Abstract:
An exemplary stiffener includes at least one polyethylene naphthalate layer. The at least one polyethylene naphthalate layer is represented by the following formula:

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to flexible printed circuit boards, particularly to a stiffener and a strengthened flexible printed circuit board having the stiffener. 
         [0003]    2. Description of related art 
         [0004]    In recent years, electronic products have achieved miniaturization in volume and diversification in function. Flexible printed circuit boards (FPCBs) are widely used for electrical connection because of their flexibility and lightness. 
         [0005]    Typically, a mass of electronic components are mounted onto a surface of a flexible printed circuit board to achieve various functions. For mounting and supporting the electronic components, a stiffener made of polyimide (PI) is usually attached onto the rear of the flexible printed circuit board via an adhesive layer to increase mechanical strength of the flexible printed circuit board, thereby obtaining a strengthened flexible printed circuit board. However, the strengthened flexible printed circuit board with the polyimide stiffener still has not enough mechanical strength to mount a mass of electronic components thereon. In addition, high cost of the polyimide stiffener may increase the cost of manufacturing the strengthened flexible printed circuit board. 
         [0006]    What is needed, therefore, is a stiffener and a strengthened flexible printed circuit board having the stiffener so as to increase mechanical strength of the flexible printed circuit board and lower cost of manufacturing the flexible printed circuit board with high mechanical strength. 
       SUMMARY 
       [0007]    One present embodiment provides a stiffener. The stiffener includes at least one polyethylene naphthalate layer. The at least one polyethylene naphthalate layer is represented by the following general formula: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0008]    Another present embodiment provides a strengthened flexible printed circuit board. The strengthened flexible printed circuit board includes a flexible printed circuit board and a stiffener attached onto the flexible printed circuit board. The stiffener includes at least one polyethylene naphthalate layer. The at least one polyethylene naphthalate layer is represented by the following general formula: 
         [0000]    
       
                 
         
             
             
         
       
