HK1084508A - Area array package with non-electrically connected solder balls - Google Patents

Abstract

An area array package includes a plurality of solder balls not used as electrical connectors. These non-connected solder balls, or "dummy balls," provide protection to solder balls connected to live pins and therefore increase reliability of the area array package. The dummy balls may be placed in the corners, along the diagonals or in other high stress locations on the area array package. To further increase reliability, a continuous copper ball land pad may be used to connect each group of corner dummy balls. Continuous copper pads help to reduce stress on the dummy balls. For center-depopulated BGA packages, an array of dummy balls may be used in the center of the package to prevent substrate bending and improve drop test reliability.

Description

Area array package with non-electrically connected solder balls

Related application

The present application claims U.S. provisional patent application No. 60/443,817 entitled "area array package with improved SOLDER joint reliability (AREA ARRAY PACKAGE WITH improved SOLDER joint reliability OF SOLDER JOINTS"), filed on 30/1/2003.

Technical Field

The present application relates to area array packages and, more particularly, to improving the reliability of solder joints in area array packages that are subjected to environmental stresses.

Background

In order to enable the semiconductor integrated circuit chip to electrically interact with the outside, the semiconductor integrated circuit chip is connected. Ball Grid Array (BGA) semiconductor chip packages use a plurality of solder balls as external terminals. BGA packages are widely used because they can implement a multi-pin structure on a limited area.

Chip devices typically have a Coefficient of Thermal Expansion (CTE) of about 3 ppm/C. These devices are relatively rigid and may exhibit brittle fracture if stressed by excessive bending. An area array package includes a chip, a package substrate, and optional molding. The CTE of an area array package is affected by each component in the package.

The CTE of the epoxy glass printed circuit board may be in the range of about 16 to 21ppm/C depending on the content of glass cloth, resin system, and copper. To obtain a useful electrical connection, it is necessary to provide a physical connection between the area array package and the printed circuit board.

The mismatch in CTE that exists between the area array package and the printed circuit board causes thermally driven stresses and can affect the reliability of the package in a number of ways. In some way, this substantial CTE mismatch affects all electronic packaging schemes involving area array packaging and printed circuit boards. Measurements of Ball Grid Array (BGA) and Chip Size (CSP) type semiconductor packages have found that solder joints near the corners of the package are subject to particularly high strain due to CTE mismatch.

As pad-pad pitch and size shrink, solder ball joint reliability typically decreases. Area array packaging requires packaging schemes to make and maintain electrical contact between the package and a printed circuit board over the entire surface of the device. There is a temperature dependent shear strain between the area array package and the printed circuit board. Solder ball connections have a very predictable limited fatigue life. Furthermore, area array packages are also subject to mechanical stresses that are independent of CTE. These stresses include mechanical bending and other environmental stresses.

The stress between the area array package and the printed circuit board makes it necessary to protect the solder balls that provide the electrical connection. The protection of the solder balls providing the electrical connection should provide sufficient stress relief to reduce the strain on the solder ball connection to an acceptable level to achieve an increase in fatigue life.

Disclosure of Invention

An area array package includes a plurality of solder balls that are not used as electrical connectors. These unconnected solder balls or "dummy balls" provide protection for solder balls connected to live pins, thus increasing the reliability of the area array package. The dummy balls may be placed diagonally at the corners or at other high stress locations on the area array package. To further improve reliability, a continuous copper ball pad may be used to connect each set of corner dummy balls. The continuous copper pads help reduce stress on the dummy balls. For center-ball-reduced BGA packages, an array of dummy balls may be used at the center of the package to prevent substrate bending and improve drop test reliability.

The use of dummy balls to improve the reliability of area array packages and the advantages of the embodiments will become more apparent upon reading the following detailed description and upon reference to the accompanying drawings.

Drawings

FIG. 1 shows an area array package connected to a printed circuit board;

FIG. 2 illustrates a dummy ball structure on the area array package;

FIG. 3 shows an alternative center dummy ball configuration for an area array package with reduced components at the center;

fig. 4 is a flow chart showing how connections are made between the area array package and the printed circuit board to improve reliability.

Detailed Description

Fig. 1 shows an area array package 100. The area array package 100 includes a substrate 105, a die 106, and optionally molding 108. The substrate 105 may be composed of different materials including ceramic, laminate, or polyimide tape. The die 106 is typically a silicon chip that is attached to the substrate 105. The die 106 may be wire bonded or flip-chip mounted to the substrate 105. A molding 108 may be arranged around the die 106 to protect the die 106. The molding 108 is typically used when the die 106 is wire bonded to the substrate 105, but the molding 108 may be used even if the die 106 is flip-chip mounted.

The area array package 100 is mounted on a printed circuit board 110 or similar component. The printed circuit board 110 is typically a laminate. The area array package 100 is connected to the printed circuit board 110 by a plurality of solder balls 115. The solder balls 115 may provide mechanical and electrical connection between the area array package 100 and the printed circuit board 100.

