Abstract:
Disclosed is a platen for a molding press for encapsulating semiconductor dies on a substrate, the platen comprising: a first mold chase having a first mold chase surface; the platen being operable to cooperate with a further platen having a second mold chase with a second mold chase surface to clamp a substrate, which is held against a substrate-facing surface relating to either the first or second mold chase surface, between the first and second mold chase surfaces to define at least one mold cavity with respect to the substrate; wherein the platen further comprises a rotational mounting device on which either the first or second mold chase is rotatable about at least one axis passing through the centre of the substrate-facing surface to adjust the relative arrangement of the first and second mold chase surfaces. Also disclosed is a molding press comprising the platen, and the further platen cooperating with the platen.

Description:
BACKGROUND 
       [0001]    The present invention relates to the field of semiconductor assembly and packaging, and more particularly, to a molding press (and a platen thereof) for applying an encapsulant to semiconductor dies on a substrate. 
         [0002]    Packaging, also known as encapsulation, is an important part of the semiconductor assembly process. Typically, encapsulation is performed by either transfer molding or compression molding. 
         [0003]    In transfer molding, the molding system includes a first platen having a supply pot which receives a molding compound, for example in the form of a solid pellet. The first platen also has a plurality of cavities. The first platen is pressed against a second platen on which a substrate carrying a plurality of semiconductor dies is held, such that the cavities of the first platen overlie the semiconductor dies. The molding compound is melted, with the application of heat and pressure, to a liquid state, and the liquefied molding compound is then forced by a plunger into runners connected between the plunger and the molding cavities to enter into the molding cavities via narrow gates. The molding compound is then cured and the encapsulated substrate then removed from the mold. 
         [0004]    In compression molding, a molding compound in the form of powder or liquid or paste resin is loaded into one or more mold cavities of a bottom platen (in the case of die-down molding) or directly onto a substrate which is held on the bottom platen (in the case of die-up molding). A heater plate in the bottom platen is then used to melt the molding compound. Next, a mold chase of a top platen is clamped against a mold chase of the bottom platen to form a mold cavity between the top and bottom platens, with the molten molding compound then being cured to form a mold cap which encapsulates the dies. 
         [0005]    In either a transfer molding or a compression molding encapsulation process, it is critical to maintain substantial parallelism between the substrate and the opposed surface of the mold cavity. Otherwise, there may be defects in the mold cap caused by incomplete filling of the mold cavity. 
         [0006]    There remains a need for a molding press which overcomes or alleviates at least one of the foregoing difficulties, or which at least provides a useful alternative. 
       SUMMARY 
       [0007]    Certain embodiments of the invention relate to a molding press for encapsulating a substrate, the molding press comprising:
       a first mold chase with a first mold chase surface; and   a second mold chase with a second mold chase surface;   the first and second mold chases being operable to clamp a substrate, which is held against a substrate-facing surface relating to either the first or second mold chase surface, between the first and second mold chase surfaces to define at least one mold cavity with respect to the substrate;   wherein the molding press further comprises a rotational mounting device on which either the first or second mold chase is rotatable about at least one axis passing through the centre of the substrate-facing surface to adjust the relative arrangement of the first and second mold chase surfaces.       
 
         [0012]    Other embodiments of the invention relate to a platen for a molding press for encapsulating a substrate, the platen comprising:
       a first mold chase having a first mold chase surface;   the platen being operable to cooperate with a further platen having a second mold chase with a second mold chase surface to clamp a substrate, which is held against a substrate-facing surface relating to either the first or second mold chase surface, between the first and second mold chase surfaces to define at least one mold cavity with respect to the substrate;   wherein the platen further comprises a rotational mounting device on which either the first or second mold chase is rotatable about at least one axis passing through the centre of the substrate-facing surface to adjust the relative arrangement of the first and second mold chase surfaces.       
 
         [0016]    By ensuring that the at least one axis of rotation passes through the centre of the substrate-facing surface, it is possible to allow the relative orientation of the first and second mold chase surfaces to be adjusted as the first and second mold chases are being clamped together, to enhance the co-planarity between the substrate and the opposed surface of the at least one mold cavity. This allows creation of a molding cavity of substantially uniform depth, and makes it possible to substantially avoid molding defects caused by incomplete filling of the mold cavity, for example. Additionally, by centering the rotation axis or axes on the substrate it is possible to avoid introducing an offset between the substrate centre and the opposed mold surface. 
         [0017]    In certain embodiments, the platen comprises a drive mechanism configured to rotate the first mold chase. 
