TESTING ELECTRONIC COMPONENTS

Abstract: A method of nondestructively testing electronic chips adapted for direct attachment to metallized pads on circuitized substrate is disclosed, wherein an electrically conductive liquid eutectic joint is formed at room temperature with low pressure between electrical terminals on the chip and pads on the substrate. The eutectic joint remains liquid at test temperature, enabling test completion. At the end thereof, chips and pads are separated and any eutectic material residue thereon removed. The eutectic is preferably gallium/indium.

Description: This invention relates to testing electronic components. More particularly, it relates to the non-destructive testing of semiconductor chips prior to incorporation into a circuit. Conventionally, semiconductor chips are first tested at the wafer level with "good" chips being subsequently wired into a variety of packages. To minimise early life failures in chips when packaged, the chips are "burnt in", that is systematically subjected to electrical and environmental stress. Test parameters for such "burn-in" testing are higher than the intended power and temperature expected during normal use but are set at such levels in order to cause failure of weak parts. In practice this technique yields 90-100% good chips for mature chip programs, and the 0-10% drop out reduces field failures in actual machines. In a typical testing process, chips are permanently mounted, burned-in and tested as a final assembly. Parts that fail out during testing are replaced using burned-in chips removed from other assemblies, which are subsequently scrapped. A disadvantage of this process is that chips can be damaged during the operation of removal and replacement, leading either to immediate faillure of faillure in subsequent operation. Attempts have been made to avoid this problem by using a fixture with multiple probes to contact chip metallurgy and run tests. An example of this approach in described in IBM Technical Disclosure Bulletin, Vol. 22, No. 4, 9/79 on page 1476. Here a test fixture requiring no bonding connects chips under test to a substrate. Mechanical forces are used to ensure contact between test probes and electrical terminals of the chip under test while this permits easy removal, the alignment of the probes to make reliable contact adds significantly to the complexity and cost of the manufacturing process. In IBM TDB, Vol. 9, No. 8, 1/67, at page 1051 silicon devices are bonded to gold pads on an insulating substrate at high temperature and pressure. To selectively remove and replace a faulty silicon device, the pad is locally heated to a temperature in excess of the gold-silicon eutectic temperature. Other approaches involve mechanical removal by shearing or breaking which can damage the solder pads on the chips. Various such techniques are disclosed in Technical Disclosure Bulletin, Vol 25, No. 9, 2/84, p. 4780, Vol 19, No. 7, 12/76, p. 2476 and Vol. 19, No. 7, 12/76, p. 2477. Burn-in testing is particularly desirable in high density multi-chip module applications. Overall yield in a multi-chip module is a function of individual chip yield (Y) and the number of chips (N). The yield for the multi-chip module is determined by multiplying the chip yield (Y) by itself N times. Detection of chip failures before the assembly process decreases ultimate waste and maximises overall yield. However, damage to the chips during the testing process can seriously reduce yield, and it is accordingly an object of the present invention to provide an improved testing process for components such as semiconductor chips. According to the invention we provide a method of testing electronic components each having a plurality of terminals for connection to conductors on a circuit substrate comprising providing a test substrate with a pattern of pads complementary to the terminals on components to be tested and attaching said components to said test substrate by bridges formed from electrically conductive eutectic material, characterised in that the melting point of said eutectic material is below the temperature at which the components are tested so that testing proceeds while the bridges are in their liquid eutectic state, and further characterised by removing successfully tested components from said substrate while the bridges are in their liquid state. For testing semiconductor chips having terminals in the form of solder bumps, the formed two metals, preferably indium and gallium, are deposited successively on the test substrate pads. Raising the temperature of the metals above eutectic melting temperature permits chips to self centre as the eutectic material forms at the junction on the two metals and remains in that state during testing. Ideally the eutectic is liquid at room temperature, whereas the individual component metals are solids at room temperature. The liquid state of the eutectic material is maintained until the test is completed. The chip is then removed from the test board and before solidification occurs, any residue of the eutectic material is removed from the chip. Brief Description of the Drawing The above and other features and advantages of the present invention become clear from the following description, taken in conjunction with the accompanying drawing wherein the same reference numerals are used throughout to designate the same parts and wherein: Fig. 1 is a schematic plan view of a chip with solder bumps; Fig. 2 is a schematic plan by diagram of a test substrate; Fig. 3 is an exploded schematic view of a chip solder bump