Fig. 2 shows solder ball connections 115 between the area array package 100 and the printed circuit board 110. Solder ball connections 115 are typically disposed on the bottom of the substrate 105 of the area array package 100. In the present invention, two types of solder ball connections 115 are used. The first type of solder ball connections 205 provide a mechanical and electrical interface between the area array package 100 and the printed circuit board 110. Thus, in addition to physically securing the area array package 100 to the printed circuit board 110, the solder ball connections 205 are also capable of transmitting electrical signals between the area array package 100 and the printed circuit board 110. This type of solder ball connection 205 is a common solder ball connection in BGA applications. The second type of solder ball connection is a dummy ball 210. The dummy balls 210 provide only a mechanical connection between the area array package 100 and the printed circuit board 110. The dummy balls 210 are not connected to any electrical signal lines.

In one embodiment of the present invention, the area array package 100 includes solder ball pads 208. Each solder ball pad 208 is adapted to receive a solder ball. In fig. 2, only a sample of solder ball pads 208 is shown for clarity, although a solder ball pad 208 is typically located in each location adapted to receive a solder ball. The solder ball pads 208 located at each corner 215, 220, 225, 230 of the area array package 100 and along the diagonal 235, 240 connecting the corners 215, 220, 225, 230 of the area array package 100 do not have electrical connections to the area array package 100. These solder ball pads 208 without electrical connections are configured to use dummy balls 210.

Measurements performed on semiconductor packages have found that solder joints near the corners of the package are subjected to particularly high stresses. By designing the area array package 100 to have no electrical connections at the corners 215, 220, 225, 230 and along the diagonals 235, 240 connecting the corners 215, 220, 225, 230 of the area array package 100, the dummy balls 210 may be used in place of standard solder joints. The use of dummy balls 210 in place of standard solder joints allows the high strain experienced in these corners and diagonals to be absorbed by the dummy balls 210. Since the dummy balls 210 do not contain any electrical connections, the strain applied to the dummy balls 210 does not break sensitive electrical connections. In addition, designing an area array package 100 without electrical connections along the diagonals 235, 240 also enables the dummy balls 210 to be placed along the diagonals 235, 240 to further absorb stress and strain without breaking any electrical connections.

Empirical tests conducted using the dummy balls 210 show that the dummy balls 210 fail earlier than conventional solder balls in both the pre-failure temperature cycling test and the kink failure test. Since the dummy balls 210 do not have electrical connections, their failure does not fail the connection between the area array package 100 and the printed circuit board 110. By locating the dummy balls 210 at high stress locations, the overall life of the area array package 100 is improved. Table 1 shows the test.

	Temperature cycling (number of cycles before failure)
			Sample(s)	I/O circuit	Dummy ball
1	1455	379
			2	2065	140
3	1759	1070
			4	1750	467
5	426	189

	Failure to bend (mm deflection)
			Sample(s)	I/O circuit	Dummy ball
1	14.5	10
			2	15.5	15
3	15	14.5
			4	19	14.5
5	15	14.5

6	1923	329
			7	916	176
8	1184	134
			9	1797	121
10	1425	211
			11	1898	1269
12	2315	417
			13	1805	737
14	768	218
			15	1849	1075
16	1809	171
			17	1946	375
18	1075	589
			19	2259	387
20	2300	125
			Average	1636	429

6	16	13.5
			7	14	12.5
8	15.5	13
			9	18.5	18
10	18	14
			11	14	8
12	16	7.5
			13	15	14
Average	16	13

TABLE 1

To further reduce stress on the dummy balls 210, a continuous copper ball pad 250 may be disposed in one or more corners 215, 220, 225, 230 under the dummy balls 210. The copper pads 250 are disposed between the dummy balls 210 and the substrate 105. The copper pad 250 provides a uniform pad area and further reduces stress on the dummy balls 210 at the corners. Modeling data conducted using copper pad 250 indicates an increased number of temperature cycles before failure. Table 2 shows the test.

	Temperature cycling before failure
			Dummy ball	Copper-free welding pad	With copper bonding pads
1	920	987
			2	3216	3750
3	3099	3793

4

2475

2617

TABLE 2

Fig. 3 shows an alternative arrangement of solder ball connections between the area array package 100 and the printed circuit board 110 for a reduced component package in the center. In the present embodiment, an array of dummy balls 350 is located in the reduced component area at the center of the area array package 100. The centrally located dummy ball array 350 prevents substrate bending during electrical testing and provides more solder joints to improve drop test reliability. The array of dummy balls 350 located at the center may also be combined with dummy balls used at the corners and diagonals to enhance reliability.

