KR100952850B1 - Printed circuit board assembly having improved thermal energy dissipation - Google Patents

Abstract

Methods and apparatus for improving thermal conductivity of a printed circuit board (PCB) assembly include a PCB mounted integrated circuit (IC). The voids are formed in the PCB and one device or material is placed in the voids having a thermal conductivity higher than that of the removal portion of the circuit board. By placing a material with a relatively high thermal conductivity in the void, the heat energy dissipation efficiency is improved.

Printed circuit board, voids, thermal conductivity, heat dissipation

Description

Printed circuit board assembly having improved thermal energy dissipation

FIELD OF THE INVENTION The present invention relates to printed circuit boards (PCBs) and, more particularly, to dissipating thermal energy generated by integrated circuits (ICs) mounted on a PCB.

Conventional IC packages typically have a sealed and / or encapsulated plastic housing having an internal lead frame, bond wires and die as well as external electrical leads for electrically connecting the die to the outside. Include. IC packages housing higher heat dissipation dies have additional integrated heat sinks formed within the package connected to the IC substrate. The heat sink typically includes a metal layer. The die is generally mounted on a metal layer with facing electrical contact pads and bond wires connecting the electrical contact pads of the die to the internal lead frame of the IC package. The lead frame provides a mechanically rigid electrical path from the die and bond wires to the external electrical leads of the IC package. The lead frame is generally molded partially in the plastic housing to bridge the electrical connection between the inner lead frame and the outer leads. Electrical leads form an electrical connection from the package to the packaged PCB.

When a high power heat dissipation IC is mounted to a PCB, the heat spreader on the PCB placed directly below the IC package is thermally connected to the heat sink of the IC package and then to the ground plane in the PCB layers. The heat spreader dissipates the heat generated by the IC to the PCB ground plane. Heat generated by the IC is also convectively transferred from the IC package to the ambient air. However, when the use of IC packages is limited to small physical locations and / or physically small compared to other dies, the air around the package circulates very poorly, resulting in little removal of heat by convection. Therefore, all of the heat generated by the IC must dissipate mostly through the IC package heat sink down and towards the heat spreader of the PCB.

In direct-chip-attach configurations, the IC package is removed and the die is mounted directly to the PCB in the inverted position so that the die-on electrical contact pads face down toward the circuit traces on the PCB. And electrically connected by electrical interconnects (eg, solder bumps) that connect the contact pads of the die to circuit traces on the PCB. PCBs used in direct-chip-mount configurations are generally physically flexible PCBs and are known as flexible PCBs or flexible circuits. Due to flexibility, the flexible PCB can be formed into small physical areas while maintaining electrical contact between the PCB and the die. These types of circuits are often used in situations where very little space is available to mount a PCB such as a read / write head that reads data from and writes data from a hard drive magnetic recording medium in a hard disk drive (HDD). These read / write heads typically sit on a stainless steel armature with a flexible PCB mounted thereon. The electrical contacts of the flexible PCB typically connect the read / write heads to the electrical contacts of the read / write preamplifier IC. This entire assembly is suspended aerodynamically on the magnetic recording medium.

6 shows a perspective view of a typical direct-chip-mount assembly 11 that includes a flexible PCB comprising various layers 12 and an IC die 13 mounted on the flexible PCB 12. The IC die 13 has electrical contact pads (not shown) on the die 13 with circuit traces on the flexible PCB 12 by electrical interconnects (eg, solder bumps) 19. And mounted to the flexible PCB 12 in the facing position so that it is easily connected to (not shown). The flexible PCB 12 is a heat sink material 14 in contact with the side of the PCB 12 opposite the die 13, an adhesive and thermally insulating dielectric material disposed on the heat sink material 14 (eg For example, a polyimide) layer 15, and a thermally insulating dielectric material (eg, polyimide) layer 17 disposed on the metal layer 16. The circuit traces described above are formed by portions of the metal layer 16 of the flexible PCB 12 that are typically made of copper. The heat sink material 14 functioning as a heat dissipator and heat path is heat sink in the die 13, as indicated by arrows 23 and 24 directed from the die 13 to the heat sink material 14. Material 14 is in the direction.

