Patent Publication Number: US-6710257-B2

Title: Circuit encapsulation

Description:
This application is a divisional of co-pending application Ser. No. 09/337,581, filed Jun. 22, 1999, which is a divisional of application Ser. No. 08/802,762, filed Feb. 20, 1997 (now U.S. Pat. No. 5,945,130), which is a continuation-in-part of application Ser. No. 08/340,162, filed Nov. 15, 1994 (now U.S. Pat. No. 5,728,600). 
    
    
     BACKGROUND 
     This invention relates to circuit encapsulation. 
     The circuit  10 , shown in FIGS. 1 a  and  1   b,  for example, has integrated circuit dies (i.e., semiconductor dies)  12 , and  14 , and other electrical components  15 ,  16 ,  17 , and  18  connected by a printed circuit board (PCB)  20 . Encapsulation of circuit  10  is done within a mold cavity  22  of a mold  24  (FIGS. 2 a - 2   e ) using a molding compound  35 . Connections to PCB  20  are made available externally to the molding compound by soldering conductive leads  26   a,    26   b,    26   c,    26   d,    26   e,  and  26   f  (FIGS. 1 a,    1   b ) of metal lead frame  26  to input/output (I/O) pads  30  of PCB  20 . 
     I/O pads  30  are also connected through conductive leads to the electrical components of circuit  10 . For clarity, only conductive leads  28  (FIG. 1 a ) and  29  (FIG. 1 b ) are shown. Metal lead  26   b  is connected to an I/O pad  30  which is connected to conductive lead  28 . A conductive pad  12   a  on semiconductor die  12  may be connected through wire  12   b  to conductive lead  28  (FIG. 2 b ) or a conductive pad (not shown) on a bottom side of die  12  could be connected to conductive lead  28 . Metal lead  26   f  is connected to an I/O pad  30  which is connected by a plated through hole  31  (FIG. 2C) to conductive lead  29 . A conductive pad (not shown) on a bottom side of electrical component  17  is directly connected to conductive lead  29 . 
     Mold  24  has a top  24   a  and a bottom  24   b  that are closed on portions  26   g  and  26   h  (FIGS. 2 a,    2   d ), and leads  26   a,    26   b,    26   c,    26   d,    26   e  and  26   f  (FIGS. 2 b,    2   c,    2   e ) of lead frame  26 . Once mold  24  is closed, transfer molding is done by pushing molding compound  35 , at 1000 psi, from a pot  36  (in mold bottom  24   b ), using a piston  40 , into a runner  38  (also in mold bottom  24   b ) and into mold cavity  22  to surround circuit  10 . After molding, circuit  10 , encapsulated in cured molding compound, is removed from mold  24 , and lead frame  26  is cut along dashed line  42  (FIGS. 1 a,    1   b ), and waste molding compound is trimmed away along dashed line  43 . 
     In one example, lead frame  26  has a thickness of approximately 0.008 inches with a tolerance of +/−0.00025 inches. When lead frame  26  is at a minimum thickness (i.e., 0.008−0.00025=0.00775), top  24   a  closes against bottom  24   b,  as shown in FIGS. 2 a,    2   b,    2   c  (i.e., they touch). The pressure of mold  24  on lead frame  26  does not damage lead frame  26 . When lead frame  26  is at a maximum thickness (i.e., 0.008+0.00025=0.00825), top  24   a  does not close against bottom  24   b  leaving a gap  44 , as shown in FIGS. 2 d,    2   e.  The maximum height of gap  44  is 0.0005 inches (i.e., maximum thickness variance, 0.00025+0.00025=0.0005). Typical molding compounds will not leak, indicated by arrow  45 , through a gap  44  of 0.0005 inches or less due to the viscosity of the molding compound. 
     Referring to FIGS. 3 a,    3   b,    3   c,  metal lead frame  26  (FIGS. 1 a,    1   b ) is not needed to make external connections to I/O pads  30  of a component  9  because only a portion of one side of PCB  20  is encapsulated in cured molding compound  50  leaving I/O pads  30  exposed. Electrical components are generally not mounted in the exposed areas. 
