Abstract:
A Printed Circuit Board Assembly (PCBA) forming an enhanced fingerprint module is disclosed. The PCBA includes a Printed Circuit Board (PCB), an image sensing chip, at least one electrode and a protection layer. An opening in a first insulation layer and a second insulation layer of the PCB together form a sensor portion so that the image sensing chip can be packaged in the opening. Thus, the thickness of the enhanced fingerprint module can be thinner than other fingerprint modules provided by the conventional package methods.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a Printed Circuit Board Assembly (PCBA). More particularly, the present invention relates to a PCBA with an image sensor mounted thereon, forming an enhanced fingerprint sensing module. 
     BACKGROUND OF THE INVENTION 
     A silicon chip, or Integrated Circuit (IC), is the core element of an electronic device and usually comes in packaged form. With the development of manufacturing technology and requirement of compact design for end products, various packaging methods were invented to meet the demand. Generally, silicon chips are sealed within a protective material such as a molding compound. There are certain cases, particularly when the silicon chip is a sensor device such as a fingerprint sensor chip embedded in a cellular phone or smart card, that the silicon chip needs to be mount on a substrate and has a surface exposed. Meanwhile, for a fingerprint reader device, the thickness of the packaged sensor must be reduced, and the surface should be maintained flat whenever possible. Several techniques, such as wire bonding, flip chip, and other non-conventional packaging methods, are used to package a fingerprint sensing chip now. However, none of them meets all the requirements: flatness of the top surface, thickness of the package, rigidness between the fingerprint sensing chip and the package material, and good circuit connectivity. 
     Conventional wire bonding is sometimes used to address the above requirements. Please refer to  FIG. 1 . A chip  1  in form of a die is mounted on a Printed Circuit Board (PCB)  2 . There are many bonding pads  3  on one surface of the chip  1 . Some connectors  4  are arranged on the PCB  2 . By wire bonding, gold or aluminum wires  5  are formed to link the related bonding pads  3  and the connectors  4 . In order to fix the chip  1  to the PCB  2 , a layer of adhesive (not shown) may be applied on the interface between the chip  1  and the PCB  2 . In general, the height of the gold wires  5  over the PCB  2  will occupy certain space above the chip surface. Moreover, when the chip  1  and the wires  5  are sealed in molding compound to protect the circuit while maintaining a flat surface, the molding compound above the chip must be thicker than the height of the bonding wires  5 . The extra sealing material above the sensing surface will cause significant performance degrade. For the electronic devices whose thickness and surface flatness are much concerned, apparently, the wire bonding for the chip  1  and the PCB  2  is not appropriate. 
     Flip chip technology is another widely used packaging method for interconnecting between a die and a PCB. Processing a flip chip is similar to the conventional IC fabrication, with a few additional steps. Please refer to  FIG. 2 . Around the end of the manufacturing process, the attachment pads  12  of a chip  11  are metalized to make them more receptive to solders. It typically consists of several treatments. Small dots of solder balls  13  are then deposited on each metalized pad  12 . The chips  11  are then cut out of a wafer as normal. To attach the flipped chips  11  onto a PCB  14 , the chip (die)  11  is inverted to bring the solder balls  13  down onto connectors  15  on the underlying PCB  14 . The solder balls  13  are then re-melted to produce an electrical connection, typically using a thermosonic bonding or alternatively using a reflow solder process. This also leaves a small space between the chip&#39;s circuitry and the underlying mounting surface. In most cases, an electrically-insulating adhesive  16  is then underfilled to provide a stronger mechanical connection. However, to provide good circuit connectivity and rigidness, the size of the solder balls  13  cannot be reduced. Moreover, the difference in height between the top surface of the chip  11  and that of the underlying PCB  14  cannot be removed when the flip chip technology is applied. Therefore, the flip chip technology is not a proper packaging method for thickness and surface flatness is concerned. 
     U.S. Pat. No. 7,090,139 of Kasuga et al. discloses a smart card including a fingerprint sensor attached to a thin wiring film, and having a window or opening above the sensing surface to expose the sensing surface. The fingerprint sensor is sandwiched by two substrates, and the electrical connection between the sensor and the wiring film is achieved by anisotropic conductive films. Certainly, an obvious step, caused by the height of the substrate above the sensor and the height of the anisotropic conductive film, exists between the top surface of the smart card and that of the sensor. Thus, even though the smart card packaging method provided by Kasuga et al. meets the thickness requirement of a smart card, a flat top surface cannot be achieved. 
