JPS61187261A - Lead frame for semiconductor and manufacture thereof - Google Patents

Abstract

PURPOSE:To reduce cost by directly forming a silver plating layer on a metallic substrate constituting a lead frame through an electroless plating method. CONSTITUTION:A metallic substrate 1 shaping a lead frame is pre-treated, and dipped in an etching liquid, and the outer surface of the substrate is etched and treated. The substrate is washed by ion-exchange water, dried, and introduced into a vacuum furnace and heated, and nitrogen gas is encapsulated into the vacuum furnace to return pressure in the furnace to atmospheric pressure, thus cooling the frame. The substrate is forwarded to a rinsing treating process 6, dipped in a rinsing liquid, washed by ion-exchange water, and dried. On the other hand, a silver plating liquid is compounded in a silver plating liquid compounding process 7, and the lead frame is plated with silver 2 as fast as possible. Accordingly, a plating treating process is simplified, and the cost of the lead frame is reduced.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a semiconductor lead frame in which a silver plating layer is directly formed on a metal substrate by an electroless plating method, and a method for manufacturing the same. This makes it possible to put into practical use low-cost lead frames made of stainless steel, iron, etc.

(Prior art) Conventionally, materials such as Nivar, 42 nickel alloy, 52 nickel alloy, copper alloy, etc. have been widely used as materials for semiconductor lead frames, and among these, copper alloy and 42 nickel alloy have It accounts for over 80%. This is because it exhibits excellent performance in terms of dimensional accuracy, mechanical properties, electrical properties, thermal conductivity, coefficient of thermal expansion, punchability, corrosion resistance, etc. required for lead frame materials.

However, the price of the nickel alloy and copper alloy is S
It is approximately twice as expensive as US 304 stainless steel and approximately four times as expensive as iron, which is a major obstacle in reducing the cost of lead frames.

On the other hand, another important characteristic that the lead frame must have is the problem of so-called wire bonding of the inner lead portion. Having good wire bondability is an essential requirement for lead frames, and for this purpose, copper, nickel, tin, or their alloys are generally plated on the metal substrate, and then the inner leads etc. A method of silver plating has been adopted (Japanese Patent Publication No. 59-36426, etc.).

However, the method described above has problems in that the plating process is a two-step process, making quality control difficult and making it difficult to reduce manufacturing costs.

(Problems to be Solved by the Invention) As mentioned above, the basic problem with conventional semiconductor lead frames is that they cannot be lowered in price due to increased costs due to the materials used and the complexity of the plating process. There is.

The present invention solves the above-mentioned problems with conventional lead frames for semiconductors, and reduces plating costs by applying silver plating directly onto the metal substrate forming the lead frame by electroless plating. In addition, the present invention provides a semiconductor lead frame and its manufacturing method that enables direct silver plating on stainless steel, etc., thereby making it possible to utilize inexpensive metal substrates and significantly reduce costs. .

(Means for Solving the Problems) The inventors previously developed a direct electroless silver plating method for stainless steel and disclosed it as JP-A-58-181857 and JP-A-9 Sho-181858. ing.

The technique mainly involves applying silver plating to the outer surface of the vacuum insulation part of a stainless steel thermos flask to form a silver mirror surface to improve insulation performance and to reduce plating processing costs.

By the way, these techniques are for plating a relatively large area, and as long as a smooth silver mirror surface can be formed, the intended purpose can be achieved even if the adhesion strength or thickness of the silver plating film is insufficient. can be fully achieved. Therefore, this technology still has many problems to be solved in terms of the adhesion of the silver plating film, the thickness of the silver plating film, the uniformity of the film thickness, etc., and the technology can be applied directly to lead frames. It is difficult to apply it to

For example, in lead frames, wire bonding is required, so the adhesion between the silver plating layer and the metal substrate must be particularly strong, and the inner frame requires a considerably thick silver plating layer. shall be. Further, in order to maintain the quality of the semiconductor chip, it is not allowed that small pores remain in the silver plating film.

Therefore, the inventor of the present application conducted numerous plating tests in order to improve the adhesion of the silver plating layer, increase the film thickness, and eliminate small holes remaining in the film, and based on the test results, it was found that In addition, a lead frame for semiconductors and a method for manufacturing the same which have excellent wire bonding properties and do not cause deterioration in quality have been developed. The basic structure is that a silver plating layer is directly formed by electroless plating.

