Patent Publication Number: US-2013241088-A1

Title: Curable silicone resin composition and optoelectronic device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This non-provisional application claims priority under 35 U.S.C. §119(a) on Patent Application Nos. 2012-055872 and 2012-136714 filed in Japan on Mar. 13, 2012 and Jun. 18, 2012, respectively, the entire contents of which are hereby incorporated by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to a curable silicone resin composition useful as material for optical devices and optical members, insulating material for electronic devices and electronic parts, coating material or the like, and an optoelectronic semiconductor device encapsulated therewith. 
     BACKGROUND ART 
     Epoxy resins are generally used as the encapsulating material for light-emitting diode (LED) elements. However, since the epoxy resin has a high modulus of elasticity, stress is induced in the epoxy resin during thermal cycling, raising problems that bonding wires are broken by the stress and the epoxy resin is cracked. The stress applied to LED element by the epoxy resin can cause to disrupt the crystal structure of semiconductor, resulting in a drop of emission efficiency. Also, since the LED element releases both light and heat during a long term of operation, a portion of the resin surrounding the element can be discolored to impede extraction of light. Improvements in light resistance and heat resistance are desired. 
     As the countermeasure to the above problem, Patent Document 1 discloses a silicone-modified organic resin, and Patent Document 2 proposes addition of microparticulate silicone to epoxy resin. However, since these measures use organic component-rich encapsulants, cracks can be formed by ultraviolet light of short wavelength from the optical semiconductor and yellowing can proceed during a long period of operation. Then, the LED device experiences a drop of luminance with the lapse of time. 
     Further, silicone resins are used as the organic component-free flexible encapsulant, as proposed in Patent Documents 3 to 5, for example. The silicone resin is superior in heat resistance, weather resistance and discoloration resistance to the epoxy resin. The silicone resin is also superior in transparency and optical properties to the epoxy resin and other organic materials. For these reasons, the use of silicone resin with LEDs, especially blue LEDs and white LEDs is recently found in many examples. Since a composition based on a silicone resin having a flexible molecular structure has rubber elasticity over a wide temperature range from low to high temperatures, it is effective for suppressing cracks against thermal shock, ensuring that an LED device remains highly reliable over a long period of time. 
     On the other hand, the silicone resin material is generally tacky or adhesive and has low material strength because of rubber nature. It is known that these drawbacks are obviated by adding fillers to silicone to enhance material strength and mitigate tack. In Patent Document 6, for example, polyorganosilsesquioxane in nano-particulate polymer form having an average particle size of up to 100 nm is added to a silicone resin composition. The resulting resin composition has high hardness and improved impact resistance while maintaining high transparency. It is thus suited for the encapsulation of optoelectronic semiconductor. 
     Also, Patent Documents 7 and 8 disclose silicone resin compositions to which is added a silicone-based polymer particles of core/shell structure comprising silicone particles (core) having an average particle size of 0.001 to 1.0 μm coated with an alkoxysilane condensate (shell). The resulting resin compositions have high hardness and improved impact resistance while maintaining high transparency. They are thus suited for the encapsulation of optoelectronic semiconductor devices. 
     However, silicone fine powder having an average particle size of the submicron order as in the above-cited patent documents is susceptible to discoloration by oxidation at elevated temperatures because of its increased specific surface area. On the other hand, since the LED element which produces a higher luminance as a result of the recent lo advancement releases a greater amount of heat, it is empirically expected that the surface temperature of the LED element reaches about 150° C. during operation. Under the circumstances, when a silicone encapsulant containing silicone submicron particles is used, the encapsulant undergoes fast discoloration due to oxidation of silicone submicron particles themselves. Then the light emitted by the optoelectronic semiconductor element is intercepted by the discolored encapsulant, giving rise to the problem of darkened emission. This means that the LED device has a shortened life or lacks long-term heat resistance. 
     Further, Patent Documents 9 to 11 disclose silicone resin compositions which are filled with silicone fine particles for the purposes of material strength improvement, light diffusion and color rendering stabilization. 
     Meanwhile, there is an increasing application of LED devices to outdoor luminaires and automobiles. When an LED device using a silicone encapsulant having high gas permeability is exposed to sulfur oxide gas (commonly referred to as SOx) in the air for a long time, the silver-plated reflector typically included in the LED device as one structural member turns to black silver sulfide, interfering with effective extraction of light emission from the LED device. This necessitates further protection around the LED device, giving negative impacts to brightness extraction efficiency and economy. The techniques of the above-cited patent documents are difficult to prevent silver from sulfiding by SOx. An improvement in this respect is desired. 
     It would be desirable to have an LED-encapsulating material which is effective by itself for preventing silver sulfiding, resistant to repetitive thermal cycling impacts, unsusceptible to cracks under rigorous thermal cycling, resistant to discoloration upon long-term temperature exposure, and reliable. 
     CITATION LIST 
     Patent Document 1: JP-A 2007-077252 
     Patent Document 2: JP-A 2008-192880 
     Patent Document 3: JP-A H11-001619 
     Patent Document 4: JP-A 2002-265787 
     Patent Document 5: JP-A 2004-186168 
     Patent Document 6: JP-A 2006-321832 
     Patent Document 7: JP-A 2007-126609 
     Patent Document 8: JP-A 2008-045039 
     Patent Document 9: JP-A 2006-339581 
     Patent Document 10: JP-A 2008-159713 
     Patent Document 11: JP-A 2011-184625 
     DISCLOSURE OF INVENTION 
     An object of the invention is to provide a curable silicone resin composition which in the cured state is highly resistant to repetitive thermal cycling impacts, unsusceptible to cracks under rigorous thermal cycling, resistant to discoloration upon long-term temperature exposure, and gas barrier (capable of preventing permeation of SOx and other gases), and which is useful as material for optical devices and optical members, insulating material for electronic devices and electronic parts, coating material or the like; a cured product of the composition; and an optoelectronic semiconductor device. 
     Since the prior art encapsulants for LED elements have a likelihood of cracking by thermal shock during electricity conduction and operation of LED as discussed above, there is a need for an encapsulating material which is unsusceptible to cracking and peeling under rigorous thermal cycling. 
     Notably, an LED-encapsulating composition based on a silicone resin having a flexible molecular structure has rubber physical properties (specifically low modulus of elasticity and high elongation) over a wide temperature range, and is effective for preventing cracks by thermal shock. This material is difficult to provide robust protection to LED elements because it is a tacky (or adhesive) rubber material. When the material is applied to outdoor luminaires and automobiles where strict reliability and a long life are required, drops of properties and economy occur. There is a need for a material which has a high hardness (or high modulus of elasticity), is resistant to cracks by thermal shock, and remains reliable. 
     As the current supply to an optoelectronic semiconductor element is increased so that the LED device may produce a higher luminance, the heat release and light emission of the element are increased. Then a thermal load which is at least 50° C. higher than in the prior art is applied to the material around the light-emitting element. Specifically, the temperature around the light-emitting element reaches about 150° C. The encapsulating material is required to be resistant to thermal discoloration at elevated temperatures. 
     As described above, there is a need for an LED-encapsulating material which has a high material strength, and is effective for suppressing cracks by thermal shock and resistant to thermal discoloration. 
     The inventors have found that the outstanding problems are solved by a composition comprising (A-1) an organosilicon compound having at least two aliphatic unsaturated radicals per molecule and optionally, (A-2) an organopolysiloxane of branched or three-dimensional network structure, (B) an organosilicon compound having at least two silicon-bonded hydrogen atoms per molecule and free of aliphatic unsaturation, (C) a hydrosilylation catalyst, and (D) silicone powder having an average particle size of 0.5 to 100 μm. This addition-curable silicone resin composition is suitable as the LED-encapsulating material. 
     The curable silicone resin composition cures into a product which is highly resistant to repetitive thermal impacts, unsusceptible to cracks under rigorous thermal cycling, and resistant to discoloration upon long-term temperature exposure. 
     In one aspect, the invention provides a curable silicone resin composition comprising 
     (A) the following component (A-1) or a mixture of components (A-1) and (A-2): 
     (A-1) a compound having at least two aliphatic unsaturated radicals per molecule, represented by the formula (1): 
     
