Description

MULTI-CHIP ELECTRONIC PACKEGE

This invention relates to a method of assembling a multi- semiconductor chip package with reduced stress.

Prior art for the construction of an electronic package with multiple semiconductor chips uses a lead frame with multiple die pads. As the coefficients of thermal expansion of the semiconductor chip and the die pad differs, chips that are sensitive to stress suffer from this manufacturing technique. This is especially so for the semiconductor chip, which incorporates a glass layer as this is sensitive to mechanical stress. Furthermore, there is difficulty in aligning the multiple die pads with lead fingers of the lead frame. The wire bonding operation requires an accurate placement of the die pad with respect to the lead fingers .

Patent WO 88/06348 discloses a method to relieve stress in a semiconductor chip by coating the semiconductor chip with silicon gel. However, this method suffers from difficulty in getting a consistent silicon gel coating.

The object of this invention is to provide a method for producing a reliable multi-chip electronic package.

An electronic component produced according to the invention comprises a lead frame, which in turn consists of lead fingers and a die paddle. A tape pad is mounted below the lead fingers and the die paddle. A first semiconductor chip is bonded to the tape pad by a layer of die adhesive, such as

epoxy resin. A second semiconductor chip is bonded to the die paddle by a layer of the die adhesive. Electrical contacts, such as electrical wires, provide electrical connections between contact areas of the semiconductor chips and between the contact areas of the semiconductor chips and the lead fingers. An encapsulating compound encloses part of the lead frame, the tape pad, the semiconductor chips and the electrical contacts .

An advantage of the invention is that the semiconductor chip placed on the tape pad has a lower height since the base of the semiconductor chip that is placed on the tape pad is lower than the base of the semiconductor chip that is placed on the die paddle. This is of advantage as it reduces the package height requirement demanded by tall semiconductor chip.

The surface of the tape pad deters any excessive die adhesive from reaching the back of the die paddle. Excessive adhesive at the back of the die paddle may tilt the die paddle out of position. This in turn could affect the attachment of electrical wires between the semiconductor chip and the die paddle.

According to the invention, the tape pad further comprises a layer of tape adhesive, which inhibits parts attached to the tape pad from shifting. Furthermore, the tape pad preferably comprises a material that has high tensile modulus, which makes the tape pad rigid. This tends to prevent the parts that are attached to it from shifting. Parts that are connected to the tape pad are the lead fingers and the die paddle of the lead frame. Shifting of these parts may affect

machine placement of the electrical contacts between the parts .

According to the invention, the layer of tape adhesive on the tape pad has an opening for the placement of the semiconductor chip. The opening allows for the provision of die adhesive on the tape pad for the direct bonding of the first semiconductor chip onto the die pad.

According to the invention, the tape pad comprises a material that can tolerate 200 ⁰ C (degrees Celsius) . Molten encapsulating compound surrounds the tape pad during package encapsulation. The encapsulation compound has a melting point of about 170 ⁰ C. This feature allows the tape pad to withstand the moulding process .

According to the invention, the die paddle is at least partly- covering the tape pad. The attachment of die paddle to the tape pad keeps the die paddle fixed with respect to the lead fingers.

According to the invention, the die paddle is preferably fully on top of the tape pad. In this embodiment of the invention the tape pad is enlarged to cover fully the die paddle and preferably large enough for all lead fingers to be attached to the tape pad. This further enhances fixing the position of the die paddle with respect to the lead fingers.

According to the invention, the first semiconductor chip includes a layer of glass. This layer of glass provides a bonding such as a hermetic seal between the upper portion of the first semiconductor chip that is above the layer of glass

and the lower portion of first semiconductor chip that is below the layer of glass. An example of the first semiconductor chip is a silicon cap above the layer of glass and a micro-electrical mechanical system device such as an acceler- ometer below the layer of glass.

In prior art, the first semiconductor chip within the encapsulation package experiences a shifting force, especially at extreme temperature, from the mismatch of thermal expansion between the first semiconductor chip and the die paddle. The advantage of the invention is that this mismatch of thermal expansion is reduced. The layer of glass is sensitive to stress exerted by the shifting force. The first semiconductor chip experiences extreme temperature during electronic component testing and end-user application.

According to the invention, the electrical contacts comprise bonding wires, which provide electrical connection between the contact areas of the semiconductor chip and the lead fingers.

According to the invention, the die adhesive is electrically conductive. This provides an electrical path between the rear surface of the semiconductor chip and the tape pad. Prefera- bly, a further electrical path is provided between the tape pad and an electrical ground.

A method of fabricating an electronic component comprises the provision of a lead frame on a tape pad. Next, a bonding compound such as epoxy resin is provided on the chip placement areas of both the die paddle and the tape pad. After this, a first semiconductor chip is attached to the chip

placement area of the tape pad and a second semiconductor chip is attached to the chip placement area of the die paddle. Following this, electrical contacts such as electrical wires are connected between contact areas of the semicon- ductor chips and the lead fingers. Then, part of the lead fingers, the semiconductor chips, the tape pad and the electrical contacts are covered with an encapsulating compound.

