The Reasons Of White Residual On The PCB After Cleaning (1)

In the production practice,the white residue are often found in the circuit board after cleaning. Especially surface mounted components. White residue on their joints are more obvious. Especially between surface mounted fine pitch device pins,white residue is very difficult to remove. 

The white residue seriously affect the appearance of the board and makes it hard for passing the final test. Additional repair work and some slit final residue which remains difficult to remove and only reduce the requirement through test. The white residue showed the appearance of loose material because after soaking with solvent. Easily absorb moisture and air among the various corrosive gases, especially in the coastal areas of high salt spray areas, there may be serious impact the reliability of the circuit board for long-term working . Moreover, the not removal of the white residue will affect late coating (such as three anti-paint coating adhesion,impact the long-term useful life and the protective effect of three anti-paint)

The composition and  reasons of the white residue:

The experts have a common point of view : Since the circuit board soldering and cleaning process, various process parameters are often volatile and too many chemicals involved, such as: Sn – Pb solder and various trace metals (aluminum , zinc, etc.), as well as their oxides, various flux: as rosin flux, water soluble flux, solder containing various activators, no-clean solder paste, etc.

Paste containing thixotropic agents, solvents and the like. Circuit board laminates material, solder resist film. Among air moisture, oxygen.A variety of cleaning solvents (chlorine, fluorine organic solvent, alcohol solvent, water, solvents, etc.). So many chemical materials under high temperature soldering and cleaning process of chemical reactions that occur in the end, what chemicals produce? Which of these is the white residue insoluble? However, so far the existing techniques and tools failed to clear all the ingredients of white residue completely, nor can absolutely understand their exact causes, and therefore it is also quite difficult to solve. Now ,we summarized some of the most common causes of white residue ,as follows

Ten reasons as follows:

1.1 oxidised rosin residue
Rosin resin composed mainly of rosin acids. Because of its molecule an unsaturated double bond, it is particularly susceptible to oxidation. When soldering the circuit board is heated, rapid oxidation rosin acid and a ketone peroxide compound. The chemical composition of the solvent is less soluble in resin than the original. Thus, the white residual will be distributed significant and irregular after cleaning.
And the part of board which severe heating is more obvious. Since the formation of the white spots are often caused by excessive heat. Thus, the white spots often appear in the ground floor there is a large area in need of special welding heat to complete the multi-layer printed circuit board. Since surface mount reflow process, the heating time is longer and the pcb will easy to cause excessive heating. So this white residue on the surface of the circuit board assembly is also not uncommon. These white residue can not be cleaned with ordinary chlorine, fluorine-containing solvent, alcohol or saponification water .

Two common ways：
(1) proper scrubbing methods, such as scrub;
(2) Since this white residue was dissolved in the molten rosin acids, and some alcohol based flux, thus heating the flux to the original re-coating, these may be dissolved oxidized white rosin, so that the normal cleaning solvent can be used for washing it. However, this approach seems not so economical and practical.

1.2 polymerized rosin residue
Another common source of the white residue is rosin flux polymerization reaction in the welding process. Rosin flux polymerization reaction is often caused by excessive heat of the circuit board. Causes is the same with the oxidized rosin . This white residue is also common in relatively thick multi-layer circuit boards and circuit boards which are prone to overheating of the surface mount reflow process them.

When polymerized rosin, the formation of longer-chain molecules, not be dissolved in any conventional solvent. Cleaning solvent will dissolve part of the flux of short-chain.The rest of the long chain part stubbornly firmly adhere to the surface of PCB, which is a polymerized rosin white residue. Polymerized rosin is recognized as the most common causes of white residue, but it needs to form the surface of the solder tin oxide as catalyst. The method of removing the Polymerized rosin white residue is generally the same with oxidized rosin .A mechanical brushing or dissolved again with heating original flux , and then use a common flux cleaning solvent for cleaning.

1.3 hydrolysis rosin residue
Rosin flux itself is insoluble in water, but the flux in the process of storage and use, which absorbs moisture in the air. When water and an aqueous alcohol solvent cleaning methods for cleaning, similarly combined with water, hydrolysis reaction. Rosin partially hydrolyzed is insoluble in the cleaning solvent. However, the formation of hydrolysis rosin itself is a reversible process, the rosin was heated slowly to hydrolysis to 100℃and it can decompose the Rosin hydrolysis,then be removed by conventional solvents. But much worse, if the heating improper control, the other will cause unpredictable complex chemical reactions to form other stubborn stains more difficult to clean.

1.4 laminates +solder + solder flux residues
Unreacted complete epichlorohydrin of laminate will cause polymerization in the rosin flux. However, the polymerization need tin oxide on the surface of the solder as catalyst, the presence of solder is a prerequisite of the polymerization reaction. The product of the polymerization reaction is also not soluble in various solvents.
To solve this problem, we must first ensure that the material is fully cured laminate. Once this kind of residue occurs, the best way is still the mechanical removal.

1.5 solder+ rosin solder flux residue
When soldering the circuit board, the solder surfaces and components of the electrode, tin, lead, copper or iron in the pin will occur in complex organic chemical reactions with Organic acids in flux, as the formation of a complex series of compounds.
Wherein the tin and lead pine rosin ester and methyl is more. They usually do not dissolve in water or any chlorine, fluorine-containing solvent. To solve the problem mainly through preventive measures, such as the welding process optimization: reduce heat rosin time, increase wave soldering reflow oven or belt speed and so on.