Patent Abstract:
A thermal bonding structure and manufacture process of a flexible printed circuit (FPC) board are disclosed, and the thermal bonding structure includes a laminated structure having a first insulating layer with a solder pad area and showing parts of a first conductive layer, the first conductive layer, a second insulating layer, a second conductive layer, and a third insulating layer with a bonding area such that a part of the second conductive layer is exposed, and at least a through hole passing through the first conductive layer to the second conductive layer for propagating heat energy to fuse a solder. Accordingly, the reduction of heat energy lost in the third insulating layer improves the bonding quality, shortens the bonding period, and maintains the material stability under high temperature resulted from high heat energy.

Full Description:
FIELD OF THE INVENTION 
   The present invention generally relates to a flexible printed circuit board, and more particularly relates to a thermal bonding structure and manufacture process of a flexible printed circuit board that can improve the issues of bonding burns and material quality. 
   BACKGROUND OF THE INVENTION 
   As our living standard is improving gradually, various electronic consumer products are introduced to meet extensive consumer requirements, and thus promoting the prosperity of various industries directly and driving the growth of related sub-industries indirectly. To further meet the consumer requirements and trends for various functions, portability, operability and appearance, in hope of improving consumer&#39;s willingness to buy and brand loyalty, various electronic consumer products tend to be designed thinner and lighter. For example, the market share of color mobile phones with a photographic function and other combined functions grows drastically, and the demand of color LCD panels and camera modules for mobile phones rises accordingly. Color LCD panel industry is divided into the area of color super twisted nematic (CSTN) LCDs and thin-film transistor (TFT) LCDs, and the key components including light emitting diodes (LEDs) and flexible printed circuit (FPC) boards also grow with the high demand for flexibility, 3-D circuit layout and light weight of a miniaturized foldable design of mobile phones. The estimated quantity of flexible printed circuit boards used in a color mobile phone is increased from 3˜4 pieces to 6˜7 pieces, and the design of flexible printed circuit boards tends to follow a high-end small circuit specification. A flexible printed circuit board is made by raw materials including a flexible insulating substrate material and a circuit conductor material (generally copper clad), and the raw materials are divided into resins, copper clads, adhesives, coverlays, and flexible copper cladded laminates (FCCL). Since polyimide (PI) has good expansibility and heat resistance, therefore PI is generally used as a resin material and serves as a middle layer and a substrate in the manufacture of flexible copper substrates and also as a coverlay film. 
   PI manufacturers can produce different PI films from different PI monomers according to different technologies in three main aspects: formula, manufacture process and processing method, and thus different manufacturers achieve different applications and performance of the materials. Further, the flexible copper substrate is divided into two main types: an adhesive three-layer structure and an adhesiveless two-layer structure, and both adopt different manufacture processes, methods and applications, and thus the properties of the materials are different. In general, the adhesive three-layer structure is usually applied to the production of a large number of flexible printed circuit board products and the adhesiveless two-layer structure is usually applied to the manufacture of high-end flexible printed circuit boards, such as the rigid and flexible printed circuit boards and some of the multi-layer boards. It is believed that the adhesiveless two-layer structure will take over some of the adhesive three-layer flexible copper cladded laminates used for the flexible printed circuit boards with high resolution and good dimensional stability. 
   Referring to  FIG. 1 , a schematic view of the relation between the raw materials and the finished goods of a prior art flexible printed circuit board is illustrated. In the manufacture of the flexible printed circuit board  150 , an insulating substrate material  100  and a circuit conductor material  110  are used to produce an adhesiveless two-layer flexible copper cladded laminate  130  first, and then a coverlay, a stiffener, and an anti-static layer are used to produce the flexible circuit board  150 . On the other hand, an adhesive three-layer flexible copper cladded laminate  140  is made of an insulating substrate material  100 , a circuit conductor material  110  and an adhesive  120 , and a flexible printed circuit  150  is made of such laminate  140 . At present, flexible printed circuit boards are generally used in electronic products, particularly mobile phones and LCDs showing a drastic a growth of using flexible circuit boards in their applications. 
