JP2699796B2 - Composite lead frame and method of manufacturing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a composite lead frame for a semiconductor device and a method for manufacturing the same.

[0002]

2. Description of the Related Art Composite leadframes in which outer leads are formed by metal lead frames and inner leads are formed by a flexible wiring board in which a fine wiring pattern of a conductor is formed on an insulating film are currently available in various structures. However, in either structure, it is necessary to electrically connect the inner lead of the flexible wiring board and the outer lead on the outer frame side by some method. Since the inner leads from the flexible wiring board are laminated on the insulating layer (usually a polyimide film), the following problems occur in joining.

[0003] That is, the bonding between the outer lead and the inner lead is performed under pressure and heat. When the temperature of the heating tool is higher than 340 ° C, a resin (for example, a polyimide resin) constituting the insulating layer or the insulating layer is used. The adhesive (for example, epoxy resin adhesive) used for bonding the inner lead and the inner lead is deteriorated by heating, and is oxidized or carbonized, thereby deteriorating insulation properties and decreasing strength.

When Pb-Sn solder is used for the purpose of joining the outer lead and the inner lead, its eutectic point (melting point: 1
83 ° C.), the above problem does not occur, but a wire bonding step performed at about 200 ° C. after joining the outer lead and the inner lead, or
Since there is a resin sealing step performed at around 00 ° C., stable bonding between the outer lead and the inner lead cannot be obtained.

On the other hand, when a commonly used Au-Sn alloy having a Sn content of 20% by weight is used for joining, the tool temperature exceeds 340.degree. C. and is as high as 380 to 540.degree. .

[0006]

Thus, the present invention provides:
With the object of providing a composite lead frame in which the outer lead and the inner lead are stably bonded to withstand a wire bonding step, a resin sealing step, and the like, and the resin or adhesive of the insulating layer does not undergo thermal deterioration, and a method of manufacturing the same. I have.

[0007]

According to the present invention, there is provided a means for solving] The outer lead constituted by metal lead frame, comprising constituted by a flexible wiring substrate having a fine wiring pattern of the conductor on the insulating film inner leads flop
In composite lead frames for plastic packages ,
The bonding layer between the outer lead and the inner lead is made of an alloy of tin and gold formed by heating a metal paste interposed between the two bonding surfaces, and the gold content of the alloy is 10 to 40% by weight. Plastic , characterized by being
A composite lead frame for a backpack package is provided.

Further, the outer lead and the inner lead of a composite lead frame for a plastic package , wherein the outer lead is constituted by a metal lead frame and the inner lead is constituted by a flexible wiring board having a fine wiring pattern of a conductor formed on an insulating film. When joining the inner lead with a gold-tin eutectic alloy having a gold content of 10 to 40% by weight, the gold having a gold content of 10 to 40% by weight between the bonding surfaces of the outer lead and the inner lead. A method of manufacturing a composite lead frame for a plastic package , comprising joining an outer lead and an inner lead by heating the paste after interposing a tin paste.

Further, the outer lead and the outer lead of the composite lead frame for a plastic package , wherein the outer lead is constituted by a metal lead frame and the inner lead is constituted by a flexible wiring board having a fine wiring pattern of a conductor formed on an insulating film. When joining the inner lead with the gold-tin eutectic alloy, after applying a gold-tin paste mainly composed of tin or tin to one of the joining surfaces of the outer lead and the inner lead and applying gold plating to the other joining surface, The outer lead and the inner lead are formed by overlapping and heating the two joining surfaces.
Formed into an alloy layer with a gold content of 10-40% by weight
A method of manufacturing a composite lead frame for a plastic package is provided.

Here, the flexible wiring board refers to a so-called flexible wiring board in which a fine wiring pattern of a conductor is formed on an insulating film (including a tape-shaped one). What is called a TAB tape is also a flexible wiring board in which a fine wiring pattern of a conductor is formed on an insulating film, and is included in the flexible wiring board according to the present invention.

In the above, a mixed powder paste composed of gold powder and tin powder or a powder paste composed of an alloy of gold and tin is used as the gold-tin paste.

The most important feature of the present invention is that the bonding between the inner lead of the flexible wiring board and the outer lead of the outer frame has a gold content of 10 to 40% by weight formed by heating the metal paste as a bonding layer. The use of a Sn-Au alloy is a particular alloy having an appropriate melting point range (210-305 ° C.) and the absence of a phase consisting of AuSn, a brittle intermetallic compound. In this case, it is not necessary to heat to a high temperature of 340 ° C. or more, and stable bonding is achieved.

