Cleaning of flux

Before the Montreal Agreement, almost all circuit boards needed to be cleaned after welding. After the Montreal Agreement, cleaning became an activity of only a small number of high-reliability assemblers, who carried out cleaning because they were required to do so by strict regulations. The cleaning equipment market has become highly specialized, with many previously known manufacturers of high-speed in-line (with conveyor belt) flux cleaning equipment either exiting the cleaning industry or merging with surviving companies. Small-batch batch cleaning systems replace larger, faster cleaning systems because they are suitable for small-batch/high-mix applications common in high-reliability environments and are highly efficient.

The introduction of lead-free regulations has once again brought shock to the cleaning industry, but this time in the opposite direction. 2008 became a banner year for many flux cleaning equipment manufacturers and chemical product suppliers. In a recession, the cleaning industry is actually growing. What factors have led to a rethink of cleaning flux throughout the industrialized world? The answer lies in two electrical components mounted on the circuit board, which are small metal crystals called dendrites.

Dendrites are metal crystals that can grow between two electrical points on a circuit board. Dendrites are made of metal and are used to conduct electricity. Unwanted conduction between two unwanted positions on the board can easily cause the board to fail. Failure of circuit boards in cruise missile products can lead to significant adverse results, which requires cleaning. However, dendrites do not apply boundaries. In fact, there are only three basic factors that produce dendrites: voltage (minimum 1.5V); Corrosive materials; Water.

Dendrites are generated on the circuit board by an electroplating process in which a conductive corrosion residue (flux residue) provides a current path between the cathode and anode. The voltage travels along the new unconscious current path, and dendrites begin to form.
So if only three elements are required for dendrite formation, why haven't we seen this before? Although only three basic elements are needed for dendrite formation, there are two other factors that help and promote dendrites, one is the increasing distance between the cathode and the anode, which we call miniaturization, and probably no other industry understands miniaturization better than the electronics industry. Another factor is lead-free welding alloys. Compared with eutectic alloys, lead-free alloys require higher reflux temperatures. What happens when a higher reflux temperature is combined with an ultra-low solid flux? Flux polymerizes very quickly during reflow soldering, impeding the resulting metal salt encapsulation when the metal is converted to liquid. Unlike traditional rosin fluxes with high solids content (previously more commonly used) that trap and encapsulate metal salts, low-solid no-clean fluxes, combined with higher heat, harden the metal salts and prevent metal salts from encapsulation. These free metal salts act as fertilizer for the growth of dendrites.
How can you prevent dendrites from forming? The answer is simple, and there are three ways to do it:
Remove the voltage from the circuit board, of course, this is just a joke.
2, prevent contact with moisture and/or moisture, this can be achieved by controlling the board environment (which is not always possible) or board conformal spraying (which requires surface cleaning to achieve proper bonding).
3. Remove conductive residue (flux).
I choose the third approach, as do most of the manufacturers in the electronics assembly industry. Over the past few years, more and more assemblers have begun cleaning components after reflow and during assembly to remove flux and other conductive residues from the circuit board casting process.
Because flux cleaning is a new field, engineers have new needs and want to learn more about modern welding solutions. In October 2008, IPC and SMTA held a large seminar on high-performance electronic assembly cleaning, which resulted in a full house. To the surprise of many, a large number of commercial assemblers attended the workshop. Failure of a circuit board due to a lack of proper post-reflow cleaning is no longer a problem exclusive to manufacturers of ultra-high reliability components. Manufacturers of hardware, medical devices, mobile phones, train control equipment, digital signage and many other applications traveled to Chicago from far and wide to participate in the two-day workshop.
Cleaning is once again a hot spot in EMS, and cleaning equipment and chemical product suppliers offer manufacturers the right options to adapt to any cleanliness, throughput, and environmental requirements. By reducing on-the-ground failures and improving reliability, our times may not feel so "troubled" anymore.