US20050231222A1 - Wafer probing that conditions devices for flip-chip bonding - Google Patents

Abstract

A probing system or process for electrical testing of a device fabricated on a wafer also conditions terminals such as solder balls on the device to improve uniformity of the heights of the terminals and improve the reliability of connections to an interconnect substrate in a flip-chip package or to a printed circuit board in a chip-on-board application. The system can employ a probe card that is a printed circuit board and/or is substantially identical to interconnect substrates used in flip-chip packaging. The probe card can be replaceable on a test head to allow for quick changes the reduce ATE downtime and to accommodate device changes such as a die shrink. Probe tips on the probe card can be the contact pads or bumps that are the normal electrical contact structures of the interconnect substrates.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS
• This patent document is a divisional and claims benefit of the earlier filing date of U.S. patent application Ser. No. 10/428,572, filed May 1, 2003, which is hereby incorporated by reference in its entirety.
BACKGROUND
• Testing of integrated circuit identifies devices that are defective and also provides information regarding the yield of or problems in the fabrication process. Preferably testing is preformed early in the fabrication process to avoid wasted processing of defected parts and to identify process problems before a correctable problem affect multiple batches. Wafer probing in particular permits early electrical testing of integrated circuit devices before the devices are separated from a wafer. The devices identified as being defective or bad can then be discarded before being packaged. Further, corrections or adjustments to the fabrication process can be made without the additional delay that would result if devices were only tested after being packaged.
• FIG. 1 illustratesconventional test equipment100 for the testing of anintegrated circuit device112 fabricated on awafer110. Wafer110 generally is a semiconductor wafer that includesmultiple devices112. For testing, a prober or other positioning system (not shown) moveswafer110 or atest head130 to align atest board120 with thedevice112 currently selected for testing. Ontest board120 arepins124 arranged to match the pattern ofelectrical terminals114 on eachdevice112. Whentest board120 is appropriately aligned with theselected device112,pins124 andterminals114 are brought together to provide electrical connections between theselected device112 andtest board120.Pins124,test board120, andtest head130 can then relay electrical signals between theselected device112 andtest electronics140.
• Test equipment100 is generally designed to avoid or minimize damage todevices112, particularly wherepins124contact terminals114. InFIG. 1,pins124 are cantilevered to provide flexibility that limits the force thatpins124 apply toterminals114. Some other similar test equipment designs use spring-loaded pins that similarly cushion or limit the force applied todevices112 during testing.
• A disadvantage ofpins124 being flexible is the ease with whichpins124 become misaligned. When one ofpins124 is bent, for example, during cleaning or use, thatpin124 will often fail to make a good electrical contact with thetarget terminal114, resulting in a failed test. Further, a difference in the thermal properties ofwafer110 andtest board120 orpins124 limits the temperature range at whichpins124 will suitably match the pattern ofterminals114. Particularly,pins124 are long relative to the size ofdevice112 and will proportionally change in length when the temperature changes.
• Damage or abrasion that testing causes onterminals114 can be a problem even whenpins124 are compliant, and such damage is particularly problematic whendevice112 is designed for flip-chip packaging.FIG. 2 illustrates a flip-chip package200 including a die210 and aninterconnect substrate220. Die210 contains adevice112 that has been separated fromwafer110 ofFIG. 1. Flip-chip packaging attaches metal bumps, which form elevatedelectrical terminals114 ondevice112, to pads224 onsubstrate220. Interconnectsubstrate220 then provides electrical connections between die210 andexternal terminals222.
• Sharptest pins124 thatcontact terminals114 before the packaging process can leavegouges216 inmetal bumps114, particularly when the contacted portion of the metal bumps are relatively soft metal such as solder.Gouges216 can trap contaminants such as oxidation or soldering flux that weaken solder joints betweenterminals114 and pads214, resulting in a less dependable package.
• Another potential problem in flip-chip packages arises fromnon-uniformity terminals114. In particular, for a reliable attachment ofterminals114 to pads214, the tops ofterminals114 and pads214 should lie in a plane corresponding to the packaging substrate.FIG. 2 illustrates the problem of aterminal114′ that fails to extend to or make a reliable connection with a corresponding pad214. The formation process forterminals114 would typically be the cause ofnon-uniform terminals114, butpins124 can abradeselected terminals114 during testing and further disrupt planarity, making reliable packaging more difficult.