The features of the present invention will be described in more detail. A eutectic alloy of gold and tin (Au 82% by weight, Sn18
(% By weight) has been used for a long time in the field of ceramic-sealed LSI packages and the like. In this method, a eutectic alloy foil of gold and tin is interposed as a brazing material between a gold-plated metal cap and a similarly gold-plated ceramic base, and 340
It is heated and melted in an Nz gas stream at a temperature of about 360 ° C. for 3 to 5 minutes and brazed. In this method, since the alloy foil is merely melted with the alloy foil interposed therebetween, the solidified bonding layer has almost the same eutectic composition as the alloy foil. This is because while the thickness of the gold plating on the metal cap and the ceramic base is as thin as about 1 μm, the alloy foil with the eutectic composition has a thickness of 3 μm.
This is because a thick foil of about 0 to 50 μm is used.

On the other hand, in the gold-tin bonding method of the present invention, the final composition of the bonding portion is changed to a normal gold-tin eutectic composition (usually Au82
% Of Sn and 18% by weight of Sn are referred to as a eutectic composition).
In that it utilizes the composition range of 0% by weight,
This is significantly different from the conventional method described above. Explaining this point, the reason why the composition of the bonding portion is a gold-tin alloy of Au 10 to 40% by weight is that in this region, Au and S
It is difficult to generate a brittle AuSn intermetallic compound in which n is one atom at a time, and the junction temperature can be lowered. Explaining this with an Au-Sn metal phase diagram, in the case of a gold-tin alloy, there is a low-temperature eutectic point of 217 ° C in a region of about 10% by weight of Au, which is 5 ° C lower than the melting point of 232 ° C of tin. That is, compared with the normal eutectic point of 82% by weight of Au at 293 ° C., the temperature is lower by 76 ° C. (293-217 = 76), and bonding at a low temperature is possible. This difference of 76 ° C. is very important. Particularly, in the case of a composite lead frame or a semiconductor device using an organic material such as a polyimide film or an epoxy-based adhesive as a package constituent material, a temperature of 200 ° C.
At higher temperatures, the higher the temperature, the more the organic material is degraded or partially carbonized, so that electrical characteristics such as insulation resistance cannot be guaranteed. In the ceramic sealed LSI package, a composition having a high melting point is selected in order to obtain heat resistance at a high temperature because an organic substance is not interposed. This is also because the package is used for aircraft communication equipment and industrial equipment that require high reliability at high temperatures. In the plastic package envisaged by the present composite frame, since the sealing plastic itself is an epoxy-based organic material, the soldering brazing material having a low temperature of 217 ° C. is sufficient.

Hereinafter, the present invention will be described in detail. here,
An embodiment in which gold plating is applied to the bonding surface of the inner lead and gold or tin paste mainly containing tin or tin is applied to the bonding surface of the outer lead by printing will be described.

When joining the inner lead of the flexible wiring board and the outer lead of the outer frame, first, the joining surface of the inner lead is plated with gold. This is a preferred embodiment. Then, tin or a gold-tin paste mainly composed of tin is applied to the joint surface of the outer lead by partial printing. When gold and tin are separately provided in this manner, the amount of brazing material such as foil interposed at the joining portion, particularly the thickness, can be reduced as a whole. Outflow can be reduced. Reducing the outflow of brazing material in this way is extremely important as a method for joining many fine leads at a narrow pitch and precisely, and depending on the degree of outflow of brazing material, each lead may be short-circuited. I will. In addition, it is preferable that one of the brazing materials is formed by gold plating in order to reduce the thickness of the brazing material.

As the heating means used for bonding, known heating means such as infrared beam heating and heating by a heater are employed, but a method of heating using a heater is preferred. When a heating heater is used, the heating temperature is set to 300 to 500 ° C. and 0.5 to 20 while applying a pressure of 31 to 94 g to the lead.
Heat for 2 seconds. When bonding is performed under such pressure, the thickness of the bonding layer can be reduced to the minimum necessary thickness, thereby increasing the bonding strength. Further, as the thickness of the bonding layer is smaller, defects such as air bubbles are reduced, and the ratio of the alloy layer of the bonding layer having a lower strength than that of the base metal is reduced, so that the bonding strength is increased.

By such heating means, tin diffuses into gold, and as a result, an alloy layer of tin and gold is formed. The gold content of this alloy layer is 10 to 40% by weight. Such a composition is achieved by appropriately adjusting the amount of gold plating and the thickness of the tin-containing paste. .
Usually, plating and printing are performed so that the ratio of the amount of gold plating to the inner leads and the ratio of tin-containing paste printing to the outer leads fall within the above ranges.

