JPH1022422A - Double layer resin-sealed integrated circuit device and manufacture thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a resin-sealed integrated circuit device which enables easy molding and reliable prevention of generation of package crack, and manufacture thereof. SOLUTION: This device includes an IC chip 1 having an integrated circuit formed thereon, and leads 2a, 2b connected with terminals of the chip. The device also has an inner resin layer 4 of low stress which encloses the chip 1 and chip lateral end portions of the leads 2a, 2b, and an outer resin layer 5 of high strength and low hygroscopicity which encloses the inner resin layer. The resin layers 4, 5 have shapes substantially analogous to each other. Manufacture of this integrated circuit device includes a first molding step of forming the inner resin layer for enclosing the chip 1 and the chip lateral end portions of the leads 2a, 2b with a low-stress resin, using a metal mold having a first cavity of a predetermined shape, and a second molding step of forming the outer resin layer 5 for enclosing the inner resin layer 4 with a resin of high strength and low hygroscopicity, using a metal mold having a second cavity of a shape analogous to and greater than the predetermined shape.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a semiconductor device and / or a semiconductor device.
Or an integrated circuit formed by other elements,
In particular, the present invention relates to an integrated circuit device in which a chip on which an integrated circuit is mounted is resin-sealed, and a method for manufacturing the same.

[0002]

2. Description of the Related Art When an integrated circuit device including an IC chip on which an integrated circuit is formed and a package for encapsulating and embedding the IC chip is surface-mounted on a printed circuit board, cracks often occur in the package body. . In general,
In a resin-sealed integrated circuit device, it is considered that the largest cause of the occurrence of package cracks is that the resin absorbs moisture due to the environmental atmosphere during the storage period of the device until surface mounting.

As a countermeasure against cracking, there is a method of indirectly reducing the area occupied by the die pad portion of the lead frame with respect to the package body to reduce the internal stress of the package body, but this tends to increase the cost. On the other hand, measures are generally taken to directly improve the sealing resin,
This measure is roughly divided into the following two, and one of them is selected according to the purpose.

(1) Enhancing strength and reducing moisture absorption of sealing resin This is because a sealing resin in which a substance having low moisture absorption is employed as a filler and a filling rate of a base resin (base resin) is increased. By adopting it, the mechanical strength of the package body is increased. (2) Reducing the stress of the sealing resin This is intended to alleviate the internal stress of the package body by dispersing a flexible agent such as silicone in the sealing resin.

The effects of these countermeasures have advantages and disadvantages depending on the package form, and are not suitable for all cases.
The reason is the recent background that demands for larger chips, narrower lead pitches, and thinner packages are increasing. That is, if the above package crack countermeasures are taken, the resin mold itself for forming a package satisfying these requirements becomes very difficult.

In detail, when a sealing resin having high strength and low hygroscopicity as described in the above (1) is used, the package crack resistance is high, but the resin viscosity is high. At the time of resin injection, problems such as a flow of a bonding wire connecting the chip and the inner lead end (wire flow) and an inclination or deviation of a die pad of a lead frame (die pad shift) are likely to occur. . In addition, when a sealing resin having a low stress as described in the above (2) is used, the resin viscosity can be suppressed low, so that the problems of wire flow and die pad shift are alleviated. Is inferior to the measure (1).

[0007] As described above, there is a problem with molding as a problem for measures against package cracks caused only by the properties of the sealing resin. On the other hand, as an example in which both of the above measures (1) and (2) are simultaneously applied, Japanese Patent Application Laid-Open No.
There is a technique described in Japanese Patent Publication No. 58-58. According to this, a plate-shaped insulating member is installed under a stage (die pad) on which a semiconductor chip on which a circuit element is formed is mounted,
The vicinity of the stage is potted with a first low-stress resin, and the periphery of the potted resin is transfer-molded with a second high-moisture-resistant and high-stress resin. A semiconductor device is disclosed.

However, in the disclosed technology,
Since the low-stress inner resin portion formed near the stage is formed by potting molding, it has a hill shape in which the top is rounded and the bottom is flat along the flat insulating member. Therefore, the inner resin part is vertically asymmetric,
When performing transfer molding of the high-stress outer resin, if the resin is injected by arranging the hill-shaped inner resin portion in the corresponding mold cavity, there is a high possibility that the injected resin will cause turbulent flow in the cavity. . Due to this turbulence, the inner resin portion is pushed down, pulled up, or tilted, which may result in the chip and the inner resin portion not being uniformly embedded in the finally formed outer resin. is expected. Such non-uniformity of the external resin gives local excessive strength and insufficient strength in the package body, and thus greatly impairs the package crack resistance.