SUMMARY
• In accordance with an aspect of the invention, a wafer probing process improves planarity or otherwise conditions the tops of metal bumps that form the electrical terminals of a semiconductor device. The probing process can thus electrically test devices and flatten or condition terminals of the device to aid in the formation of reliable bonds between the device and a packaging substrate during flip-chip packaging.
• In accordance with another aspect of the invention, a wafer probe for a device that is designed for flip-chip packaging uses a probe card that is substantially identical to an interconnect substrate that will form part of the packaged device. In one configuration, the probe card has a compliant mounting to cushion the contact force on the terminals of integrated circuits under test. Alternatively, a non-compliant mounting holds the probe card during testing. Using an interconnect substrate suitable for flip-chip packaging as the probe card for wafer probing reduces the cost of preparing the probe card. The probe card additionally provides durable alignment for testing of integrated circuits and does not require realignment for testing of the integrated circuit when the wafer (and the test board) are at an elevated temperature.
• One specific embodiment of the invention is a wafer probing system that includes a tester, a probe card, and a prober. The tester is electrically connected to the probe card, and the prober controls the relative position of a wafer and the probe card so that probe tips on the probe card contact terminals on the wafer. In accordance with an aspect of the invention, the probe tips have flat contact surfaces or contact surfaces shaped to condition the terminals for subsequent flip-chip packaging.
• The probe tips are generally non-compliant and can be mounted on a test head using either a compliant or non-compliant mounting. In either case, the probe tips can be used to flatten individual terminals and improve overall planarity of the terminals of a device. The improved planarity in turn improves the integrity of interconnect joints if the device is subsequently packaged in a flip-chip package.
• The probe card can be a printed circuit board (PCB) or an interconnect card suitable for flip-chip packaging of the device being tested. For these types of probe cards, the probe tips can be contact pads of the PCB or interconnect card or can be contact bumps on the PCB or interconnect card.
• Preferably, each probe tip has a flat area with a width that is at least half as wide as the corresponding terminal. To permit probing at different temperatures, the probe tips may have sizes that depend on distances from a center point of the terminal pattern so that the probe tips can be aligned to contact the terminals on the wafer despite differential thermal expansion of the probe card relative to the wafer.
• Another specific embodiment of the invention is a probe card for electrical testing of a device. The probe card includes a first substrate adapted for mounting on test equipment, a receptacle mounted on the first substrate, and a second substrate in the receptacle. The second substrate, which can be an interconnect substrate suitable for a flip-chip package containing the device, includes probe tips in a pattern that matches a pattern of terminals on the device. The receptacle generally permits quick replacement of the second substrate with a third substrate having probe tips in a pattern that is the same or different from the pattern of probe tips on the second substrate.
• Yet another specific embodiment of the invention is a process that includes: bringing probe tips into contact with terminals on a devices; using the probe tips to deform the terminals to improve planarity of the terminals; and electrically testing the device through electrical connections of the probe tip to the terminals. For the desired electrical contact and deformations, each probe tip can use a flat area to contact and flatten a corresponding one of the terminals.
• The probe tips are generally non-compliant and can be the bumped or unbumped contact pads of a substrate such as a printed circuit board or an interconnect substrate for a flip-chip package. The process can further include packaging the device in a flip-chip package using an interconnect substrate that is substantially identical to the substrate.
• For testing over a broad range of temperatures, the probe tips that are further from a central point of the probe card can be made wider than the probe tips that are nearer the central point. As a result, the probe tips have sizes that depend on distances from the center point, and the probe tips can be aligned to contact the terminals on the device over a range of temperatures.
