Test mounting for surface mount device packages

A test mounting for LGA packages uses curved contact fingers to make electrical contact between lands on the LGA package and surface mount circuitry on a circuit board or the like. The mounting includes a guide member with guide apertures therein spaced from a support base with corresponding guide apertures. A bending plate positioned between the guide plate and the support base is moved laterally with respect thereto to increase or decrease the distance between the ends of each contact finger and thereby connect or disconnect the ends of the contact fingers with an LGA package mounted adjacent the guide plate and circuit pads on a circuit board on which the test mounting is mounted.

This invention relates to burn-in and test of electronic devices in surface
 mount packages. More particularly, it relates to methods and apparatus for
 mounting and holding electronic device packages during burn-in and test
 and to establishing and maintaining positive electrical contact to closely
 spaced input/output terminal lands or leads on such packages without
 damaging the electronic device, the device package, its interconnection
 terminals or the test socket and without introducing signal distortion.
 Advances in microelectronics technology tend to develop electronic device
 chips and packages which occupy less space while performing more functions
 faster. As a result, the number of electrical interconnections between the
 device package and external circuitry required for the circuits in the
 chips to communicate with the outside world increases and the physical
 size of each such interconnection must decrease. In order to provide
 electrical communication between the chip and external circuitry, circuit
 chips are usually contained within a housing or package which supports
 interconnection pads or lands, leads, balls, etc., on one or more of its
 external surfaces. In order to reduce overall lead length from chip to
 external circuitry and to provide adequate spacing between input/output
 terminals on the package, high pin count devices are sometimes mounted in
 packages in which the input/output terminals are in the form of conductive
 lands or pads formed on one or more faces of the package. The lands are
 often arranged in rows parallel with and adjacent peripheral edges of one
 face with the surfaces of the lands coplanar and parallel with (but
 slightly below) the bottom surface of the package. The lands may be
 arranged in other patterns such as parallel rows which cover the entire
 bottom surface in a grid pattern; lands grouped near the center of the
 bottom surface; or various combinations of such arrangements. Such device
 packages (commonly known as land grid array or LGA packages) may thus be
 mounted on circuit patterns on the surface of a circuit board or the like
 so that the terminal lands are bonded to mating lands or pads on the
 board.
 In many cases it is desirable that the completed device package be
 subjected to test and/or burn-in prior to acceptance and assembly onto a
 circuit board. While the terminal lands may be directly and permanently
 surface mounted on a circuit board by soldering, it is much more difficult
 to establish and maintain temporary electrical contact with each land
 without destroying or damaging the land, the package or the encapsulated
 device chip. In order to reliably test and burn-in such packages, the
 package must be temporarily mounted in a re-useable socket or mounting
 which makes precision interconnection between the input/output lands and
 outside circuitry without introducing signal distortion problems and
 without physically damaging the device package.
 As the size of the package decreases and the number of lands increases, the
 size and spacing of lands become smaller. Smaller and more closely spaced
 lands are, of course, more difficult to contact with test probes or the
 like. Furthermore, long or massive contact pins cannot be used for
 connecting external circuitry to the input/output lands for testing when
 high frequency devices are involved because such contact pins,
 particularly when closely spaced in order to contact closely spaced lands,
 introduce unacceptable signal distortion.
 Conventional burn-in and test apparatus employs test sockets mounted on a
 burn-in board with the pin-out leads of the test or burn-in socket passing
 through the bottom of the socket and through holes in a circuit board in
 conventional through-hole mountings. Interconnection of high frequency
 devices with outside circuitry using such conventional mountings can
 induce unacceptable signal distortion because of the high density of
 parallel terminal leads passing through the board.
 Miniaturized surface mount device packages have now been devised which have
 very closely-spaced input/output terminal lands on one face of the
 package. Such device packages, because of their extremely small size,
 configuration and physical construction are most difficult to handle
 without causing damage, yet good electrical contact with all the
 input/output terminal lands is essential. It is therefore desirable that
 apparatus be devised in which such small packages may be easily
 temporarily mounted (preferably by automation) and tested and/or
 stress-tested by burn-in and the like without damaging the device package
 or introducing signal distortion problems.
