Metal fusion bonding – Process – With pretreating other than heating or cooling of work part...
Reexamination Certificate
2002-07-18
2004-03-30
Elve, M. Alexandra (Department: 1725)
Metal fusion bonding
Process
With pretreating other than heating or cooling of work part...
C148S023000, C148S024000, C148S025000, C148S026000, C228S206000, C228S205000, C228S223000
Reexamination Certificate
active
06712262
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a water-soluble preflux containing a water-soluble salt of a benzimidazole-based compound, which can improve the wetting coverage of fused solder during soldering in particular, a printed circuit board having a film of such a preflux formed thereon, and a process for the surface treatment of metals in printed circuit boards.
BACKGROUND OF THE INVENTION
A printed circuit board, comprising a copper-clad laminate board as an example and a printed circuit pattern formed thereon, is designed to form one single circuit unit by mounting electronic parts thereon.
For such a printed circuit board, there is often used a surface mount type that comprises a circuit pattern with circuit interconnections formed on the surface of a board and electronic parts mounted on the circuit pattern. For instance, when a chip part having electrodes at both ends is used as an electronic part, a flux is first coated on the printed circuit board, and the chip part is then soldered to the printed circuit board by means of a wave soldering process or a reflow soldering process.
For soldering of such chip parts, there is generally used a soldering process wherein after a circuit pattern is formed on a printed circuit board, i.e., a copper foil portion of a copper-clad laminate board is etched in such a way that the desired circuit interconnections are obtained, a solder resist film is coated on the obtained circuit except a portion thereof to which a chip part is soldered, i.e., so-called soldering lands, and micro etching is further carried out, immediately after which soldering is performed. In some cases, however, the steps including coating by a solder resist film and the subsequent soldering step are independently carried out, so that the soldering step can be performed after the solder resist film-coated printed circuit board is stored as a part or by other soldering experts in the process of circulation. In such a case, much time has lapsed until the soldering step is actually carried out. In this period, the exposed copper foil surface of the soldering lands is susceptible to oxidation by air, and this oxidation is more likely to occur in damp environments. To prevent oxidation of that surface, an oxidation-preventive film is formed thereon and a surface protective agent is used to this end.
Even when a printed circuit board coated with a solder resist film is used just after its fabrication, an oxidation-preventive film should be formed. This is because when a reflow soldering process, for instance, is used to mount electronic parts on both surfaces of a printed circuit board, solder powders are fused by heating at high temperatures such as 260° C. after a solder paste is coated on the soldering lands and, consequently, when one surface is soldered, other surface is exposed to high temperatures at which the copper foil surface of the soldering lands is susceptible to oxidation. In such a case, too, an oxidation-preventive film should be formed.
Either when such surface-preventive agent treatment is carried out or when the other surfaces of the soldering lands are subjected to oxidation-preventive treatment so as to prevent deterioration upon heating which may otherwise be caused in association with the soldering of one surface of the printed circuit board, so-called prefluxes are used, among which organic solvent-free, water-soluble ones are preferably used because of no risk of pollutions or fires.
So far, the exposed copper foil of soldering lands has been subjected to rust-preventive treatment. As set forth typically in Japanese patent unexamined publication No. 05-25407 and No. 05-186888, it has been known that water-soluble surface-preventive agents for printed circuit boards containing a benzimidazole-based compound are used as water-soluble prefluxes. According to these publications, a printed board with a copper foil circuit pattern formed on its surface is immersed in the water-soluble surface-protective agent for printed circuit boards to, form a heat-resistant film on the surface of copper and a copper alloy of the circuit pattern on the printed circuit board. This film has also extremely satisfactory humidity resistance even upon exposed to high humidity; it is considerably improved in terms of protection of printed circuit boards and solderability at the time of part mounting. Thus, these surface-protective agents are found to be more improved in terms of productivity, performance, etc., as compared with prefluxes containing rosin or the like, which cause coating films to be formed on portions other than copper foil portions, and make it impossible to give high reliability to circuits without rinsing or pickling boards after parts are mounted thereon.
The compound used to improve this surface-preventive agent preflux, i.e., the benzimidazole-based compound that is the primary component of the surface-preventive agent for printed circuit boards is generally insoluble in water, and so is made soluble by using inorganic acids such as hydrochloric acid and phosphoric acid or organic acids such as acetic acid, oxalic acid and p-toluene-sulfonic acid as solubilizers for forming its water-soluble salt. The acids used as such solubilizers erode a particularly sensitive-to-acid lead film to render its thickness small or oxidizes its surface, often rendering it impossible to obtain the given soldering strength for lead wires at the time of part mounting and poor soldering as well, which may otherwise give rise to some considerable mounting yield drops. Allegedly, this problem may be solved by using an amino acid as the solubilizer thereby making benzimidazole-based compounds soluble in water without recourse to the aforesaid acids, as set forth in Japanese patent unexamined publication No. 05-186888.
Even when water-soluble prefluxes composed primarily of these benzimidazole-based compounds are used, however, much is still left to be desired in terms of the wetting coverage of fused solder when a solder paste is coated on soldering lands and heated to fuse solder powders.
As recent electronic parts slim down with enhanced performance, printed circuit boards in particular are now required to have performance keeping pace with high-density packaging for mounting electronic parts at high density. With this, it is demanded to enable electronic parts to be well soldered to a soldering land array having a much narrower spacing of the order of 0.3 to 0.4 mm. As a solder paste is coated and heated on a soldering land array having a narrow spacing, however, some portions of fused solder formed by fusion of solder powders come into contact with each other on the respective soldering lands. These portions, if they are cooled down as such, often form together a soldering bridge that offers a short-circuit problem, ending up with poorly soldered, defective board products. For those who are required to achieve fierce production cost reductions, it is thus of vital importance to cut back on the incidence of soldering bridges that may occur upon fusion of the aforesaid solder paste film, thereby achieving productivity improvements.
From this point-of-view, there is thus available a process wherein a smaller amount of solder paste than usual is coated on a portion of a soldering land except both its end portions rather than all over the soldering land in such a controlled way that when solder powders are fused and concentrated centrally on that portion into a liquid droplet, the size of the liquid droplet is kept from becoming excessively large, whereby fused solder on that portion is prevented from contact with fused solder similarly formed on the adjacent lands. In this case, however, the reduced amount of solder presents another grave problem in that any desired soldering strength is not obtained.
The first object of the present invention is to provide a water-soluble preflux that enables fused solder to be well wetted to and spread over a soldering land array in general and a soldering land array having a narrow spacing in particular, a printed circuit board,
Akaike Shinichi
Nakanami Kazutaka
Ono Takao
Takahashi Yoshiyuki
Elve M. Alexandra
Ildebrando Christina
King & Schickli PLLC
Tamurakaken Corporation
LandOfFree
Water-soluble preflux, printed circuit board, and process... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Water-soluble preflux, printed circuit board, and process..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Water-soluble preflux, printed circuit board, and process... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3208030