PRINTED CIRCUIT BOARD AND METHOD OF PRODUCING THEREOF

Abstract

A printed circuit board includes a first thermal pad on a first side of the printed circuit board for a first integrated circuit having a first exposed pad and a second thermal pad on a second side of the printed circuit board for a second integrated circuit having a second exposed pad. The first thermal pad and the second thermal pad overlap at least partially to each other in a direction perpendicular to a plane of the printed circuit board.

Claims

1. A Printed Circuit Board (PCB) comprising: a first thermal pad on a first side of the PCB for a first integrated circuit (IC) having a first exposed pad; and a second thermal pad on a second side of the PCB for a second integrated circuit (IC) having a second exposed pad, wherein the first thermal pad and the second thermal pad overlap at least partially in a direction perpendicular to a plane of the PCB, a first geometric center of the first thermal pad and a second geometric center of the second thermal pad are aligned and overlap in the direction perpendicular to the plane of the PCB.

2. The PCB according to claim 1, further comprising a plurality of thermal vias, with a first end of each thermal via connecting to the first thermal pad and a second end of each thermal via connecting to the second thermal pad.

3. The PCB according to claim 1, wherein the first thermal pad and the second thermal pad are substantially identical.

4. The PCB according to claim 1, wherein the PCB further comprises the first IC, the first exposed pad of the first IC is connected to the first thermal pad of the PCB, and the second thermal pad remains unoccupied.

5. The PCB according to claim 1, wherein the PCB further comprises the second IC, the second exposed pad of the second IC is connected to the second thermal pad of the PCB, and the first thermal pad remains unoccupied.

6. The PCB according to claim 1, wherein both the first and the second thermal pads remain unoccupied.

7. The PCB according to claim 1, wherein the PCB further comprises the first IC and the second IC, and wherein the first IC and the second IC are video serializer and video deserializer, respectively.

8. A layout structure for the PCB according to claim 1, the layout structure comprising: a first footprint on a first side of the layout structure for the first IC to be populated on the PCB; and a second footprint on a second side of the layout structure for the second IC to be populated on the PCB; wherein the first footprint and the second footprint overlap at least partially in the direction perpendicular to the plane of the PCB.

9. The layout structure according to claim 8, wherein first and second geometric centers of the first footprint and the second footprint, respectively, are aligned and overlap in the direction perpendicular to the plane of the PCB.

10. A method of manufacturing a Printed Circuit Board (PCB) comprising: providing a substrate; forming a first thermal pad on a first side of the substrate; and forming a second thermal pad on a second side of the substrate, wherein the first thermal pad and the second thermal pad overlap at least partially in a direction perpendicular to a plane of the PCB.

11. The method according to claim 10, further comprising, forming thermal vias in the substrate, with a first end of each thermal via connecting to the first thermal pad and a second end of each thermal via connecting to the second thermal pad.

12. The method according to claim 10, further comprising connecting a first exposed pad of a first integrated circuit (IC) to the first thermal pad and leaving the second thermal pad unoccupied.

13. The method according to claim 10, further comprising connecting a second exposed pad of a second integrated circuit (IC) to the second thermal pad and leaving the first thermal pad unoccupied.

14. The method according to claim 11, further comprising connecting a first integrated circuit (IC) to the first thermal pad and connecting a second integrated circuit (IC) to the second thermal pad, the first IC and the second IC are video serializer and video deserializer, respectively.

15. The PCB according to claim 1, further comprising a plurality of thermal vias, the plurality of thermal vias traverse the first thermal pad, the second thermal pad and the PCB.

16. The PCB according to claim 1, wherein the first thermal pad and the second thermal pad have different dimensions.

17. The PCB according to claim 1, wherein both the first and the second thermal pads are populated with Integrated Circuits.

18. The method according to claim 10, further comprising, forming thermal vias in the substrate, the thermal vias traverse the first thermal pad, the second thermal pad and the substrate.