In addition, the entire package is warped due to molding shrinkage of the outer resin due to the asymmetrical shape of the inner resin portion and the provision of the plate-shaped insulating member on the bottom side thereof. There is also a possibility. This also becomes a factor that impairs the package crack resistance.

[0010]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned points, and an object of the present invention is to provide a resin seal which is easy to mold and can reliably prevent the occurrence of package cracks. An object of the present invention is to provide a static integrated circuit device and a method of manufacturing the same.

[0011]

A two-layer resin-sealed integrated circuit device according to the present invention is an integrated circuit device including an IC chip on which an integrated circuit is formed and leads connected to terminals of the chip. A low-stress internal resin layer that embeds a chip-side end of the chip and the lead, and a high-strength and low-hygroscopic external resin layer that embeds the internal resin layer,
The internal resin layer and the external resin layer are substantially similar in shape to each other.

A method for manufacturing a two-layer resin-sealed integrated circuit device according to the present invention is a method for manufacturing an integrated circuit device including an IC chip on which an integrated circuit is formed and leads connected to terminals of the chip. A first molding step of forming an internal resin layer that embeds the chip and the end of the lead on the chip side with a low-stress resin using a mold having a first cavity having a predetermined shape; A second mold step of forming an external resin layer that embeds the internal resin layer with a high-strength and low-hygroscopic resin using a mold having a similar and larger second cavity. Features.

A method of manufacturing a two-layer resin-sealed integrated circuit device according to the present invention includes a die bonding step of fixing an IC chip on which an integrated circuit is formed to a die pad of a lead frame; A wire bonding step of individually wire-connecting the tip of the internal lead, a die having the wire-bonded chip, the die pad of the lead frame and the tip of the internal lead using a mold having a first cavity having a predetermined shape. Molding process for forming an internal resin layer to be embedded with a low-stress resin, and high strength and low hygroscopicity using a mold having a second cavity similar to and larger than the predetermined shape. A second molding step of forming an external resin layer that embeds the internal resin layer with a resin,
And a shaping step of removing unnecessary portions from the lead frame after the formation of the external resin layer and shaping external leads of the lead frame.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a schematic sectional view of a QFP (Quad Flat Package) semiconductor integrated circuit device according to an embodiment of the present invention. In FIG. 1, an IC chip 1 on which a semiconductor integrated circuit is formed is placed on a die pad 20 of a lead frame, and a die pad is formed by forming, for example, a eutectic alloy at a joint between the two or by applying a silver paste. 20. Although the surface of the chip 1 is coated with a surface protection film such as a PSG (phosphorus glass) film (not shown), a plurality of, for example, aluminum pads serving as electrodes or input / output terminals are formed near the outer edge of the chip. . These pads are connected to the tips of the inner leads 2a of the lead frame by bonding wires 3. The inner leads 2a correspond to the respective pads of the chip 1, and extend individually and are led to the outer leads 2b. The outer lead 2b is shaped into a gull wing shape for surface mounting, that is, for contacting and fixing to the wiring pattern surface of the printed circuit board.

The feature of this embodiment is that the chip 1 and its peripheral portion are sealed with a low-stress resin layer 4, and this low-stress resin layer 4 is further sealed with a high-strength and low-hygroscopic resin layer 5. Stop 2
In addition to having a layer structure, the low-stress resin layer 4 and the high-strength and low-moisture-absorption resin layer 5 have substantially similar shapes to each other, and the low-stress resin layer 4 has a symmetrical shape in all directions. It is in.