       
         
         
             
             
         
       
     
     wherein R is each independently a substituted or unsubstituted monovalent hydrocarbon radical, Ar is each independently an aryl radical optionally containing a heteroatom, m is 0 or an integer of at least 1, and n is an integer of at least 1, 
     (A-2) an organopolysiloxane of branched or three-dimensional network structure represented by the average compositional formula (2): 
       R′ p (C 6 H 5 ) q SiO (4-p-q)/2    (2)
 
     wherein R′ is each independently a substituted or unsubstituted monovalent hydrocarbon radical, alkoxy radical or hydroxyl radical, 0.1 to 80 mol % of all silicon-bonded organic radicals being alkenyl, p and q are positive numbers meeting the range: 1≦p+q&lt;2 and 0.20≦q/(p+q)≦0.95, 
     (B) an organosilicon compound having at least two silicon-bonded hydrogen atoms per molecule and free of aliphatic unsaturation, in a sufficient amount for the composition to cure in the presence of a hydrosilylation catalyst, 
     (C) a platinum group metal based hydrosilylation catalyst, and 
     (D) silicone powder having an average particle size of 0.5 to 100 μm, in an amount of 0.1 to 500 parts by weight per 100 parts by weight of components (A) and (B) combined. 
     Preferably, in formula (1), Ar is phenyl, and n is an integer of 1 to 100. 
     In a preferred embodiment, component (B) is an organohydrogenpolysiloxane having the average compositional formula (3): 
       R″ a H b SiO (4-a-b)/2    (3)
 
     wherein R″ is each independently a silicon-bonded, substituted or unsubstituted monovalent hydrocarbon radical other than an aliphatic unsaturated radical, a and b are positive numbers meeting the range: 0.7≦a≦2.1, 0.001≦b≦1.0, and 0.8≦a+b≦3.0. 
     In a preferred embodiment, component (D) consists of particles of a polyorganosilsesquioxane resin or silicone particles covered partially or entirely on their surface with a polyorganosilsesquioxane resin. 
     In a preferred embodiment, a sub-composition composed essentially of components (A), (B) and (C), when cured, has an oxygen permeability of up to 1,000 cm 3 /m 2 /24 h/atm. 
     The composition is typically used for the encapsulation of optoelectronic semiconductor. 
     In another aspect, the invention provides a cured product obtained by curing the curable silicone resin composition defined above. 
     In a further aspect, the invention provides an optoelectronic semiconductor device obtained by encapsulating an optoelectronic semiconductor element with the curable silicone resin composition defined above and heat curing the composition. 
     Advantageous Effects of Invention 
     The curable silicone resin composition cures into a product which is highly resistant to repetitive thermal impacts, unsusceptible to cracks under rigorous thermal cycling, and resistant to discoloration upon long-term temperature exposure, and prevents sulfur oxide gas from being transmitted thereby. The composition is advantageously applied to those optoelectronic semiconductor devices used in a more rigorous environment. 
    
    
     
       BRIED DESCRIPTION OF DRAWING 
         FIG. 1  is a schematic cross-sectional view of an optoelectronic semiconductor device to which the curable silicone resin composition is applied. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The curable silicone resin composition of the invention is useful as an optical part-coating material, typically LED-encapsulating material. The composition comprises components (A) to (D), which are described below in detail. 
     Component A 
     Component (A) is component (A-1) or a mixture of components (A-1) and (A-2). Component (A-1) is a compound having at least two aliphatic unsaturated radicals per molecule, represented by the formula (1): 
     
       
         
         
             
             
         
       
     