     
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Many aspects of the present embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the present embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0010]      FIG. 1  is a schematic, cross-sectional view of a stiffener according to a first embodiment. 
           [0011]      FIG. 2  is a schematic, cross-sectional view of a stiffener according to a second embodiment. 
           [0012]      FIG. 3  is a schematic, cross-sectional view of a stiffener according to a third embodiment. 
           [0013]      FIG. 4  is a schematic, cross-sectional view of a stiffener according to a fourth embodiment. 
           [0014]      FIG. 5  is a schematic, cross-sectional view of a strengthened flexible printed circuit board with the stiffener shown in  FIG. 1 . 
           [0015]      FIG. 6  is a schematic, cross-sectional view of a strengthened flexible printed circuit board with the first stiffener shown in  FIG. 3 . 
           [0016]      FIG. 7  is a schematic, cross-sectional view of a strengthened flexible printed circuit board with the first stiffener shown in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0017]    Embodiments will now be described in detail below with reference to the drawings. 
         [0018]    Referring to  FIG. 1 , a stiffener  20  according to a first embodiment is shown. 
         [0019]    The stiffener  20  has a thickness in a range from 25 to 250 microns. Thus, the stiffener  20  has adequate rigidity to increase mechanical strength of a flexible printed circuit board and has adequate flexibility to maintain flexibility of a strengthened flexible printed circuit board with the stiffener  20  for a roll-to-roll process. 
         [0020]    The stiffener  20  is comprised of a polyethylene naphthalate (PEN) represented by the following general formula: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0021]    The polyethylene naphthalate can be prepared from ethylene glycol and one or more naphthalene dicarboxylic acids by condensation polymerization. The polyethylene naphthalate has a number of naphthalene rings in the molecular structure thereof. The naphthalene rings can increase rigidity of the molecular structure. Thus, the polyethylene naphthalate has good mechanical strength and structure stability. Moreover, coefficient of thermal expansion (CTE) for the polyethylene naphthalate is in a range from 18×10 −6  to 20×10 −6  K −1 . In general, coefficient of thermal expansion for the polyimide is in a range from 18×10 −6  to 28×10 −6  K −1 . Therefore, the polyethylene naphthalate has similar thermal expansion property to the polyimide, and has good thermal resistance and thermal stability. In addition, the polyethylene naphthalate has a lower cost than the polyimide. Cost of manufacturing the flexible printed circuit board with high mechanical strength can be reduced correspondingly. 
         [0022]    The stiffener  20  can includes an adhesive layer adhered thereto. Additionally, a stiffener can include a number of polyethylene naphthalate layers and a number of adhesive layers arranged in an alternate fashion, thereby forming a multilayer structure to further increase mechanical strength of the stiffener. 
         [0023]    Referring to  FIG. 2 , a stiffener  30  according to a second embodiment is shown. The stiffener  30  includes two polyethylene naphthalate layers  310  and an adhesive layer  320  sandwiched between the two polyethylene naphthalate layers  310 . The adhesive layer  320  is configured for adhering to the two polyethylene naphthalate layers  310  adjacent thereto. The two polyethylene naphthalate layers  310  are configured for adhering to a flexible printed circuit board. Each of the polyethylene naphthalate layers  310  has a thickness in a range from 25 to 75 microns. The adhesive layer  320  has a thickness in a range from 25 to 75 microns. Thus, the stiffener  30  has adequate rigidity to increase mechanical strength of a flexible printed circuit board and has adequate flexibility to maintain flexibility of a strengthened flexible printed circuit board with the stiffener  30  to be suitable for a roll-to-roll process. 
         [0024]    The polyethylene naphthalate layers  310  are represented by the following general formula: 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    The adhesive layer  320  can be comprised of a material selected from a thermoplastic adhesive and an ultraviolet-curable adhesive. 
         [0025]    It is noted that the stiffener  30  can includes a number of polyethylene naphthalate layers  310  and a number of adhesive layers  320  arranged in an alternate fashion. The number of layers of the polyethylene naphthalate layers  310  is one greater than that of the adhesive layers  320 . Each of the adhesive layers  320  is disposed between the two polyethylene naphthalate layers  310  adjacent thereto and is configured for adhering to the two polyethylene naphthalate layers  310 . Thus, the stiffener  30  contains two polyethylene naphthalate layers  310  on two opposite outer sides thereof, respectively. Each of the two polyethylene naphthalate layers  310  on two opposite outer sides of the stiffener  30  is configured for adhering to the flexible printed circuit board. The total thickness of the stiffener  30  is in a range from 75 to 250 microns. 
