JP3185600B2 - Solder layer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lead-free solder used for wiring connection of electronic parts and semiconductor parts. Solder (Sold
er) is a brazing alloy having a melting point of 450 ° C. or less, and is also called soft solder, and is composed of lead (Pb) -tin (Sn).
Typical alloys are typical.

Here, the composition of Pb ₃₇ Sn ₆₃ has a eutectic temperature of 183 ° C., and is used as a standard eutectic solder. Has been used. That is, the higher the Sn composition ratio, the better the adhesion, and the lower the Pb content, the lower the toxicity, but the mechanically brittle. On the other hand, Pb alone has a hardness (Brinell hardness) as small as 2.5 to 3 and is soft. However, the hardness increases by alloying. In the case of Pb-Sn system, the melting point decreases and the flow becomes better, The feature is that it can be brazed to a narrow gap.

[0003]

2. Description of the Related Art Pb-Sn solder is a typical brazing material, and is used for circuit connection between semiconductor components and conductor lines formed on a wiring board. In such a case, Pb-
Sn-Cd (cadmium) -Pb, which has a lower melting point than Sn-based
Systems and Bi (bismuth) -Sn-Pb systems have been used.

As described above, various solders have been put to practical use, but many solders are mostly two-component or three-component solders containing Pb as a constituent element. However, the use of Pb is problematic in terms of environmental hygiene and in terms of soft errors with respect to transistors, and its use is often avoided.

That is, a printed wiring board that has been discarded
If the board is exposed to rain outdoors, Pb in the solder
Elutes and contaminates groundwater, adversely affecting the human body.
Problem. Natural Pb has four isotopes.
That is,²⁰⁴Pb,²⁰⁶Pb, ²⁰⁷Pb,²⁰⁸With a mixture of Pb
Yes, Pb is thorium (Th) decay series, uranium
(U) The final life of the decay series, actium (Ac) decay series
Because it is a product, it can emit α-rays with α-decay.
Therefore, the transistor malfunctions due to this α-ray irradiation
(Soft error).

Therefore, various measures have been taken to avoid such a problem. For example, when stopping the accumulation of discarded electronic devices or mounting flip-chip type semiconductor integrated circuits (ICs), hemispherical solder bumps are patterned on a multilayer circuit board (Bump).
Although the mounting method of positioning and fusing is adopted,
In this case, in order to prevent a soft error due to α-rays emitted from the solder bumps, the solder bumps are formed in the peripheral region by displacing from the active region where the transistor is formed in a matrix form, and the irradiation of the α-rays is performed by increasing the distance. Prevention and other measures have been taken.

However, all of these methods are urgent, and practical use of lead-free solder is desired as an essential method.

[0008]

At present, two-component or three-component solder is used for soldering.
Most contain Pb as a constituent. That is, Pb is a flexible metal, and is adopted because of its features such as improving the flow of molten solder and allowing it to penetrate into narrow gaps. However, if Pb is exposed to the field, lead carbonate (Pb CO ₃ ) or lead sulfate (Pb
b SO ₄ ), which elutes and contaminates groundwater.
There is a problem that a line is irradiated and a transistor malfunctions. To eliminate these problems, it is an issue to put lead-free solder into practical use.

[0009]

SUMMARY OF THE INVENTION The above-mentioned object is to solve the problem with the substrate to be treated.
Formed on the plate surface, gold, silver, nickel, palladium, white
A wettability improving layer comprising one of gold, and the wettability improving layer
A combination of indium and bismuth formed on the surface
The problem can be solved by a solder layer having the components lead-free solder .

[0010]

[Function] The solder must be flexible enough to absorb the stress caused by the difference in the coefficient of thermal expansion between the semiconductor chip and the circuit board on which it is mounted, a DIP (Dual-in-line Package) method, QFP (Qu
(adruped Flat Package) High bonding strength as in the case of mounting a semiconductor device, and good flowability so that it can penetrate into narrow gaps. Is Pb ₃₇ S
n ₆₃ eutectic solder.

In order to put the Pb-less solder into practical use, the inventors of the present invention have studied the necessary conditions for an element that can substitute for Pb, and as a result, it is essential that they exhibit properties similar to Pb.
It was considered that a combination of metal elements which are around Pb in the periodic table of elements and have no toxicity is appropriate.

FIG. 1 shows a periodic table around Pb, in which Pb of element No. 82 is surrounded by In of element No. 49, Sn of element No. 50, Sb of element No. 51, Tl of element No. 81, Element number
There are 83 Bi's, but Tl and Sb, like Pb, fall outside of the requirement because the compounds are toxic and eventually become In-Bi.
The system was selected as a candidate for Pb-less solder. And FIG.
As a result of conducting a meniscograph test and the like and examining the solderability, good results could be obtained.