     wherein R is each independently a substituted or unsubstituted monovalent hydrocarbon radical, Ar is each independently an aryl radical optionally containing a heteroatom, m is 0 or an integer of at least 1, and n is an integer of at least 1. Differently stated, component (A-1) is a linear diorganopolysiloxane comprising recurring diarylsiloxane units as the backbone. Component (A-1) may be an organopolysiloxane alone or a mixture of two or more organopolysiloxanes having different molecular weights and/or silicon-bonded organic groups of different type. 
     In formula (1), examples of the aryl radical represented by Ar include aromatic hydrocarbon radicals such as phenyl and naphthyl, and aromatic hydrocarbon radicals containing a heteroatom (e.g., O, S or N) such as furanyl. The aryl radical may have a substituent(s) such as halogen atom (e.g., chloro, bromo or fluoro). Preferably Ar is an unsubstituted aromatic hydrocarbon radical, most preferably phenyl. 
     Examples of the substituted or unsubstituted monovalent hydrocarbon radical represented by R in formula (1) include aliphatic unsaturated radicals (to be described later) and monovalent hydrocarbon radicals other than the aliphatic unsaturated radicals, for example, alkyl radicals of 1 to 6 carbon atoms such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, sec-butyl, and tert-butyl; haloalkyl radicals of 1 to 4 carbon atoms such as chloromethyl and 3,3,3-trifluoropropyl; and aryl radicals of 6 to 10 carbon atoms such as phenyl and tolyl. Of the monovalent hydrocarbon radicals other than the aliphatic unsaturated radicals, preference is given to alkyl radicals of 1 to 6 carbon atoms and phenyl, with methyl being most preferred. 
     The aliphatic unsaturated radical represented by R in formula (1) is not particularly limited as long as it maintains the composition in a stable uncured state prior to the start of addition reaction and it permits the composition to readily cure after the start of addition reaction. Exemplary are ethylenically unsaturated radicals and acetylenically unsaturated radicals. The aliphatic unsaturated radical may be of one type or a mixture of two or more types. At least two aliphatic unsaturated radicals must be included per molecule, while they are preferably positioned at both ends. 
     As used herein, the term “ethylenically unsaturated radical” refers to an organic radical which contains a carbon-carbon double bond and optionally, a heteroatom such as oxygen or nitrogen. Examples include alkenyl radicals of 2 to 20 carbon atoms, preferably 2 to 10 carbon atoms such as vinyl, allyl, 5-hexenyl, propenyl and butenyl; alkadienyl radicals of 4 to 10 carbon atoms such as 1,3-butadienyl; combinations of the alkenyl radicals with carbonyloxy such as acryloyloxy (—O(O)CCH═CH 2 ) and methacryloyloxy (—O(O)CC(CH 3 )═CH 2 ); and combinations of the alkenyl radicals with carbonylamino such as acrylamide (—NH(O)CCH═CH 2 ). 
     As used herein, the term “acetylenically unsaturated radical” refers to an organic radical which contains a carbon-carbon triple bond and optionally, a heteroatom such as oxygen or nitrogen. Examples include alkynyl radicals of 2 to 20 carbon atoms, preferably 2 to 10 carbon atoms such as ethynyl and propargyl; and combinations of the alkynyl radicals with carbonyloxy such as ethynylcarbonyloxy (—O(O)CC═CH). 
     It is preferred from the standpoints of productivity and cost in the preparation of reactants for component (A-1), reactivity of component (A-1) and the like that the aliphatic unsaturated radical be any of the alkenyl radicals, more preferably vinyl, allyl or 5-hexenyl, and most preferably vinyl. 
     In component (A-1), n indicative of a degree of polymerization of diarylsiloxane units is an integer of at least 1, preferably 1 to 100, more preferably 1 to 20, and even more preferably 2 to 10; m is an integer of 0, 1 or more, preferably an integer of 0 to 100, and more preferably 0 to 20. 
     Component (A-1) may be prepared, for example, from a difunctional silane such as dichlorodiphenylsilane or dialkoxydiphenylsilane, by effecting hydrolysis and condensation and subsequently or simultaneously blocking at the end with an endcapping agent containing an aliphatic unsaturated radical. 
     In the curable silicone resin composition, component (A-2) is optionally added to component (A-1) for the purpose of acquiring cured hardness. 
     Component (A-2) is an organopolysiloxane of branched or three-dimensional network structure represented by the average compositional formula (2): 
       R′ p (C 6 H 5 ) q SiO (4-p-q)/2    (2)
 
     wherein R′ is each independently a substituted or unsubstituted monovalent hydrocarbon radical, alkoxy radical or hydroxyl radical, 0.1 to 80 mol % of all silicon-bonded organic radicals being alkenyl, p and q are positive numbers meeting the range: 1≦p+q&lt;2 and 0.20≦q/(p+q)≦0.95. Component (A-2) may be an organopolysiloxane alone or a mixture of two or more. 
     As understood from the range: 1≦p+q&lt;2 in average compositional formula (2), the organopolysiloxane has a branched or three-dimensional network structure containing units of one or more type selected from R′SiO 3/2  units (wherein R′ is as defined above), (C 6 H 5 )SiO 3/2  units, and SiO 2  units, in the molecule. 
     In formula (2), C 6 H 5  stands for phenyl. R′ is each independently a substituted or unsubstituted monovalent hydrocarbon radical (exclusive of phenyl), alkoxy radical or hydroxyl radical, preferably a substituted or unsubstituted monovalent hydrocarbon radical of 1 to 20 carbon atoms, more preferably 1 to 10 carbon atoms, alkoxy radical or hydroxyl radical. Suitable hydrocarbon radicals include alkyl radicals such as methyl, ethyl, propyl, butyl, pentyl, hexyl, cyclohexyl and heptyl; aryl radicals (exclusive of phenyl) such as tolyl, xylyl and naphthyl; aralkyl radicals such as benzyl and phenethyl; unsaturated hydrocarbon radicals, for example, alkenyl radicals such as vinyl, allyl, butenyl, pentenyl, hexenyl, and heptenyl; and haloalkyl radicals such as chloromethyl, 3-chloropropyl and 3,3,3-trifluoropropyl. Suitable alkoxy radicals include unsubstituted alkoxy radicals such as methoxy, ethoxy, propoxy, and phenoxy, and alkoxy-substituted alkoxy radicals such as methoxyethoxy and ethoxyethoxy. 
     In formula (2), alkenyl should account for 0.1 to 80 mol %, preferably 0.5 to 50 mol % of all silicon-bonded organic radicals. If the alkenyl content is less than 0.1 mol %, the resulting silicone resin fails to acquire the requisite hardness. If the alkenyl content exceeds 80 mol %, the resulting silicone resin becomes brittle because of too many crosslinking sites. Of the alkenyl radicals, vinyl is most preferred. 
     The subscripts p and q are positive numbers meeting the range: 1≦p+q&lt;2, preferably 1.2≦p+q&lt;1.9, and 0.20≦q/(p+q)≦0.95, preferably 0.25≦q/(p+q)≦0.90. If p+q is less than 1 or equal to or more than 2, the requisite hardness and strength are not acquired. If the phenyl content is less than the range, the resulting silicone resin fails to acquire the requisite hardness and strength. An excessive phenyl content adversely affects heat resistance. 
     Examples of the organopolysiloxane (A-2) include, but are not limited to, (PhSiO 3/2 ) 0.55 [(CH 2 ═CH)Me 2 SiO 1/2 ] 0.2  (Me 2 SiO) 0.25  and (PhSiO 3/2 ) 0.75 [(CH 2 ═CH)Me 2 SiO 1/2 ] 0.25  wherein Ph stands for phenyl and Me stands for methyl. 
     Component (A-2) is added in such an amount that the cured resin composition may have the desired hardness, typically in an amount of 0 to 500 parts by weight per 100 parts by weight of component (A-1). When used, component (A-2) is preferably added in an amount of 20 to 300 parts by weight per 100 parts by weight of component (A-1). 
     Component B 
     Component (B) is an organosilicon compound having at least two silicon-bonded hydrogen atoms (i.e., SiH radicals) per molecule and free of aliphatic unsaturation. It may also be referred to as SiH-containing organosilicon compound. It serves as a crosslinker via hydrosilylation or addition reaction with component (A). Component (B) may be an organosilicon compound alone or a mixture of two or more. Any well-known organosilicon compound may be used as component (B) as long as it has at least two SiH radicals per molecule. Examples include organohydrogenpolysiloxanes, organohydrogensilanes, organic oligomers and organic polymers having at least two SiH radicals per molecule. Inter alia, organohydrogenpolysiloxanes having at least two SiH radicals per molecule are preferred. 
     In component (B), the silicon-bonded organic radicals include unsubstituted monovalent hydrocarbon radicals free of aliphatic unsaturation, and monovalent hydrocarbon radicals substituted with halogen atoms (e.g., chloro, bromo and fluoro), epoxy-containing radicals (e.g., epoxy, glycidyl and glycidoxy), alkoxy radicals (e.g., methoxy, ethoxy, propoxy and butoxy) or the like. Suitable monovalent hydrocarbon radicals include those exemplified as the substituted or unsubstituted monovalent hydrocarbon radicals of R in formula (1) representative of component (A-1), for example, alkyl radicals of 1 to 6 carbon atoms, haloalkyl radicals of 1 to 4 carbon atoms, and aryl radicals of 6 to 10 carbon atoms. The preferred organic radicals include alkyl radicals of 1 to 6 carbon atoms and aryl radicals of 6 to 10 carbon atoms, with methyl or phenyl being most preferred. The monovalent hydrocarbon radical substituted with an epoxy-containing radical and/or alkoxy radical is effective for imparting adhesion to the cured resin composition. 
     As long as component (B) is an organohydrogenpolysiloxane having at least two SiH radicals per molecule, the molecular structure of organohydrogenpolysiloxane is not particularly limited. Any of currently available organohydrogenpolysiloxanes of linear, cyclic, branched and three-dimensional network (or resinous) structures may be used. 
     The organohydrogenpolysiloxane has at least 2 (specifically 2 to about 300), preferably at least 3 (specifically 3 to about 200, more specifically 4 to about 100) SiH radicals per molecule. Where the organohydrogenpolysiloxane is of linear or branched structure, the SiH radicals may be positioned at either one or both of the terminal and non-terminal positions of the molecular chain. 
     In the organohydrogenpolysiloxane, the number of silicon atoms (i.e., degree of polymerization) is preferably about 2 to about 1,000, more preferably about 3 to about 200, and even more preferably about 4 to about 100 per molecule. The organohydrogenpolysiloxane is preferably liquid at 25° C. and specifically has a viscosity of 1 to 1,000 mPa-s, more preferably 10 to 100 mPa-s at 25° C. as measured by a rotational viscometer. 
     The organohydrogenpolysiloxane used herein preferably has the average compositional formula (3), for example. 
       R″ a H b SiO (4-a-b)/2    (3)
 
