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In the wave soldering flux principle

The author:    Release time : 2017-01-04 16:33:38    532time to browse

By in wave soldering flux welding defects which caused by the phenomenon

(1) virtual welding

Any metal surface in the air environment will be oxygen or other oxygen gas of different degrees of chemical etching, the surface state of metals in nature are not pure state of sheet metal. Therefore, no matter what protective measures, the surface of welding performance is not ideal, only on the characteristics of the metal itself without with the help of other substances (such as the flux) reach ideal welding effect is almost impossible. Even though this may, it is after the cost of pay a high price as a result, this is clearly not.

Metal surface state, is the key to virtual welding phenomenon induced factors. Adopted in the process of soft brazing flux (liquid or gas), can with the help of the flux function to obtain the ideal clean surface. By welding surface cleanliness is a function of the flux activity. Early '60 s, before the military of the rosin alcohol as a widely used in electronics production flux, because this flux react with many metals to the natural chemical activity is weak, so product virtual welding phenomenon is particularly serious, almost become a public nuisance. Early '60 s introduced in our country from the former Soviet union XX missile terminal guidance radar line, Sudan has provided the weapon system with special flux formula of "secret". Domestic many military units also prefer to remain in the military production activity rosin flux + cleaning process, and disable active weaker no clean flux, its purpose is to avoid cold joint hidden trouble to the weapon system reliability serious harm of unpredictable consequences.

Bad diction metallized hole through hole

Weldability are achieved when the PCB and components (zero delivery time < 1 s), and the wave soldering process parameters and suitable situation of metallized hole perforated unhealthy phenomenon, its main influencing factors should be considered as the flux of the floodplains and activity are caused by poor. Hole not permeate flux and metallization in the cause of defective hole through hole. Therefore, in the case of using the same kind of flux and the flux bubble wave coating way easy occur through hole metallization hole bad phenomenon, and adopts the flux of spray coating method of metallized hole perforated adverse occur probability will be much higher. The reasons for this phenomenon is the spray coating way vulnerable to block and appear pore permeability caused by poor.

(3) bridge and icicles

Flux of the performance of wave soldering tin and icicles wait for a phenomenon also has a great influence. Good performance of the flux activity, not only has the welding needed but also has excellent protection located in stripping zone are oxidation liquid solder ability, this is to reduce the PCB and solder stripping inhibition in the process of bridge and one of the important measures of icicles, as shown in figure 1.
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In speaking we are given A discussion topic: C wire for welding work number: the ZXA10 ADS2 - CORZ (010502), the substrate capacitance C415 and C542, C413 and C540 between adjacent clearance is 0.63 mm (< 1.27 mm, belong to improper design), with A new nozzle opening double wave in shown in figure 2 A, B two bridge even occur in one hundred percent, and only the second wave of shan bo, when welding the incidence of rather bridge is very small. Is really a problem on the flux, the main reason is the:

(1) the flux in wave soldering process in the PCB stripping zone has no protection;
(2) after the first wave of flushing flux on the PCB board face is running out.
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(4) PCB board face bad cleanliness

In the process of wave soldering flux of chemical composition and quantity of spraying and ion concentration of residual after welding, surface insulation resistance, cleanliness, and reliability in the process of product use in the future there is a direct relationship.

In the wave soldering flux principle

1.1 correct flux is used to ensure that the significance of the quality of the product

Flux is an important and indispensable material in wave soldering, it is to ensure the effect of wave soldering and product quality plays a key role. Good flux materials and their functions into full play, is to improve the production efficiency, reduce the product cost, improve product reliability.

1.2 the role of flux in wave soldering

In general, is the weld metal and the molten solder alloy surface has a layer of obstruction of thin film rust forming connection interface. The film rust is corroded by the environment as a result, and due to the difference of environment and weld metal, and can be caused by oxide, sulfide, carbide or other composition of corrosion products. The role of the non-metallic corrosion products is equal to the barrier layer. Therefore, must remove it before brazing. In the process of wave soldering, the role of the flux in summary the main function is as follows:

(1) to remove rust by welding on the surface of the substrate metal membrane

By welding on the surface of the metal film rust does not dissolve in any solution, usually don't like remove grease will get rid of it, but the film rust with some material produces chemical reaction, generate soluble in liquid flux of the compounds. Can remove the rust purification by welding on the surface of the metal film to achieve. This chemical reaction can be make the flux and the generation of film rust solvent soluble in flux and flux of another kind of compounds, can also be reduced metal film rust as pure metal surface chemical reaction.

Belong to the first kind of chemical reaction flux is mainly represented by rosin base flux. Pure rosin is mainly composed of rosin acid with other points of heterogeneous diterpenoid acids. When used as flux, usually with alcohol (ipa) as a solution, when in the oxidation of copper coating on the surface when heated, the flux and rosin acid and copper oxide compound generated abietic acid copper, it is easy to be mixed with no reaction of rosin together, thus provides the solder wetting clean metal surface. Rosin acid copper to copper oxide layer below the substrate without any erosion. When using organic solvents to remove residual flux, rosin acid copper also cleared away together.

As an example of the second kind of chemical reaction is certain with a reducing gas. For example, hydrogen at high temperature can reduce metal oxide on the surface of the water and restore the pure metal surface. The chemical reaction general formula can be expressed as: MO + H2 = M + H2O

2 to prevent heating by weld metal in the process of secondary oxidation

Wave soldering, with the increase of temperature, metal surface also exacerbate the reoxidation of phenomenon. As a result, the flux must provide protection for the purification of the metal surface. The flux should be in the metal surface to form a layer of thin film, cover the metal, make it off from the air, to prevent the welding in the process of heating of brazing metal secondary oxidation.

