Water-based flux for selective soldering

The Green Line series from Emil Otto encompasses various water-based fluxes for wave soldering. Because demand for ecological fluxes is on the increase, Emil Otto wanted to offer a water-based selective flux tested under practical conditions since the selective application is being increasingly used.

'The advantages of this environmentally friendly flux are obvious', explains Emil Otto’s Markus Gessner. Fluxes based on water are not combustible unlike alcohols which have a 12 °C flash point and are thus easily ignitable. Benefits in production, transport, storage, and handling result from this, because these product aren't subject to hazardous goods classification.

The company tested various possibilities with system manufacturers. The emphasis here was on preheating. The flux's structure and the devices' configuration would have to ensure that the PCB was dry at the time of soldering, the flux exhibited sufficient activity, and no residual moisture would remain on the circuit board.

A system concept was selected based on these specifications. Flux metering was done via drop application. The amount of flux can be exactly metered using inlet pressure, transport speed, and nozzle opening time. Standard alloys both with and without silver were used as solder in the test phase. Dwell time and radiation intensity could be controlled in a separately controllable preheating area equipped with infrared radiators. Temperature on the circuit boards' upper side was measured after conclusion of the preheating phase.

Both activated nozzle attachments and those draining single-sided were used as miniwave in the test phase. The miniwaves were moreover bathed in nitrogen. A EURO-test circuit board from Emil Otto GmbH with HAL, immersion tin, and nickel-gold was used for the test runs. However reference values first had to be recorded in a base setting at the beginning of the test.

For this the system parameters relating to the HAL test board and the 'RS-4004', type 2.2.3.A EO flux were determined. These alcohol-based fluxes meet the requirements under DIN EN 61 190 1 1 and also the requirements of in-house EO tests based on studies at the Fraunhofer Institute Oberpfaffenhofen concerning the SIR behaviour of non-thermally stressed flux residues. The system parameters relative to flux quantity, preheating dwell time and radiation intensity, and the height and dwell time of the miniwave as well as of the nitrogen stream on the miniwave were defined as base setting and reference. Optimization was intentionally dispensed with.

Comparative tests were subsequently run with different alcohol-based and partially aqueous fluxes. The results exhibited only slight variations. A water-based, VOC-free flux was tested under the same conditions after that. For this Emil Otto developed the 'EO-G-001' water-based flux, classified OR/L0 under DIN EN 61190-1-1 or type 2.1.3.A pursuant to ISO 9454. This halogen-free flux meets all of the criteria already described above relative to surface resistance.

Temperatures after preheating on the top of the circuit board were measured at 90 °C to 130 °C depending on the type of machine. The circuit boards were dry, as they were during the comparative tests with non-water-based fluxes. The water's surface tension caused the flux propagation area to be less than that with the alcohol-based fluxes. Rise-through and pad wetting were not adversely affected and the soldering results were more than satisfactory. Cleanliness was equally good and sometimes better than with the alcohol-based fluxes.

'With Emil Otto's water-based "EO-G-001" flux, achieving very good results proved possible with selective solders on various machines from different manufacturers. Both the solder results and the cleanliness of the circuit board were very good. Thus we offer our users water-based fluxes in our "Green Line product range" for both wave and selective soldering. We've closed these product gaps and can thus present an additional product innovation at this year’s SMT', summarizes Geßner with satisfaction.