PCBA Tech - SMT principle and poor PCB materials lead to possible results

Principle of SMT placement system

Different types of SMT placement machines have their own advantages and disadvantages, which usually depend on the application or technology of the system, and there is a certain trade-off between their speed and accuracy. In chip placement technology, the component placement process of any chip placement machine includes PCB transmission, component pickup, support and identification, inspection and adjustment, component placement, and other steps.

1. PCB transmission

PCB transmission is the first step to install components on the smt placement machine. This is the process of accurately guiding the PCB of the component to be glued to the specified position of the assembler mainly through the transmission mechanism. After that, the PCB transmission system needs a stable PCB output with components.

2. PCB reference calibration standards

When the smt machine is running, the upper corner of the PCB (usually the lower left corner and the upper right corner) is used as the origin to calculate the component installation coordinates. Some errors may occur during PCB processing. Therefore, the PCB must be placed during the high-precision assembly process.

3. Collect components

Component collection refers to the collection of chip components from the packaging. In this process, the key is the accuracy and correctness of the collection. The factors that affect this process include collection tools and methods, component packaging methods, and related characteristics of the components themselves.

There are two collection methods, manual collection and machine collection. The machine selection includes two modes: mechanical grip and vacuum suction. Machine picking tools are more complicated than manual picking. Almost all modern SMT machines use vacuum suction methods. Only under special circumstances, such as some special-shaped components with a large volume and a special shape, can they be installed by mechanical clamping.

4. Detection and adjustment

After the core arrangement machine absorbs the components, two issues need to be determined: whether the center of the first component is consistent with the center of the mounting head. If the center of the module is inconsistent with the center of the mounting head, and no adjustment is made, the final deviation of the module will be caused;

If the second component meets the installation requirements, if the second component does not meet the requirements, they cannot be installed. These two issues must be determined through testing.

Poor PCB material leads to 6 possible outcomes

1. The heat resistance and thermal expansion characteristics of the substrate do not match the component design, solder, soldering process and temperature, resulting in PCB deformation/deformation and serious solder defects.

2. The change of the base material welding coating material or its heat resistance, welding resistance, etc. do not match the welding, the welding process temperature is high, resulting in poor wettability or excessive wettability and other low weldability.

3. The weldable lining or weldable strength material does not meet the application requirements of the relevant process and leads to corrosion of the board surface.

4. The PCB solder coating process is out of control, the surface of the copper foil is severely oxidized or contaminated, the characteristics of the solder coating material are different, or its heat resistance and solder resistance are inconsistent with the soldering temperature and soldering process, etc., causing the copper to be exposed on the pad .

5. If resistance welding or resistance welding is not performed, the wiring spacing of the wires is too small, and the PCB wiring will exceed the tolerance, resulting in conductor welding.

6. The heat resistance of the substrate is poor, and the lamination process and material quality are not controlled, resulting in PCBA lamination and foam.