Patent Publication Number: US-2007102092-A1

Title: Method for manufacturing multilayer flexible circuits

Description:
BACKGROUND OF THE INVENTION  
      1. Technical Field  
      The present invention relates to flexible circuits in general. More particularly, the present invention relates a method for manufacturing multilayer flexible circuits. Still more particularly, the present invention relates to a method for manufacturing multilayer flexible circuits having reduced lossy materials.  
      2. Description of Related Art  
      Flexible transmission media, such as flexible circuits, are commonly used in electronic packaging. The base material used to manufacture flexible circuits is a base layer dielectric carrier film, typically made of polyimide with copper laminated on both sides of the film. For multilayer flexible circuits, the base layers are generally attached to each other by means of one or more adhesive layers. The formation of through-hole contacts and conductive patterns (on copper layers) is commonly accomplished by a subtractive process (i.e., etching) that is well-known to those skilled in the art. Outer layer conductors are subsequently provided with a protective covering film.  
      As switching speeds increase and attenuation requirements continue to restrict the content of transmission line material, it is increasingly desirable to use low-loss in flexible transmission media. Ridge printed circuit board constructs have an inherent loss limitation and variation due to woven glass weave content. However, flexible transmission media, by design, do not contain the same limitation because they do not contain such woven glass structures. Unfortunately, though, flexible transmission media do contain several electrically lossy layers known as adhesive.  
      The present disclosure provides an improved method for manufacturing multilayer flexible transmission media.  
     SUMMARY OF THE INVENTION  
      In accordance with a preferred embodiment of the present invention, the cross-sectional area of an unoccupied signal layer volume is initially determined. The unoccupied signal layer includes multiple conductive elements, and the unoccupied signal layer volume is formed between two of the conductive elements. Next, the thickness of an adhesive layer for filling the unoccupied signal layer volume is determined. Finally, the thickness of the adhesive layer is adjusted such that the adhesive layer only fills the unoccupied signal layer volume while the two conductive elements come in direct contact with a dielectric layer without any adhesive in between.  
      All features and advantages of the present invention will become apparent in the following detailed written description.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention itself, as well as a preferred mode of use, further objects, and advantages thereof, will best be understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
       FIG. 1  is a cross-section diagram of a multilayer flexible circuit, according to the prior art;  
       FIG. 2  is a high-level logic flow diagram of a method for manufacturing a multilayer flexible circuit, in accordance with a preferred embodiment of the present invention; and  
       FIG. 3  is a cross-section diagram of a multilayer flexible circuit, in accordance with a preferred embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT  
      Referring now to the drawings, and specifically to  FIG. 1 , there is depicted a cross-section diagram of a multilayer flexible circuit, according to the prior art. As shown, a multilayer flexible circuit  10  includes a base layer  11  and a base layer  12 . Base layer  11  includes a dielectric carrier film  13  laminated by copper layers  14   a  and  14   b . Similarly, base layer  12  includes a dielectric carrier film  15  laminated by copper layers  16   a  and  16   b . Dielectric carrier films  13  and  15  are typically made of polyimide. Base layers  11  and  12  are generally attached to each other by means of a bonding film  17  that includes adhesive layers  18   a  and  18   b . The thickness of adhesive layers  18   a  and  18   b  are typically around 1 mil.  
      The performance of multilayer flexible circuit  30  can be improved by reducing the thickness of adhesive layers  18   a  and  18   b . This is because an adhesive layer has a poor dissipation factor that increases attenuation within the overall dielectric medium. By reducing the amount of adhesive in the adhesive layer, the reliability and the attenuation properties of a multilayer flexible circuit can be improved.  
      The thicknesses of the adhesive layers should preferably be chosen so at the pitch of the signal lines and spaces such that the adhesive layers will just fill the area on the sides of the signal line. This eliminates the thick adhesive layer on the top of the signal lines.  
      With reference now to  FIG. 2 , there is depicted a high-level logic flow diagram of a method for manufacturing a multilayer flexible circuit, in accordance with a preferred embodiment of the present invention. Starting at block  20 , the cross-sectional area of an unoccupied signal layer volume is determined, as shown in block  21 . The cross-sectional area of an unoccupied signal layer volume can be calculated by multiplying the height of a conductive element (x in  FIG. 1 ) on a conductive layer (such as a copper layer) with the gap width (w in  FIG. 1 ) between two conductive elements on the same conductive layer.  
      Then, an adhesive thickness for filling the unoccupied signal layer volume is determined, as depicted in block  22 . The adhesive thickness for filling the unoccupied signal layer volume can be calculated by dividing the cross-sectional area of an unoccupied signal layer volume by a pitch. The pitch is defined to be the distance from a first edge of a first conductive element to a first edge of a second conductive element (p in  FIG. 1 ).  
      The determined adhesive thickness is then compared to the existing adhesive film thickness, as shown in block  23 . A determination is subsequently made as to whether or not the determined adhesive thickness is equal to the existing adhesive film thickness, as depicted in block  24 . If the determined adhesive thickness is not equal to the existing adhesive film thickness, the thickness of the adhesive film is adjusted, as shown in block  25 . Otherwise, if the determined adhesive thickness is equal to the existing adhesive film thickness, the multilayer flexible circuit is manufactured as usual, as depicted in block  26 .  
      Referring now to  FIG. 3 , there is depicted a cross-section diagram of a multilayer flexible circuit, in accordance with a preferred embodiment of the present invention. As shown, a multilayer flexible circuit  30  includes a base layer  31  and a base layer  32 . Base layer  31  includes a dielectric carrier film  33  laminated by copper layers  34   a  and  34   b . Similarly, base layer  31  includes a dielectric carrier film  35  laminated by copper layers  36   a  and  36   b . Dielectric carrier films  33  and  35  are preferably made of polyimide. Base layers  31  and  32  are attached to each other by means of a bonding film  37  that includes adhesive layers  38   a  and  38   b . Preferably, the thickness of adhesive layers  38   a  and  38   b  are approximately 0.5 mil or less.  
      For the present embodiment, signal lines and spaces and adhesive thickness are chosen to only allow for 100% (or a number higher but very close to 100%) fill of adhesive on both sides of the signal lines. For example, on a ½ ounce signal line, 4 mil lines and 10 mil traces are chosen. ½ rail adhesive is chosen on the adhesive layer to attach the signal/ground layers together. The ½ mil adhesive flows to fill the 10 mil wide×0.7 rail wide void. The adhesive volume (½ mil×14 mil total pitch) is correct to fill the void. The thinner adhesive layers and natural stop of the signal line pushing past the adhesive layer of the adhesive film into the dielectric layer to provide a more uniform dielectric gap.  
      The resulting multilayer flexible circuit structure has a reduced amount of lossy/high material in the Z axis, and provides mostly low loss material on the top and bottom of the signal lines. The proper adhesive fill is calculated for each structure and ground reference design such that standard materials can be utilized to provide improvements in overall attenuation and reliability.  
      As has been described, the present invention provides an improved method for manufacturing multilayer flexible circuits having reduced lossy materials.  
      While the invention has been particularly shown and described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention.