Active solid-state devices (e.g. – transistors – solid-state diode – Lead frame – With dam or vent for encapsulant
Reexamination Certificate
2001-11-20
2004-03-30
Chambliss, Alonzo (Department: 2827)
Active solid-state devices (e.g., transistors, solid-state diode
Lead frame
With dam or vent for encapsulant
C257S669000, C257S672000, C257S673000, C257S674000, C257S676000, C257S687000, C257S690000, C257S783000, C257S784000, C257S787000, C361S772000, C361S773000, C361S776000, C361S813000
Reexamination Certificate
active
06713849
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a technique for manufacturing a resin-sealed type semiconductor device using a lead frame and more particularly to a technique effective in its application to the manufacture of a semiconductor device (a non-leaded type semiconductor device) in which external electrode terminals are exposed to mounting side faces without intentional projection sideways of a package, such as SON (Small Outline Non-Leaded Package) and QFN (Quad Flat Non-Leaded Package).
In the manufacture of a resin-sealed type semiconductor device there is used a lead frame. The lead frame is fabricated by forming a metallic plate into a desired pattern by blanking with a precision press or by etching. The lead frame has a tab for fixing a semiconductor element (semiconductor chip), a support portion called die pad, and plural leads whose front ends (inner ends) face an outer periphery of the support portion. The tab is supported by tab suspension leads extending from a frame portion of the lead frame.
In manufacturing a resin-sealed semiconductor device with use of such a lead frame, a semiconductor chip is fixed to the tab of the lead frame, electrodes on the semiconductor chip and the front ends, or the inner ends, of the leads are connected together using conductive wires, then the inner end sides of the leads, including the wires and the semiconductor chip, are sealed with an insulating resin to form a seal member (package), thereafter, an unnecessary lead frame portion is cut off and at the same time lead portions and tab suspension lead portions projecting from the package are cut off.
On the other hand, as one of resin-sealed type semiconductor devices manufactured using a lead frame there is known a semiconductor device (non-leaded type semiconductor device) wherein a package is formed by one-side molding on one side of a lead frame and leads as external electrode terminals are exposed to one side of the package, without intentional projection of the leads from a peripheral surface of the package. As this type of semiconductor devices there are known an SON wherein leads are exposed to both side edges of one surface of a package and a QFN wherein leads are exposed to four sides of one surface of a quadrangular package.
As an example of a bleeding preventing technique there is known a technique disclosed in Japanese Published Unexamined Patent Application No. Hei 11(1999)-345897. In this unexamined publication is disclosed Fan-out-BGA of a structure having been subjected to processing for the prevention of bleeding of Ag paste, such as the formation of a solder resist dam or blasting (abrasive polishing).
On the other hand, in Japanese Published Unexamined Patent Application No. 2000-196006 is disclosed a QFP (Quad Flat Package) type semiconductor device of a structure wherein lugs projecting toward the interior of a sealing resin are formed on side faces of a die pad for the purpose of improving a close adherence between the die pad and the sealing resin and also improving moisture resistance. According to the structure of this semiconductor device, the back side of the die pad is exposed from the sealing resin.
Further, in Japanese Published Unexamined Patent Application No. Hei 11(1999)-251494 is disclosed a high-frequency device having a gull wing type wire structure for use in a portable telephone or the like in which a semiconductor element mounting portion serves as ground. According to the technique disclosed therein, not only electrodes on a semiconductor element and the leads are connected together through wires, but also the electrodes on the semiconductor element and the semiconductor element mounting portion are connected together through wires because of a die pad serving as ground. This is called down bonding in the literature just referred to above. Because of down bonding, the semiconductor element mounting portion is larger than the semiconductor element, and in a mounted state of the semiconductor device the semiconductor element mounting portion projects to the outside of the semiconductor device.
SUMMARY OF THE INVENTION
From the standpoint of reducing the size of a semiconductor device and preventing bending of leads serving as external electrode terminals there is used a non-leaded type semiconductor device using one-side molding such as SON or QFN. In a non-leaded type semiconductor device, lead surfaces exposed to one surface of a package constitute a mounting surface and therefore the mounting area is small in comparison with SOP (Small Outline Package) and QFP in which leads are projected from side faces of a package.
In a non-leaded type semiconductor device of a tab exposed structure there exists a keen demand for a down bonding structure wherein electrodes on a semiconductor element (semiconductor chip) and a tab with the chip mounted thereon are connected together using wires, with a view to improving electrical characteristics of particularly a high-frequency device. Thus, it is now a pressing need to develop a package structure which permits down bonding while ensuring high reliability.
In a non-leaded type semiconductor device wherein a tab is larger than a chip, a surface (main surface) of the tab with the chip mounted thereon and resin which constitutes a packages are apt to be peeled off from each other. This peeling-off is ascribable to a one-side molded structure wherein the surface (main surface) of the tab is in contact with the resin, while the back side of the tab is exposed from the resin, and is also ascribable to a thermal stress (thermal strain) induced by a difference in thermal expansion coefficient, &agr;, among components.
As an example, the semiconductor element is formed of silicon (&agr;=3.0×10
−6
/° C.), while the tab and leads are formed of Cu (&agr;=1.7×10
−5
/° C.). An adhesive for connection between the semiconductor element and the tab is Ag paste (&agr;=3.5×10
−5
/° C.) constituted by an epoxy resin, wires connected to electrodes on the semiconductor element are Au wires (&agr;=2.63×10
−5
/° C.), and the resin which constitutes a package is a biphenyl resin (&agr;=1.2×10
−5
/° C.).
In the case of a mounting substrate with a non-leaded type semiconductor device mounted thereon, such as a mother board, e.g., FR-4, the value of &agr; is 1.5×10
−5
/° C.
Thus, due to a difference in thermal expansion coefficient among components which constitute a non-leaded type semiconductor device or due to a difference in thermal expansion coefficient of the mounting substrate before and after mounting, a strong internal stress is imposed on the tab surface-resin interface, thus making peeling-off of the tab from the resin (package) easier.
To permit down bonding, Ag plating is needed for connecting wires (Au wires) to the tab surface, but the presence of this plating film further deteriorates a close adherence between the tab surface and the resin, thus making peeling-off easier between the tab surface and the resin.
For mounting (fixing) the chip there is used an adhesive such as Ag paste, but in the case of a structure wherein Au wires are connected directly to the tab surface, a liquid component contained in the adhesive bleeds (bleeding phenomenon) and forms a film on the Ag plating, which impedes the bonding (connection) of Au wires, causing lowering of bonding strength and consequent separation. Such lowering of the wire bonding strength caused by the bleeding phenomenon is not limited to the case of Au wire and Ag plating but occurs also in the case of other wires and other plating films.
A gap is formed as a result of peeling-off between the tab surface and the resin. This gap acts as a path of water incoming from the exterior of the package, thus deteriorating the reliability of the down-bonded portion. Particularly, in a tab-exposed type package, it is difficult to ensure a sufficient strength of contact with resin because neither side of the tab is sealed with resin. Moreover, since
Danno Tadatoshi
Hasebe Hajime
Satou Yukihiro
Chambliss Alonzo
Hitachi , Ltd.
Mattingly Stanger & Malur, P.C.
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