Operating rules for PCBA patch processing:
1. No food or drinks should be present in the PCBA work area, smoking is prohibited, and no unrelated items should clutter the space. The workbench must be kept clean and organized.
2. The surfaces designated for soldering during PCBA patch processing must not be touched with bare hands or fingers, as the oils from human skin can diminish solderability and easily result in soldering defects.
3. The operational steps for PCBA and components are minimized to mitigate risks. In assembly areas where gloves are required, dirty gloves can lead to contamination; therefore, gloves must be replaced frequently as needed.
4. Avoid using skin-protecting oils or silicone-based detergents, as these can impair solderability and conformal coating adhesion. A specially formulated detergent for PCBA soldering should be utilized.
The concept and significance of PCBA mixing degree
1. Background description
In IPC-SM-782, two essential concepts, “Producibility Levels” and “Component Mounting Complexity Levels,” are distinguished but correspond across three levels. These concepts describe the intricacies of PCBA assembly. The three-tier classification is based on assembly technologies: through-hole insertion, surface assembly, and mixed mounting. However, these concepts do not entirely align with reality. While the use of plug-in components decreases, the complexity of same-side assembly for ordinary and fine-pitch packages has significantly surpassed the challenges posed by mixing plug-in and surface assembly technologies. Thus, today’s electronic manufacturing complexity primarily arises from two factors: the shrinking size of component packages and the coexistence of ordinary and fine-pitch packages on the same PCB mounting surface. Addressing this mixing of packages through appropriate component selection and layout design is the core task in PCBA manufacturability design.
2. Mixing degree
The mixing degree is a crucial concept introduced in this text. It refers to the variance in the assembly processes of different packages on the PCBA mounting surface, specifically the differences in process methods and stencil thickness. A greater disparity indicates a higher degree of mixing, resulting in a more complex and costly process. The mixing degree reflects assembly complexity; “good soldering” often comprises two layers: the presence of components with narrow process windows (like fine-pitch components) and the variation in packaging and assembly processes on the PCBA mounting surface. A higher mixing degree complicates process optimization for each package type, adversely affecting manufacturability. For instance, mobile phone PCBAs, although comprising fine-pitch or miniature components like 01005 and 0201, face similar complexity levels, allowing for optimal packaging process design and high assembly yields. In contrast, communication PCBAs, with larger components, exhibit a higher mixing degree, complicating assembly due to layout constraints and mesh-making challenges, often leading to compromise solutions that may not achieve high yield rates. This underscores the importance of the mixing degree concept. Selecting packages with similar installation process requirements is fundamental in the hardware design phase to ensure manufacturability.
3. Measurement and classification of mixing degree
The mixing degree of PCBA is quantified by the maximum difference in ideal stencil thickness for components on the same PCB assembly surface. A larger difference indicates a greater mixing degree and poorer manufacturability. Increased stencil thickness variance complicates process optimization, not due to difficulties in producing step stencils but because greater thickness discrepancies hinder achieving quality solder paste printing. Ideally, the step thickness of the stencil should not exceed 0.05mm (2mil).
4. The relationship between component pin spacing and maximum stencil thickness
Stencil thickness is evaluated from two perspectives: the coplanarity of component pin pitches and packaging. There is a correspondence between component pin spacing and the stencil window area, which essentially sets the maximum allowable thickness, while the coplanarity dictates the minimum thickness. Since stencil thickness is not solely designed based on individual component pin pitches, mixing degree cannot be simply assessed by pitch alone, but it serves as a fundamental reference for package selection. This article thoroughly discusses the PCBA mixing degree, highlighting that a greater mixing degree correlates with a more complex process and increased costs.
5. Components and PCBAs sensitive to EOS/ESD must be clearly marked with appropriate EOS/ESD labels to avoid confusion with other components. Furthermore, to protect sensitive components from ESD and EOS, all assembly and testing operations should occur on static-controlled workbenches.
6. Regular inspections of EOS/ESD workbenches are essential to ensure their effective functioning (anti-static). Various hazards associated with EOS/ESD components can arise from improper grounding methods or oxide buildup in grounding connections. Therefore, special attention should be paid to the grounding terminals of the “third line.”
7. Stacking of PCBAs is prohibited to prevent physical damage. Various specialized brackets should be utilized on the assembly work surface, organized according to component types.
In PCBA assembly processing, adherence to these operational guidelines is crucial to ensure the final product quality while minimizing component damage and associated costs.