         [0026]    Referring to  FIG. 3 , a stiffener  40  according to a third embodiment is shown. The stiffener  40  also includes a number of polyethylene naphthalate layers  410  and a number of adhesive layers  420  arranged in an alternate fashion. The number of layers of the polyethylene naphthalate layers  410  is equal to that of the adhesive layers  420 . Thus, the stiffener  40  has one polyethylene naphthalate layer  410  and one adhesive layer  420  on two opposite outer sides thereof, respectively. The one polyethylene naphthalate layer  410  and the one adhesive layer  420  on two opposite outer sides of the stiffener  40  are configured for adhering to a flexible printed circuit board. Each of the polyethylene naphthalate layers  410  has a thickness in a range from 25 to 75 microns. Each of the adhesive layers  420  has a thickness in a range from 25 to 75 microns. The total thickness of the stiffener  40  is in a range from 50 to 250 microns. Thus, the stiffener  40  has adequate rigidity to increase mechanical strength of a flexible printed circuit board and has adequate flexibility to maintain flexibility of a strengthened flexible printed circuit board with the stiffener  40  to be suitable for a roll-to-roll process. In the present embodiment, the stiffener  40  includes two polyethylene naphthalate layers  410  and two adhesive layers  420  arranged alternately. 
         [0027]    Referring to  FIG. 4 , a stiffener  50  according to a fourth embodiment is shown. The stiffener  50  includes a number of polyethylene naphthalate layers  510  and a number of adhesive layers  520  arranged in an alternate fashion. The number of layers of the polyethylene naphthalate layers  510  is one less than that of the adhesive layers  520 . Thus, the stiffener  50  has two adhesive layers  520  on two opposite outer sides thereof respectively. The adhesive layers  520  on two opposite outer sides of the stiffener  50  are configured for adhering to a flexible printed circuit board. Each of the polyethylene naphthalate layers  510  has a thickness in a range from 25 to 75 microns. Each of the adhesive layers  520  has a thickness in a range from 25 to 75 microns. The total thickness of the stiffener  50  is in a range from 75 to 250 microns. Thus, the stiffener  50  has adequate rigidity to increase mechanical strength of a flexible printed circuit board and has adequate flexibility to maintain flexibility of a strengthened flexible printed circuit board with the stiffener  50  to be suitable for a roll-to-roll process. In the present embodiment, the stiffener  50  includes two adhesive layers  520  and a polyethylene naphthalate layer  510  sandwiched between the two adhesive layers  520 . 
         [0028]    Referring to  FIG. 5 , a strengthened flexible printed circuit board  60  with the stiffener  20  is shown. The strengthened flexible printed circuit board  60  includes a flexible printed circuit board  610 , an adhering layer  620  and the stiffener  20 . The flexible printed circuit board  60  has a strengthening surface  611 . The adhering layer  620  is adhered to the strengthening surface  611  and is sandwiched between the flexible printed circuit board  610  and the stiffener  20 , thus the stiffener  20  is attached on the flexible printed circuit board  60  via the adhering layer  620 . The adhering layer  620  can be comprised of a material selected from a thermoplastic adhesive and an ultraviolet-curable adhesive. The adhering layer  620  has a thickness in a range from 25 to 75 microns. Thus, the adhering layer  620  will not affect flexibility of the strengthened flexible printed circuit board  60  with the stiffener  20  to be suitable for a roll-to-roll process. 
         [0029]    It is noted that the stiffener  30 ,  40 ,  50  can be used as a substitute of the stiffener  20  of the strengthened flexible printed circuit board  60  so as to increase mechanical strength of the flexible printed circuit board  610 . 
         [0030]    Referring to  FIG. 6 , a strengthened flexible printed circuit board  70  with the stiffener  40  is shown. The strengthened flexible printed circuit board  710  includes a flexible printed circuit board  710  and the stiffener  40 . The flexible printed circuit board  710  has a strengthening surface  711 . The adhesive layer  420  on the outer side of the stiffener  40  is directly adhered to the strengthening surface  711 , and thus the stiffener  40  is attached onto the flexible printed circuit board  710 . 
         [0031]    Referring to  FIG. 7 , a strengthened flexible printed circuit board  80  with the stiffener  50  is shown. The strengthened flexible printed circuit board  80  includes a flexible printed circuit board  810  and the stiffener  50 . The flexible printed circuit board  810  is bent so as to form two strengthening surfaces  811  face to face. The two adhesive layers  520  on the outer sides of the stiffener  50  are directly adhered to the two strengthening surfaces  811 , respectively. Thus, the stiffener  50  is attached onto the flexible printed circuit board  810  to fix the bent flexible printed circuit board  810 . 
         [0032]    It is noted that one adhesive layer  520  on the outer side of the stiffener  50  can adheres to a flexible printed circuit board, another adhesive layer  520  on the outer sides of the stiffener  50  can adheres to other component such as a rigid printed circuit board, a support and an electronic component. 
         [0033]    It is also note that the stiffener  20 ,  30 , 40 ,  50  can be used to adhere to other flexible substrates such as a flexible copper and a copper clad laminate to increase mechanical strength of the flexible substrates. Thus, due to the strength of the flexible substrates is increased, processing of the strengthened flexible substrates become easy. 
         [0034]    While certain embodiments have been described and exemplified above, various other embodiments will be apparent to those skilled in the art from the foregoing disclosure. The present invention is not limited to the particular embodiments described and exemplified but is capable of considerable variation and modification without departure from the scope of the appended claims.