Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1995-05-09
2001-05-01
Sellers, Robert E. L. (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
C525S486000
Reexamination Certificate
active
06225377
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for producing a flame-resistant, pourable, latently reactive, phenolicly curable epoxy resin molding material for encapsulating electronics components.
2. Description of Related Art
In electronics, filler-containing molding materials having a base of epoxy resins are used for the encapsulating of active and passive components. Examples of such uses are the encapsulating of capacitors, diodes, transistors, power semiconductors, optocouplers, storage chips, and microprocessors. Epoxy resin molding materials which are suitable for this purpose must satisfy high demands with regard to their processing properties and the properties of the molded articles (i.e. molded materials, or moldings). This is true, in particular, of the purity of the molding materials, as well as their rheological behavior and their curing properties upon processing as transfer molding materials by transfer molding processes and furthermore the mechanical-thermal properties of the epoxy resin molded materials and long-term stable protection of the components from corrosive environmental influences. Furthermore, the epoxy resin molded materials must satisfy the high requirements of electronics as to their non-inflammability and—in the internationally customary flammability test in accordance with UL 94—reach a grade of V-O in a layer thickness ≦1.6 mm.
The curing of epoxy resin molding materials can be effected with chemically different hardener components, for instance with carboxylic acid anhydrides, amines or phenols. For the encapsulating of electronic components by transfer molding processes, phenolicly curable epoxy resin transfer molding materials which have a high filling of silanized ground fused quartz (i.e. synthetic silica flour) have, however, gained popularity.
Phenolicly curable epoxy resin transfer molding materials generally contain 20 to 30% organic substances and 70 to 80% inorganic substances. The chemical basis of the resin components consists, in most cases, of cresol novolak epoxy resins. For the curing of the epoxy resins, phenol novolaks are predominantly used; the accelerating of the reaction is effect, for instance, with triphenyl-phosphine, phenylphosphonium borate and 2-ethyl-4-methyl imidazole. In order to improve the low-stress behavior, silicone-modified epoxy resin components are used. The flame resistance of the molding materials is obtained by means of aromatic bromine compounds, particularly tetrabromobisphenol A epoxy resins, which are chemically incorporated in the epoxy resin molding upon the curing. In addition, antimony trioxide is used as synergist and increases the effectiveness of the brominated fire-retarding agents. The flame-resistant properties of the epoxy resin moldings are furthermore supported by a high content of silanized fused-quartz fillers; in this connection, both angular (i.e. splintery) fillers and mixtures of angular and spherical fillers are used. Recently, in order to avoid so-called soft errors in highly integrated circuits, synthetically produced &agr;-radiation-free fused quartz fillers have been used. The high content of silanized fused quartz filler also serves to improve the mechanical-thermal properties of the epoxy resin moldings, in particular for adjusting the coefficient of expansion. The molding materials furthermore also contain small quantities of additives, in particular carbon black and processing aids such as stearates and waxes.
For the production of the phenolicly curable epoxy resin transfer molding materials, the resin and hardener components are, as a rule, brought into a prereacted state (B-stage) at temperatures of up to about 120° C., in particular by kneading on a cylinder mill or by extrusion, for instance by means of a screw kneader, and pulverized. For the processing of the transfer molding materials by transfer molding processes, the powdered molding material is generally tabletted and the tablets are possibly preheated to 80 to 90° C. The encapsulating of the components is generally effected with temperatures of the mold of 170 to 190° C. and with a pressure of 70 to 200 bar, the molding material being cured in the mold generally for 60 to 120 sec. Thereupon, the components are removed from the mold and, as a rule, after-cured at temperatures of 170 to 190° C.
The phenolically curable epoxy resin molding materials which have been made flame-resistant with bromine-containing flame-retarding agents and antimony trioxide, which materials have excellent flame-inhibiting properties, have proven popular in electronics. Due to the poor compatibility of the brominated aromatic compounds with the environment, however, the replacement of such epoxy resin molding materials is being increasingly called for. The reasons for this are that, in case of disturbance, i.e. upon fire or carbonizing, these compounds spslit off highly corrosive brominated gases and form biologically difficultly degradable brominated decomposition products of high toxicological-ecological potential for danger. Furthermore epoxy resin moldings which contain brominated flame-retarding agents are not suitable for recycling if a further dispersion of dangerous products is to be avoided. Furthermore, in the future, such epoxy resin moldings cannot be disposed of by burning in view of the increasingly stringent regulations as to the purity of the air, but only as special refuse, with technically expensive and economically unprofitable methods of burning. The use of antimony trioxide is objectionable inasmuch as that compound is included in the list of dangerous carcinogenic substances. The danger is negligible, to be sure, in the case of permanent retention, but in case of fire or carbonization, or in recycling processes, antimony trioxide which has been liberated—in the form of an inhalable dust—represents a high risk.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a method of producing halogen-free molding materials having a base of epoxy resins which satisfy the demands made by electronics on the flame-resistant behavior and, at the same time, satisfy the high demands made on the processing properties and the properties of the molded articles. In addition, these molding materials should have environmentally friendly properties, be suitable for recycling and be disposable by methods of combustion which protect the environment and provide for the recovery of energy.
This is achieved, in accordance with the invention, in the manner that a latently reactive, prepolymer epoxy resin mixture in powder form with no isocyanate groups is produced from a filler-containing, thermally polymerizable reaction resin mixture of polyepoxy resin, consisting of a mixture of difunctional epoxy resins, and polyisocyanate resin, with a molar ratio of the epoxy groups to the isocyanate groups of >1:1, with the use of a substituted imidazole as a reaction accelerator in a concentration of 0.5 to 2.5% by weight, referred to polyepoxy resin, at reaction temperatures of up to 200° C., and that the prepolymer epoxy resin mixture is mixed with a powdered, filler-containing phenolic resin mixture in a molar ratio of the epoxy groups to the phenolic hydroxyl groups of 1:0.4 to 1:1.1, possibly with the use of additives.
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Donner Peter
Kretzschmar Klaus
Markert Helmut
Muller Klaus
Schreyer Michael
Kenyon & Kenyon
Sellers Robert E. L.
Siemens Aktiengesellschaft
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