JP3116332B2 - Nickel-plated copper alloy lead frame for solder die bonding - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nickel-plated copper alloy lead frame in which a semiconductor chip is directly solder-die-bonded to nickel plating in a state where silver plating is not performed. The present invention relates to a nickel-plated copper alloy lead frame having good solder die bonding property and excellent wire bonding property even in a state using solder containing no.

[0002]

2. Description of the Related Art Conventionally, a surface of a copper alloy lead frame to be solder-die bonded has been subjected to silver plating in order to obtain good solder spread. The semiconductor assembling process is performed in a reducing atmosphere containing nitrogen gas or hydrogen in order to prevent oxidation of the lead frame due to heat at the time of solder die bonding. Oxygen concentration in atmosphere is 400ppm
Good solder spread can be obtained even if the height is higher than above.Therefore, regarding silver-plated copper alloy lead frames, the oxygen concentration in the semiconductor assembly atmosphere has never been a problem, and the oxygen concentration has been sufficiently studied. Never.

On the other hand, it has been considered that silver plating is omitted and solder die bonding is performed directly on nickel plating from the viewpoint of improving productivity and reducing costs. In the following, this technology is briefly described with respect to the conventional technology and its problems. First, in the case of performing direct solder die bonding on nickel plating, it is necessary to control the oxygen concentration in the reducing atmosphere to be lower (for example, 100 ppm or less) than in the case of soldering on silver plating in order to obtain good solder spread. ,
This was a factor of cost increase.

[0004] As a technique for directly bonding to nickel plating by omitting silver plating, for example, Japanese Patent Application Laid-Open No. Sho 62-2
Japanese Patent No. 91951 discloses a copper alloy lead frame that has been subjected to oxidation-resistant nickel plating containing P (phosphorus) or B (boron). However, if P or B is contained,
Nickel plating is hardened and workability is reduced, and punching after plating becomes difficult. Therefore, plating must be performed after processing into a lead frame shape, and productivity is poor. Also, recently, the amount of heat received by the lead frame has been increasing due to the high-density integration of electric components, and a nickel-plated copper alloy lead frame having excellent heat resistance has been desired. When heated for a long time after soldering, there is a problem that solder peeling occurs.

As a solder used for solder die bonding, a solder having a high lead content (P
b-5% Sn, Pb-5% Sn-1.5% Ag, etc.). This type of solder has the advantage that the melting temperature is high and the cost is low, but recently, in order to prevent environmental pollution due to lead or to reduce the temperature of the assembly process, a tin-based solder (Sn- 9% Zn, Sn-5% S
b, Sn-5% Sb-0.5Ni, Sn-8% Bi, etc.)
The use of is increasing. However, a lead-containing solder and a tin-based solder containing no lead not only have different melting temperatures, but also different growth rates and diffusion states of the alloy layer, so that conventional lead-containing solder (including Pb-Sn eutectic solder) is used. Compared with the conventional nickel plating selected by the evaluation method, the solder spread of the tin-based solder containing no lead was poor.

Recently, semiconductor chips and pellets have been increasing in size. However, in order to mount a large-sized semiconductor chip and a pellet, it is necessary to provide a lead frame which has good solder spreadability and can control solder thickness and solder spread by stirring or the like. Has become.

[0007]

SUMMARY OF THE INVENTION The present invention relates to an improvement of the technique of directly die-bonding to nickel plating by omitting the above-mentioned silver plating. More specifically, from the viewpoint of cost reduction, the present invention relates to a lead frame with silver plating. Solder die bonding can be performed even in an atmosphere having an equivalent high oxygen concentration (approximately 400 to 1000 ppm), and a nickel-plated copper alloy lead frame with good productivity can be obtained. It is an object of the present invention to obtain a nickel-plated copper alloy lead frame capable of controlling the thickness and the spread of solder and having excellent solder die bonding properties.

[0008]

A nickel-plated copper alloy lead frame is disclosed in Japanese Patent Application Laid-Open No. 62-291951.
Even in a lead frame in which oxidation of nickel is reduced as in the publication, there is a problem that the spread of solder on the surface of nickel plating is inferior to that of silver plating in a semiconductor assembly line for lead frames with silver plating. It was generally considered that nickel has a poorer solder spread than silver, but as a result of the present inventor's intensive study of the cause of the reduced solder spread in the above line, the oxygen concentration contained in the semiconductor assembly line atmosphere was high. At times, it was confirmed that the solder spread was reduced.