 In accordance with the present invention reliable precise electrical
 contact is provided between input/output lands on LGA packages and
 external circuitry by re-useable mounting apparatus which employs a
 support base, a guide plate and contact pins in the form of fingers which
 extend through guides in the guide plate to engage the input/output lands
 on an LGA package. The guide plate is formed of electrically insulating
 material and has a set of guides (such as holes or channels) arranged to
 correspond with the terminal land arrangement of the LGA package. The
 guide plate is supported on and spaced from a support base which has a set
 of guides arranged to correspond with input/output lands on external
 circuit media such as a circuit board, burn-in board or the like. An
 axially elongated contact pin or finger having first and second ends
 supported by a shank is positioned so that the first end of the contact
 finger extends through a guide in the guide plate and the second end of
 the contact finger extends through a guide in the support base to contact
 an input/output land on an external circuit board such as a burn-in board
 or the like. The shank of each contact finger extends through an aperture
 in a bending plate positioned intermediate the guide plate and the support
 base. When the bending plate is moved in a first direction with respect to
 the guide plate and support base, it bends the contact finger, thus
 drawing the ends of the fingers toward the bending plate. When moved in
 the opposite direction, the curvature of the fingers is decreased and the
 ends move outwardly through the guides in the guide plate and the support
 base. As the ends of the fingers move outwardly, they engage terminal
 lands on an LGA package mounted adjacent the guide plate and terminal
 lands on a circuit board or the like on which the support base is mounted.
 With the space between the lands on the LGA and the lands of the circuit
 board fixed by the size of the mounting apparatus, the contact pressure
 applied to the contact lands can be closely controlled by movement of the
 bending plate. Extending and retracting the ends of the contact fingers by
 bending the fingers also causes the ends of the fingers to move slightly
 laterally (in the direction opposite the direction of movement of the
 bending plate), thus causing the ends of the fingers to very slightly
 scrape the surfaces of the terminal lands to insure good electrical
 contact.

The drawing is incorporated into and forms part of the specification to
 illustrate exemplary embodiments of the invention. For clarity of
 illustration, like reference numerals designate corresponding elements
 throughout the drawing. It will be recognized that the principles of the
 invention may be utilized and embodied in many and various forms. In order
 to demonstrate these principles, the invention is described herein by
 reference to specific preferred embodiments. The invention, however, is
 not limited to the specific forms illustrated and described. Furthermore,
 the invention is not limited to use in connection with LGA packages of the
 type illustrated in FIG. 1. Sockets employing the principles of the
 invention may be designed and used to mount, test and burn-in device
 packages employing any of a wide variety input/output terminal
 arrangements. LGA terminal lands may, for example, be arranged in multiple
 rows, in centralized groups or in a full grid array. In addition, the
 mounting socket may be designed to accommodate leaded device packages
 wherein the leads, at some point during fabrication, have a configuration
 in which the leads have a surface which may be contacted from the
 underside (such as leaded packages wherein the leads are held in a molded
 carrier ring). Sockets may also be designed to contact the underside of
 leads extending from dual inline packages, gull wing packages or various
 other packages before (or after) the terminal leads are formed into their
 final configuration. It is only necessary that the package under test
 presents a plurality of coplanar lands or terminal surfaces which may be
 directly contacted from the underside of the package by vertically
 extending ends of contact fingers which electrically connect circuitry on
 the surface of an external board and the terminal land or lead on the
 device package.
 It should be understood that the invention is not limited to mounting only
 conventional encapsulated device packages. Input/output terminal lands are
 sometimes formed on bare chips or flip chip devices to form electrical
 interconnection with other chips or support media in multi-chip
 encapsulation packages. For example, a plurality of bare chips having
 terminal lands are sometimes mounted on a single support medium mounted
 within a single encapsulation package. It is often desirable to test these
 chips prior to assembly so that only "known good die" are ultimately
 mounted in the multi-chip arrangement. Such chips are characterized by
 having input/output lands on one face thereof and may be mounted in test
 sockets employing the principles of the invention. Accordingly, as used
 herein the term "LGA package" includes any package or chip, whether
 encapsulated or not, which employs a plurality of substantially coplanar
 input/output terminal lands or leads with surfaces extending in a plane
 substantially parallel with one face of the chip or package which may be
 physically contacted by the end of a contact pin or finger extending in a
 direction substantially normal to the plane of the surface of the terminal
 lands or leads. The terms "mounting apparatus" and "socket" are used
 synonymously herein to describe any device or apparatus for holding such
 LGA packages while providing electrical communication between its
 terminals and external circuitry such as a circuit board, burn-in board or
 the like.
