How to Assemble Flexible Printed Circuits with Lead Free Solder Alloy

Flexible printed circuit assembly on a supple plastic substrate is widely used for various electronic appliances such as cameras, computer panels, microwaves, cell phones etc. While the assembly of such circuits can be done with various components like conductive polyester or polyimide, PCB manufacturers prefer using lead-free solder alloys the most especially for mass production


  1. 1
    Assemble the components temporarily. To assemble flexible PCBs with wave or reflow soldering, you need to arrange the electronic components and keep them in their place temporarily with the help of adhesives.
  2. 2
    Locate pins for flexibility. Use fixed as well as sprung pallets locations pins to provide flexibility. The pins should have protrusion of approximately 0.010″ above the flex circuit.
  3. 3
    Set the base of pallet: The base of the pallet used in SMT PCB assembly needs to have thermal relief areas. These thermal relief areas help in improving the delta T at the time of reflow of lead-free solder alloy. In case you plan to using vapor phase for the reflow, you should consider the drain holes under the flex.
  4. 4
    Print the lead-free solder paste. Allow the protrusion of pallet clips by recessing a 0.006″ (150um) stencil on the base of the foil. You should recess the stencil by 0.003″. Use 2 varying sizes of apertures on 3 different footprints of electronic components for solder paste printing. For efficient SMT PCB assembly soldering, you need to place the recesses around pin protrusions from the pallet.
  5. 5
    Heat the assembly. To fix the lead-free soldered parts in place, the PCB assembly should be heated either by passing through a preheated and controlled oven, by using a hot air pencil, or with the help of an infrared lamp.


  • Always monitor the oven while heating the assembled parts. Drastic fluctuations in temperature or overheating can lead to distortion of the flexible printed circuit.
  • You can use more than one process (hand soldering, wave soldering, reflow soldering etc) for assembly of flex circuits with lead-free solder alloy. This will save your time as well as help in managing various components efficiently.
  • Use superlative quality lead-free solder powder to make your solder paste. This is important because the quality of the paste affects the overall assembly and durability of the PCBs.

Things You'll Need

  • 0.05 mm Copper Base Material (18/18 um ESPANEX)
  • Lead Free Solder Immersion of Nickel/ Gold/ Tin
  • MPR 80 Amber Solder Mask (Liquid)
  • Adhesive Dabs/ Tapes
  • Magnets
  • Vacuum Plates
  • Plastic Substrates
  • Clips
  • Pins
  • Pressure Sensitive Low Tack Tape
  • Oil Lamp/ Infrared Lamp or Oven

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Categories: Maintenance and Repair | Hardware Maintenance and Repair