In some cases, flexible PCBs include a stable device such as an aluminum hardener (not shown) disposed between the heat sink material 14 and the layer of polyimide and adhesive 15. The hardener provides mechanical stability to the flexible PCB. The heat sink material 14 need not be part of the PCB 12 but may instead be a separate device on which the PCB 12 is disposed. If a hardener is used, heat dissipation paths are provided to the heat sink material and ultimately to the armature and housing of the disk drive.

Typically, the electrical interconnects 19 connecting the circuit traces formed in the metal layer 16 and the contact pads of the die 13 are electrically contact pads (not shown) disposed on the bottom surface of the die 13. Solder or leadless bumps disposed on and heated and then placed in contact with circuit traces on flexible PCB 12. As the bumps cool and harden, the bumps form a rigid electrical connection between the pads on the bottom of the die 13 and the circuit traces formed in the metal layer 16 of the flexible PCB 12.

Once electrical connections are formed between the pads on the die 13 and the circuit traces of the flexible PCB 12, there is a slight gap between the surface of the die 13 and the surface of the PCB 12. Due to this spacing physical geometry, which typically ranges from 25 to 76 micrometers (0.001 to 0.003 inches), the spacing between die 13 and flexible PCB 12 is typically underfill, which provides mechanical stability. Filled with material 21. This prevents the application of excessive mechanical stresses on the die 13 and interconnects, which can cause electrical connections to fail. Underfill material 21 is generally provided after pads of die 13 are interconnected with circuit traces on flexible PCB 12. Underfill material 12 is typically provided using capillary flow. The underfill material 21 is heated to cure the material in a solid physical state. The underfill material 21 currently used for this purpose is Hysol® EP4549, which has poor thermal conductivity and is typically manufactured by Henkel Loctite Corporation of Dusseldorf, Germany. This particular underfill material is a very pure, low stress, liquid epoxy designed for enhanced adhesion to integrated circuit passivation materials.

When a flexible PCB assembly as shown in FIG. 6 is used on the read / write head of a disk drive, the flexible PCB assembly generally uses a large amount of current and / or voltage to allow read and write operations to be performed. These types of signals are typically very fast rise times, some less than 200 picoseconds (ps), and exhibit large slew rates of more than 700 milliamps (mA) per nanosecond (ns), which is extremely near. Forms instantaneous currents and / or voltages. These large instantaneous currents and / or voltages form a large amount of thermal energy that needs to be dissipated.

There have been several attempts to improve the efficiency of heat sinks in PCB assemblies, which have increased the copper trace area on the flexible circuit, increased the copper trace thickness on the flexible circuit, and dedicated the number of "dummy" bumps. Flip opposite the PCB to help with higher density thermally conductive interconnects such as locations, higher thermally conductive underfill, and convective cooling to ambient air and improved conductive cooling through the physical structure of the die / PCB interface Heat sinks on the side of the chip. Today, none of these techniques, used individually or together, have proven to be completely effective at significantly reducing thermal resistance while providing an efficient low cost (or no charge) solution.

When flexible PCB assemblies are used in physically very small environments, such as read / write heads of hard drives (where space and cost constraints are good), conventional methods for reducing thermal resistance are inadequate and / or impractical. In addition, flexible PCB assemblies typically use a single layer (ie, metal layer 16 with traces formed therein). In cases where it is possible and feasible to use a multilayer PCB where space and cost constraints are not an issue, a simple multilayer plating through-hole technique can be used to thermally conduct thermally down through the PCB to dissipate the generated heat. Can be used to provide paths. However, multilayer PCBs are generally considerably more expensive than single layer PCBs. Therefore, using a multilayer conductor PCB can be very expensive in some cases. In addition, due to the aerodynamics of the head armature of the disk drive, multilayer PCBs disposed on the disk drive armature are generally not suitable because the armature additional weight results in slower read and write speeds.

Thus, what is needed is a method and apparatus for effectively dissipating thermal energy in a PCB.

The present invention provides a method and apparatus for improving heat dissipation from a die to a printed circuit board (PCB) assembly. The apparatus includes a PCB assembly with improved thermal conductivity, and the method is for improving the thermal conductivity of a PCB.

