Typically water soluble fluxes have a higher activity than the rosin based fluxes so if you are looking for wetting as well as a cleaner board then water soluble might be the way to go.

Nowadays we don't see many Rosin Activated (RA) or Rosin Midly Activated(RMA) fluxes being sold as most of this business has been switched to low solids No Clean fluxes.

If you want to use a flux that doesn't necessarily have to be cleaned than a Rosin/No Clean flux might be the direction you want to go.

Rosin based fluxes use natural rosin as the activator to clean the metals so that the liquid alloy can coalesce and wet to the solder pad.

ROL0 (Rosin Low activation) is generally used in no clean applications.

Water soluble flux. There are basically two types 1. Water washable and 2. Water soluble.

Water washable flux may contain rosin and in addition a surfactant to remove the flux residue after reflow.

Water soluble generally uses organic acid activators as the flux and every component in the flux system should be soluble in water and therefore clean with only water. These types of paste are generally high activation flux.

Why use one or the other ?

It depends on what you are making. If you need high reliability for a long term then it is a good idea to use a water soluble paste as it has high activation flux and after cleaning there will be no residue or contamination left on the board.

If you are conformaly coating the board you should either use a cleaning process after soldering, chemical or water, or test the compatibility of any no clean flux used with the conformal coating. At HumiSeal we can provide a compatibility test service for no clean flux residues.

• Rosin-based fluxes contain rosin and in most cases pass no-clean requirements. No-Clean flux-residues are benign, inert, non-hygroscopic and does not need to be cleaned off after reflow.
• Water-wash fluxes contain aggressive acids and need to be cleaned off after reflow - failure to clean will cause corrosion and dendritic growth.

In general, the trend is to move to a no-clean process - this saves $$$ and eliminates an extra cleaning step.

Apart from some of the known advantages of a no-clean process over a water-wash process (longer stencil life, increased slump resistance..), the rosin in a no-clean paste acts as a normal oxidation barrier and helps mitigate some of the bigger challenges of:

In addition, with today's boards packed with low-standoff components such as QFNs & LGAs - ifa water-wash paste is used, cleaning under standoffs sub-2 mils poses real challenges.

Rosin based fluxes are widely used in military and space electronics. The reasons could be listed as:

1. They are noncorrosive at room temperature
2. Rosin type flux is hygroscopic and can act as an insulation
3. They normally cure at room temperature to entrap potentially corrosive activators

Rosin fluxes can not be removed from assemblies using water. Alkaline cleaning agents are highly recommended for removing RMA.

The important difference between water soluble and rosin fluxes is their flux activity. Partial removal of the water soluble flux residues can result in corrosion.

Compared to rosin fluxes, water soluble fluxes are aggressive also at normal temperatures in assembly processing. Therefore, it is critical to make sure complete removal of water soluble flux residues.

Because the residues of a water wash flux are to be removed typically much more aggressive activators (both acids and where used halides) are used and the level of these materials is also typically much higher than for a conventional rosin flux.

The activators are chosen for their solubility in water and are often hygroscopic ( will absorb water from the air)in nature, as a result of this the boards must be washed within a finite time after processing to prevent attack of the joints by the flux residues.

Rosin based systems by comparison are not intrinsically hygroscopic and the residues have much less aggressive and lower levels of activators, thus the residues represent a lower risk of joint attack and have a much longer cleaning window if in fact they require cleaning at all.

The extra levels of activator present in water wash systems would tend to make them more appropriate for situation where excessive levels of oxidation or contamination are present on the surfaces to be soldered.

On the other hand assemblies with open windings or catch pockets may better be served by a rosin base system because of the lower level of corrosivity of the residues which can get trapped in such areas.

These questions raise more questions! In a few words, the choice is based on how strong the flux needs to be and what kind of cleaning (removal of flux residues) can be tolerated. This, in turn, is a function of the end product.

Is it toys that will be discarded after a few months or a high-reliability assembly (Pacemaker, Aerospace) that must, in principle, last forever? In all likelihood, it is a variety of products between these extremes.