• Another probing process in accordance with an embodiment of the invention uses flat or bumped contact pads on a printed circuit board for making contact to a device under test. Accordingly, fragile compliant test pins that may be cantilevered or spring loaded are not required. The process includes connecting test equipment to a printed circuit board having a set of contact pads with a pattern that matches elevated terminals on a device to be tested. The printed circuit board and the device are then brought into contact so that the elevated terminals on the device make electrical connections with the contact pads on the printed circuit board. The test equipment can then test the device via the electrical connections of the printed circuit board to the device. The printed circuit board can be substantially identical to an interconnect substrate used in a flip-chip package containing the device.
BRIEF DESCRIPTION OF THE DRAWINGS
• FIG. 1 shows conventional test equipment for wafer probing.
• FIG. 2 shows a conventional flip-chip package containing defects that arise from testing and uneven solder bumps.
• FIG. 3 illustrates wafer-probing equipment in accordance with an embodiment of the invention.
• FIG. 4A shows a set of metal bump before wafer probing.
• FIGS. 4B, 4C, and4D show the metal bumps ofFIG. 4A after wafer probing using illustrated probe tips in accordance with alternative embodiments of the invention.
• FIGS. 5A and 5B show plots illustrating the distribution of bump heights for device before and after a wafer probing process in accordance with an embodiment of the invention.
• FIGS. 6A and 6B are perspective view of a probe card having integrated metal on pad probes in accordance with an embodiment of the invention.
• FIGS. 7A and 7B are perspective view of a probe card in accordance with an embodiment of the invention in which metal on pad probes are on a replaceable printed circuit board or interconnect substrate.
• FIGS. 8A and 8B illustrate a probe card that maintains alignment with a device under test over a range of temperatures.
• Use of the same reference symbols in different figures indicates similar or identical items.
DETAILED DESCRIPTION
• In accordance with an aspect of the invention, a wafer probing process for electrical testing of a device fabricated on a wafer also conditions the terminals on the device to improve uniformity of the heights of the terminals. The good devices when separated from the wafer are thus in better condition to provide reliable bonds to an interconnect substrate in a flip-chip package or to a circuit board when the chip is not packaged but is assembled in a “chip-on-board” application. The wafer probe can employ a probe card that is substantially identical to all or part of a printed circuit board or an interconnect substrate to which the device will be attached. Probe tips on the probe card can be the flat contact pads or bumps that are the normal electrical contact structures of the interconnect substrates. Alternatively, probe tips having a desired shape and size can be formed on the probe card to provide desired deformations of the metal bumps on the devices.
• FIG. 3 is a block diagram oftest equipment300 in accordance with an exemplary embodiment of the invention.Test equipment300 includes automatic test equipment (ATE)310, atest head320, a probe card330 including metal on pad (MOP) probes340, awafer chuck350, and aprober360.Test equipment300electrically tests devices112, which are fabricated on awafer110, and in the process alsoconditions terminals114 ofdevices112 to improve the planarity ofterminals114.
• Devices112 can generally be any type of device including but not limited to a memory, a controller, a processor, an application specific integrated circuit (ASIC), or any other type of integrated circuit or separate device. Asterminals114,devices112 have metal bumps that rise above a top surface ofwafer110 by a height that is sufficient for flip-chip packaging or attachment to a printed circuit board. For current flip-chip packaging process,terminals114 typically have an average height of between about 60 μm and about 700 μm, with 100 μm as a typical average height.Terminals114 may, for example, be solder balls or composite structures containing multiple metal layers such as stacked solder balls, a copper or other metal pillar that is capped with a solder layer, a solder ball, gold, or a gold stud.
• For a probing operation that electrically tests a selecteddevice112 onwafer110, a probe card330 having MOP probes340 in a pattern that matches the pattern ofterminals114 on adevice112, is mounted ontest head320. MOP probes340 can either be metal probes directly formed on probe card330 or can include one or more a separate printed circuit board or interconnect substrate that is attached to probe card330.Wafer110, which is typically made of silicon (Si) or another semiconductor material, is then placed onwafer chuck350.Prober360 operates to position and orientwafer chuck350 so thatterminals114 for the selected device ordevices112 are aligned with MOP probes340. As an illustrative example, the following describes testing of onedevice112 at a time, but as will be apparent to those skilled in the art, multiple devices could be simultaneously tested if desired.
