Assembly of Flexible Circuits Assembly of Flexible Circuits
with Lead
with Lead- -Free Solder Alloy Free Solder Alloy
Bob Willis
leadfreesoldering.com
Introduction to Lead
Introduction to Lead- -Free Assembly Video Clips Free Assembly Video Clips
Component
www.bobwillis.co.uk/lead/videos/components.rm Printed Boards
www.bobwillis.co.uk/lead/videos/pcb.rm Solder Paste Printing
www.bobwillis.co.uk/lead/videos/pasteprinting.rm Component Placement
www.bobwillis.co.uk/lead/videos/plaement.rm Reflow Soldering
www.bobwillis.co.uk/lead/videos/reflow.rm Wave Soldering
www.bobwillis.co.uk/lead/videos/wave.rm Hand Soldering
www.bobwillis.co.uk/lead/videos/handsoldering.rm Rework and Repair
www.bobwillis.co.uk/lead/videos/rework.rm Inspection
www.bobwillis.co.uk/lead/videos/inspection.rm
These files ar e all in RealPlayer for mat These files ar e all in RealPlayer for mat
Flexible Circuit Construction 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 Process Stages Assembly Process Stages
Flexible Circuits - Screen Printing - Solder Paste - Placement - Reflow – Profiling -
Automatical Optical Inspection - X-ray inspection -
Optical Inspection - Rework -
Rework Materials –
Flexible Circuit Construction Flexible Circuit Construction
Wetting indicator str ips for solder paste dot solder ability patter n
0402 chip component positions SOT23 component positions
SOIC16 component positions
Land Gr id Arr ay 40 pin
Assembly Pallet for Flexible Circuit 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.
Assembly Pallet for Flexible Circuit Assembly Pallet for Flexible Circuit
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
Pallet and Flex Circuit Pallet and Flex Circuit
Paste being printed/reflowed in side this area of the pallet
Assembly Pallet for Flexible Circuit Assembly Pallet for Flexible Circuit
Double sided flexible circuit on assembly pallet. Fixed and sprung pallets location pins used for the flexible. Protrusion of pins above the circuit was approximately 0.010”
Assembly Pallet for Flexible Circuit Assembly Pallet for Flexible Circuit
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
Printing of Lead- -Free Solder Paste Free Solder Paste
The 0.006” (150um) stencil was recessed on the base of the foil to allow for any protrusion of the pallet clips. The stencils was recessed by 0.003” .
Printing of Lead
Printing of Lead- -Free Solder Paste Free Solder Paste
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.
Component Placement on Flexible Circuit Component Placement on Flexible Circuit
Placement of components on 0402, SOT23 and SOIC16 paste deposits.
LGA Land Grid Array to be used in the Lead-Free Experience.
Reflow Profile of Circuit on Pallet
Reflow Profile of Circuit on Pallet
Reflow Profile of Circuit on Pallet Reflow Profile of Circuit on Pallet
Vapour Phase Reflow Profile
Vapour Phase Reflow Profile
Reflow Oven Parameters Reflow Oven Parameters
Zone Number: 1 2 3 4
Top Set points (deg C): 195 229 220 270 Bottom Set points: 195 229 220 270 Conveyor Speed 39.5 cm/minute
Product Name: Flexible Circuit Process Window Name: Sn/Ag/Cu
Oven Name: RO400FC
Example Lead
Example Lead - - Free Joints Free Joints
Sn/Ag/Cu solder joints after convection reflow in air. The paste alloy was Sn96/Ag3/Cu05. Its was a type 4 powder also being used on another rigid board design for 01005.
X- X - Ray Examination of Joints Ray Examination of Joints
Images show voiding due to the time in a liquid state. The speed of reflow and the time above liquidus temperature was related to the pallet mass.
Incomplete reflow of wetting indicators due to non relief of pallet under strips
Lead Lead - - Free PPM Flex Results Free PPM Flex Results
Results from 20 flexible circuits
Rework of Flexible Circuits with Lead
Rework of Flexible Circuits with Lead- -Free Free
Rework joints with hot air pencil
Use flux prior to reflow
Remove and replace components Set-point on tool 340-360oC
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 380oC
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.
Rework of Flexible Circuits with Lead
Rework of Flexible Circuits with Lead- -Free Free
Video clip showing the removal of SOT23 with lead-free alloy SnAgCu from the surface of a flexible circuit. Flux is applied first to aid removal