That is, the atmosphere in the semiconductor assembly line for a lead frame with silver plating is 5-20 hydrogen in order to prevent oxidation of copper and solder balls on the cut surface of the lead frame.
% Nitrogen gas (reducing atmosphere) is used. According to the investigation by the present inventors, this nitrogen gas atmosphere contains about 400 to 1000 ppm of oxygen due to entrainment of the air. And, in the nickel-plated copper alloy lead frame obtained from a normal bright watt bath, good solder spread is exhibited when the oxygen concentration in the reducing atmosphere is 300 ppm or less, but when the oxygen concentration exceeds 300 ppm, the solder spread begins to decrease. At 500 ppm, the solder hardly spreads.

On the other hand, in a reducing atmosphere containing about 10% hydrogen, it was confirmed that even if oxygen was contained at about 1000 ppm, oxidation of nickel was small, and the cause of the decrease in solder spread was not due to nickel itself. I understood that. As a result of further studies by the present inventor, they have found that the cause of the decrease in solder spread on the nickel plating surface is the effect of trace impurities contained in the nickel plating film. These trace impurities are components that are added to the nickel plating bath, such as brighteners, throwing agents, and stress relaxation agents, and are incorporated into the nickel plating film. When the oxygen concentration is high even in a reducing atmosphere, the solder spreads. Acts to lower the

The present invention has been made on the basis of the above-mentioned findings, and has a specular reflectance of nickel plating of 25% or more and a sulfur content of nickel within 0.1 μm from the surface of 0.0005 wt% or less. A nickel-plated copper alloy lead frame for solder die bonding. In this copper alloy lead frame, the nickel plating on the outermost surface contains 0.5 to 10.0 wt% of one or both of cobalt and palladium, so that the solder spread is further improved.

In the present invention, the sulfur content of the nickel layer within 0.1 μm in the depth direction from the nickel surface is set to 0.0
The reason why the content is set to 005 wt% or less is that a depth of 0.1 μm is formed from a surface which forms an alloy layer with solder and affects solder spread.
m, the sulfur element taken into the nickel layer from the brightener or the like exceeds 0.0005 wt%,
This is because the solder is repelled in an atmosphere having a high oxygen concentration, and the spread of the solder is reduced. By reducing the sulfur content to 0.0005 wt% or less in this portion, the contact angle between the lead-less solder (solder containing no lead) and the nickel surface is reduced, the solder spread is improved, and an excellent solder die is obtained. Shows bonding properties.

Further, in the present invention, the specular reflectance of the nickel plating is set to 25% or more in order to improve the wire bonding property, the resin burr removing property, and the control of the solder spread. In other words, wire bonding uses an alloy wire such as gold, aluminum, or copper to bond the chip and the lead using ultrasonic waves. However, the bonding reliability is better as the lead frame surface is smoother. The smoother the lead frame surface, the easier it is to control the spread of the solder by the solder bar during solder die bonding. Note that the smoothness of the lead frame surface has a correlation with the specular reflectance, and in the present invention, the smoothness is represented by the specular reflectance.

In the present invention, when the nickel plating on the outermost surface contains 0.5 to 10.0 wt% of one or both of cobalt and palladium, even if the oxygen concentration in the reducing atmosphere is high, the solder spread is further improved. Become. Here, the reason why the total content of cobalt and / or palladium in the nickel plating film is set to 0.5 to 10.0 wt% is that the effect of addition is small at 0.5 wt% or less.
If the content exceeds 0 wt%, the workability of the nickel plating film is reduced, and if heated for a long time after soldering, the solder adhesion is reduced.

As described above, according to the present invention, by limiting the sulfur content on the nickel surface and defining the specular reflectance, even in an atmosphere having a high oxygen concentration, the same good solder spread as in silver plating and the control of the solder spread can be achieved. It is possible to provide a nickel-plated copper alloy lead frame which is excellent in die bonding property by lead-less solder which is expected to develop in the future.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In a copper alloy lead frame,
The thickness of the nickel plating is required to be 0.5 μm or more to prevent corrosion, thermal oxidation and diffusion of copper, and usually 2 to 5 μm.
It is applied to the thickness of. On the other hand, in order to reduce the sulfur content in the nickel plating to 0.0005 wt% or less, there is a method of applying a matte nickel plating that does not contain any brightener, a uniform electrodeposition agent, a stress relaxation agent, etc. If the nickel plating is applied to a thickness exceeding 0.5 μm, there is a problem that the specular reflectance becomes 25% or less. Therefore, as specified in the present invention, in order to reduce the sulfur content in nickel within 0.1 μm from the surface to 0.0005 wt% or less and to set the mirror reflectivity to 25% or more, only the surface layer portion should be glossy. A plating method using a nickel plating solution containing no agent or the like, a method in which sulfur is concentrated on the outermost surface by heating after nickel plating, and then only the outermost surface is removed (etching or the like) can be considered.

