Abstract:
An electromechanical apparatus for testing integrated chips includes a chip holding subassembly, a power converter subassembly, and a temperature regulating subassembly, which are squeezed together in multiple sets, by respective pressing mechanisms. One benefit which is achieved with this electromechanical apparatus is that by pressing the temperature regulating subassembly against the chip holding subassembly, heat can be added/removed from the chips by conduction; and thus the temperature of the chips can be regulated accurately. Another benefit which is achieved with this electromechanical apparatus is that by pressing the power converter subassembly against the chip holding subassembly, the distance between the chips that are tested and the power supplies for those chips is made small. Consequently, the chip voltages can easily be kept constant while the chip power dissipation changes rapidly as the chips are tested. Another benefit of this electromechanical apparatus is that physical contact between the three subassemblies is made quickly, and is broken quickly, by the pressing mechanisms. This quick quick-connect/quick-disconnect feature is very useful in a chip testing environment.

Description:
RELATED CASES 
     The present invention, as identified by the above docket number and title, is related to three other inventions. Patent applications on all of these inventions were filed concurrently on Feb. 23, 2000; and they have one common Detailed Description. These three related inventions are identified as follows: 
     1. “SLIDING SPRINGY MECHANISM THAT OPENS AND CLOSES PRESSED ELECTRICAL CONTACTS WITH A FORCE WHICH IS NEARLY CONSTANT OVER A RANGE OF CLOSED POSITIONS” having U.S. Ser. No. 09/511,792; 
     2. “PIVOTING SPRINGY MECHANISM THAT OPENS AND CLOSES PRESSED ELECTRICAL CONTACTS WITH A FORCE WHICH IS NEARLY CONSTANT OVER A RANGE OF CLOSED POSITIONS” having U.S. Ser. No. 09/511,791; 
     3. “PLANAR SUBASSEMBLY FOR TESTING IC CHIPS HAVING FACES WITH PRESSED ELECTRICAL CONTACTS THAT CARRY ALL POWER AND SIGNALS FOR THE CHIPS” having U.S. Ser. No. 09/511,790. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to electromechanical apparatus for testing integrated circuit chips. More particularly, the present invention relates to chip testing apparatus in which a chip holding subassembly, a power converter subassembly, and a temperature regulating subassembly are squeezed together in multiple sets by respective pressing mechanisms which exert a substantially constant force despite several dimensional variations in the apparatus. 
     Typically, a single IC chip contains more than one-hundred-thousand transistors. Thus, a manufacturer of IC chips must test the chips to ensure that they operate properly before they are sold to a customer. This testing is usually accomplished as follows. 
     Initially, one group of chips that are to be tested are placed in respective sockets that are mounted on several printed circuit boards. Each printed circuit board has edge connectors on one edge of the board; and those connectors carry test signals, as well as DC electrical power, for the chips that are in the sockets. 
     After the chips are placed in the sockets, the printed circuit boards are inserted into fixed slots in an electromechanical apparatus where the chip testing occurs. As each printed circuit board is inserted into a slot, the edge connectors on the board plug into mating connectors that are provided in the slot. 
     Usually, several printed circuit boards are held in the slots, spaced-apart from each other, in a horizontal row. Alternatively, several printed circuit boards can be held in the slots, spaced-apart from each other, in a vertical column. 
     Multiple signal lines are provided in the chip testing apparatus which extend from the connectors in the slots to a test signal controller. This controller tests the chips by sending signals to the chips and receiving responses from them. Also, electrical power lines are provided in the chip testing apparatus which extend from the connectors in the slots to one or more power supplies. 
     Often it is desirable to perform a “burn-in” test on the chips wherein the chips are held at a high temperature while they are tested. In the prior art, that was done by enclosing the chip testing apparatus in an oven and providing fans in the enclosure which circulate hot air past the chips while they are tested. 
     However, one drawback with the above prior art chip testing apparatus is that the temperature at which the chips are tested cannot be regulated accurately. This inaccuracy is caused, in part, by variations in the temperature and velocity of the air which flows past each of the chips. Also, the inaccuracy is caused by variations in chip power dissipation which occurs while the chips are being tested, and this problem gets worse as the magnitude of the power variations increase. 
     One prior art mechanism which accurately regulates the temperature of IC chips in a product where the chips are permanently held, such as a computer, is described in U.S. Pat. No. 4,809,134, by Tustaniwskyj, et al, which is entitled “Low Stress Liquid Cooling Assembly”. That assembly includes a hollow jacket which carries a liquid coolant and the jacket contacts each IC chip. Thus the temperature of the chips is regulated accurately by conduction. 
     However, in the above &#39;134 assembly, the jackets are held in place on the chips by a beam; and several bolts must be removed before the jackets can be lifted off the chips. To use such an assembly in a chip-testing environment would be impractical because there, the jackets need to be repeatedly taken off one set of chips and put on another set of chips. 
     Also, another drawback with the above prior art chip testing apparatus is that due to the row/column arrangement of the printed circuit boards, a large distance inherently exists between the chips that are tested and the power supplies for those chips. Due to those large distances, parasitic resistances, parasitic inductances and parasitic capacitances are inherently large; and thus, the more difficult it becomes to keep the chip voltages constant while chip power dissipation changes rapidly as the chips are tested. 
     Accordingly, a primary object of the invention is to provide an improved electromechanical apparatus for testing IC chips which avoids the above drawbacks. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention, as claimed, covers the structure of an electromechanical apparatus for testing integrated circuit chips wherein a chip holding subassembly, a power converter subassembly, and a temperature regulating subassembly are squeezed together in multiple sets by respective pressing mechanisms. A major benefit which is achieved with this electromechanical apparatus is that by pressing the temperature regulating subassembly against the chip holding subassembly, heat can be added/removed from the chips by conduction; and thus the temperature of the chips can be regulated accurately. Another major benefit which is achieved with this electromechanical apparatus is that by pressing the power converter subassembly against the chip holding subassembly, the distance between the chips that are tested and the power supplies for those chips is made small. Consequently, the chip voltages can easily be kept constant while the chip power dissipation changes rapidly as the chips are tested. 
     In one embodiment, the chip holding subassemblies and the power converter subassemblies and the temperature regulating subassemblies are held by a frame in a vertical stack. Above each chip holding subassembly lies its power converter subassembly, and below each chip holding subassembly lies its temperature regulating subassembly. To press the subassemblies together, the pressing mechanisms move the temperature regulating subassemblies upward where they initially contact their chip holding subassembly, and then, the pressing mechanisms move both the chip holding subassemblies and the temperature regulating subassemblies further upward to a “closed” position where they contact their power converter subassembly. 
     All of the chips are tested while the subassemblies are pressed together in the closed position. Thereafter, when the testing is complete, the pressing mechanisms move the temperature regulating subassemblies and the chip holding subassemblies downward to an “open” position. There, the chip holding subassemblies are simply slid out of the frame, loaded with a new set of chips to be tested, and slid back into the frame. 
     All of the pressing mechanisms operate concurrently in response to a single actuator. Consequently, physical contact between the temperature regulating subassemblies and the chip holding subassemblies and the power converter subassemblies is made quickly, and is broken quickly. This quick connect/quick disconnect feature is very useful in a chip testing environment. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A shows a pictorial view of the top portion of an electromechanical apparatus for testing IC chips which constitutes one preferred embodiment of the present invention. 
     FIG. 1B shows a pictorial view of the bottom portion of the electromechanical apparatus of FIG.  1 A. 
     FIG. 1C shows a pictorial view of a chip holding subassembly, a power converter subassembly, and a temperature regulating subassembly which are held in multiples in a vertical stack within the electromechanical apparatus of FIGS. 1A and 1B. 
     FIG. 2 is a schematic diagram of the three subassemblies of FIG. 1C, plus a pressing mechanism within the electromechanical apparatus of FIGS. 1A and 1B, which squeezes the three subassemblies together. 
     FIG. 3 shows the pressing mechanism of FIG. 2 is in a closed position where the angle between two arms in the pressing mechanism can range from A 1  to A 2 . 
     FIG. 4 shows various forces which occur in the pressing mechanism of FIG. 2 when its arms are at the two different closed positions which are shown in FIG.  3 . 
     FIG. 5 shows a set of equations which are derived from FIG.  4  and which relate certain parameters in the pressing mechanism of FIG.  2 . 