         [0018]    In certain embodiments, the rotational mounting device may be configured to allow rotation of either the first or second mold chase about two orthogonal axes passing through the centre of the substrate-facing surface to adjust the relative arrangement of the first and second mold chase surfaces. The rotational mounting device may comprise roller bearings. The platen may comprise a middle portion to which the top portion is mounted by a first rotational mounting, and a base portion to which the middle portion is mounted by a second rotational mounting. The drive mechanism may be configured to rotate the top portion relative to the middle portion about the first rotational mounting, and the middle portion relative to the base portion about the second rotational mounting. 
         [0019]    The platen may comprise a plurality of load cells positioned to measure clamping forces applied during clamping of a substrate. The drive mechanism may be configured to rotate the top portion and/or the middle portion to balance said clamping forces. The load cells may be positioned between first and second parts of the top portion of the platen, or may be positioned beneath the base portion. In either case, the load cells may be aligned with the two orthogonal axes. 
         [0020]    In certain embodiments, the first mold chase may comprise a clamping plate coupled to the top portion by a plurality of springs, the clamping plate comprising a plurality of gauging sensors positioned to measure spring compression at different locations on the clamping plate. The drive mechanism may be configured to rotate the top portion and/or the middle portion to minimize any differences in the measured spring compression at the different locations. The gauging sensors may be positioned in alignment with the two orthogonal axes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    Embodiments of the invention will now be described, by way of non-limiting example only, with reference to the accompanying drawings in which: 
           [0022]      FIG. 1  is a side elevation view of a molding press, in an unclamped configuration, according to embodiments of the invention; 
           [0023]      FIG. 2  is an alternative side elevation view of the molding press of  FIG. 1 ; 
           [0024]      FIG. 3  is a side elevation view of the molding press of  FIG. 1  in a clamped configuration; 
           [0025]      FIG. 4  is a side elevation view of a lower platen of the molding press of  FIG. 1 ; 
           [0026]      FIG. 5  is a cross-sectional view through the line A-A of  FIG. 4 ; 
           [0027]      FIG. 6  is a partial exploded perspective view of the platen of  FIG. 4 ; 
           [0028]      FIG. 7  and  FIG. 8  are side elevation views of the platen showing a pitch-and-roll orientation adjustment mechanism; 
           [0029]      FIGS. 9 to 11  are schematic cross-sectional views of an alternative embodiment of a lower platen showing pitch-and-roll adjustment during a compression molding process; and 
           [0030]      FIGS. 12 to 14  are schematic cross-sectional views of an alternative embodiment of a lower platen showing pitch-and-roll adjustment during a transfer molding process. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]    Referring initially to  FIG. 1  and  FIG. 2 , there is shown a molding press  100  having an upper (first) platen  110  and a lower (second) platen  120 . The molding press is in an open or unclamped configuration. In operation, the upper platen  110  is urged towards the lower platen using a toggle mechanism  114  of known construction, in order to clamp a substrate  130  between the upper platen  110  and lower platen  120  as shown in  FIG. 3 , prior to executing a molding procedure. The lower platen  120  has an angular adjustment mechanism which can be used to adjust the relative orientation of planar surfaces of the mold chases of the upper and lower platens to account for substrate unevenness, in a manner which will be described in detail below. The molding press  100  shown in  FIG. 1-3  is particularly suitable for compression molding, but the angular adjustment mechanism described below in relation to lower platen  120  is equally applicable to the upper platen  110  and also to transfer molding processes as will become apparent. 
         [0032]    The upper platen  110  carries an upper (first) mold chase  112  to which a substrate  130  can be secured by a vacuum. The lower platen  120  carries a lower (second) mold chase  122  against which the upper mold chase  112  is clamped when the toggle mechanism  114  is actuated, as shown in  FIG. 3 . 
         [0033]    The lower platen  120  comprises a top portion  124  rotatably mounted to a middle portion  126  which is in turn rotatably mounted to a base portion  128 . The lower platen  120  also comprises a drive mechanism  140  for controlling the relative movements of the top, middle and base portions, using a microcontroller of known type coupled to a general purpose computer system (not shown), for example. 
         [0034]    As shown in  FIG. 4  and  FIG. 5 , in which the drive mechanism  140  is omitted for clarity, lower platen  120  comprises a rotational mounting device (shown as a pair of rotational mountings  220  and  222 ). Each rotational mounting  220 ,  222  comprises a roller bearing mechanism, comprising a series of cylindrical rollers in a part-circular raceway. Advantageously, the use of rotational mountings  220 ,  222  allows pivoting movement of the lower mold chase  122  without the use of bushing or guiding mechanisms or the like, which would lead to a structurally overconstrained system. 