When nickel plating is performed on two layers, it is preferable to apply bright or semi-gloss nickel plating of 1 to 5 μm on the underlayer in order to prevent copper diffusion and smooth the surface. If the thickness is less than 1 μm, diffusion of copper may not be prevented due to the presence of pinholes. If the thickness is more than 5 μm, not only productivity is reduced but also processability is reduced. Further, in a lead frame subjected to bending, it is desirable to perform semi-bright nickel plating with good workability of about 2 μm. On the other hand, it is desirable that the upper layer be plated with nickel plating containing no brightener at 0.1 to 0.5 μm. 0.1
If the thickness is smaller than μm, it is impossible to prevent the spread of the solder from being reduced due to the influence of impurities in the base. If the thickness is more than 0.5 μm, the specular reflectance starts to decrease, and the smoothness of the surface decreases (the wire bonding property decreases).

The surface hardness of the lead frame affects the workability of the lead portion, and the oxidation of nickel affects the spread of the solder. Then, by measuring the surface hardness and the oxidation state of nickel, it is possible to manage the state at the time of nickel plating production. For example, if the nickel plating bath or plating conditions change, the amount of impurities and brighteners taken in changes, and the more brighteners, the higher the surface hardness of nickel. In the present invention, the surface hardness is Hv20.
0 to 450 is desirable. If the surface hardness is lower than Hv200, the wire bonding property is reduced, and if the surface hardness is higher than Hv450, the workability is deteriorated. The oxidation of nickel is affected by the temperature of the plating solution, drying conditions and cleanliness. If the temperature rises or the plating becomes dirty, the nickel oxide peak to nickel metal peak intensity ratio (Nioxid
e / Nimetal), and if it exceeds 1.0, the solder spread is reduced. Contains hydrogen and oxygen content is 1000pp
When solder die bonding is performed in a reducing atmosphere of less than m, oxidation of nickel during storage is hardly a problem, but oxidation of nickel immediately after plating is small (Nioxid
e / Nimetal ≦ 1.0) is desirable.

[0019]

Embodiments of the present invention will be described below. [Example 1] Fe: 0.1 wt% and P: 0.03 wt%
Was plated with nickel plating having a specular reflectance of 35% at 1.7 μm, and then nickel plating containing no sulfur was given at 0.3 μm. After plating, a stamping process was performed, and the specular reflectance and the sulfur content (from the surface, 0.1 to 0.3) shown in Examples 1 to 3 in Table 1 were obtained.
A copper alloy lead frame having a sulfur content of 1 μm or less was obtained. The nickel plating layer containing no sulfur was prepared in Example 1 using a matte watt bath under the conditions of a bath temperature of 40 ° C. and a current density of 6 A / dm ² . Example 2 was prepared using a sulfamic acid bath containing no brightener.
Was prepared by plating using a sulfamic acid bath containing a trace amount of brightener, heating and removing sulfur from the surface layer. The copper alloy lead frame material was plated with a nickel sulfamate bath containing a brightener, plated with a normal matte nickel plating bath, and plated with a normal bright watt bath in Comparative Examples 1 to 3. 3, and the specular reflectance and sulfur content of each were measured. Comparative Examples 1 to 3
Is 2 μm.

[0020]

[Table 1]

The surface hardness of the copper alloy lead frame of Example 1 is Vickers hardness (load: 10 g) Hv220, and the oxidation of nickel is determined by the ratio of the nickel oxide peak intensity to the nickel metal peak intensity in ESCA analysis as (N
(oxide / Nimetal) = 0.7.

Using these copper alloy lead frames,
In a reducing atmosphere (a nitrogen gas atmosphere containing 10% of hydrogen and 700 ppm of oxygen), a tin-based solder (Sn-3.5%
The solder die bonding property was tested using Sb), and the wire bonding property test using an aluminum wire was performed. In addition, each test method in this example is as follows. (Surface hardness) Test load 10 g according to JISZ2251
At f, Vickers hardness was measured from the nickel plating surface. (Specular Reflectance) According to JIS Z8741, a light beam was incident at 45 degrees, and the ratio of the reflected light beam to the incident light beam in the specular reflection was measured. (Solder die bonding property) In a nitrogen gas atmosphere containing 10% hydrogen and an oxygen concentration of 700 ppm, a copper alloy lead frame is heated on a plate at 320 ° C. for 1 minute, and then has a diameter of 1 mm.
mm of lead-free solder (tin 96.5 wt%, antimony 3.5 wt%) balls were placed, and the area where the solder balls spread out after 10 seconds without flux and without stirring was measured. (Wire bonding property) Using an aluminum wire (99.99%, Annealed) of 5 mil (125 μm), wire bonding by an ultrasonic wave (pulse 60 kHz) application method at 25 ° C. in the air (bonding time: 1)
00 ms, a load of 250 g, and a UV output of 5.5 W). Next, the bonded aluminum wire was cut in the middle, the bonding state of the 2nd bond portion where the aluminum wire was bonded to the copper alloy lead frame was examined (tizer strength), and the wire breakage rate was calculated by the following equation. Wire breakage rate (%) = (number of broken wires / total number of test pieces) × 100

