Metal treatment – Compositions – Fluxing
Reexamination Certificate
2003-03-10
2004-04-27
Mai, Ngoclan T. (Department: 1742)
Metal treatment
Compositions
Fluxing
C075S252000, C148S025000, C420S561000
Reexamination Certificate
active
06726780
ABSTRACT:
This application claims priority to Japanese Patent Application No. 2002-077437, filed Mar. 20, 2002.
TECHNICAL FIELD
The present invention relates to a so-called lead-free solder which is capable of forming a fillet that can be utilized for improving the accuracy of inspection in the detection of a defective soldering of soldered portions of a wiring board by means of an automatic inspection apparatus which is designed to detect such a defective soldering by making use of a reflected beam. The present invention also relates to a solder paste comprising the powder of the lead-free solder.
BACKGROUND OF THE INVENTION
In recent years, concomitant with an increasing trend to multi-functionalize the wiring board of electronic devices as well as with an increasing trend of miniaturization, i.e. the reduction of the size and weight of electronic devices, a surface mounting technique has now been quickly advanced, and therefore, most of the surface mounting of electronic devices are now being performed by means of a reflow soldering method using a solder paste. As for the solder powder to be employed for this solder paste, an Sn—Pb-based powder is predominantly employed.
By the way, when electronic instruments are to be scrapped because of the reasons that the electronic instruments are no longer useful for instance, the electronic instruments may be disintegrated to recover some components thereof. However, at present, most of the substrates having electronic components mounted thereon are crushed into pieces and buried underground without being recovered, or otherwise may be discarded as they are, thus leaving them in fields.
Since the substrates discarded in fields in this manner are provided with electronic components which are attached to the substrates by means of soldering, if lead is contained in the solder, the lead may be turned into a soluble lead compound due to the effects of acid rain, etc., thus not only polluting the natural environments but also raising probabilities that water or animal and vegetable foods which are contaminated through underground water, etc. may be taken into human body. Since the toxicity of such contaminated water and foods is strong, it may raise a serious problem.
With a view to avoid such problems mentioned above, solder materials which are free from lead have been developed, and so-called lead-free solder powder made of an Sn—Ag alloy, an Si—Ag—Cu alloy, etc. is now increasingly employed.
However, since the lead-free solder powder mentioned above is relatively high in melting point, i.e. about 200° C. to 220° C., it is required, in the reflow soldering method employing a solder paste containing the aforementioned lead-free solder powder, to set the peak temperature to 230° C. to 240° C. on the occasion of heating the solder paste. Therefore, there is a problem in the employment of the lead-free solder that the properties of electronic components to be soldered may be deteriorated due to the thermal damage thereof to be caused by the soldering. By contrast, in the case of reflow soldering method where a solder paste containing a lead-based solder powder such as an Sn—Pb alloy powder is employed, it is possible to set the peak temperature to about 230° C. on the occasion of heating the solder paste because of the fact that the melting point of the eutectic composition thereof (63Sn/37Pb) is as low as 183° C., thereby enabling the soldering to be performed without damaging the electronic components which are vulnerable to heating.
In order to avoid the problems mentioned above, it is desired to develop a lead-free solder whose melting point is as low as possible. With a view to meet such demands, there have been proposed various kinds of lead-free solder such as an Sn—Ag—Cu-based lead-free solder (Japanese Patent No. 3,027,441 and U.S. Pat. No. 5,5276,28); an Sn—Ag—Cu—Bi-based lead-free solder (U.S. Pat. No. 4,879,096); etc. In Japan, there is an increasing trend at present to mainly employ an Sn—Ag—Cu-based lead-free solder.
By the way, on the occasion of mounting electronic components on a wiring board by means of a reflow soldering method, procedures are taken wherein a solder paste is coated at first on a solder land of the wiring board, a chip-like electronic component is temporarily secured to the wiring board by making use of the adhesive force of the coated paste in such a manner that the electrodes or leads of the electronic component are positioned in alignment with the solder land, and then, the solder paste is heated to melt the solder powder in the coated film of the solder paste to thereby perform the soldering. Alternatively, a solder paste is coated at first on the through-holes provided in the wiring board, and after the leads of an electronic component are inserted into the through-holes, the solder paste is heated in the same manner as described above to thereby perform the soldering.
The aforementioned soldering of an electronic component is required to be performed on a large number of locations of a single wiring board, and moreover, since 1005 pieces of chips for instance each being very small (1 mm in length and 0.5 mm in width) and light-weight are required to be soldered onto a narrow solder land, i.e. as the density of electronic components to be mounted on a single wiring board is increased further, it becomes more difficult to perform the soldering without generating defective soldering. Further, this defective soldering is versatile in features, it includes, for example, the lifting of pin (a phenomenon wherein the pin of lead is lifted up without being soldered onto the solder land), wettability failure (a phenomenon wherein a fused solder fails to fully spread all over the surface required to be coated, thus permitting the soldering to be performed only partially), solder bridge (a phenomenon wherein the space between soldering lands is bridged with a solder), solder ball(a phenomenon wherein fused solder is exuded outside the solder land and formed into a ball on the surface of a solder resist film), abnormal shaping of solder fillet (a phenomenon wherein the fillet of solder at the soldered portion is distorted), the dislocation of electronic component, and Manhattan phenomenon (a phenomenon wherein one side of an electronic component is caused to rise due to a difference in surface tension of fused solder acting on the electrodes of electronic component and on the soldering lands until the fused solder is solidified). Since these phenomena may give rise to the generation of short circuit of wiring, and to the detachment of the electronic components due to the failure of the solder to obtain a predetermined bonding strength. Therefore, when the wiring board having electronic components soldered thereon is to be delivered as a product, all of the soldered portions are subjected to inspection so as to see if there is any defective soldering.
Although this inspection can be performed by way visual inspection, this inspection is now generally performed by making use of an automatic inspection apparatus in order to improve the inspection efficiency and the inspection precision. As for the specific methods of the inspection that are adopted at present, there are known, for example, an X-ray transmission system, an ultrasonic flaw detector, a laser system, etc. Among them, the laser system is performed by making use of an image inspection apparatus wherein an LED laser (laser to be derived from a light-emitting diode) is irradiated against the fillet of the soldered portion or the periphery thereof so as to measure the reflectance of the reflected beam. Since the fillet generally exhibits metallic luster or so-called mirror luster and is semi-circular in longitudinal cross-section, it is more likely that the area of dark portion of the image (the portion where irregular reflection is not generated) relative to the area of whitish portion (the portion where irregular reflection is caused to generate) of the image would become relatively large, so that if the ratio of the whitish portion relative to the entire fillet i
Fujimori Kenji
Iwabuchi Mitsuru
Koyahara Hiroaki
Ono Takao
King & Schickli PLLC
Mai Ngoclan T.
Tamura Kaken Corporation
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