More specifically, as a material of the low-stress resin (hereinafter, referred to as an internal resin layer) 4, for example, a phenol novolak type epoxy resin may be employed, and the chip 1, the bonding wires 3, and the inner leads 2 a may be used. The end and the die pad 20 are embedded. As a material of the high-strength and low-hygroscopic resin (hereinafter, referred to as an external resin layer) 5, for example, a biphenyl-based epoxy resin can be adopted, which embeds the internal resin layer 4 and reduces the outer shape of the package body in the finished product. Carry. If the outer shape of the outer resin layer 5 is such that two trapezoids having substantially the same side surface or cross section as shown in the drawing are formed by joining the bases thereof to each other and, for example, a substantially square shape in the plane, the inner resin The layer 4 also has a shape in which two trapezoids which are also substantially the same as shown in the drawing in side surface or cross section are formed by joining their bases to each other, and has, for example, a substantially square shape in a plane. However, as can be understood from FIG.
The trapezoid on the side surface or the cross section is slightly smaller than that of the external resin layer 5, and the square in the plane of the internal resin layer 4 is slightly smaller than that of the external resin layer 5. Further, both the inner and outer resin layers 4 and 5 are formed so as to have substantially vertical and laterally symmetric shapes in side surfaces or cross sections.

According to the integrated circuit device having the substantially similar and symmetrical two-layer resin sealing structure, unlike the technique described in the above-mentioned publication, the inner resin layer 4 is also used for the outer resin layer. 5, the uniformity in the package is very high, and the package crack resistance is improved. In addition, since the inner resin layer 4 has a symmetrical shape, there is little possibility of causing the above-described turbulent flow when performing transfer molding of the outer resin layer 5. In other words, the inner resin layer 4
Is a pattern for bridging not only the bonding wires 3, the ends of the inner leads 2a and the die pad 20, but also the outer frame of the lead frame and the die pad 20 (in the finished product, the pattern for supporting the die pad 20 on all sides, for example) 1), that is, by embedding the inside of a so-called hanging pin 2c shown by a broken line in FIG.
In the transfer molding of the outer resin layer 5, the stabilization of the entire inner resin layer 4 is further promoted, and the uniformity of the outer resin layer 5 is improved.

Next, a method of manufacturing the integrated circuit device will be described. First, to clarify the overall manufacturing flow, reference is made to FIG. FIG. 2 is a flowchart mainly showing a so-called assembly process, in which dicing (step S1) for dividing chips formed on a wafer into individual chips is performed.
0). Then, die bonding for mounting the chip 1 obtained by the dicing on the die pad of the lead frame 2 via a silver paste (step S
11) is performed.

One example of the form of the lead frame 2 is shown in FIG. As can be seen from FIG. 3, the square die pad 20 is arranged at the center of the lead frame 2 and is connected to the outer frame 21 of the lead frame by hanging pins 2c extending in the respective diagonal directions. In the vicinity of the edge of the die pad 20, the tips of the inner leads 2a are arranged along the edges, and the inner leads 2a extend almost radially from the respective tips and have a resin flow stopping pattern called a dam bar or a tie bar. It is led to the external lead 2b with 2d interposed therebetween. The external leads 2b are respectively coupled to the outer frame 21.

After the die bonding in step S11, the die-bonded lead frame 2 is placed in an atmosphere of, for example, 175 ° C. for 90 minutes to harden a silver paste as a thermosetting resin on the die pad. Step S12) The processing is performed. Next, wire bonding (step S13) for individually connecting the fixed pads of the chip 1 and the corresponding tips of the inner leads 2a is performed. When all the bonding is completed, the double sealing step S14 which is a feature of this embodiment is performed.
Move to In the double sealing step S14, the above-described inner and outer resin layers 4 and 5 are formed. The details will be described later.

When the double sealing step S14 is completed, the sealed lead frame 2 is placed in an atmosphere of 175 ° C., for example, in order to solidify the internal and external resins 4 and 5, which are thermosetting resins. Cure for only time (Step S15)
Processing is performed. Then, the dam bar 2d is cut (step S16), the outer leads 2b are plated (step S17), and the process proceeds to the lead shaping step S18. In the lead shaping step, unnecessary parts such as the exposed portions of the suspension pins 2c are removed from the lead frame 2. Further, in this shaping step, the outer lead 2b is shaped into a gull wing shape and finished.

After the external leads have been shaped in step S18, they are shipped (step S21) through inspection (step S19) and packing (step S20). Next, the aspect of the double sealing step S14 will be described in detail. FIG. 4 shows a transfer mold for packaging the present integrated circuit device by a so-called low-pressure transfer molding method, a manufacturing apparatus including the same, and a lead frame 2 mounted with a bonded chip 1 set in the manufacturing apparatus. It is a schematic sectional drawing which shows a transition form.