Table 1 shows the results and evaluation of the above test. In Table 1, the evaluation of the solder spread area is as follows: ◎ is very good, ○ is good, X is bad, and the evaluation of the wire breakage rate is 100% for X and less than 100% for X. . Specular reflectance of nickel plating is 25% or more, sulfur content of nickel layer within 0.1 μm from surface is 0.000
Examples 1 to 3 satisfying the requirement of the present invention of 5 wt% or less show good solder spread without flux in a state where the oxygen concentration in the reducing atmosphere is as high as 700 ppm, that is, excellent solder die bonding properties. Wire breakage rate is 1 even with wire bonding with aluminum wire
The bonding reliability was as high as 00%. On the other hand, in Comparative Example 1, since the sulfur content is high, the solder spread is poor in an atmosphere having a high oxygen concentration, and in Comparative Example 2, the mirror reflectance is low and the surface is rough, and the wire bonding property is poor. In Comparative Example 3, since the sulfur content was high, the solder spread was poor in an atmosphere having a high oxygen concentration. In Comparative Example 2, it was difficult to control the removal of resin burrs and the spread of solder.

Example 2 Here, cobalt (Co) or palladium (P) was applied to nickel plating on the outermost surface.
The effect of adding d) (corresponding to claim 2) was tested.
First, the copper alloy lead frame material shown in [Example 1] was subjected to nickel plating containing a brightener at 1.7 μm, and Co, Pd, and Co + Pd were added thereto. (Alloy) plating containing 0.3% in various ratios. After plating, stamping was performed to obtain a copper alloy lead frame having a mirror reflectance of nickel plating of 30% and a sulfur content of 0.001% or less from the surface of 0.1 μm or less. The surface hardness of this copper alloy lead frame is Vickers hardness Hv1
It was 15.

The copper alloy lead frame was subjected to a solder die bonding property test as described in [Example 1] and a workability test as described below. (Workability test) The lead portion was repeatedly bent in the forward and reverse directions (inner radius 0.5 mm, bent at 90 °), and the number of bendings before breaking was determined.

Table 2 shows the results and evaluation of the above test. In Table 2, the evaluation of the solder spread area is the same as in [Example 1], and the evaluation of the number of times of repeated bending is ○ is good and X is bad. Examples 4 to 7 satisfying the requirement of claim 2 containing one or both of Co and Pd in an amount of 0.5 to 10.0 wt% have a large solder spread area in an atmosphere having an oxygen concentration as high as 700 ppm, and have a high repetition rate. Many bends. In other words, the characteristics were that the solder die bonding property was very excellent and the workability was hardly reduced. On the other hand, Comparative Example 4 has a low Co content, so the diffusion bonding speed with the solder is low, and the degree of improvement of the solder spread area is small. Comparative Examples 5 and 6 have a high Co or Pd content, and A decrease in sex was observed. In Comparative Examples 5 and 6, a decrease in the soldering heat-peelability was also observed.

[0027]

[Table 2]

[0028]

The nickel-plated copper alloy lead frame of the present invention exhibits good wettability with a lead-free tin-based solder, and is equivalent to a silver-plated product without flux even in an atmosphere having a high oxygen concentration during a semiconductor assembly process. The solder spread can be obtained, the solder spread can be controlled, and excellent solder die bonding property is exhibited. In addition, it has good wire bonding properties with aluminum wires, does not reduce workability, and exhibits excellent properties in productivity and workability.

Claims (2)

(57) [Claims] 1. A nickel plating for solder die bonding, wherein the nickel plating has a mirror reflectance of 25% or more, and the sulfur content in nickel within 0.1 μm from the surface is 0.0005% by weight or less. Copper alloy lead frame. 2. The nickel plating on the outermost surface is 0.5-1.
The nickel-plated copper alloy lead frame for solder die bonding according to claim 1, further comprising 0.0 wt% of one or both of cobalt and palladium.