Fig. 4 is a flowchart showing how to prepare the connection between the area array package 100 and the printed circuit board 110 to improve reliability. The process begins at START block 405. Proceeding to block 410, a package outline is then formed for the area array package 100. The package outline includes a plurality of solder balls 115, the plurality of solder balls 115 being more than the number of input/output (I/O) requirements of the chip device 106. By including a high number of solder balls 115, not every solder ball 115 is required to serve as an I/O connection. This enables some of the solder balls 115 to function as dummy balls 210.

Proceeding to block 415, high stress locations along the connection between the area array package 100 and the printed circuit board 110 are identified. The high stress locations are those locations where failure of the solder ball 115 may first occur when subjected to environmental or other stresses. The high stress locations may be identified by modeling data (computer simulation), empirical testing, or other suitable techniques.

Proceeding to block 420, for each high stress location, an appropriate number of solder balls 115 are selected as dummy balls 210. The number of dummy balls 210 to be used for each area is selected based on the target performance level and the number of I/O connections required. As described above, high stress areas are typically found in the corners and along the diagonals of the area array package 100. Therefore, dummy balls 210 are disposed in these areas as shown in fig. 2. The construction in fig. 2 is merely exemplary, and the number of dummy balls 210 to be used in each high stress region may vary depending on the target performance level and the number of I/O connections required. When a center reduced component package is used, an array of dummy balls may be optionally placed in the center of the area array package 100. The center dummy balls may or may not be used with dummy balls at corners and diagonals. The center dummy balls may be arranged in a rectangular or square layout, but other types of layouts may be used.

Proceeding to block 425, the substrate is routed so that the die pads are only connected to locations containing charged I/O solder balls. No electrical connection is made between the die pads and the dummy balls.

Proceeding to block 430, a continuous copper or other pad may be placed between the dummy balls at the corners and the area array package 100. The continuous copper pads improve the resistance of the dummy balls at the corners to environmental stresses. The use of copper pads is optional.

After the package is designed and the dummy balls are placed, empirical testing may be performed to determine whether the package meets the target stress level. If the target level is not met, further modifications may be made to the design. Once the target level is met, the process terminates in an END block 435.

Although the present device has been fully described in connection with the preferred embodiments thereof with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Such changes and modifications are to be considered as falling within the scope of the present device as defined by the appended claims.

Claims (15)

1. An apparatus, comprising:

a printed circuit board;

an area array package having corners;

a first set of solder balls electrically connecting the area array package to the printed circuit board; and

a second set of solder balls providing physical connections but not electrical connections, the second set being configurably disposed in at least one of the corners of the area array package and along a diagonal of the area array package relative to the corners.

2. The device of claim 1, wherein the second set of solder balls further comprises solder balls disposed at a center of the area array package.

3. The device of claim 1, wherein at least one of the second set of solder balls located in one of the corners of the area array package is located on a continuous ball pad.

4. The device of claim 3, wherein the continuous ball pad is a metal pad.

5. An apparatus, comprising:

a printed circuit board;

an area array package;

a first set of solder balls electrically connecting the area array package to the printed circuit board; and

a second set of solder balls providing physical connections but no electrical connections, the second set being configurably disposed at a center of the area array package.

6. A method of improving reliability between an area array package having corners and a printed circuit board, comprising:

providing a first plurality of solder balls unused for electrical connection between the area array package and the printed circuit board, wherein the first plurality of solder balls are configurably disposed in at least one of the corners of the area array package and along a diagonal of the area array package relative to the corners; and

providing a second plurality of solder balls electrically connecting the area array package and the printed circuit board.

7. The method of claim 6, further providing a third plurality of solder balls disposed at the center of the area array package.

8. The method of claim 6, further comprising positioning portions of the first plurality of solder balls disposed in the corners on a continuous ball pad.

9. The method of claim 8, wherein the continuous ball pad is a metal pad.

10. An area array package having corners, comprising:

a first set of solder ball pads configured for transmitting electrical signals to the area array package; and

a second set of solder ball pads without electrical connections, the second set of solder ball pads configured to provide only physical connections with the area array package, the second set of solder ball pads being configurably disposed in at least one of the corners of the area array package and along diagonals of the area array package relative to the corners.

11. The area array package of claim 10, wherein the second set of solder ball pads further comprises solder ball pads disposed at the center of the area array package.

12. The area array package of claim 10, wherein at least one of the second set of solder ball pads located in one of the corners of the area array package comprises a continuous ball pad.

13. The area array package of claim 12, wherein the continuous ball pad is a metal pad.

14. A method of designing an area array package having corners, comprising:

providing a first plurality of solder ball pads in at least one of the corners of the area array package and along a diagonal of the area array package relative to the corners that are not used to electrically connect the area array package; and

a second plurality of solder ball pads adapted to allow electrical connection to the area array package is provided.

15. The method of claim 14, further providing a third plurality of solder ball pads disposed at the center of the area array package not used for electrical connections.