In the production of an electronic board, the transition in the oven is one of the most sensitive part of the whole process. This is because, unlike the other part, is an indirect operation, passing through the warming in the oven with the inability to control the values of temperature in each point.
In traditional tunnel oven, in fact, temperatures are made vary, following a default thermal profile, depending on the material to be treated.
Its proper definition of such a profile, whose performance must be studied and created for each production, entails the need to devote significant time to achieve the best outcome, you must make a series of tests, also empirical, and progressive optimization of the cycle
All this, of course, takes time and, above all, involves a number of attempts, with the possibility of damaging some components mounted on the card itself.
This means that, besides reducing the efficiency of the production process, siyou have an increase in expenditure, particularly important especially in productions characterized by the use of special or expensive components.
Similar considerations, of course, have a negligible impact on production numerically significant, is quite different in the case of a few limited prorotypes in which the setup time and errors must be canceled.
To overcome the limitations of traditional ovens, which remain essential on a large scale production, were made Soldering System capable of exploiting the technology Vapour-Phase.
In this case, in fact, the air inside the oven is not heated, but it directs a stream of inert steam on the card itself.
The advantage of this approach is easily understood. From a physical point of view, in fact, under certain conditions of pressure, the liquid used for the job evaporates always at the same temperature.
This means that the card will always be at the pre-determined temperature, avoiding any risk of damage. With a perfect temperature control, the whole process can be adjusted simply by acting on the time permanence of the PCB inside the oven, thus eliminating any risk of error by the line operator.
In fact, although the board remain in the oven for a time longer time than necessary, the temperature does not exceed the values pre-determinated.
All the components on the card also will reach the same temperature, regardless of their size, thus ensuring optimum welding.
These characteristics are ideal for the production of special cards or when you need to make small batches that, with the traditional solutions, impose each time the study and definition of a proper thermal cycle.
For these reasons, the Vapor Soldering Technology was originally designed for applications in military and aerospace fields, where are often used special components to realize a limited number of PCB with absolute reliability.
To all this, must be added the benefits of a limited energy consumption.
Last but not least, in the area to be welded, due to the presence of steam, there is no oxygen, so there is no possibility of oxidation, thereby improving the quality of the weld.
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Introduction to Welding Process Vapour-Phase
Welding Process for Condensation, also known as Vapour-Phase Soldering Process, requires the use of an inert fluid, electrically non-conductive, heated to its boiling temperature. Above the fluid is created a zone of saturated vapor, chemically inert, the temperature of which is identical to the boiling point of the fluid used in the process. When you insert an electronic board in the zone of saturated vapor, it condenses on the surface of the board until the PCB reaches the temperature of the steam. Any soldering cream with a fusion point lower than the temperature of steam is then melting.
Welding Soldering Systems for Condensation, produced by ASSCON, have been developed on the principle of single phase and don’t use chemicals containing CFCs. They use a technology that reduces fluid losses.
Thanks to the physical principles of steam condensation it is possible, without great effort, achieve exceptional Welding results.
The major advantages of the soldering by condensation are:
- Process free from oxidation (0 ppm of Oxigen) in the steam-inert phase, without the use of nitrogen;
- Reproducible process conditions;
- No overheating of electronic assemblies;
- No shadow effect, thus uniform heating and blending;
- Heating of the components independent of shape and color of the product;
- Thanks to its high pressure, steam penetrates even in smaller openings. Then you can assure the welding of hidden areas (i.e. BGA);
- Weilding Profile completely reproducible even for different assembly, since the heating process is auto-regulating due to its physical properties;
- No wasted time to determining the temperature profiles;
- Process non-polluting (no value FC);
Another benefit of the reflow by condensation is the excellent capability of heat transfer through saturated steam. The coefficient of heat transfer to the fluid used varies from 500 to 700 (W m-² K-¹) and then is, up to 10 times the heating by radiation or forced convection, in air or nitrogen. In this way, is obtained a reproducible heating of the assembly in a considerably shorter time. The energy consumption of a welding machine for condensation are, thanks to its excellent efficiency in the transfer of heat through condensation, a fraction compared to the forced convection or IR reflow machinery. You can work PCB with components of different size and masses, i.e. mini-chip, QFP, BGA, heat-sink, screens for high frequency, etc., without problems and dangers of overheating.
Description of the Soldering Process
Assemblies to be welded are brought to temperature of reflow from condensation of steam, directly proportional to the difference in local temperature and thus extremely homogeneous. Because heating occurs from the beginning by steam condensation steam, the process is completely free from oxidation. The boiling point of the liquid defines the welding temperature.
The welding zone contains an inert fluid, heated until its boiling point by electric heaters. Once you reach the boiling point, the temperature of the fluid can no longer increase. Each subsequent injection of energy will be used to produce steam (enthalpy of evaporation), saturated, chemically inert, the temperature of which is identical to the boiling point of the fluid. For a gas, an added helpful variable is the enthalpy (H), defined by the sum of the internal energy (E) plus the product of pressure (p) for the volume (V).
By placing the assembly in this environment, vapor condenses on its surface because its temperature is lower than that of the steam. Condensation stops when the temperature of the product reaches that of the steam. Soldering paste whose melting point is lower than that of the steam will melt. The product will be then raised from the process chamber through a descenders.
The Fluid for the Heat Transfer
The ASSCON uses a perfluoropolyester with a high boiling point product from AUSIMONT, sold under the trade name of GaldenTM. The perfluoropolyester GaldenTM series are liquid polymers composed of Carbon atoms (C), Fluorine (F) e Oxigen (O). The bonds within the molecules C-O e C-F are extremely robust, so as to be considered among the most stable bonds in the chemistry of Carbon. Fluorine atoms, related to the central chain of the polymer, fully protect the Carbon skeleton and then defend the most vulnerable links C-C from chemical and thermal attacks. The longer Carbon chains consist of units C3; it is impossible for them to change in PFIB (perfluoroisobutylene C4F8) hazardous to health.
The liquid polymer series GaldenTM are all constructed according to this principle and exhibit properties quite extraordinary:
- High thermal stability;
- Excellent material compatibility;
- High resistance to chemicals reactive substances;
- Good dielectric properties;
- Low pressure of the steam;
- No flammability point;
- High density of the steam;
- Excellent coefficient of heat transfer;
- Low surface tension, good wetting properties;
- Not classified as hazardous materials;
- No chemistry activity (perfluorinated, eg. no atom of H or CI)
- No damage to the ozone layer.
These security features in applications of reflow by condensation have been documented in many studies and may no longer be called into question. Polymers of GaldenTM do not burn and are extremely inert against all chemical products, up to very high temperatures. Not react with acids, bases or strong oxidizing agents. They are also compatible with all synthetic materials, metals and elastomers known.
The area of the steam, due to the density of the fluid, creates an inert area protecting the assembly from oxidation. The oxygen present in the molecules is chemically bound and thus unavailable to react.
The GaldenTM polymers have been tested in many tests, in terms of process, searching for toxic products. No toxic reaction product was detected.
Other uses of these molecules are:
- As a lubrificant in vacuum and high temperature applications;
- As seal in the construction of buildings;
- As liquid separation of other fluids;
- In the Burn-in Test;
- As primary substance for ointments and cosmetics;
- As cooling liquid for high performance computer;
- As replacement of blood in medicine.