 A typical LGA package 10 having input/output terminal lands 12 arranged in
 a grid array on its bottom surface 11 is illustrated in FIG. 1B. The lands
 12 are typically highly conductive metal such as gold or the like
 electrically connected with circuitry inside the package 10 and are
 arranged on the bottom surface 11 to be aligned in register with and
 soldered to corresponding lands, solder balls or the like on a circuit
 board or the like employing conventional surface mount technology. Various
 methods for forming such terminals are known and form no part of this
 invention. The number and arrangement of lands 12 will depend, of course,
 on the configuration and size of package 10 and the chip or the chips
 encapsulated therein. As the density of lands 12 on the surface increases,
 the lands become smaller and the spacing between lands decreases.
 Obviously, making reliable temporary electrical contact to the surfaces of
 closely spaced lands requires extremely small contact pins which must be
 maintained in precision alignment.
 Operational arrangement of an LGA package 10 with the preferred embodiment
 of the mounting of the invention is illustrated in FIG. 1. The device
 package 10 has a bottom face 11 (see FIG. 1B) on which are formed a
 plurality of terminal lands 12. The LGA package 10 is positioned on or
 parallel with the top face of guide plate 21 in housing 30 and positioned
 so that the contact surface of each terminal land 12 is aligned in
 register with an aperture 22 in guide plate 21. Adjustable alignment
 templates 13 may be used to accommodate various sizes and shapes of
 packages 10 and align the lands 12 with apertures 22. A hinged lid 14 or
 other conventional closure mechanism is used to secure the LGA package 10
 in fixed relationship with the top surface of guide plate 21.
 In the embodiment illustrated guide plate 21 and support base 25 are
 supported in a housing 30 in fixed spaced relationship with each other.
 Guide plate 21 has plurality of apertures 22 extending therethrough from
 its first surface 23 to its second surface 24 which act as guides for the
 contact fingers 40. Support base 25 has corresponding guides 26 extending
 therethrough from its inner surface 27 to its outer surface 28. The guide
 plate 21 and support base 25 are supported in housing 30 in spaced apart
 parallel relationship so that inner surface 27 of support base 25 is
 spaced from second surface 24 of the guide plate. Apertures 22 in guide
 plate 21 and apertures 26 in support base 25 are aligned in fixed
 relationship with each other. In the embodiment illustrated the apertures
 22 in guide plate 21 are directly aligned with apertures 26 in support
 base 25. However, direct corresponding alignment is not necessary so long
 as a direct relationship is maintained between each pair of guides 22, 26.
 An elongated contact pin or finger 40 is supported in each pair of guides
 22, 26 so that the end portion 41 adjacent first end 42 is positioned an
 guide 22 and the end portion 43 adjacent second end 44 is positioned
 within corresponding guide 26. First end portion 41 and second end portion
 43 are connected by an elongated central portion or shank 45. A widened
 tab 46 (see FIG. 4) adjacent second end portion 43 is loosely confined
 between the inner surface 27 of support base 25 and a trap plate 50 which
 is parallel with and spaced from inner surface 27 of support base 25. Trap
 plate 50 has a plurality of apertures 51 therein aligned with guides 26 in
 support base 25. The apertures 51 and guides 26 are dimensioned so that
 shank 45 and second end portion 43 pass readily by therethrough but
 prevent passage of tab 46. The surface of trap plate 50 is spaced from
 inner surface 27 of support base 25 to loosely trap tab 46 therebetween.
 Thus axial movement of contact finger 40 is limited so that the first end
 portion 41 cannot be fully withdrawn from guide 22 in the guide plate 21
 and the second end portion 43 cannot be fully withdrawn from guide 26. End
 portions 41 and 43 may move axially within guides 22 and 26, respectively,
 but axial movement is limited and the dimensions of the guides 22, 26
 prevent other movement of the contact finger 40.
 A bending plate 60 is supported between and parallel with guide plate 21
 and support base 25. Bending plate 60 has an arrangement of apertures 61
 therein which corresponds with the arrangement of guides 22 in guide plate
 21 and guides 26 in support base 25. Bending plate 60 is positioned so
 that the shank 45 of each contact finger 40 may pass directly through an
 aperture 61. However, bending plate 60 is mounted for lateral movement in
 a plane parallel with second surface 24 of guide plate 21 and inner
 surface 27 of support base 25 in response to rotation of cam 65.
 As shown in FIG. 2 the ends 42, 44 of contact finger 40 are drawn inwardly
 (toward bending plate 60) when cam 65 is rotated to engage lobe 66 with
 the end of bending plate 60 and move the bending plate laterally (to the
 left as shown in FIG. 2). This bends the fingers 40 into a curve and
 reduces the distance between ends 42, 44 to position the fingers 40 in the
 "open" position.