     Herein R″ is each independently a silicon-bonded, substituted or unsubstituted monovalent hydrocarbon radical other than an aliphatic unsaturated radical, a and b are positive numbers meeting the range: 0.7≦a≦2.1, 0.001≦b≦1.0, and 0.8≦a+b≦3.0, preferably 1.0≦a≦2.0, 0.01≦b≦1.0, and 1.5≦a+b≦2.5. 
     In formula (3), suitable monovalent hydrocarbon radicals (exclusive of the aforementioned aliphatic unsaturated radical) of R″ include those exemplified as the substituted or unsubstituted monovalent hydrocarbon radicals of R in formula (1) representative of component (A-1), for example, alkyl radicals of 1 to 6 carbon atoms, haloalkyl radicals of 1 to 4 carbon atoms, and aryl radicals of 6 to 10 carbon atoms. R″ is preferably selected from alkyl radicals of 1 to 6 carbon atoms and aryl radicals of 6 to 10 carbon atoms, with methyl or phenyl being most preferred. 
     Examples of the organohydrogenpolysiloxane having average compositional formula (3) include cyclic compounds containing at least 4 organohydrogensiloxane units of the formula: R″HSiO, compounds of the formula: R″ 3 SiO(HR″SiO) c SiR″ 3 , compounds of the formula: HR″ 2 SiO(HR″SiO) c SiR″ 2 H, and compounds of the formula: HR″ 2 SiO(HR″SiO) c (R″ 2 SiO) d SiR″ 2 H wherein R″ is as defined above, c and d are numbers of at least 1. 
     Alternatively, the organohydrogenpolysiloxane having average compositional formula (3) may comprise siloxane units of the formula: HSiO 1.5 , siloxane units of the formula: R″HSiO, and/or siloxane units of the formula: R″ 2 HSiO 0.5  wherein R″ is as defined above. The organohydrogenpolysiloxane may also comprise monoorganosiloxane units, diorganosiloxane units, triorganosiloxane units and/or SiO 4/2  units, which are free of SiH radical. 
     In the organohydrogenpolysiloxane having average compositional formula (3), methylhydrogensiloxane units preferably account for 30 to 100 mol % of all organosiloxane units. 
     Examples of the organohydrogenpolysiloxane having at least two SiH radicals (B) include 1,1,3,3-tetramethyldisiloxane, 1,3,5,7-tetramethylcyclotetrasiloxane, tris(hydrogendimethylsiloxy)methylsilane, tris(hydrogendimethylsiloxy)phenylsilane, methylhydrogencyclopolysiloxane, methylhydrogensiloxane/dimethylsiloxane cyclic copolymers, trimethylsiloxy-endcapped methylhydrogenpolysiloxane, trimethylsiloxy-endcapped dimethylsiloxane/methylhydrogen-siloxane copolymers, trimethylsiloxy-endcapped diphenylsiloxane/methylhydrogen-siloxane copolymers, trimethylsiloxy-endcapped methylphenylsiloxane/methyl-hydrogensiloxane copolymers, trimethylsiloxy-endcapped dimethylsiloxane/methylhydrogen-siloxane/methylphenylsiloxane copolymers, trimethylsiloxy-endcapped dimethylsiloxane/methylhydrogen-siloxane/diphenylsiloxane copolymers, dimethylhydrogensiloxy-endcapped methylhydrogenpolysiloxane, dimethylhydrogensiloxy-endcapped dimethylpolysiloxane, dimethylhydrogensiloxy-endcapped dimethylsiloxane/methyl-hydrogensiloxane copolymers, dimethylhydrogensiloxy-endcapped dimethylsiloxane/methyl-phenylsiloxane copolymers, dimethylhydrogensiloxy-endcapped dimethylsiloxane/diphenyl-siloxane copolymers, dimethylhydrogensiloxy-endcapped methylphenylpolysiloxane, dimethylhydrogensiloxy-endcapped diphenylpolysiloxane, dimethylhydrogensiloxy-endcapped diphenylsiloxane/methyl-hydrogensiloxane copolymers, and substituted forms of the foregoing organohydrogenpolysiloxanes in which some or all methyl radicals are substituted by other alkyl radicals such as ethyl or propyl, organosiloxane copolymers consisting of siloxane units of R″ 3 SiO 0.5 , siloxane units of R″ 2 HSiO 0.5 , and siloxane units of SiO 2 , organosiloxane copolymers consisting of siloxane units of R″ 2 HSiO 0.5  and siloxane units of SiO 2 , organosiloxane copolymers consisting of siloxane units of R″HSiO, and siloxane units of R″ SiO 1.5  and/or siloxane units of HSiO 1.5 , and mixtures of two or more of the foregoing organopolysiloxanes. 
     Herein, R″ is as defined above. It is noted that the term “endcapped” means that a siloxane is capped at both ends of the molecular chain with the referenced radicals, unless otherwise stated. 
     Component (B) is used in a sufficient amount for the composition to cure in the presence of a hydrosilylation catalyst, typically in such an amount that the molar ratio of SiH radicals in component (B) to aliphatic unsaturated radicals in component (A) may range from 0.2/1 to 5/1, preferably from 0.5/1 to 2/1. 
     Component C 
     Component (C) is a platinum group metal based hydrosilylation catalyst. It may be any of catalysts which promote hydrosilylation or addition reaction between silicon-bonded aliphatic unsaturated radicals in component (A) and SiH radicals in component (B). Component (C) may be a catalyst alone or a mixture of two or more. Suitable catalysts include platinum group metals such as platinum, palladium and rhodium, chloroplatinic acid, alcohol-modified chloroplatinic acid, coordinate compounds of chloroplatinic acid with olefin, vinylsiloxane or acetylene compounds, platinum group metal compounds such as tetrakis(triphenylphosphine)palladium and chlorotris(triphenylphosphine)rhodium. Inter alia, platinum compounds are preferred. The catalyst may be used in a catalytic amount, which is typically 0.1 to 1,000 ppm, more preferably 1 to 500 ppm of platinum group metal based on the total weight of components (A) and (B). 
     Preferably, when a sub-composition composed essentially of components (A) to (C) as defined above is heat cured, the cured product has an oxygen permeability of up to 1,000 cm 3 /m 2 /24 h/atm. As long as the oxygen permeability of the cured composition is up to 1,000 cm 3 /m 2 /24 h/atm, the invention&#39;s effect of inhibiting permeation of sulfur oxide and other gases is advantageously exerted. More preferably the oxygen permeability is up to 500 cm 3 /m 2 /24 h/atm. With an oxygen permeability in excess of 1,000 cm 3 /m 2 /24 h/atm, there is a risk that when an LED package constructed by filling a cavity around a partially silver-plated LED device with an encapsulating resin composition and curing the composition is subjected to a sulfur exposure test, sulfur can permeate through the encapsulating resin to promote sulfiding of plated silver so that silver turns to black silver sulfide. As a result, the luminance of the LED package may be reduced. The lower limit of oxygen permeability is typically at least 0 cm 3 /m 2 /24 h/atm, though not critical. The measurement of oxygen permeability will be described later. 
     Component D 
     The curable silicone resin composition is characterized by comprising (D) silicone powder in addition to components (A) to (C) defined above. The addition of silicone powder as component (D) offers one advantage of improvement in mechanical properties. Since the silicone powder has low elasticity, it functions as a stress releaser to heat shocks repeatedly applied in turning on and off the light-emitting element. Accordingly, the encapsulating resin loaded with silicone powder is highly resistant to thermal shock and unsusceptible to cracks under rigorous thermal cycling conditions. In addition, the silicone powder can clear the problems of the resin composition that it has tack or adhesion inherent to rubber material and is difficult to provide robust protection to the LED element because of rubber nature. That is, the silicone powder serves to mitigate tack and stiffen the cured product. 
     A second advantage is an improvement in discoloration resistance. The silicone powder is known to experience least discoloration in a high-temperature environment. Thus adding silicone powder to the encapsulating resin is effective for suppressing discoloration of the resin when hot. Also, since the silicone powder is not light absorptive, it is devoid of gradual blackening upon exposure to UV light from the light-emitting element, as often found with other organic resin powders. Thus adding silicone powder to the encapsulating resin is effective for suppressing discoloration of the resin in a high-temperature environment. Then the resin can maintain transparency upon long-term exposure to UV light, yielding an optoelectronic semiconductor device having a longer lifetime. 
     Examples of the silicone powder include silicone resin particles, typically fine particles of polyorganosilsesquioxane, for example, those described in JP-B S40-16917, JP-A S54-72300, S60-13813, H03-244636, and H04-88023; composite silicone particles of the structure in which silicone rubber particles are surface covered with fine particles of polyorganosilsesquioxane resin, for example, those described in JP-A H07-196815. The silicone powder may be used alone or in admixture of two or more. The silicone powder may be prepared by any well-known methods while any commercially available silicone powder may be used. 
