JP3313006B2 - Copper alloy lead frame for bare bond - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a copper alloy lead frame for bare bond used without plating, which has excellent solder die bonding with a semiconductor chip and excellent wire bonding with an aluminum wire. The present invention relates to a copper alloy lead frame for bare bond in which a decrease in the bonding strength of an aluminum wire bonding portion due to heating is suppressed.

[0002]

2. Description of the Related Art For example, Fe: 0.02 to 0.50% by mass , P: 0.0 described in JP-B-55-2454.
Copper alloy containing 2 to 0.10% by mass and the balance essentially consisting of Cu, or Fe: 0.04 to 0.15% by mass , P: 0.02 described in JP-B-58-53057.
The copper alloy containing 5 to 0.04% by mass and containing the third additive element and the balance essentially consisting of Cu is a high-strength copper alloy strengthened by fine precipitation of Fe ₂ P,
It is widely used as a copper-based lead frame for ICs that is becoming smaller and thinner.

[0003] Lead frames made of these materials are:
Conventionally, nickel-plated or noble metal-plated (silver-plated) surfaces have been used for bonding. However, due to the development of die bonding technology and wire bonding technology, the amount of heat received by the lead frame during the bonding process has been reduced. Therefore, the growth of an oxide film that lowers the direct bonding property on the surface of the copper alloy is suppressed, and nickel plating and noble metal plating can be omitted from the viewpoint of improving productivity and reducing costs. Was. Then, first, a technique of wire bonding using a gold wire while applying an ultrasonic wave in a high-temperature (200 ° C.) inert atmosphere to ensure reliability has been put to practical use.

The gold wire used for direct bonding is 1
Since a thin wire of about 5 to 35 μmφ is used and the area of the wire bond portion is small, it is said that the surface of the lead frame should be smooth in order to stabilize the bonding strength. For example, Japanese Patent Publication No. Sho 62-46071 discloses that the maximum surface roughness (R _max ) of a copper alloy lead frame is 0.5 μm.
m is described below.
No. 41 describes that the surface roughness (R _max ) of a wire bond portion is finished to 0.2 μm or less as an IC lead frame for direct bonding.

Japanese Unexamined Patent Publication (Kokai) No. 2-173227 discloses a copper alloy having good direct bonding properties.
Fe: 0.05 to 0.5% by mass , P: 0.01 to 0.3
% By mass , optionally containing 0.001 to 2.0% by mass of a third additive to improve strength and corrosion resistance, the balance being copper and unavoidable impurities, and having a surface hardness of Hv 120 or more;
A copper alloy whose surface roughness is adjusted to a center line average roughness (R _a ) of 0.15 μm or less and a maximum height (R _max ) of 0.8 μm or less is shown. It also describes that good wire bonding properties were obtained using a fine copper wire.

[0006]

In recent years, from the viewpoint of cost reduction, it has been considered that a gold wire is changed to an aluminum wire, and the wire is wire-bonded to a copper alloy lead frame without plating in an atmosphere at room temperature. However, the literatures which point out that the surface condition such as the surface roughness of the copper alloy lead frame affects the wire bonding property (bonding strength) all use a thin gold wire or a copper wire. It is assumed that 100 to 50
The relationship between the wire bonding property of a thick aluminum wire having a diameter of 0 μm and the surface state of the lead frame has not yet been clarified.

A semiconductor chip is joined to a copper alloy lead frame by high-temperature solder. However, in recent years, fluxlessness has been promoted to improve reliability. However, when plating is omitted, there has been a problem that the spread of the solder and the copper alloy lead frame is reduced due to the suppression of the use of the flux.

For example, in order to obtain a high-strength material having a surface hardness Hv of 120 or more, which is an essential requirement in the above-mentioned Japanese Patent Application Laid-Open No. 2-173227, it is necessary to increase the elements added to copper. When the amount of the element increases, the wettability and spreadability of the solder, that is, the solder die bonding property is deteriorated, and the solder does not follow the flow of fluxlessness. At the same time, increasing the number of added elements causes a problem that the conductivity is reduced, and the adhesion of the oxide film is reduced, so that the oxide film is easily peeled off during the assembly process.

