Assembly of Flexible Circuits with Lead-Free Solder Alloy Bob Willis leadfreesoldering.com Introduction to Lead-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 These files ar e all in RealPlayer for mat Inspection www.bobwillis.co.uk/lead/videos/inspection.rm X-Ray Inspection www.bobwillis.co.uk/lead/videos/xray.rm
Flexible Circuit Construction Base Material - 0.05mm Copper 18/18 um ESPANEX from Holders Technologies Liquid Solder M ask - M PR80 Amber - Nippon Steel Solder Finishes Immersion Nickel/Gold - Aurotech, from Atotech Immersion Tin - Stannatech from Atotech Immersion Silver - Sterling from M acdermid OSP - Glicoat SMD P2 from Shikoku 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 Wetting indicator strips for solder paste dot solder ability patter n SOT23 component positions SOIC16 component positions 0402 chip component positions Land Grid Array 40 pin 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 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 Paste being printed/reflowed in side this area of the pallet
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 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 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-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 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 Vapour Phase Reflow Profile
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-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-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-Free PPM Flex Results Results from 20 flexible circuits
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 o 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 o 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. Rework of Flexible Circuits with Lead-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 followed but reflow with hot air pencil.
Next Lead-Free Process Trials Double sided reflow trials More complex components/smaller pitch Through hole components, reduced pin length Wave solder and reflow flexible circuits Trials on solder levelled circuits with other finishes Sensitivity of flex with liquid solder mask to moisture Lead-Free Experience 3 To download the FREE 85 page SMART Group Lead-Free Experience Report go to www.leadoutproject.com
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Contact details: Bob Willis bob@bobwillis.co.uk www.leadfreesoldering.com Tel 01245 351502 Fax 01245 496123 2 Fourth Ave, Chelmsford Essex CM1 4HA, England