         [0035]    The top portion  124  of the platen  120  comprises upper  124 A and lower  1248  parts, and is mounted to the middle portion  126  by the first rotational mounting  222 , the middle portion  126  in turn being mounted to the base portion  128  by the second rotational mounting  220 . The rotational mountings  220 ,  222  provide two degrees of rotational freedom for the lower platen  120  with the top, middle and base portions  124 ,  126 ,  128  being able to rotate relative to each other about two orthogonal axes  230  and  232  which meet at the centre  234  of a plane passing through the upper surface of the lower mold chase  122  ( FIG. 6 ). In cross-section, each raceway forms part of a circle with its centre at the intersection  234  of the two orthogonal axes  230 ,  232 . 
         [0036]    Accordingly, the top, middle and base units  124 ,  126  and  128  together with the mountings  220 ,  222  provide a pitch-and-roll mechanism to allow angular alignment of a substrate at the upper surface of lower mold chase  122 , via pivoting movement of the upper surface about the centre  234  as depicted in  FIG. 7  and  FIG. 8 . The pitch-and-roll mechanism may be passive, such that the top  124  and middle  126  portions naturally roll on the roller bearings in response to differences in contact forces at respective edges or corners of the clamping plate  122 . However, it is particularly advantageous to make use of an active drive mechanism  140  which drives the top  124  and middle  126  portions responsive to measured differences in the contact forces, the measured differences being obtainable using gauging sensors and/or load cells as will be described below. 
         [0037]    The lower mold chase  122  comprises a clamping plate  122 A resiliently coupled to the top portion  124  by a plurality of springs  123  extending around a periphery of the lower surface of the clamping plate  122 A. Clamping plate  122 A has an exposed interior region having a surface  122 B which faces towards substrate  130  when the substrate is clamped between the upper mold chase  112  and lower mold chase  122 . The surface of the upper mold chase  112  to which the substrate is retained thus defines a first plane while the substrate-facing surface  122 B of the lower mold chase  122  defines a second plane. When the substrate  130  is clamped, a mold cavity is defined between the substrate and the substrate-facing surface  122 B, which may be an upper surface or partial upper surface of the top portion  124  of the lower platen  120  (as shown in  FIG. 5 ), or alternatively may be an upper surface of a plunger which is mounted within the top portion  124  for reciprocating movement along an axis towards the upper mold chase  112  to compress a molding resin within the mold cavity. 
         [0038]    The lower platen  120  further comprises a plurality of load cells  204 ,  206 , and a plurality of gauging sensors  200 ,  202 , which may be inductive sensors, for example. 
         [0039]    As shown in  FIG. 4 , first gauging sensors  200  each comprise an optical beam component  201  configured to transmit an optical beam towards a target  210  and to measure the distance between the beam  201  and target  210  according to a measured reflected beam. The targets  210  extend from a first opposed pair of edges of the clamping plate  122 A and are aligned with the “roll” axis  230 . The beam components  201  extend from a first opposed pair of edges of the top portion  124  of the lower platen  120 , and are likewise aligned with axis  230  and also with respective targets  210 . Accordingly the gauging sensors  200  are positioned to detect changes in distance which are due to compression and expansion of springs  123 . More particularly, the gauging sensors  200  can detect an imbalance in the contact forces applied at the opposed edges of the clamping plate  122 A, due to differential spring compression at the edges and thus a difference in spacings between the respective beam components  201  and targets  210 , and the measured difference can then be used to adjust the angular orientation of the top portion  124 . 
         [0040]    Similarly, as shown in  FIG. 5 , second gauging sensors  202  each comprise a target  212  extending from one of a pair of opposed edges of the clamping plate  122 A and are aligned with the “pitch” axis  232 , which is orthogonal to the “roll” axis  230 . Second gauging sensors  202  each also comprise a beam component  203  extending from a respective opposed edge of the top portion  124  of the lower platen  120 , the beam components  203  being aligned with axis  230  and also with respective targets  212 . 
         [0041]    The load cells  204 ,  206  are located within the top portion  124  of the lower platen  120 , specifically between an upper part  124 A and a lower part  124 B, and provide an alternative mechanism for monitoring differences in contact forces applied to the clamping plate  122 A. The load cells are configured to measure compressive forces acting on the upper surface of the clamping plate  122 A. First load cells  204  are located approximately below first respective opposed edges of the clamping plate  122 A and second load cells  206  are located approximately below second respective opposed edges of the clamping plate  122 A. The first load cells  204  are in alignment with the “roll” axis  230  and the second load cells  206  are in alignment with the “pitch” axis  232 . 
