Transfer moulding silicone

External mould release agents, on the other hand, are sprayed into the open mould. They are often based on paraffins, soaps and oils, e.g., silicone oil. Because of the manner of treatment, these mould release agents may be found not only on the surface but also in layers near the surface. Proprietary non-transfer mould release agents such as Frekote are recommended to ensure optimum bonding surfaces. 

Further processing in which appropriate fillers, catalysts, pigments and other additives are incorporated leads to silicone greases, elastomers and resin-based transfer moulding compounds. A broad list of final silicone products is given in Table 3.1. 

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... 

Table 3.5 compares the typical ingredients of silicone and organic transfer moulding compounds. Because the silicone is naturally fire retardant, no additive for this purpose is needed, nor does the silicone require as much release agent as an organic compound. Silicone... 

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. 

Silicone moulding powders are used almost exclusively in electronic applications and especially for the encapsulation of electronic devices. They are processed by compression or transfer moulding at temperatures of 130-180°C with a minimum pressure of 2.74 MN/m. Typical properties of transfer moulded materials are given in Table 14. 

In the first application, either fluids are coated or solids are sprayed into a mould, or on to a surface, to make it non-stick . Examples of these release agents inclnde oils (organic, fluorocarbon and silicones), soaps and waxes. The choice of any given release agent is usually governed by heat or chemical resistance, the number of release cycles and the sensitivity of the released material to the transferred release agent. 

HTV and liquid silicone rubber LR, are used in moulding at elevated temperatures (press, transfer or injection moulding). 

For the injection moulding of the plastic parts, hot runners allow a waste free production. The plastic is injection moulded as in a conventional process. When sufficient solidification is reached, the mould opens, and an index plate (this is a rotating platen which takes the plastic parts from the first cavity and puts them into the liquid rubber cavity by rotating them through 180 degrees), robot, etc., extracts the plastic moulding and transfers it into the silicone cavity. 

It is enlightening to be able to handle a sample of the soUd polymer. A useful method for forming a sheet of polymer is to cast the Uquid into a silicone rab-ber mould, say 75 x 75 x 1mm deep. A silicone mould can only be used with materials that do not seriously swell the rubber, e.g. dispersions, epoxies and wax. Preparing a hand sample for a liquid that reacts with the mould material, e.g. solutions in organic solvents, is more difficult. One can allow a puddle on polyethene to dry or cast in a well made by waxed microscope slides (Bradley and Wilthew, 1984), but it is difficult to achieve uniformity between samples in this way. Equipment is commercially available for the purpose from polymer and paint test instrument manufacturers, and is usually more cost-effective and the results more transferable than trying to make one s own equipment. 

Figure 44 Package constructions used to evaluate the thermal transfer properties of the die attachment materials and substrates. TO-3 steel package is composed of steel case la, OFHC copper 2a, nickel film 3, soft solder 4a, and integrated circuit 5. TO-3 aluminium package is formed of aluminium header lb plated with nickel 3 on the top of which a silver-filled epoxy adhesive 4b is employed to bond the semiconductor device 5. The 24-lead CERDIP package is based on nickel-plated alumina substrate Ic with soft solder bonded silicon chips. In 40-lead plastic package, integrated circuits 5 are bonded to leadframes 2b by using silver-filled epoxy adhesive 4b and then encapsulated in moulding...

Mould release agents can also be the cause of low adhesion if they are silicone or PTFE based and are transferred across from the mould tool. 

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