Furthermore, the amount of heat received by the lead frame after assembly recently tends to further increase, and it is necessary to keep the conductivity high from the viewpoint of heat dissipation, and
There is a need for a technique for improving the reliability of an aluminum wire joint when exposed to a temperature of 75 ° C to 200 ° C for a long time. However, materials with high conductivity, such as copper with tin, copper with phosphorous deoxidation, and copper with zirconium, have low reliability of the aluminum wire joint, furthermore, the oxide film is easily peeled off, and the solder wettability is poor. Have a problem. In addition, in order to improve the reliability of the aluminum wire joint, there has been reported a technology that slightly melts the aluminum wire during the assembly process to improve the bonding strength between the aluminum wire and copper. Problems such as reduced productivity and new capital investment.

The present invention has been made in view of such a conventional problem, and has a high conductivity in a copper alloy lead frame containing Fe and P and strengthened by precipitation with an Fe ₂ P intermetallic compound. Bare bond copper excellent in solder die bonding (solder wet spreadability), wire bonding by aluminum wire (wire bonding strength), reliability of aluminum wire bonding (retention of bonding strength after heating), and adhesion of oxide film The purpose is to obtain an alloy lead frame.

[0011]

SUMMARY OF THE INVENTION The present invention relates to a copper alloy lead frame containing Fe and P which is wire-bonded using an aluminum wire without plating.
Zn: 0.005 to 0.150 mass %, Ni: 0.00
5 to 0.050 mass %, containing at least one kind in total of 0.20 mass % or less, having a conductivity of 85% IACS or more, a surface hardness of less than Vickers hardness (Hv) 120, and a specular reflectance of a die bond portion. 30% or more, and the thickness of the oxide film on the lead frame surface is 80A (A = angstrom,
The present invention also relates to a copper alloy lead frame for bare bond having excellent solder die bonding property, wire bonding property, and joint reliability.

[0012]

According to the present invention, a copper alloy lead frame containing Fe and P and strengthened by precipitation with an Fe ₂ P intermetallic compound is provided.
The five conditions of trace added elements, conductivity, surface hardness, specular reflectance, and oxide film thickness are interrelated, and solder die bonding (solder wet spread), aluminum wire bonding (wire bonding strength), The four most necessary characteristics for bare bond lead frames such as the reliability of the aluminum wire bonding portion (maintaining bonding strength after heating) and the adhesion of an oxide film are improved.

It has been experimentally confirmed that all of the above characteristics can be improved by setting the conductivity of the copper alloy lead frame containing Fe and P to 85% IACS or more, preferably 90% IACS or more. Was. To obtain the copper alloy lead frame of the present invention containing Fe and P and having a conductivity of 85% IACS or more, the mass % ratio of Fe to P (Fe /
P) is controlled to 2 to 4, and the P content is 0.05
It is desirably set to not more than mass %. Then, such a copper alloy material is cold-rolled at a cross-sectional area ratio of 50% or more, and then heat-treated at a temperature of 450 to 550 ° C. as a final refining process to precipitate Fe ₂ P uniformly and finely. Obtainable.

Here, when the heat treatment is performed at a temperature exceeding 550 ° C., the precipitate of Fe ₂ P becomes coarse and the distribution state becomes coarse, so that desired characteristics cannot be obtained. In addition, P which does not precipitate
The conductivity sharply decreases according to the amount of, the type and amount of other additive elements and impurities. For example, Japanese Patent Application Laid-Open No.
No. 7, even in a copper alloy containing 0.05 to 0.5% by mass of Fe and 0.01 to 0.3% by mass of P and the balance consisting of copper and inevitable impurities, the mass of Fe and P Depending on the% ratio (Fe / P) and the production conditions, and when the content of Fe or P is large, the conductivity becomes less than 85% IACS, and 0.001 to 2.0% by mass of As, Sb, or the like is added as an accessory component. In this case, the conductivity is further reduced.

P, which is not precipitated as Fe ₂ P, or contains a large amount of other additional elements or impurities (such as Sn), has a conductivity of 85% I.
The reason why the above characteristics deteriorate when the value is less than ACS is that a concentrated layer of these elements is formed on the surface oxide film, which serves as a starting point of oxide exfoliation, and that oxides of impurities such as Sn are bonded by die bonding. It is considered that the reduction is not sufficiently reduced by the previous reduction heat treatment, which impairs the solder wettability and the wire bonding property and also forms an unbonded portion.

