US 20050030052 A1
An innovative chip testing system and method includes controlling temperature and condensation during testing. Coarse temperature is controlled by providing a desired fluid flow rate and fluid temperature to a cold plate. Fine temperature control is provided by a feedback loop which controls the power dissipation of cartridge heaters installed within the cold plate. Condensation control is provided by insulating various components of the system, manipulation of dry compressed air in enclosures to reduce surface dew point temperatures, usage of cartridge heaters in a card backside stiffener plate, and by providing a heatsink assembly which prevents condensation on the insulation.
1. A system for testing integrated circuits, comprising:
a source of chilled fluid;
a cold plate connected to the source of chilled fluid such that the temperature of the cold plate is reduced by the chilled fluid;
at least one heater connected to the cold plate such that the at least one heater supplies heat to the cold plate;
wherein the cold plate is placed in proximity to the integrated circuit to change the temperature of the integrated circuit.
2. The system of
3. The system of
4. The system of
5. The system of
wherein the surface of the cold plate is smaller in area than the surface of the integrated circuit; and wherein the surface of the cold plate area is as large as possible while still being smaller than the surface of the integrated circuit.
6. The system of
7. A method of testing an, integrated circuit, comprising the steps of:
supplying a chilled fluid to a cold plate, the cold plate being movable to be in contact with an integrated circuit;
supplying energy to at least one heater, the at least one heater being connected to heat the cold plate;
wherein the integrated circuit is maintained at a first temperature by varying energy supplied to the at least one heater.
8. The method of
9. The method of
10. The method of
11. The method of
12. The method of
wherein the surface of the cold plate is smaller in area than the surface of the integrated circuit; and wherein the surface of the cold plate area is as large as possible while still being smaller than the surface of the integrated circuit.
13. The method of
1. Technical Field
The present invention relates generally to integrated circuit (IC) processing and testing, and particularly to temperature and condensation control in a chip tester.
2. Description of Related Art
High performance processor chips or integrated circuits (ICs) often must be tested and subsequently sorted based on their performance and matched to a given class of machine. Individual chips are tested prior to installation on modules in machines in order to identify chip failures, allowing higher machine manufacturing productivity and improving product quality. Test equipment developed to support performance, productivity and quality is usually required to meet functional test limits defined by several boundary variables such as chip/module voltage, clock speed, power dissipation, and temperature.
Chip testing often requires the chips to be cooled to low temperatures. For example, some processor chip test requirements establish a need for cooling devices to temperatures as low as −20 C. Accurate temperature control over a wide range of temperatures during chip testing processes allows verification of whether a processor functions, and more specifically how many of its processor cores function. It also detects more of the faulty chips early in the manufacturing process. For example, two conductors can be touching each other with sufficient force that an electrical connection is made, even though the parts are not mechanically interconnected. Such a part may pass tests and reach a customer, but will fail at a higher frequency rate than is acceptable. If a chip is tested at low temperatures, this marginal interconnection may separate, and thereby identify the faulty part.
Similarly, some chip failure modes are found only at elevated temperatures, while other early use failures which might not have occurred until after reaching a customer may be found during testing at elevated temperatures. Varying temperature testing also allows more precise determination of chip speed, helping to minimize the guardband (i.e., extra margin) that is put into the acceptance criteria for a good chip, allowing an increase in the manufacturing yield.
Alternatives to a precisely controlled thermal system result in less thermal control and hence fewer faults are found, and more guardband must be allowed to ensure the chip will function in the customer environment.
However, testing at varying temperature also raises certain problems. For instance, the manufacturing environment in which some chip testers operate contain atmospheric conditions with a dew point of 17 C, for example. Clearly, any tester surfaces exposed to this environment and operating at −20 C contains mechanical components with surface temperatures that are well below the dew point and will cause condensation of the water vapor in the air on those surfaces. Condensed water within the machine could cause electrical shorts, destroying the tester and/or the tested chip or module. In addition, condensed water escaping from the tester would disrupt manufacturing operations due to safety and related issues.
The present invention is directed to a temperature and condensation control system for chip testing environments. In a brief summary of a preferred embodiment, multiple features are implemented to incorporate a highly selectable temperature control system while reducing exposure of test equipment and chips to condensation from the environment, particularly at low test temperatures. These features preferably include passing a refrigerated liquid (e.g., 0 C to −20 C) at a predetermined flow rate through a cold plate which is spring loaded against the chip to be tested. The temperature of the chip is measured by OCTS (On Chip Temperature Sensors) and is controlled via a feedback loop which responds by modulating the power applied to cartridge heaters embedded in the cold plate. Other features of the temperature control aspect of the innovative testing system are described further below.