 Surface mounting of test and burn-in sockets can minimize signal distortion
 resulting from cross-talk, reactive capacitance, etc., by spreading the
 interconnection circuitry across the surface of the board and reducing
 parallel lead lengths. Accordingly, mounting 30 is mounted directly on the
 surface of a circuit board 70, burn-in board or the like which has surface
 mount contact pads 71 on the surface thereof arranged in a pattern
 corresponding to the pin-out footprint of the LGA package. The mounting 30
 thus provides direct contact between pads 71 on the board and lands 12 on
 the LGA package. It will be appreciated, of course, that mounting 30 may
 be designed to spread the second ends 44 so that the pin-out footprint is
 enlarged.
 With the mounting 30 mounted on a board 70, an LGA package is positioned
 adjacent the top surface of guide plate 21 with its lands 12 aligned in
 register with guides 22 in guide plate 21. With cam 65 rotated so that
 lobe 66 urges bending plate 60 to the left as shown in FIG. 2., the ends
 42, 44 of contact fingers 40 are aligned with but slightly spaced from
 lands 12 and pads 71, respectively. The mounting socket is closed by
 rotating cam 65 so that bending plate 60 moves in the opposite direction
 (to the right as shown in FIG. 3). The bending plate may be moved to the
 closed position by any suitable means such as springs (not shown) or cams
 such as cam 65. With proper choice of materials, the fingers 40 may have
 sufficient memory to move the bending plate 60 in returning to the relaxed
 (unbiased) position. As the bending plate 60 moves to the right as shown
 in FIG. 3., the ends 42, 44 move outwardly and, guided by guides 22, 26,
 engage the faces of lands 12 and pads 71, respectively.
 It should be noted that as the ends 42, 44 move outwardly, they also move
 very slightly laterally because the curvature of fingers 40 is reduced.
 The ends 42, 44 thus lightly scrape the surfaces they engage to remove any
 oxides and assure good electrical contact with the contact finger 40.
 Although the bending plate 60 moves all the fingers 40 simultaneously, the
 pressure exerted by each finger is independent of the other fingers. By
 selection of the composition, size, shape, etc., of the fingers 40 and
 control of movement of the bending plate 60, the pressure exerted at the
 contact ends 42, 44 of each contact finger may be precisely controlled.
 In the embodiment illustrated, fingers 40 are thin flat ribbons or strips
 and guides 22, 26 are correspondingly-shaped and sized apertures which
 limit movement of the contact fingers therethrough to axial movement only.
 It will be readily recognized, however, that both the fingers 40 and
 guides 22, 26 may take other shapes so long as the guides 22, 26 control
 the direction of movement of the ends of the fingers 40.
 In the preferred embodiment illustrated, the bending plate 60 is positioned
 approximately midway between the guide plate 21 and the support base 25
 and is moved laterally with respect to the guide plate and the support
 base. It will be readily recognized, however, that other arrangements will
 produce similar results. For example, the bending plate 60 and either the
 guide plate 21 or support base 25 could held stationary and the other
 moved with respect thereto. Similarly, the function of the trap plate 50
 may be performed by the bending plate 60 if the tabs and the bending plate
 are appropriately sized and positioned.
 It will be recognized that all components of the mounting apparatus of the
 invention may be fabricated from readily available materials using
 conventional techniques. Of course, when the mounting apparatus is to be
 used as a burn-in socket, materials must be chosen which will withstand
 the temperatures involved as well as repeated usage. It will also be
 recognized that loading and unloading of device packages 10 may readily be
 automated using conventional techniques.
 Likewise, while the embodiment illustrated employs templates 13 and lid 14
 to orient and hold an LGA package of the type shown in FIG. 1 with respect
 to the guide plate 21, various other structures may be employed to receive
 other types of packages or chips such as bare die, flip chips, etc., and
 leaded packages configured to be an LGA package as that term is defined
 herein.
 It will be apparent from the foregoing that the principles of the invention
 may be used to mount and form temporary electrical contact with the
 terminal leads or lands of various device packages without risk of
 damaging the device or its input/output terminals. Because of the unique
 structure, mounting apparatus employing the invention may be used for
 burn-in and/or test of high frequency devices without concern for signal
 distortion introduced by the test socket or its interconnection with the
 burn-in board or other support media. It is to be understood, however,
 that even though numerous characteristics and advantages of the invention
 have been set forth in the foregoing description together with details of
 the structure and function of various embodiments, this disclosure is to
 be considered illustrative only. Various changes and modifications may be
 made in detail, especially in matters of shape, size, arrangement and
 combination of parts, without departing from the spirit and scope of the
 invention as defined by the appended claims.