     FIG. 6 shows a set of steps which use the equations of FIG. 5 to select parameters for the pressing mechanism of FIG.  2 . 
     FIG. 7 shows a numerical example of the parameters that are selected by the steps in FIG.  6 . 
     FIG. 8 shows how two forces Fx and Fy vary over a range of angles in the pressing mechanism of FIG. 2 when that mechanism has the parameters in FIG.  7 . 
     FIG. 9 shows a second embodiment of a pressing mechanism which has some similarity to the pressing mechanism of FIG.  2 . 
     FIG. 10A shows a third embodiment of a pressing mechanism which has some similarity to the pressing mechanism of FIG.  2 . 
     FIG. 10B shows a fourth embodiment of a pressing mechanism which has some similarity to the pressing mechanism of FIG.  2 . 
     FIG. 11 shows a fifth embodiment of a pressing mechanism which has some similarity to the pressing mechanism of FIG.  2 . 
     FIG. 11A shows an equation which relates certain parameters in the pressing mechanism of FIG.  11 . 
     FIG. 11B illustrates the various parameters which are in the equation of FIG.  11 A. 
     FIG. 12 shows a sixth embodiment of a pressing mechanism which is substantially different than the pressing mechanisms of FIGS. 2,  9 ,  10 A,  10 B, and  11 . 
     FIG. 13 shows various forces which occur in the pressing mechanism of FIG. 12 when its arms are at two different closed positions. 
     FIGS. 14A and 14B show a set of equations which are derived from FIG.  13  and which relate certain parameters in the pressing mechanism of FIG.  12 . 
     FIG. 15 shows a set of steps which use the equations of FIGS. 14A and 14B to select parameters for the pressing mechanism of FIG.  12 . 
     FIG. 16 is a numerical example of the parameters that are selected by the steps in FIG.  15 . 
     FIG. 17 shows how two forces Fx and F 2   y  vary for a range of angles in the pressing mechanism of FIG. 12 when that mechanism has the parameters of FIG.  16 . 
     FIG. 18 shows a seventh embodiment of a pressing mechanism which has some similarity to the pressing mechanism of FIG.  12 . 
     FIG. 19 shows an eighth embodiment of a pressing mechanism which has some similarity to the pressing mechanism of FIG.  12 . 
    
    
     DETAILED DESCRIPTION 
     With reference now to FIGS. 1A, IB, IC and  2 , one preferred embodiment of the present invention will be described. This embodiment is an electromechanical apparatus  10  for testing IC chips on multiple printed circuit boards which are held in a vertical stack and which have faces with pressed electrical contacts that carry all power and all signals to/from the chips. 
     The apparatus  10  is comprised of six different types of subassemblies  11 - 16 ; and each subassembly includes several components. All of the components of any one particular subassembly are identified by the same reference numeral with a different letter appended to it. For example, components  11   a - 11   g  are in subassembly  11 . Each subassembly  11 - 16 , and their respective components, will now be described. 
     Subassembly  11  is a frame that includes components  11   a - 11   g . Component  11   a  is a horizontal base of the frame which has several legs  11   b  that are rigidly connected to the base  11   a . Components  11   c - 11   f  are four vertical columns which are rigidly connected to the base  11   a ; and component  11   g  is a top of the frame which is rigidly connected to the columns  11   c - 11   f.    
     Subassembly  12  is a chip holding subassembly which includes components  12   a - 12   d . From one to fourteen of these chip holding subassemblies  12  are held by the frame  11 . Component  12   a  is a printed circuit board which has one face  12   a - 1  and an opposite face  12   a - 2 . Face  12   a - 1  is seen only in FIG. 2, and attached to it are N sockets  12   b , each of which holds one IC chip  12   c  that is to be tested. Here, N is any desired number, such as sixteen or thirty for example. Attached to face  12   b - 1  are N sets of electrical contacts  12   d , and each set carries all of the electrical power and all signals for one of the chips  12   c . Each socket  12   b  is connected to one set of contacts  12   d  by microscopic conductors (not shown) that pass thru the printed circuit board  12   a.    
     Subassembly  13  is a power converter subassembly which includes components  13   a - 13   c . A separate power converter subassembly  13  is held by the frame  11  above each chip holding subassembly  12 . Component  13   a  is a printed circuit board which has one face  13   a - 1  and an opposite face  13   a - 2 . Face  13   a - 1  is seen only in FIG. 2, and attached to it are N sets of electrical contacts  13   b , each of which mates with one set of the contacts  12   d  on the chip holding subassembly  12 . Attached to face  13   a - 2  are N DC-DC power converters  13   c . Each power converter  13   c  supplies power to one set of the contacts  13   b , and it is connected to those contacts by microscopic conductors (not shown) that pass through the printed circuit board  13   a.    
     Subassembly  14  is a temperature regulating subassembly which includes components  14   a - 14   d . A separate temperature regulating subassembly  14  is held by the frame  11  below each chip holding assembly  12 . Component  14   a  is a flat rigid plate which has one face  14   a - 1  and an opposite face  14   a - 2 . Attached to face  14   a - 2  are N springy components  14   b , and each springy component  14   b  holds one temperature regulating component  14   c  such that it is aligned with one chip  12   c  in the chip holding assembly  12 . 
     Each temperature regulating component  14   c  can be of a type which removes heat from the chips  12   c  by conduction, such as a heat sink; or it can be of a type which adds heat to the chips  12   c  by conduction, such as an electric resistor; or it can be a combination of both types. Several stops  14   d  are attached to the face  14   a - 2 , and they are aligned with the spaces between the sockets  12   b  in the chip holding assembly  12 . 
     These stops  14   d  limit the force with which the temperature regulating components  14   c  can be pressed against the chips  12   c . This is achieved by limiting the amount by which the springy components  14   b  get compressed when the subassemblies  12 - 14  are squeezed together. Preferably, the stops  14   d  have a length which is selectable within a predetermined range so that the temperature regulating components  14   c  are pressed against the chips  12   c  with a force that can be adjusted up or down. 
     Subassembly  15  is a pressing mechanism which presses the subassemblies  12 ,  13  and  14  together. In order to press those subassemblies  12 - 14  together, the power converter subassembly  13  is held stationary in the frame  11 , and the pressing mechanism  15  moves the temperature regulating subassembly  14  upward. This upward movement causes the chip holding subassembly  12  to be squeezed between the temperature regulating subassembly  14  and the power converter subassembly  13 . 
     For each chip holding subassembly  12  that is held in the frame  11 , two copies of the pressing mechanism  15  are provided. One copy is held in the frame by columns  11   c  and  11   d , while the other copy is held in the frame by columns  11   e  and  11   f . Several different embodiments of the pressing mechanism  15  are described in detail herein in conjunction with FIGS. 2-19. 
     Subassembly  16  is an actuator for all of the pressing mechanisms  15  which are in the frame  11 , and it includes components  16   a - 16   f . Component  16   a  is a plate which moves up and down in the frame between columns  11   c  and  11   d . Component  16   b  is identical to plate  16   a , and it moves up and down in the frame between columns  11   e  and  11   f . Plate  16   a  has a separate pair of slots  16   a - 1  for each pressing mechanism  15  that is held by the frame columns  11   c  and  11   d , and plate  16   b  has a separate pair of slots  16   b - 1  for each pressing mechanism  15  that is held by the frame columns  11   c  and  11   f.    
     As the plates  16   a  and  16   b  move, the slots  16   a - 1  and  16   b - 1  act as tracks which cause all of the pressing mechanisms  15  to move. When the plates  16   a  and  16   b  move down, the pressing mechanisms  15  move to an open position where the subassemblies  12 ,  13  and  14  are spaced-apart. Conversely, when the plates  16   a  and  16   b  move up, the pressing mechanisms  15  move to a closed position where the subassemblies  12 ,  13  and  14  are pressed together. 
     Component  16   c  is an electric motor. Component  16   d  is a linkage between the motor  16   c  and plate  16   a ; and component  16   e  is a linkage between the motor  16   c  and plate  16   b . These components  16   c - 16   e  move the plates  16   a  and  16   b  up, and move the plates down, in response to control signals that are sent on conductors  16   f  to the motor  16   c  from manually operated control switches (not shown). 
     How the chip holding subassembly  12 , the power converter subassembly  13 , the temperature regulating subassembly  14 , and the pressing mechanism  15  are held relative to each other by the frame  11  is shown schematically in FIG.  2 . In addition, FIG. 2 schematically illustrates how the pressing mechanism  15  squeezes the chip holding subassembly  12  between the power converter subassembly  13  and the temperature regulating subassembly  14 . 