In order to reduce problems caused by condensation in the innovative testing system, water vapor in the ambient air is prevented from condensing on tester mechanical components. The chilled fluid passed through the cold plate causes a decrease in temperature of hoses, fittings, manifolds, the cold plate itself, and related mechanical parts so that frost formation or liquid water condensation occurs unless measures are taken. An enclosure is used around the test equipment and certain subparts thereof (e.g., the nest) such that dry compressed air may be injected into the enclosure to maintain atmospheric relative humidity within the enclosure at levels preventing condensation on surfaces. Compressed air inlets are positioned so that a rotational air flow pattern is preferably established, covering the entire test surface more efficiently and allowing prevention of condensation with less dry air. Such air flow is also preferably limited to the times when chips are actually in test mode. Other features of the condensation control aspect of the present innovative testing system include limiting air flow to enclosures containing low temperature components, closed cell foam pipe insulation to control condensation and frost formation on coolant hoses and fittings, and polymer coolant hose clamps to provide thermal properties preventing condensation. Additionally, one or more cartridge heaters are preferably inserted in the card backside socket retention stiffener plate to control condensation on the bottom of the card, and a metal heat sink with hose insulation retention devices control condensation at locations where insulated hoses touch fitting junctures. Further features and details are described below.
The novel features believed characteristic of the invention are set forth in the appended claims. The invention itself, however, as well as a preferred mode of use, further objectives and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:
The present invention is described with reference to the several figures.
Cold plate 304 also includes cartridge heaters 306 which are preferably connected to increase the temperature of cold plate 304 in response to a feedback loop. The feedback loop is intended to detect the temperature of the chip at various locations by the use of OCTS embedded in the chip, and this information is used to maintain a constant or desired temperature by varying the power supplied to the cartridge heaters. Broadly speaking, for a given test condition chilled fluid is flowed through the cold plate at a constant flow rate and temperature to provide gross temperature control, while cartridge heaters 306 respond to the OCTS (OCTS cables 312 shown) via a feedback loop to provide automatic fine sense temperature control of the chip. For the next test condition, the fluid temperature and/or flow rate can be adjusted, and the feedback control continues. The cold plate is preferably loaded against the chip, its surface flattened to provide good contact between plate and chip surface. Other specific features of the cold plate are further described below.
Fluid lines 308 supply chilled fluid to the cold plate 304 while air cylinder 310 provides pneumatic pressure driving the cold plate suspension against the module, electrically engaging the processor under test to the pogo-pin socket and ultimately to the processor card.
Nest unit 300 preferably includes an enclosure surrounding the socket for the chip and cold plate 304. FIGS. 4A-C show various views of the nest and card.
Backside heaters 418 are preferably positioned on the backside of the card 422 to prevent temperature there from becoming low enough that frost or condensation forms. This is discussed further below. Cold plate heaters 420 of the cold plate are also shown. These respond to the feedback loop using information from the OCTS, and provide temperature control to the chip.
The functions of the temperature control system of the innovative testing unit include several features. A functional tester, referred to as ERIF (Early Run-in Functional), is employed in manufacturing to ensure the quality and robustness of processors before they are placed in a server and shipped to customers. Many processors have multiple processor cores on a single silicon chip. The goals of such testing includes verifying how many of the processor cores are good, sorting the processors into different buckets so they can be used in the corresponding speed rated product, and providing some burn-in of the chips to find early faults.
The innovative testing equipment can perform such tests in 1-4 hours for each chip. During this time, differing software applications are run on the chip. Each of the applications results in differing power dissipation within the chip. It has been found that different modes of chip failure are evidenced more easily at differing temperatures. Additionally, the speed sorting of the chips must be done at the temperature they will run at in the actual product. Hence, the innovative ERIF testing unit must be able to control the chip under test to a wide range of pre-specified temperatures at differing parts of the test.
To achieve these requirements, the innovative thermal control system comprises a low temperature chiller, a nest assembly, and a thermal control system. The chiller provides a low temperature source of liquid. For instance, flow rates up to 1.6 gallons/minute of a 50/50 mixture of ethylene glycol and water at a temperature of −20 C can be provided. The cold liquid flows from the chiller through insulated lines to the thermal control box (see
The chilled fluid flows through the cold plate, which in a preferred embodiment is made of copper for its heat conducting properties. The air-actuated cylinder drives the copper cold plate to the down position, electrically engaging the processor under test to the socket of the processor card. This completes the thermal path from the chip to the cold plate to the cold fluid within the cold plate.