The gold plating on the inner leads is usually 0.3 to 6.0 μm in thickness, preferably 0.5 to 2.0 μm.
The plating is performed so as to have a thickness of 0.1 to 1.5 μm. The printing thickness of the tin-containing paste on the outer lead is usually 15 to 2
It is performed so as to have a thickness of 0 μm. The composition of the tin-containing paste is
Since it is composed of a metal (tin) powder, rosin as a vehicle, a petroleum-based solvent and an activator, the components other than the metal are volatilized at the time of heat bonding to complete the bonding. For this reason, the printing thickness of the tin-containing paste is usually set to twice the thickness when tin is formed, for example, by plating alone. The tin-containing paste printed to a thickness of 15 to 20 μm is about 7 to 10 after volatilization of the solvent, rosin, activator and the like.
It has a thickness of μm.

The plating method of gold, tin or nickel is performed by a method known per se, for example, an electroplating method. For gold plating, semi-bright plating using a neutral or acidic bath is employed.

FIG. 1 shows an example of the composite lead frame of the present invention.
This will be described with reference to FIG.

The lead frame has an outer frame 13 from which outer leads 15 (15a, 15b) extend toward the vicinity of the center of the outer frame 13.

At the center of the outer frame 13 of the lead frame, a flexible multilayer wiring board 17 (hereinafter referred to as "multilayer wiring board") is arranged. Layer 19;
On the lower side, there are two layers, for example, a grounding and power supply conductor layer 21 made of copper.

The insulating film layer 19 is a polyimide film with a single-sided (ie, upper) copper foil.

This copper foil is later etched to form inner leads 27. The insulating film layer 19 has a grounding and power supply hole 23 opened by press punching or the like. The conductor layer 21 is adhered to the insulating film layer 19 with an adhesive or the like. This multilayer wiring board 17
The inner lead 27 is formed by, for example, etching or vapor-depositing copper on the upper surface of the insulating film layer 19. A good conductor such as gold is plated on an Sn—Ni base, for example, at the inner end of the inner lead 27 so that the wire bonding connection is made well.

If gold plating is also applied to the inner tip of the inner lead for wire bonding as described above, the outer lead and the inner lead are connected by the gold-tin eutectic alloy method in the present invention as described above. The fact that gold plating is applied to the inner lead side is very convenient in terms of plating work because the above-described respective gold plating operations can be performed simultaneously on the inner lead. A good conductor such as gold is also plated on the hole 23 of the multilayer wiring board 17 on, for example, a Sn—Ni base.

Outer lead 15 of the above lead frame
The electrical connection between b and the inner leads 27 of the multilayer wiring board 17 is achieved by the heat and pressure bonding by the method described above.

In this figure, the outer leads of the lead frame are respectively extended from, for example, four corners of the outer frame 13 as grounding and power supply outer leads 15a, and the others are used as signal outer leads 1a.
5b. The signal outer lead 15b is connected to the inner lead 27 as described above, but the ground and power supply lead 15a is connected to the conductor layer 21b.
To ground directly.

A semiconductor element 33 is mounted on such a multilayer lead frame, and a bonding wire 35 is provided between a signal terminal of the semiconductor element 33 and a plating terminal of the inner lead 27.
And the semiconductor element 3
A bonding wire 35 connects between the ground terminal 3 and the portion of the conductor layer 21 that is exposed from the ground and power supply holes 23 of the multilayer wiring board 17. Finally, the semiconductor device can be manufactured by resin sealing so as to wrap the inner leads 27.

The structure of the composite lead frame of the present invention may be such that the flexible wiring board has a single layer other than that described above, that is, does not have the conductor layer 21 of FIG. Further, a conductor layer may be further laminated with a structure having a conductor layer for grounding and a conductor layer for power supply.

[0031]

(Example 1) First, a 0.15 mm thick Fe
The outer frame 13 and the outer lead 15 as shown in FIGS. 2 and 3 are manufactured using a −42% Ni alloy. Here, the pitch at the tip of the outer lead 15 is 0.
37 mm. Next, the grounding and power supply holes 23 are formed in the 0.05 mm thick polyimide insulating film layer 19.
And a two-layer multilayer wiring board 17 in which a 0.10 mm-thick 42 alloy (Fe-42% Ni alloy) foil is adhered to one surface of the polyimide insulating film layer. A copper foil having a thickness of 0.018 mm is attached to the multilayer wiring board 17 and etched to form inner leads 27 having a pitch of 0.12 mm.
Is formed, and Au / Ni is formed on the outer end of the inner lead 27.
Plating (Au: 2.0 μm, Ni: 0.5 μm)
m) A tin paste made of tin powder was printed on the outer end of the inner lead 27 (Sn paste thickness: 15 μm)
m) The signal lead 15b of the outer lead 15 is heated and pressed by a tool (heater). Here, the ratio of the plating amount of Au to Sn is 26% by weight of Au,
Is 74% by weight. Reference numeral 37 denotes an Au / Sn junction layer.