     The thickness of PCB  20  is approximately 0.020 inches and has a tolerance of +/−0.0025 inches. The maximum thickness variation of 0.005 inches (i.e., 0.0025+0.0025=0.005) makes it difficult to use conventional molds to fully encapsulate circuit  10  (i.e., top, bottom, and sides) with the exception of I/O pads  30  (component  9 , FIGS. 4,  5   a,  and  5   b ). 
     SUMMARY 
     In general, in one aspect, the invention features an apparatus for encapsulating a circuit on a circuit board. The apparatus has a first mold section configured to close on one side of the board. The first mold section has an exposed first conduit. The apparatus also has a second mold section configured to close on another side of the board. The second mold section has a second conduit for pushing molding compound into a mold cavity in at least one of the mold sections. The second conduit has a side opened to the first mold section when the first and section mold sections are closed on the board. The apparatus also has a piston slidably mounted inside the first conduit and configured to extend toward the second mold section to close the side of the second conduit. 
     Implementations of the invention may include one or more of the following. The board may include a portion that extends over the side of the second conduit, and the portion of the board is crushed by the piston. The piston may have a face configured to close the side of the second conduit and a rim extending from the face and configured to crush the portion of the board. The rim may be arcuate. 
     The second mold section may have a depression for receiving the board. The piston has a knife extending from a face, and the knife is configured to exert force on the board to seat the board against an end stop of the depression when the piston contacts the portion of the board. The depression may have another knife configured to exert force on the board to seat the board in the depression when the piston contacts the portion of the board. One or both of the knives may be asymmetric. 
     The second mold section may have a depression for receiving the board. The depression has a knife extending from the depression. The knife is configured to exert force on the board to seat the board in the depression when the piston contacts the portion of the board. 
     In another aspect, the invention features encapsulating a circuit on a circuit board. A first mold section is closed on a first mold section on one side of the board. The first mold section has an exposed first conduit. A second mold section is closed on another side of the board. The second mold section has a second conduit for pushing molding compound into a mold cavity in at least one of the mold sections. The second conduit has a side opened to the first mold section when the first and section mold sections are closed on the circuit board. A piston is extended through the first conduit to close the side of the second conduit. 
     Implementations of the invention may include one or more of the following. The board may partially extend over the side of the second circuit, and a portion of the board extending over the side of the second conduit is crushed by the piston. The piston may be used to exert lateral forces on the board to seat the board within the second mold section. 
     Advantages of the invention may include one or more of the following. Unused portions of the circuit board are minimized. A tight seal is formed between the mold sections and the circuit board. Tolerances in the thickness of the circuit board are accomodated. 
     A variety of other advantages and features will become apparent from the following description and from the claims. 
    
    
     DESCRIPTION 
     FIGS. 1 a  and  1   b  are plan views of a top and a bottom, respectively, of a circuit connected to a metal lead frame. 
     FIGS. 2 a,    2   b,    2   c,    2   d,  and  2   e  are cross-sectional side views of the structure of FIGS. 1 a  and  1   b  inserted within a mold. 
     FIG. 3 a  is a cross-sectional side view of a circuit having a portion of one side of a substrate encapsulated in molding compound. 
     FIGS. 3 b  and  3   c  are top and bottom plan views, respectively, of the circuit of FIG. 3 a.    
     FIG. 4 is a cross-sectional side view of a circuit having a portion of a top, a bottom, and sides of a substrate encapsulated in molding compound. 
     FIGS. 5 a  and  5   b  are cross-sectional side views of the structure of FIG.  4 . 
     FIGS. 6 a  and  6   b  are plans of the unencapsulated circuit of FIG.  4 . 
     FIGS. 7,  8   a,    8   b,  and  9  are cross-sectional side views of the circuit of FIGS. 6 a  and  6   b  inserted in a mold. 
     FIG. 10 is an exploded perspective view of the circuit of FIGS. 6 a  and  6   b  and the mold of FIGS. 7,  8   a,    8   b,  and  9 . 
     FIGS. 11 and 12 are cross-sectional side views of a press. 
     FIGS. 13 a,    13   b,  and  13   c  are cross-sectional side views of presses including compliance mechanisms attached to molds. 
