CA2338157A1 - Method for brazing by solder reflow electronic components and brazing device therefor - Google Patents

Abstract

The invention concerns a method for brazing by solder reflow electronic components (22) on a support, for example a printed circuit wafer (20) which consists in: arranging a solder alloy on the support (20) in sites for connecting the components (22); placing the components on the connection sites and brazing the components (22) properly speaking by a thermal treatment of the support, by contacting it, at a pressure close to atmospheric pressure, with a treating atmosphere comprising excited or unstable species substantially free from electrically charged species, the atmosphere being obtained by passing an initial treating gas in an electrical discharge, and the thermal treatment of the support being obtained by using the species thus heated by the action of the discharge.

Description

REF'QSION BRAZING PROCESS
OF EI ~ ECTRONIC COMPONENTS AND DEVICE
OF BRAZING FOR IDA IMPLEMENTING ~ TIRE OF A TEI ~ PROCESS
The present invention relates to a method of reflow soldering of electronic components on a support such as a printed circuit board, and also relates to a soldering device for placing using such a method.
The two most commonly used methods for perform a brazing operation, are the "brazing to the wave "(wave soldering an English) and" soldering by refusion "(reflow soldering in English).
In the first case, i.e. brazing with wave, soldering consists in bringing the plates carrying electronic components to be brazed in contact with one or more several waves of liquid solder alloy obtained by circulation of a solder bath contained in a tank, at by means of a nozzle.
Generally, the platelets are previously streamed in an upstream area, using a stream spray or flow foam, then preheated to activate previously deposited fluxes, in order to clean surfaces brazing, for the removal of oxides, contaminants organic, etc ...
In the second "reflow" brazing technique, a quantity of brazing alloy is available in component connection locations on the circuit, for example by screen printing a solder paste containing a mixture of metallic alloy and flux on the circuit printed before depositing the components to be soldered on.
Another recent technology is that the brazing alloy was pre-deposited) on the support, on connection locations of components, pre-deposits) recast) ("reflow"), this being usually followed by a planarization operation surface of previously remelted pre-deposits (formation of "bumps" in English, we will refer to the SIPADT processes "or

2 still OPTIPADT "'industrially available), or even by the fact that the brazing alloy has been the subject for some components of a pre-deposit on terminations of components themselves.
It is only then that we proceed to the positioning of the components on the support.
The support provided with the components is then inserted in a reflow oven so as to bring the amount of heat necessary to allow the fusion of the metal alloy, as well as the activation of the element fluxing agent contained in the paste or the pre-deposit.
As in the case of wave soldering, this technique requires the use of flux in order to clean the surfaces to be brazed. (, 'the use of these flows presents a a number of drawbacks, notably due to their cost, and the residue they leave on the cards, which tends to generate reliability issues for cards electronics thus designed. I1 is therefore necessary, with these techniques, to provide an additional step of card cleaning after soldering, which uses the most often chlorinated solvents, the use of which tends to be strongly limited by the regulations in force. In in addition, this additional cleaning step tends to significantly increase the cost of manufacturing circuits.
In addition, there is a strong trend of the microelectronics industry to move towards a miniaturization and integration rate on circuits extremely high (number of inputs / outputs of components), with in particular the appearance of new types of components with very high number of connections and complex connection geometries on the circuit, such as components called in this business BGA, or Flip Chip.
We know for example that the BGA components present extremely attractive performances for industry but also a number disadvantages, particularly related to the fact that the connections WO 00/0633

3 P'C1 '/ FR99 / 01657 and solder joints are not lateral to the component but located under the component, making it very difficult both cleaning than any repairs.
We will refer to these questions new components, and new deposition technologies solder, to the various articles of the journal "Advanced Packaging ", July / August 1997.
Documents EP-658391 and EP-747159 in the name of the Applicant had proposed processes and installations dry fluxing, before soldering or tinning, at using gas mixtures containing chemical species excited or unstable and substantially lacking electrically charged cash.
The work carried out by the Applicant has showed that these processes still needed to be improved in the case of reflow soldering, not only to improve the soldering conditions of the special components but also generally the flow conditions avoiding the cleaning operation of the supports by downstream.
The object of the invention is to provide an answer technical to the above mentioned problems.
It therefore relates to a brazing process by reflow of electronic components on a support, during which.
- there is a quantity of brazing alloy, - the components are placed on locations of support connection, and - a brazing operation is carried out properly said components using said brazing alloy, by support heat treatment, and is characterized in that we carry out this audit heat treatment of the support to perform the brazing of components using the brazing alloy, by setting support contact, at a pressure close to the pressure atmospheric, with a treatment atmosphere comprising excited or unstable chemicals and