Each condition in the joining was as follows.

Temperature: 320 ° C. Pressure: 40 g / lead time: 10 seconds The alloy composition of the formed bonding layer 37 was 15% Au.

Outer lead 15 for grounding and power supply
a was also simultaneously bonded with the same tool as in the case of the signal outer lead 15b. In this case, the printing thickness of the tin paste on the grounding and power supply outer leads 15a is sufficient to be the same as the thickness on the signal outer leads 15b.

In the composite lead frame thus obtained, no carbonized or oxidized polyimide resin or adhesive near the joint between the inner lead and the outer lead was observed.

The peel strength of the joint was measured for the purpose of confirming that the joint strength between the inner lead and the outer lead was stable. The initial value was about 45 gf / mm even after leaving at 150 ° C. for 1000 hours. There was little change.

A series of experimental results will be described for a better understanding of the present invention.

FIG. 3 is a cross-sectional view before bonding in which gold is plated on the inner lead 27 and tin paste is printed on the lower part of the outer lead 15, while FIG. 4 is a cross-sectional view after pressure bonding. And 37 is a bonding layer made of an alloy of tin and gold.

When the total thickness of gold and tin was fixed (7.5 μm) (in this case, the tin thickness was reduced by heating the tin paste in FIG. 3 to complete the volatilization of rosin, solvent, activator, etc.). (Note: This is the same in FIGS. 5, 6, 7, and 8.)
As shown in FIG. 5, the thickness of the Au / Sn junction cross section is Au
A tendency was observed to increase as the plating thickness increased.
This is because the Au layer which does not react when the thickness of gold is large because the temperature at the time of pressure bonding is low remains as it is.

<Note> In the actual joining operation, no de-vehicle heating is performed on the tin paste before joining, and the vehicle (rosin, solvent, activator, etc.) is volatilized simultaneously with the joining by heating during joining. ing.

On the other hand, when the gold plating thickness is constant at 1.0 μm, as shown in FIG. 6, the thickness of the bonding section is almost constant (4.0 μm) regardless of the thickness of the tin layer. Was.

Next, focusing on the tensile strength of the joint, as shown in FIG. 7, there was a region having a large tensile strength where the Au plating thickness was 0.5 to 2.0 μm.

FIG. 8 plots the relationship between Au and Sn thickness by performing a quantitative analysis of the joint. FIG. 8 shows that the region having a high tensile strength has a gold content of 10 to 4 in relation to FIG.
It can be seen that the alloy composition is 0% by weight.

Au- consisting of an alloy having a gold content of 20% by weight
FIG. 9 shows that the Sn junction was analyzed by X-ray diffraction.
And a brittle intermetallic compound represented by AuSn was not formed. AuSn was not formed in the alloy of 10% by weight of gold or the alloy of 40% by weight of gold.

On the other hand, for the purpose of a reliability test of a joint made of an alloy of gold and tin, the composition of the treated joint subjected to a high-temperature storage test at 150 ° C. was Sn-10 wt% Au to Sn-40 wt%.
In the Au range, no decrease in peel strength was observed even after 1000 hours. An example is shown in FIG. In addition, a temperature cycle test (−30 ° C. × 30 min.
(Holding at 30 ° C. for 30 minutes) was carried out for 1,000 cycles, but no peeling occurred and there was almost no decrease in peel strength.

(Example 2) In Example 1, a mixed powder paste containing gold and tin powders was applied to the joint surface of the outer lead by printing. In this case, gold plating was applied to the bonding surface and the wire bonding surface of the inner lead at 0.5 μm. The reason why the thickness of the gold plating is set to 0.5 μm is to allow wire bonding to work normally. The weight ratio of gold and tin in the mixed powder paste is 10% gold,
Tin was 90%. Therefore, the composition of the alloy layer of gold and tin in the joint portion was such that the gold component ratio was increased by the weight of the gold plating on the inner lead side, and as a result of composition analysis, Au was 23%. Others are the same as the first embodiment.