In FIG. 4, a first manufacturing unit for molding the internal resin layer 4 mainly includes an upper mold (upper mold) 61 and a lower mold (lower mold) 71 serving as a mating mold. An injection mechanism for injecting the liquid resin into the cavity formed by these molds, and a void removing mechanism for removing bubbles or voids from the injected resin in the cavity. The injection mechanism includes a cull hole 81, a transfer plunger 91, a runner R1, and a gate G1, and the void removing mechanism includes an air vent V1.

The upper die 61 and the lower die 71 are heated to a predetermined temperature in advance, and hold the lead frame 2 to be sealed fixed at a predetermined position (clamping). On the other hand, as described above, the low-stress resin 40 previously formed into a tablet shape as a raw material of the internal resin layer 4 is subjected to high-frequency heating, and is filled in the cull hole 81. Alternatively, a raw material (tablet) is put into a cull hole of a mold that has been heated to a predetermined temperature in advance, preheated and filled. Then, the transfer plunger 91 is operated to squeeze the raw material resin 40 and melt and press-inject it into the cavity 610 of the upper mold 61 and the cavity 710 of the lower mold 71 through the runner R1 and the gate G1.

FIG. 5 shows details of the positional relationship between the lower mold 71 and the lead frame 2 at this time. Lead frame 2
Is fixed such that the die pad 20 is located at the center of the lower mold cavity 710. On the bottom surface side of the package of the lead frame 2, the internal resin layer 4 is formed in a region surrounded by a broken line in FIGS. 3 and 5 by the resin injected into the lower mold cavity 710 through the gate G1. The upper mold cavity 610 has almost the same shape as the lower mold cavity 710, and at the same time as the package bottom side of the lead frame 2,
The internal resin layer 4 is also formed on the package top surface side of the lead frame 2 in a region surrounded by the broken line by the resin injected into the upper mold cavity 610 through the gate G1.

In the upper and lower mold cavities 610 and 710 at the beginning of the resin injection, voids are mixed in the injected resin. The air vent V1 is a groove for extruding or extracting the void to the outside, and eliminates a final mixed void in the sealing resin. The second manufacturing unit for forming the external resin layer 5 is similarly configured and operates.

That is, the second manufacturing section mainly injects the liquid resin into the upper mold (upper mold) 62 and the lower mold (lower mold) 72 as the mating molds and the cavities of these molds. And a void removing mechanism for removing bubbles or voids from the injected resin in the cavity. Such an injection mechanism includes a cull hole 8.
2, the transfer plunger 92, the runner R2, and the gate G2, and the void removing mechanism includes an air vent V2.

The upper die 62 and the lower die 72 are heated to a predetermined temperature in advance, and sandwich the lead frame 2 to be sealed fixed at a predetermined position (clamping). On the other hand, the raw material of the outer resin layer 5 is subjected to the same
Is filled. Then, the transfer plunger 92
Is operated to squeeze the raw resin 50, and the cavity 620 of the upper die 62 and the lower die 72 through the runner R2 and the gate G2.
And melt pressure injection.

FIG. 6 shows details of the positional relationship between the lower mold 72 and the lead frame 2 at this time. Lead frame 2
Is fixed such that the internal resin layer 4 is located at the center of the lower mold cavity 720. The external resin layer 5 is formed on the bottom surface side of the package of the lead frame 2 in a region surrounded by a dashed line in FIGS. 3 and 6 by the resin injected into the lower mold cavity 720 through the gate G2. The upper mold cavity 620 has substantially the same shape as the lower mold cavity 720, and the package G of the lead frame 2 and the package bottom surface of the lead frame 2 are formed at the same time.
2 in a region surrounded by the same dashed line by the resin injected into the upper mold cavity 620 through the outer resin layer 5.
Is formed.

The air vent V2 performs the same function as the air vent V1. It should be noted here that the lead frame 2 to be sealed is processed in cooperation from the first manufacturing unit to the second manufacturing unit. In other words, a pair of mating dies 61, 71 in the first manufacturing section and a pair of mating dies 62, 72 in the second manufacturing section are arranged in parallel, and immediately after the processing of the first manufacturing section is finished, 2 Mold 6 in the manufacturing department
The lead frame is set at 2, 72. The former pair of dies 61 and 71 and the latter pair of dies 62 and 7
2, the lead frame 2 to be sealed can be easily transported by, for example, a vacuum transporter.