4 substantially free of cash electrically charged, the atmosphere being obtained by passing a gas initial treatment in an electric shock, and the heat treatment of the support being obtained by means of chemical species thus heated under the action of dump.
The process according to the invention can also include one or more of the following steps.
- prior to the component deposition step, polymerizable glue is deposited on locations of bonding of the support, and we proceed to the polymerization of said glue in order to stick the components on the support to level of bonding locations;
- the brazing alloy is available by proceeding a screen print of solder paste on the support in component connection locations;
- the brazing alloy is available by the fact that the brazing alloy had been the object on the support of pre-deposit (s), on. the connection location (s) components, pre-deposits) having then undergone an operation of reflow, then having then advantageously been the subject an operation to flatten their surface;
- the brazing alloy is available by the fact that the brazing alloy was pre-deposited) on locations / terminations of the components properly said, pre-deposits) having then undergone an operation of reflow;
- before or after the stage of deposit of glue on the bonding locations of the support, or after depositing the components on the support, performs a prior fluxing operation of the support by treatment of the latter, at a pressure close to the atmospheric pressure, by means of a fluxing atmosphere containing excited or unstable chemical species and substantially free of cash electrically loaded;

- the fluxing atmosphere of the support is obtained by passing an initial fluxing gas through a landfill electric, the initial fluxing gas comprising advantageously a reducing gas mixture comprising

5 hydrogen;
- the initial treatment gas comprises a mixture reducing gas comprising hydrogen;
- the treatment atmosphere is obtained at a gas outlet from an excited species forming apparatus or unstable, in which the initial gas of treatment ;
- the flux atmosphere is obtained at an outlet gas from an excited species forming apparatus or unstable, in which the initial gas of fluxing;
- the steps for placing the components and brazing are performed on the two large faces of the support, said processing operation being carried out using two chemical species forming devices excited or unstable, each arranged opposite one faces of the support;
- The method further includes a step of cooling of the support, after said treatment, by passing the latter in an atmosphere of cooling with neutral gas.
The invention also relates to a device for reflow soldering of electronic components on a support, for example a printed circuit board, for the implementation of a process as defined above, characterized in that it comprises a device for conveying supports carrying, on at least one of their faces the components to be soldered on connection locations, locations where an alloy of soldering, the conveying member ensuring the transfer of supports next to first means allowing the polymerization of glue points present on locations for gluing the support, and next to seconds

6 means comprising at least one apparatus for forming a treatment atmosphere with chemical species excited or unstable and being substantially lacking of electrically charged species, processing atmosphere whose temperature makes it usable to achieve, at a pressure close to atmospheric pressure, a treatment thermal support, for the actual soldering of components.
The device advantageously comprises, interposed with along the conveyor upstream of the first means or between the first means and second means, at least one device for forming a fluxing atmosphere comprising excited or unstable chemical species and being substantially free of cash electrically charged, fluxing atmosphere usable to achieve a fluxing in the gas phase of the support.
The expression "pressure close to the atmospheric pressure "according to the invention, a pressure advantageously situated in the interval X 0.1 x 105 Pa, 3 x 105 PaJ.
As will be understood from reading what precedes the "support" on which the components are soldered according to the invention can be very varied in nature, depending on in this the different supports used in the industry, as well as on the electronic components to be soldered. AT
illustrative title, it could be supports of the type printed circuits (whatever their surface condition or finishing), or for example ceramic supports metallized such as hybrid circuits, or even backgrounds boxes on which circuits must be soldered in a global encapsulation process.
Similarly, the components concerned may be extremely varied, from electronic components traditional passive or active down to more components complex and delicate handling, whether encapsulated or bare chips (BGA, MCM, Flip Chips etc.). The