     FIG. 14 is a cross-sectional side view of the circuit of FIG. 15 within a mold. 
     FIG. 15 is plan view showing a circuit. 
     FIG. 16 is a plan showing the circuit of FIG. 15 partially encapsulated. 
     FIG. 17 is a cross-sectional side view of the structure of FIG.  16 . 
     FIG. 18 is a plan of a larger circuit including the circuit of FIG.  4 . 
     FIG. 19 is a plan of a substrate including three unencapsulated circuits. 
     FIG. 20 is a cross-sectional side view of the substrate of FIG. 19 inserted within a mold. 
     FIGS. 21 and 22 are top and bottom plan views, respectively, of a substrate showing a circuit having four areas with conductive pads. 
     FIG. 23 is a cross-sectional view of a mold. 
     FIG. 24 is a cross-sectional view taken along lines  24 — 24  of FIG.  23 . 
     FIG. 25 is a bottom view of the piston of FIG.  23 . 
     FIGS. 26 and 27 are detailed views of the region of contact between a printed circuit board and the piston. 
    
    
     Referring to FIGS. 4,  5   a,  and  5   b,  a component  9  includes a circuit  10 , having electrical components  12 ,  14 ,  15 ,  16 ,  17 , and  18  mounted on both sides of printed circuit board (PCB)  20 , that is only partially encapsulated in cured molding compound  50 . Input/Output (I/O) pads  30 , located on a portion  52  of PCB  20 , are exposed (i.e., not encapsulated). This reduces the size of component  9  by allowing connections to be made directly with I/O pads  30  (i.e., without leads  26   a,    26   b,    26   c,    26   d,    26   e,  and  26   f,  FIGS. 1 a,    1   b ). The size of component  9  is also reduced by having components mounted on both sides of PCB  20 . 
     For clarity, only conductive leads  28  and  29  of PCB  20  are shown. Wire  12   b  connects conductive pad  12   a  of semiconductor die  12  to conductive lead  28  which is connected to an I/O pad  30  (FIGS. 5 a,    6   a,    8   a ). A conductive pad (not shown) on a bottom side of component  17  is connected to conductive lead  29  which is connected to an I/O pad  30  through a plated through hole  31  (FIGS. 5 b,    6   b,    8   b ). 
     In order to provide component  9  as shown in FIGS. 4,  5   a,  and  5   b,  unencapsulated circuit  10  is inserted into a mold  60 , shown in FIGS. 7-10. Referring to FIGS. 6 a,    6   b,  instead of a lead frame  26  (FIGS. 1 a,    1   b ), unencapsulated circuit  10  has PCB waste portions  20   a,    20   b,    20   c,  and  20   d.    
     As shown in FIGS. 7-10, a top  60   a  of a mold  60  has a ridge  62  that extends, in a direction T tr  (FIG.  10 ), for example, approximately 0.001 inches, above a surface  64  of top  60   a  and surrounds a cavity  68   a  within top  60   a  (FIGS. 8 a,    8   b ). When the top  60   a  and a bottom  60   b  of mold  60  are closed around circuit  10 , ridge  62  pushes down (arrows  70 , FIGS. 7,  8   a,    8   b ) onto waste portions  20   b,    20   c,  and  20   d  (along dashed line  72 , FIG. 6 a ) and down (arrows  74 , FIGS. 8 a,    8   b ) onto the area between PCB  20  and waste portion  20   a,  including I/O pads  30  (between dashed lines  72 ,  76 , FIG. 6 a ). 
     Bottom  60   b  of mold  60  has a ridge  78  that extends, in a direction T br  (FIG.  10 ), for example, approximately 0.001 inches, above a surface  80  of bottom  60   b  and around a first part of a cavity  68   b  within bottom  60   b  (FIGS. 8 a  and  8   b ). Bottom  60   b  also has a second ridge  82  that forms an end to a runner  84  within bottom  60   b  that is connected to a pot  86  also within bottom  60   b  (FIG.  9 ). Ridge  82  extends around a second part of bottom cavity  68   b.  When top  60   a  and bottom  60   b  of mold  60  are closed around circuit  10 , cavities  68   a  and  68   b  form cavity  68  and ridge  78  pushes up (arrows  90 , FIGS. 7,  8   a,    8   b ) against waste portions  20   a,    20   c,  and  20   d  (along dashed line  72 , FIG. 6 b ). 