         [0042]    If load cells  204 ,  206  are used for detecting an imbalance in clamping forces and to thereby provide feedback to drive mechanism  140  for angular alignment purposes, for example in a transfer molding process, the resilient coupling of clamping plate  122 A to the top portion  124  may be replaced with a rigid connection, and the gauging sensors  200 ,  202  may be omitted. 
         [0043]    An example of angular alignment of surfaces of the upper and lower mold chases during a compression molding process will now be described with reference to  FIG. 9-11 . Referring firstly to  FIG. 9 , a molding press  300  has an upper (first) platen with an upper (first) mold chase  310  to which a substrate  130  is secured by a vacuum at a surface  313 . Molding press  300  also has a lower (second) platen  320  with an angular alignment mechanism which operates in similar fashion to that described above in relation to lower platen  120 . Lower platen  320  has a rotational mounting device comprising a pair of rotational mountings, as for lower platen  120 , although only one rotational mounting  316  (equivalent to rotational mounting  220  of lower platen  120 ) is shown for illustrative purposes. A top portion  324  of the lower platen  320  is mounted to a middle portion (not shown) which is in turn mounted to a base portion  328  by the rotational mounting  316 , such that the top portion  324  and middle portion can rotate relative to the base portion  328  on rotational mounting  316 . 
         [0044]    Extending from opposed edges of the top portion  324  are beam components  302 A and  302 B of respective gauging sensors. Corresponding targets  312 A and  312 B extend from opposed edges of a clamping plate  322  of a lower (second) mold chase of the lower (second) platen  320 , the clamping plate  322  being resiliently coupled to the top portion  324  by springs  323 A and  323 B at its periphery. The beam components  302 A and  302 B are aligned with their respective targets  312 A and  312 B and are also aligned with an axis (not shown) passing through the centre of a plane defined by the upper surface of the clamping plate  322 , as previously described. The top portion  324  has a packing plunger  340  with an upper surface  342  which faces the substrate  130  when the substrate  130  is clamped between the upper and lower mold chases. The packing plunger  340  may be a stationary component or may be mounted for reciprocating movement within the top portion  324  to compress a molding resin loaded in a mold cavity  350  defined between the plunger upper surface  342  and the substrate  130  ( FIG. 10 ). 
         [0045]    In  FIG. 9 , the molding press  300  is in an unclamped configuration and the springs  323 A and  323 B are compressed only by the weight of the clamping plate  322 . The distance between beam component  302 A and target  312 A is A, and between beam component  302 B and target  312 B is B. Prior to clamping, A will differ from B by an amount a=|A−B|. 
         [0046]    The substrate  130  retained at surface  313  of upper mold chase  310  has a surface  131  facing the lower mold chase which is not parallel to surface  313  or to upper surface  342  of the packing plunger  340 . Accordingly, even though a first plane defined by the surface  313  of upper mold chase is parallel to a second plane defined by the upper surface  342 , when the molding press is placed in a clamped configuration with substrate  130  pressed against clamping plate  322 , the mold cavity  350  formed between the surface  131  and the upper (substrate-facing) surface  342  of the plunger will be uneven ( FIG. 10 ). 
         [0047]    To address the unevenness of the mold cavity  350 , a microprocessor coupled to a drive mechanism (similar to drive mechanism  140 , for example) receives feedback from the gauging sensors ( 302 A,  312 A) and ( 302 B,  312 B) as the substrate  130  is clamped against the clamping plate  322 . The substrate  130  is slightly thicker at edge “A” of the clamping plate  322 , so that edge of the substrate will contact the clamping plate  322  first. As it does so, it begins to compress spring  323 A such that distance A begins to decrease. The upper mold chase  310  continues to press down on clamping plate  322  until the opposite edge of the substrate contacts edge “B” of the clamping plate  322 . At this point, the gauging sensors measure distances A′, which is less than A, and B′, which may be larger than B due to the movement of the clamping plate  322 . Since the microprocessor now sees a difference |A′−B′| which is no longer equal to a, it will be apparent that there is unevenness in the substrate surface  131  and that correction is required to ensure parallelism between the substrate surface  131  and the packing plunger surface  342  during encapsulation. In this case, the microprocessor detects that there is greater compression at edge “A”, so that the drive mechanism  140  should be actuated to rotate top portion  324  on rotational mounting  316  towards edge “A”, until the difference between distances A″ and B″ is close to the originally measured difference a ( FIG. 11 ). This causes the orientation of the plane defined by the plunger upper surface to change such that the plunger upper surface  342  is parallel to substrate surface  131  as shown in the mold cavity  350 ′ of  FIG. 11 . At this point, resin in the mold cavity  350 ′ can be heated, the packing plunger  342  urged towards the substrate surface  131 , and the resin then cured to encapsulate dies on the substrate surface  131  with a substantially even molding cap thereby being formed on the dies. 