The surface hardness of the pure aluminum wire is about Vickers hardness (Hv) of about 50, and the closer the hardness of the copper alloy lead frame to the aluminum wire, the better the bondability. Therefore, the surface hardness of the lead frame is determined by Vickers hardness (Hv).
The hardness is set to be smaller than 120. At this hardness, good wire bonding strength can be obtained by ultrasonic wedge bonding at room temperature in an aluminum wire with air. However, if the copper alloy lead frame itself is too soft, there is a problem such as bending at the time of handling, so that the hardness (Hv) is desirably 85 or more.

Recently, aluminum alloy wires obtained by adding a small amount of nickel or magnesium to aluminum for the purpose of improving corrosion resistance and strength have been increasing, and the copper alloy lead frame of the present invention is also effective for such aluminum alloy wires. It is. In addition, the copper alloy lead frame is heated at a temperature of 300 ° C. or more in a reducing atmosphere before solder die bonding to remove organic substances and oxide films on the surface, but the surface hardness is reduced by Vickers hardness ( Hv)
The hardness may be smaller than 120.

Next, the specular reflectivity of the copper alloy lead frame is related to the die bonding property, that is, the solder wet spreading property. By setting the specular reflectivity of the die bond portion to 30% or more, the solder wet spreading property without flux is obtained. Is improved. The specular reflectance of the die bond portion can be increased by processing at the time of stamping or by strengthening the processing at the time of rolling.

With respect to the surface properties of the copper alloy lead frame, the surface roughness is specified in the above-mentioned JP-A-64-57741 and JP-A-2-173227. No clear relationship has been found with wet spreadability. This is because the effect on the wettability and spreadability of solder pits differs in the polishing pits existing in the rolling direction in the copper alloy manufacturing process and the oil pits generated perpendicular to the rolling direction in the final finish rolling process, and the surface roughness is different from the polishing pits and oil. This is because it is not possible to clarify which of the pits is being measured, so that sufficient solder wettability cannot be guaranteed even if the surface roughness is specified.

The specular reflectance also relates to the wire bonding property, and it is desirable that the wire bond portion has as little dirt, rust preventive film, oxide film, etc. as possible. In the above publication, the surface roughness of the copper alloy lead frame is specified, and it is considered that the smoother the surface, the better the wire bonding property. This is because, when the surface of the wire bond portion becomes rough, the ultrasonic wave during wire bonding concentrates only on the convex portion, and the bonding area between the wire and the lead frame decreases, which causes a decrease in bonding strength. In order to stabilize the strength, it has been recognized that it is necessary to smooth the lead frame surface, but this is applied to wire bonding using a thin gold wire or copper wire having a diameter of 15 to 35 μm. However, it is not immediately applied mainly to wire bonding of an aluminum wire mainly using a thick wire having a diameter of 100 to 500 μm.

Conversely, since a high load can be applied to an aluminum wire (thick wire), the R _{max of the} wire bond portion is increased.
In some cases, a rougher one having a roughness of about 0.8 to 1.0 μm may improve the bonding property due to an anchor effect or the like. That is, 100 ~
In the case of a copper alloy lead frame using an aluminum wire having a diameter of about 500 μm, since the area of the wire bond portion is large and the load at the time of bonding is large, R _max is 0.2 μm.
Since a sufficient bonding strength can be obtained on the surface of a normally produced copper product having a size of 1 μm or less, there is no need to specify the surface roughness in the present invention.

Next, the thickness of the oxide film is related to the bonding strength of the wire bond portion. In other words, the smaller the initial oxidation of the copper alloy lead frame, the easier it is to reduce in the reduction heating treatment in the assembly step, and the thinner the oxide film at the time of wire bonding, the higher the bonding strength of the wire bond portion. As a result of measuring the thickness of the oxide film by an instrumental analysis or a cathode reduction method, if the thickness of the oxide film before the assembly process is 80 A or less, the bonding strength of the wire bond portion becomes good, and conversely, the thickness of the oxide film exceeds 80 A Not only takes a long time for reduction, but also lowers the bonding strength of the wire bond portion.