According to one embodiment of the present invention, a PCB assembly has a thermal conductivity higher than that of a PCB having voids formed where a portion of the PCB is removed, an integrated circuit (IC) mounted on the side of the PCB, and the thermal conductivity of the removed portion of the PCB. Material or device disposed within the void having a void. The PCB includes at least one circuit trace. The die is interconnected to the PCB by at least one electrical interconnect that interconnects the circuit traces of the PCB and the electrical contact pads on the die.

According to another embodiment of the present invention, a PCB assembly is a PCB having a void formed where a portion of the PCB is removed, an IC die mounted in a direct-chip-mount configuration and electrically interconnected to the PCB, and adjacent to the void Material and devices attached to or adjacent to the die protruding toward it. The material or device bonded or adjacent to the die has a thermal conductivity higher than that of the removal portion of the circuit board. The PCB includes at least one circuit trace. The die is interconnected to the PCB by at least one electrical interconnect that interconnects the circuit traces of the PCB and the electrical contact pads on the die.

The method of the present invention for improving heat dissipation in a PCB assembly includes removing a portion of the PCB to form a void in the PCB, placing a device or material in the void having a thermal conductivity higher than that of the removed portion of the circuit board. And mounting the die to one side of the PCB in a direct-chip-mount configuration.

These and other features and advantages of the invention will be apparent from the following detailed description, drawings, and claims.

1 is a cross-sectional view of a PCB assembly of the present invention in accordance with an embodiment where a portion of the PCB is removed and the removed portion is filled with a material having a higher thermal conductivity than the removed portion of the PCB.

2 is a cross-sectional view of a PCB assembly of the present invention in accordance with another embodiment in which a portion of the PCB is removed and a portion of the heat sink material of the PCB is raised and / or inserted into the void so as to be closer to the bottom surface of the die.

3 shows that a portion of the PCB has been removed to form voids in the PCB and a high thermally conductive material (eg, metal rivets) protrudes towards and / or from the void, thus reducing the physical distance between the die and the heat sink material. Cross-sectional view of a PCB assembly of the present invention in accordance with another embodiment reducing and increasing the thermal path of the thermal path.

4 shows another embodiment in which a portion of the PCB is removed to form voids in the PCB and a high thermal conductivity (eg copper) material is disposed on the bottom surface of the die to further increase the thermal conductivity of the thermal path. Cross-sectional view of the PCB assembly of the present invention according to.

5 is a flow chart of a method of the present invention in accordance with a preferred embodiment.

6 is a cross-sectional view of a conventional direct-chip-mount configuration including a PCB and an IC die mounted on the PCB.

According to the invention, the PCB assembly comprises an IC die mounted on the PCB. IC dies can be mounted in a direct-chip-mount configuration. Optionally, the IC die can be encapsulated in an IC package and the IC package is mounted to the PCB. As used herein, PCB includes any substrate having a dielectric material that is electrically and thermally insulating. Therefore, as used herein, PCB includes, but is not limited to, the flexible circuits described above that are single layer printed circuit boards. The invention is not limited to the type of material from which the PCB is formed. Although polyimide and fiber resins are the most commonly used materials for forming PCBs, other materials such as Teflon®, for example, used for forming rigid PCBs, can be used as the PCB material. However, the present invention applies to multilayer PCBs. To facilitate discussion and illustration, the present invention will be described with reference to a PCB, which is a flexible circuit that includes certain materials.

1 shows a cross-sectional view of a PCB assembly 20 of the present invention in accordance with an embodiment. The PCB assembly 20 includes a PCB 30 and an IC die 40 mounted on the PCB 30 in a direct-chip-mount configuration, ie a flip chip configuration. The direct-chip-mount configuration allows die-side electrical contact pads facing downward toward the PCB to face the circuit traces on the PCB by electrical interconnects (e.g., solder bumps, leadless bumps, bias, etc.). The IC package is removed and mounted directly to the PCB with the IC die facing so as to be electrically interconnected. According to the embodiment shown in FIG. 1, the PCB 30 is a flexible circuit, and as described above, the present invention is not limited to the PCB. The invention is not limited to direct-chip-mount configurations, but may apply to IC packages mounted on PCBs such as, for example, single layer PCBs.