   Referring to  FIG. 2 , a top view of a flexible printed circuit board and a cross-sectional view of a bonding head according to a prior art are illustrated. The flexible printed circuit board  2  comprises a first insulating layer  200 , an adhesive layer  210 , a conductive layer  220  and a second insulating layer; wherein the first insulating layer  200  and the second insulating layer  240  are made of the same material or different materials, and the first insulating layer  200  includes a solder pad area  270  and the second insulating layer  240  includes a bonding area  250 , such that a bonding head  260  is in direct contact with the bonding area  250  for soldering the flexible printed circuit board  2  with another flexible printed circuit board. In actual practices, the boding area  250  is usually situated at a position substantially parallel to the solder pad area  270 , so that heat energy can be conducted from the bonding area  250  to the adhesive layer  210 , conductive layer  220  and solder pad area  270  for bonding. However, it is necessary to increase the temperature of the bonding head  260  for bonding, and the high temperature will burn the first insulating layer  200  and the adjacent adhesive layer  210  black, and thus causing poor bonding quality and appearance of the product, or even deteriorating the materials in the bonding area. For example, a bonding machine sets a temperature for the bonding head for a thermal compression, and the temperature of the bonding head is set to 330° C. for a predetermined time (such as 3 seconds for temperature rise) and then the operating temperature of the bonding machine is set to 470° C. for another predetermined time (such as 3.5 seconds for the bonding), then the solder will be fused to complete the bonding process. However, the first insulating layer  200  and its adjacent adhesive layer  210  will be burned black at the temperature of 470° C., and such phenomenon is particularly severe for lead-free solders because the melting point of lead-free solders is higher than that of lead solders. For example, the melting points of the solders of the Sn—Ag—Cu series and Sn—Cu—Ni series are 227° C. and 217° C. respectively. Compared with the melting point 183° C. of solder of the Sn—Pb series, there is a difference of 34˜44° C. Therefore, it is necessary to increase the temperature of the bonding head  260  for lead-free solders in compliance with the environmental protection and international standard requirements. As a result, the burning effect produced in the bonding area  250  becomes obvious and severe. 
   Therefore, developing a thermal bonding structure and manufacture process for a flexible printed circuit board to overcome the foregoing shortcomings of the prior arts, improving the burning situation in the bonding area, and further conducting heat energy to the solder so as to lower the bonding temperature and supply less heat energy for saving bonding time and costs are important topics for manufactures and users and demand immediate attentions and feasible solutions. The inventor of the present invention based on years of experience on related research and development of the optoelectronic component industry to invent a thermal bonding structure and manufacture process for flexible printed circuit boards to overcome the foregoing shortcomings. 
   SUMMARY OF THE INVENTION 
   Therefore, it is a primary objective of the present invention to provide a thermal bonding structure of a flexible printed circuit board that comprises: a laminated structure and the laminated structure includes a first insulating layer, a first conductive layer, a second insulating layer, a second conductive layer and a third insulating layer in sequence; at least a through hole for passing through the first conductive layer, the second insulating layer and the second conductive layer. The first insulating layer includes a solder pad area for exposing the first conductive layer, and the third includes a bonding area for exposing the second conductive layer, such that the bonding head is in direct contact with the bonding area, and the heat energy can be conducted to the solder pad area through the through hole quickly for bonding. 
   Another objective of the present invention is to provide a flexible printed circuit board comprising a first area, and the first area includes a laminated structure having a first insulating layer, a first conductive layer, a second insulating layer, a second conductive layer and a third insulating layer arranged in sequence; a second area including a laminated structure coupled to the first area and having a first insulating layer, a first conductive layer, and a second insulating layer arranged in sequence; and a third area including a laminated structure disposed away from the first area and coupled to the second area and having the foregoing first insulating layer, first conductive layer, second insulating layer, second conductive layer and third insulating layer arranged in sequence; and at least one through hole passing through the foregoing first conductive layer, second insulating layer and second conductive layer. The first insulating layer in the third area includes a solder pad area for exposing the first conductive layer and being in contact with the solder, and the third insulating layer includes a bonding area for exposing part of the second conductive layer to define a bonding area, such that the heat energy of the bonding head can be conducted to the solder pad area through the through hole quickly to reduce bonding time and heat supply costs. 
   A further objective of the present invention is to provide a manufacture process of a flexible printed circuit board that comprises the steps of: providing a laminated structure and the laminated structure is divided into a first area, a second area and a third area, and the second area is disposed between the first area and the third area, and the laminated structure in the first and third areas includes a first insulating layer, a first conductive layer, a second insulating layer, a second conductive layer and a third insulating layer arranged in sequence, and the second area includes a first insulating layer, a first conductive layer, a second insulating layer, and at least one through hole formed in the first area and third area separately and passing through the first conductive layer, the second insulating layer and the second conductive layer; removing a part of the third insulating layer to expose the second conductive layer and define a bonding area; removing a part of the first insulating layer in the third area to expose the first conductive layer and define a solder pad area; and removing a part of the third insulating layer in the third area to expose the second conductive layer and define a bonding area. 