A thermal feedback loop, with input from the OCTS, maintains the required temperature setpoints by varying the power dissipated at the cartridge heaters which are mounted into the cold plate. In a preferred embodiment, there are two such cartridge heaters positioned to provide even heat distribution to the cold plate and ultimately the chip. The interface resistance between the cold plate and chip is minimized by a gimbaled cold plate, high pressure spring load, and controlled flatness of the cold plate. Additionally, as previously described, helium is injected at the cold plate/chip interface to replace the air that would normally occupy small spaces between the two surfaces, improving the interface thermal resistance. The cold plate pedestal must be smaller than the surface of the chip to mitigate cracking of the chip when pressure is applied. However, in preferred embodiments, the cold plate is as large as possible to minimize temperature gradients. Temperature gradients are also minimized by the preferred choice of copper for the cold plate material, which has a high thermal conductivity.
Testing of the chip is preferably performed at multiple different temperature setpoints. These temperatures are measured preferably using the OCTS of the chip itself. A predetermined fluid flow rate is chosen for each setpoint and provided by the chiller, manifold, and series of valves with associated interconnections. The setpoint temperature is preferably maintained by a temperature feedback loop which controls how much power is dissipated by the cartridge heaters in the cold plate. Hence, the chilled fluid flow provides large scale or gross temperature control during testing, while the cartridge heaters controlled by the feedback loop provide small scale or fine temperature control. Temperature control of the cartridges is preferably based on a proportional, integral, derivative (PID) loop.
When a given phase of the testing is completed, the flow rate and/or temperature of the chilled liquid is changed for the next phase to adjust the OCTS temperature to within range of the next desired setpoint. This next phase is typically associated with a different software application, chip voltage, and/or chip frequency, which all contribute to a different chip power dissipation level. The new desired temperature setpoint is maintained via the temperature feedback loop previously described.
This process is simply described in the flow chart of
Having both gross and fine control of the temperature is valuable because large power swings or spikes occur during a give part of the test, while even larger swings occur between different phases of a test. Hence, the innovative testing apparatus provides temperature control when power dissipation varies dramatically, e.g., 20 W up to 225 W. Note that other testing procedures, such as a typical burn-in, employ significantly different temperature control schemes because the power and temperature setpoints remain largely constant.
The innovative testing system also preferably includes measures for controlling and reducing condensation on the various surfaces and equipment. The condensation control apparatus prevents water vapor in the ambient air surrounding the unit from condensing on tester mechanical components. The liquid chiller reduces the temperature of the heat transfer fluid significantly below dew point. In turn, the chilled fluid lowers the temperature of hoses, fittings, manifolds, the cold plate and related mechanical parts so that frost formation or liquid water condensation occurs on the surfaces unless prevented. In a preferred embodiment, heat transfer and thermodynamic properties of insulation, compressed air, structural polymers, and metal are used to prevent frost or liquid condensation from forming on tester components.
In preferred embodiments of the innovative testing system, some or all of the following features or groups of features are implemented.
Dry compressed air is injected into the enclosure(s) to maintain the atmospheric relative humidity within the enclosure at levels preventing condensation on metal and polymer surfaces. Compressed air inlet nozzles are positioned so as to establish a rotational air flow pattern within a cold plate housing ensuring dry air contacts all sides of the low temperature cold plate. The air flow of the system is also controlled by an air flow control system that limits the duration of compressed air flow to time periods when chips are actually in test.
In addition to the compressed air features, the room ambient air flow paths into enclosures containing low temperature components are minimized, reducing the necessary dry compressed air flow rates. Closed cell foam pipe insulation is used to control condensation and frost formation on coolant hoses and fittings, while a polymer coolant hose clamp preferably provides thermal properties preventing condensation.
Additionally in preferred embodiments, cartridge heaters are placed in the card backside socket retention stiffener plate, to control condensation on the bottom of the card (see
As described previously, the primary elements of the cooling system used to control the temperature of the chip under test are the liquid coolant chiller, the thermal control box, the nest assembly, and hoses that transport coolant from the chiller to the TCB, from the TCB to the nest, and from the nest back to the chiller. The chiller cools a heat transfer liquid to, e.g., −20 C for the specific processor test application, and an internal pump provides pressure to move the chilled fluid through components in the fluid loop and returning it to the chiller for reuse. The devices in the chilled fluid loop require mechanisms to prevent frost or condensation formation. In a preferred embodiment, from the chiller the coolant moves through ethylene propylene diene/rubber (EPDM) hoses to the TCB. The TCB preferably controls coolant flow throughout the cooling fluid loop, compressed air flow to the nest air cylinder, cold plate to chip interface gas flow, cartridge heater function, and temperature control functions. From the TCB, chilled fluid passes through EPDM hose to the nest. The nest contains the metal cold plate which absorbs heat from the chip in test and transfers it to the chilled fluid via a set of internal channels. The cold plate position is controlled by an air cylinder which linearly moves the cold plate from an open position which provides an open space through which the module with the chip is manually installed, to a closed position in which heat is transferred from the chip to the cold plate. After the heat transfers from the tested chip to the cold plate and then to the coolant fluid, the fluid is transported by the insulated hose to the TCB and finally back to the chiller where it is again cooled for reuse.