The flexible PCB 30 of the present invention typically comprises a heat sink material 31, a first layer 32 of electrically and thermally insulating dielectric material (eg, polyimide) disposed on the heat sink material 31. A metal layer 33 having one or more circuit traces disposed thereon and formed on the adhesive polyimide layer 32, and an electrically thermal insulating material layer 34 disposed on top of the metal layer 33 (eg, , Polyimide). The heat sink material 31 functions as a heat dissipator and the heat path is from the die 40 to the heat sink material, as indicated by arrows 43 and 44 directed from the die 40 to the heat sink material 31. It is in the direction of (31).

The metal layer 33 is typically a copper layer. As described above with reference to FIG. 1, metal layer 33 has circuit traces of flexible circuit 30 formed therein. A portion of the layer of dielectric material 34 that is electrically and thermally insulating is removed such that circuit traces can be formed in the metal layer 33. The die 40 is typically in a direct-chip-mount configuration with the metal layer 33 of the PCB 30 via electrical interconnects 36 such as solder bumps disposed at the periphery of the die 40. Placed in electrical contact. Electrical interconnects 36 interconnect circuit traces formed in metal layer 33 and electrical contact pads on die 40.

The heat sink material 31 may be part of the PCB assembly 20 or may be independent of the PCB assembly 20. The heat sink material 31 is typically metal and may be, for example, aluminum or stainless steel. In addition, a stabilization device such as aluminum is commonly used in flexible PCB assemblies to provide mechanical stability and can be disposed between the layer of adhesive electrically insulating insulating dielectric material 32 and the heat sink material 31.

In accordance with the present invention, a portion of the PCB 30 is used to create a void 50 in the PCB 30 between the lower surface 41 of the die 40 and the upper surface 38 of the heat sink material 31. Removed. Preferably, the void 50 is formed by removing portions of all layers 32-34 down to the top surface 38 of the heat sink material 31. From the comparison of FIGS. 1 and 6, it can be seen that the void 50 shown in FIG. 1 is not provided in FIG. 6. The void 50 is preferably formed at a position to be disposed directly below the die and centered on the die face area. According to the embodiment shown in FIG. 1, the voids 50 are filled with underfill material 60 which exhibits a relatively higher thermal conductivity than the removal portion of the PCB 30. Underfill material 60 is represented in FIG. 1 by regions filled with black dots. In today's industry standards, underfill materials commonly used to provide mechanical stability (reference 21 in FIG. 6) are very poor thermal conductors and typically have the same thermal conductivity as the PCB substrate material. Typical ranges of thermal conductivity for the underfill material 21 used in the flip chip assembly shown in FIG. 6 are approximately 0.3 watts per meter-kelvin (W / m-K) and 0.9 W / m-K.

According to the embodiment shown in FIG. 1, the thermal conductivity of the underfill material 60 preferably used to fill the void 50 is approximately 3.0 W / mK (ie, the minimum of the thermal conductivity of the removal portion of the PCB dielectric material). 10 times). Therefore, the thermal conductivity of the underfill material 60 is preferably equal to or greater than approximately 3.0 W / m-K. However, the present invention is not limited to any particular minimum or maximum value or range of values for the thermal conductivity of the underfill material 60. The underfill material can be, for example, Namics 8449-3 manufactured by Namics Corporation of Nigorikawa, Niigata City, Japan. This particular underfill material is a liquid epoxy comprising a thermally conductive small grain fill material. Grain-filled materials with low thermal conductivity are typically silver or blond nitride. It should be noted that any fill material with suitable physical properties such as adhesion and thermal conductivity may be suitable for this purpose, for example.

By removing the poor thermal conductivity material to form the voids 50 on the PCB 30, the voids 50 are removed with an underfill material 60 having a higher thermal conductivity than the removed portion of the PCB 30. By charging, an improved thermally conductive path is provided between the die 40 and the heat sink material 31 in relation to the removal portion of the PCB 30, which heats the heat sink material 31 away from the die 40. "Channel" energy. Carrying heat energy in this way improves the efficiency of the heat sink and thus lowers the maximum temperature of the die. In addition, since the underfill material has already been used in flip chip assemblies of the type shown in FIG. 6, providing the underfill material 60 in the assembly 20 shown in FIG. 1 may not require additional processing steps. have. The only additional processing step required may be to form voids 50 that, when initially physically placed into the flexible PCB 30, add additional cost to processing. This is because the PCB 30 typically requires other openings, or voids, to be formed therein for the purpose of mechanically mounting the PCB (eg, read / write head armature, or heat sink material, Or on some other devices). As will be described in more detail below with reference to FIGS. 2-4, the void-free area in the PCB may be filled with underfill material and other materials to improve the thermal conductivity of the path from the die to the heat sink material. In addition, the location of the void or voids may be selected to further improve the efficiency of the heat sink as described in more detail below.

