Method for soldering printed-circuit boards under low pressure

Metal fusion bonding – Process – With pretreating other than heating or cooling of work part...

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Details

228220, 228221, 228 42, H05K 334, B23K 120

Patent

active

054922658

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND AND SUMMARY OF THE INVENTION

The present invention relates to a method for soldering electronic components to printed circuit boards or other elements to be connected together by soldering.
The use of electronic modules, designed in the form of printed circuit boards populated with pluggable, mountable, or surface-mountable electronic components (Surface Mounted Devices, SMDs), which must then be soldered, is growing steadily. To manufacture these modules, suitably prepared printed circuit boards are populated with the corresponding components, these components being held in place possibly with an adhesive or soldering paste in preparation for the soldering process, and finally the printed circuit boards and the components are connected together permanently and conductively by a soldering process. This soldering process is currently performed, firstly, in a conventional manner with wave soldering systems, in which solder is applied to corresponding areas by one or more solder waves that contact the printed circuit boards from below, thus soldering the corresponding parts. In a second manner, components mounted with soldering paste, soldering preforms or even solder deposits require only a melting of the solder component of the paste or the solder (reflow soldering).
In both cases however, flux must be used for a good quality solder connection. Either the flux is applied prior to the actual soldering process, as in the case of wave soldering, or the soldering paste or soldering preform contains both solder and flux. Flux aids the soldering process primarily by destroying the metal oxide layer on the workpiece that adversely effects soldering, and prevents oxidation of the metals involved during the soldering process. In addition, the surface tension of the liquefied solder is reduced. Fluxes are therefore multifunctional and are accordingly comprised of a number of substances. However, after the soldering process, flux residues remain on the printed circuit boards and can lead to disadvantageous consequences. These include reduction of the insulation resistance between the traces on the printed circuit board as well as premature corrosion of the traces. For this reason, the flux residue must be removed. It is known that this can be done by treating the boards with chlorofluorocarbons or chlorocarbons to clean them after the soldering process. The use of these substances, which are Known to be environmentally hazardous (the chlorofluorocarbon ozone problem) should however be minimized or completely avoided today.
In a recent process, the necessity for fluxes is at least reduced by preceding the soldering of the boards with a pretreatment of the boards with a low-pressure plasma, this plasma being generated from a process gas using microwaves and/or glow discharge. This is shown in German Patent Document 39 36 955. After this pretreatment, however, the soldering process is still performed under normal pressure and under air or only a local, limited supply of protective gas. During the pause between pretreatment and soldering it therefore happens that since oxygen is not excluded, oxide deposition and/or oxide formation can occur at the soldered locations, which has a negative influence on the soldering results. For this reason, as a rule it is not possible in this known method to completely avoid using fluxes or additives that have the same effect.
An object of the invention is therefore to provide a soldering method that permits soldering populated printed circuit boards without using flux, with a low reject rate and high quality.
This object is achieved according to the present invention by providing that the soldering process and any additional previous, intermediate, or subsequent steps are performed under low pressure and under the plasma effect of a special process gas atmosphere. There are advantages to performing previous, intermediate, and subsequent processing steps, for example heating and cooling steps, under the same low pressure as the soldering process, namely a low pressure of less

REFERENCES:
patent: 4921157 (1990-05-01), Dishon et al.
patent: 5192582 (1993-03-01), Liedke et al.
patent: 5193739 (1993-03-01), Liedke et al.
patent: 5409543 (1995-04-01), Panitz et al.

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