I have an unusual low-volume application requiring the manufacture of very large flexible circuits using a pseudo-SMT process. The circuits will be approximately 15" wide x 70" long (yes, that's 70 inches long!). This is definitely not a fine-pitch application, but the large size is quite a challenge. 

The substrate is Cu-clad polyimide. The flex will be rigidized on FR-4 for processing. Solder paste will be hand-stenciled, followed by placement of devices. Reflow will be accomplished using a Heller 1500N convection reflow oven. Because of the size of the circuit and the limited print-life of the solder paste, I must place and reflow components on the circuit in partial sections. Previously reflowed sections will be shielded as necessary to prevent secondary reflow. This means that conductor traces on some areas of each board will see as many as six (6) thermal excursions before soldering, because the entire board must pass through the reflow oven to reflow each individual section.

Now my questions: What surface finish among Ni/Au, HASL, OSP, electroless Pd, etc. will allow the solderability of the traces to be maintained after exposure to this many thermal excursions? Will nitrogen inerting allow me to use any of these finishes with equally good results? Which finish will be the easiest to apply to a very large flex circuit? Do you have any recommendations for vendors in the United States that can apply any of these finishes to very large flex circuits?

I know that I will face many other problems in addition to surface finish as I attempt to implement a successful process. Any other input relative to this interesting, yet hellish, task will be greatly appreciated.

Thanks to you all in advance,

Kent Asmus
Senior Materials & Processes Engineer
TRW, Inc.
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