Description

DRAWINGS

[0021] In order that the disclosure may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:

[0022] FIG. 1 is a perspective view showing two integrated circuits having exposed pads;

[0023] FIGS. 2A and 2B are exemplary layout structures of PCBs according to prior art;

[0024] FIG. 3A is an exemplary layout structure of a top side of a PCB in accordance with the principles of the present disclosure;

[0025] FIG. 3B is an exemplary layout structure of a bottom side of the PCB of FIG. 3A in accordance with the principles of the present disclosure;

[0026] FIG. 4A is a cross sectional view of a portion of the PCB of FIG. 3A with thermal pads according to one or more embodiments;

[0027] FIG. 4B is a cross sectional view of a portion of another PCB with thermal pads according to one or more embodiments;

[0028] FIG. 5 is a three-dimensional view of a portion of the PCB with thermal pads according to one or more embodiments;

[0029] FIG. 6A is a cross-sectional view sequentially illustrating a process of fabricating a PCB according to one or more embodiments; and

[0030] FIG. 6B is a cross-sectional view sequentially illustrating a process of fabricating a PCB according to one or more embodiments.

[0031] The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

DETAILED DESCRIPTION

[0032] The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

[0033] There now follows, by way of example only, a detailed description of exemplary embodiments of the present disclosure. These examples are described in reference to the figures identified above, which are included to provide further detail and clarity. It should be understood that the examples described are not limiting, and that other examples falling within the scope of the disclosure may exist.

[0034] Referring to FIG. 1, there is shown a first integrated circuit (also known as chip or microchip) 102 and a second integrated circuit (IC) 106. The first and the second ICs 102 and 106 may be mounted on a PCB to create functional electronic devices. Both ICs 102 and 106 are provided with exposed pads 104 and 108, respectively. The exposed pads 104 and 108 are exposed metal plates on the IC package that is left exposed on the underside the IC 102 and 106, respectively. The exposed pads 104, 108 may also serve as heat sinks. The exposed pads 104 and 108 are used to improve the thermal performance and electrical grounding of the ICs. They may be connected to a ground plane or power plane on the PCB. By providing a larger area for heat dissipation, the exposed pad helps to inhibit overheating and improves the reliability and lifespan of the ICs.

[0035] In one or more embodiments, the ICs 102 and 106 may be components both including exposed pads for thermal dissipation. The ICs 102 and 106 may be ICs used in similar applications. For example, the first and the second ICs 102 and 106 may be video serializer and video deserializer, radio frequency modulator and demodulator, digital-to-analog converter and analog-to-digital converter, wireless transmitter and receiver, or the like. In addition to this, or as another option, it is possible that the first and the second ICs 102 and 106 rely on similar or identical supporting components to enable them function properly, and/or are used in a comparable PCB layout. Alternatively, any ICs can be used based on actual needs.

[0036] Referring to FIG. 2A, there is provided a layout structure 200a of a PCB 204a used in the prior art. When the PCB 204a is designed, a layout structure 200a is created to plan for incorporating all necessary components, electrical connections, and other features required for an intended application. One important aspect of the layout structure 200a is the creation of footprint for each component. A footprint is a pattern or layout on the PCB 204a that defines the location and physical dimension of a component and its electrical connections. The footprint provides that the component has its own place to be soldered to the PCB, and it is designed to match the specific dimensions and pin locations of the component being used.

[0037] To implement (also known as populate) the first IC 102 shown in FIG. 1, a footprint 202 is created for the first IC 102 on the layout structure 200a at the top side of a PCB 204a. The footprint 202 illustrates locations and dimensions of solder pads and a thermal pad which will be created on the PCB 204a. Solder pads are the areas on the PCB where the IC's leads or contacts are attached by using solder. The thermal pad is a copper pad on the PCB where the IC's exposed pad is attached by using solder to dissipate heat generated by the IC. The copper pad is generally connected to a ground plane through vias to improve heat dissipation. The layout structure 200a may also include footprints for other electronic components or ICs that are illustrated as dashed rectangles.

[0038] Similarly, referring to FIG. 2B, to implement the second IC 106 shown in FIG. 1, a further layout structure 200b of a PCB 204b is designed. On the layout structure 200b, a footprint 206 is created for the second IC 106 at the top side of the PCB 204b. The footprint 206 on the layout structure 200b illustrates solder pads and a thermal pad which corresponds to the leads and the exposed pad 108 of the second IC 106.

[0039] In the case of the prior art shown in FIGS. 2A and 2B, two separate layout structures for two PCBs have to be created for the first IC 102 and the second IC 106, respectively. Specifically, since both the first IC 102 and the second IC 106 have exposed pads to meet heat dissipation needs, the opposite side (bottom side) of the PCBs underneath the ICs cannot be used to accommodate any additional components and must be kept free (unoccupied).Consequently, during manufacturing, two PCBs 204a and 204b have to be manufactured to implement the two ICs. Such process involves high production and certification costs.

