Abstract:
Disclosed is a method of bonding integrated circuit chips to lead frames where the leads are later soldered. A solution of an adhesive polyimide is used. The polyimide is the reaction product of an aromatic dianhydride and a mixture of an aromatic diamine and an aliphatic diamine.

Description:
This application is a continuation of application Ser. No. 08/784,220, filed Jan. 16, 1997, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a polyimide adhesive that is useful in bonding integrated circuit chips to a lead frame, where the leads are subsequently soldered. In particular, it relates to a solution of a polyimide made from a dianhydride and a mixture consisting essentially of an aromatic diamine and an aliphatic diamine. 
     In manufacturing microelectronic components, integrated circuit chips are bonded to lead frames using a polyimide resin. Later, the leads from the chips are soldered to the leads from the lead frames. A persistent problem in the industry has been the failure of the solder to &#34;wet&#34; the leads and form a good electrical connection. 
     SUMMARY OF THE INVENTION 
     I have discovered that the failure of solder to &#34;wet&#34; leads is due to the use of polyimide adhesive resins that contain siloxane groups. I have found that when these resins are heated, they evolve silicon-containing compounds that condense on the leads, preventing the solder from wetting the leads. I have further found that a polyimide adhesive made from a dianhydride and a mixture of aromatic and aliphatic diamines is an excellent adhesive for this application and will not prevent the solder from wetting the leads because it evolves almost no volatiles during bonding. 
     Surprisingly, I have also found that the peel strength after pressure cooking of the adhesives of this invention is better than for identical adhesives made with a siloxane-containing diamine instead of an aliphatic diamine. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The adhesives of this invention are prepared by reacting in an organic solvent an aromatic dianhydride with a mixture of aromatic and aliphatic diamines. 
     Organic Solvent 
     The organic solvent should be a low boiling solvent so that the adhesive can be dried at moderate temperatures; a boiling point of less than 180° C. is preferred. Suitable organic solvents include N-methylpyrrolidone (NMP), dimethylacetamide (DMAC), 2-vinyl pyrrolidone, acetone, benzene, toluene, xylene, &#34;Cellosolve&#34; (glycol ethyl ether), &#34;Cellosolve acetate&#34; (hydroxyethyl acetate glycol monoacetate), diethyl ether, dichloromethane, dimethyl formamide (DMF), ethyl alcohol, methyl isobutyl ketone, methyl ethyl ketone, sulfolane, dimethyl sulfoxide (DMSO), hexamethylphosphoramide (HMPA), tetramethyl urea (TMU), diethyleneglycol diethyl ether, 1,2-dimethoxyethane (monoglyme), diethylene glycol dimethyl ether (diglyme), 1,2-bis(2-methoxyethoxy) ethane (triglyme), bis  2-(2-methoxyethoxy) ethyl)! ether (tetraglyme), bis(2-methoxyethyl) ether tetrahydrofuran, m-dioxane, and p-dioxane. NMP and DMAC are preferred as they are good solvents for polyamic acids. About 5 to about 50 wt % (based on total solvent weight) of a cosolvent, such as toluene or xylene, may be useful with NMP, DMAC, or other solvents to aid in water removal. 
