JPH0964264A - Lead frame partial plating method - Google Patents

Abstract

(57) [Abstract] [PROBLEMS] To use a mask having resistance to a plating solution and immerse only a predetermined region of a lead frame in the plating solution in a state where the mask is exposed to the plating solution, and at least an inner lead portion is made of a precious metal. In a partial plating method of a lead frame to be plated, an attempt is made to provide a partial plating method for increasing current density and plating efficiency. SOLUTION: While immersing a lead frame for manufacturing a semiconductor device in a plating solution with a predetermined area of the lead frame exposed to the plating solution using a mask having resistance to the plating solution, This is a partial plating method for a lead frame in which at least the inner lead portion is plated with a noble metal, and plating is performed while forcibly applying agitation to the plating solution by low-frequency vibration.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plating method for plating a noble metal on at least an inner lead portion of a lead frame used in a semiconductor device.

[0002]

2. Description of the Related Art Conventionally, a (single layer) lead frame used as an assembly member of a resin-sealed semiconductor device is formed by a pressing method or an etching method, and generally, as shown in FIG. A die pad 712 for mounting a semiconductor element, and an inner lead 713 for connecting to a semiconductor element provided around the die pad 712.
And an outer lead 714 for connecting to the external circuit continuously with the inner lead 713, a dam bar 715 serving as a dam when resin-sealing, a frame portion 716 supporting the entire lead frame 710, and the like. I have it.
Then, the lead frame (single layer lead frame) 710
Is usually made of a metal having excellent conductivity such as Kovar, 42 alloy (42% nickel-iron alloy), and copper alloy.
As shown in FIG. 7B, the semiconductor element 720 is mounted on the die pad 712, and the terminals (pads) of the semiconductor element 720 are mounted.
The semiconductor device 700 was manufactured by connecting the wire 721 and the tip of the inner lead 713 with a wire 730 such as gold and then sealing with a resin 740. As described above, since the terminals (pads) 721 of the semiconductor element 720 and the tips of the inner leads 713 are connected by the wire 730 such as gold, silver plating having excellent conductivity and strong bonding force with the wire is used. , At least on the inner lead tip,
Partial silver plating was commonly used.

Initially, the silver plating treatment was carried out by immersing the lead frame in a highly alkaline alkaline cyanide bath and then silver-plating the entire surface of the lead frame. This method has a current density of several A / Only dm ² can be obtained, productivity is low, and it is necessary to use toxic cyanine at a high concentration, so there is a problem in terms of safety, and silver plating is also applied to unnecessary parts. There was a problem. The reason why the current density can be only several A / dm ^{2 in} such a plating method by immersion will be briefly described below. When performing normal plating, an electric double layer (Helmholtz layer and diffusion double layer) is formed on the cathode side to be plated by metal ions and free electrons and electrolyte ions on the solution side, and the deposited metal ions pass through this interface. It progresses when an electric current flows. For this reaction on the cathode side, supply of substances involved in the reaction to the metal / solution interface,
And the process of mass transfer such as removal from the metal / solution interface. It is considered that (a) diffusion due to a concentration gradient, (b) migration due to a potential gradient, and (c) movement of a substance due to convection of a solution are considered for the movement of this substance. The reaction rate of the reaction on the cathode side is equivalent to the current, and it is considered that this reaction proceeds at a constant rate because electroplating is performed at a constant current. However, since the cathode side where metal ions are deposited is inside the electric double layer and a potential drop occurs through the diffusion layer, thinning the diffusion layer prevents the potential drop, and as a result, the deposition rate of metal is increased. Will lead to faster.