     The silicone powder should have an average particle size in the range of 0.5 to 100 μm, preferably 1 to 15 μm. Particles with an average particle size of less than 0.5 μm will agglomerate together when dispersed in the resin composition, inviting a drop of strength of the cured resin composition. Since such submicron particles have an increased specific surface area, they undergo discoloration by oxidation at elevated temperatures, indicating a lowering of thermal discoloration resistance. An average particle size in excess of the range is undesirable because the step of uniformly dispensing and dispersing such particles in the resin composition is interfered, specifically giving rise to problems of stringing and dispensing nozzle clogging. 
     It is noted that the average particle size is determined as cumulative weight average value D 50  (or median diameter) upon particle size distribution measurement by the laser light diffraction method. 
     The silicone powder is used in an amount of 0.1 to 500 parts, preferably 1 to 100 parts by weight per 100 parts by weight of components (A) and (B) combined. Outside the range, a lower content of silicone powder fails to provide the cured resin composition with desired properties including thermal shock resistance, least thermal discoloration and gas barrier whereas a higher content provides the resin composition with a high viscosity which is industrially undesirable because the working (including casting and curing) of the composition is adversely affected. 
     Silicone powders are commercially available. Exemplary silicone resin powders include KMP590, KMP701, X-52-854, and X-52-1621, and exemplary composite silicone powders include KMP600, KMP601, KMP602, KMP605, and X-52-7030, all commercially available from Shin-Etsu Chemical Co., Ltd., but are not limited thereto. 
     Other Components 
     In addition to components (A) to (D) defined above, other components may be optionally compounded in the composition of the invention insofar as the objects of the invention are not impeded. Optional components are described below. Each component may be a single compound or a mixture of two or more compounds. 
     Aliphatic Unsaturation-Bearing Compound Other than Component (A) 
     Besides component (A), an aliphatic unsaturation-bearing compound capable of addition reaction to component (B) may be compounded in the composition. The aliphatic unsaturation-bearing compound other than component (A) is preferably one that participates in formation of a cured product, typically an organopolysiloxane other than the organopolysiloxane having at least two aliphatic unsaturated radicals per molecule (A-1). Its molecular structure may be linear, cyclic, branched or three-dimensional network, for example. 
     In the composition, an aliphatic unsaturation-bearing oraanic compound other than the aforementioned organopolysiloxane may be compounded. Suitable aliphatic unsaturation-bearing compounds include monomers such as butadiene and diacrylates derived from polyfunctional alcohols; polyolefins such as polyethylene, polypropylene, and copolymers of styrene with other ethylenically unsaturated compounds (e.g., acrylonitrile and butadiene); oligomers and polymers derived from functionality-substituted organic compounds such as esters of acrylic acid, methacrylic acid and maleic acid. The aliphatic unsaturation-bearing compound other than component (A) may be either liquid or solid at room temperature. 
     Addition Reaction Inhibitor 
     In the composition, an addition reaction inhibitor may be added for providing a pot life. The addition reaction inhibitor is not particularly limited as long as it is effective for suppressing the cure promotion of the hydrosilylation catalyst (C). Any of well-known inhibitors may be used. Exemplary inhibitors include phosphorus-containing compounds such as triphenylphosphine, nitrogen-containing compounds such as tributylamine, tetramethylethylenediamine, and benzotriazole, sulfur-containing compounds, acetylene compounds such as acetylene alcohols, e.g., 1-ethynylcyclohexanol and 3,5-dimethyl-1-hexyn-3-ol, compounds having at least two alkenyl radicals, hydroperoxy compounds, and maleic acid derivatives. 
     The degree of cure suppression that the addition reaction inhibitor exerts depends on its chemical structure. Thus, the amount of inhibitor added is preferably adjusted to optimum for a particular addition reaction inhibitor used. When an optimum amount of the addition reaction inhibitor is added, the composition has a good balance of long-term shelf stability at room temperature and heat curability. 
     Silane Coupling Agent 
     A tackifier may be added to the composition for improving its adhesion. Typical tackifiers are silane coupling agents and hydrolytic condensates thereof. Suitable silane coupling agents include epoxy-containing silane coupling agents (e.g., γ-glycidoxypropyltrimethoxysilane), (meth)acrylic silane coupling agents, isocyanate-containing silane coupling agents, isocyanurate-containing silane coupling agents, amino-containing silane coupling agents, and mercapto-containing silane coupling agents. The silane coupling agent is preferably added in an amount of 0.1 to 20 parts, more preferably 0.3 to 10 parts by weight per 100 parts by weight of components (A) and (B) combined. 
     Other Optional Components 
     Also, any well-known antioxidants such as 2,6-di-t-butyl-4-methylphenol may be added to the composition for the purpose of preventing the cured composition from coloration and oxidative degradation. Photo-stabilizers such as hindered amine stabilizers may be added to the composition for the purpose of imparting resistance to photo-degradation. Inorganic fillers such as fumed silica and nano-size alumina may be added to the composition for the purposes of improving strength and preventing particle settlement as long as the transparency of the cured composition is not affected. If desired, dyes, pigments, flame retardants or the like may be added to the composition. 
     Cured Product 
     The silicone composition may be cured under well-known conditions by any standard methods. Specifically, the composition may be cured by heating at a temperature of 80 to 200° C., preferably 100 to 160° C. The heating time may be about 0.5 minute to about 5 hours, preferably about 1 minute to about 3 hours. A relatively long cure time is preferred where an accuracy is required as in the case of LED encapsulation. The form of the cured composition is not particularly limited. The cured product may be of gel, elastomer or resin form, for example. 
     Optoelectronic Device Encapsulant 
     Like cured products of conventional addition-curable silicone compositions, the cured product of the composition is improved in heat resistance, freeze resistance and electrical insulation. The composition is used as encapsulant for optoelectronic devices including, for example, LED, semiconductor laser, photo-diodes, photo-transistors, solar cells, and CCD. The optoelectronic package may be obtained by coating an optoelectronic element with the composition (i.e., encapsulant), and curing the composition by any standard methods under well-known conditions, specifically under the above-mentioned conditions. 
     EXAMPLE 
     Examples of the invention are given below by way of illustration and not by way of limitation. In Examples, all parts are by weight. A silicone oil or silicone resin is represented by an average compositional formula wherein symbols denote the following units. The molar amount associated with a silicone oil or silicone resin is the molar amount of vinyl or SiH radicals contained in each component. 
     M H : (CH 3 ) 2 HSiO 1/2    
     M: (CH 3 ) 3 SiO 1/2    
     M Vi : (CH 2 ═CH)(CH 3 ) 2 SiO 1/2    
     M Vi3 : (CH 2 ═CH) 3 SiO 1/2    
     D H : (CH 3 )HSiO 2/2    
     D Ph : (C 6 H 5 ) 2 SiO 2/2    
     D: (CH 3 ) 2 SiO 2/2    
     D Vi : (CH 2 ═CH)(CH 3 )SiO 2/2    
     T Ph : (C 6 H 5 )SiO 3/2    
     Q: SiO 4/2    
     Formulation Example 1 
     A silicone sub-composition #1 was prepared by thoroughly milling the following components:
     100 parts of a silicone oil of average compositional formula: M Vi   2 D Ph   2.8  as component (A),   51.3 parts of methylhydrogensiloxane of average compositional formula: M H D H   2 D Ph   2 M H  as component (B),   0.06 part of a toluene solution of chloroplatinic acid/1,3-divinyltetramethyldisiloxane complex (1 wt % Pt) as component (C),   0.05 part of ethynylcyclohexanol, and   3 parts of γ-glycidoxypropyltrimethoxysilane.   