One of the main objectives of the present invention faced by all the embodiments described herein is to establish a low thermal resistance path (underfilled void) parallel to the surrounding high thermal resistance path (ie, PCB dielectric material layers). The result is to reduce the overall thermal resistance from the die to the PCB base (eg, heat sink material). This is analogous to placing a low value resistor in parallel with a high value resistor such that the value of the total total effective resistance is close to the low value resistor.

2 illustrates a cross-sectional view of a PCB assembly 70 of the present invention in accordance with another embodiment. According to this embodiment, the PCB 80 is a flexible circuit of the type shown in FIG. The flexible circuit 80 is on the top of the heat sink material 81, the adhesive electrically and thermally insulating dielectric material (eg polyimide) disposed on the heat sink material 81, the layer 82. A disposed metal layer 83 and an electrically and thermally insulating dielectric material (eg, polyimide) layer 84 disposed over the metal layer 83. Metal layer 83 has one or more circuit traces formed therein. The die 90 is connected to the PCB through the electrical interconnects 86 (eg, solder bumps, leadless bumps, bias, etc.) that interconnect the electrical contact pads on the die to the circuit traces of the PCB 80. Disposed in electrical contact with circuit traces of metal layer 83 of 80. The heat sink material 81 may be part of the PCB assembly 70 or may be independent of the PCB assembly 70. In addition, reinforcements made of higher electrically and thermally conductive materials are commonly used in flexible PCB assemblies to provide mechanical stability and are placed between the adhesive electrically thermal insulating dielectric material layer 82 and the heat sink material 81. Can be. The materials used to form the PCB assembly 70 shown in FIG. 2 may be the same as the materials used to form the PCB assembly 20 shown in FIG. 1.

As with the embodiment represented by FIG. 1, in accordance with the embodiment shown in FIG. 2, a portion of the PCB 80 has been removed to create the void 100 in the PCB 80. The void 100 is preferably formed by removing portions of the layers 82-84 of the PCB 80 down towards the top surface 88 of the heat sink material 81. The heat sink material 81 is, for example, aluminum or stainless steel. The void 100 is filled with an underfill material 110 having a relatively higher thermal conductivity than the removal portion of the PCB 80, such as a Namics 8449-3 underfill material, or a metal having a higher thermal conductivity than the removal portion of the PCB 80. do. The void 100 may be filled with any material having a higher thermal conductivity than the removal portion of the PCB including the dielectric materials. Additionally, according to this exemplary embodiment, the top surface 88 of the heat sink material 81 reduces the distance between the top surface 88 of the heat sink material and the bottom surface 91 of the die 90. Can be raised to let. This may be accomplished by modifying the heat sink material 81 in some way, for example by stamping treatment. Thus, a portion of the top surface 88 of the heat sink material 81 protrudes into the void 100 and is closer to the die 90 than other portions of the heat sink material 81. By reducing the physical distance between the dies 90 and the surfaces 88 and 91 of the heat sink material 81, the thermal conductivity of the path between the die 90 and the heat sink material 81 is improved, so that the heat sink Improve overall efficiency.

Although the underfill material 110 preferably has a relatively higher thermal conductivity than the removal portion of the PCB material 80, this is not necessary for this embodiment. Because the distance between the lower surface 91 of the die 90 and the upper surface 88 of the heat sink material 81 is reduced, the underfill material 110 has higher thermal conductivity than the removal portion of the PCB 80. Is not necessary. Although standard underfill material is used in this embodiment (eg, Hysol® FP5449), the reduced distance between the surfaces 88 and 81 provides improved heat dissipation.

