Re: Reflow soldering a Flexi-Rigid pcb| 5 April, 1998
| We are looking at reflow soldering some Flexi-rigid | pcbs. I have heard that IR reflow soldering can make the Polyimide "self heat" and cause de-lamination. | Does anyone have any info.
The thermal problems associated with polyimide rigid/flex designs primarily are related to the adhesive used in the fabrication of this type circuitry. It often is an acrylic film adhesive material that thermally "bonds," without molecularly cross linking with, to circuitry and the polyimide film or resin/glass materials. It does not provide the higest level of laminate integrity as do resin based systems. There is no such thing as "self heating." The problems are simply a matter of delamination caused by "ungluing" the multilayer structure where the film is used as the bonding medium. Also, polyimide resin structures can be over cured when the highest possible Tg is sought. It is recommended that a lower Tg, with adequate cure (determined using TMA or DSC), be assured or delamination may result in a shorter time. This means instead of going for a maximum 270 degrees C., one of approximately 250 should be effected. Again, acrylics and polyimides (whether film or resin systems) do not have high bond strengths compared with epoxies. However, polyimides are the primary materials used to produce rigid/flex circuitry. All this means very precise reflow, or any other type, soldering process management must be effected only on circuitry fabricated to precise requirements under highly controlled conditions.