     Included within the pressing mechanism  15  of FIG. 2 are components  15   a - 15   g . Component  15   a  is a rail which is rigidly attached to the frame columns lie and  11   f . This rail  15   a  lies below the temperature regulating subassembly  14  and is parallel to face  14   a - 1  of the plate  14   a . Components  15   b  and  15   c  are a pair of arms that are coupled together with a pivotal joint  15   d  which presses against face  14   a - 1  of the plate  14   a . These arms  15   b  and  15   c  also have slidable joints  15   e  and  15   f  which slide on the rail  15   a . Component  15   g  is a spring which is coupled between the slidable joint  15   f  and the frame  11 . All of the components  15   b - 15   g  are duplicated in the pressing mechanism  15  as shown in FIG.  2 . 
     Both of the slidable joints  15   e  fit into one pair of the slots  16   b - 1  in the plate  16   b . The slots  16   b - 1  of each pair are close together at their top and far apart at their bottom. Thus, as the plate  16   b  move down, the joints  15   e  slide close together to an “open” position. There, the angle “A” between each pair of arms  15   b  and  15   c  is large; and so the pivotal joints  15   d  have moved down. Consequently, the three subassemblies  12 ,  13 , and  14  are spaced apart from each other. 
     Conversely, as the plate  16   d  moves up, the joints  15   e  slide far apart to a “closed” position. There, the angle “A” between each pair of arms is small; and so the pivotal joints  15   d  have moved up. Consequently, the three subassemblies  12 ,  13 , and  14  are squeezed together. 
     When the arms  15   b  are in the closed position, the angle “A” which is between each pair of arms  15   b  and  15   c  does not have a single value. Instead, the angle “A” in the closed position is a variable which ranges from A 1  to A 2 ; and why this is so is illustrated in FIG.  3 . 
     In FIG. 3, the dimension Y 1  is the distance from the top of the rail  15   a  in the pressing mechanism  15  to face  13   a - 1  of component  13   a . Due to various manufacturing tolerances in components  11   e  and  11   f , the dimension Y 1  can vary between a minimum of Y 1 MIN and a maximum of Y 1 MAX. 
     Also in FIG. 3, the dimension Y 2  is the combined thickness of the components  13   b ,  12   d ,  12   a  and  14   a  plus the length of component  14   d . The thicknesses of components  13   b ,  12   d ,  12   a  and  14   a  can vary due to manufacturing tolerances; and, the length of component  14   d  is selectable in order to adjust the force with which the temperature regulating components  14   c  are pressed against the chips  12   c . Thus the dimension Y 2  will vary between a minimum of Y 2 MIN and a maximum of Y 2 MAX. 
     When Y 1  has its maximum value and Y 2  has its minimum value, the angle A between the arms  15   b  and  15   c  in the closed position has its smallest value which is the angle A 1 . Conversely, when Y 1  has its minimum value and Y 2  has its maximum value, the angle A between the arms  15   b  and  15   c  in the closed position has its largest value which is the angle A 2 . 
     As the angle A between the arms  15   b  and  15   c  increases from the angle A 1  to the angle A 2 , the spring  15   g  gets compressed by an increasing amount. Thus, as the angle A increases from angle A 1  to angle A 2 , the spring  15   g  exerts a force on the sliding joint  15   f  which increases monotonically. 
     However, in accordance with one feature of the pressing mechanism  15 , the force with which the pivotal joint  15   d  squeezes the subassemblies  12 - 14  together does not monotonically increase as the angle A between the arms  15   b  and  15   c  increases from A 1  to A 2 . Instead, that force initially increases and then decreases; and the reason for this will now be described in conjunction with FIGS. 4-8. 
     In FIG. 4, the arms  15   b  and  15   c  are again shown just like they are in FIG.  3 . However, in FIG. 4, a force vector Fx is shown which pushes against the sliding joint  15   f , and that force vector is caused by the spring  15   g . Similarly, in FIG. 4, a force vector Fy is shown which pushes against the pivotal joint  15   d , and that force vector is due to the subassemblies  12 - 14  being squeezed by the pivotal joint  15   d . Thus, both of the arms  15   b  and  15   c  are in compression. 
     Also in FIG. 4, two angles B 1  and B 2  are shown; and they respectively equal one-half of the angles A 1  and A 2  that are shown in FIG.  3 . These half angles B 1  and B 2  are shown in FIG. 4, rather than the full angles A 1  and A 2 , because they are more useful in the analysis which is made by several equations that are shown in FIG.  5 . 
     In equation 1, an expression is given for the spring constant K of the spring  15   g . Equation 1 indicates that the spring constant K is equal to the force which is exerted by the spring  15   g  at the angle B 2  minus the force which is exerted by the spring  15   g  at the angle B 1 , divided by a distance Δx. That distance Δx, as is shown in FIG. 4, is the distance by which the spring  15   g  is compressed as the angle B increases from B 1  to B 2 . 
     An expression for the distance Δx is given by equation 2. There the parameter L is the length of each of the arms  15   b  and  15   c . Equation 2 is obtained from the geometries in FIG.  4 . Then, equation 3 is obtained by substituting equation 2 into equation 1. 
     Inspection of equation 3 shows that the numerator contains two terms which represent the forces Fx that are exerted by the spring  15   g . Then, by equation 4 through equation 10, the two force terms Fx are translated to the corresponding orthogonal forces Fy which are exerted by the pivotal joint  15   d  on the subassemblies  12 - 14 . 
     To begin the above translation, equation 4 states that the force Fy which is exerted against the pivotal joint  15   d  is equal to the force Fa which is exerted by each one of the arms  15   b  or  15   c , times the cosine of the angle B, times “2”. Equation 4 is obtained by summing the forces in the vertical direction “y” on the pivotal joint  15   d . Force Fa as exerted parallel to the longitudinal axis of each arm; and thus its component in the y direction is given by the cosine term. Also, the factor of “2” occurs in equation 4 because each of the arms  15   b  and  15   c  push the pivotal joint  15   d  upward with the same force Fa. Next, equation 5 relates the force Fa, which is exerted by each of the arms  15   b  and  15   c , to the force Fx which is exerted by the spring  15   g . According to equation 5, the spring force Fx is equal to the force Fa times the sine of the angle B. Equation 5 is obtained by summing the forces which occur in the horizontal direction “x” on the slidable joint  15   f.    
     By dividing equation 4 with equation 5, equation 6 is obtained. In that division, the force Fa in the numerator cancels out with the force Fa in the denominator. Then, equation 6 can be rewritten as equation 7 which says that the force Fx is equal to one half of the tangent of the angle B times the force of Fy. 
     When the angle B equals the particular angle B 1 , equation 7 can be rewritten as equation 8. Likewise, when the angle B has the particular value B 2 , equation 7 can be rewritten as equation 9. Then, equations 8 and 9 can be substituted into the numerator of equation 3, and the result yields a new expression for the spring constant K which is given by equation 10. 
     Inspection of equation 10 shows that the denominator contains the parameter L which is the length of each of the arms  15   b  and  15   c . But, by utilizing equations 11 and 12, the parameter L can be removed from equation 10 and replaced with another parameter Δy. 
     In equation 11, Δy is the vertical distance by which the pivotal joint  15   d  moves as the angle B varies from B 1  to B 2 . Equation 11 is obtained from the geometries that are shown in FIG.  4 . By rearranging the terms which are in equation 11, an expression for L is obtained as shown by equation 12. Then, substituting equation 12 into equation 10 yields equation 13. 
     Equation 13 is a complex expression for the spring constant K of the spring  15   g ; and that expression includes two force terms which are Fy(B 1 ) and Fy(B 2 ). In equation 14, both of those force terms are set equal to the same force Fy(MIN). Then, substituting equation 14 into equation 13 yields equation 15. By selecting the spring constant K in accordance with equation 15, the result of Fy(B 1 ) equal to Fy(B 2 ) will be achieved. 
     This means that the pivotal joint  15   d  will press the subassemblies  12 - 14  together with the same force Fy(MIN) when the angle between the arms  15   b  and  15   c  is either B 1  or B 2  as shown in FIG.  4 . And, this result occurs even though the spring  15   g  is compressed by two different amounts at the angles B 1  and B 2 . 