3 shows a top cross-sectional view of a PCB assembly 120 of the present invention in accordance with another embodiment. PCB assembly 120 includes PCB 130 and die 140 mounted on PCB 130 in a direct-chip-mounted arrangement, ie, a flip chip configuration. According to this embodiment, the PCB 130 is a flexible circuit of the type shown in FIGS. 1 and 2. The flexible circuit typically comprises a heat sink material 131, a first layer 132 of an adhesive and electrically and thermally insulating dielectric material (eg, polyimide) disposed on the heat sink material 131. ), A metal layer 133 disposed on top of the layer 132 and an electrically and thermally insulating dielectric material (eg, polyimide) layer 134 disposed on top of the metal layer 132. do. Flip chip die 140 typically includes PCB 130 through electrical interconnects 136 (eg, solder bumps, leadless bumps, bias, etc.) disposed around the periphery of die 140. And are in electrical contact with circuit traces formed in the metal layer 133. Conductive interconnects 136 interconnect circuit traces formed in metal layer 133 of PCB 130 and electrical contact pads on die 140. The materials used to form the PCB assembly 120 shown in FIG. 3 may be the same materials used to form the PCB assemblies 20 and 70 shown in FIGS. 1 and 2.

As with the embodiments represented by FIGS. 1 and 2 according to this embodiment, a portion of the PCB 130 is located between the bottom surface 141 of the die 140 and the top surface 138 of the heat sink material 131. It was removed to create voids 150 in the PCB 130. The void 150 is preferably formed by removing portions of the layers 132-134 of the PCB 130 down to the top surface 138 of the heat sink material 131. Void 150 is preferably filled with underfill material 160 having a higher thermal conductivity than the removal portion of PCB 130. The Namics 8449-3 underfill material described above may be suitable for this purpose. Additionally, in accordance with this embodiment, the top surface 138 of the heat sink material 131 is made of a relatively high thermally conductive material, such as metal rivets or slugs that protrude toward, for example, the void 150. The device 170 is made and reduces the heat path length between the heat sink material 131 and the die 140. Since the elongation or protrusion 170 reduces the thermal path length, it improves the thermal conductivity of the assembly 120, which improves the overall efficiency of the heat sink.

In addition, filling at least a portion of the voids 150 with a device having a relatively high thermal conductivity (ie, higher than the thermal conductivity of the removal portion of the PCB) is a thermal path between the die 140 and the heat sink material 131. To increase its thermal conductivity. This is true regardless of whether the device or material 170 partially or fully fills the void 150, and whether the device or material 170 reduces the thermal path length between the surfaces 138 and 141. . Devices such as metal rivets or metal slugs are suitable for this purpose because the metal has absolute high thermal conductivity. Therefore, using the device greatly increases the thermal conductivity of the thermal path between the die 140 and the heat sink material 131, which may be aluminum or stainless steel, for example. However, it should be noted that if the device comprising the dielectric material has a higher thermal conductivity than the removal portion of the PCB 130, the device including the dielectric material may be suitable for placement in the void 150.

When the length of the thermal path is reduced using the device 170, the underfill material 160 does not need to have a higher thermal conductivity (eg, Namics 8449-3) than the removal portion of the PCB 130. Care should be taken. Although standard underfill materials are used (eg Hysol® FP5449), the reduced path length provides a more effective heat sink.

The amount of improved efficiency of thermal energy dissipation depends on various factors. For example, the improved efficiency amount depends on the ratio of the thermal conductivity of the material or device disposed within and adjacent the void to the relative thermal conductivity of the PCB material surrounding the void. As this ratio increases, the amount of improved efficiency increases. This is true as described with reference to FIGS. 2 and 3, whether the higher thermally conductive material as described above with reference to FIG. 1 is an underfill material or some other material or device, such as a portion of the heat sink material formed in a particular manner. This is true no matter what some other devices like metal rivets.

The improved amount of efficiency of dissipating thermal energy is the area (or volume) of the void filled with higher thermally conductive material or device versus the area (or volume) of the PCB dielectric material (eg polyimide) disposed below the die. According to the ratio. As this ratio increases, the amount of improved efficiency increases. This is an example as described above with reference to FIGS. 2 and 3, whether the higher thermally conductive material is an underfill material as described above with reference to FIG. 1, or another material or device such as a portion of the heat sink material formed in a particular manner. This is true whether or not some other device like metal rivets or slugs for example.

