Alloys or metallic compositions – Lead base
Patent
1983-10-31
1985-04-23
O'Keefe, Veronica
Alloys or metallic compositions
Lead base
420557, 420558, 420566, 420570, 751225, B32D 1502
Patent
active
045129509
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to Pb alloys suitable for use as a soft solder in assembling various parts to fabricate not only ordinary ICs but also VLSIs such as large-capacity memories including 64 KRAM and 256 KRAM.
BACKGROUND ART
An IC with ceramic package, which is shown in cross section in FIG. 1, is extensively used in semiconductor devices. This ceramic package consists of a ceramic case 1 with a cavity of a predetermined configuration, a silicon chip 2 soldered on the bottom of this cavity, aluminum bonding wires 3 and a sealing plate 4 also soldered on the top of the ceramic case 1. This ceramic package is assembled by die-bonding silicon chip 2 and sealing plate 4 with Au-Si alloy or Au-Sn alloy. Solders of these Au alloys ensure high device reliability, but on the other hand, they are costly.
DISCLOSURE OF INVENTION
We have therefore made various studies to develop a solder that ensures high device reliability comparable to the conventional Au alloys and which yet is less expensive to the latter. As a result, we have obtained a Pb alloy that contains 1 to 65 wt% of Sn or In or both, and optionally 1 to 10 wt% of Ag, the balance being Pb and incidental impurities which include less than 50 ppb (ppb stands for parts per billion) of radioisotopes and the count of radioactive alpha-particles being not more than 0.5 CPH (counts/hr)/cm.sup.2. When this Pb alloy is used as a solder in assemblying semiconductor devices, it exhibits good wettability and adhesive strength, and at the same time, it ensures an extremely high reliability without causing any memory error. As a further advantage, the cost of this alloy is very low since it contains no precious metals such as Au.
The present invention is based on this finding, and the criticality of the contents of the respective components is as follows.
(a) Sn and In
These components provide a soft solder with good wettability and ensure high adhesive strength. If the content of each element is less than 1 wt%, their desired properties are not obtained. If their content exceeds 65 wt%, no corresponding improvement is achieved by their additions. Therefore, Sn or In or both are used in an amount ranging from 1 to 65 wt%.
(b) Ag
This element is effective in providing further improved wettability and adhesive strength and may be contained if particularly high values of these properties are required. However, if the Ag content is less than 1 wt%, its desired properties are not obtained. If the Ag content exceeds 10 wt%, no further improvement is achieved by its addition. Therefore, from an economical viewpoint, Ag is contained in an amount ranging from 1 to 10 wt%.
(c) Radioactive isotopes as incidental impurities
Lead ingots produced by the conventional refining method contain at least 50 ppb of radioactive isotopes such as uranium and thorium, and this amount is equal to several to several hundred CPH/cm.sup.2 in terms of the count of radioactive alpha-particles. If Pb alloys containing Pb of such high radioactive isotope contents are used as a soft solder in the assembling of semiconductor devices, it emits radioactive alpha-particles which will cause memory errors and reduce the reliability of the devices. Therefore, in order to avoid this deleterious effect, the content of radioactive isotopes as incidental impurities must be reduced to less than 50 ppb, thereby reducing the count of radioactive alpha-particles to not more than 0.5 CPH/cm.sup.2.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a schematic cross section of the conventional IC ceramic package.
BEST EMBODIMENT OF CARRYING OUT THE INVENTION
Example
The advantages of the soft solder of Pb alloy of the present invention are hereunder described in comparison with the conventional products.
Melts of Samples No. 1 to No. 12 and comparative samples Nos. 1 to 12 of the Pb alloy having the formulations indicated in Table 1 were prepared by the conventional melting process. The melts were cast into ingots under conventional conditions and rolled into sheets 0.05 mm. The sheets wer
REFERENCES:
patent: 2711484 (1955-06-01), Knapp et al.
patent: 2811721 (1957-10-01), Fritts et al.
patent: 2883330 (1959-04-01), Teitel
patent: 4396677 (1983-08-01), Intrater et al.
ASM, Metals Handbook, 9th Ed., vol. 6, Soldering, Tables V and VI, pp. 1069-1076, Am. Soc. Metals, Metals Park, Ohio, 44073, 1983.
Hosoda Naoyuki
Kawanaka Ryusuke
Uchiyama Naoki
Mitsubishi Denki & Kabushiki Kaisha
Mitsubishi Kinzoku Kabushiki Kaisha
O'Keefe Veronica
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