• Withwafer110 properly aligned,prober360 drives chuck350 up untilterminals114 on the selecteddevice112 make electrical contact with MOP probes340 and MOP probes340 begin to inelasticallydeform terminals114. ATE310 then applies electrical input signals throughtest head320 and probe card330 to theterminals114 and measures the resulting output signals from the selecteddevice112 to determine whether thedevice112 is functional and provides the required performance.
• ATE310 andprober360 can be standard test equipment that is available commercially from a variety of suppliers including Agilent Technologies, Inc., Teradyne, Inc., and LTX Corporation. ATE310 generally performs the electrical testing ofdevices112 in a conventional manner that depends on the type ofdevice112.Prober360 which controls the positioning ofwafer110 relative to MOP probes340 is preferably capable of measuring a distance between the top surface ofwafer110 and probe card330 or capable of precisely controlling an amount of upward movement ofwafer110 after the initial contact with probe card330. Alternatively, probe card330 can be moved to control the relative position ofwafer110. The ideal distance between the top surface ofwafer110 and the MOP probes340 during testing will depend on the height ofterminals114 above the surface ofwafer110 as described further below.
• In accordance with an aspect of the invention, MOP probes340 on probe card330 have limited compliancy to facilitate deformation ofterminals114 during probing. Probe card330 can, for example, be a bumped or unbumped interconnect substrate that is suitable for use in a flip-chip package containing adevice112 being packaged. Such interconnect substrates are typically made of an organic material such as polyamide or other insulating material and contain conductive traces that electrically connect bumps or contact pads on one side of the interconnect substrate to a contact pads and/or a ball grid array (BGA) on an opposite side of the interconnect substrate. Alternatively, probe card330 can be a printed circuit board or another structure on which one or more interconnect substrates are mounted. The bumps or contact pads on the interconnect substrate or substrates form MOP probes340, whichcontact terminals114 of thedevice112 for electrical testing and are able to apply sufficient pressure to cause deformation ofterminals114. The BGA or other terminals on the opposite side of the interconnect substrate provide electrical connections to thetest head320.
• Probe card330 and MOP probes340 could be one homogeneous/integrated structure or separable elements. Test heads320 are generally standard, and a base part of probe card can be designed according to that standard and attached to testhead320. However, in the illustrated embodiment of the invention, MOP probes340 can be on a separate substrate attached as a removable part of probe card330. This permits use of probe card330 with different MOP probes340 for testing different devices. A probe card330 with replaceable MOP probes340 further has the advantage of permitting quick replacement of a damaged probe tips so that downtime of ATE310 is minimized.
• Probe card330 can be rigidly mounted or spring mounted ontest head320 to provide a limited compliancy to probe card330 as a whole. The amount of compliancy can range from 0 for a non-compliant or rigid mounting up to about 15 mils or more for a spring mounting. The desired deformation or planarization ofdevice terminals114 during probing, as described further below, will generally control selection of a fixed or compliant mounting, the maximum travel distance of a compliant mounting, the number of springs or other compressible structures betweentest head320 and probe card330 in a compliant mounting, and the spring constant or modulus of the compressible structures in a compliant mounting.
• MOP probes340, which can be created using printed circuit board technology, have the advantage of being easily configured to match a specific device or multiple devices for parallel testing. In contrast, a probe card having cantilevered or spring loaded probes must typically be larger than the device to accommodate the size of the probes, and arranging the probes to match one or more devices can be complicated.
• Another advantage of compact and non-compliant MOP probes340 is their durability when compared to needle, spring, or cantilever probes used in conventional probing equipment. MOP probes340 thus maintain proper alignment without requiring adjustment and without fear of bending. MOP probes340 can also be cleaned, for example, with a brush or other mechanical cleaning techniques without damaging or misaligning the probes.
• MOP probes340 also have relatively large flat contact areas, as described further below. The flat contact areas, beside being less likely to be damaged during use and cleaning, do not have protrusions or sharp points that pick up and hold particles. As a result, MOP probes340 can continue to provide low contact resistance to the device under test even after prolonged use without clean.
