Electrolytic cleaning method for metal surfaces of electronic components

Abstract: This invention relates to a method for the electrolytic removal of plastic mold flash or bleed from the metal surfaces of semiconductor devices and similar electronic components by immersing such components in a water based solution of one or more organic solvents, conducting salts and a wetting agent and whereby the components to be cleaned are connected to the negative pole of a source of direct current, of which the positive pole is connected to an anode in the same solution and whereby during passing a current through the solution the pH of the liquid film directly adjecent to the cathodic metallic surface is raised to a value at which the solvent causes a softening action on the plastic mold flash or bleed, which is sufficient to enable the hydrogen gas, generated simultaneously on the cathode to remove the plastic mold flash or bleed from the metal surface. The invention relates also to a composition of a water based solution to be used with this method.

Description: The invention relates to a method of the electrolytic removal of plastic mold flash or bleed from the outer metal leads of an encapsulated semiconductor device comprising the steps of immersing said device in a water based solution of one or more organic solvents, conducting salts and a wetting agent, connecting said device to the negative pole of a direct current source of which the positive pole is connected to a counter electrode in said water based solution, such that a current with a density between 25-40 A/dm 2 passes through said water based solution. This method is known from US-A-4 781 804, decribing a method to remove plastic bleed from the leads of semiconductor components by means of an electrolytic treatment in a solution containing an alkali metal hydroxide, detergent, a glycol compound, alkali metal bicarbonate, ethoxylated furfuryl alcohol, tetrahydrofurfuryl alcohol and a fluorinated surfactant in water, whereby the components are connected as a cathode to a source of direct current so that hydrogen is generated on the metal surface upon current passages. In practice it appears that this solution contains aggressive components attacking the material of the plastic encapsulation. From US-A-4 968 397 an electrolytic anodic process is known with which embedded blasting particles can be removed, whilst simultaneously metal bleed, such as silver, extending on the leads outside the plastic encapsulation is dissolved, resulting in a clean surface of the leads. To accomplish this effect a solution of lactic acid and a metal hydroxide is used, in which the components are connected to the positive pole of a source of direct current and whereby simultaneously an improved surface condition of copper based metal leads is obtained. In the manufacturing process of the majority of semiconductors, integrated circuits, transistors and the like, for briefness's sake further called semiconductor components, the following production steps can be distinguished. A carrier, called a leadframe is stamped or etched from a copper alloy, nickel-iron alloy or another metal. A leadframe comprises generally a pad and a number of leads. The central part of the leadframe consisting of pad and leadtips is subsequently selectively electroplated with metals such as gold, silver, palladium, copper or nickel. In the next step a chip is bonded to the leadframe pad and electric wire connections are made between chip and leadtips (wirebonding), followed by encapsulating the central area containing the chip and wirebonded leadtips by injection molding with a plastic, usually an epoxy based plastic. The manufacturing process is further explained by the enclosed drawing in which an integrated circuit (IC) is shown manufactured as described above and of which the outer leads are contaminated with plastic bleed. In the drawing, the plastic encapsulation, which contains the bonded chip and the wirebonded leadtips is indicated with numeral 1, the outer leads with 2. Numeral 3 indicates spaces which usually fill up with plastic during molding of encapsulation 1, whilst numeral 4 indicates places where very thin films of plastic (normally called "bleed") can contaminate the metal surface of the outer leads and which must be removed. Numeral 5 indicates parts of the leadframe serving interconnection during manufacturing and which will be removed before the IC is used. The method of this invention is designed to remove the plastic bleed as indicated in the drawing with numeral 4. Such plastic bleed films on the metal surface of the leads are thin, usually less than 10 micrometer, but must be carefully removed to avoid uncoated areas during subsequent tin-lead plating, necessary to enable reliable solder connections to be made to other parts of an electronic circuit. According to the present invention an improved method is obtained for removing plastic mold of flash bleed from outer metal leads of an encapsulated semiconductor device as disclosed in the preamble of the description, which method is characterised in that said water based solution comprises 10-20% by volume alkyl lactate as said organic solvent and said water based solution having a pH-value of between 6.5-7.5, whereby said current causes the pH-value of the film of said water based solution directly adjacent to the exposed cathodic metal leads surfaces to rise to a value exceeding 10 at which said water based solution causes a softening of said plastic mold flash or bleed, which is sufficient to enable the hydrogen gas, generated simultaneously on the exposed cathodic metal leads surfaces to remove said plastic mold flash or bleed from said metal outer lead. At the high pH in the layer of metal-liquid, the solvents presenting the electrolyte, supported by the wetting agent, soften the plastic bleed to such an extend that the hydrogen gas generated at the metal surface simultaneously is able to mechanically scrub the surface clean of the softened