the hybrid cooling system of the present invention enables a true universal slot architecture for a semiconductor testhead 20 , where different instruments may be universally compatible with any of the instrument slot locations within the testhead . this is made possible by providing a first cooling assembly 40 to distribute a first cooling medium proximate the instruments , and a second cooling assembly 50 for selective application of a second cooling medium to each of the instrument slot locations . referring now to fig1 , a semiconductor tester according to one form of the present invention , generally designated 10 , includes a main console 12 to house various electronic sub - assemblies that support the operation of the tester instruments . the testhead 20 couples to the console via a cable bundle 14 and serves as an extension of the tester in an effort to place the tester instruments as close to the device - under - test ( dut ) as possible . a manipulator 16 disposed adjacent the console carries and positions the testhead with respect to a handling apparatus ( not shown ), such as a packaged - device handler or wafer prober . the instrument boards adapted for installation within the testhead 20 resemble large rectangular circuit boards , with a size on the order of approximately 17 inches by 26 inches . the boards may comprise digital instuments such as waveform digitizers , digital signal generators , analog instuments such as rf signal generators and other analog waveform analyzers , memory test instrumentation or the like . because different boards require varying levels of cooling , the testhead is configured for maximum cooling flexibility while maintaining low cost . with reference to fig2 , the testhead 20 in accordance with one form of the present invention , includes a card cage area 22 for installing one or more card cages . the card cage area is configured to support respective upstream and downstream card cages 24 and 26 , depending on the instrument requirements of the user . as seen more clearly in fig3 , the card cages are formed with a sheet - metal frame and provide space for a linear air cooling path ( shown by the flow arrows ). referring to fig2 and 3 , each of the card cages 24 and 26 includes a plurality of spaced - apart slots 30 ( fig3 ) or guides that are adapted for receiving the instrument boards ( not shown ). respective inlet and outlet safety / emi shield screens 32 and 34 are disposed on each end of the card cage cooling path . in the likely event that both card cages are utilized , an optional heat exchanger 36 ( in phantom ) is employed between the card cages to improve the cooling performance for a first cooling assembly 40 , more fully described below . the first cooling assembly 40 comprises an air - cooling scheme that couples to the downstream card cage 26 and cooperates with the spaced - apart nature of the cardcage slots along the cooling path to effect a desirable level of air cooling . the first cooling assembly includes a remotely mounted impeller 42 , or fan , coupled through a flexible accordion - shaped duct 44 to a transition duct section 46 . the transition section attaches to the downstream card cage outlet screen 34 . ambient air is the preferred coolant medium for the first cooling assembly , being drawn serially through the card cages 24 and 26 , along the cooling path between the boards , and into the transition and flexible ducts 46 and 44 , where it is discharged from the tester 10 back into the outside operating environment . preferably , the fan and ducting are configured to provide an airflow rate of approximately 1700 cfm distributed uniformly across the boards to provide approximately 12000 watts of cooling . while the first cooling assembly 40 described above adequately cools instrument boards that dissipate relatively low power in the range of around 200 to 500 watts , high power digital instrument boards that dissipate power above 500 watts are not well served by this cooling technique alone . in an effort to enable the use of high power instrument boards within the testhead 20 , and to provide the flexibility for allowing the installation of these boards into any slot , the testhead provides for a second cooling assembly 50 to selectively provide cooling by a second cooling medium , such as liquid . referring now to fig3 , the second cooling assembly 50 provides a source of liquid coolant for those boards equipped with cold plates or liquid cooling modules . an example of a liquid cooling module , or lcm , is described in u . s . pat . no . 5 , 871 , 042 , assigned to the assignee of the present invention , and expressly incorporated herein by reference . further referring to fig3 , the second cooling assembly 50 includes a liquid coolant source , such as a chiller 52 ( fig2 ), to pump a coolant to and from the testhead 20 . the chiller preferably comprises a closed scheme having a pump ( not shown ) without rotating seals , such as a submerged or magnetically driven design . while the chiller heat load capacity may very , depending on the number of high - power boards employed in the testhead , the inventors have determined that for many applications a heat load capacity of approximately 18 kw with a flow rate capacity of approximately 30 gpm provides sufficient cooling . the chiller 52 , as noted above , pumps a liquid coolant such as hfe , through a flexible metal hose 54 to a rear manifold assembly 56 . hfe - 7100 , available from the 3m corporation , is a preferred coolant based on its dielectric and low toxicity properties . the rear manifold assembly 56 is formed of an hfe compatible metal , such as copper , brass , stainless steel or aluminum , and spans the lateral length of the downstream card cage 26 . the manifold includes a pair of elongated internal chambers ( not shown ) that distribute and discharge the coolant flow to and from an array of access and discharge ports terminated by respective quick - disconnect fittings 58 proximate each of the cardcage slots . one of the manifold chambers provides freshly chilled coolant to one set of fittings , while the other chamber provides a discharge area for heated coolant that has already circulated through the assembly . a flexible metallic effluent hose 60 returns the discharged coolant back to the chiller 52 where it is chilled and recirculated back into the second cooling assembly 50 . with continued reference to fig3 , the second cooling assembly 50 further includes a front manifold assembly 62 configured similarly to the rear manifold assembly 56 . the front manifold assembly couples to the coolant inlet hose 54 via a manifold - to - manifold line 64 to provide a similar array of access and discharge ports for the upstream cardcage 24 . in effect , each of the card cage slots has the availability of liquid cooling inlet and discharge lines for an instrument board cold plate or lcm — if needed . in circumstances where only one of the two card cages 24 and 26 are needed for instrument installations , the optional heat exchanger 36 may be omitted to further lower costs . fig4 illustrates such a configuration . in this application , the impeller 42 may be replaced by a pair of vertically aligned axial fans 70 and 72 , having an approximate diameter of around 13 inches to provide sufficient cooling airflow . a plenum space 74 separates the fans from the upstream card cage 24 to allow for maximum airflow distribution . prior to operation , the user is free to arrange the tester instrument boards in any desired configuration without the need to customize liquid cooling lines for a particular high - power board type . moreover , each slot 30 need not include a pre - installed lcm assembly . for boards that operate sufficiently with air cooling , that feature is provided automatically by the first cooling assembly 40 . where liquid cooling is needed , the availability of the second cooling assembly 50 to provide a liquid cooling line proximate each cardcage slot allows for a minimal plumbing modification to activate liquid cooling for that slot . as a result , the cost of liquid cooling is scaled in line with actual need and usage . those skilled in the art will appreciate the many benefits and advantages afforded by the present invention . of significant importance is the availability of selectable cooling modes for each slot in the testhead , allowing for a true universal slot architecture . this provides the user with flexibility in assembling both low power and high power instrument boards consistent with a single platform tester approach to testing semiconductor devices . a further advantage provided by the present invention involves the low cost that is achievable by scaling the liquid cooling requirements with the number of boards actually requiring liquid cooling . additionally , because of the implementation of the air cooling assembly for all of the boards , the liquid cooling assembly requirements may be lower than a stand - alone liquid cooling system . as a result , lower performing components and sub - assemblies may be utilized , correspondingly reducing costs . while the invention has been particularly shown and described with reference to the preferred embodiments thereof , it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention .