Metal treatment – Compositions – Fluxing
Reexamination Certificate
1996-01-11
2002-07-23
Elve, M. Alexandra (Department: 1725)
Metal treatment
Compositions
Fluxing
C228S248100, C257S736000, C257S779000, C420S558000
Reexamination Certificate
active
06423154
ABSTRACT:
The invention relates to a method of solder-coating a metallic pad provided on a substrate, whereby at least the surface of the pad is provided with a deposit of solder paste, after which the substrate is at least partially subjected to a heating process, which heating process induces localised precipitation of an essentially continuous metallic solder layer out of the solder paste, which localised precipitation occurs on the pad but not upon the substrate surface outside the borders of the pad, subsequent to which any unwanted solder paste debris upon the substrate surface outside the borders of the pad is removed.
The invention also relates to a solder paste suitable for use in a method of this type.
Solder-coated metallic pads as hereabove referred to can be used as fixation-bases for the legs of electrical components. Commonly used printed circuit boards (and flexible printed circuit foils) comprise a substrate which is provided with a plurality of metallic pads arranged according to definite patterns and selectively interconnected via a network of conducting tracks. When the pads of such a circuit board have been solder-coated in the above-described manner, appropriate electrical components can be fixed to the board using a so-called “reflow” soldering process. This process entails placement of the components at pre-determined locations on the board, arrangement of their legs on the intended solder-coated metallic pads, and subsequent application of heat. The solder layer between each leg and the corresponding pad then melts, whereby the leg is soldered to the pad upon subsequent cooling. Further information on reflow soldering is given by C. Lea in “A Scientific Guide to Surface Mount Technology”, Electrochemical Publications Limited (1988), ISBN 0 901150 22 3, in particular Chapter 7.
Modern printed circuit boards (and foils) are often highly complex, and frequently involve a very large packing-density of components. This means that the metallic pads on such a board will often be present in very large numbers, and in very close proximity to one another. In this respect, it is advantageous that the selective solder-coating method as hereabove described can efficiently coat all the pads (and none of the substrate) in one go. This eliminates the need to provide solder on a tedious and meticulous pad-by-pad basis.
A solder-coating method as described in the opening paragraph, together with a solder paste for use therein, are elucidated in un-examined Japanese Patent Disclosure (Kokai) JP-A-2-310991 (Fukunaga et al.). According to this state-of-the-art document, a solder paste comprising a mixture of Sn powder and organic acid Pb salts is painted onto the metallic pads of a printed circuit board, without the need to confine the paste exclusively to the pads. The pasted circuit board is then exposed for several minutes to a temperature of about 220° C., whereby a localised chemical reaction occurs between the said Sn powder and Pb salts, with consequent precipitation of SnPb alloy out of the paste. This reaction and the said attendant precipitation occur only on the metallic surfaces of the pads, and not on the naked non-metallic substrate surface outside the borders of the pads.
Since the SnPb alloy precipitated on the pads in this manner is rather Sn-rich, a second process step is enacted, whereby a seed oil flax containing Pb ions is brought into contact with the alloy-coated pads, and subsequently heated. This induces a chemical reaction which enriches the Pb-content of the existing SnPb alloy, at the expense of its Sn-content. The final solder alloy composition should then have the desired ratio of Sn to Pb.
Appraisal tests conducted by the present inventors have shown this known method to have certain disadvantages, some of which can be summarised as follows:
(1) The employed organic acid Pb salts, such as lead naphthenate, are particularly toxic. Special precautions are therefore required to protect the health of process operators;
(2) Due to the relatively high temperatures and lengthy thermal exposure involved, the required heating process can result in serious discolouration of the printed circuit board, unless heating is conducted in a chemically inert atmosphere;
(3) Solder-coverage of the metallic pads can prove unsatisfactory unless the printed circuit board is etch-cleaned before painting it with solder paste. This adds a further step to the total coating process;
(4) The risk of unwanted short circuits caused by solder blobs on the metallic pads increases to unacceptable levels when the mutual separation of the metallic pads is 200 &mgr;m or less. In view of the modern trend in the electronics industry towards the use of multi-legged surface-mounted components with ever decreasing leg-pitches, such unacceptable solder-coverage places a lower limit on the leg-separations of components intended for use on solder-coated printed circuit boards produced by the known method.
It is an object of the invention to provide a solder-coating method as elucidated in the opening paragraph, without reliance on the use of highly toxic organic acid Pb salts. It is a further object of the invention that such a method should employ thermal processing conditions which are mild enough to impede serious substrate discolouration. Moreover, the invention aims at the satisfactory application of such a method at mutual pad separations smaller than 200 &mgr;m, and without the necessity of etch-cleaning pads prior to solder-coating them.
These and other objects are achieved in a method as described in the opening paragraph, characterised in that a solder paste is used which comprises a suspension of metallic solder particles which, when molten, have a surface energy lower than the critical surface energy of the metallic pad but higher than the critical surface energy of the substrate surface outside the borders of the pad, that the heating process causes the metallic solder particles within the paste lying upon the pad to melt and fuse together into an essentially continuous metallic solder layer, and that the metallic solder particles within any paste lying upon the substrate surface outside the borders of the pad do not fuse together into a layer but are instead deposited as mutually-isolated solder beads, which beads can be subsequently removed from the substrate surface after completion of the heating process.
In such a method, the metallic solder particles contained in the employed solder paste already have the chemical composition which is desired of the final solder coating. Once these particles have been deposited as a layer, therefore, such a layer win require no further enriching or other compositional modification. In contrast to the known method, there is thus no need to derive a Pb-content from toxic organic acid Pb salts, or from a separately-applied flax containing Pb ions.
In the inventive method, the mechanism by which a solder coating is caused to precipitate out of the solder paste is mediated by physical processes rather than chemical reactions. On the one hand, melted solder particles in the paste located on the metallic pad can successfully “wet” the pad's surface by amalgamating together into a thin film. This effect is attributable to the fact that the surface energy (surface tension) of the solder particles is lower than that of the pad, so that such “wetting” is energetically favourable. On the other hand, melted solder particles in any paste deposits located on the naked substrate surface outside the borders of the pad will not amalgamate together and “wet” the substrate surface, but will instead lie on it in the form of small mutually-isolated solder beads. This is due to the fact that the surface energy of the solder particles exceeds that of the naked substrate surface, so that “wetting” of such a surface by the particles is energetically unfavourable. For the purpose of further clarification and illustration only, such surface energy effects can be likened to those exhibited by rainwater deposited on the surface of a partially waxed automobile. Whereas
De Langen Michael T. W.
Van Gerven Johannes A. H.
Bartlett Ernestine C.
Elve M. Alexandra
Koninklijke Philips Electronics , N.V.
Stoner Kiley
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