7 "components" according to the invention may also consist in circuits that must be soldered on another support or still on a bottom of the shoemaker before encapsulation.
Other features and advantages will emerge of the following description, given only as example, and made with reference to the accompanying drawings on which .
- Figure 1 is a schematic view of a brazing device for implementing the method according to the invention; and - Figure 2 is a schematic sectional view of a example of a module for training excited chemical species or unstable entering into the constitution of the Figure 1.
In Figure 1 there is shown a general view a reflow soldering device suitable for implementation of the method according to the invention. These measures comprises a conveying member 10 comprising a belt 12, shown in phantom, arranged in an enclosure 14 and extending between two guide rollers, 16 and 18, of which the less one is driving.
As seen in this figure, on the mat 12 are arranged here a set of circuit boards printed, such as 20, on at least one of the large faces of which are arranged a set of components electronic, such as 22, which should be soldered.
Electronic components are arranged at level of connection locations to which is applied solder paste, for example an alloy of tin and lead.
Furthermore points of a suitable polymerizable glue for the intended use, have been deposited on points adequate bonding of the wafer (for example on polymer surfaces of the printed circuit board, between two connection locations), ensuring that the components in place during soldering.
The mat 12 ensures the transfer of the platelets from printed circuit 20 carrying the components 22 opposite a

8 first station 24 allowing by thermal transfer the polymerization of the glue deposited (for example by radiation), then through a second station 28 heat treatment, at which the soldering properly said components 22 is performed.
Finally, the mat 12 ensures the transfer of platelets 20, downstream of the second treatment station 28 thermal to a level 30 cooling station from which the wafers are arranged in an atmosphere nitrogen.
As seen in this figure 1, the second heat treatment station 28, can be considered as combined with a fluxing station 26, both made up by a module 32 for forming a treatment gas and fluxing with excited chemical species or unstable and substantially free of species electrically charged.
In this figure 1, it has been considered that the components 22 are arranged on one of the large faces of the wafer 20, the species training module 32 excited or unstable being turned towards this face. But as will be apparent to those skilled in the art, in the components 22 are arranged on the two large mutually opposite sides of each wafer 20, the device can include two training modules 32 of excited or unstable chemical species each arranged in look of one of the large faces of the plate 20.
The function of module 32 is to pass a gas initial treatment, advantageously comprising a mixture reducing gas, for example based on nitrogen and hydrogen, supplemented, if necessary with water vapor, through a electric discharge, inside which the gas initial is transformed, so as to generate, at the output of module gas, the process gas that contains the species gaseous excited or unstable and which is substantially free of electrically charged cash (situation of post-discharge).

WO 00/06333 PC'T / FR99 / 01657

9 As will be understood from reading what precedes, according to the circuits to be brazed, and according to the alloy of solder considered, the temperature of the species leaving discharge will generally be directly sufficient (> 190 ° C).
However in some special cases (some metallic eutectics for example), we could consider somewhat preheat the initial gas before entering discharge.
There is shown in Figure 2 a sectional view of an example of module 32 capable of generating such chemical species.
We see in this figure that the module 32 includes a first tubular electrode 34, formed for example by an internal face of a metal block 36, and in which is concentrically arranged an assembly formed of a tube in dielectric material 38, for example ceramic, on the internal face of which is deposited, by metallization, a second electrode 40, excessively thickened in FIG. 2, in order to improve clarity.
The dielectric tube 38 and the second electrode 40 define, with the first electrode 34, a passage gas tubular 42, and internally an internal volume 44 in which a coolant is circulated.
The block 36 comprises, diametrically opposite, two longitudinal slots 46 and 48, respectively forming the entry of the initial treatment gas to be transformed (excite) in passage 42 and the flow exit treatment atmosphere including gaseous species excited or unstable but substantially devoid electrically charged cash.
Slots 46 and 48 extend the full length axial of the cavity 42.
Furthermore, block 36 further comprises, advantageously, at the periphery of the first electrode 34, a plurality of conduits, such as 50, for the passage a refrigerant, for example water.