     Aromatic Dianhydride 
     Any aromatic dianhydride or combination of aromatic dianhydrides can be used as the dianhydride monomer in forming the polyimide. Examples of suitable aromatic dianhydrides include: 
     1,2,5,6-naphthalene tetracarboxylic dianhydride; 
     1,4,5,8-naphthalene tetracarboxylic dianhdyride; 
     2,3,6,7-naphthalene tetracarboxylic dianhydride; 
     2-(3&#39;,4&#39;-dicarboxyphenyl) 5,6-dicarboxybenzimidazole dianhydride; 
     2-(3&#39;,4&#39;-dicarboxyphenyl) 5,6-dicarboxybenzoxazole dianhydride; 
     2-(3&#39;,4&#39;-dicarboxyphenyl) 5,6-dicarboxybenzothiazole dianhydride; 
     2,2&#39;,3,3&#39;-benzophenone tetracarboxylic dianhydride; 
     2,3,3&#39;,4&#39;-benzophenone tetracarboxylic dianhydride; 
     3,3&#39;,4,4&#39;-benzophenone tetracarboxylic dianhydride (BTDA); 
     2,2&#39;,3,3&#39;-biphenyl tetracarboxylic dianhydride; 
     2,3,3&#39;,4&#39;-biphenyl tetracarboxylic dianhydride; 
     3,3&#39;,4,4&#39;-biphenyl tetracarboxylic dianhydride (BPDA); 
     bicyclo- 2,2,2!-octen-(7)-2,3,5,6-tetracarboxylic-2,3,5,6-dianhydride; 
     thio-diphthalic anhydride; 
     bis (3,4-dicarboxyphenyl) sulfone dianhydride; 
     bis (3,4-dicarboxyphenyl) sulfoxide dianhydride; 
     bis (3,4-dicarboxyphenyl oxadiazole-1,3,4) paraphenylene dianhydride; 
     bis (3,4-dicarboxyphenyl) 2,5-oxadiazole 1,3,4-dianhydride; 
     bis 2,5-(3&#39;,4&#39;-dicarboxydiphenylether) 1,3,4-oxadiazole dianhydride; 
     bis (3,4-dicarboxyphenyl) ether dianhydride or 4,4&#39;-oxydiphthalic anhydride (ODPA); 
     bis (3,4-dicarboxyphenyl) thioether dianhydride; 
     bisphenol A dianhydride (BPADA); 
     bisphenol S dianhydride; 
     2,2-bis (3,4-dicarboxyphenyl) hexafluoropropane dianhydride or 5,5- 2,2,2-trifluoro-1-(trifluoromethyl)ethylidene! bis-1,3-isobenzofurandione) (6FDA); 
     hydroquinone bisether dianhydride; 
     bis (3,4-dicarboxyphenyl) methane dianhydride; 
     cyclopentadienyl tetracarboxylic acid dianhydride; 
     cyclopentane tetracarboxylic dianhydride; 
     ethylene tetracarboxylic acid dianhdyride; 
     perylene 3,4,9,10-tetracarboxylic dianhydride; 
     pyromellitic dianhydride (PMDA); 
     tetrahydrofuran tetracarboxylic dianhydride; and 
     resorcinol dianhydride. 
     The preferred dianhydrides and BTDA, BPDA, ODPA, and 6FDA, as they are readily available and have been found to give superior properties. The dianhydrides can be used in their tetraacid form or as mono, di, tri, or tetra esters of the tetra acid, but the dianhydride form is preferred because it is more reactive. 
     Aromatic Diamine 
     To prevent the polyimide from being too soft, about 50 to about 95 mole % of the diamine content of the polyimide should be a non-siloxane containing aromatic diamine; preferably, about 75 to about 90 mole % of the diamine content is non-siloxane containing aromatic diamine. Examples of suitable aromatic diamines include: 
     m-phenylenediamine; 
     p-phenylenediamine (PDA); 
     2,5-dimethyl-1,4-diaminobenzene or 2,5-dimethyl-1,4-phenylenediamine (DPX); 
     2,4-diaminotoluene (TDA); 
     2,5- and 2,6-diaminotoluene; 
     p- and m-xylenediamine; 
     4,4&#39;-diaminobiphenyl; 
     4,4&#39;-diaminodiphenyl ether or 4,4&#39;-oxydianiline (ODA); 
     4, 4&#39;-diaminobenzophenone; 
     3,3&#39;,3,4&#39;, or 4,4&#39;-diaminophenyl sulfone or m,m-, m,p- or p,p-sulfone dianiline; 
     4,4&#39;-diaminodiphenyl sulfide; 
     3,3&#39; or 4,4&#39;-diaminodiphenylmethane or m,m- or p,p-methylene dianiline; 
     3,3&#39;-dimethylbenzidine; 
     α,α&#39;-bis(4-aminophenyl)-1,4-diisopropyl benzene or 4,4&#39;-isopropylidenedianiline or bisaniline p; 
     α,α&#39;-bis(4-aminophenyl)-1,3-diisopropyl benzene or 3,3&#39;-isopropylidonedianiline or bisaniline m; 
     1,4-bis(p-aminophenoxy)benzene; 
     1,3-bis(p-aminophenoxy)benzene; 
     4,4&#39;-bis (4-aminophenoxy)biphenyl; 