     Sub-composition #1 as cured was measured to have an oxygen permeability of 210 cm 3 /m 2 /24 h/atm. 
     Formulation Example 2 
     A silicone sub-composition #2 was prepared by thoroughly milling the following components:
     23 parts of a silicone oil of average compositional formula:   MD 3.4 D Vi   6.5 D Ph   8.6 M and 80 parts of a silicone oil of average compositional formula: M Vi   2 D Ph   2.8  as component (A),   30 parts of methylhydrogensiloxane of average compositional formula: M H D H   2 D Ph   2 M H  as component (B),   0.06 part of a toluene solution of chloroplatinic acid/1,3-divinyltetramethyldisiloxane complex (1 wt % Pt) as component (C),   0.05 part of ethynylcyclohexanol, and   3 parts of γ-glycidoxypropyltrimethoxysilane.   

     Sub-composition #2 as cured was measured to have an oxygen permeability of 400 cm 3 /m 2 /24 h/atm. 
     Formulation Example 3 
     A silicone sub-composition #3 was prepared by thoroughly milling the following components:
     a mixture of 65 parts of a silicone resin consisting of M, M Vi , and Q units wherein a molar ratio of M units to M Vi  units is 6.25, and a molar ratio of the sum of M units and M Vi  units to Q units is 0.8, and 35 parts of a silicone oil of average compositional formula: M Vi3 D 298 M Vi3  (which mixture is a viscous liquid at room temperature) as component (A′),   8 parts of methylhydrogensiloxane of average compositional formula: MD H   80 M as component (B),   0.06 part of a toluene solution of chloroplatinic acid/1,3-divinyltetramethyldisiloxane complex (1 wt % Pt) as component (C),   0.05 part of ethynylcyclohexanol, and   3 parts of γ-glycidoxypropyltrimethoxysilane.   

