Patent Publication Number: US-5891566-A

Title: Adhesive tapes

Description:
This a division of application Ser. No. 08/739,930, filed Oct. 30, 1996, now issued U.S. Pat. No. 5,683,806, which is a continuation of application Ser. No. 08/417,457, filed Apr. 5, 1995, now abandoned, which is a continuation of application Ser. No. 08/137,608, filed Oct. 14, 1993, now abandoned, which is a division of application Ser. No. 07/814,236, filed Dec. 23, 1991, now issued U.S. Pat. No. 5,277,972, issued Jan. 11, 1994, which is a continuation of application Ser. No. 07/414,012, filed Sep. 28, 1989, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to adhesive tapes for electronic parts, for example resin-sealed-type semiconductor devices, in particular, those used for fixing inner lead pins for lead frames in resin-sealed-type semiconductor devices. 
     2. Description of Prior Art 
     Adhesive tapes used in resin sealed semiconductor devices include tapes for fixing lead frames, die pad tapes, tab tapes and the like. In the case of, for example, adhesive tapes for fixing lead frames, the adhesive tapes are used for fixing inner lead pins and for increasing the yield and productivity of the lead frames themselves and of whole assembly procedures for the manufacture of the semiconductor devices, therefore they are generally attached on the lead frames by the manufacturer of the lead frames and conveyed to the manufacturer of semiconductor devices where they are sealed with a resin after mounting integrated circuits (IC&#39;s). In other words, the adhesive tapes are incorporated in the package of the semiconductor devices. Therefore, adhesive tapes, for example, those for fixing the lead frames, are required to have adhesion strength high enough not only at room temperature immediately after the taping but also after heating in IC assembly procedures as well as have general reliability for the level of semiconductors and operability or ease of handling at the time of taping. 
     Heretofore, there have been used as the adhesive tapes for fixing inner lead pins of the lead frames various adhesive tapes including, for example, those composed of a base film such as polyimide film on which an adhesive is coated which comprises polyacrylonitrile resin, polyacrylate resin or synthetic rubber resins such as acrylonitrile/butadiene copolymers alone or modified or mixed with other resins, and converted to the B stage. 
     As is well known, semiconductor devices are subjected to reliability tests under severe conditions such as in pressure cooker test (hereafter, &#34;PCT&#34;). Naturally, the adhesive tapes for fixing the lead frames in the resin molded package must surely have enough reliability under the same conditions. Among various reliabilities which the materials used in the semiconductor devices must have, particularly important for adhesive tapes for fixing lead frames is electric current leak characteristics. This corresponds to a phenomenon that electric current flows through the tape portion where the adhesive tape for fixing lead frame is attached, and smaller quantity of the electric current means better electric current leak characteristics. 
     On the other hand, the degree of integrity of semiconductor elements is recently improving remarkably and also miniaturization of semiconductors has been proceeding rapidly, resulting in that the number of lead frame pins has increased and the respective inner lead pins of the lead frames have become thinner. There is a tendency for the distance between the pins to become shorter. This leads to more severe requirement for accuracy of the positioning of the respective pins. 
     Under the circumstances, the semiconductor devices with the conventional adhesive tapes for fixing the lead frames do not show sufficient insulation properties between the inner lead pins and thus their electric current leak characteristics are poor. Moreover, it has been quite unclear which factors the electric current leak characteristics of the conventional adhesive tapes depend upon and how they can be improved. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of this invention to provide adhesive tapes having improved electric current leak characteristics for use with electronic parts. 
     With view to overcoming the defects of the conventional adhesive tapes, intensive investigation has been made by the present inventors on the relationship between the composition and various properties of the adhesive tapes and the electric current leak characteristics using a heat resistant base film having approximately acceptable electric current leak characteristics as a support and an adhesive layer composed of generally acrylonitrile/butadiene copolymer/phenolic resin coated on the surface of the base film, as the result, it has now been found that the above object can be achieved using adhesive tapes with specified compositions. 
     This invention is based on the above discoveries and provides an adhesive tape for electronic parts, comprising 
     (a) a heat resistant base film, and 
     (b) an adhesive layer laminated on at least one surface of said base film, said adhesive layer comprising 
     (i) a resol phenol resin, and 
     (ii) an acrylonitrile/butadiene copolymer, 
     in a proportion of from 100 to 500 parts by weight of said resol phenol resin per 100 parts by weight of said acrylonitrile/butadiene copolymer, 
     said resol phenol resin being selected from the group consisting of bisphenol A type resins, alkylphenol type resins, phenol resins and their co-condensed type phenol resins which contain as the phenol component at least one member selected from the group consisting of bisphenol A, alkylphenols and phenol. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematical cross-sectional view of the adhesive tape for electronic parts according to this invention, in which the adhesive layer is laminated on one surface of the heat resistant base film; 
     FIG. 2 is a schematic cross-sectional view of the adhesive tape for electronic parts according to this invention, in which the adhesive layer is laminated on one surface of the heat resistant base film and a protective layer is further laminated on the adhesive layer; 
     FIG. 3 is a schematic cross-sectional view of the adhesive tape for electronic parts according to this invention, in which the adhesive layer is laminated on each surface of the heat resistant base film; 
     FIG. 4 is a schematic cross-sectional view of the adhesive tape for electronic parts according to this invention, in which the adhesive layer is laminated on each surface of the heat resistant base film and a protective layer is further laminated on the respective adhesive layers; 
     FIG. 5 is a plan view of a lead frame illustrating the state in which an adhesive tape is attached on the lead frame; and 
     FIGS. 6(a) and 6(b) are partial enlarged view of the lead frame shown in FIG. 5. 
     FIG. 7 is a view illustrating the method of peeling tests used in examples of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, this invention will be described in detail with reference to the attached drawings, in which FIGS. 1 through 4 are schematical cross-sectional views of adhesive tapes for electronic parts according to this invention. In FIGS. 1 and 2, a heat resistant base film 1 is laminated on one surface thereof with an adhesive layer 2. In FIG. 2, a protective layer 3 is further laminated on the surface of the adhesive layer 2. In FIGS. 3 and 4, adhesive layers 2 and 2&#39; are laminated on the opposite surfaces of the heat resistant base film 1. In FIG. 4, protective layers 3 and 3&#39; are laminated on the adhesive layers 2 and 2&#39;, respectively. 
     The construction of the respective layers of the adhesive tape of this invention will be explained hereafter. 
     A. Heat Resistant Base film 
     As the heat resistant base film, there can be used organic base film, more particularly heat resistant base films composed of polyimides, polyether-ether-ketones, polyetherimides, polyphenylene sulfides, and polyparabic acid, and composite heat resistant base films such as epoxy resin-glass cloth, and epoxy resin-polyimide-glass cloth, which base films have a thickness of from 5 to 300 μm, preferably from 10 to 150 μm, more preferably 12.5 to 150 μm, and most preferably 25 to 75 μm. 
     B. Adhesive Layer 
     The adhesive layer is composed of a mixture of an acrylonitrile/butadiene copolymer (hereafter, &#34;NBR&#34;) and a resol type phenol resin. 