• FIG. 4A illustrates a portion of adevice400 fabricated in and on asubstrate410.Device400 includesbumps420 and422 that can be solder balls or other conductive structures that act as the electric terminals. Ideally, all ofbumps420 and422 rise to the same height H above the surface ofsubstrate410, but bumps420 and422 are subject to manufacturing variations that may cause somebumps422 to differ by a distance Z1 from the standard height H. If the distance Z1 is too large for anybump422, a weak or defective joint will result during flip-chip bonding as described above in regard toFIG. 2.
• FIG. 4B illustrates how bringing aprobe card430 havingprobe tips440 that are sharp and rigid into contact with allbumps420 and422 on a device can gouge bumps420. In particular, when aprobe tip440 travels a sufficient distance to electrically contact anundersized bump422,other probe tips440 sink intolarger bumps420, creatingnarrow gouges425.Narrow gouges425 thus formed in thelarger bumps420 can trap contaminants and weaken the electrical connections to the larger bumps420. Additionally, thesharp probe tips420 do little or nothing to improve the disparities in the heights ofbumps420 and422, so that the inherent variation in the heights ofbumps420 and422 may still result in weak or defective electrical connections in a flip-chip package.
• FIG. 4C illustrates a system including aprobe card432 in accordance with an embodiment of the invention havingflat probe tips442.Flat probe tips442 preferably have a width that is at least one half of the diameter ofbumps420 and422. In one embodiment of the invention,probe card432 is a printed circuit board and probetips442 are contact pads or metal traces on a surface of the printed circuit board.Probe tip442 should be made of a metal capable of avoiding inelastic deformation while applying the forces required for inelastic deformation of the device terminals. A material such as copper is suitable forprobe tips442 when the device terminals contain a malleable material such as a solder.
• FIG. 4C showsprobe tips442 as being level with the surface ofprobe card432, but alternatively probetips442 may rise above the surface ofprobe card432 or even be recessed relative to the remainder of the surface ofprobe card432. However,probe card432 should allow the bottoms ofprobe tips442 to reach the desired separation from the top ofwafer410.
• For the probing operation usingprobe card432, a prober first driveswafer410 and/orprobe card432 so that the bottoms ofprobe tips442 contact at least some of thecorresponding bumps420, and the top surface ofwafer410 is about distance H from the bottoms ofprobe tips442. The prober then further driveswafer410 and/orprobe card432 closer by an overtravel distance Z2. This process flattensbumps420 that are height H or taller and bumps422 that are at least a height H2 (H2=H−Z2). The resultingdeformed bumps424 and426 are more uniformly of the same height H2. The tops ofbumps424 and426 thus have better planarity than dobumps420 and422, and the improved planarity can enhance the interconnect joint integrity in a flip-chip package or a chip-on-board application containing the probed device.
• For this embodiment of the invention,probe card432 can be the same as the interconnect substrates (e.g.,interconnect substrate220 ofFIG. 2) that will be used in the flip-chip packaging of the device after testing. Probetips442 then are the same as the contact pads that are soldered tobumps424 and426, for example, during a conventional reflow operation that electrically connects the device to the interconnect substrate of a flip-chip package.
• Overtravel distance Z2 generally must at least be sufficient to provide a low contact resistance at each terminal424 and426 to permit electrical testing of the device. Even a small overtravel distance Z2 (e.g., the minimum overtravel required for electrical testing) generally results in a flattening of the largest of the bumps, improving the overall planarity of the bumps and therefore improving the integrity of interconnection joints in a subsequently-created flip-chip package. Larger amounts of overtravel may provide further improvements in planarity until the overtravel distance Z2 provides some flattening of allbumps420 and422. After the point where each of thebumps420 and422 is at least partially flattened, the variations in the planarity ofbumps424 and426 depends on the variations in the planarity and the compliancy ofprobe tips442.
• FIG. 5A shows an example of adistribution510 of bump heights before probing and adistribution520 of bump heights after probing. In this example, the fabrication process creates bumps that nominally have a height and width of about 90 μm, but with a variation such that a few bumps may be as tall as about 105 μm or as short as about 75 μm. The probing process, for this example, then drives the wafer and the probe card so that the average separation between the wafer and the bottoms of the probe tips is about 80 μm. When the probe card is withdrawn, the resultingdistribution520 includes bumps taller and shorter than 80 μm because of the tolerances and because theshorter terminals422 may only undergo elastic deformations.