and a conductive pad on a circuitized substrate; Fig. 4 is the same view as Fig. 1 after solder bump and pad metallization. Fig. 5 shows the eutectic between a solder bump and pad; Fig. 6 is a test substrate including solder dams; Fig. 7-8 schematically illustrates an embodiment of the present invention wherein the liquid eutectic is formed on the test substrate. Fig. 9 schematically illustrates the chip placed on the containing molten metal. Description of the Preferred Embodiment Fig. 1 represents a single semiconductor chip 2, such as a memory chip, having a plurality of solder bumps 4 arranged thereon. The solder bumps are of the type known in the art as controlled collapse chip connection (C4). Fig. 2 represents a test circuit having a plurality of electrical pads 8 arranged in a complimentary pattern to the C4 bumps 4 on chip 2. Refer now to Fig. 3. An electronic chip 2 is schematically illustrated. To perform burn-in testing, chip 2 must be attached to circuitized substrate 6, a portion only of which is shown. Electrical contact is to occur between the C4 bump 4, one shown, and conductive pad 8 on substrate 6. Fig. 4 illustrates the chip and substrate of Fig. 1 after metallization in accordance with the present invention. Metals 10 and 12 are deposited on C4 bump 6 and conductive pad 8, respectively. Any of several well known application techniques may be used to deposit metals 10 and 12 on C4 bump 6 and conductive pads 8, respectively. Those having skill in the art will recognise, for example, that vapour deposition or chemical vapour deposition through a mask are processes which may be readily performed to deposit metals 10 and 12. Metals 10 and 12 are chosen from those which form an electrically conductive eutectic material at room temperture 25 deg. C or slightly above this temperature. In this preferred illustrative embodiment, metal 10 is gallium and metal 12 is indium. When gallium and indium are brought into contact at room temperature with slight pressure, a liquid eutectic is formed at their interface. As a practical matter, it is sufficient that the melting point of the eutectic solder formed be lower than the temperature at which the device will be burned-in, generally at 125 deg. C. Ideal values for the eutectic melting point will range from 10 deg. C. to 30 deg. C. We have found gallium-indium eutectic forms spontaneously at approximately 25 deg. C with very little pressure and will remain in a liquid state. In general, metal 10, galium, is coated in a relatively thin layer, preferable about one mil. Metal 12, indium, is deposited to a thickness of about one to three (1-3) mils. The small amount of gallium at the joint facilitates reuse of a test card several times before the melting point will be such that a permanent liquid metal is formed. Referring now to Fig. 5, the resulting material is represented at 16. Eutectic material 16 remains in the liquid state while burn-in testing of chip 2 takes place. At the completion of the burn-in test, chip 2 is removed from substrate 6. The separation occurs at the eutectic material 16. Any eutectic material remaining on C4 bump 4 and/or metallized pad 8 may be readily removed, for example by a high pressure air stream. Fig. 6 represents substrate 6 and a single electrical pad 8 with the addition of solder dam 20. Those skilled in the art will understand that each contact pad 8 such as shown in Fig. 2 may be surrounded by a solder dam 20. It is conventional to design circuit cards so that a protective coating of a polymeric material covers the card, with windows through the protective coating where solder contacts are to be made to metal pads. The walls of the polymeric coating at the windows constitute solder dams. A suitable material for solder dam 20 is a layer of polymer such as epoxy or polyimide with high temperature stability. Solder dams 20 provide the function of retaining liquid eutectic material 16. It will be recalled that burn-in typically occurs at an elevated temperature, around 125 deg. C. Eutectic material 16 has a melting temperature in the range of 10 to 30 deg. C in the preferred embodiment. Therefore, burn-in testing conducted in an oven at the elevated temperature eventually causes material 16 to melt. Solder dams form a containing barrier to prevent flow of molten metal. Figs. 7-9 illustrate an alternative embodiment of the present invention. In Fig. 7 eutectic material 16 is to be formed as a result of depositing metals 10 and 12 as described in connection with Fig. 3, sequentially on electrical contact pad 8 on substrate 6. In Fig. 8, the application of heat has caused liquid eutectic material 16 to form from metals 10 and 12. Fig. 9 shows a C4 hump on chip 2 in testing contact with eutectic material 16. Our process for carrying out burn-in of a given chip having a pattern of solder bumps thereon, is preferably practised as described in the following sequence of steps. A card is fabricated so as to have pads which match the chip solder bump footprint, such that when the chip is placed on the card, electrical connection can be accomplished. This is a requirement since the chip must be electrically active during the burn-in process. A first metal, (a) for example indium, is deposited on the card pads. This is done by means of a physical evaporation process whereby (a) is heated under vacuum above its vaporisation point of which depends upon the pressure used. For example if atmospheric pressure (1 atm) is used, the vaporisation point of indium is 2000 deg. C. This deposition is continued until the required thickness is achieved; it