     1,3-bis(3-aminophenoxy)benzene (APB); 
     2,4-diamine-5-chlorotoluene; 
     2,4-diamine-6-chlorotoluene; 
     2,2-bis(4 4-aminophenoxy!phenyl)propane (BAPP); 
     trifluoromethyl-2,4-diaminobenzene; 
     trifluoromethyl-3,5-diaminobenzene; 
     2,2&#39;-bis(4-aminophenyl)-hexafluoropropane (6F diamine); 
     2,2&#39;-bis(4-phenoxy aniline) isopropylidene; 
     2,4,6-trimethyl-1,3-diaminobenzene; 
     4,4&#39;-diamino-2,2&#39;-trifluoromethyl diphenyloxide; 
     3,3&#39;-diamino-5,5&#39;-trifluoromethyl diphenyloxide; 
     4,4&#39;-trifluoromethyl-2,2&#39;-diaminobiphenyl; 
     2,4,6-trimethyl-1,3-diaminobenzene; diaminoanthraquinone; 
     4,4&#39;-oxybis 2-trifluoromethyl)benzeneamine! (1,2,4-OBABTF); 
     4,4&#39;-oxybis 3-trifluoromethyl)benzeneamine!; 
     4,4&#39;-thiobis (2-trifluoromethyl)benzeneamine!; 
     4,4&#39;-thiobis (3-trifluoromethyl)benzeneamine!; 
     4,4&#39;-sulfoxylbis (2-trifluoromethyl)benzeneamine!; 
     4,4&#39;-sulfoxylbis (3-trifluoromethyl)benzeneamine!; 
     4,4&#39;-ketobis (2-trifluoromethyl)benzeneamine!; 
     4,4&#39;- (2,2,2-trifluoromethyl-1-(trifluoromethyl)-ethylidine)bis(3-trifluoromethyl)benzeneamine!. 
     The preferred aromatic diamines are APB, BAPP, DPX, and bisaniline P, or a combination of these, due to their excellent properties. 
     Aliphatic Diamines 
     About 5 to about 50 mole % of the diamine monomer, and preferably about 10 to about 25 mole %, is an aliphatic diamine that does not contain siloxane groups. If less aliphatic diamine is used, the polyimide becomes less soluble, has less adhesion, and is more difficult to process, and more aliphatic diamine will lower the Tg. The aliphatic diamines useful in this invention preferably have the formula H 2  N-R-NH 2 , where R is hydrocarbon from C 2  to C 20  ; preferably, R is a straight chain hydrocarbon from C 6  to C 12  as those diamines are more readily available. Also, the aliphatic diamine preferably is an α, ω-diamine because they are more reactive. Examples of suitable aliphatic diamines include: 
     hexamethylenediamine or 1,6-hexanediamine (HD); 
     heptamethylene diamine; 
     monomethylenediamine; 
     decamethylenediamine; 
     1,12-dodecamethylenediamine (DDD); 
     2-methyl-1,5-pentanediamine (MDP); 
     1,4-cyclohexanediamine; and 
     bis(4-aminocyclohexyl) methane. 
     The preferred aliphatic diamine is DDD because it has been found to work well. 
     Preparation of Polyimide 
     Generally, stoichiometric quantities of diamine and dianhydride monomers are used to obtain polyimides of the highest molecular weight, but the equivalent ratio of dianhydride to diamine can range from 1:2 to 2:1. The solution of the monomers in the solvent is preferably about 5 to about 20 wt % solids, where &#34;solids&#34; means components other than the solvent. 
     Upon the addition of the monomers to the solvent, polymerization will occur at room temperature to form a polyamic acid. The polyamic acid is then imidized. This can be accomplished chemically by, for example, the addition of acetic anhydride, or by heating, preferably at about 130 to about 200° C. (The polyimide is preferably fully imidized.) 
     The solution of the polyimide is spread on a non-adhering surface, such as glass coated with a release agent, and heated at 250° C. to remove solvent. The resulting film is peeled from the surface and is positioned between surfaces to be bonded. (Bonding can also be done by applying the solution to a surface, drying by evaporating the solvent, and covering the polyimide with a second surface.) Also, a paste of 30 to 70 wt % polyimide in an organic solvent, which also contains about 1 to about 10 wt % of a thixotrope, such as silica, can be screen printed onto one of the articles to be bonded. Bonding can be achieved by heating at about 250 to about 450° C. for less than a second. 
