The semiconductor structure with a sealing device miniaturization of electronic products, especially mobile products, but also embarked on a compact size of the road, and to expand the scope of the structure of the surface mounted equipment. Liquid encapsulating material (LE) to protect the semiconductor chip from the external environment of the next generation packaging material, when used in a surface mount structure, serve to enhance the use of the connection between the semiconductor chip and the power equipment.
In previous liquid packaging materials available, conventional epoxy molding compound semiconductor packaging materials as widely used, but the beginning of the 1990s, with the miniaturization of semiconductor products, traditional epoxy molding compound is no longer able to meet the spacing between the metal wire shorter, reducing the thickness of the packaging unit and other harsh conditions. Since then, as an alternative to traditional epoxy molding compound with epoxy or silicone synthetic liquid packaging materials trading and students.
At room temperature, the EMC as a solid and a liquid-like LE to differentiate according to the state, but its product types are semiconductor packaging materials. Recently, with the semiconductor package device technology to highly integrated, high-efficiency development, mobile display market yield greatly developed, materials manufacturers actively to the development of LE products, uses the type and amount of semiconductor manufacturers are expanding.
Liquid epoxy encapsulation material microelectronic packaging technology is a revolutionary change in the representation of the Third packaging materials, is one of the key packaging materials required for the package ball array package (BGA) and Chip Scale Package (CSP), including FC / BGA / CSP end filling with liquid epoxy material (Underfill) and liquid epoxy chip encapsulation materials into two categories. Underfill is mainly used for filling FC / BGA / CSP in the gap between the chip and the substrate is formed by the collapse of solder ball connections. Currently, Underfill include two types: type Underfill flow and no-flow Underfill. Liquid epoxy chip encapsulation material is mainly used FC / BGA / CSP and other flexible packaging and coated ultra-thin chip package.
30 years ago, IBM pioneered the concept Flip chip (flip chip) interconnect technology, the chip face down to the substrate interconnection, so that the bump electrode of the chip pad and the substrate to become a wiring layer with a permanently welded. It provides a higher packing density, shorter interconnect distance, better electrical performance and higher reliability. In recent years, with the C4 (controlled collapse chip connection) technology development has become a major Flip chip packaging technology.
Underfill (underfill) is a circuit suitable for flip-chip material, which is filled in the slit between the IC chip and the organic substrate, and the connecting pads sealed protected. The purpose of Underfill encapsulation comprising: reducing CTE between the silicon chip and the organic substrate does not match; protect the device from moisture, ionic contamination, radiation, and such as mechanical stretching, shearing, twisting, vibration, and other harmful operating environment ; enhanced Flip chip package reliability. Underfill materials are requirements: excellent electrical, physical and mechanical properties; Production easily applied; excellent resistance to moisture absorption and resistance to contamination. Current underfill material is mainly silicon filled epoxy resin matrix material, which improves the performance is determined by the following three factors: (1) improve the constraint of the chip, reducing the weld shear stress, and the additional adhesive a curved surface is also a tendency to decrease the chip; (2) when the elastic modulus is very close to the elastic modulus of the solder, epoxy resin to form a relatively quasi-continuous welding area, and therefore is reduced in the interface of the chip and substrate to improve the welding surface is formed at an acute angle related to the stress; (3) the solder is actually being sealed and isolated from the environment. Mechanical cycling tests showed that when used in a vacuum a layer of grease or coating protects the solder fatigue life can be improved, which is due to avoid the oxidation of the crack endpoint, slow crack growth.
Underfill filling time is: tfill = 6ηL2 / σh cosθ.
Proportional to the square of time and the chip size of the padding, and the viscosity of the liquid is proportional and inversely proportional to the surface tension of the liquid, and inversely proportional to the size of the gap. Trends in the development of flip chip die size is increasing, and the gap is getting smaller and smaller chip and substrate. Therefore, in order to reduce the time required for the liquid underfill to improve the fluidity of the liquid, reducing the viscosity is essential.
FC-BGA / CSP underfill material used (Underfill) is a spherical silica powder filled low-viscosity liquid epoxy encapsulation material, mainly used for filling down the slit between the chip and the substrate assembly and welding, reinforced joints and convex the connection strength of the substrate, the sealed protruding joints, improved package reliability FC circuit. Currently, Underfill encapsulation material and the solid material of similar composition, mainly composed of a liquid of low viscosity cycloaliphatic epoxy resin, the spherical silica powder, epoxy curing agents and accelerators, silica powder, and other surface treating agents and other functional additives by a suitable prepared by the process. Meanwhile, the liquid is desired epoxy underfill material should have a low viscosity and high surface tension, the bottom of the chip to shorten the filling time. Therefore, Underfill performance requirements include: adequate liquidity, low curing temperature, fast curing, the cured resin no defects, no bubbles, good heat resistance, low coefficient of thermal expansion, low modulus, high bonding strength, the internal stress is small, and so a small degree of warpage. .
At present, the resin system is mainly used in low viscosity at room temperature of the liquid and the liquid acid anhydride epoxy curing system, including alicyclic epoxy-epoxy, bisphenol A, bisphenol F diglycidyl ether and other (aliphatic) type epoxy, alicyclic epoxy compound which has excellent heat resistance and chemical resistance, excellent mechanical and electrical properties and excellent processing performance, does not contain an aromatic ring such strong UV chromophore can be good be resistant to UV radiation. Alicyclic epoxy resin having a low viscosity before curing, after curing, and good adhesion characteristics; in particular they are made of the cycloolefin after the organic peroxy acid is prepared by the epoxidation, instead of epichlorohydrin and phenol condensation, no organic chlorine, does not have the presence of very small amounts of organic chlorine to produce micro-circuit corrosion applications in microelectronic packaging in recent years attracted people's attention.
Because the chip CTE of about 3ppm / ℃, CTE of the substrate to about 20ppm / ℃, convex solder CTE of about 25ppm / ℃, in order to make underfill material to reduce CTE between the silicon chip and the organic substrate does not match the purpose of its the method should match the CTE of the convex pad, reaching about 25ppm / ℃, and pure resin system CTE of about 60-70ppm / ℃, the most simple and effective to reduce CTE is to add low expansion filler - fused silica. When the weight content of the molten silicon powder reaches 65% -70%, CTE underfill material will reach the range of what we need. However, with increasing filler content, the viscosity of the material will be rapidly increased, the flow performance. From a rheological point of view, the spherical silica having a random better flow, because the former has a smaller radius of the flow, the higher the tacticity spherical, the lower the viscosity of the material.
