cost performance. The package form is divided into: small outline transistor SOT; small outline integrated circuit SOIC; plastic packaged lead chip carrier PLCC; small outline J package; plastic flat package PQFP.

In order to effectively reduce the PCB area, SOIC with a pin count of 20 or less, a PLCC with a pin count of 20-84, and a PQFP with a pin count of more than 84 are preferred in the case of the same device function and performance.

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