Metal fusion bonding – Process – With subsequent treating other than heating of bonded parts...
Patent
1997-03-07
1998-09-29
Ryan, Patrick
Metal fusion bonding
Process
With subsequent treating other than heating of bonded parts...
228 37, 228 46, 228260, H05K 334
Patent
active
058135952
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention concerns a method for wave-soldering printed circuit boards in which the printed circuit boards are artificially cooled after passing through the solder wave.
BACKGROUND OF THE INVENTION
A method of this kind is disclosed in German Patent Specification DE 36 11 180 C1, in which an apparatus is described for cooling soldering workpiece carriers and soldered materials in soldering facilities by means of delivery of cool air. In the case of the known method, the printed circuit boards are cooled on their underside, after passing through the solder wave, with air from nozzles that are arranged on either side of the transport path. Smooth and also rapid cooling is thereby achieved, so that when cleaning subsequently occurs almost no cleaning agent evaporates; flux residues also essentially do not evaporate further.
It is moreover commonly known among those skilled in the art of wave-soldering printed circuit boards that solder globules settle onto the printed circuit boards during soldering; these are undesirable since they can lead to serious malfunctions in electrical or electronic devices equipped with printed circuit boards of this kind.
Attempts have therefore already been made to prevent the formation of solder globules early on, during wave-soldering the printed circuit boards themselves, by using special solder resists; however, modification of solder resists in order to eliminate solder globules on printed circuit boards is possible only to a limited extent, since the solder resists must retain the ability to adhere to the copper coatings of the printed circuit board. It is therefore not possible to manufacture solder resists that completely eliminate adhesion of solder.
At present, therefore, it is accepted that solder globules will settle onto printed circuit boards during wave-soldering of the printed circuit boards. Since these solder globules must not, however, remain on the printed circuit boards during assembly into an electrical or electronic device, the printed circuit boards are mechanically brushed off on the soldered side in a production step subsequent to wave-soldering. To this end, defined brush arrangements with specific brush materials are required, in order not only to reliably remove the adhesively attached solder globules from the solder resist on the printed circuit boards, but also to prevent damage to the printed circuit boards during this operation. The brushed printed circuit boards must then be subjected to a visual inspection to ensure that ultimately, only printed circuit boards that are free of solder globules are used.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method for wave-soldering printed circuit boards with which printed circuit boards free of solder globules can be manufactured.
According to the invention, to achieve this object in a method of the kind indicated initially, cooling of the particular printed circuit board to room temperature, achieving differential contraction of a solder resist mask usually located on the printed circuit board and of the solder applied onto the printed circuit board, occurs with an approximately constant temperature gradient of approximately 20 K/sec. In the method according to the invention, the wave-soldered printed circuit boards are therefore artificially cooled with the indicated temperature gradient after passing through the solder wave, which impedes the adhesion of solder globules onto the soldered side of the printed circuit boards.
The invention is based on recognition of the following correlations:
During wave-soldering of, in particular, mixed-population printed circuit boards, i.e. printed circuit boards that are populated with SMD (surface-mounted device) components and components equipped with connecting wires, the flow characteristics of the solder wave are substantially disturbed by the components and their connections. This causes the homogeneous solder wave to be separated by the flow impediments as it passes along
REFERENCES:
patent: 3605244 (1971-09-01), Osborne et al.
patent: 5388752 (1995-02-01), Kawakatsu
patent: 5685475 (1997-11-01), Jairazbhoy et al.
Soldering in Electronics! 1991, pp. 508-510.
Albrecht Hans-Jurgen
Strick Gunter-Werner
Knapp Jeffrey T.
Ryan Patrick
Siemens Aktiengesellschaft
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