     In order to construct the pressing mechanism  15  such that equation 15 is met, the steps S 1 -S 5  which are listed in FIG. 6 can be performed. Initially, in step S 1 , the force Fy(MIN) in equation 15 is selected as one design constraint, and the distance Δy in equation 15 is selected as another design constraint. 
     Here, the force Fy(MIN) is selected based on the minimum force with which the mating electrical contacts  12   d  and  13   b  need to be pressed together. For example, suppose that the total number of the contacts  12   d  is 110, and suppose that each contact  12   d  needs to be pressed against a corresponding contact  13   b  with a minimum force of 2 pounds. Also, suppose that the subassemblies  12  and  14  each weigh 10 pounds. Then the force Fy(MIN) which needs to be exerted by each of the four joints  15   d  is set equal to 110 times 2 plus 20, divided by 4, or 60 pounds. 
     Likewise, the distance Δy is selected based on the manufacturing tolerances and selectable length variations that were described in conjunction with FIG.  3 . For example, if the stops  14   d  have a selectable length that varies by 0.17″ and components  11   f ,  13   b ,  12   d    12   a , and  14   a  have a combined manufacturing tolerance of 0.03″, then the distance Δy is set equal to 0.20″. 
     Next, the angles B 1  and B 2  are selected for equation 15. In step S 2  of FIG. 6, the angles B 1  and B 2  are selected such that the force Fx which is exerted by the spring  15   g  on the pivotal joint  15   f  at each of the angles B 1  and B 2  is less than the force Fy which is exerted by the pivotal joint  15   d  on the subassemblies  12 - 14 . To meet this constraint, equations 8 and 9 are used since they relate the force Fx to the force Fy at each of the angles B 1  and B 2 . 
     By performing step S 2 , a mechanical advantage is obtained which reduces the total force that needs to be applied by the actuator 16 in order to move the slidable joint  15   e  of all of the pressing mechanisms  15  from the open position to the closed position. The magnitude of this mechanical advantage is equal to Fy divided by Fx. Here Fx is the force which is exerted by the spring  15   g  on the slidable joint  15 ; and, that spring force Fx is equal in magnitude to the force which must be exerted by the actuator slots  16   a - 1  on the slidable joint  15   e.    
     In equations 8 and 9, the tangent of the angles B 1  and B 2  decreases as those angles decrease; and this suggests that the angles B 1  and B 2  should be as small as possible in order to maximize the mechanical advantage. However, as the angles B 1  and B 2  get smaller, the amount by which the joint  15   d  moves in the vertical direction, for each degree of change from angle B 1  to angle B 2 , gets smaller. Thus, in order to meet the design constraint of Δy, the angles B 1  and B 2  should not be made too small. Preferably, the angles B 1  and B 2  are selected to be from 10° to 40°. 
     Suppose for example, that the angle B 1  is selected to be 20° and the angle B 2  is selected to be 29°. Then, for the angle B 1 =20°, the mechanical advantage is Fy(20°) divided by Fx(20°); and it can be calculated from equation 8 as being equal to 5.49. Likewise, the mechanical advantage at angle B 2 =29° is Fy(29°) divided by Fx(29°); and it can be obtained from equation 9 as being equal to 3.61. 
     Next, in accordance with step S 3  of FIG. 6, the spring constant k is calculated from equation 15 by utilizing the parameters of Fy(MIN), Δy, B 1  and B 2  that were selected in steps S 1  and S 2 . Also, those same parameters can be used in conjunction with equation 12 to calculate the length L of each of the arms  15   b  and  15   c ; and this is done by step S 4  in FIG.  6 . 
     What remains to be done after step S 4  is to determine the amount by which the spring  15   g  needs to be compressed when the angle B equals B 1 ; and this is done in FIG. 6 by step  5 . There, the force which is exerted by the spring  15   g  when the angle B equals B 1  is determined from equation 8; and that force is set equal to the spring constant k times a distance Δx 0 , which is the distance by which the spring  15   g  is compressed at the angle B 1 . That distance Δx 0  is the only unknown term that occurs in step S 5 ; and so it can be calculated from all of the other terms. 
     A numerical example of the above steps S 1 -S 5  is shown in FIG.  7 . There, in step S 1 , the minimum force Fy(MIN) is set equal to 60 pounds and the parameter Δy is set equal to 0.20 inches. Also, in step S 2 , the angles B 1  and B 2  are set equal to 20° and 29° respectively. 
     Utilizing the above selections, steps S 3  and S 4  are performed whereby the spring constant k is calculated to be 6.45 pounds per inch and the length L of each of the arms  15   b  and  15   c  is calculated to be 3.1 inches. Then, by step S 5 , the distance Δx 0  by which the spring  15   g  is compressed at the angle B 1  is determined to be 1.693 inches. 
     When the pressing mechanism  15  is constructed with the parameters that are given in FIG. 7, the forces Fx and Fy which occur for various angles B are listed in FIG.  8 . There the units for the angle B is degrees, and the units for the forces Fx and Fy is pounds. Inspection of FIG. 8 shows that at each of the angles of B 1 =20° and B 2 =29°, the force Fy which is exerted by the pivotal joint  15   b  equals the desired minimum force of 60 pounds. This occurs even though the force Fx which is exerted by the spring  15   g  at the angle B 1 =20° is completely different than the force Fx which is exerted by the spring at the angle B 2 =29°. 
     Inspection of FIG. 8 also shows that as the angle B decreases from the angle B 2  to the angle B 1 , the force Fx decreases monotonically, whereas the force Fy initially increases and then decreases. This decrease in the force Fy after the initial increase is important because it reduces the maximum force with which the subassemblies  12 - 14  are pressed together; and that prevents any of the subassembly components from being overstressed and permanently damaged. For example, if the force Fy gets too large, the printed circuit boards  12   a  and  14   a  could get bent. 
     Inspection of FIG. 8 further shows that as the angle B decreases from the angle B 1  to the angle B 2 , the mechanical advantage with which the actuator  16  moves the slidable joint  15   e  monotonically increases. This mechanical advantage equals Fy/Fx as was previously described. However, as the angle B decreases from B 1  to B 2 , the force Fx with the spring  15   g  pushes the arms together monotonically decreases. This decrease in the force Fx counteracts the increase in the mechanical advantage, and that causes the force Fy to decrease after its initial increase. 
     Turning now to FIG. 9, a second embodiment of the pressing mechanism  15  will be described. This second embodiment of the pressing mechanism  15  is similar to the first embodiment which was described above in conjunction with FIGS. 2-8; and, the similarities can be seen by comparing FIG. 2 with FIG.  9 . 
     In the FIG. 9 pressing mechanism, all of the components  15   a - 15   g  from FIG. 2 are repeated, but the coupling to the spring  15   g  is changed. More specifically, in the FIG. 9 pressing mechanism, another member  15   h  is included which is rigidly attached to the rail  15   a ; and the spring  15   g  is coupled between that member  15   h  and the slidable joint  15   f  of the arm  15   c.    
     In operation, the spring  15   g  is stretched by an increasing amount as the arm  15   b  moves from the open position to the closed position. By comparison, in the embodiment of FIG. 2, the spring  15   g  is compressed by an increasing amount as the arm  15   b  moves from the open position to the closed position. 
     All of the analysis that was given by equations 1-15 of FIG. 5 also applies to the pressing mechanism of FIG.  9 . Consequently, all of the steps S 1 -S 5  of FIG. 6 should be followed in order to construct the pressing mechanism of FIG. 9 such that the force Fx (which is exerted by the spring  15   g ) increases monotonically, while the force Fy (which squeezes the subassemblies  12 - 14  together) initially increases and then decreases. 
     Next, with reference to FIGS. 10A and 10B, a third embodiment and a fourth embodiment of the pressing mechanism  15  will be described. In both of these embodiments, all of the components  15   a - 15   g  from FIG. 2 are again repeated; but, the coupling to the spring  15   g , as well as the coupling to both of the joints  15   e  and  15   f , is changed. 
     Specifically, the embodiments of FIGS. 10A and 10B each include a member  15   i  which is rigidly attached to the rail  15   a ; and, the joint  15   f  is pushed against that member  15   i  such that it can pivot, but not slide. Further, in the embodiments of FIGS. 10A and 10B, the spring  15   g  is coupled between joint  15   e  of the arm  15   b  and one of the slots  16   a - 1  of the actuator  16 . In the embodiment of FIG. 10A, the spring  15   g  is compressed by an increasing amount as the actuator moves the spring from the open position to the closed position; whereas in the embodiment of FIG. 10B, the spring  15   g  is stretched by an increasing amount as the actuator moves the spring from the open position to the closed position. 
