Conductive-adhesive gluing

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. 

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]... 

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 primary joining techniques used in MID technology are soldering for chips with solder bumps and, for chips with stud bumps, gluing with isotropic conductive adhesive (ICA), anisotropic conductive adhesive (ACA), or nonconductive adhesive (NCA) [81]. 

Figure 5.22 shows a specimen process flow for flip-chip gluing with ICA. Conductive adhesive is stenciled on to the substrates. The devices are then placed on the chip, and the conductive adhesive is thermally cured. As in flip-chip soldering, underfill Is applied as mechanical protection and cured. 

The Bresle method is described in the international standard ISO 8502-6. A patch with adhesive around the edges is glued onto the test surface. This patch has a known contact area, usually 1250 mm. A known volume of deionized water is injected into the ceU. After the water has been in contact with the steel for 10 minutes, it is withdrawn and analyzed for chlorides. There are several choices for analyzing chloride content titrating on-site with a known test solution using a conductivity meter or where facilities permit, using a more sophisticated chloride analyzer. Conductivity meters cannot distinguish between chemical species. If used on heavily... 

When devices are glued with electrically conductive or nonconductive adhesive, the mechanical strength of the bond is the result of thermal curing of the adhesive. Considerably lower than process temperatures for soldering, adhesive curing temperatures have to be chosen to suit the adhesive system, the substrate material, and required process time. 

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