Conference: Main Conference
Forum: SMT Production & Design
Thread: Assemby of flexible printed circuits

Date: May 16, 2012 03:40 PM
Author: davef
Subject: Assemby of flexible printed circuits

Here's some notes from a Bob Willis presentation

Assembly of Flexible Circuits with Lead-Free Solder Alloy [Bob Willis leadfreesoldering.com] * Flexible Circuit Construction ** Base Material - 0.05mm Copper 18/18 um ESPANEX from Holders Technologies ** Liquid Solder Mask -MPR80 Amber - Nippon Steel ** Solder Finishes – *** Immersion Nickel/Gold - Aurotech, from Atotech *** Immersion Tin - Stannatech from Atotech *** Immersion Silver - Sterling from MacDermid *** OSP - Glicoat SMD P2 from Shikoku * Assembly Pallet for Flexible Circuit ** Thermal relief areas on the base of pallet to improve delta T during reflow. Also consider drain holes under the flex if using vapour phase for lead-free reflow. ** Modified flexible pallet with thicker recess in the areas of the circuits and solderability test strips. Drain holes for fluid are positioned under the flexible when using vapour phase reflow soldering. ** Fixed and sprung pallets location pins used for the flexible. Protrusion of pins above the circuit was approximately 0.010.” ** Circuits do lift on the edge of the tooling holes which does not cause a problem during assembly. This may be due to the copper being relieved around the hole causing the flex film to distort when clamped. * Printing of Lead-Free Solder Paste ** 0.006” (150um) stencil was recessed on the base of the foil to allow for any protrusion of the pallet clips. The stencil was recessed by 0.003.” ** Printing of lead-free solder paste with a 0.006” (150um) stencil. Two different aperture sizes were used on three different component footprints. Specific design rules used on the solderability wetting strips. Recesses were placed around pin protrusions from the pallet. * Rework of Flexible Circuits with Lead-Free ** Rework joints with hot air pencil ** Use flux prior to reflow ** Remove and replace components ** Set-point on tool 340-360*C ** Use lead-free wire to set-up air pencil distance from joints for reflow ** Removal of solder shorts with solder wick ** Shorts purposely added to SOIC16 leads for testing ** Set-point on iron 380*C ** No issues of delamination or pad lift on parts after two rework operations. Pad dimensions for the parts used could be reduced to improve use of the surface area but adhesion of the pads would be reduced. Modifications can be made, but only when this is really necessary.  

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