The method of fabricating according to the invention preferably includes the further step of providing a layer of tape adhesive on the tape pad. This step is preferably taken prior to the step of providing a lead frame on a tape pad.

Figure 1 shows a cross section view of an electronic component according to the invention, Figure 2 shows a top view of the electronic component of figure 1,

Figure 3 shows a top view of a further electronic component according to the invention.

Figure 1 shows a cross section view of an electronic component 10 according to the invention and figure 2 shows a top view of the said electronic component 10.

The bottom of figure 1 shows a horizontal tape pad 1. Over the tape pad 1 is placed a lead frame 12. A first semiconductor chip 3 is provided above the central portion of the tape pad 1. The lead frame 12 comprises a die paddle 4 and lead fingers 2. Above the die paddle 4 is provided a second semiconductor chip 5. A layer of encapsulation compound covers the tape pad 1, the die paddle 4, the semiconductor

chips 3 and 5, and part of the lead fingers 2. The encapsulation compound is not shown in the figure 1.

As one can see in figure 2, the lead fingers 2 and the die paddle 4 are placed on the periphery of the tape pad 1. The lead fingers 2 are partly placed over the left, top and bottom edge of the tape pad 1. The die paddle 4 is partly placed over the right edge of the tape pad 1.

The first semiconductor chip 3 is taller than the second semiconductor chip 5 due to its sensor function. However, since the chip 3 is placed directly above the tape pad 1, the bottom surface of the chip 3 is lower than the bottom surface of the chip 5. This reduces the overall package height requirement of the electronic component 10.

The first semiconductor chip 3 comprises a sensor unit 13 that is located at the bottom of the first semiconductor chip 3. Above the sensor unit 13 is placed a layer of glass 14 and above that is placed a silicon cap 15. The sensor unit 13 measures the acceleration of its own motion. The glass layer 14 provides a hermetic seal between the silicon cap 15 and the sensor unit 13. The silicon cap 15 provides a shield over the sensor unit 13.

A thermal expansion mismatch between the first semiconductor chip 3 and the tape pad 1 is less than the mismatch between the chip 3 and the die paddle 4. The different coefficients of thermal expansion between the chip 3 and the tape pad 1 at elevated temperature results in a shift of the chip 3. However, the encapsulation compound covering the chip 3 restrains this shift. Hence, a shifting force is exerted on

IB2006/000526

7 the chip 3 at elevated temperature. The glass layer 14 is sensitive to stress exerted by the shifting force. The first semiconductor chip 3 experiences extreme temperature during electronic component testing and end-user application.

Both semiconductor chips 3 and 5 contain electrical circuits and contact pads 6 that are connected to the electrical circuits . Electrical wires 11 are attached between the contact pads 6 of the semiconductor chips 3 and 5 and between the contact pads 6 of the second semiconductor chip 5 and the lead fingers 2.

The tape pad 1 comprises a polymer material such as polyimide that tolerates 200 ⁰ C (degrees Celsius) and has a high tensile modulus of about 5 000 megapascal. Molten encapsulation compound surrounds the tape pad 1 during package encapsulation. Molten encapsulating compound has a temperature of approximately 170 ⁰ C. The high tensile modulus of the tape pad 1 means that it is stiff, which in turn means that shift of the parts attached to it is reduced.

A layer of third adhesive 9 with a central opening is placed between the top surface of the tape pad 1 and the bottom surface of the lead frame 12. The third adhesive 9 bonds the lead fingers 2 and the die paddle 4 to the tape pad 1 and deter them from shifting. Shifting may affect the machine placement of electrical wires 11 between the lead fingers 2 and the semiconductor chips 3 and 5.

A layer of first adhesive 8 is provided below the first semiconductor chip 3. Similarly, a second adhesive 7 is provided below the second semiconductor chip 5. The first and

second a&hesives 7 and 8 provide bonding and electrical conductivity between the chips 3 and their substrate. The first and second adhesives 7 and 8 comprise silver filled epoxy resin, which has strong adhesion and electrical conductivity. The broad surface of the tape pad 1 prevents any excessive adhesives 7 and 8 from spilling and reaching the back of the die paddle 4

Figure 3 shows a top view of a further electronic component 10' according to the invention. The further electronic component 10' has features similar to the features of electronic component 10 of figures 1 and 2.

The distinction between the further electronic component 10' and the electronic component 10 is the larger tape pad 1 ' and the corresponding larger surface area of the third adhesive 9' that is above the tape pad 1' . The layer of third adhesive 9 ' provides bonding between the tape pad 1 ' and the lead fingers 2 as well as the die paddle 4 and keeps them from shifting.

Reference numbers

1 tape pad

1' tape pad

2 lead finger

3 first semiconductor chip

4 die paddle

5 second semiconductor chip

6 contact pad

7 second adhesive

8 first adhesive

9 third adhesive

9' third adhesive

10 electronic component

10' electronic component

11 electrical wire

12 lead frame

13 sensor unit

14 glass layer

15 silicon cap