There is no one answer for every application. If the flex circuit remains sufficiently planar during processing, including reflow, a standard SMT print-placed-reflow process will work well.

If the flex circuit has planarity problems, it may be necessary to hold the part down during reflowwith a small weight to counteract warp in the flex circuit. When properly designed, such re-usable weights can be pick-and-placed like any other SMT component from tape or matrix tray.

The best technique involves the use of a fixed temperature and variable power soldering iron. I would suggest using a hoof style tip to provide the maximum power transfer to the assembly while keeping the temperature as low as possible, which is typically in the 600F or lower range.

How thin is thin?

I have soldered components to flexible circuits using ovens and infrared lamps, and both worked well. I did have some very thin copper that had been sputtered onto Kapton and I believe the copper was only a micron or so thick. In this case the copper dissolved into the solder and I was left with a hole in the metal coating.

Be careful with how long the parts will be at temperature and look

The obvious difficulty in soldering a fine pitch surface mount connector to a thin flexible circuit is creating enough support to print paste, place the surface mount connector and reflow to ensure good solder-ability.

One of the easiest and most straight forward methods to create proper support is to have a fixture built for your flex circuit that can hold and support the circuit through the entire process. Fixtures vary in size and are often referred to as carriers or pallets.

Fixtures can be built for a single circuit, but are often built for multiple circuits to reduce handling. Prices for fixtures can vary depending upon the complexity of the fixture needed; however fixtures in general are considered a low-cost solution to avoid a potentially frustrating and non-productive problem.

Soldering to flex circuitry can be accomplished in several ways. Flex circuits are prone to de-lamination, a direct result of excessive or pro-longed heat cycles, so whichever approach you choose for assembly, care must be taken when developing the thermal profile to ensure you avoid such damage.

For fine pitch devices, you may consider hot-bar technology which can fuse all leads simultaneously. In this case, heat remains relatively local to the device being applied.

You can also successfully reflow flex circuits as long as you observe thermal criteria for the flex assembly. Often when reflow soldering flex circuitry, a fixture or heat-sinking "mask" such as an aluminum template can be utilized to protect the un-processed areas.

This assembly method may work very well for your application where the connector area can be exposed, and remaining circuitry remains protected from excessive heat exposure.

While not necessarily suggested for bonding connectors, flex circuits can also be assembled using conductive epoxies. These materials are processed under curing profiles which are much cooler than the standard reflow profiles utilized for solder pastes, and as a result are much safer for the flex material.

Conductive epoxies/adhesives can have other drawbacks as well, so before proceeding, make sure you check with your material supplier on your specific application.

Defining solder attach for flex circuit is quite common. There are a couple of tricky issues that need to be understood when talking specifically about fine pitch components (QFN, uBGA, etc)

1. Substrate support. It is very important to keep the flex circuit as flat as possible during the attachment of all components. A common method of placing components is using a vacuum jig to hold the flex circuit down.
2. Substrate support relative to fine pitch areas. When placing fine pitch components, the vacuum structure has a significant influence. If the vacuum is directly under some of the leads of a fine pitch component, there is a chance the vacuum will "pull" the flex into the hole and prevent the component from making contact with the flex circuit lead. This could result in an "open circuit". BGA and like components are more forgiving when it comes to the strategic vacuum design because of the volume of solder.
3. Solder Printing. Again, the substrate support fixture is a critical element throughout the process of SMT assembly on flex circuits. It is important to make sure the flex does not bend after solder has been applied. If there is any flex within the flex circuit, there is a chance the solder could bridge from one pad to another. This will result in "game over" before a single component is placed on the flex circuit and is even more of an issue with fine pitch components.