On the other hand, in a partial plating system (also called high speed jet plating system) in which a plating solution is sprayed from a nozzle or the like onto a predetermined portion of the lead frame, the plating solution is appropriately flown from the nozzle to the cathode side. There is an advantage in that the diffusion layer is thinned and the deposition rate is increased by ejecting with the method of 1., and the partial silver plating treatment of the lead frame has come to be performed by this method. The conventional partial silver plating process of this system is a plating apparatus 50 as shown in FIG.
No. 0 is used, the area other than the plated area of the lead frame 510 is covered with the masking jig 520 and pressed, and the plating solution 580 ejected from the nozzle 540 is applied to the plated area to perform plating. It was mainly done. Incidentally, as the plating bath, it is general to use a neutral cyan bath or a weak alkaline bath which is close to neutral. In this partial plating method, a jig is required for each type of lead frame, and it takes a long time to manufacture the jig, and wear and fatigue occur as it is used, so replacement is necessary, which results in a decrease in productivity. It was also a problem in terms of cost and cost. In addition, when considering the quality of plating, the positioning is performed by the positioning pins, and the plating is performed after holding the single lead frame between the upper and lower jigs, so that the plating quality such as plating position accuracy and plating thickness uniformity can be improved. It is easily affected by the accuracy of the manufactured jig and the accuracy of mounting. Further, there is a problem that plating is likely to be deposited on the side surface and the back surface of the lead frame, which originally does not require plating, and requires a highly empirical technique for adjustment. Furthermore, due to the increase in the number of input / output terminals of semiconductor elements due to the progress of semiconductor processes and the shrinking of the inner lead part due to the shrinking of the package size, the dimensional accuracy of the plated part is becoming more severe. Is becoming difficult to deal with.

For this reason, a jig is not required, and it is possible to cope with the increase in the number of terminals of semiconductor elements and the narrowing of inner lead portions. As shown in the figure, using a photosensitive electro-deposition resist having resistance to the plating solution, the lead frame is plated while it is immersed in the plating bath by masking only the predetermined area of the lead frame exposed to the plating solution. The silver plating method has come to be adopted. This partial silver plating method will be briefly described with reference to FIG.
Incidentally, FIG. 6 shows a cross section of a part (characteristic part) of the lead frame in order to explain the partial silver plating method using the electrodeposition resist. First, the entire lead frame 610 is electrolytically degreased, pickled and chemically polished (see FIG. 6).
(A)), copper strike plating 620 as a base plating is applied to the entire surface of the lead frame 610 (FIG. 6).
(B) After that, an electrodeposition resist 630 is formed on the entire surface.
(FIG. 6C) Next, a predetermined portion is exposed with ultraviolet rays (exposure light) 650 using a predetermined photomask 640 to cure only the exposed portion (FIG. 6D), and then a developing process is performed. Then, the electrodeposition resist 630 in the unexposed portion is removed to expose the plated portion. (FIG. 6 (e)) Next, after performing acid cleaning on the exposed plated portion 660,
The entire lead frame is immersed in a silver plating solution, plating is performed at a predetermined current density and for a predetermined time, and a silver plating (coating) 690 having a desired film thickness is obtained on the plating part 660. (FIG. 6
(F) After that, the electrodeposition resist 630 is peeled off with a peeling solution to obtain a lead frame 610A having silver plating (coating) 660 only on a predetermined region. (FIG. 6 (g)) As a pretreatment for the plating treatment, in addition to acid cleaning, alkali cleaning or the like is performed as necessary to the extent that the electrodeposition resist is not damaged. Also, copper strike plating is
If the lead frame material is a copper alloy, Sn,
Since it contains impurities such as Ni, it is thinly copper-plated as a base metal for silver plating, but it is not always necessary. In some cases, the lead frame material is directly plated with silver. In this case, the silver plating base metal is a lead frame material.

Recently, instead of silver plating, palladium or a palladium alloy as a noble metal has been formed on the lead frame by a plating method. In this case, however, the entire surface of the lead frame can be plated. Since it is relatively expensive, plating is applied only to the necessary areas, and as in the case of silver plating, in addition to the masking method using a jig, a photosensitive material that is resistant to the plating solution is used. A masking method using a positive electrodeposition resist has come to be adopted.

[0007]

However, as described above, in the masking method using the photosensitive electrodeposition resist having resistance to the plating solution, the entire lead frame is covered with the electrodeposition resist and the predetermined amount is obtained. Is exposed through an exposure mask in which the pattern shape of is formed, an unnecessary portion is dissolved with a predetermined developing solution, and drying is performed to form a mask with an electrodeposition resist having a predetermined shape, and then silver, A film of a noble metal such as palladium or a palladium-based alloy is formed. In this method as well, since the dipping method is basically used, the current density is only several A / dm ² , and the productivity is low and the toxicity is low. Since it was necessary to use a certain cyan at a high density, there was a problem in terms of safety. Under the circumstances, the present invention uses a photosensitive resist or the like having resistance to a plating solution, and immerses it in a plating solution while masking only a portion that does not require plating adhesion to perform partial plating. In the partial plating method to be performed, the present invention aims to provide a plating method that increases the current density and the plating efficiency.