[0040] Referring to FIGS. 3A and 3B, there is provided an exemplary layout structure 300 of a PCB 306 according to one or more embodiments of the present disclosure. FIG. 3A shows a first side (top side) of the layout structure 300 of the PCB 306, and FIG. 3B shows a second side (bottom side) of the layout structure 300 of the PCB 306.

[0041] As can be seen in FIG. 3A, a first footprint 302 is created for the first IC 102. The first footprint 302 illustrates locations and dimensions of solder pads and a first thermal pad 308 (can be seen from FIG. 4) which will be created on the PCB 306 for the first IC 102. Solder pads are the areas on the PCB where the first IC 102's leads or contacts are attached by using solder. The first thermal pad 308 may be a copper pad on the PCB 306 where the IC's exposed pad 104 is attached by using solder to dissipate heat generated by the IC 102. The thermal pad 308 may be connected to a ground plane through vias to improve heat dissipation.

[0042] As shown in FIG. 3B, on the second side (bottom side) of the layout structure 300 of the PCB 306, a second footprint 310 is created for the second IC 106. Similarly, the second footprint 310 illustrates locations and dimensions of solder pads and a second thermal pad 304 (FIG. 4) which will be created on the PCB 306 for the second IC 106. The second thermal pad 304 may be a copper pad on the PCB 306 where the second IC 106's exposed pad 108 is attached by using solder to dissipate heat generated by the second IC 106. The first thermal pad 308 and the second thermal pad 304 can be observed from FIG. 4, which is a cross sectional view of the PCB 306 along line AA.

[0043] The layout structure 300 may also include footprints for other electronic components or ICs that are illustrated as dashed rectangles. It goes without saying that the layout structure 300 is for illustration purpose only. The number and variety of electronic components or ICs can be selected as needed, and they can be positioned anywhere on the PCB to fulfil actual application needs.

[0044] FIG. 4A is a cross sectional view of the portion of the PCB 306 with thermal pads according to one or more embodiments. The first thermal pad 308 is provided on the first side (top side) of the PCB 306, and the second thermal pad 304 is provided on the second side (bottom side) of the PCB 306. The first thermal pad 308 at least partially overlaps with the second thermal pad 304 in a direction perpendicular to the plane of PCB 306. Preferably, a plurality of thermal vias 406 may be provided in the PCB 306 to facilitate heat dissipation. The first thermal pad 308 and the second thermal pad 304 are connected by the plurality of thermal vias 406, with one end of each thermal via connecting to the first thermal pad 308 and the other end connecting to the second thermal pad 304. This provides that the openings of the thermal vias 406 at the surface of the PCB are in direct contact with the corresponding thermal pads, establishing a reliable thermal connection.

[0045] Alternatively, according to one or more embodiments as shown in FIG. 4B, the thermal vias 406 can be designed as through-holes traversing the thermal pads and the PCB, with the openings of the vias ends at the outer surfaces of the thermal pads. Such design can simplify the production of the PCB and reduce the production costs subsequently. In one or more embodiments, the thermal vias 406 may be designed as such that only a portion of them penetrate the PCB and/or the thermal pads, while the remaining portion may connect to one or more thermal planes within the PCB. In other words, the design of the thermal vias 406 can be modified as desired based on actual needs.

[0046] A three-dimensional view of the structure of the thermal pads 304, 308 and thermal vias 406 on the PCB, particularly in the area where the thermal pads 304, 308 overlap, can be observed from FIG. 5. In one or more embodiments, the first thermal pad 308 and the second thermal pad 304 fully overlap with each other (have a same dimension). In one or more embodiments, the dimensions of the two thermal pads 304, 308 do not have to be identical. For example, the first thermal pad 308 may have a dimension of 5 mm*5 mm and the second thermal pad 304 may have a dimension of 7 mm*7 mm. In such case, heat dissipation of the IC soldered on the smaller thermal pad may be further facilitated by the larger unoccupied thermal pad positioning on the opposite side of the IC.