     All of the analysis which is made by equations 1-15 of FIG. 5 also applies to the embodiments of FIGS. 10A and 10B. Consequently, to construct the embodiments of FIGS. 10A and 10B such that the force Fx (which is exerted by the spring  15   g ) increases monotonically, while the force Fy (which squeezes the subassemblies  12 - 14  together) initially increases and then decreases, steps S 1 -S 5  of FIG. 6 should be followed. 
     Next, with reference to FIG. 11, a fifth embodiment of the pressing mechanism  15  will be described. This fifth embodiment of FIG. 11 is similar to the second embodiment of FIG. 9; and the similarities can be seen by comparing those two FIGS. 9 and 11. 
     One difference between the embodiments of FIGS. 9 and 11 is that in the embodiment of FIG. 11, a single spring  15   g  is stretched between one arm  15   c  and another arm  15   c  of two different pair of arms  15   b  and  15   c . By comparison in the embodiment of FIG. 9, a single spring  15   g  is stretched between arm  15   c  and member  15   h  for each pair of arms. Thus, the FIG. 11 embodiment has half as many springs  15   g  as the FIG. 9 embodiment. 
     Also, the embodiment of FIG. 11 illustrates another modification which is that the arms  15   b  and  15   c  have different lengths L 1  and L 2 . This modification applies not just to the embodiment of FIG. 11; but it also can be incorporated into each of the embodiments of FIGS. 2,  9 ,  10 A and  10 B. 
     When the arms  15   b  and  15   c  have the different lengths of L 1  and L 2 , the expression for the spring constant k as given by equation 15 in FIG. 5 must be modified; and, that modification is given by equation 15′ in FIG.  11 A. Equation 15′ is derived by following the same process which generated equation 1 thru equation 14 of FIG. 5 while using the lengths L 1  and L 2  to reflect the different arm lengths. 
     Equation 15′ of FIG. 11A contains four new variables which are angles B 1   a , B 1   b , B 2   a  and B 2   b . All of these angles are defined as shown in FIG.  11 B. For example, angle B 1   a  is the angle of arm  15   c  relative to the vertical axis Y when the total angle A between the arms  15   b  and  15   c  in the closed position has the minimum value A 1 . Similarly, angle B 2   a  is the angle of arm  15   c  relative to the vertical axis Y when the total angle A between the arm  15   b  and  15   c  in the closed position has the maximum value A 2 . 
     Referring next to FIG. 12, a sixth embodiment of the pressing mechanism will be described. This embodiment includes components  17   a - 17   h , and it is substantially different than the embodiments of FIGS. 2-11. All of the remaining components which are shown in FIG. 12 are the same as those which were previously shown and described in conjunction with FIG. 2, and they are identified with their previous reference numerals. 
     Components  17   a  and  17   b  are a pair of arms which are coupled together by a pivotal joint  17   c . A spring  17   d  is coupled between the pivotal joint  17   c  and one of the slots  16   a - 1  of the previously described actuator  16 . As the actuator moves from the open position to the closed position, the spring  17   d  is stretched by an amount which monotonically increases. 
     Arm  17   a  has a pivotal joint  17   e  which is coupled to the frame column  11   f ; and, arm  17   b  has a pivotal joint  17   f  which is coupled to a vertically moveable base member  17   g . When the actuator  16  is in the open position, the base member  17   g  rests on a rail  17   h  which is rigidly coupled between the frame columns  11   e  and  11   f.    
     All of the components  17   a - 17   f  are replicated for each of the four columns  11   c - 11   f  in the frame  11 . FIG. 12 shows how two copies of the components  17   a - 17   f  are coupled to the frame columns  11   e  and  11   f . Two other copies of the components  17   a - 17   f  are coupled to the frame columns  11   c  and  11   d  in the same fashion. 
     By comparing the pressing mechanism  17  of FIG. 12 to the previously described pressing mechanisms of FIGS. 2-11, four major differences can be seen. First, in the pressing mechanism  17  of FIG. 12, each of the arm joints  17   c ,  17   e , and  17   f  pivot; but none of those joints slide. Second, in the pressing mechanism  17  of FIG. 12, the spring  17   d  is coupled to the pivotal joint  17   c  between the arms  17   a and  17   b , as opposed to being coupled to a sliding joint of a single arm. Third, in the pressing mechanism  17  of FIG. 12, the subassemblies  12 - 14  are squeezed together by a single arm  17   b  which lifts the base member  17   g  vertically. Fourth, as the subassemblies are squeezed together, arm  17   b  is in tension and arm  17   a  is in compression, as opposed to both arms being in compression. 
     In FIG. 12, the pressing mechanism  17  is shown in an open position where the subassemblies  12 - 14  are spaced apart from each other. As the actuator slots  16   a - 1  move the pressing mechanism  17  from the open position to a closed position, the amount by which the spring  17   d  is stretched increases monotonically. That causes the force Fx which is exerted by the spring  17   d  on the pivotal joint  17   c  to increase monotonically, and thus the base  17   g  moves upward and squeezes the subassemblies  12 - 14  together. 
     Now, the distance by which the base  17   g  moves upward from the open position to the closed position is not fixed. Instead, that distance is a variable as shown in FIG.  13 . There, the pivotal joint  17   f  moves by a minimum distance of Yo from the open position to the closed position, and moves by a maximum distance of Yo plus Δy from the open position to the closed position. The distance Δy is caused by variations in the length of the stop  14   d  which are selectable, and by manufacturing tolerances of the components  11   e ,  11   f ,  13   b ,  12   d ,  12   a  and  14   a.    
     Due to the above variation Δy, the joint  17   c  has a variable location in the closed position which ranges from one point “a” to another point “b” as shown in FIG.  13 . When the joint  17   c  is located at point “a”, the angle C which occurs between the arms  17   a  and  17   b  has a value Ca, whereas when the joint  17   c  is located at point “b”, the angle C has a value Cb. 
     Likewise, when the pivotal joint  17   c  is located at point “a”, the angle D which occurs between the arm  17   a  and the vertical axis Y has a value Da, and the angle E which occurs between the arm  17   b  and the vertical axis has a value Ea. By comparison, when the pivotal joint  17   c  is located at point “b”, the angle D between arm  17   a  and the vertical axis has a value Db, and the angle E between arm  17   b  and the vertical axis has a value Eb. 
     As the joint  17   c  moves from point “b” to point “a”, the spring  17   d  is stretched by an increasing amount. Thus, as the joint  17   c  moves from point “b” to point “a”, the spring  17   d  exerts a force on the joint  17   c  which increases monotonically. However, in accordance with one feature of the pressing mechanism  17 , the force which squeezes the subassemblies  12 - 14  together does not monotonically increase as the joint  17   c  moves from point “b” to point “a”. Instead, that force initially increases and then decreases; and the reason for this will now be described in conjunction with the equations of FIGS. 14A and 14B. 
     Equation 21 of FIG. 14A gives an expression for the spring constant k of the spring  17   d . That expression says that the spring constant k is equal to one force Fxa minus another force Fxb, divided by a distance Δx. Here, Fxa is the force which is exerted by spring  17   d  on joint  17   c  when that joint is located at point “a”; and Fxb is the force which is exerted by spring  17   d  on joint  17   c  when that joint is located at point “b”. The distance Δx is the distance between point “b” and point “a” in the horizontal direction, and that is the added amount by which the spring  17   d  is stretched in moving from point “b” to point “a”. 
     An expression for the distance Δx is given by equation 22. There, the terms L 1 , Da, and Db are terms which are described above and shown in FIG.  13 . Equation 22 is obtained from the geometries in FIG.  13 . Then, equation 23 is obtained by substituting equation 22 into equation 21. 
     Next, by equation 24 thru equation 38, the two force terms Fxa and Fxb in equation 23 will be translated into corresponding orthogonal forces F 2   ya  and F 2   yb ; and, the end result is given by equation 39. Force F 2  is exerted by arm  17   b , along its longitudinal axis, on joints  17   c  and  17   f . Arm  17   b  is in tension, and thus force F 2  occurs in the direction shown in FIG.  13 . 