     A packaging of a fingerprint sensor and a method thereof, such as disclosed by U.S. Pat. No. 8,736,001, is shown in  FIG. 3 . The finger sensor  30  includes a substrate  35 , a finger sensing IC  34  mounted on the substrate  35 , and bond wires  32  coupling the substrate  35  and the finger sensing IC  34 . The finger sensing IC  34  includes a finger sensing area on an upper surface thereof. The finger sensor  30  includes an encapsulating layer  33  encapsulating the finger sensing IC  34  and covering the finger sensing area. The encapsulating layer  33  includes a recessed portion  37  for receiving the finger of the user. The encapsulating layer  33  also includes a peripheral flange portion  38  on the substrate  35  and surrounding the finger sensing IC  34  and the bond wires  32 . The finger sensor  30  includes a bezel  31  on the encapsulating layer  33 . The bezel  31  may be coupled to circuitry to serve as a drive electrode for driving the finger of the user. The finger sensor  30  includes conductive traces  36  on the substrate  35  for coupling the bezel  31  thereto. The bezel  31  may comprise a metal or another conductive material. In some examples, ESD protection circuitry may be coupled to the bezel  31 . That the bezel  31  is affixed on an uppermost surface of the encapsulating material (at the level higher than that of the highest point of the bond wire) means that a step between the surface of the sensing area and top surface of the bezel is subject to the loop height of the bond wire, which is around 100 μm in normal cases. Thus, the usage of the bezel  31  may protect the finger sensing IC  34  from mechanical and/or electrical damages, the bezel  31  is not suitable for the products that are needed to be flat and/or thin, such as a smart card or a smart phone. 
     Also, U.S. Pat. No. 8,933,781 of Desnoyers et al. discloses additional electrically conducting surface parts positioned adjacently to the sensitive surface of the sensor in a smart card to enhance the signal received by the fingerprint sensor. No particular method of the connection between the sensor and the circuit in the smart card mentioned implies some conventional linkage method is used. Using conventional linkage method and the lack of emphasis on the flatness of the packaging implies that not only the flatness of the top surface but a thicker protective coating layer for ESD is the purpose of &#39;781. 
     Both the bezel mentioned in U.S. Pat. No. 8,736,001 and the electrically conducting surface mentioned in U.S. Pat. No. 8,933,781 are exposed to environment. Although the bezel combines the driving electrode with ESD protective circuitry, it is still an exposed element. There are two disadvantages. First, size of the human body makes an antenna like device that can pick up radiation signals which may interfere with the fingerprint sensing function. Secondly, from the industrial design point of view, exposed conductive material, such as a reflective metal surface, may not be the choice of the designer, i.e. the designer may want to choose any suitable color or grain. 
     Furthermore, U.S. Pat. No. 8,736,080 of Arnold et al. discloses a low profile integrated circuit assembly which comprises: an integrated circuit, a substrate where the integrated circuit is disposed on, a conductive layer disposed in the signal trench and coupling to an integrated circuit signal pad, a bond wire configured to couple the conductive layer to an external pad. The substrate comprises at least one signal trench which is proximate to the integrated circuit signal pad and extending to one edge of the substrate. The bond wire, the at least one signal trench and the conductive layer are formed below a surface plane of the integrated circuit. This method successfully reduces the height of the package, and further provides a flat top surface. This method may provide an Integrated Circuit assembly having a flat top surface. However, the manufacturing processes, involving a deep etching step to form the trench and an additional metal plating step to form the conductive layer, require more manufacture time and additional cost. 
     Therefore, a low-cost and improved PCBA structure with a flat top surface, an embedded signal transmitting part(s), and a chip, especially a fingerprint sensor chip, mounted on a PCB over an opening is still desired. More particularly, both the embedded signal transmitting part(s) and the chip are sealed under a single protection layer which forms a flat top surface. 