Further, in the second invention of the present application, after etching the surface of the metal substrate constituting the lead frame with a treatment liquid containing hydrofluoric acid and nitric acid as main components, the metal substrate is heated to a temperature of 500.
After the heat treatment, heat treatment is performed in an atmosphere containing hydrogen gas at a temperature of ℃ or higher and a pressure of I The basic structure is to contact the metal substrate and form a silver plating layer directly on the metal substrate. ○ (Function) To form a silver plating layer directly on the metal substrate without providing an intermediate plating layer. , the effects of thermal expansion characteristics and heat transfer resistance of the intermediate plating layer are completely eliminated, making wire bonding and resin molding easier.

In addition, since the outer surface of the metal substrate is etched with a mixed acid solution mainly composed of hydrofluoric acid and nitric acid, and then subjected to vacuum heating or reduction heat treatment, the outer surface of the metal substrate is extremely high-grade clean. At the same time, minute irregularities are formed on the outer surface. This increases the adhesion strength of the plating layer by -1 in the layer direction, and facilitates increasing the layer thickness by stacking the plating layers. Further, there is no gas released from the outer surface of the metal plate during plating, and pinholes in the plating layer are prevented.

(Example) Figure 1 shows stainless steel (SUS304) according to the present invention.
Fig. 2 is a plan view of a lead frame made of mild steel, Fig. 3 is a plan view of a lead frame made of 42 nickel alloy, and Fig. 4 is a portion of the inner frame portion of the lead frame. FIG.

The entire outer surface of the metal substrate forming each frame is one
It is covered by a silver plating layer 2 with a thickness of ~311 m. In addition, the inner frame part B and chip fixing part A of the lead frame have a thicker silver plating layer.
A silver plating layer 2 with a thickness of 5 to 10 μm is formed.

FIG. 5 is a diagram showing a silver plating process using the electroless plating method for a lead frame of the present invention, where 3 is a lead frame pretreatment process, 4 is an etching process, 5 is a heat treatment process in vacuum, and 5 is a hydrogen plating process. A heat treatment process in an atmosphere containing gas, 6 a rinsing process, 7 a silver plating solution preparation process,
8 is a silver plating process.

First, the lead frame 1 molded into a predetermined shape is degreased by cleaning with an alkaline processing liquid, and further cleaned with a Freon 113 solvent containing a surfactant to perform pretreatment of the frame 1. Next, in the etching step 4, 7 VOL% of fluorocarbon, 18 VQ, % of nitric acid, 28 VOI-% of glacial acetic acid, and the remainder were immersed in an etching solution consisting of ion-exchanged water at a temperature of 65° C. for about 30 minutes. Then, etching is performed on the outer surface of the substrate. Next, this was washed with ion-exchanged water, dried, and then placed in a vacuum furnace at a vacuum level of 2 x 10 Torr and 900°C for 30 minutes.
After heating for a minute, the vacuum furnace is filled with nitrogen gas to return it to atmospheric pressure and the frame is cooled.

It has been experimentally confirmed that a good silver plating layer cannot be obtained if the heating temperature of the vacuum furnace is less than about 500° C. and the degree of vacuum is more than IXIO Torr.

Further, in this example, heat treatment is performed by vacuum heating, but instead of this, 200 VOL% of hydrogen gas and 95% of inert gas such as argon gas are used.
You may make it heat for 30 minutes at the temperature of ℃ or more. Even in this case, if the heating temperature becomes 200℃ or less,
6' It is impossible to obtain a satisfactory silver plating layer.
It has been certified as a birth.

When the heat treatment process is finished, it is sent to the rinsing process 6,
Immerse in a rinsing liquid made of tin and an appropriate hydrating agent (specifically, RNA solution (trade name, manufactured by London Laboratory, USA) diluted 100,000 times with ion-exchanged water, etc.), and then rinse with ion-exchanged water. Wash with water and pat dry. Note that the rinsing process may be omitted depending on the case.

Meanwhile, in the silver plating solution preparation step 7, silver nitrate and hydroxide are used.) A silver wave containing IJium (specifically, the above-mentioned London Labora, USA), a product name ATA solution manufactured by IJ-Inc., and a product name ATS solution. etc.) and a reducing solution containing glucose, drunkenness, and alcohol (specifically, product name 2XKR-3L, etc. manufactured by London Laboratory, USA) are mixed in equal amounts to prepare a silver plating solution. After the preparation of the silver plating solution is completed, the silver plating solution is used to silver plate the lead frame that has been subjected to the rinsing process as soon as possible.