[0008]

According to the method for partially plating a lead frame of the present invention, a predetermined area of a lead frame for a semiconductor device, which has been externally processed using a mask having resistance to a plating solution, is masked. A partial plating method for a lead frame, in which at least the inner lead portion is plated with a precious metal while the lead frame is immersed in a plating solution, characterized in that plating is performed while stirring the plating solution by low-frequency vibration. Is. And
The mask having resistance to the plating solution is a photosensitive resist, and the photosensitive resist is an electrodeposition resist.
The precious metal plating described above is a plating of one kind of precious metal selected from gold, silver, palladium, and palladium alloys. Further, in the above, all or part of a sheet or hoop to which a plurality of lead frames are attached is immersed in a plating solution to perform plating. In addition, the above
Low-frequency vibration refers to vibration below 200 HZ,
A stirring effect is obtained at 20 HZ or more.

In the method of partially plating the lead frame of the present invention, the lead frame material is 42 alloy (4
2% nickel-iron alloy), Kovar, and various alloys thereof can be used, and the lead frame may be in the form of a series, a hoop, or a sheet. The plating resist for coating the entire surface of the lead frame is a photosensitive resist, which may be either a positive type or a negative type, and an electrodeposition resist is preferable. The electrodeposition resist may be either anion precipitation type or cation precipitation type, and the electrodeposition resist is deposited by applying a predetermined voltage and current, and heat treated to form an insulating film without pinholes or the like. You can The patterning of the photosensitive resist is performed by exposing and developing using a pattern for exposure having a predetermined shape to expose the metal part (base metal part) of the plated region. still,
Exposure is used according to the type of resist used,
It is performed by using an ultraviolet lamp having an emission spectrum in the exposure sensitivity region of the resist film. Further, the development is performed under a developing solution and developing conditions suitable for the resist to dissolve and remove the resist film in the plated region.

As the noble metal plating, gold plating, silver plating, palladium plating, palladium alloy plating or palladium composite plating can be used. As each plating solution, a commercially available one can be used, and the plating method and plating conditions are the methods and conditions suitable for the plating solution. Although the resist is peeled off after the plating film is formed, it is removed under a peeling solution and peeling conditions suitable for the resist used, and further degreased and washed if necessary.

[0011]

The function of the lead frame partial plating method of the present invention is as follows.
With such a configuration, a predetermined area of the lead frame is masked using a mask having resistance to the plating solution, and while only the desired plating area is exposed to the plating solution, the plating solution is immersed in the plating solution. In the partial plating method of the lead frame in which at least the inner lead portion is plated with a noble metal, it is possible to increase the plating current density and the plating efficiency. Specifically, the plating current is increased and the plating efficiency is increased by performing the plating while forcibly agitating the plating solution with low frequency vibration of 20 to 200 HZ. As a result, it is not the sparger type partial plating method of plating while spraying the plating solution from a nozzle while masking with a conventional jig, but the immersion type partial plating of plating while immersing in the plating solution. In addition, it is possible to increase the plating efficiency, and in the case of the partial plating method using a sparger type jig, there is a problem of unnecessary plating deposition on the aspects of productivity, cost, position accuracy, side surface and back surface. In addition, it is said that it is possible to cope with the miniaturization of the lead frame.

[0012]

EXAMPLES A method of partially plating a lead frame of the present invention will be described based on examples. FIG. 1 is an apparatus for carrying out the partial plating method of the lead frame of the embodiment, and FIG. 2 is a process drawing for explaining the partial plating method of the lead frame of the present embodiment, showing the cross section of the characteristic part. Is. In FIG. 1, 100 is a plating device, 110 is a lead frame (cathode), 120 is an anode, 130 is a plating tank, 140 is a plating solution, 150 is a mount, 160 is a vibration motor, and 161.
Is an inverter, 162 is a spring, 163 is a stirring blade, 164
In FIG. 2, 111 is a die pad, 112 is an inner lead, 113 is an electrodeposition resist, 114 is a plating pattern plate, 115 is a plating area, and 116 is a plating film. In this embodiment, as shown in FIG.
QFP (Quad) made of 0.15-mm thick copper alloy
A sheet 400 manufactured by an etching method in a single sheet in a state in which a plurality of lead package 110 of the Flat Package type having 160 pins are connected by a hanging bar 410 and a frame portion 420.
In a state in which a predetermined area of each lead frame 110 is masked with a photosensitive electrodeposition resist having plating resistance, it is immersed in the plating tank 130 shown in FIG. 1 and the plating solution is stirred by low frequency vibration. While being plated.