     SUMMARY OF THE INVENTION 
     This paragraph extracts and compiles some features of the present invention; other features will be disclosed in the follow-up paragraphs. It is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims. 
     In accordance with an aspect of the present invention, a printed circuit board assembly (PCBA) forming an enhanced fingerprint module is disclosed. The PCBA includes: a Printed Circuit Board (PCB), including: a first insulation layer having an opening formed therein; a first electrically conductive layer, forming a first specific circuit on a portion of a top surface of the first insulation layer and forming a plurality of contacts; a second electrically conductive layer, forming a second specific circuit on a portion of a bottom surface of the first insulation layer; a second insulation layer, formed below the second electrically conductive layer, wherein the second specific circuit is formed on a portion of a top surface of the second insulation layer; and a third electrically conductive layer, forming a third specific circuit on a portion of a bottom surface of the second insulation layer; an image sensing chip, having a sensing area and a plurality of bonding pads on a top surface thereof, fixed in the opening with the sensing area facing external environment and each bonding pad connected to one corresponding contact; at least one electrode, formed close-to or around the opening in the first electrically conductive layer, for providing an excitation signal to an object which has a detected surface being detected by the image sensing chip; and a protection layer, formed completely over the at least one electrode and the top surface of the image sensing chip, and formed partially or completely over the top surface of the first insulation layer and a top surface of the first electrically conductive layer. The opening in the first insulation layer and the second insulation layer together form a recessed sensor portion on the PCB. The protection layer forms a flat top surface of the printed circuit board assembly (PCBA) over the image sensing chip and the at least one electrode. 
     According to the present invention, the excitation signal is a capacitively coupled excitation signal sent out from the image sensing chip to the object through the at least one electrode. The at least one electrode has a total contact area larger than 20 mm 2 . The at least one electrode may be a metal strip, metal strips, or a metal ring. 
     The bonding pad and a corresponding contact are connected by an electrically conductive material. The electrically conductive material may be thermal cured conductive paste or solder paste. The thermal cured conductive paste may be silver paste, copper paste, or nickel paste. The bonding pad and a corresponding contact are connected by metal plating. Gaps between sidewalls of the opening and peripherals of the image sensing chip are filled by an electrically non-conductive material. The electrically non-conductive material is epoxy resin. 
     The protection layer may be formed by an electrically non-conductive material. The electrically non-conductive material is epoxy resin. The protection layer may be made of organic coating material. 
     The image sensing chip is a fingerprint reader sensor chip. The image sensing chip has a low-power design with a wake-up function. The shape of the first opening is the similar to that of the image sensing chip but large enough in size so that the image sensing chip is able to be allocated in the opening. 
     Preferably, a step gap between a level of the top surface of the image sensing chip and that of the first insulation layer and/or the first electrically conductive layer after the image sensing chip is fixed into the sensor portion is less than 0.1 mm. 
     In accordance with another aspect of the present invention, a method for manufacturing the PCBA mention above includes the steps of: providing the PCB; providing the image sensing chip; gluing the sensor portion; placing the image sensing chip into the sensor portion; filling the gap between the sidewalls of the opening and the image sensing chip with the electrically non-conductive material; forming circuit patterns linking associated bonding pad and contact by the electrically conductive material; and forming the protection layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is used to illustrate a conventional wire bonding process. 
         FIG. 2  is used to illustrate a conventional flip chip technology process. 
         FIG. 3  shows a cross-sectional view of the finger sensor in a prior art. 
         FIG. 4  is a top view of an embodiment of a printed circuit board assembly including an image sensing chip according to the present invention. 
         FIG. 5  is a top view of another embodiment of a printed circuit board assembly including an image sensing chip according to the present invention. 
         FIG. 6  is a cross-sectional view of the printed circuit board assembly. 
         FIG. 7  is a side view of the printed circuit board assembly and an object which has a detected surface being detected by the image sensing chip. 
         FIG. 8  to  FIG. 12  are used to illustrate each step in a procedure of manufacturing the printed circuit board assembly. 
         FIG. 13  is a flow chart of the procedure for manufacturing the printed circuit board assembly according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be described more specifically with reference to the following embodiments. 