WO 00/06333 PC'T / FR99 / 01657 1 ~
It can also be seen in FIG. 2 that the slot 46 gas inlet communicates with a room homogenization 52, formed in a case 54 attached to the block 36 and comprising a gas supply pipe 56 initial.
The module is completed by an electric generator 58 high voltage and high frequency intended to generate a discharge in the gas mixture circulating in the gas passage 42 so as to cause, by ionization, an excitation of the gas molecules entering its constitution and thus generate chemical species excited or unstable, in particular H ~ or Hz ~ radicals, which come to deoxidize and decontaminate the external surface printed circuit boards 20 (Figure 1).
The soldering process for electronic components on a printed circuit board is carried out here from the next way.
We will consider later that we use pads 20 on which solder paste has been screen printed, at connection locations components. We could also have used an alloy of solder having been pre-deposited on the wafer on component connection locations, and a pre-reflow, usually followed by an operation for leveling the surface of the pre-deposits thus obtained (industrially available processes well known from a person skilled in the art of microelectronics). He could also be components that themselves have locations / terminations on which a pre-deposit of brazing alloy was made.
The invention is deposited as described above, glue dots on so as to maintain the components during the soldering.
Then place the components on the slots of connection then we have the pads as well prepared on the conveyor, in particular on the carpet 12.
The latter transfers the platelets from loading station to the first station 24 of polymerization of the glue, and the circuit is thus preheated.
During the next phase, the platelets are transferred to module 32 of cash training excited or unstable chemicals, previously described in reference to FIG. 3, at which the plates, on the one hand, undergo a fluxing operation, under the action of the chemical species delivered by module 32 and, on the other hand and at the same time, under the action of heat transmitted to these excited species under the effect of discharge, the components are soldered to the wafer.
The process continues here with a step of platelet cooling, under the action of a flow of nitrogen supplied to the cooling station 30.
In the embodiment described with reference to Figure 1, we considered that the first step of polymerization and preheating heat treatment of the brochure is made by means of a station 24 specific.
It would however be possible, as a variant, to carry out this preliminary heating by means of the heated gas issued as output from a module 32 for cash production excited chemicals.
It would also be possible to plan a step prior fluxing of platelets 20 using cash excited or unstable chemicals delivered by a module identical to module 32, before coating the slots bonding with glue.
Finally, and according to an embodiment not shown, it would also be possible to carry out the fluxing of supports and brazing heat treatment not by means of a single module but by means of two separate modules 32.

It is understood that the invention which has just been described, using a reflow / heat treatment step using excited or unstable chemical species and substantially free of cash electrically charged, heated under the action of a discharge allows significantly improve the quality of the solder as well while avoiding subsequent solvent stripping chlorine - the actual heat treatment of reflow is carried out with active species (cleaning, deoxidation ..) to avoid any phenomenon oxidation and therefore substantially improve the wetting performance;
- it is also understood that in use with low flux solder paste (low activity / low residue level on the support after brazing) or better still using alloy pre-deposition systems from brazing without flux in suitable locations on the support or component terminations, the method according to the invention combines brazing performance while quite satisfactory, while avoiding the subsequent cleaning operation of the supports.
Although the present invention has been described in relationship with particular embodiments, it does not is not limited however, but is on the contrary subject to modifications and variations which will appear to those skilled in the art in the context of claims below.
Thus, if the invention was particularly exemplified in the above by positioning on the path of supports an atmospheric formation module processing or fluxing (or even one module per side), we can of course consider the presence of several modules in series and / or in parallel to obtain the effect required, but we can also consider (depending on the case of each user site) the fact of not setting up, look of one or each side of the support, that a module (whether processing or fluxing), by performing successive ironing next to the module to obtain the desired effect.