   Thus, the thermal bonding structure and manufacture process of a flexible printed circuit board in accordance with the present invention has the following advantages. Since the heat consumption at the insulating layer is reduced, therefore the bonding head can achieve the bonding effect with less heat energy and the cost for the bonding process can be lowered. Furthermore, the bonding head is applied to the bonding area, and the through hole is used to conduct heat energy to the solder pad area to accomplish the bonding process, and thus the temperature of the bonding head can be controlled to improve the burning phenomenon caused by the high temperature of the bonding head and occurred at the bonding area, so as to enhance the soldering process, the material quality, and the appearance of the product. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of the relation between the raw materials and the finished goods of a prior art flexible printed circuit board; 
       FIG. 2  shows a top view of a finished goods of a flexible printed circuit board and a cross-sectional view of a bonding head according to a prior art; 
       FIG. 3  shows a top view of a thermal bonding structure of a flexible printed circuit board and its corresponding cross-sectional view according to a preferred embodiment of the present invention; 
       FIG. 4  shows a top view of a flexible printed circuit board and its corresponding cross-sectional view according to a preferred embodiment of the present invention; and 
       FIG. 5  shows a flow chart of the manufacture process of a flexible printed circuit according to a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   To make it easier for our examiner to understand the objective of the invention, its innovative features and performance, a detailed description and technical characteristics of the present invention are described together with the drawings as follows. 
   Referring to  FIG. 3 , a top view of a thermal bonding structure of a flexible printed circuit board and its corresponding cross-sectional view according to a preferred embodiment of the present invention are illustrated. In the preferred embodiment, a thermal bonding structure  3  of a flexible circuit board comprises: a laminated structure having a first insulating layer  310 , an adhesive layer  320 , a first conductive layer  330 , an adhesive layer  320 , a second insulating layer  340 , an adhesive layer  320 , a second conductive layer  350 , and a third insulating layer  360 ; and at least one through hole  380  passing through each layer between the first conductive layer  330  and the second conductive layer  350 . The first insulating layer  310  includes a solder pad area  390  for exposing the first conductive layer  330 , and the third insulating layer  360  includes a bonding area  365  for exposing the second conductive layer, and the through hole  380  is formed beyond the range of the solder pad area  390  and the bonding area  365 . In other words, there is a gap between the solder pad area  390  and the bonding area  365 . Referring to  FIG. 3  for a cross-sectional view of a thermal bonding structure of a flexible printed circuit board according to a preferred embodiment of the present invention, the through hole  380  is formed beyond the range of the solder pad area  390  and the bonding area  365 . It is noteworthy that each conductive layer can be stacked on top of each insulating layer directly as disclosed in another embodiment, since the laminated structure of the thermal bonding structure of the flexible printed circuit board can only have a first insulating layer  310 , a first conductive layer  330 , a second insulating layer  340 , a second conductive layer  350  and a third insulating layer  360 . The through hole  380  of this embodiment includes an electric conductive material on its internal wall, and the materials used for making the first conductive layer  330  and the second conductive layer include a copper clad, and the surface of the second conductive layer  350  of the bonding area  365  further includes a metal layer for protecting the second conductive layer  350 . The metal layer could be single-layer or multiple-layer and made of gold, nickel, tin, other metal, or an alloy of the foregoing metals. In this embodiment, a nickel layer and a gold layer are formed in sequence on the surface of the second conductive layer  350  of the bonding area  365 , and the materials used here are provided for the purpose of describing the present invention and not intended to limit the invention. Further, the quantity and size of the through holes  380  vary with the speed and time of the heat conduction, and thus the positions of the through holes and the bonding area described in this embodiment are provided for example only, and not limited to the same number and size of the through holes  380  as depicted in  FIG. 3 . 
   A lithographic etching is adopted to remove a part of the third insulating layer  360  to define a bonding area  365 , so that heat energy can be conducted directly and quickly from the second conductive layer  350  to the first conductive layer  330  and fuse a solder such as a solder paste in the solder pad area  390 . With the same conditions as the prior art, a solder can be fused to complete a bonding process with a lower temperature, if the temperature of the bonding machine is set to 330° C. for a predetermined time (such as 3 seconds for temperature rise) and the operating temperature of the bonding machine to 400° C.,and the bonding head  370  is operated for a bonding time (such as 3.5 seconds), and thus improving or avoiding the burning phenomenon at the bonding area  365  and the third insulating layer  360 . 