     Please refer to  FIG. 4  to  FIG. 13 .  FIG. 4  is a top view of a printed circuit board assembly (PCBA). The PCBA includes a Printed Circuit Board (PCB)  100  and an image sensing chip  200 . There are many image sensing chips can be used. In this embodiment, the image sensing chip  200  is a fingerprint reader. The image sensing chip  200  may further includes a metal grid (not shown) formed inside, for providing ESD protection for itself. Please notice that the sketches in the drawings may not be made according to the proportion. They are used for illustration only. The image sensing chip  200  may occupy less space than the PCB  100  in the PCBA. It should be noticed that in  FIG. 4 , a cross-sectional line AA′ is marked. For a better understanding, the cross section cut by line AA′ is used for illustration in some following drawings. 
     Since the PCBA works as a fingerprint reader module, the PCB  100  has several key parts. Please see  FIG. 6 ,  FIG. 6  is a side view of the PCBA along the cross-sectional line AA′ in  FIG. 4 . The key parts are a first electrically conductive layer  110 , a first insulation layer  120 , a second electrically conductive layer  130 , a second insulation layer  140  and a third electrically conductive layer  150  shown in sequence from top to bottom in  FIG. 6 . The first electrically conductive layer  110  forms a first specific circuit on a portion of a top surface of the first insulation layer  120 . As one can see, the first electrically conductive layer  110  shown in  FIG. 6  is in form of discontinuous conductors. The conductors are made of copper or other metals, such as alloys. Although the conductors are not connected to each one in the cross section, they are linked to form the first circuit when the first electrically conductive layer  110  is taken out from the PCB  100 . Circuit layout is a common technique. It is not described here. It should be noticed that the second electrically conductive layer  130  and the third electrically conductive layer  150  have similar structure and are illustrated in the same way. In addition, an electrical link between the first electrically conductive layer  110  and the second electrically conductive layer  130 , or that between the second electrically conductive layer  130  and the third electrically conductive layer  150  can be achieved by vias  160 . 
     The first electrically conductive layer  110  contains both the first specific circuit and at least one electrode  111 . Please refer to  FIG. 4 ,  FIG. 6  and  FIG. 8 . The contacts  112  are formed from the first specific circuit and around an opening  122  (please refer to  FIG. 8 ), and are used to electrically connect to the bonding pads  220  in the image sensing chip  200 . There is one electrode  111  in the embodiment and it is formed around the opening  122  in the first electrically conductive layer  110 . Function of the electrode  111  is to provide an excitation signal to an object which has a detected surface being detected by the image sensing chip  200 . The shape of the electrode  111  may vary. For example, the electrode  111  is a metal ring as shown in  FIG. 4 . It can also be a metal frame enclosing the opening  122 . According to the spirit of the present invention, the number of the electrode  111  is not limited to one and the electrode  111  can be close-to the opening  122 . In another embodiment shown in  FIG. 5 , there are two electrodes  111  in the form of metal strips parallel to each other. 
     Please refer to  FIG. 7 . According to the present invention, the image sensing chip  200  has a low-power design with a wake-up function. When the approach of an object  500  is detected by the image sensing chip  200 , the image sensing chip  200  wakes up and an excitation signal is sent to the object  500  which has a detected surface  510  being detected by the image sensing chip  200  through the electrode  111 . In this embodiment, the image sensing chip  200  is a fingerprint reader sensor chip. Thus, the object  500  refers to a finger while the detected surface  510  contains a fingerprint. 
     The excitation signal is a capacitively coupled excitation signal sent out from the image sensing chip  200  to the object  500  through the electrode  111 . The total area of the electrode(s)  111  should be large enough to provide sufficient signal intensity. Preferably, the total area of the electrode(s)  111  is larger than 20 mm 2 . In addition, the capacitively coupled excitation signal may reduce the interference of signals at low-frequency (around 60 Hz), further reducing the noise of the output signal of the image sensing chip  200 , and prevent the image sensing chip  200  from malfunction. 
     The first insulation layer  120  is used to separate the first electrically conductive layer  110  from the second electrically conductive layer  130 . Meanwhile, the first insulation layer  120  also provides enough hardness to the PCB  100 , preventing from breaking off. The first insulation layer  120  has the opening  122  (please refer to  FIG. 8 ) formed therein. The second electrically conductive layer  130  forms a second specific circuit on a portion of a bottom surface of the first insulation layer  120  and a portion of a top surface of the second insulation layer  140 . 
