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Figure 4-44.5.  Foil Repair
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Resistance Soldering.
TM 9-254 4-17.  Printed Circuit Board - Continued Figure 4-44.6.  Damaged Conductor (Trace) Repair (a) Fill the void with a compatible epoxy mixture and allow to cure after the welding is completed. (b) Level the cured epoxy mixture with a chisel or mill to match the surface finish of the printed circuit board. m.    Flexible  Circuits.    Flexible  circuits/cables  are  designed  to  provide  efficient  and  practical  methods  for  making connections  and  utilizing  space.    The  etched  circuit  is  composed  of  flat  copper  conductors  enclosed  in  insulating materials The conductor to, Isolation bonding provides a barrier against moisture and gases, and is very durable.  The product may be bent, coiled, twisted, and formed to follow the various outlines of multiple systems.  Assemblies may be  hinged,  pulled-out  and  flexed  for  maintenance  or  inspection  even  when  still  in  operation.    Flex  circuitry/cables provide high reliability with less distortion or noise than conventional wiring.  in addition, various methods can be used for final assembly, such as, welding and soldering. Figure 4-44.7.  Flexible Circuit l.  Multiwire Technology.  A multiwire PWA/PCB is a computer tape controlled series of conductor wires laid down on an  adhesive  coated  double-sided  printed  circuit  board.   The  two  sides  of  the  board  are  used  for  voltage  and  ground plane  applications  The  voltage  and  ground  planes  are  etched  to  both  sides  providing  a  semicured  base  for  laying  in the conductor wires.  The circuit is formed by using a pressure head, that imbeds the conductor wires in the adhesive. Holes are then drilled in the board followed by the plating of these holes.  Multiwire PWA/PCBs can be hand, wave, or dip  soldered  using  conventional  soldering  techniques.    Multiwire  printed  circuit  boards  are  recommended  for  high frequency applications. m.    Additive  Plating  Technology.    Additive  plating  may  be  defined  in  this  manner:    Additive  plating  is  a  process whereby  printed  circuit  boards  are  manufactured  by  selective  depositing  of  a  conductor  material  on  an  unclad  base material  (no  conductor  material  anyplace  on  the  base  material).    The  circuit  is  developed  by  applying  a  background negative-image  resist  pattern,  which  leaves  conductor  areas  and  holes  to  be  plated  through,  exposed  for  metal deposit.  Additive plating is an economy oriented process. n.  Leadless Components. A new trend in the manufacture of PCBs is to use so-called "LEADLESS COMPONENTS." These are passive components such as resistors or capacitors which are made on semiconductor substrate material and  are  greatly  reduced  in  size.    They  do  have  axial  leads,  however,  and  are  removed  and  replaced  in  a  similar manner to flat pack ICs (para 4-17.k.1).  Use of leadless components can reduce a multilayer board to a single layer and reduce its size by as much as one third. 4-46.4  Change 6

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