Stock material or miscellaneous articles – All metal or with adjacent metals – Composite; i.e. – plural – adjacent – spatially distinct metal...
Reexamination Certificate
2002-02-07
2004-02-10
Koehler, Robert R. (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Composite; i.e., plural, adjacent, spatially distinct metal...
C420S557000, C420S558000, C420S560000, C428S648000
Reexamination Certificate
active
06689488
ABSTRACT:
BACKGROUND OF INVENTION
1. Field of Invention
The present invention relates to lead-free solder, more particularly to lead-free solder suited for a ball-grid array and the like. In addition, the present invention relates to a solder joint having improved thermal fatigue property.
2. Description of Related Art
The binary Sn—Ag alloy used as the lead-free solder has a eutectic point of Ag=3.5 mass % at temperature of 221° C. The binary Sn—Ag alloy having the eutectic composition has such a structure that the Sn—Ag
3
Sn eutectic disperses in the grain boundaries of Sn primary crystals.
The known Sn—Ag based alloy includes Sn-3.5% Ag-0.7% Cu, that is, 0.7% of Cu is added to the eutectic Sn-3.5% Ag. Allegedly, the melting point of the Sn-3.5% Ag-0.7% Cu is 217-219° C. and is hence lower than that of the binary Sn—Ag alloy.
It is described in U.S. Pat. No. 5,527,628 that the Sn-4.7% Ag-1.7% Cu corresponds to the eutectic composition at approximately 217° C. In this patent, it is proposed that the Sn, Ag and Cu contents are shifted from the eutectic composition in such a range that the melting point does not become higher than the eutectic point by 15° C. or more. It is described that at least two inter metallic compounds, for example, Cu-rich Cu
6
Sn
5
and Ag-rich Ag
3
Sn, are finely dispersed in the &bgr; Sn phase, and increase the strength and fatigue strength of the alloy.
The present assignee and Toyota Central Research Co., Ltd. have developed a highly reliable Sn-2.5Ag-3.0Bi-1.0In-0.2Cu (5
th
Symposium on “Microjoining and Assembly Technology in Electronics” Feb. 4-5, 1999, pages 403-408). This alloy has melting point of from 202 to 216° C. and improved resistance against thermal fatigue.
Since Bi has an outstanding effect to lower the melting point of Sn—Ag based alloy and to strengthen the alloy, Bi is frequently added into the Pb-free Sn—Ag based solder alloy (for example, Japanese Patent No. 2805595, Japanese Unexamined Patent Publications (kokai) Nos. 8-132277, 8-187590, 8-206874 and 10-34376) When the solder joint undergoes thermal fatigue, the stress is repeatedly imparted to the joint portions due to the difference in the coefficients of thermal expansion of the constructional parts of the solder joint. “Materia” Vol. 38, No. 12 (1999) pages 942-946 reports research on evaluation of the thermal fatigue property by a tensile test method under varying stress speeds. According to this report, a composition similar to the Sn-2.5Ag-3.0Bi-1.0In-0.2Cu alloy mentioned above is tested, and improved resistance of this alloy against thermal fatigue is confirmed.
Japanese Domestic Publication (tokuhyo) 2001-504760 of PCT/US98/02022 (International publication WO98/34755 dated Aug. 13, 1998) describes that, since the ternary eutectic Sn-4.7% Ag-1.7% Cu incurs disadvantageous growth of the intermetallic compound layer, Ni, Co, Fe and the like are added to this alloy. When a Cu conductor is soldered with the Sn-based solder, an intermetallic compound layer, such as a Cu
6
Sn
5
layer, is thin at the interface. According to the theory proposed in this domestic publication, the solidified intermetallic compound layer is thin due to the addition of Ni and the like. Ni and the like modify the shape of the intermetallic compound layer in such a manner as to suppress its growth.
Minute balls of 0.1 to 1.2 mm in size, referred to as the BGA balls, are used in electric bonding of electronic parts. Since the BGA balls are appropriate for the multi-pin bonding method, they are being used more and more in recent years. The Sn—Ag based solder free of lead is used for the BGA ball solder as well. In this bonding method using the BGA balls, the difference in the coefficient of thermal expansion of the constructional parts induces stress in the bonding portions of the solder balls and the Ni or Cu land.