The plating solution 140 has the same composition as in the partial plating method using a sparger type jig, and has a silver concentration of 50 g / l, a KCN concentration of 1.0 g / l,
A plating solution with a liquid temperature of 45 ° C. and pH 8.0 to which a small amount of a brightening agent was added was used, but the liquid temperature was in the range of 40 to 50 ° C. and the pH was in the range of 7.8 to 8.2. preferable.

In the plating apparatus 100, the lead frame (cathode) 110 and the anode are immersed in the plating solution 140 in the plating solution tank 130, and plating is performed with the cathode side at the -potential and the anode side at the + potential. The plating solution is continuously or intermittently stirred by the vibration motor 160 by the stirring blade 163 driven via the shaft rod 164 by low-frequency vibration. The vibration motor 160 causes a frequency vibration of 20 to 200 HZ, is driven by an inverter 161, and is fixed to the pedestal 150 via a spring 162. The reason why the vibration motor 160 is driven in the range of 20 to 200 HZ is as follows. As described above, when performing normal plating, an electric double layer (Helmholtz layer and diffusion double layer) is formed on the cathode side (cathode side) by metal ions and free electrons and electrolyte ions on the solution side. , The deposited metal ions are plated by the current flowing through this interface, and the cathode side where the metal ions are deposited is inside the electric double layer, and a potential drop occurs through the diffusion layer. Thinning prevents potential drop, and as a result, increases the metal deposition rate.
At a frequency of Z or less, the plating solution is not sufficiently stirred and the diffusion layer on the cathode side (lead frame side to anode) does not become thin, so that the current density cannot be kept high. Also,
If the frequency is higher than 200 HZ, the electrodeposition resist will be partially peeled off. In the case of this example, as shown in FIG. 3, a limiting current density of 10 A / dm ² or more was obtained at 20 to 60 HZ. The effect of stirring when such low-frequency vibration is applied to the plating solution and agitated varies depending on the structure of the plating tank, the capacity, and the object to be plated, but when the plating apparatus 100 shown in FIG. Compared with the limiting current density when stirring by frequency vibration is not performed,
As shown in FIG. 3, a current density of 5 times or more was obtained. However, the plating conditions were as follows. As the plating solution, the solution temperature was 40 °, the silver concentration was 50 g / l, and the KCN concentration was 1.
The one with 0 g / l and PH 8.0 was used. The plating bath 130 is made of heat-resistant polyvinyl chloride, stirring rod 163, shaft rod 16
4 is stainless steel. Generally, in the plating by the immersion method, only a current density of ^{2 to} 3 A / dm ² can be passed, but in the present embodiment, 10 to 25 A / dm ² is possible, and the plating time is about 1/3 to 1/12. It will be shortened.

Next, the method of partially plating the lead frame of this embodiment will be described below with reference to FIG. The partial plating is, as described above, a QFP (Qua) made of a copper-based alloy having a thickness of 0.15 mm produced by the etching processing method.
This is performed using the sheet 400 shown in FIG. 4 in which a plurality of d flat package) lead frames 110 are connected, and FIG. 2 includes the die pad portion 111 and the inner lead 112 of one of the lead frames 110. The state of each step in the cross section is shown. First, the lead frame 110 obtained by etching
Is electrolytically degreased, pickled and chemically polished (Fig. 2
(A)), copper strike plating 220 as base plating is applied to the entire surface of this lead frame (see FIG. 2).
From (b)), the lead frame 110 is dipped in an electrodeposition resist bath and energized to deposit a resist,
A film of the electrodeposition resist 113 having a thickness of about 20 μm was formed by drying with hot air. (FIG. 2 (c)) Next, ultraviolet rays (exposure light) 115 were contact-exposed using a plating pattern plate that shields only the plated portions to cure only the exposed portions. (FIG. 2 (d)) As the electrodeposition resist, a negative type electrodeposition resist (product number 2100 manufactured by Shipley Co.) was used. Next, a development process was performed to remove the electrodeposition resist 113 in the unexposed portion, and the plated portion was exposed. (FIG. 2 (e)) As a developing solution, 2005 (manufactured by Shipley) was used.
After that, the entire lead frame is immersed in a silver plating solution, and plating is performed at a predetermined current density and time while stirring by low frequency vibration to obtain a silver plating (film) 1 having a desired film thickness.
17 was obtained. (FIG. 2 (f)) In the present embodiment, the frequency of the low-frequency vibration was set to 40HZ, but the frequency is not particularly limited to this, and it is 20 to 20 as described above.
It is determined according to the device and conditions in the range of 0HZ. After that, the electrodeposition resist 113 is removed with a stripping solution 2007 (manufactured by Shipley).
Then, the lead frame 110A having the silver plating (coating) 117 only on a predetermined area was obtained. (Fig. 2 (g))