In addition, an increase in the thickness of the oxide film causes a reduction in the spreadability of the solder. That is, in the bare bonding, the copper alloy lead frame is usually placed in nitrogen containing hydrogen (hydrogen content: 4 to 15%, oxygen: 1000 ppm or less) in the assembly process at 300.
The substrate is subjected to a reduction heating treatment at 380 ° C. for 15 seconds or more. However, when the oxide film is thick and the reduction time is long, impurities contained in the copper alloy lead frame diffuse to the surface, which lowers the solder wettability. Therefore, it is indispensable to reduce the initial oxidation in the copper alloy lead frame for bare bond from the viewpoint of the spreadability of the solder. By setting the thickness of the oxide film to 80 A or less, good solder wettability can be ensured. Further, as the thickness of the oxide film increases, the adhesion of the oxide film deteriorates. However, in the present invention, the initial oxide film thickness on the surface of the copper alloy lead frame is set to 80A or less to suppress the peeling. As a method for reducing the initial oxidation, polishing, acid cleaning, etching, reduction treatment and the like can be considered.

The copper alloy lead frame of the present invention has a Zn:
0.005 to 0.150 mass %, Ni: 0.005 to
At least one of 0.050% by mass is used in a total amount of 0.1%.
Although these elements are contained in an amount of 20% by mass or less, these elements have an effect of improving the reliability of the aluminum wire joint (retaining the joint strength after heating).

As described above, the amount of heat received by the lead frame after assembly tends to increase in recent years. When an aluminum wire is used, the temperature of the wire bond portion is 175 ° C.
A decrease in bonding strength when exposed to temperatures of up to 200 ° C. for a long time has been a problem. The present inventor has found that in a wire bond portion using an aluminum wire, when heated at 200 ° C., an aluminum-copper alloy layer of about 1 μm after heating for 500 hours and about 2 μm after heating for 2000 hours, and this alloy layer is formed of aluminum. Knowing that it is more brittle than wire and cause a decrease in bonding strength, and as a result of intensive studies based on this knowledge, it was found that a small amount of nickel and / or zinc was added to a copper alloy lead frame containing Fe and P. Thus, it has been found that a decrease in the joining strength of the aluminum wire joint can be suppressed.

When the amount of Zn added is 0.005% by mass or less, the effect of improving the reliability of the aluminum wire joint is small, and 0.15% by mass or less.
When the content exceeds 0% by mass , the conductivity becomes smaller than 85% IACS, the solder wettability decreases, and the surface hardness increases.
0.150 mass %. In addition, the amount of Ni added is 0.00
5 wt% less aluminum wire joint reliability improvement effect in the following, since the solder wetting property is lowered when it exceeds 0.05 mass%, Ni addition amount from 0.005 to 0.
050 mass %. When both Ni and Zn are added, the total is 0.01 to 0.20% by mass , but also in this case, the conductivity is adjusted so as to be 85% IACS or more. These additional elements do not need to be added when performing wire bonding with a gold wire or a copper wire without using an aluminum wire.

Also, trace amounts of Pb, Ag, Cr, Mg, Zr, Ti, B, T
e, Co, Mn, etc. may be added, but if the addition amount increases, the conductivity decreases and the wettability of the solder decreases, so the addition amount must be within the range satisfying the conductivity of 85% IACS or more. must not. Since Sn is oxidized by heating at the time of die bonding, the solder is not uniformly wetted and spread, and the oxide film is easily peeled off.
% By mass or less is desirable.

As described above, in a copper alloy lead frame containing Fe and P and precipitation-strengthened with an Fe ₂ P intermetallic compound, the addition of trace elements, high conductivity, low surface hardness, high specular reflectance, and thin oxidation By satisfying the five conditions of film thickness, it is possible to obtain a copper alloy lead frame for bare bond, which shows better properties than conventional phosphorus deoxidized copper, copper with Sn, copper with zircon, etc. Suitable for fluxless solder die bonding and wire bonding with aluminum wire,
The present invention can supply a copper alloy lead frame for bare bond having excellent reliability of an aluminum wire joint.