[0047] In one or more embodiments of the present disclosure, the centroids (geometric center) of the two thermal pads 304, 308 are aligned and overlap in the direction perpendicular to the plane of the PCB, so that heat dissipated by the thermal pads would not affect the function of components on the opposite side of the PCB. Also, such arrangement may provide an efficient way of using the areas on the PCB.

[0048] In one or more implementations in which the first IC 102 is desired, it is soldered on the top side of the PCB 306 at a specific location that corresponds to the first footprint 302. The exposed pad 104 of the first IC 102 is applied (soldered) on the first thermal pad 308 through a first solder 311. In this case, the second thermal pad 304 remains unoccupied (free from being connected to any component). As such, the thermal pad 308 along with the thermal vias 406 dissipates heat generated by the first IC 102. In addition, the heat dissipation effect is further improved by the existence of the second thermal pad 304.

[0049] Similarly, in one or more implementations in which the second IC 106 is desired, it is soldered on the bottom side of the PCB 306 at a specific location that corresponds to the second footprint 310. The exposed pad 108 of the second IC 106 is applied (soldered) on the second thermal pad 304 through a second solder 312. In this case, the first thermal pad remains unoccupied (free from being attached to any component). As such, the thermal pad 304 along with the thermal vias 406 dissipates heat generated by the second IC 106. In addition, the heat dissipation effect is further improved by the existence of the first thermal pad 308. It goes without saying that in the implementations, contact leads of the first IC 102 and the second IC 106 are also soldered on corresponding solder pads of the PCB.

[0050] In the present disclosure, while it is an option to populate only one of the thermal pads with an IC at a time to improve heat dissipation, it is conceivable that both thermal pads can be used if the ICs produce minimal heat during operation.

[0051] In one or more embodiments, the ICs 102 and 106 may be a video serializer and a video deserializer having exposed pads with same or different dimensions, respectively. For example, if the video serializer is implemented, it is mounted on the side of the PCB that is dedicated for the video serializer, and the other side of the PCB is left free. Similarly, if the video deserializer is implemented, it is mounted on the side of the PCB that is dedicated for the video deserializer, and the other side of the PCB is left free. Other ICs, such as radio frequency modulators and demodulators, digital-to-analog converters, analog-to-digital converters, or wireless transmitters and receivers, may be selected based on specific needs. In other words, the ICs that can be populated are not limited to any type.

[0052] In one or more embodiments, it may be possible that none of the ICs 102 and 106 are desired. In such case, both the first thermal pad 308 and the second thermal pad 304 may be unoccupied. Based on the present disclosure, it is also conceivable that further thermal pads may be included on the PCB, for example, third and fourth thermal pads, the further thermal pads may be arranged in a similar way as described above.

[0053] The PCB layout according to the present disclosure takes advantage of both the top and bottom sides of the board to include thermal pads for ICs in an overlapping arrangement. This design and the produced PCB make efficient use of space that would otherwise go unused due to the thermal characteristics of an IC having an exposed pad. Specifically, traditionally, the side of the PCB opposite to an IC with an exposed pad is intentionally left unused to enable effective heat dissipation during operation. The present disclosure adapts a single PCB for multiple applications involving using different ICs having exposed pads. Consequently, the PCB can be made more compact, and the production and certification cost of PCBs can be significantly reduced.

[0054] FIGS. 6A and 6B are cross-sectional views sequentially illustrating the process of fabricating a PCB according to one or more embodiments. At step 600a, a layer of photoresist 608 is applied on a substrate. The photoresist may be a positive photoresist or a negative photoresist. The substrate may be a copper clad laminate including a laminate 602 sandwiched by two copper layers 604 and 606. In addition to the copper clad laminate shown in FIGS. 6A and 6B, another laminate with multiple layers of copper pads may also be used. The choice of the substrate depends on specific requirements of the PCB, such as desired electrical properties, mechanical strength, and environmental conditions.