[0016]

EFFECTS OF THE INVENTION The lead frame partial plating method of the present invention is configured as described above, and masks with a resist to perform partial plating. It is possible to provide a plating method that raises the plating efficiency. More specifically, by performing plating while forcibly giving stirring to the plating solution by low-frequency vibration of 20 to 200 HZ, plating at a current density of 10 to 25 A / dm ² is possible, and as a result, the plating time is 1/3 to 1 / compared with the conventional case where frequency vibration is not performed only by immersion
It can be shortened to 15. Further, the method of partially plating the lead frame of the present invention does not use a jig, and does not plate the side surface and the back surface of the unnecessary lead frame,
It is possible to plate only the required area,
In particular, it is effective for expensive silver, palladium, palladium alloys and the like.

[Brief description of drawings]

FIG. 1 is a schematic view of a plating apparatus for carrying out the lead frame partial plating method of Example 1.

FIG. 2 is a process schematic diagram of the lead frame partial plating method according to the first embodiment.

FIG. 3 is a diagram for explaining the relationship between the plating current density and the frequency vibration frequency in the present embodiment.

FIG. 4 is a diagram for explaining a sheet that has been subjected to an etching process by imposing a plurality of lead frames.

FIG. 5 is a schematic view of a masking type partial plating apparatus using a jig.

FIG. 6 is a process schematic diagram of a conventional immersion plating method.

FIG. 7 is a plan view of a lead frame (single layer lead frame).

[Explanation of symbols]

100 Plating Equipment 110 Lead Frame (Cathode) 110A Silver Plated Lead Frame 111 Die Pad 112 Inner Lead 113 Electrodeposition Resist 114 Plating Pattern Plate 115 Ultraviolet (Exposure Light) 116 Plating Area 117 Plating Coating 120 Anode 130 Plating Tank 140 Plating Solution 150 160 Vibration motor 161 Inverter 162 Spring 163 Stirring blade 164 Shaft bar 220 Copper strike plating 400 Sheet 410 Connecting bar 420 Frame (frame) part 430 Positioning hole part 500 Plating device 510 Lead frame 520 Masking jig 530 Press jig 530A Pressed material 530B Elastic material 540 Nozzle 550 Constant current source 560 Anode electrode 570 Cathode electrode 610 Lead frame 611 Die pad 612 a Inner lead 620 Copper strike plating 630 Electrodeposition resist 640 Photo mask (pattern plate) 650 Ultraviolet (exposure light) 660 Plating area 690 Silver plating film 700 Semiconductor device 710 Lead frame 712 Die pad 713 Inner lead 714 Outer lead 715 Dam bar 716 Frame (frame) part 720 Semiconductor element 721 Terminal (pad) 730 Wire 740 Resin

Claims (5)

[Claims] 1. A predetermined area of a lead frame for a semiconductor device, which is externally processed using a mask having resistance to a plating solution, is masked, and at least the inner lead portion is immersed while the lead frame is immersed in the plating solution. A method of partially plating a lead frame, wherein the plating is carried out while agitating the plating solution by low-frequency vibration. 2. A partial plating method for a lead frame, wherein the mask having resistance to the plating solution according to claim 1 is a photosensitive resist. 3. A partial plating method for a lead frame, wherein the photosensitive resist according to claim 2 is an electrodeposition resist. 4. The method for partially plating a lead frame, wherein the noble metal plating according to claim 1 is plating of one kind of noble metal selected from gold, silver, palladium and palladium alloys. 5. The method for partially plating a lead frame according to claim 1, wherein all or a part of the sheet or hoop to which a plurality of lead frames are attached is immersed in a plating solution for plating. .