[0029]

EXAMPLE In this example, Fe: 0.1% by mass , P:
A copper alloy containing 0.03% by mass and further containing either or both of Ni and Zn is washed, polished, finish-rolled from the ingot through hot rolling, cold rolling, annealing, etc. To obtain a lead frame in which Fe ₂ P intermetallic compound is finely deposited in a high-purity matrix.
After examining the surface hardness, specular reflectance, oxide film thickness, and oxide film adhesion, and further subjecting the copper alloy lead frame to a reduction heating treatment, the aluminum wire bondability (bonding strength, wire breaking rate), The solder die bonding property (solder spread area) and the reliability of the aluminum wire joint (joining strength after 1000 hours) were examined.

In this embodiment, lead frames having different electrical conductivity, surface hardness, specular reflectance, or oxide film thickness are
It was made by changing annealing conditions, finish rolling conditions, cleaning / polishing conditions, stamping conditions (coining strength), and the like. Further, the reduction heat treatment is performed in a hydrogen: 10
% Nitrogen atmosphere (oxygen: 50 to 70 ppm or less)
In the inside, it was performed at 350 ° C. for 30 seconds. In addition, the numerical values of each characteristic in the following table are measured by a test method described later.

Table 1 shows that Fe: 0.1% by mass , P: 0.0
3% by mass , Ni: 0.020%, and Sn: 0.
It shows the relationship between the electrical conductivity, surface hardness, and specular reflectance of the copper alloy lead frame containing 010% by mass and the bonding strength, wire breakage rate, and solder spread area. Examples 1 to 3 are those in which all of the specular reflectances fall within the range specified by the present invention, and Comparative Examples 1 to 3 are those in which any of the characteristics are out of the range specified by the present invention.
As shown. However, in each case, the oxide film thickness is 40
A.

[0032]

[Table 1]

In Table 1, the conductivity is 85% IACS or more, the surface hardness (Hv) is less than 120, and the specular reflectance is 30.
% Or more of the lead frames (Examples 1 to 3) are excellent in all of the bonding strength, the wire breaking ratio, and the solder spread area. In addition, there was no adhesion of the oxide film to the adhesive tape, and the adhesion to the oxide film was good. On the other hand, in Comparative Example 1, since the conductivity was low, the bonding strength, the wire breaking ratio, and the solder spread area were all inferior, and the adhesion of the oxide film was poor. In Comparative Example 2, since the surface hardness was as high as Hv125, the bonding strength and the wire breaking ratio were low, and a decrease in aluminum wire bondability was confirmed. In Comparative Example 3, the specular reflectivity was low, so the spread area of the solder was small, and a decrease in the solder die bonding property was confirmed.

Table 2 shows that Fe: 0.1% by mass , P: 0.0
In addition to 3% by mass , 0.050 to 0.250% of Zn
The conductivity of the copper alloy lead frame changed within the range of
Zn content, and the thickness of the oxide film, and shows the relationship between the bonding strength, wire breakage, and solder spread area, conductivity, Zn content, and oxide film thickness are all within the range specified in the present invention Those included are shown in Examples 4 and 5, and those in any of which are out of the range specified in the present invention are shown as Comparative Examples 4 and 5. However, in each case, the surface hardness is Hv115,
The specular reflectance is 35%.

[0035]

[Table 2]

In Table 2, the lead frames having a conductivity of 85% IACS or more, a Zn content in the range of 0.005 to 0.150%, and an oxide film thickness of 80 A or less (Examples 4 and 5) are shown in FIG. Excellent in joint strength, wire breakage rate and solder spread area. On the other hand, Comparative Example 4 had a problem that the oxide film was thick and the bonding strength, the wire breaking ratio, and the solder spread area were reduced. Comparative Example 5 has a conductivity of 85% IACS.
Below, the zinc content was high, and a decrease in the solder spread area was confirmed.