Here, it is desirable that the eutectic solder has the lowest melting point and the same alloy composition as those of these two-component solders.
Binary alloys having different composition ratios gradually precipitate as in the case of using a solder having a composition ratio of Sn of 10 to 90% by weight in the system solder. Even if the eutectic alloy finally precipitates, there is no problem.

Next, the meniscograph test shown in FIG. 2 will be described. After applying a flux to the test piece 1 to be soldered, the test piece 1 is mounted on a meniscograph tester equipped with a pressure sensor (step 1). 2) Solder bath 2
When it is inserted into the test piece, a repulsive force 3 appears at first, at which the molten solder tries to pop out the test piece 1 (stage 2), and FIG.
The repulsive force 3 is recorded as shown in FIG. 3, but as the oxide film on the surface of the test piece 1 is removed by the heated flux, the wettability is generated and the test piece 1 is drawn into the solder bath 2. Force 4 occurs (stage 4). Here, the meniscograph test is a method of evaluating the time from the insertion of the test piece 1 into the solder bath 2 until the force applied to the test piece 1 changes from the repulsion force 3 to the pull-in force 4 as the wetting time.

Next, when soldering the substrate to be processed, there are metals that cannot be soldered beforehand.
Preliminary treatment for improving solderability is required because some metals have poor wettability.

For example, metals having a passivation film on the surface, such as iron (Fe), titanium (Ti), chrome (Cr), and aluminum (Al) cannot be soldered as they are.
Further, although gold (Au) has extremely good wettability with respect to PbSn-based solder, there is a problem that an alloy is easily formed with PbSn-based solder and melts in (commonly known as erosion). Is often subjected to a surface treatment of a three-layer structure of a wettability improving layer / barrier layer / adhesion improving layer.

That is, Ti or Cr is placed at the soldering position of the substrate to be processed by means such as sputtering or vacuum evaporation.
For example, a metal film having excellent adhesiveness to a substrate to be processed is formed as an adhesion improving layer, and a soldering property such as nickel (Ni) is good but the melting point is high (Ni has a melting point of 14
(55 ° C.) A metal film that does not easily cause solder erosion is formed as a barrier layer, and furthermore, a solder film such as Au, silver (Ag), copper (Cu), palladium (Pd), platinum (Pt) is formed thereon. A metal film having good wettability is formed as a wettability improving layer. The thickness of the wettability improving layer / barrier layer / adhesion improving layer is often about 3000/1000/1000.

Therefore, in the case of the Pb-less solder according to the present invention, the same processing as that of the three-layer structure used for the conventional PbSn-based solder was performed, and as a result, good solderability was obtained.

[0019]

【Example】

Example 1: (In-Bi-based) As a test piece, a silicon (Si) wafer having a thickness of 500 μm was cut into a size of 40 × 5 mm, and titanium (Ti) was added to the Si piece as an adhesion improving layer. Is sputtered to a thickness of 1000 mm, and nickel (Ni) is sputtered to a thickness of 1000 mm as a barrier layer. Then, two kinds of metals, gold (Au) and silver (Ag), are sputtered as a wettability improving layer. The test pieces were used to examine solderability.

Next, as a solder bath, In- as shown in FIG.
In Bi phase diagram, the eutectic composition Lee and In- _{34 wt} Bi indicating the (eutectic temperature 72 ° C.), an In ₂ Bi alloy composition (melting point 89 ° C.)
And three kinds of solder baths having an In Bi alloy composition (melting point: 110 ° C.), a meniscograph using a sample in which each bath temperature was kept at 130 ° C., 150 ° C. and 160 ° C. and Ag was used as a wettability improving layer. The test was performed.

As a result, the average values of the wetting times of the three types of solder baths were 2.23 seconds, 1.52 seconds and 1.57 seconds, and good results could be obtained.

[0022]

[0023]

[0024]

As a result of a meniscograph test on a sample in which Pb ₃₇ Sn ₆₃ is used as a solder bath and Au used in the examples is used as a wettability improving layer, the average value of the wetting time is 0.46.
In seconds, also can achieve the same results with the an In-Bi system in comparison with this, it became clear that can be used this way as a lead-free solder.

[Brief description of the drawings]

FIG. 1 is a part of a periodic table.

FIG. 2 is an explanatory diagram of a meniscograph test.

FIG. 3 is an In-Bi system state diagram.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Test piece 2 Solder bath 3 Repulsion force 4 Pull-in force

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Kozo Shimizu 1015 Kamiodanaka, Nakahara-ku, Kawasaki City, Kanagawa Prefecture Inside Fujitsu Limited (56) References JP-A-8-224689 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) B23K 35 / 20,35 / 26,35 / 34

Claims (1)

(57) [Claims] 1. A wettability improving layer formed on a surface of a substrate to be processed and made of one of gold, silver, nickel, palladium, and platinum, and indium and bismuth formed on the surface of the wettability improving layer. A solder layer having a combined two-component lead-free solder.