Temperature epoxy molding compounds

In this paper we examine moisture sorption in an epoxy molding compound formulation used for semiconductor encapsulation. In particular, we will be concerned with moisture uptake as a function of relative humidity. The effects of temperature, sample thickness, and processing history will be systematically examined for a single commercially important material. 

Epoxy molding compounds, used to encapsulate microelectronic devices, contain bromine to provide flame retardancy to the package. This bromine, typically added as tetrabromo bisphenol-A or its epoxy derivative, has been found to contain many hydrolyzable bromides. These bromides, along with the presence of chloride impurities, are detrimental to the life of the electronic component. Bromine especially has been suspected (proven) to cause wire bond failure when subjected to moisture and/or high temperatures. With the addition of a more thermally and hydrolytic stable bromine compound, flame retardancy does not have to be compromised to increase the device reliability. Stable brominated cresol epoxy novolac, when formulated into a microelectronic encapsulant, increases the reliability of the device without sacrificing any of the beneficial properties of present-day molding compounds. 

Because of the thermal contraction mismatch of a silicon chip, metal lead-frame and silica-filled epoxy molding compound integrated circuit (IC) packages bow or warp when cooled to room temperature after manufacture. The magnitude of the bow in an IC package can be determined quantitatively by... 

Phenohc resins (qv), once a popular matrix material for composite materials, have in recent years been superseded by polyesters and epoxies. Nevertheless, phenohc resins stiU find considerable use in appHcations where high temperature stabiHty and fire resistance are of paramount importance. Typical examples of the use of phenoHc resins in the marine industry include internal bulkheads, decks, and certain finishings. The curing process involves significant production of water, often resulting in the formation of voids within the volume of the material. Further, the fact that phenoHcs are prone to absorb water in humid or aqueous conditions somewhat limits their widespread appHcation. PhenoHc resins are also used as the adhesive in plywood, and phenohc molding compounds have wide use in household appliances and in the automotive, aerospace, and electrical industries (12). 

Semiconductor Grade Silicone-Epoxy. TGA, DSC, and EGA analyses revealed no difference between the FR and non-FR compounds below 200°C. The FR moieties again decomposed only in the temperature range above 350°C. There was very little Cl or Br in the aqueous extract, and no CH2CI or CH2Br was detected in the EGA product profiles. This shows the capability of material formulators to supply very clean semiconductor grade molding compounds. 

Thermoset molding compounds, when contained within a hardened steel mold, require heat and pressure to be polymerized into a solid mass. Molds may be heated by steam, electricity, or hot oil to temperatures of 280° to 425°F, depending entirely on the type of material and method of molding. Molding pressures may vary from a low of 50 p.s.i. to 15,000 p.s.i. Epoxy materials will mold at 50 p.s.i. whereas, phenolic fabric-filled material may require excessive pressures. Again, the method of molding dictates molding pressures. 

The multifunctionality contributes higher reactivity and cross-link density. These factors are especially critical when formulating systems that require improved thermal performance over conventional epichlorohydrin—bisphenol A systems. The melt viscosity of these resins, which are solids at room temperature, decreases sharply with increasing temperature. This affords the formula tor an excellent tool for controlling flow of molding compounds, and facilitating the incorporation of ECN resins into other epoxies, eg, for powder coatings. 

The magnitude of the applications for polymeric substrates has been estimated (in tons) for 1987 on a worldwide basis as phenolic resin, 78K epoxy resin, 130K polyester fiber, 1,010 polyimide film, 235 molding compounds, 330 polymers for high-frequency applications, 300 and high-temperature polymers, 1,440 (4). 

All samples were prepared from a commercially available epoxy cresol novolac-phenol formaldehyde novolac-tertia-ry amine based molding compound. Pelletized preforms were heated to 85°C in a RF preheater prior to being transfer molded at 180°C/68 atm. for 90 sec. Molded samples were cooled in air to room temperature and stored in a desiccated environment until testing or subsequent thermal treatment. Post mold curing, PMC, was accomplished in a gravity oven at 175°C for a period of 4 hours. Samples without post mold curing are designated by NPMC. 

The largest stresses are observed as shear stresses at the corners of the die at the lowest temperature. Three commercially available epoxy-based molding compounds were studied. Two of these materials are standard packaging formulations for smaller devices. Both strain gauge and beam bending experiments showed comparable stress levels with these two materials. The third material is a rubber modified, low stress material. As expected, stress levels in devices packaged with this material, as well as stresses observed in the beam bending apparatus, were considerably lower than those for the other two materials. 

In order to provide the required flame retardancy to the molding compound, an encapsulated formulation usually contains brominated resins and antimony oxide. The brominated resins used in the encapsulated formulation are mainly tetrabromobisphenol A (TBBA) based epoxy resin or brominated epoxy novolac. These bromine-containing additives were reported to cause bond degradation at high temperature through accelerated void formation in the gold-aluminum intermetallic phases (1-4). 

Table III shows the reactivity properties of the molding powder and the physical properties of the cured compound. This table indicates there are virtually no differences in gel times, flow properties, flexural properties, moisture absorption, flame retardancy, and glass transition temperatures between the compound based on stable bromine CEN and the compound based on the standard system of CEN and the epoxy of TBBA.

The electronics industry desires improved flame suppressant additives for microelectronic encapsulants due to bromine induced failure. Epoxy derivatives of novolacs containing meta-bromo phenol have exhibited exceptional hydrolytic and thermal stability in contrast to standard CEN resins with conventional TBBA epoxy resins. When formulated into a microelectronic encapsulant, this stable bromine epoxy novolac contributes to significant enhancements in device reliability over standard resins. The stable bromine CEN encapsulant took about 30% more time to reach 50% failure than the bias pressure cooker device test. In the high temperature storage device test, the stable bromine CEN encapsulant took about 400% more time to reach 50% failure than the standard compound. Finally, the replacement of the standard resins with stable bromine CEN does not adversely affect the desirable reactivity, mechanical, flame retardance or thermal properties of standard molding compounds. 

VE polyurethane resins have mechanical properties similar or superior to those of conventional VE and epoxies. Characteristics include a heat distortion temperature of 120C (248F). Ultimate elongation of an unreinforced molding compound without fillers is 5.5% tensile strength is 80 MPa and flexural strength 150 MPa. The resins can be custom-formulated. Applications include customized automobile parts, recreational vehicles, outdoor equipment, tubs/showers and electrical parts. The resins are suitable for standard molding processes some were specifically developed for pultrusion, RIM, foam, adhesive, and polymer concrete applications. 

Adherent Technologies Inc. [8] has developed a process for the reclamation of carbon fibers from carbon/epoxy composites. It has studied the depolymerization of thermoset carbon fiber reinforced epoxy matrix composites using a low temperature (20 min at 325°Q catalytic tertiary recycling reclamation process and has been able to obtain a product with 99.8% carbon and 0.2% residual resin, with only a loss of about 8.6% in fiber tensile strength. The process can be economically viable, provided sufficient scrap feedstock is available. Possible applications for the recovered fiber include thermoplastic and thermoset molding compounds. 

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