Silicon transfer

Another important application area for PSAs in the electronic industry focuses on the manufacturing, transport and assembly of electronic components into larger devices, such as computer disk drives. Due to the sensitivity of these components, contamination with adhesive residue, its outgassing products, or residue transferred from any liners used, needs to be avoided. Cleanliness of the whole tape construction becomes very critical, because residuals like metal ions, surfactants, halogens, silicones, and the like can cause product failures of the electronic component or product. Due to their inherent tackiness, acrylic PSAs are very attractive for this type of application. Other PSAs can be used as well, but particular attention has to be given to the choice of tackifier or other additives needed in the PSA formulation. The choice of release liner also becomes very critical because of the concern about silicone transfer to the adhesive, which may eventually contaminate the electronic part. 

The mechanism proposed for this transformation is outlined in Scheme 24 (235). The slow step of this reaction is silyl transfer from the copper alkoxide 353. This step may occur through the intermediacy of an external silicon source (intermolecular) or by internal transfer of the silyl group (intramolecular). To probe this issue, these workers conducted a double-crossover experiment involving two distinct nucleophiles with different silyl groups, 342a and 359, and examined the products prior to desilylation. The results show conclusively that silicon transfer has a significant intermolecular component, and is somewhat sensitive to the solvent, Eq. 199. 

The role of tetrahydrofuran in facilitating silicon transfer from metal to oxygen is further discussed in Section III,F,2. 

Mukaiyama aldol reactions using a catalytic amount of a Lewis acidic metal salt afford silylated aldols (silyl ethers) as major products, but not free aldols (alcohols). Three mechanistic pathways which account for the formation of the silylated aldols are illustrated in Scheme 10.14. In a metal-catalyzed process the Lewis acidic metal catalyst is regenerated on silylation of the metal aldolate by intramolecular or intermolecular silicon transfer (paths a and b, respectively). If aldolate silylation is slow, a silicon-catalyzed process (path c) might effectively compete with the metal-catalyzed process. Carreira and Bosnich have concluded that some metal triflates serve as precursors of silyl triflates, which promote the aldol reaction as the actual catalysts, as shown in path c [46, 47]. Three similar pathways are possible in the triarylcarbenium ion-catalyzed reaction. According to Denmark et al. triarylcarbenium ions are the actual catalysts (path b) [48], whereas Bosnich has insisted that hydrolysis of the salts by a trace amount of water generates the silicon-based Lewis acids working as the actual catalysts (path c) [47]. Otera et al. have reported that 10-methylacridinium perchlorate is an efficient catalyst of the aldol reaction of ketene triethylsilyl acetals [49]. In this reaction, the perchlorate reacts smoothly with the acetals to produce the actual catalyst, triethylsilyl perchlorate. 

Hale, P.S.,Kappen,R,Rrissanaroon,W,Brack,N.,Rigram, R.J., Liesegang, J. (2007) Minimizing silicone transfer during micro-contact printing. Appl. Surf. Sci., 253, 3746-3750. 

By analogy with lower lithium and calcium borides, lower lithium (LuSi, Lii4Si6) and calcium (Ca2Si) silicides may, in our opinion, partially dissociate in ionic-electronic melts and form silicon anions of the hypothetical composition Si". These anions may serve as the silicon carriers from the silicon powder or higher silicides to the metal. This is followed by the formation of refractory silicides of rf-transition metals of the 4th and 5th groups. The hypothetical scheme of the silicon transfer through the salt melt is similar to the scheme given above for boron. 

Porous Silicon Transfer of the Thin Silicon Films... 

Table 3.2 lists some of the most common of curing reactions. In the first of these (a) the reaction of hydroxyl (OH) groups extends and crosslinks the Si-O-Si chain and is characteristic of many silicone resins, including the resin constituent in silicone transfer moulding compounds which are described later. Usually this reaction is induced by heating the uncured resin which contains some Si-OH groups, in... 

However, silicone transfer moulding compounds have only been used to a limited extent for packaging integrated circuits. This is due to the less effective lead seal of the silicone resins compared with epoxies, a factor which is more important for a multi-lead IC than for a discrete device. This lead-seal deficiency can be overcome by a technique... 

Fig. 3.7. Power resistors encapsulated in a silicone transfer moulding compound. (C.G.S. Resistors.)...

A range of silicone transfer moulding compounds is available, but to illustrate their general performance Table 3.4 shows the properties of... 

Davis, J. and Jones, G. M., The role of silicone transfer moulding compounds in the packaging of semiconductors and passive electronic components. Proceedings of International Macroelectronic Conference, Electronica, Munich, 1982. 

Silicon Transfer Reagent Bis(trimethylsilyl) sulfide can transform alcohols, acids, and amines into their silylated counterparts. Enol silanes are also formed under the influence of this... 

Conjugated esters undergo sequential additions to form polymers (group transfer polymerization). The molecular weight and the end group functionality of the polymer can be controlled by this method Mechanistic studies indicate an associative intramolecular silicon transfer process via (2), with concomitant C-C bond formation during the polymer growth (eq 6). 

In the last few years, the use of NHC-Cu complexes in catalysis has grown exponentially, particularly for the transfer of carbon and heteroatom-based nucleophiles to various electrophilic substrates. Copper-catalyzed boron and silicon transfers have recently been reported, thus expanding the scope of NHC-copper-catalyzed reactions. Notably, the design of new chiral NHC ligands has enabled the successful development of efficient C-C and C-H bond forming enantioselective reactions. 

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