     As NBR, there can be used a nitrile rubber having a relatively high nitrile content as high as from 10 to 45% by weight, preferably from 20 to 45% by weight and a molecular weight of from 20,000 to 1,000,000, preferably from 100,000 to 500,000. The NBR may optionally contain a vulcanizing agent such as quinones, dialkyl peroxides, and peroxyketals in order to exhibit self cross-linking property upon heating. 
     The resol phenol resin includes bisphenol A type resins, alkylphenol resins, phenol type resins and their co-condensed phenol resins which contain as the phenol component at least one member selected from the group consisting of bisphenol A, alkylphenols and phenol. 
     As the resol phenol resin in which the phenol component is bisphenol A, there can be used those which are synthesized using bisphenol A as a starting compound and which have the following basic skeleton and whose softening point as measured by the ring and ball method is from 70° to 90° C. ##STR1## 
     As the resol phenol resin in which the phenol component is the alkylphenol, there can be used those which are synthesized using as a starting compound an alkylbenzene having an alkyl group mainly at the ortho- or para-position to the phenolic hydroxyl group. Examples of the alkyl group include a methyl group, an ethyl group, a propyl group, a t-butyl group, a nonyl group, and the like. For example, there can be used p-t-butylphenol resol phenol resins which have the following basic skeleton and whose softening point as measured by the ring and ball method is from 80° to 100° C. ##STR2## 
     The resol phenol resins in which the phenol component is phenol are those which are synthesized using unsubstituted phenol and formaldehydes as starting compounds. 
     When the above-described resol phenol resins are used in the adhesive layer, the resulting adhesive tapes exhibit excellent operability and adhesion and also good electric current leak characteristics. 
     In the adhesive layer of the adhesive tape of this invention, the phenol component of the resol phenol resin may contain other phenol components, for example, phenol resins of co-condensed derived from p-phenylphenol type or biphenyl phenols. Also, a small amount of novolak phenol resin or epoxy resin may be blended with the resol phenol resin. 
     Fillers can be mixed with the resol phenol resins, if desired. 
     The proportion of the resol phenol resin to NBR in the adhesive layer is from 50 to 500 parts by weight, preferably from 100 to 500 parts by weight, more preferably 100 to 300 parts by weight, per 100 parts by weight of NBR. When the content of the resol phenol resin is less than 50 parts by weight per 100 parts by weight of NBR, the leak current value is too high to be acceptable and when it is more than 500 parts by weight per 100 parts by weight of NBR, adhesive tapes having sufficient adhesion and good operability cannot be obtained. 
     In this invention, the adhesive layer having the above-described composition has a dry and after B-stage coating thickness of from 2 to 50 μm, preferably 7 to 30 μm, and more preferably 10 to 30 μm. 
     C. Protective layer 
     The adhesive tape for electronic parts of this invention may be provided with a protective layer on the adhesive layer, if desired. As the protective layer, there can be used polyethylene film, polyethylene terephthalate film, polypropylene film, fluororesin film, paper and the like, which may be imparted peeling property with silicone resin or the like, if desired. The protective layer has a thickness of from 1 to 200 μm, preferably from 5 to 100 μm, more preferably from 10 to 100 μm, and most preferably from 12.5 to 50 μm. 
     When the adhesive tape for electronic parts of this invention has a construction such that the adhesive layer is laminated on one surface of the heat resistant base film, the tape preferably has the following characteristics (1), (2) and (3). 
     (1) The electric conductivity of an extract containing organic and inorganic ionic impurities extracted from the adhesive tape under the conditions set forth below is not higher than 70 μs/cm. 
     (2) The concentration of sulfate ions in the adhesive tape for electronic parts is not higher than 300 ppm. 
     (3) When the surface of the heat resistant base film is brought in contact with the molding compound, the adhesive tape has an adhesion strength of not lower than 200 g/1.5 cm upon 180° peeling test. 
     In this invention, the above-described characteristics (1), (2) and (3) refer to those measured under the following conditions. 
     1) Conditions for Measuring Electric Conductivity 
     1-a) Conditions for Pretreatment of Tapes 
     The adhesive tape, after stripping off the protective layer, if any, is cut to pieces of a size of about 1 cm square, with care lest the tape should be contaminated with ionic impurities. 
     1-b) Conditions for Extraction 
     1-b-1) In a sufficiently washed heat resistant round bottom glass flask is charged 10 g of a sample and 100 g of deionized water having an electric conductivity of not higher than 5 μs/cm is added thereto. 
     1-b-2) To the glass flask is fitted a sufficiently washed reflux condenser and the content is heated at 100° C. and left to stand for 20 hours. 
     1-c) Conditions for Measurement 
     The electric conductivity of the extract is measured using a conductometer. 
     2) Conditions for Measurement of Concentration of Sulfate Ions 
     2-a) Conditions for Pretreatment of Tapes 
     The adhesive tape, after stripping off the protective layer, if any, is cut to pieces of a size of about 3 cm square, with care lest the tape should be contaminated with ionic impurities. 
     2-b) Conditions for Preparing Samples for Measurement 
     The sample (1 g) after the above-described pretreatment is burnt by oxygen bomb or cylinder method and absorbed in 25 ml of deionized water to form a sample solution. 
     2-c) Conditions for Measurement 
     The concentration of sulfate ions in the sample solution is determined by means of ion chromatography, and the results obtained are converted into the concentration of the sample. 
     3) 180° Peeling Test 
     3-a) Preparation of Samples for Measurement 
     3-a-1) After peeling off the protective layer, if any, the sample tape is adjusted to a size of 15 mm in width×150 mm in length. 
     3-a-2) On a metal plate as a support is placed a molding compound before curing so as to have a uniform thickness. 
     3-a-3) On the surface of the molding compound is bonded the sample tape prepared in 3-a-1) above at its surface to be measured, and is heated at 120° C., followed by pressing with a roll of a self-weight of 2 kg. 
     3-a-4) The sample prepared in 3-a-3) above is heated at 175° C. for 5 hours in a drier to cure the molding compound to form a sample for measuring adhesion to the molding compound. 
     3-b) PCT Conditions 
     Using a saturated type pressure-cooker tester, the above-described sample is treated for 50 hours under the conditions of 121° C. and 2 atm. 
     3-c) Method for Measurement 
     The 180° peeling strength of the sample is measured using a universal testing machine. 
     In the adhesive tape for electronic parts of this invention, its electric current leak characteristics is excellent when the above-described characteristics are satisfied. For example, satisfactory electric current leak characteristics are obtained when the adhesive tape is applied to quad flat package (QFP) with not less than 168 pins and the like. In addition, when the above-described characteristics (3) is satisfied, the fluctuation of the measured values of the electric current leak can be minimized. 
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     I. In a preferred embodiment this invention, a combination of the acrylonitrile/butadiene copolymer having a specified molecular weight distribution and a specified resol phenol resin is used. 
     That is, as the resol phenol resin is used a resin selected from the group consisting of bisphenol A resins, alkylphenol type resins and their co-condensed phenol resins which contain as the phenol component at least one member selected from the group consisting of bisphenol A, and alkylphenols. On the other hand, as the acrylonitrile/butadiene copolymer, there is used one which has a number average molecular weight (MN) of from 20,000 to 200,000 and the ratio of its weight average molecular weight (MW) to its number average molecular weight (MN) is in the range of MW/MN≧2.5. 
     In this embodiment, the molecular weight is measured by gel permeation chromatography (GPC) under the conditions shown in Table 1 below. 
     