Claims (15)

14 1. Component reflow soldering process electronics (22) on a medium (20), during which:
- we have a quantity of solder alloy, - the components (22) are placed on locations support connection, and - a soldering operation of the components (22) using said solder alloy by heat treatment of the support (20), characterized in that said processing is carried out heat of the support to perform said soldering of the components using said solder alloy, by placing contact with the support, at a pressure close to the pressure atmospheric, with a treatment atmosphere comprising excited or unstable chemical species and substantially devoid of electrically species charged, the atmosphere being obtained by passing a gas initial treatment in an electric discharge, and the heat treatment of the support being obtained by means of chemical species thus heated under the action of dump. 2. Method according to claim 1, characterized in that prior to the stage of deposition of the components, one application of polymerizable glue on locations of bonding of the support (20) and the polymerization is carried out of said glue in order to glue the components on the support at the bonding locations. 3. Method according to claim 1 or 2, characterized in that said solder alloy is available by one or more of the following methods.
i) by screen printing solder paste on the support (20) at connection locations of the components;
j) the solder alloy had been pre-prepared deposit(s) on the support in the location(s) of connection of the components, pre-deposit(s) having then undergone a reflow operation;

k) the solder alloy had been pre-prepared deposit(s) on component locations/terminations themselves, pre-deposit(s) having then undergone reflow operation. 4. Method according to claim 3, characterized in what said pre-deposits) had then been the subject of a flattening of its surface. 5. Method according to one of the claims preceding, characterized in that, before or after the step of depositing glue on the gluing locations of the support, we proceed to a fluxing operation of the support (20) by treatment of this last, at a pressure close to the pressure atmospheric, by means of a fluxing atmosphere containing excited or unstable chemical species and substantially devoid of electrically species loaded. 6. Method according to one of claims 1 to 4, characterized in that, after the step of depositing the components on the support, a fluxing operation of the support (20) by treatment of the latter, at a pressure close to atmospheric pressure, by means of a fluxing atmosphere comprising chemical species excited or unstable and substantially devoid electrically charged species. 7. Method according to claim 5 or 6, characterized in that the fluxing atmosphere is obtained by passing an initial fluxing gas through a landfill electric. 8. Method according to claim 7, characterized in that the initial fluxing gas comprises a gaseous mixture reducer comprising hydrogen. 9. Method according to any one of the claims above, characterized in that the initial gas of treatment comprises a reducing gas mixture comprising hydrogen. 10. Process according to any of the preceding claims, characterized in that the treatment atmosphere is obtained at a gas outlet an apparatus for forming excited or unstable species, into which the initial treatment gas was transformed. 11. Process according to any of the claims 5 to 8, characterized in that the atmosphere of fluxing is obtained at a gas outlet of a formation of excited or unstable species, in which has been transformed the initial fluxing gas. 12. Method according to claim 10, characterized in that the steps of component placement and brazing are carried out on the two large faces of the support (20), said heat treatment operation being performed using two training apparatus (32) of excited or unstable chemical species, each arranged opposite one of the sides of the support. 13. Process according to any of the preceding claims, characterized in that it comprises furthermore a step of cooling the support, subsequent to the said heat treatment, by passing the support in a cooling atmosphere comprising a neutral gas. 14. Component reflow soldering device electronics (22) on a support (20) using an alloy solder, for carrying out the method according to one any one of claims 1 to 13, characterized in that that it comprises a member (10) for conveying supports (20) carrying, on at least one of their faces, the components (22) to be soldered onto connection locations, the member (10) conveyor ensuring the transfer of supports in view of first means (24) allowing the polymerization glue dots present on glue locations of the support (20), and facing second means comprising at least one apparatus (28) for forming an atmosphere of treatment comprising excited chemical species or unstable and being substantially devoid of species electrically charged, treatment atmosphere whose temperature makes it usable to produce, at pressure close to atmospheric pressure, a treatment heat transfer of the support, with a view to the actual brazing of the components. 15. Device according to claim 14, characterized in that it comprises, interposed along the conveying member, upstream of the first means or between the first means and the second means, at least one apparatus (28) for forming a fluxing atmosphere containing excited or unstable chemical species and being substantially devoid of electrically species charged, fluxing atmosphere usable to produce a gas-phase fluxing of the support (20).