• FIG. 5B shows anotherexample distribution530 of bump heights before probing and a resultingdistribution540 of the bump heights after probing. Before the probing operation, the bumps have an average height near 88 μm, and the shortest bump height is about 82 μm. In the example ofFIG. 5B, the probing operation drives the wafer and the probe card until the average separation between the wafer and the bottoms of the probe tips is less than the shortest bump height before the probing. As a result, when the probe card is withdrawn, all of the bumps have heights that are shorter than the shortest of the bump heights in thepre-probing distribution530.Distribution540 thus includes shorter bump heights but is also much narrower thandistribution530, indicating that the bumps have improved planarity after probing.
• The overtravel distance Z2 used during probing may need to be limited to avoid damaging the device because compression ofbumps420 and422 can cause damaging stress on underlying portions of the device. The amount of stress introduced generally depends on overtravel distance Z2 and the structure ofbumps420.Bumps420 and422 made of a malleable material such as lead-based or eutectic solder can be inelastically deformed without creating stress that damages the underlying structure of a typical semiconductor device. Bumps containing a less-malleable solder and/or more rigid structures such as copper (Cu) pillars will tolerate smaller overtravel distance Z2 before the risk of damaging the underlying structure becomes too great.
• Another factor in choosing an overtravel distance Z2 for the probing/planarization operation is the desired deformed profile of bumps. Overtravel distances that are larger than necessary to provide good electrical contact may provide more flattening and larger flat areas at the tops ofbumps424 and426. For attachment of the device to an interconnect substrate, the flat areas at the tops ofbumps424 and426 are brought into contact with the contact pads or bumps on the interconnect substrate. A reflow process then at least partially liquefies the solder, and each flattened solder bump tends to reshape itself into a spherical shape to minimize surface tension. The flattened solder balls thus naturally extend toward the interconnect wafer during the reflow process.
• The conditioning of the tops ofbumps420 and422 during probing is not limited to flattening the top surface of the bumps. Instead, bumps420 and422 can be imprinted or coined with any desired shape.FIG. 4D illustrates an example of aprobe card434 havingprobe tips444 that extend above the surface of theprobe card434 and are smaller than the diameter of thebumps420. Probetips444 may be, for example, about 50 μm wide whilebumps420 have diameters of about 90 μm.Such probe tips444 may result from using a bumped interconnect substrate asprobe card434.
• During the probing process, probetips444 create a flattened top surface on asmaller bump426 where the flattened area ofbump426 is smaller than the area ofprobe tip444. Probetips444 however create concave top surfaces inlarger bumps428. Such concave surfaces are acceptable when the resulting cavities are neither narrow nor deep enough to entrap contaminants such as oxidation or soldering flux. The cavities can aid in the alignment of the device with a bumped interconnect substrate during the packaging process. The integrity of the soldered connections can thus be improved through improved alignment and the natural expansion of the deformed solder balls during a reflow process.
• As mentioned above in regard to testequipment300 ofFIG. 3, a probe card having probe tips in accordance with the invention can either be a single integrated structure including the probe tips or a compound structure from which a part including the probe tips can easily be removed and replaced.
• FIG. 6A shows anintegrated probe card600 in accordance with an embodiment of the invention.Probe card600 includes asubstrate610,conductive traces610, and probetips620.Substrate610, which can be a printed circuit board, is made of an insulating material on and through which conductive traces620 run. As illustrated,conductive traces620 electrically connectprobe tips630 tovias640 that lead to electrical contacts (not shown) on the side ofsubstrate610 that connects to a probe head. Probetips620, as shown in greater detail inFIG. 6B, can be carefully formed as flat-topped metal bumps on pad portions of conductive traces620. Alternatively, the pad portions ofconductive traces610 can function as probe tips as described above. In either, case the probe tips provide flat non-compliant surfaces that can be used during wafer probing to improve the planarity of the device terminals.
• An integrated probe card such asprobe card600 has an advantage in that the connections between the device being tested and the probe head (e.g., probetips630,conductive traces620, and vias640) can be optimized to provide minimum impedance, which may be important for RF circuits or high frequency testing. However, probetips630, being fixed tosubstrate610 can only be used to test devices that have terminals in a pattern that matches the pattern ofprobe tips630. Theentire probe card600 must be changed when the test equipment begins testing another type of device or ifprobe tips620 are damaged.
