1. Field of the Invention
The present invention relates to the operation of fluxing metallic surfaces which is generally involved before operations of soldering or tinning in electronics. It thus applies in particular to the fluxing involved before the operations:
of soldering components onto a circuit (both in the case of inserted components and surface-mounted components), PA1 of soldering contact strips onto electronic supports, making it possible to connect the support in question onto another support (mention may here be made of the example of a hybrid circuit or of a printed circuit which is to be inserted using these contacts into a printed circuit, or else a hybrid or printed circuit which can be plugged, using its connection edge, into a connector), PA1 of soldering circuits into bottoms of packages (involved during encapsulation of such circuits), PA1 of soldering involved during packaging closure procedures, PA1 of soldering bare chips onto supports such as a printed circuit, hybrid circuits or else multilayer interconnection substrates such as the substrates commonly called MCM (multi-chip modules), PA1 tinning terminations of electronic components. PA1 its efficiency (ratio of power input for creating the plasma to the density of species produced which actually interact with the support to be treated), or else the power density obtained (in the case of corona discharge, it reaches only a few watts (W) per unit surface area of dielectric), which, if they were to be enhanced, might permit shorter treatment times, PA1 and also the fact of limiting "geometrical" factors: in the case of corona discharge, the electrode/sample distance is highly critical and must be kept very small, which may cause problems in the case of substrates whose surface structure is relatively convoluted; in the case of microwave discharge, it gives rise to the formation of a plasma-generating spot which has defined dimensions limited by the plasma source, PA1 moreover, a plasma as created in this document contains, by definition, ionic species and electrons (and therefore electrically charged species) which are still difficult to use on electronic components. PA1 to operate substantially at atmospheric pressure, PA1 to provide a high degree of flexibility in the distance between the object to be treated and the device used for carrying out this treatment, PA1 to avoid contact of the articles with charged species, PA1 to provide an improved power density, making it possible to achieve an increased treatment speed.
2. Description of the Related Art
The role of fluxing is then to clean the metallic surfaces to be soldered or tinned (degreasing, deoxidation, decontamination of adsorbed layers, etc.), this being with the aim of facilitating subsequent wetting of these surfaces by the solder.
This fluxing operation is most commonly carried out using chemical fluxes which are often obtained from resin bases, supplemented in particular by acidic compounds. After soldering, flux residues remain on the article, often requiring the manufacturer to carry out a cleaning operation, most often using chlorinated solvents, which cleaning operation proves particularly problematic in the case of bare chips because they are very fragile, and is also highly controversial, in the context of the "Montreal Protocol" which strictly regulates or even, in the case of some, depending on the countries, completely prohibits such solvents.
Two of the methods most commonly used for carrying out the soldering operation are called "wave soldering" and "reflow soldering".
In the first case (wave soldering machines), the design of these machines is such that the articles to be soldered or to be tinned are brought into contact with one or more waves of liquid solder which are obtained by circulation of the solder bath contained in a vat through a nozzle.
The articles (for example the circuits onto which the components have been attached, or else the components to be tinned) have generally been fluxed beforehand in an upstream zone of the machine, using a flux spray or a flux foam, the fluxing operation being followed by a so-called preheating operation which is carried out in order to activate the fluxes previously deposited on the circuit and in order to preheat the circuits or the components before they reach the hot soldering zone. A conveyor system is present for moving the articles from one zone of the machine to another.
These machines are traditionally open to the ambient air atmosphere.
In the case of the second type of method (reflow soldering), which term moreover encompasses a plurality of techniques, use is made no longer of a liquid solder bath but of a soldering paste containing the solder alloy (paste formulation in which the flux is included) which is deposited on the support (circuit before deposition of the components, edges of the package to be closed, or else package bottom) and to which a certain quantity of heat is supplied, making it possible to melt the metallic alloy. This heat transfer is most often carried out in a continuous oven.
In either case (wave or reflow), the problem arises of the abovementioned cleaning operation after soldering, which most often uses chlorinated solvents which are strongly regulated by the Montreal Protocol and its subsequent revisions.
This problem has therefore provided the motivation for a number of research efforts throughout the world over the past few years in order to try to provide a solution replacing use of these compounds.
Among the solutions envisaged, mention may be made of plasma cleaning of surfaces before soldering, thereby avoiding using chemical fluxes and therefore the actual requirement of a downstream cleaning operation. The mixtures envisaged used, in particular, hydrogen.
In this field, mention may be made of document EP-A-0,427,020, which proposes treating assembly parts to be soldered using a plasma of a process gas, recommending the use of low pressures for this treatment "with the aim of avoiding thermal damage to the assembly parts". All the examples given, in conjunction with the figures provided, relate to pressure conditions varying in the range of 30-100 Pa.
The same comment can be made in regard of document EP-A-0,371,693, which relates to a method of cleaning metallic surfaces before soldering using microwave plasma containing hydrogen. Here again, it is recommended (and illustrated throughout the examples) to use low pressures "in order to make it possible to limit the level of residual oxygen in the plasma".
This consensus of opinion in favour of the use of low pressure conditions for carrying out these plasma cleaning operations, despite the drawbacks which are in particular linked with the cost of obtaining such pressures or else the difficulty of implanting the corresponding infrastructures in a production line, is undoubtedly linked with the technical and technological difficulty of obtaining, at atmospheric pressure, plasmas which give performance comparable to that which is traditionally obtained at low pressure.
In this context, the Applicant Company has, in document FR-A-2,697,456, the content of which is incorporated herein by way of reference, recently proposed a method of plasma fluxing metallic surfaces before soldering, at atmospheric pressure and using, in order to create the plasma, a microwave source or else a corona discharge transferred via slots placed suitably in a dielectric layer placed above the article to be treated. Although this application provides an advantageous solution to the problem in question, the Applicant Company has demonstrated the fact that the proposed method might be improved, especially as regards:
In parallel, the Applicant Company has recently proposed, in document FR-A-2,692,730, the content of which is assumed to be incorporated herein by way of reference, a device for formation of excited or unstable gas molecules which operates substantially at atmospheric pressure and which offers improved energy density.