     Piston  90  (FIGS. 7,  9 ) is used to push liquid, uncured molding compound  35  from pot  86  into runner  84  and into mold cavity  68  at about 1000 psi. 
     Top  60   a  and bottom  60   b  of mold  60  close on printed circuit board on all sides of circuit  10  forming a seal around circuit  10  with the exception of I/O pads  30 . A top of pot  86  and runner  84  is defined by PCB waste portion  20   b  (FIG.  9 ). As a result, variations in the thickness of the PCB do not cause a gap (gap  44 , FIGS. 2 d,    2   e ) through which molding compound  35  can leak. 
     Ridge  62  of mold top  60   a  and ridge  78  of mold bottom  60   b  extend up to a maximum of 0.002 inches into PCB  20  and waste portions  20   a,    20   b,    20   c,  and  20   d.  PCB  20  is compressible, and in the area of I/O pads  30 , PCB  20  is not crushed or otherwise permanently damaged by ridges  62  and  78 . Ridges  62  and  78  also prevent molding compound from leaking in the direction indicated by arrows  92  (FIGS. 7,  8   a,    8   b ) and “resin bleeding” (i.e., epoxy resin leakage) which can occur with a gap  44  (FIGS. 2 d,    2   e ) of 0.0005 inches or less. 
     Mold  60  is part of a press  130 , shown in FIG.  11 . After PCB  20  is inserted into mold cavity  68 , a top  130   a  of press  130  closes mold top  60   a  on PCB  20  and the waste portions. Press top  130   a  and bottom  130   b  hold the mold closed while the molding compound  35  is pushed into mold cavity  68 . 
     Press  130  provides an even profile of pressure across PCB  20  (the edges of waste portions  20   c  are shown) through mold  60 . If two molds  60  are inserted into press  130  and the thickness T 1 , T 2  of each PCB is different (i.e., T 1  is less than T 2 ), press  130  will not provide an even profile of pressure across the thinner PCB (i.e., T 1 ), as indicated in FIG. 12 by gap  132 . 
     In order to accommodate multiple molds holding PCBs of different thicknesses, a press  134  is provided (FIG. 13 a ) with springs  136  attached to each mold top  60   a.  Springs  136  have a spring constant of approximately 100 Kg per 0.001 inch, and the compliance of the springs allows the press to apply an even pressure across each mold  60  even where the molds hold PCBs of different thicknesses. One or more springs may be attached to each mold top  60   a,  and instead of springs, hydraulic cylinders  150  filled with high pressure fluid  154  and having pistons  152  (FIG. 13 b ) or pneumatic cylinders  156  filled with high pressure gas  158  and having pistons  160  (FIG. 13 c ) can be used. 
     Referring to FIGS. 14 and 15, to stabilize (i.e., prevent movement of) PCB  20  within mold cavity  68  during encapsulation, the mold can be configured to close on a portion (i.e., in between dashed lines  142 ,  146  of FIG. 15)  140  of PCB  20  in an area of PCB  20  without pads  30 . As a result, portion  140  will not be encapsulated in cured molding compound  50 , shown in FIGS. 16 and 17. Ridges  62  and  78  can also be configured to close on multiple portions of PCB  20  in areas of PCB  20  without pads  30 . 
     After molding, component  9  is removed from mold  60 , and waste portions  20   a,    20   b,    20   c,  and  20   d  (along with waste, cured molding compound, i.e., cull) are removed by cutting along dashed line  94  (FIGS. 6 a,    6   b ). As shown in FIG. 18, exposed conductive pads  30  of component  9  (cured molding compound  50  surrounds circuit  10 ) are then used to connect circuit  10  into a larger circuit  96  on a PCB  98 . Cured molding compound  50  also serves as an electrical insulator providing a high voltage breakdown and, thus, allowing component  9  to be placed in close proximity to other electrical components. 