     Sub-composition #3 as cured was measured to have an oxygen permeability of 20,000 cm 3 /m 2 /24 h/atm. 
     Formulation Example 4 
     A silicone sub-composition #4 was prepared by thoroughly milling the following components:
     a mixture of (A-1) 31 parts of a silicone oil of average compositional formula: M Vi   2 D Ph   2.8  and (A-2) 59 parts of branched chain organopolysiloxane of formula: T Ph   0.75 M Vi   0.25  (solid at 25° C., silicon-bonded vinyl content=20 mol % and silicon-bonded phenyl content=50 mol % based on all silicon-bonded organic radicals, weight average molecular weight=1600 versus polystyrene standards) as component (A),   6.4 parts of methylhydrogensiloxane of average compositional formula: M H D H   2 D Ph   2 M H  as component (B),   0.06 part of a toluene solution of chloroplatinic acid/1,3-divinyltetramethyldisiloxane complex (1 wt % Pt) as component (C),   0.05 part of ethynylcyclohexanol, and   3 parts of γ-glycidoxypropyltrimethoxysilane.   

     Sub-composition #4 as cured was measured to have an oxygen permeability of 250 cm 3 /m 2 /24 h/atm. 
     Example 1 
     A composition (a) was prepared by mixing 100 parts of 
     Sub-composition #1 with 10 parts of composite silicone powder having an average particle size of 5 μm (KMP-600 by Shin-Etsu Chemical Co., Ltd.) until uniform. 
     Example 2 
     A composition (b) was prepared by mixing 100 parts of Sub-composition #1 with 10 parts of silicone resin powder having an average particle size of 5 μm (X-52-1621 by Shin-Etsu Chemical Co., Ltd.) until uniform. 
     Example 3 
     A composition (c) was prepared by mixing 100 parts of Sub-composition #2 with 5 parts of composite silicone powder having an average particle size of 0.8 μm (X-52-7030 by Shin-Etsu Chemical Co., Ltd.) until uniform. 
     Example 4 
     A composition (d) was prepared by mixing 100 parts of Sub-composition #2 with 5 parts of silicone resin powder having an average particle size of 0.8 μm (X-52-854 by Shin-Etsu Chemical Co., Ltd.) until uniform. 
     Example 5 
     A composition (e) was prepared by mixing 100 parts of Sub-composition #4 with 5 parts of silicone resin powder having an average particle size of 0.8 μm (X-52-854 by Shin-Etsu Chemical Co., Ltd.) until uniform. 
     Comparative Example 1 
     Sub-composition #1 was used as composition (f) without adding silicone powder. 
     Comparative Example 2 
     A composition (g) was prepared by mixing 100 parts of Sub-composition #1 with 10 parts of silicone resin powder (obtained by cutting off a coarse fraction of X-52-854 by Shin-Etsu Chemical Co., Ltd. so that the remaining fraction had an average particle size of 0.4 μm) until uniform. 
     Comparative Example 3 
     Sub-composition #3 was used as composition (h) without adding silicone powder. 
     Comparative Example 4 
     A composition (i) was prepared by mixing 100 parts of Sub-composition #1 with 600 parts of silicone resin powder having an average particle size of 5 μm (X-52-1621 by Shin-Etsu Chemical Co., Ltd.) until uniform. 
     Silicone compositions (a) to (i) prepared in Examples and Comparative Examples were evaluated by the following methods. 
     [Evaluation Methods] 
     Optoelectronic Semiconductor Package 
     The optoelectronic semiconductor element used was an LED chip having a light-emitting layer of InGaN and a main emission peak at 450 nm. An optoelectronic semiconductor package as shown in  FIG. 1  was constructed by mounting the LED chip  2  on a housing  1  in the form of SMD3020 or SMD5050 package (I-Chiun Precision Industry Co., resin section PPA) with a die bonder  5 , bonding gold wires  6  to the chip and lead electrodes  3  and  4 , dispensing an encapusulating resin composition  7 , and curing the resin composition at 150° C. for 4 hours. 
     Oxygen Permeability 
     It is preferred that silicone resin compositions (Sub-compositions #1 and #2) within the scope of the invention have an oxygen permeability of up to 1,000 cm 3 /m 2 /24 h/atm in lo the cured state. The oxygen permeability of a sample of 1 mm thick was measured according to JIS K7126-2 at a temperature of 23° C. 
     Package Sulfiding A 100-ml transparent glass bottle was charged with 0.2 g of sulfur powder and the LED package (encapsulated with each composition) and tightly sealed. The sealed bottle was placed in a dryer at 70° C. and taken out after 24 hours. The plated silver portion in the package was observed for color change under a microscope. A sample which turned black was rated “discolored.” 
     Tack 
     Each silicone composition was evaluated for tack by dispensing the silicone composition to the package, heat curing it, and touching the surface of the package at room temperature (25° C.). 
     Thermal Cycling Test 
     A thermal cycling test was performed in a temperature range between −40° C. and 125° C. over 1,000 cycles. Ten optoelectronic semiconductor packages, prepared as above, were placed in a thermal cycling tester. After 1,000 cycles, the number of LED packages failed (could not be turned on) was counted. The value of 0 in the row of “Number of LEDs failed after thermal cycling” in Table 1 means that all ten packages could be turned on (to emit light) and remained reliable, whereas the value of 10 means that all ten packages were non-operative and less reliable. 
     Ten optoelectronic semiconductor packages, prepared as above, were measured for total luminous flux by using a total luminous flux measurement system HM-9100 (Otsuka Electronics Co., Ltd.) and conducting current IF=20 mA. An average of ten measurements was computed. The measured flux value was compared as follows, to determine whether the efficiency of light extraction associated with the addition of a different silicone powder was good or poor. 
       Efficiency of light extraction=[(total luminous flux after powder addition)/(total luminous flux prior to powder addition)]×100−100 (%)
 
