Adhesive Gluing

So-called white glues are particularly important. The chemical basis is polyvinyl acetate (PVA), the bonded joints - or traditionally adhesive gluings - are characterized by very high adhesive strengths, which usually result in adherend fractions after destructive testing. In order to avoid possible tensions, it is recommended to let the bonded joint set under even pressure (clamp) for more than the indicated... 

Figure 7 Series of monosaccharides used for testing of bacterial adhesion (Glu=Glucose, Gal=Galactose, Man=Mannose, GlcNAc iV-Acetylglucosamine, Fuc Fucose).

For adhesive gluing, the surfaces to be joined must be adequately treated to improve adhesion. Flaming, eorona diseharge and primer are used for surface treatment. The methods for surfaee treatment are similar to those used for decorating and printing (Section 7.3). Adhesives used for bonding of PP are eontact adhesives based on polyurethanes or synthetie rubber, two-pack adhesives based on epoxy resins, polyurethanes, ete., vinyl aeetate eopolymer hot melt adhesives, pressure-sensitive adhesives and pol5mrethane eontact cements. 

Adhesive gluing Difficult to bond plastic, surface preparation very crucial. Proper selection of glue necessary. Consider improved design to avoid the need for gluing or use welding. 

Very widely differing connection technologies necessitate metallization layers of varying composition. Each process (soldering, conductive-adhesive gluing, wire bonding, pin contacts) has its own set of requirements for the surface finish. Cu-Ni-Au metallization is the conventional approach, but electroless Cu/Sn and Cu/Ag metallizations are other possibilities. 

The processes generally employed to fix electronic components on three-dimensional circuit carriers are reflow soldering and electrically conductive-adhesive gluing. The first step in reflow soldering is to print solder paste on to the pads of the circuit trace (Fig. 4.1). The electronic components are subsequently placed in these solder depots. The module, complete with its electronic components, is then heated in a reflow oven, the solder particles of the paste melt, and the molten solder flows over... 

FIGURE 4.1 Process chain of reflow soldering or electrically conductive-adhesive gluing processes with application of the interconnection medium, placement of electronic components, and remelting of the solder or curing of the conductive adhesive... 

The steps in the conductive-adhesive gluing process are not unlike those in reflow soldering. Conductive adhesive Is applied to the pads of the circuit carriers by a conventional printing process. Placement of the electronic components is followed by curing of the adhesive. The three-dimensionality of interconnect devices entails elevated requirements for medium application and for component placement, particularly in terms of process automation. 

Broadly speaking, the connection mediums and techniques familiar from conventional printed-circuit hoard technology can be used for mechanically locating and electrically contacting electronic components on MID. Soldering, conductive-adhesive gluing, bonding, and flip-chip placement are all in successful use under series-production conditions. 

So substituting conductive adhesive for solder paste is unlikely to contribute much to direct cost savings. Since, however, switching to conductive-adhesive gluing for an MID application can permit the use of more cost-effective base materials on account of the lower process temperatures, certain indirect cost benefits could accrue. Conductive-adhesive technology opens up considerable cost benefits for applications characterized by large base-material volume combined with few conductive joins. [48]... 

The mechanical strength of conductive-adhesive gluing is achieved by thermal curing, while the electrical connection is established by the three-dimensional mesh of touching particles embedded in the polymer matrix (Fig. 5.13). Silver is the most common filler, on account of its good electrical conductivity and high thermal conductivity [109]. Curing temperatures are low (80 to 150 °C), so this process can be used on thermally sensitive materials. 

Connections made with insulating adhesives free of fillers are electrically conductive if the partners are in contact with each other in adhesive no thicker than the surface roughness (< 10 pm). In principle the process is very similar to that of anisotropic adhesive gluing. The glue is applied by a dispenser or by print application of a paste. The electrically nonconductive adhesive can be applied allover across multiple connections. This implies low requirements for the process as such and therefore good affinity for fine-pitch applications. Once the partners to be joined have been positioned, the adhesive cures under pressure and temperature within a matter of seconds. 

The pressure sensor is a two-shot molding. Production volume is on the high side of 10 million units per year. The printed-circuit board is contacted on the MID by conductive-adhesive gluing (isotropic conductive adhesive). Nonconductive adhesive is also applied for mechanical location. The contact pins of the MID are tiny bumps to which the conductive adhesive is applied in the dispensing process. [143]... 

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