According to such a manufacturing mode, it is advantageous in terms of quality and economy. In other words, since both resin layers are formed by transfer molding, the dimensional accuracy is extremely high, and the occurrence of defective products can be prevented.In addition, merely changing the mold will cause other process routines to be different from normal transfer molding. Since there is no transfer molding mechanism, an existing transfer molding mechanism can be used, and there is an advantage that capital investment for realizing the present invention can be saved.

The upper die cavity 610 and the lower die cavity 710 in the preceding stage have a symmetrical shape in the vertical, horizontal, height and depth directions. Arrange in the center. Therefore, the inner resin layer 4 formed by the mating cavity is also embedded vertically around the chip 1.
The shape becomes symmetrical in width, height and depth. The upper and lower mold cavities 620 and 720 of the latter stage are also symmetrical in length, width, height and depth and have a shape similar to that of the former cavity, and the matching cavity also has the internal resin layer 4 disposed at the center. Therefore, when the external resin material 50 is injected into the upper and lower mold cavities 620 and 720 in the subsequent stage, there is little possibility of causing a turbulent flow such that the internal resin layer 4 is largely vibrated in the cavities.

In addition, the internal resin layer 4 is provided between the frame 21 of the lead frame 2 and the die pad 20 in addition to the bonding wires 3, the ends of the inner leads 2 a and the die pad 20 in a region surrounded by a dotted line in FIG. By embedding the inside (die pad carrying pattern) of the suspension pin 2c that bridges the cable, fixation to the lead frame 2 and fixation during external resin molding are achieved.

Therefore, the high-viscosity external resin raw material 50 is supplied to the subsequent upper and lower mold cavities 620 and 720 at a high injection pressure.
, The vibration of the internal resin layer 4 in the cavity is suppressed. The external resin layer 5 molded in this way can obtain high uniformity. From another aspect, since the internal resin layer 4 is formed of a low-stress resin exhibiting a low viscosity, the occurrence of the wire flow and the die pad shift as described above can be suppressed. When the external resin layer 5 is formed, the wires and the die pad are embedded in the internal resin layer 4, so that the high viscosity and the high strength can be used without worrying about the problem of the wire flow and the die pad shift. In addition, transfer molding of the resin having low hygroscopicity and high injection pressure into the cavity can be performed. As described above, the injection pressure at the time of molding the external resin layer 5 can be increased by the presence of the internal resin layer 4. Therefore, as the external resin layer 5, a resin having a higher filler filling rate (ie, Higher strength and lower hygroscopic resin) can be adopted.

Further, with respect to the package crack resistance in the present embodiment, generally, in the surface mounting of a packaged integrated circuit device on a printed circuit board,
A shear stress is generated due to a difference in linear expansion coefficient generated between a die pad, a chip, and a resin due to infrared reflow heating. However, in the device of this embodiment, since the internal resin layer 4 has a low stress, the stress relaxing effect is large. In addition, the external resin layer 5
Uses high-strength and low-moisture-absorbing resin, so that moisture absorption inside the package can be suppressed. Then, even if a crack is generated inside the package due to the infrared reflow heating, there is an effect of preventing the high-strength external resin layer 5 from extending the crack to the outside of the package.

In the above-described embodiment, the QFP type package has been described. However, the present invention is not limited to this, and can be applied to various forms such as a DIP type. The form of the external lead is also limited to a gull wing shape. Not something. In addition, various means and steps have been described in the above embodiment,
It can be modified as appropriate within a range that can be designed by those skilled in the art.

[0037]

As described in detail above, according to the present invention,
It is possible to provide a resin-sealed integrated circuit device that can be easily formed and reliably prevent the occurrence of package cracks, and a method for manufacturing the same.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a schematic structure of an integrated circuit device according to an embodiment of the present invention.

FIG. 2 is a flowchart illustrating a method of manufacturing the integrated circuit device of FIG. 1;

FIG. 3 is a diagram showing a form of a lead frame applied to the integrated circuit device of FIG. 1;

FIG. 4 is a transfer mold for packaging the integrated circuit device of the present embodiment by a low-pressure transfer molding method, a manufacturing apparatus including the same, and a lead frame 2 mounted with a bonded chip 1 set in the manufacturing apparatus. FIG. 3 is a schematic cross-sectional view showing a transition mode of FIG.

