JPH0846125A - Lead frame - Google Patents

Abstract

PURPOSE:To prevent crack at the time of reflow mounting and during actual use by forming a mixture film of a metal and a metal oxide having a specified thickness on the surface and the rear of a die pad surrounded by leads thereby enhancing the bonding strength between a lead frame and a seal resin. CONSTITUTION:A die pad 3 surrounded by leads 15 is coated, on the surface and rear thereof, with a mixture film or an inclination film 21. the thermal spraying method is most effective as a method for forming the film. The film material being employed is based on Al-Al2O3, Si-SiO2, Si-Al2O3 or Al-SiO2. In case of the mixture film, the mixing ratio of metal is preferably set in the range of 30-70% in order to satisfy the adhesion between the metal and die pad and between the metal oxide and resin. Since the adhesion effect is not achieved when the coating is too thin, the thickness of the coating is set at 1mum or above and the upper limit is set at 30mum in viewpoint of cost and labor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mountability of a lead frame mounted with a semiconductor chip and used as a central part of a semiconductor device, in which peeling from a resin during molding and resin cracking during reflow do not occur. It is about a good lead frame.

[0002]

2. Description of the Related Art In general, a lead frame has a die pad for mounting a semiconductor chip in the center by pressing or etching a thin metal made of a conductive material such as 42Ni alloy, Al or Cu. It is formed and processed into a shape in which a predetermined number of leads are piped around, and is used as a key part of a resin-sealed semiconductor device.

A resin-sealed semiconductor device is die-bonded to a semiconductor chip 2 with a bonding material 4 such as Ag paste on a die pad 3 in the center of a lead frame 1, as shown in FIG. , The electrode pad 5 on the chip and the A on the inner lead 1a of the lead frame 1
Bonding wire 6 such as Au thin wire with g plating 14
The structure formed by bonding is inserted into a package and resin-sealed 7 to manufacture the package.

The semiconductor device thus manufactured is
The tip (outer lead) 1b of the lead frame 1 to which the solder 8 is attached is subjected to a so-called reflow treatment in which the solder is melted by heat irradiation or heating in order to firmly fix the circuit board 9 by soldering. Generally, a sealing resin for a semiconductor device has a hygroscopic property, and therefore tends to absorb moisture over time. There is also moisture adhering to the components. Such contained water mainly destroys the interface with the resin, and the water accumulated in the gap causes rapid vaporization by the reflow heating and expands to cause the resin destruction 10. On the other hand, potentially at room temperature, distortion occurs due to the difference in thermal expansion between the resin and the components, especially the die pad,
It is known that shear peeling occurs at the interface, and water vapor generated in the peeled gap portion during reflow heating promotes peeling. In particular, peeling is large at the end portion of the die pad where stress concentration is likely to occur, and cracks 10 as shown in the figure occur in combination with the action of vapor pressure. Therefore, the sealing performance of the semiconductor device is deteriorated and the reliability is impaired.

Therefore, when the semiconductor device is shipped, special packaging for moisture proofing is required, it is requested that the board mounting time is shortened, and further, the hygroscopic product is baked in air before the board mounting. Although measures can be taken, these methods are not preferable because they are extremely troublesome and need to be dealt with by the customer. On the other hand, in order to prevent peeling between the sealing resin and the die pad, for example, JP-A-58-1995.
Japanese Unexamined Patent Publication No. 48 discloses that the back surface of the die pad (tab) 3 is formed into an uneven shape 11 as shown in a cross section in FIG. Further, Japanese Patent Laid-Open No. 55-4983 discloses that a die pad (island) 3 shown in FIG. 6 is provided with a partial cutout portion 12, and further Japanese Laid-Open Patent Publication No. 60-97645. , A semiconductor device in which a thermosetting or thermoplastic organic resin (polyimide-based) 13 having a thermal expansion coefficient similar to that of the sealing resin is attached to the back surface of the die pad (tab) 3 as shown in cross section in FIG. ing.

A method has also been proposed in which the die pad is coated with a metal or a metal oxide alone to prevent the occurrence of cracks.

[0007]

SUMMARY OF THE INVENTION In the above-mentioned conventional method, the die pad is mechanically processed to increase its surface area or to be caught, thereby increasing the contact surface with the sealing resin and shear peeling. Although it is intended to suppress the occurrence of cracks and cracks, the bond strength of the die pad itself with the encapsulation resin has not been completely improved. Occasionally, a rapid expansion occurs and the sealing resin is destroyed.

Further, when a polyimide-based organic resin is used, the resin itself also has a hygroscopic property, which causes a problem. The method of coating the metal or the metal oxide alone is more effective in preventing the generation of cracks than the above method.
However, in the case of metal coating, the adhesion to the resin is insufficient, and in the case of metal oxide coating, the adhesion to the die pad is still insufficient, which is insufficient for solving the problem.