                       TABLE 1
______________________________________
Item        Conditions
______________________________________
Eluant      Tetrahydrofuran
Detector    RI
Column      SHODEX GPC KF-80M X 2 (Hard, spherical
            porous polystyrene gel)
Temperature Room temperature
Method of   Calibration curve is prepared standard
determining polystyrene gel manufactured by Toyo
calibration Soda Manufacturing Ltd.having the
curve in terms
            following molecular weights:
of polystyrene
            3,840,000, 775,000, 107,000, 16,700,
            2,800.
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     When the number average molecular weight of the acrylonitrile/butadiene copolymer is less than 20,000, the surface of the adhesive layer after being converted to B-stage becomes relatively sticky and the improvement in the surface property is not noticeable. On the other hand, when it is more than 200,000, improvement in the adhesion of the adhesive tape, particularly that when the adhesive tape is bonded at low temperatures, is unsatisfactory. Also, no further improvement in the adhesion is obtained with the ratio of the weight average molecular weight to the number average molecular weight (MW/MN) being outside the above-defined range. 
     In this embodiment, the acrylonitrile/butadiene copolymer may have one or more peaks of GPC chromatogram in its molecular weight distribution. 
     Also, in this embodiment, it is preferred to use the resol phenol resin in an amount of from 50 to 300 parts by weight per 100 parts by weight of the acrylonitrile/butadiene copolymer. With the content of the resol phenol resin per 100 parts by weight of NBR being less than 50 parts by weight, the blocking of the surface of the adhesive layer increases and when it is more than 300 parts by weight, decrease in the adhesion strength is observed. 
     Other conditions may be the same as described above. 
     In this embodiment, among various properties required for the adhesive tapes for use in resin sealed type semiconductor devices, are particularly improved the adhesion strength immediately after taping and that after thermal hysteresis, which is encountered by the adhesive tapes attached on the lead frame when the semiconductors are subjected to die bonding procedure in which the semiconductor chips are bonded to the lead frames at a temperature of from 150° to 250° C. for from 1 to 3 hours or when they are subjected to wire bonding procedure in which the extreme point of the inner lead of the lead frame is connected to the wiring pad on the semiconductor chip with a gold wire at a temperature of from 150° to 350° C. for from about 1 to about 30 minutes. 
     II. In another preferred embodiment, the heat resistant base film is used which has additionally the following properties. 
     Coefficient of thermal expansion: an average coefficient of linear thermal expansion along the width thereof at from 20° to 300° C. (as measured according to ASTM D-696-44) of from 0.4×10 -5  to 2.5×10 -5  cm/cm/°C. 
     Degree of thermal shrinkage: coefficient of dimensional change along the width thereof after heating at 300° C. for 3 minutes (as measured according to JIS C2318-6.3.5) being not higher than 0.15%. 
     (1) Coefficient of thermal expansion 
     As the material for the heat resistant base film, the same material as described above can be used. However, the following treatments are conducted for obtaining the average coefficient of linear thermal expansion defined as above because various organic films generally have average coefficient of thermal expansion at from 20° to 300° C. which is in the range of 1.0×10-5 to 15×10-5 cm/cm/°C., and therefore almost all of them except for only a part of them have the average coefficient outside the above-described range required for this preferred embodiment. 
     (1-i) An inorganic filler is added to the organic base film. 
     (1-ii) As the main component material for constituting the organic base film, there is used one having a low coefficient of thermal expansion, for example, a polyimide based material containing a repeating unit having the following formula: ##STR3## 
     (2) Degree of Thermal Shrinkage 
     Generally, the thermal shrinkage of various organic films is caused by the following two causes. 
     (2-1) In the organic films based on epoxy resin, polyimide or the like thermosetting resin, heat curing reaction of the resin constituting the film is not sufficient and therefore the film shrinks upon subsequent heating. 
     (2-2) When the resin is stretched to form a film, deforming stress is generated in the film due to stretching or the like, and the deforming stress is relaxed by heating to cause the film to shrink. 
     Therefore, the thermal shrinkage due to the heating during the assembly step can be reduced to a value within the above-described range by subjecting the film to preliminary heat treatment. The heat treatment can be conducted, for example, by passing the organic films through two opposing infrared heaters or passing between a plurality of heated rollers heated at a temperature of from 280° to 360° C. for from 1 to 10 minutes with keeping the films in contact with the surface of the rollers. 
     According to this preferred embodiment, the so-called lead shift is caused by the deviation of the positions of the inner lead pins which is because of the thermal hysteresis encountered during the assembly procedure of the semiconductor devices as explained previously. 
     The lead shift phenomenon is explained with reference to the attached drawings in which FIG. 5 illustrates a 4-directional multi-pin lead frame on which an adhesive tape for fixing the lead frame is attached. In FIG. 5, the adhesive tapes 11a and 11b are attached to the inner lead pins 12 so as to substantially form a square. FIGS. 6(a) and 6(b) are partial enlarged views of the lead frame shown in FIG. 5. FIG. 6(a) shows the state before the lead frame undergoes the thermal hysteresis, and FIG. 6(b) shows the state after the lead frame undergoes the thermal hysteresis. In FIGS. 6(a) and 6(b), the adhesive tapes 11a and 11b are attached to the inner lead pins 12 so as to substantially form a square. Reference numeral 13 indicates a support bar which is connected to the die pad of the lead frame. When heated at 300° C. for 3 minutes, the lead frame in the state shown in FIG. 6(a), the distance between the support bar 13 and the inner lead pin 12a adjacent thereto (indicated by symbol A in FIG. 6(a) and by B in FIG. 6(b)) is extended as shown in FIG. 6(b), thus causing deviation of the positions of the inner lead pins. As the result, the wire bonding step in which the semiconductor chips and the inner lead pins are connected to each other cannot be conducted appropriately. This leads to a decrease in the efficiency of the wire bonding step during the assembly procedure, or to short-circuit between the wirings caused by the shifted inner lead pin contacting the adjacent inner lead pin, resulting in that the semiconductor device is no longer usable. 
     As stated above, this problem which would otherwise occur can be prevented with the above-described countermeasure according to the preferred embodiment of this invention. 
     III. In a still another embodiment of this invention, the adhesive layer further contains an organic or inorganic filler. 
     Examples of the inorganic filler which can be used include pulverized silica, molten silica, alumina, titanium oxide, beryllium oxide, zirconium oxide, magnesium oxide, titanium nitride, boron nitride, silicon nitride, titanium boride, tungsten boride, silicon carbide, titanium carbide, zirconium carbide, molybdenum carbide, and these substances treated on their surfaces with a trimethylsiloxyl group or the like. As the organic filler, there can be cited polyimides, resol phenol resins, epoxy resins, acrylic resins, styrene resins, silicone resins, nylons, polyamindeimides, polyether-ether-ketones, polyetherimides, polyesterimides, and the like. The organic and inorganic fillers may have an average particle size of from 0.1 to 50 μm. However, it is preferred that the maximum particle size thereof does not exceed the thickness of the adhesive layer. 
     In this embodiment, the organic or inorganic filler must be contained in the adhesive layer in an amount of from 0.5 to 80% by weight based on the total weight of the adhesive layer. When the amount of the filler is less than 0.5% by weight, the effect of the filler is not obtained. On the other hand, the adhesive layer becomes brittle, thus failing to give appropriate operability when the amount of the filler is more than 80% by weight. 
     The adhesive layer used in this embodiment can be prepared by mixing the adhesive with the filler by agitating them with a sand mill, three-roll mill, Banbury mixer or the like, or by dispersing them using a disperser, and coating the resulting mixture in a thickness of from 1 to 150 μm, preferably from 5 to 50 μm, and curing it to B-stage. Various thermoplastic resins such as polyether-ether-ketones, polyesters, polyamideimides and the like can be added alone or as a blend with the thermosetting resin. 
     The addition of the organic or inorganic filler in the adhesive layer is useful in controlling the dielectric constant and adhesive properties of the adhesive used as well as improving the lead shift characteristics as described above. 
     EXAMPLES 
     This invention will be described in greater detail with reference to the following examples. However, the invention should not be construed as being limited thereto. Unless otherwise indicated specifically, all parts and percentages are by weight. 
     Example 1 
     Using materials shown in Table 2 below and methyl ethyl ketone (hereafter, &#34;MEK&#34;) as a solvent, adhesive solutions 1 through 8 are prepared which have blend ratios shown in Table 3 below. 
     