     The second insulation layer  140  basically has the same functions as the first insulation layer  120 . It is formed below the second electrically conductive layer  110 . A sensor portion  170  is formed on the PCB  100  by combining the opening  122  in the first insulation layer  120  and the second insulation layer  140 . The third electrically conductive layer  150  forms a third specific circuit on a portion of a bottom surface of the second insulation layer  140 . 
     The image sensing chip  200  has a number of bonding pads  220  on its top surface. The image sensing chip  200  is fixed in the sensor portion  170  (the opening  122 ) with each bonding pad  220  connected to one corresponding contact  112 . The image sensing chip  200  also has a sensing area  210  facing up to external environment after the image sensing chip  200  is fixed in the sensor portion  170  of the PCB  100 . 
     The PCBA further includes a protection layer  300 . The protection layer  300  forms a flat top surface of the PCBA over the image sensing chip  200  and the electrode  111 . Please see  FIG. 6  again. The protection layer  300  forms over some or all portions of the top surface of the first insulation layer  120  and that of the first electrically conductive layer  110 , and on the sensing area  210  of the image sensing chip  200 . The protection layer  300  also covers the electrode  111  to insulate the electrode  111  from the object  500 , such that the signal sent out from the electrode  111  is capacitively coupled to the object  500 . The protection layer  300  is formed by an electrically non-conductive material. Preferably, the electrically non-conductive material is epoxy resin. The protection layer  300  can be also made by organic coating material. 
     For linkage of some components mentioned above, there are many suitable ways. For example, the bonding pad  220  and the corresponding contact  112  are connected by an electrically conductive material  402 . Preferably, silver paste is used as the electrically conductive material  402  for its conductivity. In practice, the electrically conductive material  402  can be a copper paste, nickel paste (thermal cured conductive paste) or even a solder paste. In addition, the bonding pad  220  and the corresponding contact  112  may be connected by metal plating. The electrically conductive material  402  is printed on the area between each bonding pad  220  and the corresponding contact  112 , over the top of each bonding pad  220  and that of the corresponding contact  112 . Besides, gaps between sidewalls of the first insulation layer  120  and peripherals of the image sensing chip  200  are filled by an electrically non-conductive material  404 . Epoxy resin is a good choice for the electrically non-conductive material  404 . The electrically non-conductive material  404  can assist to fix the image sensing chip  200  into the opening  122  while no more current leakage may occur. 
     According to the present invention, the shape of the first opening  122  is similar to that of the image sensing chip  200  but large enough in size so that the image sensing chip  200  can be allocated in the opening  122 . This is to reduce the risk that the image sensing chip  200  may slide when a finger exerts force. A step gap between a level of the top surface of the image sensing chip  200  and that of the first insulation layer  120  and/or the first electrically conductive layer  110  after the image sensing chip  200  is fixed in the sensor portion  170  is less than 0.1 mm. Thus, the protection layer  300  can be attached well without being torn off due to a large step gap. 
     The PCBA has a procedure to manufacture. Please refer to  FIG. 8  to  FIG. 13  at the same time.  FIG. 13  is a flow chart of the procedure for manufacturing the PCBA according to the present invention.  FIG. 8  to  FIG. 12  are used to illustrate steps in the procedure. 
     First, provide the PCB  100  having the sensor portion  170 , at least one electrode  111 , a number of contacts  112  and an image sensing chip  200  having a number of bonding pads  220  (S 01 ). Then, glue the sensor portion  170  with the electrically conductive material  400  (S 02 ). Next, place the image sensing chip  200  into the sensor portion  170  with the sensing area faced up to face the external environment (S 03 ). Afterwards, fill the gaps with the electrically non-conductive material  404  (S 04 ). For the next step, form circuit patterns linking associated bonding pad  220  and contact  112  by electrically conductive material  402  (S 05 ). Finally, form the protection layer  300  over the top surface of the first insulation layer  120  and the sensing area  210  (S 06 ). 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.