The present invention solves the above-mentioned conventional problems, and in order to increase the adhesive strength between the lead frame and the sealing resin, the die pad is provided with a metal / metal oxide mixed coating or a gradient. It is intended to provide a lead frame for obtaining a reliable resin-encapsulated semiconductor device that does not cause problems such as cracking during reflow mounting and practical use by coating the film. is there.

[0010]

In order to achieve the above object, the present invention has the following points. However, where M and M'are metal, M = M 'or M ≠ M'
Is. (1) A mixed coating film of a metal (M) and a metal oxide (M′-O) having a thickness of 1 to 30 μm is formed on the front surface and the back surface of a die pad surrounded by leads. Lead frame to

(2) The front and back surfaces of the die pad surrounded by the leads have a thickness of 1 to 30 μm, and the interface is 10
A lead frame, wherein the mixing ratio of the metal oxide is gradually increased toward the coating surface with 0% metal, and the coating surface forms a gradient composition coating film of 100% metal oxide.

[0012]

According to the present invention, the front surface and the back surface of the die pad of the lead frame, which is one of the constituents of the semiconductor device, are modified by applying a mixed coating film of metal and metal oxide. That is, in the resin-sealed semiconductor device using this lead frame, the interface having high adhesive strength can be formed by the following mechanism.

The oxide coating present on the surface of the coated coating mixture, M'-O or M'-OH, is effective as an intermediate layer to improve substrate / resin adhesion. That is, the bonding mode of -CO, which is a constituent molecule of the resin, and the bonding mode of M'-O, M'-OH, which are oxide films existing on the surface of the coated mixed film, are similar. Therefore, excellent adhesive strength can be realized.

The metal and metal oxide in the coating are M'-O.
They are strongly connected with an interaction such as M (-: original bond, ...: interaction). The lead frame is made of Fe-Ni alloy, Cu, or the like, and has a higher adhesiveness with a metal than a metal oxide. In the mixed coating, the metal in the coating enhances the adhesiveness with the lead frame, and the above-mentioned metal oxide can suppress reflow cracking together with the effect of enhancing the adhesiveness with the resin.

The graded coating is a coating in which 100% metal oxide is present on the surface in contact with the resin and 100% metal is present on the surface in contact with the lead frame as the metal ratio gradually increases toward the inside of the coating. Therefore, the roles of the metal and the metal oxide are further promoted, and the above characteristics can be further exhibited. Although the two-layer coating of metal and metal oxide has promoted the division of roles between the two, the bond between the interfaces of the two is much stronger in the graded coating, and not only against reflow cracks but also against damage within the coating. But even more effective.

The mixed coating and the gradient coating that cover the shear strain generated at the resin / substrate interface during reflow can be relaxed, and thus the true interface strain between the resin and the substrate can be reduced. In this way, reflow cracks can be suppressed by reducing the strain. Further, the relaxation by the coating at this time means elastic relaxation by the coating having a thermal property intermediate between the lead frame base material (42Ni alloy etc.) which is a metal and the resin, or relaxation by the composition change of the coating. To do. Since the mixed coating and the gradient coating have thermal characteristics intermediate between those of metal and metal oxide, they have an advantageous effect in terms of strain relaxation.

The present invention will be described in detail below. FIG. 1 shows a cross section of a lead frame 1 of the present invention, in which a mixed coating or a gradient coating 21 is coated on both the back surface and the front surface of a die pad 3 surrounded by leads 15. The most effective method for forming the coating is by the thermal spraying method. The thermal spraying method is economically effective because it has the fastest film formation rate, and the film surface is relatively rough and the surface area increases, so that the adhesion to the mold resin is good.

As the material for the coating, Al--Al₂O₃system,
Si-SiO₂System, Si-Al₂O ₃System or Al-
SiO₂Use the system. This is the bond strength of the coating,
This is because the adhesion strength of the resin is excellent. Mixed coverage
In the case of a film, the mixing ratio of metals is preferably 30 to 70%.
Yes. This is due to the adhesion of the metal to the die pad, metal acid
This is because the adhesiveness of the compound with the resin is compatible.

According to the present invention, both front and back surfaces of the die pad are coated. However, when the coating is applied to only one side by a thermal spraying method, the die pad warps due to the difference in coefficient of thermal expansion between the die pad and the coating. This is to prevent If warpage occurs, it becomes difficult to assemble the semiconductor, and the adhesiveness with the resin becomes unstable. According to the present invention, since the same film quality is formed on both the front and back surfaces of the die pad, the die pad is not deformed and extremely stable molding is possible.

FIG. 2 shows an example in which a thermal spraying method is applied as a method for coating the lead frame of the present invention. The lead frame is housed in the storage rack 16 and automatically pulled out to the thermal spraying device. In the thermal spraying device, a pair of masks 17 are installed opposite to each other with a space provided for the lead frame 1 to pass through.
The mask is provided with an exposed portion of the die pad, which is arranged in tandem in the traveling direction of the lead frame. On the exposed part, a spray nozzle 18 and a powder feeder 20 for supplying a powdery coating material to the nozzle and a suction hood 19 are installed so as to face each other, and the exposed parts are alternately arranged in opposite directions. There is.