has been found by experimentation that 2 mils of metal will accommodate typical tolerance in the assembly. A second metal, (b) for example gallium, is deposited on the chip bumps through an appropriate mask typically of polyimide with holes to match the chip footprint. This deposition process can be accomplished by heating (b) above its vaporisation point which is 2237 deg. C. at 1 atm pressure for gallium. This process is continued until the required thickness is achieved, typically approximately 1 mil. This is a thickness that will ensure essentially complete consumption of all the gallium to form the eutectic interface. The chip is placed on the pads bringing the two metals into contact, and the temperature is raised to the appropriate burn-in temperature by convection heating, typically 125 deg. C. for memory applications. The interface between the two metals will now be a conductive liquid eutectic. The eutectic composition is approximately 75.5% gallium and 24.5% indium by weight with a melting point of 15.7 deg. C. After the burn-in process is complete, the chip is easily removed, and excess material is cleaned from the pads while the eutectic is in liquid form. The chips that pass the test are now ready to be mouted in their final package and the inoperative chips are discarded. Removal of the chips while the eutectic is in liquid form prevents damage to the bumps since the process is carried out without physical stress to the joints. There is a possibility of using a ternary (three metal) eutectic metal systems for this process; however this complicates the process somewhat. An example of such a ternary system is 62.5% Ga, 21.5% In and 16% Sn with a melting point of 10.7 deg. C. Further, a single metal system using only gallium is also feasible but has the disadvantages of a melting point 4 deg. C. above room temperature. Under such conditions it would not be possible to pre-test the chips on the test substrate at room temperature, prior to burn-in, since good electrical contact is not made. Those skilled in the art will appreciate that our preferred embodiment has inherent practical advantages. Test substrate 6, as shown in Fig. 6-9, provided with a solder dam and having eutectic material 16 formed entirely thereon, may be used and reused as each chip or set of chips 2 undergoes burn-in test for any set of chips 2 may be conducted in a conventional manner. However, upon completion of testing, chips 2 may be removed from their electrical testing contact with eutectic material 16 very easily. Test substrate 6 may be reloaded with additional chips 2 for burn-in while the previously tested chips may be, if tested good, easily cleaned as here and above described, and used. Chip burn-in in accordance with the present invention includes applying metals to the surfaces of the chip bumps and substrate pads, bringing the chips and substrate into contact so that a eutectic forms at the interface of the metals. The electrical characteristics of the eutectic allow testing to be completed. The liquid state allows easy separation and subsequent cleaning. The temporary nature of this joint facilitates reuse of the test substrate. While the present invention has been described having reference to a particular preferred embodiment, those having skill in the art will appreciate that the above noted and other modifications to form and detail may be made without departing from the spirit and scope of the invention.

Claims: A method of testing electronic components each having a plurality of terminals for connection to conductors on a circuit substrate comprising providing a test substrate with a pattern of pads complementary to the terminals on components to be tested and attaching said components to said test substrate by bridges formed from electrically conductive eutectic material, characterised in that the melting point of said eutectic material is below the temperature at which the components are tested so that testing proceeds while the bridges are in their liquid eutectic state, and further characterised by removing successfully tested components from said substrate while the bridges are in their liquid state. A method as claimed in claim 1 in which said bridges are formed by applying a first metal to said pads and a second metal to said terminals, the first and second metals being such as to form said electrically conductive eutectic material on contact with each other, and placing the components with their terminals in contact with corresponding pads on said substrate. A method as claimed in claim 1 in which said bridges are formed by successively depositing first and second metals on said pads, heating the test substrate to form said eutectic material, and placing said components on said substrate with their terminals in contact with the eutectic material on corresponding pads. A method as claimed in claim 2 or claim 3 in which the two metals are indium and gallium. A method as claimed in claim 2 in which said first metal is indium and said second metal is galluim. A method as claimed in claim 3, for testing semiconductor chips having terminals in the form of bumped contacts including the steps of providing solder dams on said test substrate at said pads and coating said pads first with indium to a depth between approximately 1 mil and approximately 3 mils and overcoating the indium with gallium to a depth of approximately 1 mil.

Inventors: POPE, KEITH RANDAL, SCHROTTKE, GUSTAV
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1991
Main classification: H01L21
IPC codes: G01R1, H01L21, G01R31, H05K3