                       TABLE 2
______________________________________
Resin    Trade Name Manufacturer
______________________________________
NBR (1)  NB56       Nippon Zeon Co., Ltd. Note 1
NBR (2)  NIPOL 1001 Nippon Zeon Co., Ltd. Note 2
Phenol   CKM-908    Showa Kobunshi Kogyo Co., Ltd.
resin (1)           Note 3
Phenol   Tackirol 201
                    Taoka Kagaku Kogyo Co., Ltd.
resin (2)           Note 4
Phenol   BKM-2620   Showa Kobunshi Kogyo Co., Ltd.
resin (3)           Note 5
Phenol   KP732 E-2  Arakawa Kagaku Kogyo Co., Ltd.
resin (4)           Note 6
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     Note 1: NBR synthesized by Nippon Zeon Co., Ltd. The polymerization catalyst, pH adjusting agent after the polymerization and the like are considered. Particularly, the concentrations of calcium ions and of sulfate ions are lowered. Mooney viscosity: 50, bonded acrylonitrile content: 37%. 
     Note 2: NBR synthesized by Nippon Zeon Co., Ltd. which has the following properties. Mooney viscosity: 78, bonded acrylonitrile content: 41%. 
     Note 3: Bisphenol A type resol phenol resin produced by Showa Kobunshi Kogyo Co., Ltd., having a softening point of 73° to 90° C. 
     Note 4: Alkylphenol type resol phenol resin produced by Taoka Kagaku Kogyo Co., Ltd., having a softening point of 75° to 90° C. 
     Note 5: Cresol type resol phenol resin produced by Showa Kobunshi Kogyo Co., Ltd., having a softening point of 80° to 100° C. 
     Note 6: Resol phenol resin produced by Arakawa Kagaku Kogyo Co., Ltd. which is a liquid low molecular weight resin containing phenol type structure as a basic structure, having the following properties. Volatile component with methanol/isopropyl alcohol about 3.5%. Viscosity at 25° C.: 245 cps. Gelling time at 140° C.: about 8 minutes. 
     