         [0048]    An example of angular alignment of surfaces of the upper and lower mold chases during a transfer molding process will now be described with reference to  FIG. 12-14 . In  FIG. 12 , a transfer molding press comprises an upper (first) platen having an upper (first) mold chase  412 , and a pair of lower (second) platens  420 A and  420 B, each having a lower (second) mold chase comprising a substrate-facing surface  422 A and  422 B respectively. Respective substrates  130 A and  1308  can be retained against respective substrate-facing surfaces  422 A and  422 B by vacuum. 
         [0049]    Upper mold chase  412  comprises a plunger pot and runner system  460  having channels through which molten resin can be injected, by plunger  470 , into mold cavities formed between surface  414 A and substrate  130 A, and between surface  414 B and substrate  1308 , during a transfer molding process when the substrates  130 A and  1308  are clamped. It will be appreciated that any number of recesses can be formed in the upper mold chase  412  to cooperate with corresponding lower mold chases of respective lower platens  420 A,  420 B, etc. In some embodiments only a single lower platen, cooperating with a single recess of the upper mold chase  412 , may be employed, although it is operationally more efficient to provide multiple lower platens per upper platen. 
         [0050]    As shown in  FIG. 13 , the upper platen and upper mold chase  412  are urged towards the lower platens  420 A,  420 B (or vice versa if preferred, in the direction indicated by the solid arrows). As the upper mold chase  412  contacts the substrates  130 A and  130 B retained on lower mold chases  422 A,  422 B, it encounters surface unevenness for each substrate. Substrate  130 A is thicker at one edge  138 A than the other edge  136 A, such that edge  438 A of the first recess of the upper mold chase  412  contacts substrate edge  138 A before edge  436 A contacts substrate edge  136 A. Thus, as can be seen from  FIG. 13 , the mold cavity  450 A between surface  414 A and substrate  130 A is not properly formed since substrate  130 A is not clamped at edge  136 A. Similarly, substrate  1308  is not properly clamped at edge  1388  such that mold cavity  450 B is not properly formed at the second lower mold chase  422 B. 
         [0051]    In order to correctly form the mold cavities, load cells  402 A and  404 A of the first lower platen  420 A and load cells  402 B and  404 B of the second lower platen  420 B are used to measure differences in compression force at the respective edges and to provide feedback to respective independent drive mechanisms of the lower platens. At lower platen  420 A, load cell  402 A measures a lower force than load cell  404 A, such that the drive mechanism is actuated to rotate top portion  424 A and middle portion  426 A relative to bottom portion  428 A on the roller bearing  416 A, in the direction of edge  138 A of substrate  130 A. The drive mechanism is operative until the forces at load cells  402 A and  404 A are balanced. Similar principles apply to the second lower platen  420 B, except that the force at load cell  402 B is measured to be higher than that at load cell  404 B, so that the top portion and middle portion  424 B,  426 B need to be rotated in the opposite direction to the corresponding portions of the first lower platen  420 A. Once the forces are balanced, properly formed mold cavities  450 A′ and  450 B′ are obtained, with the surfaces  414 A and  414 B being parallel to respective substrates  130 A and  130 B. At this point, a charge  472  of resin loaded in the pot can be heated and the molten resin pushed through plunger pot/runner system  460  into the cavities  450 A′ and  450 B′ and cured to complete the molding process. 
         [0052]    Although particular embodiments of the invention have been described in detail, many modifications and variations are possible within the scope of the invention, as will be clear to a skilled reader. For example, the rotational mountings  220 ,  222  of the lower platen  120  as described above can be incorporated within the upper platen  110  instead, so as to achieve co-planarity between the substrate and the opposed surface of the mold cavity. Alternatively, both the upper and lower platens  110 ,  120  may comprise corresponding rotational mountings to achieve the desired objective. For example, each platen  110 ,  120  may comprise a rotational mounting device which comprises at least one rotational mounting such as a roller bearing. Yet further, the upper mold chase  112  or the lower mold chase  122  is rotatable on the rotational mounting device about at least one axis passing through the centre of the substrate-facing surface to adjust the relative arrangement of the upper and lower mold chase surfaces for clamping the substrate in-between. In one variant, the upper mold chase  112  may be rotatable about a first axis while the lower mold chase  122  is rotatable about a second axis orthogonal to the first axis. In another variant, each of the upper  112  and lower  122  mold chases may be rotatable about two orthogonal axes passing through the centre of the substrate-facing surface.