FIG. 5 is a perspective view showing an arrangement relationship between a lower mold and a lead frame when an internal resin layer is formed in a first manufacturing unit.

FIG. 6 is a perspective view showing an arrangement relationship between a lower mold and a lead frame when an external resin layer is formed in a second manufacturing unit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 IC chip 2 Lead frame 20 Die pad 21 Outer frame 2a Inner lead 2b Outer lead 2c Hanging pin 2d Dam bar 3 Bonding wire 4 Internal resin layer 5 External resin layer 61,62 Upper die 71,72 Lower die 610,620 Upper die cavity 710 , 720 Lower mold cavity 81, 82 Cull part 91, 92 Plunger R1, R2 Runner G1, G2 Gate V1, V2 Air vent 91, 92 Resin tablet

Claims (12)

[Claims] 1. An integrated circuit device comprising: an IC chip on which an integrated circuit is formed; and a lead connected to a terminal of the chip, wherein a low stress embeds the chip and a chip-side end of the lead. An internal resin layer, and a high-strength and low-hygroscopic external resin layer that embeds the internal resin layer, wherein the internal resin layer and the external resin layer are substantially similar in shape to each other. A two-layer resin-sealed integrated circuit device. 2. The integrated circuit device according to claim 1, wherein the cross section of the internal resin layer is substantially symmetrical in the vertical and horizontal directions. 3. The integrated circuit device according to claim 1, further comprising a die pad on which the chip is mounted, wherein the internal resin layer also embeds the die pad. 4. The integrated circuit device according to claim 3, further comprising a carrier pattern for supporting said die pad, wherein said internal resin layer also embeds said carrier pattern. 5. A method for manufacturing an integrated circuit device including an IC chip on which an integrated circuit is formed, and leads connected to terminals of the chip, the method comprising using a mold having a first cavity having a predetermined shape. A first molding step of forming an internal resin layer that embeds the chip and the chip-side end of the lead with a low-stress resin;
A second molding step of forming an external resin layer embedding the internal resin layer with a high-strength and low-hygroscopic resin by using a mold having a second cavity having a shape similar to and larger than the predetermined shape; A method for manufacturing a two-layer resin-sealed integrated circuit device, comprising: 6. The method according to claim 5, wherein a cross section of the first cavity is substantially symmetrical in the vertical and horizontal directions. 7. The integrated circuit device according to claim 1, further comprising a die pad on which the chip is mounted, wherein the first molding step includes forming the internal resin layer so as to embed the die pad. Item 7. The method according to Item 5 or 6. 8. The integrated circuit device according to claim 1, further comprising a support pattern for supporting the die pad, wherein the first molding step includes forming the internal resin layer so as to embed the support pattern. The manufacturing method according to claim 7, wherein 9. A die bonding step of fixing an IC chip on which an integrated circuit is formed to a die pad of a lead frame, and a wire bonding step of individually wire connecting between the chip pad and the tip of an internal lead of the lead frame. And forming an internal resin layer that embeds the wire-bonded chip, the die pad of the lead frame, and the tip of the internal lead with a low-stress resin using a mold having a first cavity of a predetermined shape. Forming a first molding step and forming an outer resin layer that embeds the inner resin layer with a high-strength and low-hygroscopic resin by using a mold having a second cavity having a shape similar to and larger than the predetermined shape. A second molding step,
A forming step of removing unnecessary portions from the lead frame and forming external leads of the lead frame after the formation of the external resin layer. . 10. The first molding step includes performing transfer molding in which the first cavity is formed by a pair of mating dies, and a molten low-stress resin is injected into the first cavity. Item 10. The production method according to Item 9. 11. The second molding step includes performing transfer molding in which the second cavity is formed by a pair of mating dies, and molten high-strength and low-hygroscopic resin is injected into the second cavity. The method according to claim 9 or 10, wherein 12. The first molding step includes performing transfer molding in which the first cavity is formed by a first pair of mating dies, and molten low-stress resin is injected into the first cavity. In the step, the second cavity is formed by a second pair of molds, and transfer molding is performed to inject the molten high-strength and low-hygroscopic resin into the second cavity. The method according to claim 9, wherein the second pair of mating dies is arranged in parallel.