Humid air access paths from the tester room ambient through the Lexan™ housing to the dry air space near the cold plate is limited. EPDM hose and its insulation passes through holes on each side of the housing, as shown in
When the cold plate is lowered by the air cylinder into its operational position on the chip, the cold plate suspension assembly moves with respect to the Lexan™ housing to its final position. After all components in the assembly have reached final position, each hole in the Lexan™ housing is concentrically aligned with each fitting that channels coolant through the cold plate and with the fitting insulation. This concentricity provides two advantages: first, it maintains the slight compression of the insulation by the housing hole at the entire perimeter interface of Lexan™ and insulation which effectively blocks ambient air access; second, concentric alignment of holes and insulation balances the external forces acting on the cold plate, which improves cold plate/chip thermal interface.
In a preferred embodiment the clamp is counter-bored to provide a tight interface to the hose insulation and to block humid air access to below dew point hose and clamp components, thereby preventing condensation. Preferred embodiments fix the location of the coolant hoses with a hose clamp attached to a bulkhead within the tester. The preferred material for such a clamp is a structural polymer with a low thermal conductivity to mitigate condensation.
Providing dry compressed air to the housing inlet nozzles requires a control function to maintain a given air flow rate of dry air only during the time period when the air cylinder has lowered the cold plate to the chip for test. A control algorithm and the TCB modify coolant flow and activate cartridge heaters in the cold plate and backside card stiffeners to elevate nest component temperatures above the dew point before the cold plate is raised to the open position by the air cylinder. To conserve the compressed air injected into the housing, it is only released to the nest inlets when the air cylinder is activated to lower the cold plate. The control system for this function is shown in
Dry compressed air at preferably 85-90 psi is provided to the two compressed air inlets 702. One inlet directs air routed from the TCB to the nest air cylinder fitting supplying compressed air that moves the cold plate to the down position. The other inlet feeds air through the control system to the nest inlet nozzles. The air provided to the nest is also called purge air due to its function of purging humid air from within the Lexan™ housing. From this second inlet location, purge air passes through a manual shutoff valve 704 used for system maintenance, air flow calibration or emergencies. The air is then regulated to 80 psi by pressure regulator 706, and then sent to the pneumatic control valve 708. The normally closed pneumatic valve is opened by air pressure in air line 712. Airline 712 is connected to “T” fitting 714 which passes air from the TCB at inlet 702 through 714 to the air cylinder port driving downward movement of the cold plate and related hardware. Pneumatic control valve 708 allows purge air to pass through the air flow meter 710 when air pressure is in line 712 and the air cylinder is actuated downward.
During calibration, this meter 710 is set to the flow rate needed to support nest condensation control. The meter also serves as a diagnostic tool to verify dry air flow. From the flow meter, the purge air is divided by a “T” fitting to support flow to both the front and rear inlet nozzles attached to the nest Lexan™ housing (not shown). Air flow control components are connected as necessary by flexible polymer tubing.
In a preferred embodiment, compressed air is provided to the enclosure only when the cold plate is in the down position. As described above, this is preferably achieved by a valve positioned so that when air is supplied to the cylinder to move it to the down position, air is also supplied to the purge lines, flooding the enclosure with dry compressed air. When the cylinder is not in the down position (i.e., when the chip is not in test and the cooling system is not engaged), air is cut off from the enclosure, conserving dry compressed air so it is only provided when needed or during test, as appropriate.
In addition, the coolant hose insulation preferably extends only to the top of the mounting plate 813. The hose preferably mates with a polymer barbed fitting at this point. The mounting plate 813 ensures that insulation mates with the TCB to prevent humid air from gaining access to the low temperature hose or fitting, causing condensation. For serviceability, it is preferable to provide a mechanism to facilitate removal of the insulation and hoses from the TCB. To retain the hose insulation, a polymer cavity plate contains properly sized orifices that establish a tight boundary with the hose insulation. The mounting plate is retained to the top of the TCB using the same nut that secures the barbed hose fittings to the TCB. Retention of the hose insulation against the mounting plate coupled with the close fitting orifices in the cavity plate provide the needed condensation control while simultaneously enabling disassembly for service.
The description of the present invention has been presented for purposes of illustration and description, and is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art. The embodiment was chosen and described in order to best explain the principles of the invention, the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.