                       TABLE 3
______________________________________
Adhesive
       Blend Ratio of Materials (Parts by Weight)
solution      NIPOL   CKM   Tacki-
                                  BKM   KP732
Number NB56   1001    -908  rol 201
                                  2620  E-2   MEK
______________________________________
1      100    --      200   --    --    --    500
2      100    --      --    200   --    --    500
3      100    --      --    --    200   --    500
4      100    --      --    --    --    200   500
5      --     100     200   --    --    --    500
6      --     100     --    200   --    --    500
7      --     100     --    --    200   --    500
8      --     100     --    --    --    200   500
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     The resulting adhesive solutions 1 through 8 are coated on one surface of respective polyimide films having a thickness of 50 μm (&#34;Kapton 200H&#34;, produced by E. I. Du Pont de Nemours) to a thickness of 20±3 μm, and dried at 150° C. for 5 minutes to prepare adhesive tapes 1 through 8. These adhesive tapes are used for measuring electric current leak characteristics, and the electric conductivity, concentration of sulfate ions and adhesion to the molding compound are measured in the above-described manner. The results obtained are shown in Table 4 below. 
     
                       TABLE 4
______________________________________
          Electric   Concentration
                                 Adhesion to
Adhesive  Conductivity
                     of Sulfate Ions
                                 Molding Material
Tape Number
          (μs/cm) (ppm)       (kg/1.5 cm)
______________________________________
1          40        220         420
2          35        200         420
3          45        140         420
4          80        180         420
5         120        960         420
6         140        840         420
7         130        720         420
8         160        660         420
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     Using the adhesive tapes, the change in the electric current characteristics after PCT tests are measured under the following conditions. 
     a. An adhesive tape having a size of 1 mm in width×5 mm in length is attached between the pins to be measured of 100-pin QFP lead frame. 
     b. The lead frame in a above is molded and dam bar is cut. 
     c. Voltage of 10V is applied between the pins to be measured, and initial leak current value is measured. 
     d. The samples for the measurement are treated with PCT (saturated type, 121° C./2 atm.) for 500 hours, and the leak current values after PCT 500 hours are measured in the same manner as in c above. 
     The results obtained are shown in Table 5 below. 
     
                       TABLE 5
______________________________________
Adhesive   Initial Current
                       Current Leak Value
Tape       Leak Value  After PCT 500 Hours
Number     (A)         (A)
______________________________________
1          1 × 10.sup.-13  or less
                       1 × 10.sup.-12
2          1 × 10.sup.-13  or less
                       5 × 10.sup.-12
3          1 × 10.sup.-13  or less
                       8 × 10.sup.-10
4          1 × 10.sup.-13  or less
                       4 × 10.sup.-10
5          1 × 10.sup.-13  or less
                       5 × 10.sup.-10
6          1 × 10.sup.-13  or less
                       7 × 10.sup.-9
7          1 × 10.sup.-13  or less
                       5 × 10.sup.-9
8          1 × 10.sup.-13  or less
                       4 × 10.sup.-9
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     Example 2 
     Using NBR (1) and Phenol resin (4) shown in Table 2 above as the NBR and resol type phenol resins, test adhesive solutions 9 through 12 are prepared. The adhesive solutions are coated on Apical (polyimide film produced by Kanegafuchi Kagaku Kogyo Co., Ltd.) and Ultem (polyetherimide film produced by Mitsubishi Gas Chemical Co., Ltd.) each having a thickness of 50 μm so as to form a layer of a dry thickness of 25±5 μm, and dried at 165° C. for 3 minutes to prepare adhesive tapes 9 through 16. Table 6 shows the relationship between the heat resistant films used in the samples and the adhesive solutions. 
     
                       TABLE 6
______________________________________
Adhesive      Name of   Adhesive
Tape Number   Base Film Solution Number
______________________________________
 9            Apical 50AH
                         9
10            Apical 50AH
                        10
11            Apical 50AH
                        11
12            Apical 50AH
                        12
13            Ultem 50 μm
                         9
14            Ultem 50 μm
                        10
15            Ultem 50 μm
                        11
16            Ultem 50 μm
                        12
______________________________________
 
    
     Adhesive Solution 9 
     a. Deionized water (100 parts) is charged in a stainless steel vessel, to which is added 1 part of phenol resin (4). The mixture is stirred well and the phenol resin component which settles on the bottom of the vessel is collected, and placed in a vacuum drier at 50° C. under reduced pressure to remove water content. MEK (100 parts) is added to 100 parts of the phenol resin obtained after the removal of water to dissolve it well to obtain a phenol resin solution A. 
     b. To 100 parts of NBR is added 400 parts of MEK, and the mixture is stirred to well dissolve to obtain an NBR solution A. 
     c. To 100 parts of NBR solution A is added 250 parts of phenol resin solution A, and the mixture is well stirred. 
     d. The NBR-phenol resin mixed solution prepared in c above is introduced in a pressurized reactor, and stirred at 80° C. for 2 hours to obtain an adhesive solution 9. (The ratio of NBR: phenol resin is 100:625 in terms of the solid content.) 
     Adhesive Solution 10 
     a. To 100 parts of the NBR solution A used in the preparation of Adhesive Solution 9 above is added 100 parts of phenol resin solution A, and stirred well. 
     b. The NBR-phenol resin mixed solution prepared in a above is introduced in a pressurized reactor, and stirred at 80° C. for 2 hours to obtain an adhesive solution 10. (The ratio of NBR: phenol resin is 100:250 in terms of the solid content.) 
     Adhesive Solution 11 
     a. To 100 parts of the NBR solution A used in the preparation of Adhesive Solution 9 above is added 20 parts of phenol resin solution A, and stirred well. 
     b. The NBR-phenol resin mixed solution prepared in a above is introduced in a pressurized reactor, and stirred at 80° C. for 2 hours to obtain an adhesive solution 11. (The ratio of NBR: phenol resin is 100:50 in terms of the solid content.) 
     Adhesive Solution 12 
     a. To 100 parts of the NBR solution A used in the preparation of Adhesive Solution 9 above is added 77 parts of phenol resin solution (4), and stirred well. 
     b. The NBR-phenol resin mixed solution prepared in a above is introduced in a pressurized reactor, and stirred at 80° C. for 2 hours to obtain an adhesive solution 12. (The ratio of NBR: phenol resin is 100:250 in terms of the solid content.) 
     The adhesive tapes thus prepared are measured for the electric conductivity of extracted water, concentration of sulfate ions contained in the system, and adhesion strength between the heat resistant film and the molding compound are measured in the above-described manner. The results obtained are shown in Table 7 below. 
     