The plating method in the silver plating process may be a method of immersing the lead frame in a silver plating solution, or a method of flowing a silver plating solution onto the lead frame. By carrying out the silver plating treatment, a silver plating layer is formed by a so-called silver mirror reaction, and the thickness thereof is about 1 to 2 μm.

The lead frame that has undergone the plating process 8 is washed with water and then dried.

In the above embodiment, the entire surface of the lead frame is plated with silver, but the inner lead portion B and chip holding portion A of the lead frame may be plated with silver. In this case, the silver plating solution may be dropped by an appropriate method only on the required areas for plating, or
A sponge-like band may be impregnated with a silver plating solution, and the band may be pressed against a desired portion of the lead frame.

Furthermore, if it is desired to thicken the silver plating layer 2 on the inner lead portion B or the chip holding portion A, the plating solution may be repeatedly dropped onto these portions. For example, when the silver plating solution was dropped onto the inner lead portion B five times using a dropper, the thickness of the silver plating layer increased to 4 to 6 μm.

Furthermore, it is also possible to increase the thickness of the entire plating layer by applying silver plating by electrolytic plating to a lead frame that has been silver plated by electroless plating.

(Effects of the Invention) Since the semiconductor lead frame according to the present invention is directly silver plated without providing an intermediate plating layer, the plating process is simplified and the cost of the lead frame can be significantly reduced. There is no distortion of the plating layer due to the difference in thermal expansion characteristics between the intermediate plating layer and the silver plating layer, and no peeling due to this. Mano 2: Since the intermediate plating layer is omitted in the chip mounting area, the chip and lead frame are bonded via a silver plating layer with good thermal conductivity.
Cooling characteristics are further improved. Furthermore, when the silver plating layer only on the inner lead portion is made thicker, the wire bonding performance can be improved without increasing the plating cost.

On the other hand, the method for manufacturing a semiconductor lead frame of the present invention includes:
Since the plating process is performed after etching the metal substrate, the adhesion of the silver plating layer is greatly improved compared to the case where no etching process is performed. In addition, by repeatedly contacting the silver plating solution at intervals, the thickness of the silver plating layer can be increased locally very easily and reliably, greatly improving the wire bonding properties of inner leads etc. Towards the end.

Furthermore, since the outer surface of the metal substrate is etched and then heat-treated, gas molecules existing near the outer surface of the substrate are more reliably etched than when heat-treated directly without etching. will be removed. As a result, no gas is released from inside the substrate during the formation of the silver plating layer, making it possible to obtain a silver plating layer with extremely few pinholes.

As mentioned above, the present invention has excellent practical utility.

[Brief explanation of the drawing]

Fig. 1 is a plan view of a stainless steel glued frame according to the present invention, Fig. 2 is a plan view of a mild steel lead frame, and Fig. 3 is a plan view of a 42 nickel alloy lead frame. be. FIG. 4 is a partial vertical sectional view of the inner lead portion. FIG. 5 is a process diagram regarding silver plating of the lead frame of the present invention. ■ Metal substrate A Chip fixing part 2 Silver plating layer B Inner frame part 2 Silver plating layer for inner lead parts etc. 3 Pretreatment process 4 Rinse treatment process 5 Vacuum heat treatment process 5 Reducing gas heat treatment process 6 Rinse treatment process 7 Plating solution preparation process 8 Silver plating process

Claims (4)

[Claims] (1) A lead frame for a semiconductor, characterized in that a silver plating layer is directly formed on a metal substrate constituting the lead frame by electroless plating. (2) In the semiconductor lead frame according to claim 1, the metal substrate is made of iron or stainless steel, and the thickness of the silver plating layer on the inner frame portion of the metal substrate is set to be smaller than that of other portions. A semiconductor lead frame with a thickness greater than that of the silver plating layer. (3) After etching the surface of the metal substrate constituting the lead frame with a treatment solution mainly composed of hydrofluoric acid and nitric acid, the metal substrate is etched at a temperature of 500°C or higher and a pressure of 1×.
In addition to heat treatment in a vacuum of 10 Torr or less or in an atmosphere containing hydrogen gas at a temperature of 200°C or more,
A lead frame for a semiconductor, characterized in that a silver plating solution, which is a mixture of a silver solution containing silver nitrate and a reducing solution containing glucose, is brought into contact with the heat-treated metal substrate to directly form a silver plating layer on the metal substrate. manufacturing method. (4) In the method for manufacturing a semiconductor lead frame according to claim 3, a silver plating solution is repeatedly brought into contact with an inner lead portion on a metal substrate made of iron or stainless steel. A method for manufacturing semiconductor lead frames with a thick silver plating layer.