Principles and methods for selecting electronic components during SMT processing

SMT processing overview

The selection and design of SMT 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 conditions of the equipment and process and the overall design requirements to determine the package form and structure of the surface mount components. Solder joints of surface mount are both mechanical joints and electrical joints, reasonable choices have a decisive influence on improving PCB design density, manufacturability, testability and reliability.

Functionally of the surface mount components are no different from the plug-in components, the difference is in the package of the components. Surface mount packages must withstandthe high temperatures during soldering and the components must have a matching coefficient of thermal expansion. These factors must be considered in the product design. 

Choosing the right package has the following advantages:

1. Effectively save PCB area;
2. Provide better electrical performance;
3. Protect the inside of components from environmental influences such as moisture;
4. Provide good communication links;
5. Helps dissipate heat and facilitate transfer and testing.

Surface mount component selection

Surface mount components are divided into active components and passive component. It is divided into gull wing type and "J" type according to the shape of the pin. The following classified expound the selection of components.

1. Active components

There are two main types of surface mount chip carriers: ceramics and plastics.

1.1 The advantages of ceramic chip packaging are: 

⑴ Good air tightness and good protection for internal structure; 

⑵ Short signal path, significant improvement in parasitic parameters, noise, and delay characteristics; 

⑶ Reduce power consumption.

The disadvantage is that the CTE mismatch between the package and the substrate can lead to cracking of the solder joint during soldering because of the stress generated by the lead-free absorption solder paste. The most commonly package is the leadless ceramic chip carrier (LCCC).

1.2 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 leaded chip carriers (PLCC), plastic quad 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.

2. Passive components

Passive devices mainly include monolithic ceramic capacitors, tantalum capacitors and thick film resistors, which are rectangular or cylindrical in shape. The cylindrical passive device is called “MELF”, it is easy to roll when reflow soldering. It needs special pad design, commonly should be avoided use. Rectangular passive components are called "CHIP", 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.