                       TABLE 7
______________________________________
          Electric   Concentration
                                 Adhesion to
Adhesive  Conductivity
                     of Sulfate Ions
                                 Molding Material
Tape Number
          (μs/cm) (ppm)       (kg/1.5 cm)
______________________________________
 9        28         130         420
10        23         100         420
11        25          70         420
12        78         110         420
13        18         220         150
14        14         180         150
15        10         210         150
16        40         220         150
______________________________________
 
    
     Using the adhesive tapes, the change in the electric current characteristics after PCT tests are measured under the same conditions as in Example 1. 
     The results obtained are shown in Table 8 below. 
     
                       TABLE 8
______________________________________
            Initial Current
                        Current Leak Value
Adhesive    Leak Value  After PCT 500 Hours
Tape Number (A)         (A)
______________________________________
 9          Impossible to measure because of striping off of
            the adhesive layer from the base film.
10          1 × 10.sup.-13  or less
                         2 × 10.sup.-12  to 8 × 10.sup.-12
11          1 × 10.sup.-13  or less
                        1 × 10.sup.-10  to 1 × 10.sup.-9
12          1 × 10.sup.-13  or less
                        2 × 10.sup.-10  to 3 × 10.sup.-9
13          Impossible to measure because of striping off of
            the adhesive layer from the base film.
14          1 × 10.sup.-13  or less
                        1 × 10.sup.-12  to 7 × 10.sup.-9
15          1 × 10.sup.-13  or less
                        8 × 10.sup.-10  to 5 × 10.sup.-7
16          1 × 10.sup.-13  or less
                        3 × 10.sup.-10  to 4 × 10.sup.-9
______________________________________
 
    
     Example 3 
     Using materials shown in Table 9 below and MEK as a solvent, adhesive solutions 13 through 17 are prepared which have blend ratios shown in Table 10 below. 
     
                       TABLE 9
______________________________________
Resin    Trade Name Manufacturer
______________________________________
NBR (3)  NIPOL 1031 Nippon Zeon Co., Ltd. Note 1
Phenol   CKM-908    Showa Kobunshi Kogyo Co., Ltd.
resin (1)           Note 2
Phenol   CRM-0803   Taoka Kagaku Kogyo Co., Ltd.
resin (5)           Note 3
Phenol   KP732 E-2  Arakawa Kagaku Kogyo Co., Ltd.
resin (4)           Note 4
______________________________________
 
    
     Note 1: NBR synthesized by Nippon Zeon Co., Ltd. which has the following properties. Mooney viscosity: 63, bonded acrylonitrile content: 41%. Weight average molecular weight/number average molecular weight=4.2 
     Note 2: Bisphenol A type resol phenol resin produced by Showa Kobunshi Kogyo Co., Ltd., having a softening point of 70° to 90° C. 
     Note 3: p-Nonylphenyl type resol phenol resin (alkylphenol type resol phenol resin) produced by Showa Kobunshi Kogyo Co., Ltd., having a softening point of 82° to 90° C. 
     Note 4: Resol phenol resin produced by Arakawa Kagaku Kogyo Co., Ltd. which is a liquid low molecular weight resin containing phenol type structure as a basic structure, having the following properties. Volatile component with methanol/isopropyl alcohol: about 3.5%. Viscosity at 25° C.: 245 cps. Gelling time at 140° C.: about 8 minutes. 
     
                       TABLE 10
______________________________________
Adhesive Blend Ratio of Materials (Parts by Weight)
solution NIPOL     CKM    CRM    KP732
Number   1031      -908   0803   E-2   MEK .
______________________________________
13       100       150    --     --    500
14       100       --     150    --    500
15       100       --     --     200   500
16       100        20    --     --    500
17       100       500    --     --    500
______________________________________
 
    
     The resulting adhesive solutions 13 through 17 are coated on one surface of respective polyimide films having a thickness of 50 μm (&#34;Upilex 50S&#34;, produced by Ube Kosan Co., Ltd.) to a thickness of 20 μm, and dried at 160° C. for 3 minutes to prepare adhesive tapes 17 through 21. Simulating the taping with adhesive tapes represented by those for fixing lead frame, the adhesive tapes thus-obtained are attached to 42-pin 42-alloy lead frame and pressurized for 0.3 second under the conditions of an application size of 1.5 mm in width×10 mm in length at a temperature of 200° C. and a pressure of 4 kg·f/cm 2  to perform taping. The adhesion strength at room temperature immediately after the taping and decrease in the adhesion strength after heating at 250° C. for 1 hour (hereafter, &#34;adhesion strength after thermal hysteresis&#34;) are examined by measuring the peel strength of the tapes using a universal testing machine in the manner as shown in FIG. 7. That is, an adhesive tape composed of the heat resistant film 1 having laminated thereon the adhesive layer 2 is attached to a plurality of pins 4 fixed on the lead frame and the adhesive tape is hung on a member for engagement or hook 5 positioned between the pins in the central portion of the lead frame as shown in FIG. 7. Then the hook 5 is pulled up in the direction at right angles to the plane of adhesion to measure the peel strength of the adhesive tapes. The results obtained are shown in Table 11 below. 
     
                       TABLE 11
______________________________________
Adhesive
        Adhesion at Room
                      Adhesion
Tape    Temp. After Taping
                      After Thermal
Number  (g)           Hysteresis  Evaluation
______________________________________
17      580           570         Fair
18      530           610         Fair
19      250           190         Poor
20      450           580         Poor Surface
                                  Property
21      150           100         Poor
______________________________________
 
    
     As will be clear from the results in Table 11 above, the adhesive tapes 17 and 18 of this invention exhibit sufficient adhesion and fair surface properties. On the other hand, the adhesive tapes of the comparative examples 19 and 21 show poor adhesion. The adhesive tape of the comparative example 20 shows sufficient adhesion but the surface of its adhesive layer is blocking and the surface property thereof is poor and practically unacceptable. 
     Example 4 
     Using materials shown in Table 12 below and MEK as a solvent, adhesive solutions 18 and 19 are prepared which have blend ratios shown in Table 13 below. 
     
