The increasing assembly challenges to SMT using flip chip technology

Because of the use of flip chip technology continues to increase, to understand the challenges it faces in the SMT assembly becomes critical.

Semiconductor packaging and electronic manufacturing services company is tending to their ability in polymerization, and investment to draw close to each other. In the context of growing industrial demand, many companies are offering "complete solutions", so this aggregation is expected, but this is a challenge for both sides.

For example, flip chip, BGA, or SIP such as the use of advanced packaging technology products, from the printed circuit board assembly to device assembly, who seems not very important factors may be important: interconnection of different stress; compatibility materials increase; process change. Whether you need to design their own new products or the use of flip chip technology, or currently only need to know whether the flip chip as your investment goals, understand the challenges of flip chip technology is very important.

Flip chip technology

Flip chip technology is divided into a variety of processes, each of which has many variations and applications. For example, according to the technical requirements of printed circuit board or substrate type organic, ceramic or flexible assembly decide the choice of materials (such as bump type, solder and underfill material), process equipment and decided to be in a certain extent. The company must decide which technology to use to determine which processes are to be identified, and determine which research and development resources are needed in order to meet future product requirements and minimize the investment and operating costs.

In the SMT environment, the most common and the most appropriate way to achieve is the solder flip chip assembly process, we will describe in more detail. There are many variants of this technology that must be fully considered to ensure manufacturability, reliability, and cost objectives. Today's widely used flip chip technology is mainly defined by the interconnection structure. For example, the flexible bump (compliant-bump) process is the process of using a conductive polymer bump or a gold-plated polymer / elastomer bump. The short column bump (Stub-bump) technique uses ball bonding (mainly with gold wire) or electroplating, and then uses an isotropic conductive adhesive to assemble. When the anisotropic conductive film or paste is used, the bonding pad of the integrated circuit does not need to be changed. The solder bump formation technology includes evaporation, electroplating, electroless plating, stencil printing, spraying, etc..

The selected interconnect technology determines whether the necessary processes are reflow soldering, hot pressing, hot pressing, or transient liquid phase bonding. Each process has its advantages and disadvantages, which are usually used to decide. However, from the ease of integration to the standard SMT process, the solder flip chip assembly process is the most versatile, and it is shown to be fully compatible with the SMT.

Solder flip chip assembly

The main steps of the traditional solder flip chip assembly process (Figure 1) include: applying flux, placing chip, solder reflow and underfill. However, in order to ensure the success and reliability of flip chip assembly, it is also necessary to understand and implement other guidelines and requirements, and success usually begins with design.

Figure 1 Typical flip chip assembly process flow.

The first consideration is the structure of the solder bumps and bumps. The goal is that between the interconnection and the integrated circuit bonding pad transfer stress to a minimum. If the interconnect is properly designed, the failure will occur only in the solder as predicted by the * * * model. This can be achieved by properly designing the bonding pad structure of the integrated circuit, including its passivation / polyimide opening and the bump down (UBM) structure. The passivation opening should be designed to reduce the current density, reduce the stress concentration, improve the life of the electron transfer, and maximize the cross section of the UBM and solder bumps.

The placement of bump locations is another design consideration. The solder bump position should be as symmetrical as possible unless there is a clear positioning feature (e.g., no bump in a corner). Layout must also consider the downstream process of wafer slicing process to ensure that no conflict occurs. The placement of solder bumps on the power supply area of the integrated circuit will also be governed by the electrical sensitivity and performance of the integrated circuit. Of course, there are other factors that need to be taken into account in the design of integrated circuits, wafer bumping companies have specific integrated circuit pads and layout design guide. These design guidelines must be taken into account in order to ensure the formation of a * * * * * * * * * * *, thereby forming a * * * interconnect.

The main considerations in the design of printed circuit board include the size of the metal pad and the corresponding solder mask opening. It is important to make the PCB pad position of the wetted area reaches the maximum, so as to form a strong connection. However, consideration must be given to the size of the area of the printed circuit board to match the diameter of the UBM, which helps to form a symmetrical interconnection, avoiding the interconnection of one end to the other end of the state of high stress. In fact, the design of PCB pad diameter more than UBM is smaller than the diameter of some will increase the stress of PCB side, and reduce the stress of the weak side of the integrated circuit.

The control of the wetting area on the pad position of the printed board is achieved by properly designing the opening of the solder mask, as shown in figure 2. The two design methods defined by the solder mask defined and non solder film may be used, but the combination of the two may be the most feasible. Using the rectangular opening on the printed circuit board