[0055] At step 600b, the substrate is exposed to UV light through a photomask 610, which is a mask that has the pattern of the desired electrodes, e.g. the solder pads and the thermal pad of the first IC 102. The process shown in FIGS. 6A and 6B uses a positive photoresist. Thus, after exposure, the photoresist dissolves away at the areas that were exposed to light, leaving behind a patterned layer on the substrate, as shown in 600c. Afterwards, the copper layer 604 is etched to form the pattern shown in step 600d. At step 600e, the remained photoresist 608 is dissolved and the first thermal pad 308 is formed. The process is repeated for the other side of the PCB. At step 600f, the photoresist 612 is applied on the copper layer 606. At step 600g, the substrate is exposed to UV light through a photomask 610, which is a mask that has the pattern of the desired electrodes, e.g. the solder pads and the thermal pad of the second IC 106. After exposure, the photoresist dissolves away at the areas that were exposed to light, leaving behind a patterned layer on the substrate, as shown in 600h. Afterwards, the copper layer 606 is etched to form the pattern shown step 600i. At step 600j, the remained photoresist 608 is dissolved and the second thermal pad 304 is formed.

[0056] At step 600k, thermal vias is formed in the PCB. The formation of thermal vias may be achieved through various methods, including mechanical drilling, laser drilling, or any other suitable method. Mechanical drilling involves the use of a drill bit to create holes in the PCB. The size and shape of the holes may vary depending on the design and requirements of the PCB. Alternatively, laser drilling may also be used to create the thermal vias. This method uses a high-powered laser to remove material from the PCB and create the desired holes. Once the holes are formed, copper plating may be applied to the inner walls of the vias to enhance thermal conductivity. The plating material used may vary, depending on the design specifications of the PCB. The PCB produced in this process is similar to that shown in FIG. 4B, in which the thermal vias transverse the thermal pads and the PCB. Alternatively, according to the present disclosure, the step 600k may also be performed before forming the thermal pads and the contact pads. That is, before the step 600a. In other words, the thermal vias 406 (along with other vias) can be drilled before forming the thermal pads and the contact pads.

[0057] It goes without saying that the methods are not limited and other methods may be applied. For example, to produce the PCB shown in FIG. 4A, a laminate material may be firstly drilled to form the thermal vias. Followed a layer of copper may be applied on the laminate by sputtering and electro-chemical deposition. Afterwards, lithography process similar as described above may be used to form the thermal pads and contact pads.

[0058] In the present disclosure, it is preferable that the first thermal pad 308 may be formed in a way in which it at least partially overlaps with the second thermal pad 304 in a direction perpendicular to the plane of PCB 306. It may be possible that the first thermal pad 308 and the second thermal pad 304 may be formed in a way that they fully overlap in the direction perpendicular to the plane of the PCB 306. This may be the case when the first thermal pad 308 and the second thermal pad 304 have a same dimension. It goes without saying that the dimensions of the two thermal pads do not have to be identical. Optionally, the centroids (geometric center) of the two thermal pads are aligned and overlap in the direction perpendicular to the plane of the PCB, so that heat dissipated by the thermal pads would not affect the function of components on the opposite side of the PCB. In one or more embodiments, further thermal pads may be formed on the PCB by the process as described above or by similar processes.

[0059] It is conceivable that the steps shown in FIGS. 6A and 6B are only a subset of the overall process required to fabricate a functional PCB. In other words, there are additional steps that may be performed beyond those shown in the figure, depending on specific application requirements. For example, after patterning the electrode on the substrate as shown in FIGS. 6A and 6B, additional processing steps may be performed to apply a solder mask, which is a protective coating applied to the surface of the PCB to protect the copper traces and prevent solder from bridging between adjacent traces. The process may include applying a surface finish to protect the exposed copper and improve the electrical conductivity of the traces. The process may further include testing and inspection to provide that the final device meets the desired specifications. This may involve a range of tests, such as electrical testing, mechanical testing, and visual inspection. Overall, the skilled person in the field is familiar with the additional steps that may be required and can adapt the process as needed to achieve desired outcomes.

[0060] As one skilled in the art would appreciate, one or more embodiments disclosed in the present disclosure can be combined in any combination. In other words, the features, components, and methods described can be combined to suit specific needs and requirements of a particular application.

[0061] Unless otherwise expressly indicated herein, all numerical values indicating mechanical/thermal properties, compositional percentages, dimensions and/or tolerances, or other characteristics are to be understood as modified by the word about or approximately in describing the scope of the present disclosure. This modification is desired for various reasons including industrial practice, material, manufacturing, and assembly tolerances, and testing capability.

[0062] As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A OR B OR C), using a non-exclusive logical OR, and should not be construed to mean at least one of A, at least one of B, and at least one of C.

[0063] The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.