     After this measurement, the ten optoelectronic semiconductor packages were held stationary in a dryer at 180° C. for 24 hours. They were similarly measured for total luminous flux by using the total luminous flux measurement system HM-9100 and conducting current IF=20 mA. An average of ten measurements was computed. The measured flux value was compared to determine a total luminous flux retentivity (%) after the heating test, as follows. 
       Total luminous flux retentivity after heating test=[(total luminous flux after 180° C./24 hr heating)/(initial total luminous flux)]×100 (%)
 
     The total luminous flux retentivity after the heating test was compared to determine whether or not heat resistance was improved by addition of silicone powder. 
       Ratio of total luminous flux retentivity after heating test=[(total luminous flux retentivity of powder-laden sample after heating test)/(total luminous flux retentivity of non-powder-laden sample after heating test)]×100−100 (%)
 
     It is seen from these equations that for both the efficiency of light extraction and the ratio of total luminous flux retentivity, samples giving positive values indicate bright as compared with the reference, and samples giving negative values indicate dark as compared with the reference. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                 Example 
                 Comparative Example 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 1 
                 2 
                 3 
                 4 
                 5 
                 1 
                 2 
                 3 
                 4 
               
               
                   
               
               
                 Composition 
                 a 
                 b 
                 c 
                 d 
                 e 
                 f 
                 g 
                 h 
                 i 
               
               
                 Sub-composition No. 
                 #1 
                 #1 
                 #2 
                 #2 
                 #4 
                 #1 
                 #1 
                 #3 
                 #1 
               
               
                 Type of silicone 
                 composite 
                 resin 
                 composite 
                 resin 
                 resin 
                 — 
                 resin 
                 — 
                 resin 
               
               
                 powder 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Amount of silicone 
                 10 
                 10 
                 5 
                 5 
                 5 
                 0 
                 10 
                 0 
                 600 
               
               
                 powder (pbw) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Average particle 
                 5 
                 5 
                 0.8 
                 0.8 
                 0.8 
                 — 
                 0.4 
                 — 
                 5 
               
               
                 size of silicone 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 powder (μm) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Ease of dispensing 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 good 
                 NG 
               
               
                 Oxygen permeability 
                 210 
                 210 
                 400 
                 400 
                 250 
                 210 
                 210 
                 20000 
                 210 
               
               
                 of Sub-composition 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 (cm 3 /m 2 /24 h/atm) 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Discoloration of 
                 no 
                 no 
                 no 
                 no 
                 no 
                 no 
                 no 
                 discolored 
                 no 
               
               
                 plated silver upon 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 sulfur exposure 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Tack after cure 
                 no 
                 no 
                 no 
                 no 
                 no 
                 no 
                 no 
                 tacky 
                 no 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Number of 
                 3020 
                 0 
                 0 
                 0 
                 0 
                 0 
                 10 
                 0 
                 0 
                 — 
               
               
                 LEDs failed 
                 package 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 after thermal 
                 5050 
                 0 
                 0 
                 0 
                 0 
                 0 
                 10 
                 0 
                 0 
                 — 
               
               
                 cycling 
                 package 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Efficiency 
                 3020 
                 1.0 
                 0 
                 1.5 
                 1.0 
                 0 
                 ±0 
                 −36 
                 — 
                 — 
               
               
                 of light 
                 package 
                   
                   
                   
                   
                   
                 (reference) 
                   
                   
                   
               
               
                 extraction 
                 5050 
                 2.0 
                 1.0 
                 2.0 
                 3.0 
                 1.0 
                 ±0 
                 −34 
                 — 
                 — 
               
               
                 (%) 
                 package 
                   
                   
                   
                   
                   
                 (reference) 
                   
                   
                   
               
               
                 Ratio of total 
                 3020 
                 3.0 
                 1.0 
                 2.0 
                 9.0 
                 9.0 
                 ±0 
                 −8.0 
                 — 
                 — 
               
               
                 luminous flux 
                 package 
                   
                   
                   
                   
                   
                 (reference) 
                   
                   
                   
               
               
                 retentivity 
                 5050 
                 2.5 
                 1.5 
                 1.5 
                 5.0 
                 13.0 
                 ±0 
                 −5.4 
                 — 
                 — 
               
               
                 after heating 
                 package 
                   
                   
                   
                   
                   
                 (reference) 
                   
                   
                   
               
               
                 test (%) 
               
               
                   
               
            
           
         
       
     
     As seen from Table 1, in all Examples 1 to 5, the number of LEDs failed after thermal cycling is 0. That is, crack resistance is good even under thermal cycling between high and low temperatures, no peeling occurs, and the effect of light extraction is satisfactory. Also in the sulfiding test, discoloration is equally prevented as compared with the sample prior to addition of silicone powder. These silicone resins are excellent with respect to the efficiency of light extraction and the total luminous flux retentivity after heating test. 
     By contrast, the samples of Comparative Example 1 give satisfactory results in the sulfiding test, but failed to emit light after the thermal cycling test because the resin cracked in all packages. It is thus evident that LED packages using prior art encapsulating material are unreliable. 
     The samples of Comparative Example 2 wherein silicone powder having an average particle size below the range is added give satisfactory results in the sulfiding test and good crack resistance under thermal cycling conditions, but degraded results for both the efficiency of light extraction after addition of silicone powder and the total luminous flux retentivity after heating test. It is thus evident that LED packages using prior art encapsulating material suffer a loss of luminance. 
     Comparative Example 3 using a resin composition containing a silicone outside the scope of formula (1) as component (A) and having a high oxygen permeability demonstrated that in the sulfiding test, the plated silver turned black in all packages. These results indicate that the use of a resin composition having a high oxygen permeability is not effective for preventing permeation of sulfur oxide gas. It is proven that a composition using a silicone within the scope of formula (1) as component (A) is useful. 
     The composition of Comparative Example 4 caused nozzle clogging during dispensing and could not be applied to packages in a consistent manner. 
     It is thus demonstrated that the addition-curable silicone resin composition within the scope of the invention has sufficient thermal shock resistance to ensure crack unsusceptibility and a high efficiency of light extraction, experiences a least discoloration when hot, and maintains luminance or brightness. Thus the composition is advantageous for use with LED packages which are subject to rigorous thermal cycling. 
     While the invention has been illustrated and described in typical embodiments, it is not intended to be limited to the details shown. Any modified embodiments having substantially the same features and achieving substantially the same results as the technical idea disclosed herein are within the spirit and scope of the invention. 
     Japanese Patent Application Nos. 2012-055872 and 2012-136714 are incorporated herein by reference. 
     Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.