Process for selective soldering

Details Process for selective soldering Process for selective soldering

1999-02-26

2001-12-11

Dunn, Tom (Department: 1725)

Metal fusion bonding

Process

Preplacing solid filler

C228S180220, C228S225000

Reexamination Certificate

active

06328200

ABSTRACT:

The present invention relates to the selective formation of mouldings of solder material on a substrate, wherein the surface of the substrate is covered with a template.
Template soldering processes are basically known in connection with the production of circuit boards that are used in SMD (Surface-Mounted Device) technology. The templates that are used in this technology serve, on the one hand, for positioning solder deposits at the desired points on the surface of the circuit board and, on the other hand, for defining the shape of the solder deposits via the template openings which are designed to correspond to the shape of the solder deposits. With a view to filling the deposit spaces pertaining to the template which are formed by the template openings, the solder material is frequently applied in pasty form onto the surface of the template and is filled into the deposit spaces by means of a suitable displacement device. After the deposit spaces have been filled with solder material the template is removed. Remelting of the solder deposits with a view to establishing an electrically conductive and mechanically secure connection of components on the circuit board is effected only after the appropriate components with their supply leads have been inserted into the still pasty solder deposits. In this process the shape of the remelted solder deposits is unimportant, as long as the aforementioned electrically conductive and mechanically secure connection is guaranteed.
In contrast with the aforementioned solder deposits that are mainly used in the field of SMD technology, raised contact metallisations, which are also known in technical language by the term “bumps” and which are formed from mouldings of solder material connected to terminal areas of a substrate, also perform a spacing function. Bumps, which serve as a rule for direct connection of components via the terminal areas thereof, exhibit to this end a raised formation projecting above the surface of the components. Performance of the spacing function furthermore presupposes a substantially unyielding formation of the bumps. Therefore, as distinct from solder deposits, bumps have already been remelted and solidified at the time of their contacting, whereas, as already explained above, the solder deposits are remelted only after the contacting process.
By reason of the spacing function explained above, particular importance is assumed by the shaping process in the course of the formation of bumps. With a view to the shaping of bumps it is known to utilise the liquid meniscus which forms in the melted state substantially independently of the solder material and which, as a result of the surface tension of the melted solder material, defines the shape in which the solder material solidifies after cooling. With the known processes for producing bumps that have a meniscus shape the solder material is applied in the already molten state onto the terminal areas to be wetted. This proves to be very cost-intensive, particularly with a plurality of terminal areas distributed over an area.
With other known processes for forming bumps on terminal areas of a substrate use is made of galvanic or chemical deposition processes which require the formation of an irrecoverable mask with mask openings above the terminal areas on the surface of the substrate. For instance, in the case of the galvanic deposition process it is conventional to arrange a mask, which is formed from a photoresist, on the surface of the substrate, the photoresist mask being pulled off after formation or deposition of the bumps on the terminal areas and thereby becoming unusable. Therefore with the known processes using masks or templates it is necessary to form a new template or mask for each substrate to be provided with contact metallisations.
The object underlying the present invention is to propose a process for the selective formation of mouldings of solder material on a substrate, said process enabling selective formation of the mouldings of solder material with comparatively little effort.
This object is achieved by means of a process having the features of Claim
1
or Claim
2
.
With the process according to the invention as specified in Claim
1
for the selective formation of contact metallisations on terminal areas of a substrate, the surface of the substrate is covered with a template in such a manner that template openings forming deposit spaces are arranged above the terminal areas and subsequently the deposit spaces are filled with a solder material. The contact metallisations are formed from the fused solder material immediately afterwards, in the deposit spaces that inhibit wetting or that are non-wettable at least in regions of contact with the solder material.
With the process according to the invention the use of a template in the course of application of the solder material on the terminal areas and the fusing of the solder material which is filled into the deposit spaces when the template is still arranged on the surface of the substrate enable use to be made of solder material having a very largely arbitrary consistency, since the positioning of the solder material in relation to the individual terminal areas is defined by the template, at least for such time until, as a result of the formation of a liquid meniscus by the molten solder material on the wettable terminal areas, on the one hand an adhesion of the solder material on the terminal areas and, on the other hand, a dimensional stability of the contact metallisations is achieved. The non-wettable formation of the contact regions of the deposit spaces ensures that detachment of the template from the surface of the substrate is made possible already in the fused state of the contact metallisations or only after the solidification thereof, without it being possible for this to be impaired by an adhesion of the contact metallisations to the template.
In contrast with the “irrecoverable” templates that are used in the deposition processes described above, after it has been detached from the surface of the substrate the template that is used in the process according to the invention can be re-used in connection with the selective formation of contact metallisations on a subsequent substrate. Furthermore, detachment of the template after the contact metallisations have been fused enables immediate contacting of the substrate that is provided with contact metallisations in this manner, for example in the flip-chip process, without there being any need for a template, consisting for example of a photoresist lacquer, to be eliminated beforehand in elaborate manner by means of an etching process or such like. There are no particular restrictions as regards the template material, as long as the regions of contact with the solder material have a surface that inhibits wetting or that is non-wettable. This can also be effected by means of appropriate coatings in the contact regions. Rigid plastics are suitable in principle by way of template material, as are flexible films. Moreover, the use of semiconductor materials, for example, particularly the use of anisotropically etched semiconductors, is also possible.
Besides the process as set forth above for the selective formation of mouldings of solder material that are connected to terminal areas and designated here as contact metallisations, it is also possible for the idea of using a template for the application of solder material onto a substrate surface as a positioning aid during the fusing of the solder material to be applied to the formation of freely manageable mouldings of solder material that are not connected to terminal areas. With the further process according to the invention as specified in Claim
2
, the surface of a substrate is covered with a template in such a manner that template openings forming deposit spaces are arranged above non-wettable deposit areas and the deposit spaces are filled with a solder material. Subsequently fusing of the solder material is effected with a view to forming the mouldings of solder material in the

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