     Force F 2   ya  is the vertical component of the force F 2  when joint  17   c  is at point “a”, and force F 2   yb  is the vertical component of force F 2  when joint  17   c  is at point “b”. These vertical forces F 2   ya  and F 2   yb  are exerted by the joint  17   f  on the base member  17   g  which in turn squeezes the subassemblies  12 - 14  together. 
     To begin the above translation, the force F 2  is partitioned into two components, F 2   x  and F 2   y  which respectively are parallel to the X and Y axis. Equation 24 says that the force F 2   x  is equal to the force F 2  times the sine of the angle E. Likewise, equation 25 states that the force F 2   y  is equal to the force F 2  times the cosine of the angle E. These equations 24 and 25 are obtained from the geometries of arm  17   b  in FIG.  13 . 
     By dividing equation 24 with equation 25, equation 26 is obtained. In that division, the force F 2  in the numerator cancels with the force F 2  in the denominator. Then, equation 26 can be re-written as equation 27 which says that the force F 2   x  is equal to the force F 2   y  times the tangent of the angle E; and this equation 27 will be used subsequently in equation 35. 
     Next, by equation 28, the forces which are exerted on the pivotal joint  17   c  are summed in the vertical direction Y. Arm  17   a  exerts a force F 1  on joint  17   c  which occurs along its longitudinal axis and has a vertical component F 1   y . Arms  17   a  is in compression, and thus the force F 1  occurs in the direction shown in FIG.  13 . Similarly, arm  17   b  exerts the force F 2  on joint  17   c  which has vertical component F 2   y . By substituting the terms F 1   y  and F 2   y  into equation 28, equation 29 is obtained; and equation 29 will be used subsequently in equation 33. 
     Next, the force F 1  is partitioned into two components F 1   x  and F 1   y  which respectively are parallel to the X and Y axis. Equation 30 says that the force F 1   x  equals the force F 1  times the sine of the angle D. Likewise, equation 31 says that the force F 1   y  is equal to the force F 1  times the cosine of the angle D. These equations 30 and 31 are obtained from the geometries of arm  17   a  in FIG.  13 . 
     By dividing equation 30 with equation 31, equation 32 is obtained. In that division, the force F 1  in the numerator cancels with the force F 1  in the denominator. Then, equation 32 can be rewritten as equation 33. 
     However, the force F 1   y  in equation 33 is related to the force F 2   y  by equation 29. Thus, when equation 29 is substituted into equation 33, the result of equation 34 is obtained; and equation 34 will be used subsequently in equation 35. 
     Next, by equation 35, the forces which occur on the pivotal joint  17   c  are summed in the horizontal direction X. Equation 35 says that the force Fx is equal to the force F 1   x  minus the force F 2   x . Here, Fx is the force which is exerted by the spring  17   d  on the joint  17   c ; F 1   x  is the horizontal component of the force which occurs along the longitudinal axis in the arm  17   a ; and F 2   x  is the horizontal component of the force which occurs along the longitudinal axis of the arm  17   b.    
     An expression for the force F 1   x  is given by equation 34, and an expression for the force F 2   x  is given by equation 27. Thus, substituting equations 34 and 27 into equation 35 yields equation 36. 
     Utilizing equation 36, two other equations 37 and 38 are obtained. Equation 37 is the same as equation 36 except that it applies only to the specific case where the pivotal joint  17   c  is at point “a”. Similarly, equation 38 is the same as equation 36 except that it applies only to the specific case where the pivotal joint  17   c  is at point “b”. 
     Next, equation 39 is obtained by substituting the two equations 37 and 38 into equation 23. In equation 39, an expression is given for the spring constant K of the spring  17   d ; and in that expression, the only force terms which occur are the forces F 2   ya  and F 2   yb . Force F 2   ya  exerted by joint  17   f  against the subassemblies  12 - 14  when joint  17   c  is at point “a”; and force F 2   yb  is exerted by joint  17   f  against the subassemblies  12 - 14  when joint  17   c  is at point “b”. 
     Next, by equation 40, both of the forces F 2   ya  and F 2   yb  are set equal to a predetermined force of F 2   y (MIN). That force F 2   y (MIN) is the minimum force with which the subassemblies  12 - 14  should be squeezed by each joint  17   f  in order to ensure that a proper electrical connection is made between the mating contacts  12   d  and  13   b . Then, substituting equation 40 into equation 39 yields equation 41. 
     By selecting the spring constant K for the spring  17   d  in accordance with equation 41, the result of F 2   ya  and F 2   yb  being equal to F 2   y (MIN) will be achieved. This means that with the pressing mechanism  17  of FIG. 12, each joint  17   f  will press the subassemblies  12 - 14  together with the same force F 2   y (MIN) when the angle between the arms  17   a  and  17   b  is either Ca or Cb as shown in FIG.  13 . And, this result occurs even though the spring  17   d  is stretched by two different amounts at the angles Ca and Cb. 
     Equation 41 can be used to construct the pressing mechanism  17  by performing a series of steps S 11 -S 17  which are listed in FIG.  15 . Initially, in step S 11 , the force F 2   y (MIN) in equation 41 is selected as one design constraint, and the distance Δy as shown in FIG. 13 is selected as another design constraint. 
     Here, the force F 2   y (MIN) is selected based on the minimum force with which the mating electrical contacts  12   d  and  13   b  need to be pressed together. An example of this step was previously given in conjunction with FIG. 6 wherein the force Fy(MIN) was set equal to 60 pounds. 
     Likewise, the distance Δy is chosen based on the selectable variations which can occur in the length of the stop  14   d , and based on the manufacturing tolerances of the components  11   e ,  11   f ,  13   b ,  12   d ,  12   a , and  14   a . An example of this step was also previously given in conjunction with FIG. 6 wherein Δy was set equal to 0.20 inches. 
     Next, step S 12  of FIG. 15 is performed. There, the angles Ea and Da are selected such that the force Fxa which is exerted by the spring  17   d  on joint  17   c  at point “a” is less than the force F 2   ya  which is exerted by the joint  17   f  against the subassemblies  12 - 14 . By meeting the constraint of Fxa being less than F 2   ya , the subassemblies  12 - 14  are squeezed together at point “a” with a mechanical advantage which is equal to F 2   ya  divided by Fxa. To choose the angles Ea and Da such that the force Fxa is less than the force F 2   ya , equation 37 is used. That equation indicates that the force Fxa will be less than the force F 2   ya  as long as the tangent of the angle Ea minus the tangent of the angle Da is less than one. 
     Next, step S 13  of FIG. 15 is performed whereby the length L 1  of arm  17   a  is selected and the length L 2  of arm  17   b  is selected. In selecting these lengths, one constraint to meet is that arm  17   a  must be long enough to couple joint  17   e  to one of the frame columns  11   c - 11   f , and arm  17   b  must be long enough to couple joint  17   f  to the vertically moveable base member  17   g.    
     By performing the above steps S 11 , S 12 , and S 13 , the following parameters in FIG. 13 are established: 1) the location of point “a”, 2) the length of the arms  17   a  and  17   b  which extend from point “a”; and 3) the respective angles Da and Ea at which those arms extend from point “a”. Thus, the angle by which arm  17   a  must pivot on joint  17   e  in the counter clockwise direction in order for the joint  17   f  to move upward by the distance Δy can be calculated. That angle as shown in FIG. 13 is the angle Da minus the angle Db; and it is calculated in step S 14  from the geometries in FIG.  13 . 
     If the length L 1  of arm  17   a  is selected to be too short in step S 13 , then it may not be possible to move joint  17   c  by the distance Δy. Likewise, if the angle Da is selected to be too small in step S 12 , then it may not be possible to move joint  17   c  by the distance Δy. However, these problems are overcome simply by repeating one or more of the steps S 11 -S 13  in an interactive fashion for different arm lengths L 1  and L 2 , and different angles Da, until joint  12   f  does move by the distance Δy. 
     Next, by step S 15 , the angles Db and Eb in FIG. 13 are calculated. Angle Db is simply the angle Da minus the angle which is calculated in step S 14 . Then, once the angle Db is determined, the angle Eb can be determined from the geometry of the arms  17   a  and  17   b  which occur when joint  17   c  is at point “b”. 
     Following the above step, step S 16  is performed wherein a value is calculated for the spring constant K of the spring  17   d . That calculation is made by utilizing equation 41 of FIG. 14 b . The right hand side of equation 41 includes all of the parameters F 2   y (min), Da, Db, Ea, Eb, and L 1 ; and values for those parameters are provided by the above-described steps S 11 -S 16 . 