                       TABLE 12
______________________________________
Resin     Trade Name Manufacturer
______________________________________
NBR (4)   DN1031     Nippon Zeon Co., Ltd. Note 5
NBR (5)   D-8161     Nippon Zeon Co., Ltd. Note 6
Phenol    BKM-2620   Showa Kobunshi Kogyo Co., Ltd.
resin (5)            Note 7
______________________________________
 
    
     Note 5: NBR synthesized by Nippon Zeon Co., Ltd. which has the following properties. Mooney viscosity: 63, bonded acrylonitrile content: 41%. Number average molecular weight: about 73,000. Weight average molecular weight/number average molecular weight=4.15 
     Note 6: NBR synthesized by Nippon Zeon Co., Ltd. which has the following properties. Softening point: 70° to 90° C. Mooney viscosity: 73, bonded acrylonitrile content: 40%. Weight average molecular weight/number average molecular weight=4.15 
     Note 7: Cresol type resol phenol resin (alkylphenol type resol phenol resin) produced by Showa Kobunshi Kogyo Co., Ltd., having a softening point of 80° to 100° C. 
     
                       TABLE 13
______________________________________
Adhesive  Blend Ratio of Materials (Parts by Weight)
solution  DN-
Number    1031   D-8161     BRM-2620
                                    MEK.
______________________________________
18        100    --         200     500
19        --     100        200     500
______________________________________
 
    
     The resulting adhesive solutions 18 and 19 are coated on one surface of respective polyimide films having a thickness of 50 μm (&#34;Apical 50AH&#34;, produced by Kanegafuchi Kagaku Kogyo Co., Ltd.) to a dry thickness of 20 μm, and dried at 160° C. for 3 minutes to prepare adhesive tapes 22 and 23. Simulating the taping with adhesive tapes represented by those for fixing lead frame, the adhesive tapes thus-obtained are attached to 42-pin copper-alloy lead frame and pressurized for 0.3 second under the conditions of an application size of 1.5 mm in width×10 mm in length at a temperature of 200° C. and a pressure of 4 kg·f/cm 2  to perform taping. The adhesion strength at room temperature immediately after the taping and decrease in the adhesion strength after heating at 250° C. for 1 hour (hereafter, &#34;adhesion strength after thermal hysteresis&#34;) are examined by measuring the peel strength of the tapes using a universal testing machine in the same manner as in Example 3. The results obtained are shown in Table 14 below. 
     
                       TABLE 14
______________________________________
         Adhesion at
Adhesive Room Temp.   Adhesion
Tape     After Taping After Thermal
Number   (g)          Hysteresis Evaluation
______________________________________
22       550          470        Fair
23       230          110        Poor
______________________________________
 
    
     As will be clear from the results in Table 14 above, the adhesive tape 22 of this invention exhibits sufficient adhesion as compared with the comparative adhesive tape 23. 
     Example 5 
     On various base films shown in Table 15 is coated an acrylic based adhesive (Teisan Resin SG-70L, produced by Teikoku Kagaku Sangyo Co. Ltd.) as the adhesive, and the coated base films are dried in a drier in which hot air is circulated at 150° C. for 10 minutes to form B-stage adhesive layers of a thickness of 15±2 μm. The adhesive sheets thus obtained are cut to pieces of a size of 20 mm and 17 mm in width, respectively, along the direction of running of the base film (the direction of the machine) to obtain adhesive tapes, which are used as samples for the measurement of lead shift. Using the adhesive tapes, the lead shift is measured in the following manner. The results obtained are shown in Table 16 below. 
     (1) Samples 
     Adhesive tape to be measured is cut to a width of 17 mm or 20 mm along the direction of running of the base film (the direction of the machine). 
     (2) Lead frame to be measured 
     As the lead frame for measuring the lead shift used 128-pin QFP lead frame. The distance (A) between the portions of the lead frame shown in FIG. 6(a) is measured with a measurement limit of 1 μm prior to measurement of the lead shift. The value obtained is named &#34;A&#34;. 
     (3) Taping 
     Taping is conducted using 4-directional taping machine produced by Sumitomo Kinzoku Kozan Co., Ltd. as a lead frame taping machine. 
     (4) Heat Treatment 
     The lead frame subjected to the taping is then heat treated at 300° C. for 3 minutes on a hot plate, supposing thermal hysteresis of the semiconductor devices in the assembly step. 
     (5) Measurement of Lead Shift 
     The distance (B) between the portions shown in FIG. 6(b) is measured with a measurement limit of 1 μm, and the value obtained is named &#34;B&#34;. Then, the lead shift ratio α (%) is calculated according to the following equation. ##EQU1## 
     
                       TABLE 15
______________________________________
                                 Thickness
Sample Base Film      Composition
                                 (μm)
______________________________________
1      Upilex S       Polyimide  50
       (Ube Kosan
2      Upilex S       Polyimide  50
       (Ube Kosan)
       Treated at 300° C.
       for 5 minutes
3      Kapton 200 H
       (Toray Du Pont)
                      Polyimide  50
4      Apical 50 AH
       (Kanegafuchi Kagaku)
                      Polyimide  50
______________________________________
                      Degree of
       Coefficient of Thermal
       Thermal Expansion
                      Shrinkage
Sample (cm/cm/°C.)
                      (%)        Note
______________________________________
1      1.6 × 10.sup.-5
                      0.33
2      1.6 × 10.sup.-5
                      0.10       Both surfaces
                                 heat treated
                                 with infrared
                                 heater
3      3.4 × 10.sup.-5
                      0.25
4      3.2 × 10.sup.-5
                      0.03
______________________________________
 
    
     
                       TABLE 16
______________________________________
Sample   Ratio of
Number   Lead Shift   Evaluation
                               Note
______________________________________
1        11.5         Poor     Comparison
2        4.0          Fair     Invention
3        14.3         Poor     Comparison
4        13.2         Poor     Comparison
______________________________________
 
    
     The results shown in Table 16 clearly indicate that the sample of this invention has low lead shift ratio. 
     Example 6 
     On various base films shown in Table 17 is coated a nylon based adhesive (Toresin FS-410, produced by Teikoku Kagaku Sangyo Co. Ltd.) as the adhesive, and the coated base films are dried in a drier in which hot air is circulated at 150° C. for 10 minutes to form B-stage adhesive layers of a thickness of 25±2 μm. The adhesive sheets thus obtained are cut to pieces of a size of 20 mm and 17 mm in width, respectively, along the direction of running of the base film (the direction of the machine) to obtain adhesive tapes, which are used as samples for the measurement of lead shift. Using the adhesive tapes, the lead shift is measured in the same manner as in Example 5. 
     The results obtained are shown in Table 18 below. 
     