• FIG. 7A shows aprobe card700 in accordance with an embodiment of the invention that facilitates rapid changes or replacement of the probe tips to minimize test equipment downtime.Probe card700 includes afirst substrate710, areceptacle750, and asecond substrate760.Receptacle750 is mounted onsubstrate710, andconductive traces720 in and onsubstrate710 electrically connectreceptacle750 to vias740 leading to the electrical connections (not shown) for the test head.Substrate760 fits in or plugs intoreceptacle750 and has affixedprobe tips730. Accordingly, probetips730 electrically connect to the test equipment throughsubstrate760,receptacle750,conductive traces720, vias740, and electrical contact (not shown) on the back ofsubstrate710.
• Each ofsubstrates710 and760 can be made using conventional printed circuit technology, and in an exemplary embodiment of the invention,substrate760 is identical to an interconnect substrate used in a flip-chip package for the device to be tested.Receptacle750 can be any type of receptacle that can accommodate and provide electrical connections tosubstrate760. In the illustrated embodiment ofFIG. 7B,receptacle750 includespads725 at the ends ofconductive traces720 that match and electrically connect to terminals (not shown) on the bottom ofsubstrate760. A hinged clamp then holdssubstrate760 in place.
• An advantage ofprobe card700 is the ease with whichsubstrate760 can be changed whilesubstrate710 remains attached to the test equipment.Substrate760 can, for example, be unclamped or unplugged and then removed fromreceptacle750 without the need for unsoldering or any complicated disassembly.Substrate760 can thus quickly be replaced with a new substrate whenever test equipment switches to testing devices having a different terminal pattern (e.g., after a die shrink) or whenprobe tips730 are damaged. Such quick changes minimize the downtime of the test equipment.
• Electrical probing and testing of a wafer at extreme temperatures is sometimes desired to identify and eliminate unreliable devices, to bin or categorize devices by performance or specification standards, or to qualify a device under specific operating temperatures or application conditions. An advantage of the probe card having compact probe tips instead of cantilevered or spring pins is the improved thermal stability of the probe card. A temperature change such as heating for an at-temperature test can cause a large change in the pattern of the conventional test pins because long pins expand considerably with increasing temperature. In contrast, the pattern of the probe tips thermally expands or contracts with the expansion or contraction of the relatively small interconnect substrate.
• If the temperature conditions for probing are known, a probe card can be designed that will mechanically match up with the terminals of the device at the testing temperature. Such probe card design would take into account for the physical properties of the device, e.g., the coefficient of thermal expansion (CTE) of a silicon wafer, the CTE of the probe card, and the temperature of the testing. However, a probe card that matches a device at one temperature (e.g., room temperature) may not adequately match the device at a significantly different test temperature (e.g., 120° C. or higher). As a result, using the probe card at the higher temperature may provide a higher contact resistance or in the extreme case an open contact. Accordingly, a second probe card can be designed to match the device at the elevated temperature.
• In accordance with a further aspect of the invention, the pattern and size of probe tips on one probe card can make proper contact with the terminals of a device over a wide range of temperatures.FIGS. 8A and 8B show aprobe card810 havingprobe tips811,812, and813 that increase in size with distance from the center ofprobe card810. For probing, the center ofprobe card810 is aligned with the center of adevice820.FIG. 8A illustrates howterminals822 ofdevice820 then align withprobe tips811,812, and813 at a first temperature (e.g., room temperature.) An “at-temperature” test can be performed at an elevated temperature (e.g., at 120° C.). As a result, thermal expansion ofdevice820 may differ from expansion ofprobe card810 because of a difference in their respective coefficients of thermal expansion (e.g., a difference between the CTE of a silicon wafer and the CTE of a printed circuit board.) Normally, a differential expansion ofdevice820 would move each terminal822 relative to thecorresponding tip811,812, or813, with the amount of movement being proportional to the distance between the terminal822 and the center ofdevice820. To compensate for the differential expansion,pads811,812, and813 extend over the range of positions of thecorresponding terminals822 so thatpads811,812, and813 remain aligned withterminals822 even at the elevated temperature illustrated inFIG. 8B.
• Although the invention has been described with reference to particular embodiments, the description is only an example of the invention's application and should not be taken as a limitation. Various adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.