As noted above, the voids may have a high thermal conductivity underfill as well as some kind of metal rivets or devices disposed on the voids or portions of the heat sink material. In this case, the improved amount of efficiency of dissipating thermal energy is increased by increasing the ratio of the area (or volume) of the heat sink material or other material or device disposed in the void to the area (or volume) of the underfill material disposed or adjacent to the void. do.

The extent to which the present invention improves the efficiency of dissipating thermal energy depends on the positional relationship of the voids with respect to the "hot spots" of the die. In other words, the die does not produce thermal energy uniformly across the surface area. Rather, if there is a portion of the die circuit that consumes a very large amount of power compared to other portions of the die, the portion of the die that consumes the greatest amount of power will exhibit the highest temperature. Thus, placing voids in the hot spots as closely as possible will provide the shortest heat path to the heat sink material, thereby improving the overall efficiency of heat energy dissipation.

4 illustrates a cross-sectional view of a PCB assembly 170 of the present invention in accordance with another embodiment. PCB assembly 170 includes a PCB 180 and an IC die 190 mounted on PCB 180 in a direct-chip-mount device, ie, a flip chip configuration. According to this embodiment, the PCB 180 is a flexible circuit of the type shown in FIGS. 1,2 and 3. As noted above, the flexible circuit typically includes a heat sink material 181, a first layer 182 of an adhesive electrically thermally insulating dielectric material (eg, polyimide) disposed on the heat sink material 181. , A metal layer 183 disposed over the layer 182, and an electrically thermally insulating dielectric material (eg, polyimide) 184, disposed over the metal layer 182, and the metal layer 183. Die 190 is typically a metal layer of PCB 180 via electrical interconnects 186 (ie, solder bumps, leadless bumps, bias, etc.) disposed around the periphery of die 190. Disposed in electrical contact with circuit traces formed in 183. Interconnects 186 interconnect contact pads on die 190 with trace circuits formed in or on PCB 180. The materials used to form the layers of the PCB assembly 170 shown in FIG. 4 are the materials used to create the layers of the PCB assemblies 20, 70 and 120 shown in FIGS. 1,2 and 3, respectively. May be the same as

As with the embodiments provided by FIGS. 1,2 and 3, in accordance with this embodiment, a portion of PCB 180 may have a lower surface 191 of die 190 and an upper surface 188 of heat sink material 181. ) To remove the voids 200 within the PCB 180. The void 200 is preferably formed by removing portions of the layers 182-184 of the PCB 180 down to the top surface 188 of the heat sink material 181. The void 200 is then filled with underfill material 210 having a higher thermal conductivity than the removal portion of the PCB 180. Additionally, in accordance with this embodiment, the top surface 188 of the heat sink material 181 preferably at least partially protrudes into the void 200, for example a metal device (eg metal rivets or slugs). It has a device 220 inserted in the same void. In addition, according to this embodiment, the bottom surface of die 190 has a relatively high thermally conductive material or device 240 disposed thereon or closely disposed thereon. The material or device 240 has a higher thermal conductivity than the removal portion of the PCB 180. Preferably, the material or device 240 is made of metal. Thus, the material or device 240 may be a metal plate, such as a metal plate of copper. Material or device 240 may be disposed on or adjacent to bottom surface 191 of die 190 and may be formed by any suitable process, such as, for example, plating, deposition, sputtering, and the like. Material or device 240 may be attached to the top surface of device 220 by, for example, solder 250.

The portion of the void 200 between the lower surface 191 of the die 190 and the upper surface 188 of the heat sink material 181 is completely filled with a material of higher thermal conductivity than the removal portion of the PCB 180. It can be seen from FIG. 4. For example, all devices or materials 220, 240, and 250 may be partially or fully metal. Thus, the thermal path between die 190 and heat sink material 181 may be made entirely of high thermally conductive material. Providing a connection between the bottom surface 191 of the die 190 and the top surface 188 of the heat sink material 181 with high thermal conductivity greatly increases the thermal conductivity of the thermal path.

5 is a flow chart of the method of the present invention in accordance with a preferred embodiment for improving the efficiency of a heat sink of a PCB assembly. As noted above, a portion of the PCB is removed to provide voids to the PCB as indicated by block 271. The thermal conductivity of the thermal path between the die and the heat sink material reduces the length of the thermal path and / or increases the thermal conductivity of the material disposed in the void and / or between the lower surface of the die as indicated by block 272. To improve.