plastic bleed. Possible remnants of the softened plastic bleed can be easily removed by a subsequent pressure rinse or by a pressurised air jet. As a wetting agent 2-amino 2-methyl propanol proved to be extremely suitable since it combined good wetting properties with low foaming characteristics. This wetting agent can be used in concentrations of 0.5 - 2.5% by volume. As solvents, esters of lactic acid are added to the water based solution. Preferred esters are methyl, ethyl, isopropyl or butyl lactate. The alkyl esters with the lower number of carbon atoms are best soluble in water. At a pH value over 7 relative rapid hydrolysis of the lactate esters takes place resulting in the formation of lactic acid. Salts which must be added to obtain sufficient conductivity of the solution can ben organic or inorganic but are preferably potassium, sodium or ammonium salts of acetic or lactic acid, whilst suitable inorganic salts are the potassium, sodium and ammonium salts of sulphuric, phosphoric or carbonic acids. Chlorides would also be suitable but are usually not permitted in the manufacturing process of semiconductors. The concentration of conducting salts must be adequate to provide a pH between 10 - 13 in the catholyte when a direct current is applied with a cathodic current density of 25 - 40 A/dm 2 and at which the pH of the bulk of the electrolyte is maintained between 6 and 7. Preferably lactates are used as conducting salts to reduce the number of ions in the solution and to reduce the hydrolyses of the alkyl lactate. The softening effect of alkyl esters of lactic acid on plastic bleed could be further improved by the addition of other water soluble solvents, particularly by addition of 0.5 - 2.5 ml/liter 4-methyl-2-pentanol. The invention will be further explained by means of the following examples and experiments. Example I Several solutions were prepared and examined on their ability to soften plastic encapsulations of semiconductor components as indicated on the enclosed drawing. These components were immersed in the following solutions: 1) Pure ethyl lactate. 2) 98% ethyl lactate with 2% 4-methyl-2-pentanol. 3) 90% ethyl lactate with 10% water. 4) 90% ethyl lactate with 10% of a concentrated KOH solution. (appr. 250 g/l KOH). 5) 88% ethyl lactate with 2% 4-methyl-2-pentanol and 10% concentrated KOH solution. The pH of solutions 1, 2 and 3 was well below 7 whilst the pH of solutions 4 and 5 was over 12. The immersion of the components in the solution took place at ambient temperature. After exposure for 1 hour the components were rinsed and the black encapsulation plastic was rubbed with soft white tissue paper. The intensity of the black contamination on the tissue paper after 4 rubs was considered as a measure for the degree of attack on the plastic. In freshly prepared solutions 1, 2 and 3 no attack was visible after 1 minute exposure, nor after 1 hour exposure. Only after 24 hours immersion a very slight attack on the encapsulation material was observed. The freshly made up solutions 4 and 5 showed considerable attack of the plastic encapsulating already after 1 minute exposure. The degree of attack was considered sufficient for solution 4 and very good for solution 5. When the test with solutions 4 and 5 were repeated the next day, no attack of the plastic was noticeable after one minute nor after one hour exposure. Even after 24 hours, attack of the plastic was hardly noticeable. This phenomenon can probably be explained by the hydrolyses of the ethyl lactate at the high pH of solutions 4 and 5, rendering them inactive. When passing a direct current through an electrolyte the pH in the cathode film (catholyte) will increase and in the anode film (anolyte) will decrease compared to the pH of the bulk of the solution, the deviation being dependent upon the type of solution and upon the local current density (A/dm 2 ). The higher the C.D., the larger the pH difference will be between the bulk of the solution and catholyte or anolyte. The method of the present invention utilises this phenomena for removal of plastic bleed from metal surfaces by employing a solution with a pH between 6.5 and 7.5. At this pH, hydrolysis of alkyl lactates is negligeable. When direct current is passed through the solution at which the components are cathodic and the C.D. has a value between 30 - 40 A/dm 2 the pH in the catholyte will increase to a value well over 12. These high pH values are created only in the areas where metal is exposed, such as in the pores of the plastic bleed and the immediate area surrounding the pores. There will be no pH increase at the surface of the bulk of plastic encapsulating material. The result of this is zero attack on the encapsulating plastic, whilst the bleed, which normally is less than 10 micrometer thick and hence is very porous will be exposed to a high pH environment which almost instantly softens the bleed. The vigorous evolution of fine hydrogen bubbles formed simultaneously acts as a scrubber and removes the plastic bleed at large. Any remaining loosely adhering remnants of the plastic bleed are completely removed in a subsequent pressurised water rinse or air jet. The method for removing plastic bleed from metal surfaces utilises a solution with a temperature between 35 and 50 °C, preferably at appr. 45 °C for appr. 1 minute. Even after longer exposure times no attack of plastic encapsulating material will take place, in areas other than the plastic bleed film which must be removed. Example II Leadframes for integrated circuits were manufactured and assembled in the standard industrial way, and finally encapsulated with an epoxy based plastic. These leadframes showed plastic bleed on the outer lead metal surfaces. This plastic bleed film was removed using the method of this invention as will be explained hereafter. The process time for removing the plastic bleed was 1 minute and 15 seconds and followed by a pressurised water rinse during 5 seconds, after which a final treatment took place in the