Example 3 In Example 2, an alloy powder paste composed of an alloy of gold and tin was applied to the joint surface of the outer lead by printing. The composition of the alloy powder was 10% by weight of gold and 90% by weight of tin, and was the same as that of the mixed powder paste. Others are the same as the second embodiment.

Example 4 In Example 3, printing of the alloy powder paste was performed by local printing on the bonding surface of the inner lead instead of the bonding surface of the outer lead. Others are the same as the third embodiment.

[0049]

According to the present invention, the inner lead and the outer lead are stably bonded even when subjected to the wire bonding step and the resin sealing step. In other words, the bonded portion is heated by the step. Since it does not melt and flow, it is very suitable for joining fine patterns, and since the joining temperature is in an appropriate range, it is easy to obtain a composite lead frame without thermal degradation of the resin and adhesive used for the insulating layer. Can be

According to the present invention, when the inner lead and the outer lead are connected by the gold-tin eutectic alloy method, gold plating is applied to the inner lead side. In the case where gold plating is applied to the inner end of the lead, these gold plating operations can be easily performed, for example, simultaneously.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view illustrating an example of a composite lead frame according to the present invention.

FIG. 2 is a plan view illustrating an example of a composite lead frame according to the present invention.

FIG. 3 is a cross-sectional view showing a state before joining an inner lead and an outer lead.

FIG. 4 is a cross-sectional view showing a state after the inner lead and the outer lead are joined.

FIG. 5 is a graph showing a change in the thickness of the bonding cross section when the Au plating thickness and the Sn plating thickness are changed.

FIG. 6 is a graph showing a change in the thickness of the junction cross section when the thickness of the Sn layer is changed while the Au plating thickness is kept constant.

FIG. 7 is a graph showing the strength of the bonding layer when the thickness of the Au plating and the thickness of the Sn layer are changed.

FIG. 8 is a graph showing the composition of an Au / Sn junction layer.

FIG. 9 is a view showing an X-ray diffraction pattern of a bonding layer.

FIG. 10 is a graph showing the heat resistance of the strength of the Au / Sn junction.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Lead frame 13 Outer frame 15a Grounding and power supply outer lead 15b Signal outer lead 17 Flexible multilayer wiring board 19 Insulating film layer 21 Grounding and power supply conductor layer 23 Grounding and power supply hole 27 Inner lead 31 Bump 33 Semiconductor Element 35 Bonding wire 37 Sn-Au alloy layer 38 Hillet discharged by pressure bonding

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Hiroki Tanaka 3550 Kida Yomachi, Tsuchiura City, Ibaraki Prefecture Inside Hitachi, Ltd.System Materials Research Laboratories (72) Kenji Yamaguchi 3550 Kida Yomachi, Tsuchiura City, Ibaraki Prefecture (56) References JP-A-2-168661 (JP, A) JP-A-55-16732 (JP, A) JP-A-54-122653 (JP, A)

Claims (3)

(57) [Claims] 1. A composite lead frame for a plastic package, wherein an outer lead is constituted by a metal lead frame, and an inner lead is constituted by a flexible wiring board in which a fine metal pattern of a conductor is formed on an insulating film. The bonding layer with the inner lead is made of an alloy of tin and gold formed by heating the metal paste interposed between the two bonding surfaces,
A composite lead frame for a plastic package, wherein the alloy has a gold content of 10 to 40% by weight. 2. The composite package according to claim 1, wherein the outer lead is formed of a metal lead frame, and the inner lead is formed of a flexible wiring board having a fine wiring pattern of a conductor formed on an insulating film. When joining the inner lead with a gold-tin eutectic alloy having a gold content of 10 to 40% by weight, the gold having a gold content of 10 to 40% by weight between the bonding surfaces of the outer lead and the inner lead. A method for manufacturing a composite lead frame for a plastic package , comprising joining an outer lead and an inner lead by heating the paste after interposing a tin paste. 3. The composite package according to claim 1, wherein the outer lead comprises a metal lead frame, and the inner lead comprises a flexible wiring board having a fine wiring pattern of a conductor formed on an insulating film. When joining the inner lead with the gold-tin eutectic alloy, after applying a gold-tin paste mainly composed of tin or tin to one of the joining surfaces of the outer lead and the inner lead and applying gold plating to the other joining surface, joining of the outer lead and the inner lead by heating by superimposing the both joint surfaces
Method for producing a plastic package for a composite lead frame layer is gold content, characterized in the this <br/> forming the alloy layer of 10 to 40 wt%.