Selective Radiation Soldering For Flex-Circuits

Mylar films used for membrane keyboards have a limited resistance against high temperatures. Therefore, electronic components, such as LED, cannot be connected using standard SMT solders. Instead, conductive glue or low-temperature solders are used. A European keyboard manufacturer went one step further in protecting sensitive materials by inventing selective soldering with infrared radiation.

The process requires a specific combination of solder, foil and infrared wavelength. If the combination is right, the foil is nearly transparent for the IR. Therefore, only the solder joints are heated; the foil itself stays cool. Testing such a new process in the laboratory is one thing, getting it into production is another. As a result, the keyboard manufacturer requested that Essemtec builds a special type of machine for their new process.

However, a new process does not always require a completely new machine. Essemtec, the Swiss manufacturer of production machines, can adapt standard machines to exceptional tasks. As such, new solutions can be realized quickly and efficiently. The selective soldering oven for the keyboard manufacturer has been developed in exactly this manner.

Dispensing and Placement
The complete production line for membrane keyboards includes several process steps. One of them is the population of the keyboard foils with electronics and switching elements. For this, an FLX2011-VL flexible production cell from Essemtec was used. In one run, the machine can dispense and place without moving the foil. The FLX can dispense up to two different liquids and can place metal domes, LED and other electronic components.

For security and easy handling, foils are fixed in carriers. These carriers stay with the foil from the first dispensing operation until after soldering. This way, the foils cannot warp and placed components will not shift during handling and soldering.

Conditioning, Soldering and Cooling
After dispensing and placement, the foils are soldered using the selective IR soldering process described above. A standard RO300FC reflow system with process-specific adaptations is used for soldering. The oven features five zones, where the first and last ones are required for handling the foil carriers. The three zones in between are process zones.

The primary process zone's function is to condition the foils for proper soldering. It can preheat or temper the foils using a convection heating module that guarantees homogeneous temperature distribution. The second process zone is the soldering zone. The standard convection heating modules have been replaced by special IR radiation modules. These modules emit IR radiation with a wavelength in which the Mylar foil is nearly transparent. Therefore, only the solder joint is heated and the foil stays cool.

In the third process zone, solder joints are cooled using an air convection cooling module. The foils are then processed further into the output handling zone. Carriers are controlled by sensors that ensure that only one carrier is inside the process tunnel or on the output platform. Another carrier will be released at input only if process and output zones are free.

Vapour Exhaust and Energy Saving
Heating the solder paste creates vapor that may have two disadvantages: First, it is not good for the health and second, vapors may condense inside the oven and contaminate the IR emitters. This would reduce their efficiency and as well as their lifetime.

For this reason, all potentially harmful vapor must be exhausted from the oven. However, the exhaust air flow must be in sync with the convection heating and cooling modules. Essemtec has invested great effort to precisely analyze flows inside the oven tunnel and has invented special flow control systems that prevent vapor while guaranteeing the efficiency of the convection heating and cooling modules.

Heat energy shall only be produced when required. Therefore, the RO300FC is equipped with a standby mode. If no carrier is inside the oven, both the IR radiation modules and the transport band are stopped. Only if a carrier is placed on the input loading platform will the heating and transport modules be restarted. With this feature, energy is saved and the system’s lifetime is extended.

Graphical Control

RO-CONTROL software is used for process setup and control. This PC software features a fully graphical, easy to understand display of the oven status and the temperatures in the zones. When connected to the network, soldering profiles also can be stored externally and remote support is available via the Internet.

RO-CONTROL allows the simulation of soldering profiles and the comparison to solder paste specifications. Furthermore, temperatures inside the zones or on the substrate can be recorded, documented and compared with the simulation. Therefore, RO-CONTROL makes it easy to set up and monitor processes.

Special Solution as Reliable as a Standard Machine
The selective soldering oven described in this article is based on a RO300FC standard convection oven that has been adapted to the customer's special requirements. For Essemtec, such modifications are feasible because all of its machines are designed to be highly flexible, modular and adaptable.

Essemtec often realizes special solutions based on standard machines, for example pick-and-place systems, dispensers, printers and reflow ovens. Such solutions offer great advantage compared to a complete rebuild of a new machine. The special machine inherits the reliability and the spare parts from the standard machine and, furthermore, a redesign is faster than a completely new development.