Metal fusion bonding – Process – Plural joints
Reexamination Certificate
2002-04-29
2004-08-24
Stoner, Kiley (Department: 1725)
Metal fusion bonding
Process
Plural joints
C228S225000, C228S230000, C228S260000
Reexamination Certificate
active
06779710
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims priority under 35 USC §119 to Japanese Patent Application No. 2001-132295 filed on Apr. 27, 2001, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to soldering technology that performs reflow and flow soldering to both side surfaces, respectively, of a baseboard with a lead-free solder alloy, and more particularly, to a method capable of suppressing peeling of a reflow soldering section when flow soldering is performed, in an electronic circuit baseboard, and an electronic instrument.
2. Description of the Related Art
In the past, a Sn—Pb (tin-lead) type solder, including a large quantity of Pb (lead), is generally utilized when electronic parts are mounted. However, when a circuit baseboard is soldered with an Sn—Pb type solder and the lead is discarded, the solder sometimes fuses out therefrom, giving undesirable effects to an ecological system and thereby causing environmental pollution. As a result, usage of a Pb-free type solder alloy is highly desirable.
After investigations of various Pb-free solder alloys; three components such as an Sn—Ag—Bi (tin-silver-bismuth) based material is a prevailing candidate for a Pb-free type solder alloy rather than an Sn—Pb type solder.
The reason is that various compositions formed by a two component type solder alloy have already been examined as Pb-free solder alternatives. For example, since Sn-3.5 weight % Ag has a fusing point of 221° C. and Sn-5 weight % Sb (antimony) has a fusing point of 199° C., respectively, these fusing points are too high in comparison to the Sn-37 weight % Pb solder alloy. The Sn-37 weight % Pb has a fusing point of 183° C. Accordingly, these two component type materials are not employed as Pb-free solders for a conventional glass epoxy baseboard.
In addition, even though Sn-9 weight % Zn (zinc) has a low fusing point of 199° C., the solder's surface is easily oxidized. The solder's surfaces wetting performance, with regard to an electrode comprising Cu (copper) or Ni (nickel), is particularly lower in comparison to that of an Sn—Ag or Sn—Sb type solder. As a result, Sn-9 weight % Zn is not employed either as a Pb-free type solder. Furthermore, since Sn-58 weight % Bi has a fusing point of 138° C. and is hard and brittle, this two component type alloy has problems associated with its structural integrity and is thus, difficult to employ.
Sn-52 weight % In (indium) also has a lower fusing point of 117° C. than Sn-37 weight % Pb which has a fusing point of 183° C. This difference in fusing point temperatures causes an additional problem of a weakening intensity in the solder connection section at high temperatures. In contrast, the fusing point can be approximated more closely to 183° C. (e.g., the fusing point of Sn-37 weight % Pb,) when a three component type Pb-free alloy, such as Sn—Ag—Bi is employed, as compared to when a two component type Pb-free alloy is employed.
However, when seeking prescribed materials whose fusing points approximate to 183° C., in the three component type Pb-free alloy, a perfect eutectic composition is not obtained. A composition should have a solid and liquid coexisting temperature (e.g., a solid phase line temperature lower than 183° C. and a liquid phase line temperature higher than 183° C.).
Thus, when a flow soldering process is performed after parts are connected by a reflow soldering process, and air cooling is performed without a blower for the baseboard, the respective temperatures decline at different rates in these added parts and the baseboard. As a result, a large temperature difference arises in the solder of the connecting sections since the connected parts have different heat capacities from that of the glass epoxy baseboard. In these situations, when a solder is utilized having a wide temperature range of a solid and liquid coexistence, the solder coagulates, because a phase having a low fusing point (e.g., a hard and brittle phase largely including Bi) is segregated at a higher temperature side. As a result, the connecting intensity of various parts that complete the segregation phase after receiving a reflow soldering process is readily weakened.
Current technlology relating to a Pb-free type solder alloy such as Bi (bismuth) is disclosed in Japanese Patent Application No. 11-221694. This application refers to a reflow soldering process performed on both surfaces of an organic baseboard with a Pb-free type solder, that includes Sn as a principal component together with 0~3.0 weight % Bi, 0.5~4.0 weight % Ag, and a total 0~3.0 weight % Cu and/or In. Japanese Patent Application No. 2001-36233 discloses technology capable of avoiding a temperature difference connecting a section through a soldering process by employing a heat conductive material between the baseboard and the parts body.
Specifically, to resolve the above-mentioned problems, a three component type alloy such as Sn—Ag—Cu as a Pb-free solder alloy is desirable. This three component type alloy has a high fusing point, an inferior wetting performance, and superior connecting structural integrity after soldering processes. As a result, the use of Sn—Ag—Cu as a Pb-free solder alloy has received much attention.
FIGS. 23A-C
are charts illustrating various types of Pb-free solders with varying characteristics, such as fusing points, machine performance, wetting performance, oxidizing performance, processing performance, and cost.
FIGS. 24A-B
are charts illustrating typical types of solder alloys and their respective fusing points, such as their solid and liquid phase line temperatures.
However, it has been discovered that even when using Sn—Ag—Cu, which is believed to have good structural integrity, the segregation of Pb included in a parts lead terminal frame as illustrated in
FIG. 21
, arises when a flow soldering process is performed with a flow solder on the second side surface after a reflow soldering process on the first surface is performed. As a result, peeling off arises in the soldering section as illustrated in FIG.
22
.
The peeling off phenomenon occurs because the temperature of the reflow soldering section approaches the fusing point of the solder alloy of the soldering section (i.e., between the solid and liquid phase line temperatures) due to heat conducted from the second side when a flow soldering process is performed.
To avoid this problem, it is desirable that the temperature of the soldering section is not raised to the solid phase line level of the fusing alloy. Particularly, the reflow soldering section is controlled so it does not fuse again. It is desirable that a eutectic crystal composition is utilized which will not cause the segregation of a low fusing point component even when an alloy fuses. It is also desirable to obtain a liquid phase line temperature that exceeds the high fusing point alloy temperature and makes the alloy coagulate again through cooling so that the segregation of the low fusing component does not occur even when the alloy fuses. In addition, it is desirable to perform a cooling process so that a temperature difference does not occur in the soldering section during a coagulation process when an alloy fuses.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to resolve the above noted problems and provide a new mounting parts peel suppressing system. The present invention provides a novel soldering method including the steps of performing a reflow soldering process on one side surface of a baseboard, performing a flow soldering process while contacting the jet flow solder to the other side surface of the baseboard, producing alloys in soldering sections when reflow and flow soldering processes are performed, and differentiating a composition point or a fusing point of each of the alloys employed.
In one embodiment, the reflow soldering process uses a solder material comprising Sn—Pb eutectic as a base and is obtained by blending compositions, and the flo
Igarashi Minoru
Isoda Masashi
Kobayashi Takayuki
Mukai Katsuhiko
Ricoh & Company, Ltd.
Stoner Kiley
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