Typical "no-clean" fluxes differ from fluxes that are intended to be cleaned in two important ways. First, noclean fluxes employ acids with lower activity levels. Secondly, they contain lower quantities of non-volatile solids. It is important to identify the characteristics of noclean fluxes as follows:

• The flux cannot reduce heavy oxides from boards or component termination.
• The flux may not promote thermal transfer, through convection.
• The flux residue may not inhibit formation of oxides at high temperatures.
• The flux may not enhance surface tension of the solder, nor promote wetting of the metals to be connected. Low residue means reduced surfactant properties.
• The flux residue may be more active when improperly processed. Many low solid flux formulas have very low viscosity. This can present problems in selective and hand soldering applications where the entire assembly may not see process temperatures required to properly render the flux.

Process Recommendations
1. Components must be solderable at the start of the soldering process. Optimize handling and storage by verifying that boards and components satisfy the requirements of J-STD-002 and J-STD-003 before you put them in stock.

2. In addition to solderability, verify that boards and componentsare clean.

3. Flux performance can change from one component or board finish to another. Some fluxes seem to work better on nickel under gold compared to immersion tin. Others seem to work better with immersion silver and OSP (Organic Solder Preservative). Choose a flux that works well with Pb-Free finishes.

4. Increase soldering process control. Establish a time temperature profile for each assembly. Oxides form quickly at high temperature. If the flux is cooked off too early in the soldering process, new oxides may form at the connection site. Some fluxes form an electrically insulating varnish (polymerization) between the carrier and the activator. If complete polymerization takes place before reflow, formation of the solder connection may be inhibited. The machine settings should be established by a proven process. Validate solder process settings for preheat controls, conveyor controls, solder pot controls, machine recipes and inert atmosphere controls.

5. Train hand soldering and rework technicians. Sometimes the methods used with one formula will not
work with another. The transfer of heat into the work piece is accomplished by means of radiation, convection, conduction, or a combination of the above. Excessive heat can cause the flux to be boiled away from the solder connection. This results in the loss of its contribution to the process. Technicians may need the following alternative approaches to be successful:

• Consider lower tip temperatures.
• Consider the benefits of hot air rework.
• Validate manual soldering and rework techniques.
• Establish rules for hand soldering and rework cosmetics that everyone can live with.