Claims (21)

1. A wafer probing system comprising:

a probe structure having probe tips with flat contact surfaces;

a tester electrically connected to the probe structure; and

a prober adapted to position a wafer relative to the probe structure so that the flat contact surfaces contact terminals on the wafer.

2. The system ofclaim 1, wherein each contact surface has a width that is at least one half a width of a corresponding one of the terminals.

3. The system ofclaim 1, wherein the probe structure comprises an interconnect card suitable for flip-chip packaging of a device contained in the wafer.

4. The system ofclaim 3, wherein the probe tips are contact pads of the interconnect card.

5. The system ofclaim 3, wherein the probe tips are contact bumps of the interconnect card.

6. The system ofclaim 1, wherein the probe structure comprises a plurality of interconnect cards positioned to contact a plurality of devices contained in the wafer, each of the interconnect cards being suitable for flip-chip packaging of a respective one of the devices contained in the wafer.

7. The system ofclaim 1, wherein the probe structure comprises a printed circuit board having contact pads that form the probe tips.

8. The system ofclaim 7, wherein the contact pads of the printed circuit board include metal bumps.

9. The system ofclaim 7, wherein the printed circuit board simultaneously contacts a plurality of devices contained in the wafer for parallel testing.

10. The system ofclaim 1, wherein the probe tips are non-compliant.

11. The system ofclaim 1, wherein the probe structure comprises:

a probe card;

a test head; and

a mounting that attaches the probe card to a test head.

12. The system ofclaim 11, wherein the mounting is non-compliant.

13. The system ofclaim 11, wherein the mounting is compliant.

14. The system ofclaim 11, wherein the probe card comprises a printed circuit board having contact pads in a pattern that matches a pattern of the terminals on the wafer that correspond to a device.

15. The system ofclaim 11, wherein the probe card comprises:

a first substrate;

a receptacle mounted on the first substrate; and

a second substrate in the receptacle, wherein the probe tips are on the second substrate.

16. The system ofclaim 11, wherein the probe tips have sizes that depend on distances from a center point so that the probe tips can be aligned to contact the terminals over a range of temperatures.

17. The system ofclaim 1, wherein the probe tips electrically connect the tester to the terminals and condition the terminals for flip-chip packaging.

18. The system ofclaim 17, wherein conditioning the terminals for flip-chip packaging comprises flattening individual terminals to improve planarity of the terminals.

19. A probe card for electrical testing of a device, comprising:

a first substrate adapted for mounting on test equipment;

a receptacle mounted on the first substrate; and

a second substrate in the receptacle, wherein the second substrate includes probe tips in a pattern that matches a pattern of terminals on the device.

20. The probe card ofclaim 19, wherein the second substrate is an interconnect substrate suitable for a flip-chip package containing the device.

21. The probe card ofclaim 19, wherein the receptacle permits replacement of the second substrate with a third substrate have probe tips in a pattern that differs from the pattern of probe tips on the second substrate.

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