In IPC-SM-782 there are two important concepts "Producibility Levels" and "Component Mounting Complexity Levels " (Component Mounting Complexity Levels), which are different but are divided into three levels and correspond to each other. Both concepts are used to describe the complexity of PCBA assembly, and the three levels are based on the technology used for assembly - through-hole cartridge technology, surface assembly technology, and hybrid mounting technology.
These two concepts do not fully correspond to reality. On the one hand, the use of cartridge components is becoming less and less; on the other hand, the difficulty and complexity brought about by the same surface assembly of common pitch and fine pitch packages has far exceeded the difficulty and complexity brought about by the mixed application of cartridge and surface assembly technologies. In other words, the complexity of today's electronics manufacturing mainly from two challenges: First, the component package size is getting smaller and smaller; second is the common pitch and fine pitch package on the same mounting surface of the PCB mixed use. This is also the biggest challenge facing the design of PCBA manufacturability today, the core task of PCBA manufacturability design is to solve the problem of mixed use of common pitch and fine pitch packages on the same mounting surface through package selection and component layout and other design means.
Mixed degree, which is an important concept proposed in this book, refers to the degree of difference between the various types of packaging assembly process on the PCBA mounting surface, specifically the difference between the process used in the assembly of various types of packaging and stencil thickness, the greater the degree of difference between the assembly process requirements, the greater the mixed degree, and vice versa. The greater the degree of mixing, the more complex the process, the higher the cost.
The mixing degree of PCBA reflects the complexity of the assembly process. We usually talk about PCBA "good or bad welding", actually contains two layers of meaning, one layer means that there is no PCBA on the process window is very narrow components, such as fine pitch components; another layer means that the PCBA mounting surface of various types of packaging assembly process difference degree.
The higher the mixed degree of PCBA, the more difficult it is to optimize the assembly process of each type of package, the worse the process. As an example, such as cell phone PCBA, although the components used on the cell phone board are fine pitch or small size components, such as 01005, 0201, 0.4mm CSP, PoP, each package is very difficult to assemble, but their process requirements belong to the same level of complexity, the process of mixed assembly degree is not high, the process of each package can be optimized design, the final The final assembly yield will be very high. In the case of communication PCBAs, although the size of the components used is relatively large, the process mix is higher and a ladder stencil is required for assembly. Due to the difficulty of component layout gap and stencil production, it is difficult to meet the individual needs of each package, and the final process solution is often a compromise solution that takes care of the requirements of various packaging processes, rather than the optimal solution, and the assembly yield will not be very high. This is the significance of the concept of mixed assembly degree.
The basic requirement for package selection is to install packages with similar process requirements on the same assembly surface. In the hardware design stage, establishing the appropriate package is the first step in the design of manufacturability.
PCBA's mixed degree of assembly, with the same assembly surface of the PCB components used in the ideal stencil thickness of the maximum difference to say, the greater the difference, that the greater the degree of mixed assembly, the worse the process.
The larger the stencil thickness difference, the more difficult to optimize the process. The difficulty of the process does not mean that the stepped stencil is more difficult to produce, but that the greater the thickness of the stepped stencil, the more difficult it is to guarantee the printing quality of the solder paste. Ideally, the step thickness of the step stencil should not exceed 0.05mm (2mil)
The design of the stencil thickness is mainly considered from two aspects, namely the component pin spacing and the coplanarity of the package. Component pin spacing and stencil window area have a certain correspondence, basically determining the maximum thickness value that can be used, and the coplanarity of the package determines the minimum thickness value that can be used. Since the stencil thickness is not designed according to the single component pin spacing, the mixed degree cannot be determined simply by the size of the spacing, but it can be used as a basic reference for component package selection.
This concept has an important guiding meaning for the selection of component packages and the layout of components. We hope that the smaller the variability of the package process on the same assembly surface, the better.
Zhejiang NeoDen Technology Co., Ltd. has been manufacturing and exporting various small pick and place machines since 2010. Taking advantage of our own rich experienced R&D, well trained production, NeoDen wins great reputation from the world wide customers.
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