Summary Of The Flip Chip Package Design

Posted by crescentled on February 6th, 2019

The term "flip chip package design" includes many different methods. Each method is different in many ways, and the application is different. For example, the choice of type of circuit board or substrate, whether it is organic, ceramic, or flexible, determines the choice of assembly materials (bump type, flux, bottom filler, etc.) and, to a certain extent, the choice of equipment required. In the current situation, each company must decide which technology to adopt, which process components to choose, which research and development to conduct to meet the needs of future products, and how to minimize capital investment and operating costs. The most common and appropriate method in SMT environment is solder paste flip chip assembly process. Even so, many variations in the technology should be considered in order to ensure manufacturability, reliability, and cost objectives. At present, the flip chip package design method is mainly based on the interconnection structure. For example, the implementation of a compliant convex point technique uses a gold-plated conductive polymer or polymer/elastomer convex point.

Bonding technology

Solder ball bonding (mainly gold wire) or electroplating technology is used for the realization of the protruding point technology of the welding column, and then the assembly is completed with conductive isotropic adhesive. The process must not affect the bonding point of the integrated circuit (1C). In this case, an anisotropic conductive film is required. Solder paste bump technology includes evaporation, electroplating, electroless plating, stencil printing, jet and so on. Thus, the choice of interconnection determines the required bonding technique. In general, alternative bonding techniques include reflow bonding, thermosonic bonding, thermocompression bonding and transient liquid phase bonding. Each of these techniques has advantages and disadvantages, and is often application-driven. However, the solder paste flip chip assembly process is the most common in terms of standard SMT processes and has proven to be completely suitable for SMT.

Solder paste flip chip package design

Traditional solder paste flip chip assembly process includes: flux coating, chip cloth, solder paste reflows and bottom filling, and so on. But other things must be paid attention to in order to ensure success. Often, success begins with design. Primary design considerations include solder bump and lower bump structures designed to minimize stress on interconnect and IC bonding points. Known reliability models can predict solder paste problems if the interconnection design is appropriate. The IC bond structure, passivation, polyimide opening and lower convex metallurgy (UBM) structure can be designed reasonably to achieve this purpose. Passivation openings must be designed to reduce current density, decrease the area of concentrated stress, increase the lifetime of electromigration, maximize the cross-section area of UBM and solder bumps. The convex spot layout is another design consideration. The solder convex spots should be as symmetrical as possible, with the exception of identifying directional features (removing one corner convex spot). Layout design must also consider downstream slicing operations without any interference. The placement of solder bumps in the active region of IC also depends on the electrical properties and sensitivity of IC circuits. In addition, there are other IC design considerations, but wafer bump fabrication companies have specific IC solder joints and layout design guidelines to ensure the reliability of bump, thus ensuring the reliability of interconnection.  

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