Flex Assembly Tips : 

Array Options:

There are many ways to panelize a flex circuit for assembly. For your array design keep in mind the following:

  • Tab routing is the best way for flex, instead of scoring
  • Use local fiducials within the flex circuit
  • Use lots of copper for stability
  • Optimal sizes depending on the size of the flex and component feature. We recommend: 5 x 5”, 5 x 10”, 8 x 10”, 10 x 10”, 10 x 16”
  • Methods of singulation of parts: Xacto knife, Laser cutting, Die-Punch, Routing
  • Singulation process should not induce stress in the solder joints

Here are some different array examples:

Guide to ordering Stencils:

  • Typically 4 mil thick
  • Occasionally 3 mil thick or step down stencil may be needed
  • Coated stencil needed for micro BGA
  • Add fiducials on stencil to align properly
  • Mini stencil for offline printing is commonly utilized and sometimes components are loaded by hand
  • Stiffeners or FR-4 guides are used to align mini-stencils

Baking Guidelines:

  • Polyimide laminates and adhesives absorb up to 3% by weight of moisture
  • All flex circuits should be prebaked at 125℃ for 2 to 6 hours
  • Moisture uptake for Acrylic adhesives at 80% relative humidity and 23℃:

●  0.65% by weight in 2 hours

●  1% in 10 hours

●  1.2% in 2 days

  • Moisture in flex creates very high vapor pressure during re-flow:

●  At 220℃ the vapor pressure of water is about 340 psi

●  At 230℃ the vapor pressure of water is about 400 psi

●  At 260℃ it increases to approximately 700 psi

  • Maximum storage time at 50% humidity is 8 hours

Reflow Parameters:

  • Ramp rate is slower, approximately 2.5℃ per second
  • Peak temperature for pure flex for RoHS assembly is 220℃
  • Soak time is kept at minimum – 30 seconds or less
  • Rules for rigid-flex are similar to rigid PCB
  • Always use thermocouples to profile the reflow oven

Challenges you can expect with assembling Flex:

  • Dimensional stability of the flex material – thinner is worse
  • Warpage or deformation after one side assembly
  • Registration of solder paste
  • Pick and Place programming – sometimes have to manually teach each array at a time if proper fiducials are not present
  • Handling
  • Moisture
  • Solid copper shields
  • Component Misalignment
  • Rework
  • Productivity and Cost – NRE and labor costs are higher