Table 3 shows that Fe: 0.1% by mass , P: 0.0
In addition to 3% by mass, the conductivity of a copper alloy lead frame in which Zn is changed up to 0.250% and Ni is changed up to 0.065%, the contents of Zn and Ni, the bonding strength,
The relationship between the bonding strength after 1000 hours and the spread area of the solder is shown, and the conductivity and the contents of Zn and Ni are all within the ranges specified in the present invention.
8 to 8 are shown as Comparative Examples 6 to 8 when any one of them is out of the range defined in the present invention. However, in each case, the surface hardness was Hv115, the oxide film thickness was 40A, and the specular reflectance was 35.
%.

[0038]

[Table 3]

In Table 3, the lead frames containing 85% IACS or more of electrical conductivity and containing Zn and Ni in the specified ranges (Examples 6 to 8) showed a small decrease in bonding strength after heating at 200 ° C. for 1000 hours, and the solder spread. The area was also excellent. On the other hand, since Comparative Example 6 did not contain zinc and nickel in appropriate amounts, peeling occurred at the interface between the aluminum wire and copper,
Bonding strength after heating for 1000 hours decreased. Comparative Example 7,
8 contains a large amount of zinc and nickel, so that the electrical conductivity is reduced and the solder spread area is reduced.

As shown in Tables 1 to 3, the copper alloy lead frames of the examples of the present invention show good solder wettability without flux after heating in a reducing atmosphere, Also, high bonding strength was exhibited in wire bonding in air. In addition, good aluminum wire bonding strength was maintained even after heating at 200 ° C. for 1000 hours. In Tables 1 to 3, △, △, and △ indicate evaluations for each numerical value, ◎ indicates that it is excellent, ○ indicates that it can be used, and △ and X indicate that it cannot be used.

The test method in this embodiment will be described below. The joining strength was examined as follows. First, 5
mil (125 μm) diameter aluminum wire (99.99
%, Annealed, and wire bonding (bonding time: 100 ms, load: 250 g, U) at 25 ° C. in the atmosphere by ultrasonic waves (pulse 60 kHz).
(V output: 5.5 W), and then the joined aluminum wire was cut in the middle, and the joining strength of the 2nd bond portion where the aluminum wire was joined to the copper alloy lead frame was examined (twister strength). Also, check the break point,
The wire breaking ratio was determined by the following equation. Wire breakage rate (%) = (number of broken wires / total number of test pieces) × 100

The spread area of the solder was determined by reducing heat treatment (the copper alloy lead frame was treated with an oxygen concentration of 50 to 7 containing 10% hydrogen).
3 on a plate at 350 ° C. in a nitrogen atmosphere of 0 ppm
After heating for 0 seconds), a solder having a diameter of 1 mm (Sn: 5% by mass ,
Ag: 2.5% by mass , lead: balance) A ball was placed, and the area where the solder ball had spread by 5 seconds was measured without flux and without stirring.

The bonding strength after 1000 hours was measured by storing the sample wire-bonded with an aluminum wire in a furnace at 200 ° C., and after 1000 hours, measuring the wire bonding strength. The adhesion of the oxide film was determined by heating a copper alloy lead frame at 350 ° C. for 1 minute on a hot plate in the air, and then using an adhesive tape (trade name, 3M No. 81).
When 0) was attached and then peeled off, it was determined whether or not an oxide film was adhered to the adhesive tape.

[0044]

According to the present invention, even if plating is omitted, sufficient solder wettability is exhibited without flux, and good bonding strength is obtained even with the use of an aluminum wire. In addition, it is possible to obtain a copper alloy lead frame for die bonding which is excellent in the adhesiveness of the oxide film. As a result, costs can be reduced, and productivity and workability can be improved.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H01L 23/50 C22C 9/00

Claims (1)

(57) [Claims] 1. A copper alloy lead frame containing Fe and P which is wire-bonded using an aluminum wire without plating, wherein Zn: 0.005 to 0.1
50 wt%, Ni: 0.005 to 0.050 contain mass% 0.20 mass% in total of at least one of the following, conductivity 85% IACS or more, the Vickers hardness of the surface hardness (Hv) 120 Less, the specular reflectance of the die bond portion is 30% or more, and the thickness of the oxide film on the lead frame surface is 8
A copper alloy lead frame for bare bond, which is excellent in solder die bonding property, wire bonding property, and joint reliability, characterized in that it is set to 0 A (angstrom) or less.