Chip-bonding adhesives

CBA. [HernonMfg.] Chip bonding adhesives for bonding surface mounted devices to printed circuit board assemblies. 

Electrically Conductive Adhesives (Chip-bonding Adhesives)... 

The adhesives are suitable for use in flip chip bonding of semiconductor parts. They exhibit good shelf stability, productivity, strength properties and heat resistance. The electrical properties, such as dielectric constant and dielectric loss tangent are highly satisfactory. 

As indicated previously, a significant feature of these resins is the very low content (<0.1%) of the monomers, phenol and formaldehyde. The molecular weights of resins used in plywood manufacture are higher, and the viscosity is in the range of 700 (wet process) or 450 mPa s (dry process) (9). In the manufacture of particle boards, which consist of —95% by weight of wood chips bonded together by the adhesive, the viscosity of the resin used is much lower, about 130 mPa s (9). In both cases, the resin has a dry solids content of 40-50%. 

After mechanical attachment to a substrate, a leadframe, or to the inside of a package, bare die or chip devices are electrically connected by one of five methods wire bonding, flip-chip bonding, TAB, solder attachment, or attachment with electrically conductive adhesives. Fig. 1.6 shows some of these methods. 

Several approaches using electrically conductive adhesives instead of solder have been explored and are proving successful. Anisotropic adhesives (See Chapter 1), for example, have been used to connect flex circuits and cables from the separate driver circuits to the panel, avoiding the use of solder connections. More importantly, integrated-circuit chips can be bonded directly to the ITO conductor traces on the panel, a technology called chip-on-glass (COG). IC chips can be flip-chip bonded, then underfilled with a stress-free underfill adhesive if necessary. For protection, the chips may then be encapsulated with epoxy (glob-topped). 

One of the more common difficulties in bonding pine veneers and chips is adhesive dryout. Dry-out is associated with the high liquid absorbancy of pine sapwood and it appears especially during long assembly times. This problem can be overcome by using resins modified through reaction with alkylated phenols, especially 3,4-xylenol [52]. Another technique used to achieve similar results is the manipulation of synthesis procedures used in preparing a standard PF resin [52]. The dry-out resistance imparted by alkylated phenols is due to an initial semithermoplastic character in the resin. This is derived from their monomer bifunctionality and the linear polymer that is consequently formed. 

Laser cutting allows for almost free design of the microfluidic channels in the tape gasket. Figure 5 shows laser-cut double-sided adhesive tapes with microfluidic channels. The tape adhesive is still protected by its liners. The tape cutting can also be depth sensitive, i.e., kiss-cuts down to the carrier liner can be done if the tape roll is used in further automated roll-to-chip or manual tape-to-chip bonding processes. 

These syringe applicators with very fine needles may apply very small dots or drops down to one hundredth of a milligram. They are widely used in electronics for surface mounting, chips bonding and in jewellery and watches. The application of the adhesive may be controlled with a special camera or an optical fibre detector. The viscosities of the adhesives may range from 100 mPas (cyanoacrylates) to 20,000 mPas (epoxies, silicones). 

Z. Lai and J. Liu, Anisotropically Conductive Adhesive Rip-Chip Bonding on Rigid and Flexible Printed Circuit Substrates, IEEE Transactions on Components Packaging and Manufacturing Technology, Part B, Vol 19, 1996, p 644-660... 

A. Seppala and E. Ristolainen, Study of Adhesive Rip Chip Bonding Process and Failure Mechanisms of ACA Joints, Microelectronics Reliability, Vol 44, 2004, p 639-648... 

Besides holding the chip in place, the die bonding adhesives generally must conduct heat from the chip to the heat sink. This requires a high thermal conductivity and minimum of voids in the bond line. Electrically conductive adhesives are used where the back of the chip... 

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