(3) reduce the surface tension of the liquid solder

Brazing flux, in the area in which to promote solder flowing way affect the surface energy balance. Reduce the surface tension of the liquid solder, decrease the contact Angle.

Metal surface oxide layer, the liquid solder are condensed into a ball, not with metal wetting. The solder wetting the harmful effect on oxide, is because there is a tension than metal oxide on the surface of the metal itself is much lower the surface tension of the reason. Gamma SF > gamma LF is the basic condition of liquid wetting solid. Covered with oxide film of solid metal surface than no clean surface of the oxide film, greatly reduce the surface tension, the gamma SF, < gamma LF and appear no wetting phenomenon.

Used in the welding flux can remove solder and oxide film on the surface of the weld metal, improve the wettability. And when the liquid solder and weld metal surface covered with a layer of flux, the interfacial tension between them changed, as shown in figure 3.
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Liquid solder end to balance in the flowing equation is as follows:

Gamma SF = gamma LF + gamma LS cosine theta
Cosine theta = (gamma SF - gamma LF)/gamma LS

Type:
Gamma SF - be welded metal and the interfacial tension between flux;
Gamma LF - liquid solder and flux on the interface of interfacial tension;
Gamma LS - liquid solder and interfacial tension between the weld metal.

By type, want to improve the wettability (i.e., reduce the Angle of theta), must increase the gamma SF, or reduce the gamma LF and gamma LS. Flux has been in addition to remove the surface of the weld metal oxide increased gamma SF, another important function is to reduce the interfacial tension between the liquid gamma LF.

(4) heat transfer

Generally, there are a lot of clearance brazing joint department in the process of brazing, the clearance of the air to play the role of a heat insulation, resulting in poor heat transfer. If the clearance is flux fill, can accelerate the heat transfer, quickly reach thermal equilibrium.

5] promote flowing liquid solder

After preheating, the glue shape after contact with wave solder flux activity increased, the viscosity fell sharply in the weld metal surface to form a second time flowing, and in the weld metal surface spreading quickly. Flux flowing force, which is formed by the second sheetflood process attached to the liquid solder from the drag of the liquid metal flowing process, as shown in figure 4.
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2 ideal flux in the wave soldering process mechanism and mode

2.1 the role of the ideal flux model

Analysis of the physical and chemical process of wave soldering, flux although participated in the whole process, but it is in the role of each interval is different, as shown in figure 5. And different types of flux, its carrier involved in the physical and chemical process is also different, here we only with rosin flux, active rosin flux and no clean type flux respectively to explain the specific physical and chemical process.

2.2 rosin flux in the mechanism in the process of wave soldering

Rosin flux, the mechanism and model description as shown in figure 6.
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2.3 activity rosin flux in the mechanism and pattern of wave soldering

Rosin flux activity is weak, on the weld metal surface clean ability is poor, when the weld metal surface weldability is not ideal, the common virtual welding, welding defects such as bridging. In order to overcome these defects, and improve the welding quality and efficiency of the virtual welding and bridge phenomenon as much as possible to a minimum, the current domestic also widely used in military equipment activity rosin flux of the why.

Activity and the functional mechanism of rosin flux in wave soldering and the description of the model as shown in figure 7.
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2.4 no clean flux in the wave crest welding mechanism and pattern

Active rosin flux, high solid content after wave soldering residue, and the decomposition of residues may also contain not end of the activity of ionic substances, if not carefully remove will stick around. Due to erase the extensive use of ODS, VOC or water consumption and pollution, to protect the earth environment. Therefore, at present in some universal electronics production is heavily promoted application of no clean flux.

No clean flux in the wave crest welding mechanism and the description of the model as shown in figure 8.
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No clean flux of process temperature specification is strict, only fully meet the requirements under the condition of its properties, can give full play to its role to help welding. Therefore, the supplier must provide the temperature of the complete application features. IF Belgium, for example, INTERFLUX ELECTRONICS company produces 2005 m no clean flux is complete application temperature standard value is given as follows:

Preheat temperature: 95 ℃ to 130 ℃ (component side);
Solder bath temperature: the minimum is 235 ℃ and normal is 250 ℃, the highest at 275 ℃;
PCB with molten solder contact time for 4 seconds.

Throughout the soft brazing of modern electronic equipment (manual soldering, wave soldering, reflow soldering), flux from beginning to end played a very key role. Through the above description of wave soldering process, enough to prove that in the soft brazing process how to emphasize the importance of flux are however.

3 how to evaluate in wave soldering flux capacity index

3.1 how to evaluate the stand or fall of flux flux performance is usually adopt the role of the ability to describe the following two aspects:

(1) activity: for effective soft brazing, flux must be welding metal surface by chemical reactions to clean up, only in full after purification, the surface is between weld metal and molten solder to form effective metallurgical connection, to the eradication of welding defects such as. When evaluating flux activity is, therefore, must be sufficient attention.

2 protection function: visible in the above analysis in wave soldering flux is another very important role in the process of the protective function of flux. The realization of the function of protection in rosin flux is implemented through the medium of rosin, and in no clean flux by high boiling solvent of this medium to persistent. No clean flux in the strength of the protection function relationship in the success or failure of the wave soldering is very large. And the function must be run through the computer to study.

3.2 how to understand the flux of corrosive

From the chemical point of view, each a kind of effective flux are inevitably corrosive to a certain extent, otherwise, it cannot from the weld surface cleaning off the oxidation film. What we call the corrosive attention refers to the completion of brazing residual flux in the assembly and its residue of chemical risk, and thus determine the flux of the physical and chemical index requirements.

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