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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Many thanks davef

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Hi,

Since you're looking information about flexible printed circuits,a book entitled "Printed Circuit Boards R. S. Khandpur" could give you lot of information about flex circuits from Design, Fabrication, and Assembly Phase.

The book contains comprehensive single volume self-teaching guide to the art of printed circuit board design and fabrication -- covering the complete cycle of PCB creation, design, layout, fabrication, assembly, and testing.

You can read some info inside the book from Amazon at this link :

http://www.amazon.com/Printed-McGraw-Hill-Electronic-Engineering-ebook/dp/B000YAT1RO
Or you can read more on Google books at this link :

http://books.google.com.ph/books?id=m8sJBIMtETgC&pg=PA452&dq=IPC-FA-251+Assembly+Guidelines+for+Single+and+Double+Sided+Flexible+Printed+Circuits&hl=tl&sa=X&ei=0BS3T7yiCML6mAW8pNDNCQ&ved=0CDQQ6AEwAA#v=onepage&q=IPC-FA-251%20Assembly%20Guidelines%20for%20Single%20and%20Double%20Sided%20Flexible%20Printed%20Circuits&f=false

For Industry Standards, IPC-FA-251 Assembly Guidelines for Single and Double Sided Flexible Printed Circuits provides you a more detailed explanation on this field.

You can also search and read articles from electronics companies or any EMS since they are expert on that field. Maybe you can visit Bittele electronics for PCB fabrications, Assembly, and other PCB related concerns.

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Polymide is like a sponge and absorbs moisture much faster than rigid substrates. Pre-bake your substrate for 2-4 hours at 125C and perform your soldering within 1 shift thereafter or seal in moisture barrier bags.

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There is also a good reference book on flexible circuits from Joe Fjelstad.

http://flexiblecircuittechnology.com/flex4/
This book covers everything from creating the flex circuitry itself to the assembly of flex circuits.

Also, roll-to-roll assembly of flex circuits is definitely possible, although there can be some limitations. However, it can be cheaper and usually much more efficient. The up front equipment investment is usually more than most people want to deal with, so usually flex circuits are processed in panels. We currently do both panel and roll-to-roll flex circuits and whevever possible we try to do a project in continuous rolls.

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