What about the PCBs and the components? Do they have the best solderability or must you be able to solder whatever is provided?

Rosin fluxes are based on solutions of rosin - a natural product obtained from trees - in an organic vehicle, typically isopropyl alcohol. Rosin contains small amounts of Abiatic Acid that, when heated, creates a reducing environment that can have a cleaning effect on tarnishes such as copper oxide.

This effect is very limited and, for more severe tarnishes, varying amounts of stronger, so-called activators are added to enhance the cleaning capabilities of the flux. These fluxes are classified as R (Rosin), RMA (Rosin Mildly Activated) and RA (Rosin Activated).

Removal of rosin flux residues after soldering requires an organic solvent such as a chlorinated or fluorinated hydrocarbon, to dissolve the rosin, and a polar ingredient, such as alcohol, to dissolve the ionic portion of the residue (the activator).

Rosin is not soluble in water but some rosin fluxes are formulated to be soluble in water plus a saponifier that converts rosin to a soluble soap-like material.

Water soluble fluxes are solutions of active chemicals, usually proprietary formulae, in solvents - usually non-aqueous. Typically, these fluxes are relatively strong and can deal with tarnishes more easily than rosin fluxes.

The residues generally are soluble in water - probably the best cleaning agent for ionic residues - but, of course, all the components on the assembly must be sufficiently sealed to withstand immersion in water.

However the cleaning is performed, the assembly must be free of ionic residues that can be very dangerous.

When this debate first arose, Rosin fluxes were not as aggressive as the more active water soluble fluxes. The thought process was "flux was your friend", and the more aggressive the flux was the easier it was to solder components to the products.

It was also identified that these water soluble fluxes could be cleaned with just water. Quite a difference between the existing solvent materials being used to clean rosin fluxes, such as Trichloroethylene (TCE), 1,1,1-Trichloroethane, Chloroethane, D-Sol, MEK (Methyl Ethyl Ketone), Freons (CFCs) and HCFC, which by the way were found to be carcinogenic and ozone depleting materials.

Rosin base fluxes were only cleanable in solvents designed for their removal. The rosin has to be dissolved and the activators have to be removed especially with Rosin Activated fluxes.

Mildly Activated Rosin (RMA) fluxes do not need to be removed but due to their appearance and difficulty they pose in electrical testing, most manufacturers also have processes in place to remove them from printed circuit boards. They usually don't remove these RMA residues from cable assemblies.

Another way to remove rosin fluxes was to convert them to rosin soap with an alkaline saponifier and rinse them off with deionized water, whereas the water soluble fluxes were more aggressive material and theoretically were easier to remove with the traditional water cleaning systems.

With the introduction of surface mount in the 1980s, entrapment areas were more prevalent as spaces beneath components were smaller and the ability of these solvents to penetrate beneath these components became susceptible.

The surface tension of the cleaning medium was too high to get beneath the components hence the fluxes were not washed off, leaving behind some ionic residues from the halide activators being used in those fluxes.

Although this is a short history of the fluxes, the questions remain the same, which flux works best for you, and can you clean it all off the product. The question of "How Clean is Clean?" must still be answered.

The question of recycling the solvents and aqueous solution must be answered. The value added option of cleaning still must be answered. In other words, is all of this worth the cost, or should the process be changed to low solid content fluxes and not worry about any cleaning processes.

I know there are more issues than those stated such as the compatibility of flux residues with conformal coating, the residual flux behavior on the end use of the product, the need to educate the customer to seeing residues and films on their products, the types of testing which need to be conducted to verify the goodness of the products.

One also has to consider the cost of the equipment, not just the capital cost but the cost of materials, floor space, venting, effluent discharge etc. which must be added to the equations for determining the value of cleaning printed circuit boards.

I would say the best flux to use is the one which is the best process for you and your customers, i.e. good quality products and which will be less costly to you and your customer and have the least impact on the environment.

If there are more questions, please don't hesitate to contact me off line and we can discuss this further as this topic is very difficult to cover in a couple of paragraphs.