Metal fusion bonding – Process – Plural joints
Patent
1995-08-02
1997-02-11
Heinrich, Samuel M.
Metal fusion bonding
Process
Plural joints
228209, 228253, 148 24, 437183, H05K 334, B23K 3534
Patent
active
056012284
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a solder-precipitating composition and a mounting method using the composition, and more particularly to a composition and a mounting method using the composition, which are capable of forming, in a relatively short time, an uniform and relatively-thick solder layer on each of conductive portions (pads) of a circuit board, and pre-coating solder without creating a solder bridge even when the distance between the conductive portions is narrow.
BACKGROUND ART
A conventional solder composition (solder paste) used in a surface mounting technique is in the form of a paste which is obtained by dispersing, in a flux, solder powder as Sn-Pb alloy, and mixing necessary additives therein.
To mount an electronic device on a circuit board using a solder composition, the solder composition is put on a pad which serves as a mounting portion, and the device is placed on the board such that the terminal of the device is positioned on the solder composition. Then, the resultant structure is heated to melt the solder composition and electrically connect the device to the pad.
In the above structure, the device can be electrically connected to the pad in a reliable manner if they are sufficiently separated from each other. However, where they cannot sufficiently be separated in a high-density mounting, it is possible that portions of solder are connected to each other at the time of putting the solder composition on the pad or melting the same, thereby forming a solder bridge. When the solder bridge has been formed, short-circuiting will occur which degrades the circuit function. Moreover, repairing the solder bridge requires high cost.
As a solder-precipitating composition free from the above problem, U.S. Pat. No. 5,145,532 (Jpn. Pat. Appln. KOKAI Publication No. 1-157796) discloses a solder-precipitating composition which contains Sn powder and an organic acid salt of Pb.
This solder-precipitating composition has the following solder deposition mechanism:
1) The organic acid salt of Pb is separated into organic acid ions and Pb ions by heating;
2) Pb ions are displaced with nonionic Sn which has an ionization tendency higher than Pb, with the result that Pb ions are reduced to nonionic Pb and part of nonionic Sn is oxidized to Sn ions; and
3) The reduced Pb is dispersed into the other part of Sn (i.e. its nonionic portion), thereby forming a solder.
It was found that the above-described solder precipitation process shows behaviors which significantly differs from those shown in the case of melting solder at its melting point. For example, in the case of pre-coating pads of a printed circuit board with solder, using the conventional solder composition, adjacent portions of solder paste may be connected to each other after heating unless an appropriate amount of solder paste which is proportional to the area of the pad is supplied, thus forming a solder bridge. On the other hand, in the case of the solder-precipitating composition disclosed in U.S. Pat. No. 5,145,532, it is not necessary to supply the solder-precipitating composition by the printing method using a stencil corresponding to a pad pattern. This solder-precipitating composition is characterized in that even if the solder-precipitating composition is supplied on the overall surface of the board, solder is coated only on conductive pads, and no solder bridges are formed. This feature has now been highlighted under the circumstances that high-density circuit boards have been developed in accordance with downsizing of electronic parts. In such high-density circuit boards, it is extremely difficult to supply a solder composition by the printing method so as to avoid occurrence of solder bridges. In the solder-precipitating composition of U.S. Pat. No. 5,145,532, however, the above-described solder precipitation mechanism exerts on each Sn particle. It has been found that in Sn powder having a certain particle size distribution, the smaller the diameter of a particle and the higher the surface area ratio of the same, the
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Fujiwara Takahiro
Fukunaga Takao
Higasa Kazuhito
Katayama Noriko
Kumamoto Seishi
Harima Chemicals Inc.
Heinrich Samuel M.
The Furukawa Electric Co. Ltd.
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