The lead frame is automatically pulled out from the storage rack 16, moves intermittently between the masks, and stops when the die pad reaches the exposed portion to form a film on each surface. Since the lead is covered with the mask 17, it is not covered. Further, since the suction hood 19 operates during coating, it is possible to prevent the sprayed particles from wrapping around other than a necessary portion.

The graded film can be easily obtained by the method disclosed in Japanese Patent Publication No. 5-63549. If the thickness of the coating film formed by the above method is too thin, the adhesion effect cannot be obtained, so at least 1 μm or more is necessary. Further, if it is too thick, the effect is not changed but the cost and labor are required, so the upper limit is 30 μm.

[0023]

EXAMPLES Example 1 Al-Al by a thermal spraying method₂O₃Example of creating a mixed coating
Show. Al, Al₂O ₃The sprayed powder of
Even if you spray what you had, it will be fed separately into the spray frame.
It does not matter which method is used to mix in the frame.
Yes. Al and Al₂O₃Mixing ratio of resin, die pad
It was set to 50% in consideration of the adhesiveness with. The film thickness is 20
μm. Lead with mixed coating formed in this way
Assemble the frame into the semiconductor device as shown in FIG.
Table 1 in the atmosphere maintained at 30 ° C and 85% humidity
After leaving it for a seed time, the same soldering condition as the circuit board is used.
The same was heated at 215 ° C. for 90 seconds. Film for comparison
A similar test was conducted for the one without the material. Inside the resin after heating
The presence or absence of peeling and cracks was investigated. Table 1 shows the results.

[0024]

[Table 1]

Example 2 An example of forming a Si-SiO ₂ gradient coating film by a thermal spraying method will be described. Spray powder of Si-SiO ₂ is carried out by a method of mixing in a frame by feeding in separately spraying flame. First S
Only i is sprayed, then SiO ₂ is gradually mixed to increase the ratio, and the feed amount of Si is simultaneously reduced.
Finally, only SiO ₂ is sprayed, and a film is formed so that SiO ₂ exists on the outermost surface. Total thickness of coating is 30μ
m. The lead frame on which the mixed coating was formed in this manner was assembled into a semiconductor device as shown in FIG. 3 and left in an atmosphere kept at a temperature of 30 ° C. and a humidity of 85% for various times shown in Table 2 and then soldered to a circuit board. The heating was performed at 215 ° C. for 90 seconds, which is the same as the attachment conditions. After heating, the presence or absence of peeling and cracks inside the resin was investigated. Table 2 shows the results.

[0026]

[Table 2]

[0027]

As described above, according to the present invention, a lead frame having one surface or both surfaces mixed or gradient-coated with metal-ceramics is manufactured and used in a resin-sealed semiconductor device, whereby a lead resin Adhesion is very strong, and even if moisture is absorbed, the adhesive does not peel off or break, so moisture does not accumulate at the interface, so the rapid expansion of moisture during reflow mounting has been a problem in the past. The occurrence of cracks does not occur. Further, since the peeling of the adhesive does not occur, the stress concentration at the corners of the die pad does not occur, and a highly reliable semiconductor device can be provided. The conventional method can also be applied to dry coating, and inexpensive manufacturing is possible.

[Brief description of drawings]

FIG. 1 is a sectional view of a lead frame of the present invention.

FIG. 2 is a cross-sectional explanatory view showing an example of a lead frame manufacturing apparatus of the present invention.

FIG. 3 is a cross-sectional view of a semiconductor device using the lead frame of the present invention.

FIG. 4 is a cross-sectional view of a semiconductor device using a conventional lead frame.

FIG. 5 is a morphological view of a conventional die pad.

FIG. 6 is a morphological view of a conventional die pad.

FIG. 7 is a morphological view of a conventional die pad.

[Explanation of symbols]

 1 Lead Frame 1a Inner Lead 1b Outer Lead 2 Semiconductor Chip 3 Die Pad 4 Bonding Material 5 Electrode Pad 6 Bonding Wire 7 Resin Sealing 8 Solder 9 Circuit Board 10 Resin Breakage (Crack) 11 Concavo-convex 12 Cutout 13 Organic Resin 14 Ag Plating 15 Lead 16 Storage Rack 17 Mask 18 Spray Nozzle 19 Suction Hood 20 Powder Feeder 21 Mixing or Gradient Coating

Claims (2)

[Claims] 1. A lead frame comprising a die pad surrounded by leads, wherein a mixed coating film of a metal and a metal oxide having a thickness of 1 to 30 μm is formed on the front and back surfaces of the die pad. 2. The front and back surfaces of a die pad surrounded by leads have a thickness of 1 to 30 μm, and the interface is 100%.
A lead frame, characterized in that the mixing ratio of metal oxide gradually increases toward the surface of the coating with metal, and the coating surface forms a gradient composition coating of 100% metal oxide.