     Lastly, a calculation is made to determine the amount by which the spring  17   d  must be stretched when the joint  17   c  is at point “b”. This calculation is made by step S 17  in FIG.  15 . There, Δx 0  represents the amount by which the spring  17   d  is stretched; and, it is determined from the equation of Fxb=KΔx 0 . In that equation the only unknown is Δx 0  since a value for the force Fxb can be obtained from equation 38, and a value for the spring constant K was calculated in step S 16 . 
     A numerical example of the above steps S 11 -S 17  is shown in FIG.  16 . There, by step S 11 , the minimum force F 2   y (min) is set equal to 60 pounds and the distance Δy is set equal to 0.20 inches. Next, by step S 12 , the angles Da and Ea are set equal to 29° and 2.722° respectively. Then, by step S 13 , the arm lengths L 1  and L 2  are selected to be 3 inches and 4 inches respectively. Due to these selections, the horizontal offset between joint  17   e  and joint  17   f  is 1.268 inches. 
     Next, by step S 14 , the angle of Da minus Db is calculated. That angle is the amount by which arm  17   a  must rotate in the counter-clockwise direction in order for joint  17   f  to move upward by the distance Δy. In step S 14 , the angle of Da minus Db is calculated to be 9.077°. Then, by step S 15 , the angles of Db and Eb are calculated to be 20° and −3.468° respectively. Here, the negative angle indicates that point “b” in FIG. 13 lies to the left of joint  17   f.    
     Next, step S 16  is performed wherein the spring constant K for the spring  17   d  is calculated. This calculation is made by substituting the above values for the parameters F 2   y (min), Da, Db, Ea, Eb, and L 1  into equation 41. By that calculation, the spring constant K is set equal to 11.66 pounds per inch. Then, step S 17  calculates the amount by which spring  17   d  must be stretched when joint  17   c  is at point “b”. By step S 17 , Δx 0  is determined to be 2.184 inches. 
     When the pressing mechanism  17  has the parameters that are given in FIG. 16, the forces Fx and F 2   y  which occur, for various positions of the arms, are listed in FIG.  17 . There the units for the angles D and E is degrees, and the units for the forces Fx and F 2   y  is pounds. Inspection of FIG. 17 shows that at each of the angles Da and Db, the force F 2   y  which is exerted by the joint  17   f  equals the desired minimum force of 60 pounds. This occurs even though the force Fx which is exerted by the spring  17   d  at the angle Da is completely different than the force Fx which is exerted by the spring at the angle Db. 
     Inspection of FIG. 17 also shows that as the angle D decreases from the angle Da to the angle Db, the force Fx decreases monotonically, whereas the force F 2   y  initially increases and then decreases. This decrease in the force F 2   y  after the initial increase is important because it reduces the maximum force with which the subassemblies  12 - 14  are pressed together. 
     Inspection of FIG. 17 further shows that as the angle D decreases from Da to Db, the mechanical advantage with which the actuator  16  moves the pivotal joint  17   e  monotonically increases. This mechanical advantage equals the force F 2   y  divided by Fx. However, as the angle D decreases from Da to Db, the force Fx with which the spring  17   d  pulls the arms together monotonically decreases. This decrease in force Fx counteracts the increase in the mechanical advantage, and that causes the force F 2   y  to decrease after its initial increase. 
     Referring next to FIG. 18, a seventh embodiment of the pressing mechanism will be described. This seventh embodiment includes components  17   a - 17   j , and it is obtained by modifying the sixth embodiment of FIG.  12 . 
     One change in the FIG. 18 pressing mechanism is that the two arms  17   a  and  17   b  extend downward and from their respective joints of  17   e  and  17   f , rather than upward. Consequently in FIG. 18, joint  17   c , which connects the two arms, is below both of the other two joints  17   e  and  17   f ; whereas in FIG. 12, joint  17   c  is above both of the joints  17   e  and  17   f.    
     Another change in the FIG. 18 pressing mechanism occurs in the coupling between joint  17   c  and the actuator slots  16   a - 1 . This is seen in FIG. 18 wherein—a) each joint  17   c  is connected to a pulley wheel  17   i ; b) a single spring  17   d  is located between each pair of the pulley wheels  17   i ; and c) the spring  17   g  is stretched by a cable  17   j  which wraps approximately halfway around the pulley wheels where it is then pulled by the actuator slots  16   a - 1 . 
     All of the analysis which is provided in FIGS. 13,  14 A and  14 B for the embodiment of FIG. 12 can be easily modified to apply to the embodiment of FIG.  18 . To do that, the forces Fxa and Fxb, which are shown in FIG. 13 as being exerted on joint  17   c  by the spring, become changed to the forces Fxa and Fxb which are exerted on joint  17   c  by the pulley wheel  17   i.    
     Cable  17   j  extends from the pulley wheel  17   i  at its top and its bottom, and thus cable  17   i  exerts two forces of equal magnitude on the pulley wheel  17   i . Thus, about half of the forces Fxa and Fxb that are exerted on joint  17   c  occur within the cable  17   j . Spring  17   d  is stretched by the cable  17   j , and so about half of the forces Fxa and Fxb are exerted by the spring  17   d . Also, joint  17   c  moves by half the distance which cable  17   i  is moved by the actuator slots  16   a - 1 . In all other respects, the analysis of FIGS. 13,  14 A and  14 B applies directly to the pressing mechanism of FIG.  18 . Thus, equation  41  of FIG. 14B can be used to determine the spring constant k for the spring  17   d . Likewise, steps S 1 -S 17  can be used to select all of the other parameters of F 2   y (MIN). ΔY, Da, Ea, L 1 , L 2 , Db, Eb, and Δxo. 
     Next, with reference to FIG. 19, an eighth embodiment of the pressing mechanism will be described. This embodiment of FIG. 19 is the same as the previously described embodiment of FIG. 12 except that spring  17   d  is eliminated and replaced by a different spring  17   k  at a different location. 
     More specifically, in the embodiment of FIG. 19, the spring  17   k  is a torsion spring; and it is coupled as shown between joint  17   e  and slot  16   a - 1  of the actuator. As the actuator moves from the open position to the closed position, the amount of torque which is exerted by the spring  17   k  on joint  17   e  monotonically increases. That torque increase then causes the base  17   g  to move upward and squeeze the subassemblies  12 - 14  together. 
     Several preferred embodiments of the present invention have now been described in detail. In addition however, the following changes and modifications can be made to these details without departing from the nature and spirit of the invention. 
     To aid in the description of one modification, reference should now be made back to FIGS. 7 and 8. Those figures show an embodiment of the pressing mechanism  15  wherein the mating electrical contacts  12   d  and  13   b  are pressed together with the minimum force of Fy(min) when the angle B between the arms is at the two limits of B 1  and B 2  for the closed position. Having the minimum force occur at the closed position limits of B 3  and B 2  is desirable because it causes the maximum value of the force Fy to occur close to midway between B 1  and B 2 ; and, that tends to minimize the difference between the minimum force and the maximum force with which the subassemblies  12 - 14  are pressed together. 
     However, as an alternative, the angle B at which the maximum Fy force occurs can be shifted either towards the angle B 1  or towards the angle B 2 . In fact, the angle B at which the maximum Fy force occurs can be shifted past the angle B 1  or past the angle B 2 . Such shifting is achieved simply by altering the amount Δxo by which the spring  15   g  is compressed at the angle B 1 , and/or altering the spring constant K, from the preferred values which are determined by steps  33  and  35  in FIG.  6 . 
     When the above shifting is performed, the mechanical advantage of Fy/Fx will still increase as the angle B between the arms decreases from B 2  to B 1 ; and, that increase in the mechanical advantage will still be counter-acted by a decrease in the force Fx as the angle B varies from B 2  to B 1 . Consequently, the total amount by which the force Fy varies from angle B 2  to angle B 1  will still be reduced. 
     The above modification to the embodiment of FIG. 8 can also be incorporated into all of the other illustrated embodiments. In FIGS. 16 and 17, for example, angle D in the closed position varies from Da to Db, and at each of those angular limits, the force F 2   y  equals the minimum force of F 2   y (min). Thus, the maximum value of the force F 2   y  occurs close to midway between the angles Da and Db. However, as a modification, the angle D at which the maximum F 2   y  force occurs can be shifted either towards angle Db, or towards angle Da, or past those angles. This shift is accomplished simply by altering the amount Δxo by which the spring  17   d  is stretched at position Db; and/or altering the spring constant K, from the preferred values that are determined by steps S 16  and S 17  in FIG.  15 . 