                       TABLE 17
______________________________________
                                 Thickness
Sample  Base Film     Composition
                                 (μm)
______________________________________
5       CCLHL800      Glass cloth
                                 100
        (Mitsubishi
        Gas Chemical)
        Treated at 330° C.
        for 30 minutes
6       Upilex 50 R   Polyimide  50
        (Ube Kosan)
7       Kapton 100 V
        (Toray Du Pont)
                      Polyimide  25
______________________________________
                      Degree of
        Coefficient of
                      Thermal
        Thermal Expansion
                      Shrinkage
Sample  (cm/cm/°C.)
                      (%)        Note
______________________________________
5       1.5 × 10.sup.-5
                      0.10       Heat treated
                                 in a drier in
                                 which hot air
                                 is circulated
6       8.2 × 10.sup.-5
                      0.63
7       3.2 × 10.sup.-5
                      0.12       Heat treated,
                                 low shrinkage
                                 grade
______________________________________
 
    
     
                       TABLE 18
______________________________________
Sample   Ratio of
Number   Lead Shift   Evaluation
                               Note
______________________________________
5        5.9          Fair     Invention
6        24.3         Poor     Comparison
7        12.1         Poor     Comparison
______________________________________
 
    
     The results shown in Table 18 clearly indicate that the sample of this invention has low lead shift ratio. 
     Example 7 
     On base films made of Apical 50AH (polyimide films, thickness: 50 μm, produced by Kanegafuchi Kagaku Co., Ltd.) is coated a mixture of an acrylic based adhesive (Teisan Resin SG-70L, produced by Teikoku Kagaku Sangyo Co. Ltd.) as the adhesive blended with the filler shown in Table 19 below and dispersed therein. The coated base films are dried in a drier in which hot air is circulated at 150° C. for 10 minutes to form B-stage adhesive layers of a thickness of 15±2 μm. The adhesive sheets thus obtained are cut to pieces of a size of 20 mm and 17 mm in width, respectively, along the direction of running of the base film (the direction of the machine) to obtain adhesive tapes, which are used as samples for the measurement of lead shift. Using the adhesive tapes, the lead shift is measured in the same manner as in Example 5 except that the thickness of the adhesive layer is 5 μm in samples 8 through 11 and 0.5 μm in sample 5. The results obtained are shown in Table 20 below. 
     
                       TABLE 19
______________________________________
                               Ratio of
                                     Method
Sample Ad-                     Filler
                                     of Dis-
Number hesive  Filler          Added persion
______________________________________
8      Teisan  ZrB.sub.2 -F (Zirconium
                               20    Sand mill
       Resin   boride)
       SG70-L  Nippon Shin-Kinzoku
               Co., Ltd.
9      ditto   MBN-250 (Boron nitride)
                               20    Sand mill
               Mitsui Toatsu Kagaku
               Co., Ltd.
10     ditto   RSG (Magnesium oxide)
                               20    Sand mill
               Tateho Kagaku Co., Ltd.
11     ditto   MT-10 (Silica)  20    Sand mill
               Tokuyama Soda Co., Ltd.
                               20    Sand mill
12     ditto   Tosvall20 (Silicone
                               --    --
               resin powder
               Toshiba Silicone
               Co., Ltd.
______________________________________
 
    
     
                       TABLE 20
______________________________________
Sample   Ratio of
Number   Lead Shift   Evaluation
                               Note
______________________________________
8        5.0          Fair     Invention
9        4.8          Fair     Invention
10       5.2          Fair     Invention
11       4.5          Fair     Invention
12       5.9          Fair     Invention
13       13.2         Poor     Comparison
______________________________________
 
    
     The results shown in Table 20 above clearly show that the addition of the filler reduces the lead shift ratio to a greater extent. 
     Example 8 
     On various base films shown in Table 21 is coated a nylon based adhesive (Toresin FS-410, produced by Teikoku Kagaku Sangyo Co. Ltd.) as the adhesive, and the coated base films are dried in a drier in which hot air is circulated at 150° C. for 10 minutes to form B-stage adhesive layers of a thickness of 25±2 μm. The adhesive sheets thus obtained are cut to pieces of a size of 20 mm and 17 mm in width, respectively, along the direction of running of the base film (the direction of the machine) to obtain adhesive tapes, which are used as samples for the measurement of lead shift. Using the adhesive tapes, the lead shift is measured in the same manner as in Example 7. 
     The results obtained are shown in Table 22 below. 
     
                       TABLE 21
______________________________________
Sample   Base Film     Composition
                                 Thickness
______________________________________
14       Kapton 200 V  Polyimide 50
         (Toray Du Pont)
15       Kapton 200 V  Polyimide 50
         (Toray Du Pont)
16       Kapton 200 V  Polyimide 50
         (Toray Du Pont)
17       Upilex 50 S   Polyimide 50
         (Ube Kosan)
18       Upilex 50 S   Polyimide 50
         (Ube Kosan)
19       Upilex 50 S   Polyimide 50
         (Ube Kosan)
20       CCLHL800      Glass cloth/
                                 100
         (Mitsubishi   polyimide
         Gas Chemical)
21       CCLHL800      Glass cloth/
                                 100
         (Mitsubishi   polyimide
         Gas Chemical)
22       CCLHL800      Glass cloth/
                                 100
         (Mitsubishi   polyimide
         Gas Chemical)
______________________________________
 
    
     
                       TABLE 21 (Continued)
______________________________________
Sample                      Filler
Number      Filler          Added
______________________________________
14          SE-5 (Fused silica)
                            30
            Tokuyama Soda Co., Ltd.
15          SE-5 (Fused silica)
                            10
            Tokuyama Soda Co., Ltd.
16          --              0
17          SE-5 (fused silica)
                            30
            Tokuyama Soda Co., Ltd.
18          SE-5 (fused silica)
                            10
19          --              0
20          SE-5 (fused silica)
                            30
            Tokuyama Soda Co., Ltd
21          SE-5 (fused silica)
                            10
22          --              0
______________________________________
 
    
     
                       TABLE 22
______________________________________
Sample   Ratio of
Number   Lead Shift   Evaluation
                               Note
______________________________________
14       7.9          Fair     Invention
15       9.0          Fair     Invention
16       18.6         Poor     Comparison
17       8.4          Fair     Invention
18       8.8          Fair     Invention
19       19.5         Poor     Comparison
20       5.2          Fair     Invention
21       6.7          Fair     Invention
22       16.6         Poor     Comparison
______________________________________
 
    
     The results shown in Table 22 clearly indicate that the samples 14, 15, 17, 18, 20 and 21 of this invention has low lead shift ratios.