Pb-free solder alloy, and solder material and solder joint using same

Abstract

A solder alloy based on an Sn—Zn—In—Ag system contains, in weight, 3.0%<Zn<5.0%, 0.1%<In<4.0%, 0.1%<Ag<0.4%, and the balance Sn. Therefore, the current Sn—Pb soldering method can be employed as it is. Further, a Pb-free solder material having a solder characteristic with excellent bonding strengths of the parts can be provided. Still further, since a difference between a solidus temperature and a liquidus temperature is small, floating of the parts leads can be suppressed, even in case where packaging processes are performed many times over. Still further, when the joint is exposed to the high temperature and high humidity atmosphere, the bonding strength can be prevented from being lowered.

Description

FIELD OF THE INVENTION [0001] The present invention relates to a Pb-free solder alloy, and a solder material and a solder joint using same. BACKGROUND OF THE INVENTION [0002] Recently, the problem of the toxicity of lead (Pb) has invoked a strong movement in regulating the disposal of lead to an environment. Thus, as a bonding material for parts of electronic products, a Pb-free solder has been substituted for a conventional Sn—Pb solder. [0003] As for characteristic properties of an alloy necessary as a solder material, there are melting temperature, tensile strength, ductility (or elongation property), wettability, bonding strengths of parts joints, and the like. [0004] A melting temperature of a solder is preferably to be set at approximately 200° C. If a melting point of the solder is too high, it will exceed heat resistance temperatures of the parts in a reflow soldering, whereby a current soldering method may possibly incur damages on the parts. On the other hand, if a melting...

AI Technical Summary

Benefits of technology

[0018] Particularly, it is an important object to improve solder joint reliability under the condition of the high temperature and high humidity.

Problems solved by technology

A melting temperature of a solder is preferably to be set at approximately 200° C. If a melting point of the solder is too high, it will exceed heat resistance temperatures of the parts in a reflow soldering, whereby a current soldering method may possibly incur damages on the parts.

On the other hand, if a melting point of the solder is too low, the solder becomes most likely melted such that the parts may fall down or be peeled off in case when the environmental temperature surrounding the parts becomes high.

However, the melting points of these alloys are higher than that of a conventional Sn—Pb solder by 30° C. to 40° C. As a result, under a temperature condition of reflow soldering by using these alloys, the melting points thereof may exceed heat resistance temperatures of the parts.

It is technically difficult to increase heat resistances of corresponding parts up to a temperature where reflow soldering can be performed by using the aforementioned solder.

However, group II alloys are highly oxidized in a melting state in the air, and technically difficult to be applied in the flow soldering method at this point.

However, it has been considered that wettability to a joint base material of a solder is not good.

Further, it has been confirmed that bonding strengths of the parts are significantly deteriorated if the joint to be soldered with a Cu base material is exposed to a high temperature and a high humidity condition, even after the reflow soldering.

Further, it is likely that Zn is eluted from a solder into a flux, possibly incurring problems such as a lowered insulation resistance and a generation of migration, since Zn is employed in solder.

It has been confirmed that the compound's phase grows large as time passes and becomes fragile, whereby an interface strength becomes lowered.

Such a technical trend draws a concern that deformation of the solder may cause short circuits.

Further, since the solder contains a large amount of rare and expensive Indium (In), the material cost amounts high and the continuous future supply may not be secured.

Here, the problematic point is that a place soldered once is peeled off in subsequent soldering processes.

However, the Ag concentration described in reference 1 is too high by as much as 1% in weight or more.

As a result, under a reflow soldering condition same as that of the Sn—Pb solder, it is likely that the solder is not sufficiently melted down.

If the solder is not sufficiently melted down, fluidity of the solder is deteriorated, whereby a joint is not fully formed.

In that case, voids in the solder remain to thereby lower the bonding strength.

The conventional Pb-free solder may incur various problems such as poor wettability due to Zn, which is a problem in the Sn—Zn(—Bi,—Al) based solder, and a lowered bonding strength with Cu electrode under the condition of the high temperature and high humidity.

Further, using rare metals such as In and Ag becomes a problem in the Sn—In—Ag—Bi based solder alloy.

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