Metal fusion bonding – Process – Plural joints
Patent
1985-03-12
1986-07-08
Godici, Nicholas P.
Metal fusion bonding
Process
Plural joints
228214, 228260, 228 37, 228 40, 118422, H05K 334, B23K 108
Patent
active
045988587
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for protecting the surface of a soldering bath and for removing metal oxides in accordance with the generic part of claim 1.
When printed circuit boards are soft soldered by machine, efficient production depends to a large extent on a good soldering bath. In order to protect the surface as much as possible from oxidation and to remove the oxidation products of the liquid solder that are generated primarily in the soldering wave, simple fats, vegetable or mineral oils, esters in the form of oils, or organic acids dissolv in mineral oil have been used as covering means for the soldering bath. The effect of these known covering means on the production process is, however, unsatisfactory, as they either thicken after a short time of use so that they can no longer be used or contaminate the soldering machines with resinous residues. For this reason the covering means must be replaced frequently and the cleaning expense is very high.
It is the purpose of the present invention to create a method for protecting the surface of a soldering bath and removing metal oxides in accordance with the generic part of claim 1, which substantially extends the useful life of the covering means and reduces the material, cleaning and maintenance expenses for the soldering machines as well as the corresponding repair and down times.
This purpose is achieved by the characteristics of claim 1. Solutions of additional partial problems are indicated in the subsequent claims.
The method of the invention is very economical because the useful life of the covering means is several times longer than that of the known covering means, because the time and material required for replacing the covering means are reduced, because of the absence of resinous deposits in the soldering machine and because the viscosity of the covering means does not increase substantially during its use.,
The method is explained in more detail with the aid of a drawing.
The only FIGURE of the drawing shows schematically a soldering machine consisting of tank 1 for the solder 2 and a covering means for the solder 2, which covering means consists of bottom phase 3 and top phase 4, a duct 5, connected to a pump (not shown in the drawing), for generating a soldering wave 6 and a collecting tank 7. Through the overflow 8, which is located at some distance above the limit of the bottom phase 3 of the covering means, top phase 4 of the covering means can overflow into collecting tank 7. Space 9 is provided at the bottom of collecting tank 7 for accumulating the metal soaps generated in the soldering process. Higher up there is opening 10 of pipe 11, through which top phase 4 is recirculated by means of pump 12 and pipe 13 leading back to tank 1. A printed circuit board 14 is being soldered in the soldering machine.
A heating device (not shown in the drawing) for heating solder 2 is provided in tank 1, which heats the solder to a soldering temperature of, for example, 250.degree. C. In order to protect liquid solder 2 and to remove its oxidation products, provided on top of said solder is bottom phase 3, which is solid at room temperature but liquid at the temperature of liquid solder 2, and liquid top phase 4 of the covering means.
In the method of the invention, phases 3 and 4 of the covering means are separated from each other. The components of the two phases 3 and 4 are not soluble in each other. Bottom phase 3 of the covering means has a higher specific gravity than top phase 4. Bottom phase 3 of the covering means consists of a chemically active component with low vapor pressure, which is able to bind metal oxides of the metal being soldered, and in particular those of solder 2. Organic acids, unmixed or mixed, or their esters, particularly sebacic acid or suberic acid, are suitable for this purpose. They are used in sufficient quantity to ensure that the surface of solder 2 is completely covered by a continuous layer. The amount of non-consumed suberic acid which might be brought in with the flux is not sufficient for this purpose.
REFERENCES:
patent: 1719512 (1929-07-01), Krembs
patent: 2026598 (1936-01-01), Taylor
patent: 3914481 (1975-10-01), Bostrom
patent: 4171761 (1979-10-01), Boldt
Hershberger, Immersible Solder Wave System, IBM Technical Disclosure Bulletin, vol. 20, No. 6, 11/1977.
Fehr Peter
Pisinger Milos
Stratil Tomas
Godici Nicholas P.
LGZ Landis & Gyr Zug AG
McKee C.
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