     Next, to aid in the description of another modification, reference should be made to FIG.  2 . There, joint  15   d  in the pressing mechanism  15  pushes against the temperature regulating subassembly  14  in order to press the subassemblies  12 - 14  together. However, when certain types of chips  12   c  are tested, their temperature may not need to be regulated; and so in that case, the temperature regulating subassembly  14  can be eliminated. Then, joint  15   d  of the pressing mechanism  15  can push directly against the chips carrying subassembly  12  in order to press together the mating electrical contact  12   d  and  13   b . This modification can also be made to all of the other illustrated embodiments. 
     Next, to aid in the description of another modification, reference should again be made to FIG.  2 . There, the spring  15   g  is shown as being attached directly to the slidable joint  15   f . However, as a modification, the spring  15   g  can be coupled to arm  15   c  at any other point. This modification also applies to all of the embodiments of FIGS. 9 through 11. Similarly, in the embodiment of FIG. 12, the spring  17   d  is shown as being attached directly to joint  17   c . However, as a modification, the spring  17   d  can be coupled to arm  17   a  at any other point. This modification also applies to the embodiment of FIG.  18 . 
     Next, to aid in the description of another modification, reference should again be made to FIG.  2 . There, the stop  14   d  is shown as being attached to member  14   a  in the temperature regulating subassembly  14 . However, as a modification, the stop  14   d  can be attached to member  12   a  of the chip holding subassembly  12 . This modification can also be made to all of the other illustrated embodiments. 
     Next, to aid in the description of another modification, reference should be made to FIG.  1 A. There, the actuator  16  is shown including an electric motor  16   c  which operates in response to control signals that are sent on conductors  16   f . Those control signals were described as being generated by manually operated control switches (not shown); however, as a modification the control signals for the motor  16   c  can be generated from any other source. For example, a digital computer with a control program can generate the control signals on the conductors  16   f  automatically. 
     Next, to aid in the description of another modification, reference should be made to FIG.  2 . There, a separate DC-DC power converter  13   c  is shown in the subassembly  12  for each chip  12   c  that is held in the subassembly  12 . However, if the chips  12   c  use a relatively small amount of power, then each DC-DC converter  12   c  can supply power to more than one chip. Conversely, if the chips  12   c  use a relatively large amount of power, then two or more DC-DC power converters can supply power to each chip. Further, the power converters  13   c  are not limited to being DC-DC power converters; but as an alternative, they can be any circuit which converts AC power to DC power. 
     Next, to aid in the description of another modification, reference should be made to FIG.  1 B. That figure shows the plates  16   a  and  16   b  which move up and down, and it shows the slots  16   a - 1  in the plates which cause the joints  15   e  in the pressing mechanisms  15  to slide. In FIG. 1B, the slots  16   a - 1  as illustrated lie at an angle of about  450  with respect to the direction in which the joints  15   e  slide. However, as a modification, the angle of the slots  16   a - 1  can be either increased or decreased. As the angle of the slots  16   a - 1  is decreased with respect to the direction in which the joints  15   e  slide, then the force which the motor  16   c  must exert to move the plates  16   b  decreases. However, as the angle of the slots  16   a - 1  is decreased, the distance which the motor  16   c  must move the plates  16   a  and  16   b  in order to open the contacts  12   d  and  13   b , increases. 
     Next, to aid in the description of another modification, reference should be made to FIGS. 11A and 11B. FIG. 11B shows an embodiment of the pressing mechanism  15  wherein the legs  15   b  and  15   c  have different lengths of L 1  and L 2  respectively; and, FIG. 11A provides an equation 15′ which expresses the spring constant k in terms of various parameters that are shown in FIG.  11 B. Inspection of FIG. 11B shows that the two slidable joints  15   e  and  15   f  are illustrated as sliding on a single axis  15   x . But as a modification, the joints  15   e  and  15   f  can slide on separate axes which are spaced apart and parallel to each other. With this modification, equation 15′ of FIG. 11A can still be used in conjunction with steps S 1 -S 5  of FIG. 6 to calculate the spring constant k for the spring  15   g.    
     Next, to aid in the description of another modification, reference should be made to FIGS. 1C and 2. Those figures illustrate the mating electrical contacts  12   d  and  13   b  which are pressed together by any one of the pressing mechanisms in FIGS. 2-19. The contacts  12   d  and  13   b  can be any type of pressed electrical contacts that are commercially available. For example, each contact  13   b  can be a springy contact such as a “fuzz-button”, and each contact  12   d  can be a non-springy metal pad for a corresponding fuzz button. Alternatively, each contact  12   d  can be the springy contact, and each contact  13   b  can be the non-springy metal pad. 
     Also, the planar members  12   a  and  13   a  on which the mating electrical contacts  12   d  and  13   b  are mounted are not limited to being a printed circuit board. Instead, the planar members  12   a  and  13   a  can be made of any electrical insulator such as a ceramic or a plastic or epoxy glass; and the electrical conductors which carry signals to and from the contacts  12   d  and  13   b  can be printed conductors that are integrated into the planar members  12   a  and  13   a , or separate wires that are attached to the planar members. 
     Preferably, each of the electrical contacts  12   d  and  13   b  have a contact resistance which is so small that a negligible IR drop occurs through the contacts. Having such a low contact resistance is especially important for the contacts which carry electrical power to the chips; and, this is because certain high power chips can draw a large amount of current, such as 50 amps. To decrease the resistance of a contact  12   d  or  13   b , the area which the contact occupies on the face of the substrates  12   a  and  13   a  should be increased. A large amount of room is available on the substrates  12   a  and  13   a  to increase the contact areas, as desired, and this is seen from FIGS. 1C and 2. 
     Next, to aid in the description of another modification, reference should be made to FIG.  6 . There, in step S 5 , the amount Δx 0  by which the spring  15   g  is compressed (or stretched) at the angle B 1  is determined from the equation Fx(B 1 )=kΔxo. However, as a modification, the spring  15   g  can be constructed with a “preload” such that an initial force of Fo must be exerted on the spring in order to start to compress it (or stretch it). In that case, step S 5  is modified such that Δxo is determined from the equation Fx(B 1 )−Fo=kΔxo. Likewise, this same modification can be incorporated into step S 17  of FIG.  15 . 
     Next, to aid in the description of another modification, reference should be made to FIGS. 1A and 1B. There, the subassemblies  12 - 15  are shown as being held in a vertical stack by the frame  11 . However, as a modification, the frame  11  of FIGS. 1A and 1B can be tipped over by ninety degrees. With this modification, the subassemblies  12 - 15  are held by the frame in a horizontal row. 
     Next, to aid in the description of another modification, reference should be made to FIG.  2 . There, the power converter subassembly  13  is shown as being held stationary in the frame  11 , and the chip holding subassembly  12  as well as the temperature regulating subassembly  14  are shown as being moved by the pressing mechanism  15 . However, as a modification, all of the subassemblies  12 - 14  in FIG. 2 can be rotated 180 degrees in the plane of the figure. With this modification, the pressing mechanism  15  pushes against and moves the power converter subassembly  13 , and the temperature regulating subassembly  14  is held stationary by the frame  11 . This modification can also be made in combination with the previously described modification where the temperature regulating subassembly  14  was eliminated. In that case, the pressing mechanism  15  would push against and move the power converter subassembly  13 , and the chip holding subassembly  12  would be held stationary by the frame  11 . 
     Next, to aid in the description of another modification, reference should be made to FIG.  2 . There, joint  15   d  in the pressing mechanism  15  pushes against the temperature regulating subassembly  14  in order to press the subassemblies  12 - 14  together. However, the pressing mechanism  15  can be used to push the temperature regulating subassembly  14  against the chip holding subassembly  12  even if power is supplied to the chips in a conventional fashion through an edge connector on the chip holding board  12   a . In that case, the power converter subassembly would be eliminated. This modification also can be made to all of the other illustrated embodiments. 
     Accordingly, in view of the above modifications, it is to be understood that the present invention is not limited to the details of any one of the illustrated preferred embodiments, but is defined by the appended claims.