     Referring to FIG. 19, several circuits  110 ,  112 ,  114  (with similar electrical components  12 ,  14 ,  16 , and  18  ( 15  and  17  are included but not shown) connected through similar PCBs  20 ) can be molded simultaneously. Each circuit has conductive pads  30  through which electrical connections can be made to the components  12 ,  14 ,  15 ,  16 ,  17 , and  18  (only conductive lead  29  is shown for clarity). PCB waste portions  20   a,    20   b,    20   c,    20   d,    20   e,    20   f,    20   g,    20   h,    20   i,  and  20   j  connect the circuits and are cut away along dashed lines  116 , after molding, to provide encapsulated circuits with exposed conductive pads  30 . 
     As shown in FIG. 20, mold  120  has a cavity  122  for each circuit. Ridges  124  of mold top  80   a  surround each circuit, by extending down against waste portions  20   b,    20   c,    20   d,    20   e,    20   f,    20   g,    20   h,    20   i,  and  20   j  and by extending down against the areas between waste portions  20   a,    20   d,  and  20   g  and PCB  20 , including I/O pads  30 . Similarly, ridges  126  of mold bottom  120   b  surround each circuit by extending up against waste portions  20   b,    20   c,    20   d,    20   e,    20   f,    20   g,    20   h,    20   i,  and  20   j  and by extending up against the areas between waste portions  20   a,    20   d,  and  20   g  and PCB  20 , including I/O pads  30 . As a result, ridges  124  and  126  form a seal around PCBs  20 . The bottom side of waste portions  20   b,    20   f,  and  20   i  define tops of runners and pots indicated by dashed lines  128  in FIG.  19 . Molding is accomplished as described above with reference to FIGS. 7-10 and  13 . 
     A PCB&#39;s thickness can vary as much as 0.002 inches from waste portion  20   a  to waste portion  20   j.  Hence, conventionally molding several circuits simultaneously is difficult because although the mold may apply the proper pressure and not form a gap  44  (FIGS. 2 d,    2   e ) at one end of the PCB, it may not apply the proper pressure or form a gap  44  at the opposite end of the PCB. The variation in the thickness of the PCB is accommodated for in mold  120  (FIG. 20) by ridges  62  and  78 . In areas where the PCB is thickest, ridges  62  and  78  compress the PCB approximately 0.002 inches. In areas where the PCB is the thinnest, ridges  62  and  78  close against, but do not compress the PCB. As a result, no gap is created through which molding compound can leak out of mold  120 . 
     Many different circuit shapes can be molded in this way, and the printed circuit boards may have only one area where conductive pads are mounted or multiple areas (FIG. 21) where conductive I/O pads  30  are mounted. The conductive pads may also be mounted on both sides of the PCB (FIG.  22 ). 
     Similarly, a variety of compliant and non-compliant substrates can be used instead of PCB  20  and waste portions  20   a,    20   b,    20   c,  and  20   d,  shown in FIGS. 6 a,    6   b.  A non-compliant substrate such as ceramic, for example, could be used. Where non-compliant substrates are used, mold top  60   a  and mold bottom  60   b  (FIGS. 7-10) would not include ridges  62 ,  78 , respectively. Instead, surfaces  64  and  80  of mold top  60   a  and bottom  60   b,  respectively, would close flush against the non-compliant substrate. 
     As shown in FIG. 23, in another scheme, a different mold  200  may be used to encapsulate circuits on a printed circuit board (PCB)  206 . The mold  200  has a top mold section  202  which closes on the top of the PCB  206  and a bottom mold section  204  which closes on the bottom of the PCB  206 . The mold sections  202  and  204  are held together to form the mold  200  by two mold presses  224  and  226 . Because the sections  202  and  204  close on opposite sides of the PCB  206  (i.e., the sections  202  do not contact each other to form the seal of the mold  200 ), variations (e.g., +/−0.0025 inches) in the thickness of the PCB  206  do not affect the seal integrity of the mold  200 . 