   Referring to  FIG. 4 , a top view of a flexible printed circuit board and its corresponding cross-sectional view according to a preferred embodiment of the present invention are illustrated. In this embodiment, the flexible printed circuit board  4  is divided into a first area  410 , a second area  430  coupled to the foregoing first area  410  and a third area  450  disposed away from the first area  410  and coupled to the second area  430 . The flexible printed circuit board  4  comprises a laminated structure, and the first area  410  (which is a connecting area for components such as the light emitting diodes) and the third area  420  include a first insulating layer  411 , a first conductive layer  412 , a second insulating layer  413 , a second conductive layer  414  and a third insulating layer  415  arranged in sequence, and the second area  430  includes a first insulating layer  431 , a first conductive layer  432 , a second insulating layer  433  arranged in sequence and considered as a circuit area; and at least one through hole  458  passing through the first conductive layer  452 , the second insulating layer  453  and the second conductive layer  454 . The first insulating layer  451  in the third area  450  includes a solder pad area  456  for exposing the first conductive layer  452 , and the third insulating layer  455  includes a bonding area  457  for exposing the second conductive layer  454  and contacting a bonding head  459 . The surface of the second conductive layer  454  exposed from the bonding area  457  could include a metal layer, which is a single-layer metal layer such as a gold layer, or a double-layer metal layer such as a nickel layer and a gold layer. These materials are used for example to describe the present invention and not intended to limit the invention. 
   It is noteworthy that the through hole  458  is formed beyond the range of the solder pad area  456  and the bonding area  457 , and the through hole  458  includes an electric conductive material such as nickel or any other substance having a thermal conductive property. In addition, the materials used for the first conductive layer  452  and the second conductive layer  454  could include a copper clad, and an adhesive layer could be included between layers. 
   Referring to  FIGS. 4 and 5 , a flexible printed circuit board and a flow chart of a manufacture process of a flexible printed circuit board according to a preferred embodiment of the present invention are illustrated. In the embodiment, the manufacture process of a flexible printed circuit board  4  comprises the steps of: (Step S 51 ) providing a laminated structure, which is a four-layer structure as shown in the figure, and the laminated structure is divided into a first area  410 , a second area  430  and a third area  450 , and the second area  420  is disposed between the first area  410  and the third area  450  and includes a first insulating layer  411 , a first conductive layer  412 , a second insulating layer  413 , a second conductive layer  414  and a third insulating layer  415 ; and at least one through hole  458  being formed at the first area  410  and the third area  450  and passing through the first conductive layer  412 , second insulating layer  413  and second conductive layer  414 ; (Step S 52 ) removing a part of the first insulating layer  411  in the third area  450  to expose the first conductive layer  412  to define a solder pad area  456 ; and (Step S 53 ) removing a part of the third insulating layer  415  in the third area  450  to expose the second conductive layer  414  to define a bonding area  457 . 
   In another preferred embodiment of the present invention, a manufacture process of a flexible printed circuit board further comprises the steps of removing the second conductive layer  414  and the third insulating layer  415  in the second area  430 , such that the laminated structure in the first area  410  and the third area  450  includes a first insulating layer  411 , a first conductive layer  412 , a second insulating layer  413 , a second conductive layer  414  and a third insulating layer  415  arranged in sequence, and the laminated structure in the second area  430  includes a first insulating layer  411 , a first conductive layer  412  and a second insulating layer  413  arranged in sequence; depositing an electric conductive material in at least one through hole  458  after forming at least one through hole  458  that passes through the first conductive layer  412 , the second insulating layer  413  and the second conductive layer  414 ; and forming a metal layer on the surface of the bonding area  457  after forming the bonding area  457 . It is noteworthy that the laminated structure further comprises at least one adhesive layer disposed between the first insulating layer, the first conductive layer  412 , the second insulating layer  413 , the second conductive layer  414  and the third insulating layer  415 . 
   While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures. 
   In summation of the description above, the present invention is novel and useful and definite enhances the performance over the conventional structure and further complies with the patent application requirements and is submitted to the Patent and Trademark Office for review and granting of the commensurate patent rights.

Technology Classification (CPC): 7