form of electroplating with tin-lead alloy. After electroplating the samples were inspected for detection of bare spots in the electroplated coating, which would indicate incomplete removal of the plastic bleed film. In a parallel test using a known composition for electrocleaning metal surfaces (containing sodium hydroxyde, potassium phosphate, sodium gluconate and sodium pyrophosphate) 30 - 40% of, the original plastic bleed area was not removed and showed up bare spots after tin-lead plating. According to the invention different compositions 1 - 4 as indicated in the table were used. Ingredient Sol. 1 Sol. 2 Sol. 3 Sol. 4 Ethyl lactate 10% -- 10% 20% Isopropyl lactate -- 15% -- -- Tri potassium phosphate 60 g/l -- -- 12 g/l Sodium acetate -- 75 g/l -- -- Potassium lactate -- -- 20% 20% 2-Amino-2-methylpropanol 1% 2% 2% 1% 4-Methyl-2-pentanol 0.5 ml/l 1 ml/l 2 ml/l 1 ml/l Leadframes were obtained with the solutions 1 - 4 listed in the table which were completely free of plastic bleed on the metal surfaces of the outer leads. During the treatments, the temperature of the solutions was maintained at 40 - 50 °C. The method could also be successfully used at lower temperatures at which higher bath voltages were needed. At the temperatures of 40 - 50 °C the current densities applied were between 25 - 35 A/dm 2 . At C.D. below 20 A/dm 2 the removal of the plastic bleed was not complete. All experiments were carried out at pH values between 6.5 and 7. When pH was below 6.5 a slight attack of the anode material was noticed, so it is recommended to maintain a pH value not below 6.5, whilst at pH values over 7.5 danger exists for the hydrolysis of the alkyl lactates. The treatment time was varied between 30 seconds and 2 minutes; whereby a treatment of 30 seconds gave already adequate results. Example for comparison The tests described in example II were repeated under identical conditions and solution compositions, except that the encapsulated leadframes with plastic bleed on the outer lead metal surfaces were connected to the positive pole of the direct current source and hence were exposed as anodes. It was estimated that the pH of the anolyte, the electrolyte film directly adjacent to the metal surface, was well below 4. After the treatment the leadframes were electroplated with tin-lead and inspected for bare spots. All leadframes showed numerous bare spots and were identical to a sample which was tin-lead plated without being submitted to a deflash treatment. Surprisingly it was also found that the plastic bleed residues had hardened which is believed to be caused by the reaction of the oxygen, developed at the anode surface, with the epoxy bleed films. It was also noted that the subsequent tin-lead coatings were of an inferior coarser structure than those on the leadframes of example II, which was to all probability caused by the etching of the metal surfaces during the anodic treatment.

Claims: A method of electrolytic removal of plastic mold flash or bleed from outer metal leads of an encapsulated semiconductor device comprising the steps of immersing said device in a water based solution of one or more organic solvents, conducting salts and a wetting agent, connecting said device to the negative pole of a direct current source of which the positive pole is connected to a counter electrode in said water based solution, such that a current with a density between 25-40 A/dm 2 passes through said water based solution, characterised in that: said water based solution comprises 10-20% by volume alkyl lactate as said organic solvent and said water based solution has a pH-value of between 6.5-7.5, whereby said current causes the pH-value of the film of said water based solution directly adjacent to the exposed cathodic metal leads surfaces to rise to a value exceeding 10 at which said water based solution causes a softening of said plastic mold flash or bleed, which is sufficient to enable the hydrogen gas, generated simultaneously on the exposed cathodic metal leads surfaces to remove said plastic mold flash or bleed from said metal outer lead. A method according to claim 1, further characterized in that the alkyl group of said alkyl lactate comprises 1-4 carbon atoms. A method according to claims 1 or 2, further characterized in that water based solution comprises 0.5-2.5 ml/l 4-methyl-2-pentanol as said organic solvent. A method according to claims 1-3, further characterized in that said water based solution comprises a sodium-, potassium- or ammonium salt of an inorganic acid as said conducting salt. A method according to claims 1-3, further characterized in that said water based solution comprises a sodium-, potassium- or ammonium salt of an organic acid as said conducting salt . A method according to claim 5, further characterized in that said water based solution comprises sodium-, potassium- or ammonium acetate as said conducting salt. A method according to claim 5, further characterized in that said water based solution comprises potassium-, sodium- or ammonium lactate as said conducting salt. A method according to claim 7, further characterized in that said water based solution comprises between 5-25 % by weight of said potassium-, sodium- or ammonium lactate. Method according to claims 1-8, further characterized in that the amount of said wetting agent in said solution is between 0.5-2.5 % by volume. A method according to claim 9, further characterized in that said wetting agent is 2-amino-2-methyl propanol. A method according to claim 10, further characterized in that said potassium-, sodium- or ammonium lactate is at least partly replaced by a phosphate. A method according to claim 10, further characterized in that said potassium-, sodium- or ammonium lactate is at least partly replaced by an acetate.

Inventors: VAN DER HEIJDEN HENRICUS JOHAN, VAN DER HEIJDEN, HENRICUS JOHANNES
Applicants: MECO EQUIP ENG, MECO EQUIPMENT ENGINEERS B.V.
Application year: 1993
Main classification: H01L21
IPC codes: H01L21, H01L23, C25F3, C25F1