     The PCB  206  does not cover all exposed cavities of the bottom mold section  204 . The bottom mold section  204  has a molding compound injection cylinder, or pot  210 , that is neither closed by the top mold section  202  nor by the PCB  206  (i.e., the pot  210  is exposed). For purposes of closing the pot  210  where it is exposed, a spring-loaded cylindrical piston  220  is slidably mounted inside a cylinder  218  formed in the top mold section  202 . When the mold sections  202  and  204  are assembled on the PCB  206 , a spring  222  forces the piston  220  downwardly to close the pot  210  where the pot  210  is exposed. To prevent the piston  220  from dropping out of the mold section  202  when the mold  200  is not assembled, the piston  220  has an annular extension  223  that slides within a cylinder  219  of the mold section  202 . The cylinder  219  is coaxial with and has a larger diameter than the cylinder  218 . A shoulder  225  is formed where the two cylinders  218  and  219  meet. The shoulder  225  serves as a stop to limit the translational freedom of the extension  223  and thus, the piston  220 . 
     The conduit  210  provides a channel for delivering the molding compound to internal cavities  205  and  207  of the mold  200 . Exposed channels, or runners  208  (one for each encapsulated circuit), formed in the bottom section  204  direct the molding compound from the pot  210  into the top mold cavity  205  (formed in the top mold section  202 ) and the bottom mold cavity  207  (formed in the bottom mold section  204 ). Each otherwise exposed runner  208  is partially covered by the PCB  206  and partially covered by the piston  220 . At the end of each runner  208  farthest from the pot  210  is an inclined ridge  209   a  which extends upwardly toward the PCB  206  and creates a narrow injection point  203   a  at the entrance of the bottom mold cavity  207 . Another inclined ridge  209   b  extends upwardly from the runner  208  to create another narrow injection point  203   b  where the pot  210  and runner  208  meet. To force the molding compound into the cavities  205  and  207 , a piston  212  is slidably mounted inside the pot  210  and is used to force the molding compound from the pot  210  into the bottom cavity  207 . The molding compound flows into the top cavity  205  via holes in the PCB  206 . 
     As shown in FIG. 24, the piston  220  closes on, or contacts, the bottom mold section  204 , forming a metal-to-metal seal over an annular area  215  between the outer perimeter  213  of the piston  220  and the outer perimeter  211  of the pot  210 . Thus, the piston  220  closes part of the exposed runners  208 . For purposes of sealing off the annular space between the piston  220  and the top section  202  (over the runners  208 ), the PCB  206  extends beyond the mold cavities  205  and  207  and is located beneath this annular space. The piston  220  crushes an arcuate region  239  of the PCB  206  where the PCB  206  extends under the piston  220 . The crushed PCB forms a seal between the piston  220 , the top mold section  202 , and the PCB  206 . The crushing of the PCB  206  by the piston  220  removes any thickness variation of the PCB  206 . 
     As shown in FIG. 25, a face  216  of the piston  220  (used for closing the conduit  210  and crushing the PCB  206 ) has an annular downwardly extending rim  230  which forms the arcuate region  239 . As shown in FIG. 26, the bottom mold section  204  has a lower region, or depression  237 , for holding the PCB  206  and a higher region  238  for forming the metal-to-metal seal with the piston  210 . A shoulder  234  separates the depression  237  and the higher region  238 . A distance D between the top surface of the depression  237  and the top surface of the region  238  (i.e., the height of the shoulder  234 ) is equal to the minimum potential thickness of the PCB  206 . The rim  230  crushes the PCB  206  such that the top surface of the crushed region  239  of the PCB  206  is substantially flush with the top surface of the region  238 . 
     For purposes of forcing an edge of the PCB  206  against the shoulder  234  to form a tight seal between the PCB  206  and the bottom mold section  204 , the face  216  has two downwardly extending asymmetric knives  232 . The knives  232  are aligned with two asymmetric knives  233  which upwardly extend from the depression  237 . As shown in FIG. 27, when the piston  220  exerts force on the PCB  206 , the edge of the PCB  206  is forced against the shoulder  234  by the contact of the PCB  206  with the knives  232 . 
     Other embodiments are within the scope of the following claims. For example, one or more of the knives may be symmetric instead of asymmetric.