    
    
     The following examples further illustrate this invention. 
     EXAMPLE 1 
     Various polyamic acids were prepared in a solvent mixture of 85 wt % NMP-15 wt % toluene by adding monomers at 17 wt % solids to the solvent mixture and letting the resulting solution stand at room temperature for 16 hours. (This was followed by imidization at 165 to 175° C. for 2 to 4 hours.) Formulations 1 to 4 and 6 to 9 are comparative examples (&#34;C&#34;) as they contain siloxane diamines. The siloxane diamines have the formula: ##STR1## and are designated &#34;G m  &#34;. The following table gives the formulations prepared and their Tg as determined by dynamic mechanical thermal analysis (DMTA): 
     
         __________________________________________________________________________ Aromatic Aromatic Aliphatic                            Siloxane                                    TgFormulation Dianhydride (m)          Diamine (m)                   Diamine (m)                            Diamine (m)                                    (°C.)__________________________________________________________________________1C    BPDA     (0.0215)          BAPP              (0.0205)      G.sub.9                               (0.0031)                                    239 BTDA     (0.0092)          DPX (0.0063)      G.sub.1                               (0.0016)2C    BPDA     (0.1031)          BAPP              (0.0949)                   DDD (0.0092)                            G.sub.1                               (0.0072)                                    247 BTDA     (0.0427)          DPX (0.0292)      G.sub.25                               (0.0053)3C    BPDA     (0.1031)          BAPP              (0.0949)                   DDD (0.0161)                            G.sub.25                               (0.0055)                                    247 BTDA     (0.0427)          DPX (0.0292)4C    BPDA     (0.1031)          BAPP              (0.0949)      G.sub.1                               (0.00167)                                    244 BTDA     (0.0427)          DPX (0.0292)      G.sub.25                               (0.0050)5     BPDA     (0.1031)          BAPP              (0.0949)                   DDD (0.0217)     242 BTDA     (0.0427)          DPX (0.0292)6C    ODPA     (0.1758)          APB (0.1023)                   DDD (0.0210)                            G.sub.25                               (0.0118)                                    1637C    ODPA     (0.1365)          APB (0.1122)      G.sub.25                               (0.0143)                                    1768C    ODPA     (0.1357)          APB (0.1025)      G.sub.1                               (0.0217)                                    157                            G.sub.25                               (0.0115)9C    ODPA     (0.0333)          APB (0.0250)      G.sub.9                               (0.0083)                                    14810    ODPA     (0.1727)          APB (0.1296)                   DDD (0.0431)     16211    ODPA     (0.1727)          APB (0.1345)                   MPD (0.0447)     18712    ODPA     (0.1727)          APB (0.1345)                   HD  (0.0447)     182__________________________________________________________________________ 
    
     The formulations were tested for peel strength as bonded and after one and two days at 121° C. and 100% relative humidity (RH) (pressure cooker test). The bond was for 60 seconds at 3.4 MPa (500 psi) to Alloy 42 (an alloy of 42 wt % nickel and 58 wt % iron). Samples of formulations were placed in a thermal desorber-gas chromatograph. The samples were heated to 400° C. for about 5 minutes and the wt % of siloxane-containing volatiles emitted was determined. The following table gives the results: 
     
         ______________________________________   PEEL STRENGTH (kg/cm) Bond           After 1 day                        After 2 days                                Wt % SiloxaneForm- Temp.          121° C./                        121° C./                                Volatilesulation (°C.)         Initial                100% RH 100% RH (ppm)*______________________________________1C    275     1.20   1.47    0.99    25002C    275     0.93   0.77    0.71    20603C    275     0.94   0.78    0.72    19904C    275     1.06   0.81    0.67    25005     275     1.73   1.80    1.67    06C    250     1.49   0       0       42007C    250     0.64   0       0       49108C    250     1.39   0       0       46609C    250     1.84   0.16    0       500010    225     1.89   0.90    1.46    011    250     1.80   1.87    1.70    012    250     1.97   1.96    1.70    0______________________________________ *based on wt of polymer C = comparative example 
    
     The above table shows that the best retention of peel strength was in the formulations of this invention, formulations 5, 10, 11, and 12.