It should be noted that the present invention is described with reference to some exemplary embodiments and that the present invention is not limited to these embodiments. The embodiments described herein are intended to convey the principles and concepts of the present invention and are not intended to represent exclusive embodiments for carrying out the present invention. For example, in each of the embodiments, the present invention has been described with reference to removing some of all layers of the PCB down to the top surface of the heat sink material. It should be noted that removing all non-layered portions of the PCB to form a void and filling the void with a material or device with higher thermal conductivity than the removed portion may be sufficient to improve the heat dissipation required for a particular application. do. In addition, the invention is not limited in terms of the type of material or device that is not disposed in the void or adheres to the bottom surface of the die. In addition, the present invention is not limited to any particular type of PCB. For example, the present invention is applicable to single layer PCBs and multilayer PCBs. Also, while all embodiments show a heat sink material in contact with the bottom surface of the PCB, the heat sink material may be in other locations. For example, in a multilayer PCB, the heat sink material may be a ground plane disposed between the outermost layers of the PCB. Other variations may refer to the embodiments described herein and all such variations are within the scope of the present invention.

Claims (10)

In a circuit board (CB) assembly, A CB comprising a dielectric material layer and one or more electrically conductive circuit traces formed on the dielectric material layer; An integrated circuit (IC) die mounted to the side of the CB in a direct-chip-attach configuration, the interconnecting one of the one or more circuit traces of the CB with an electrical contact pad on the die. The integrated circuit (IC) die electrically interconnected to the CB by at least one electrical interconnect; And A relatively high thermally conductive material or device disposed on or adjacent to the die, wherein the material or device disposed on or adjacent to the die has a thermal conductivity higher than that of the dielectric material layer of the CB. Having a circuit board (CB) assembly. The method of claim 1, Wherein the die is part of an IC package and the CB is a single layer CB. The method of claim 1, And further comprising a heat sink material in contact with the CB side opposite the CB side on which the die is mounted, the layer of dielectric material having a void extending therethrough, the void having the dielectric of the CB. Circuitry having a material or device disposed therein that has a higher thermal conductivity than a material layer, wherein the material or device disposed within the void is in contact with or adjacent to the material or device disposed on or adjacent to the die Substrate (CB) Assembly. The method of claim 3, wherein The material or device disposed within the void is in contact with a bonding material, the bonding material is also in contact with the material or device disposed on the die, and the bonding material has a higher thermal conductivity than the dielectric material layer. Substrate (CB) Assembly. A method for improving heat dissipation in a circuit board (CB) assembly, wherein the CB assembly includes an integrated circuit (IC) die mounted on the CB, wherein the CB is a dielectric material layer and the dielectric material. A method for improving heat dissipation comprising at least one electrically conductive circuit trace formed on a layer, Placing a device or material on or near the die having a thermal conductivity higher than that of the dielectric material layer of the CB; And Mounting the die on the CB by attaching the die to the side of the CB in a direct-chip-mount configuration, wherein when the die is mounted on the CB, placement on or adjacent to the die Wherein the device or material is in contact with or adjacent to the device or material disposed on the CB, and the device or material disposed on the CB has a higher thermal conductivity than the dielectric material layer of the CB. . The method of claim 5, Prior to mounting the die on the CB, forming voids in a portion of the CB; And Disposing a device or material within the void having a thermal conductivity higher than that of the dielectric material layer of the CB, and when the die is mounted to the CB, the device or material disposed within the void is A method for improving heat dissipation in contact with or adjacent to the device or material disposed on or adjacent to a die. The method of claim 6, Prior to mounting the die on the CB, placing a heat sink material in contact with the CB side opposite the CB side mounted on the die; And Changing the shape of the portion of the heat sink material such that a portion of the heat sink material protrudes toward the void to be closer to the die than the other portion of the heat sink material, the apparatus disposed within the void or The material corresponds to the portion of the heat sink material that has changed shape. The method of claim 7, wherein And a portion of the heat sink material protruding toward the void is changed in shape by a stamping process. The method of claim 7, wherein And a portion of the heat sink material protruding toward the void is changed in shape by an embossing process. delete

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