The thermal fatigue fracture of solder occurs both in the bulk of the solder and the bonding interface. “Materia” Vol.38, No.12(1999) pages 942-946 elucidates the former thermal fatigue fracture. Meanwhile, there is a published investigation that, since Sn-3.5Ag-5Bi is strengthened by means of Bi addition, the fatigue fracture is attributable to the intermetallic compounds in the bonding interface (Surface Mounting Pocket Handbook, Lead-Free Solder Technique, edited by Tadatomo Suga, Apr. 28, 2000, first edition, second print, pages 90-91). The present inventors researched the thermal fatigue of BGA balls and discovered that the thermal fatigue of BGA balls is related to the growth process of the intermetallic compound, which is formed by a reaction which occurs between the solder and the land during the solder bonding in the joining regions
SUMMARY OF INVENTION
In the BGA bonding, since horizontal stress is imparted to a narrow bonding region of a solder ball at the circumferential surface of the bonding portion, initial thermal fatigue is very liable to occur. More specifically, this stress accelerates the diffusion of Ni or Cu into the solder bulk, which Ni or Cu is contained in high concentration in the bonding region formed during the solder bonding. The intermetallic compound grows, therefore, thicker than that formed at the bonding, with the result that fatigue fracture occurs in the formation region of the intermetallic compound.
It is, therefore, an object of the present invention to suppress the Ni or Cu diffusion from the bonding interface into the bulk of the solder and to provide a lead-free solder alloy of BGA balls having improved resistance against thermal fatigue.
It is also an object of the present invention to suppress the Ni or Cu diffusion from the bonding interface into the bulk of the solder of BGA balls and to provide a lead-free solder joint having improved resistance against thermal fatigue.
In accordance with the objects of the present invention, there is provided a lead-free solder, which contains, by mass percentage, from 1.0 to 3.5% of Ag, from 0.1 to 0.7% of Cu, and from 0.1 to 2.0% of In, the balance consisting of unavoidable impurities and Sn. This composition may be referred to as the first composition.
There is also provided a lead-free solder, which contains, by mass percentage, from 1.0 to 3.5% of Ag, from 0.1 to 0.7% of Cu, from 0.1 to 2.0% of In, and at least one element selected from the group consisting of from 0.03 to 0.15% of Ni, from 0.01 to 0.1% of Co, and from 0.01 to 0.1% of Fe, balance consisting of unavoidable impurities and Sn. This composition may be referred to as the second composition.
In accordance with the objects of the present invention, there is provided a solder joint, which comprises: an Ni-based conductor; a bulk of lead-free solder, which has the first or second composition and has a structure essentially consisting of an Sn matrix with Cu solute and an Sn—Ag eutectic structure, in which a Cu—Sn based intermetallic compound and an Ag—Sn intermetallic compound are dispersed; and, an Ni—Sn based intermetallic compound layer formed at interface between the Ni-based conductor and the bulk of lead-free solder.
There is also provided a Cu-based conductor; a bulk of lead-free solder, which has the first or second composition and has a structure essentially consisting of an Sn matrix with Cu solute and an Sn—Ag eutectic structure, in which a Cu—Sn based intermetallic compound and an Ag—Sn intermetallic compound are dispersed; and, a Cu—Sn based intermetallic compound layer formed at an interface between the Cu-based conductor and the bulk of the lead-free solder.
DESCRIPTION OF PREFERRED EMBODIMENTS
In the present invention, the fundamental system is Sn—Ag eutectic and hypo-eutectic. This fundamental system has more improved thermal fatigue property than that of Sn—Pb eutectic. The additive elements are selected and are added in a specific amount in such a manner that the interface reaction is suppressed while the mechanical properties and wettability of the bulk are not adversely affected as compared with the Sn—Ag eutectic. The percentage of the composition is mass % hereinbelow.
In the present invention, Ag is added for the purpose o
Tanaka Yasuhisa
Yoshitome Daisuke
Armstrong Kratz Quintos Hanson & Brooks, LLP
Koehler Robert R.
Taiho Kogyo Co. Ltd.
LandOfFree
Lead-free solder and solder joint does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Lead-free solder and solder joint, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Lead-free solder and solder joint will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3279514