JPH11274711A - Package assembly - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a package assembly which can improve the reliability of a mounted BGA(ball grid allay) package and print substrate. SOLUTION: A package assembly is provided with a package 3 having a plurality of outer lead pads 9, a printer substrate 13 having a plurality of package pads 15, and a plurality of connectors for connecting the outer lead pads 9 and the corresponding package pads 15. In this case, the connectors 5 are melted and bonded to the outer lead pads 9 and the package pads 15, and connectors 5a are melted and bonded merely to the outer lead pads 9. The connectors 5a can shift on the corresponding package pads 15.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a BGA (ball
The present invention relates to a package assembly consisting of a grid array (Ball Grid Array) package and a printed circuit board.
The present invention relates to a technology effective when applied to mounting of a GA package and a printed circuit board.

[0002]

2. Description of the Related Art Conventionally, a QFP (Quad Flood Package) has been widely used as a typical multi-pin LSI package. However, in recent years, I / O (Input / Outpu
t) With the increase in the number, it is becoming increasingly difficult for QFP to respond.

That is, in the case of the QFP, in order to increase the number of I / Os, it is necessary to reduce the lead pitch of the lead frame or to increase the external dimensions of the package. This is because they are easily deformed and the defect rate when soldering to a printed circuit board is increased, and when the external dimensions of the package are increased, the mounting density is reduced.

[0004] Recently, a BGA package has attracted attention as a package capable of solving the above problems of the QFP. This BGA package is a package in which conductive balls or bumps are mounted in a matrix on the back surface of a package on which a semiconductor chip is mounted. Since a lead frame is not used unlike a QFP, it is easy to increase the number of pins, and There is an advantage that the mounting area can be reduced.

[0005]

FIG. 7 is a diagram showing a state of a connection portion when a conventional semiconductor device package (BGA package) is mounted on a printed circuit board 13. As shown in FIG. In the BGA package shown in FIG. 7, the semiconductor chip 1 is mounted by a wire bonding method on the BGA package 3 having the conductive balls 5 attached to the back surface. All of the conductive balls 5 are welded and fixed to the electrode pads 9 provided on the back surface of the package 3 by solder 11. On the other hand, all the conductive balls 5 are welded and fixed to the electrode pads 15 on the printed circuit board 13 by solder 17. In this manner, the BGA package 3 is
Is mounted on the printed circuit board 13.

Generally, the material forming the BGA package 3 and the material forming the printed circuit board 13 have different coefficients of thermal expansion. Therefore, the difference in thermal expansion coefficient causes a difference in thermal expansion between the BGA package 3 and the printed circuit board 13 at the time of mounting, and as a result, mechanical stress is reduced.
It will be added to the package 3 and the printed circuit board 13. In particular, this stress tends to concentrate on the connection between the BGA package 3 and the printed circuit board 13. Therefore, this stress is caused by the conductive balls 5 themselves and the solders 11, 1
7 and the conductive ball 5, the solder 11, 1
There is a possibility that the conductive ball 5 may be peeled off at the interface between the conductive ball 7 and the conductive ball 5 or a crack may be generated in the conductive ball 5. Similarly, the electrode pads 9 and 15 may be peeled off from the BGA package 3 and the printed circuit board 13. These results in poor electrical connection between the BGA package 3 and the printed circuit board 13, which in turn reduces the reliability of package mounting.

For example, B using 35 mm square FR4
Consider a case where a GA package is mounted on a printed circuit board. In this case, the difference in the linear expansion coefficient between the BGA package and the printed circuit board is about 12 ppm / K. Melting point 18
When mounting on a printed circuit board using 3 ° C. solder, the temperature change until normal temperature is 183 ° C. if normal temperature is 18 ° C.
-18 ° C = 165 ° C. Therefore, from the difference between the linear expansion coefficient and the temperature difference, the calculation is shifted by about 200 μm from 35 mm square in the above case.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a package assembly capable of improving the mounting reliability when a BGA package is mounted on a printed circuit board. Is to do.

[0009]

In order to achieve the above object, the present invention is characterized by a package having a plurality of outer lead pads, a printed board having a plurality of package pads, and the outer lead pad. And a plurality of connectors for electrically connecting the corresponding package pads, and a portion of the connectors is welded to both the outer lead pads and the package pads by soldering, and the remaining A package assembly in which a connection body is welded to only one of the outer lead pad and the package pad by soldering.

That is, according to the present invention, all of the connecting bodies for connecting the outer lead pads and the package pads are not welded to both electrode pads, and a part thereof is connected to only one of the electrode pads. It is welded and is movable with respect to the other electrode pad. Therefore, the mechanical stress caused by the difference in the coefficient of thermal expansion between the package and the printed circuit board can be reduced as compared with the case where all the connecting members are welded and fixed to both electrode pads. Thereby, mechanical stress caused by a temperature change during mounting is reduced, and mounting reliability is improved.

Here, "welding by soldering" does not necessarily mean that the connection body does not need to be completely melted.
In order to be welded by solder, depending on the material of the connection body, at least the surface of the connection body at the atomic layer level and the solder melted by a predetermined reaction such as an alloying reaction, a solid layer diffusion reaction, and a surface catalysis reaction mutually. It just needs to be attached.

Further, the connection body which is welded to only one of the outer lead pad and the package pad by soldering may be selected, for example, from the outer periphery of the package. That is, it is preferable that the connection body disposed near the center of the package is welded to and fixed to both electrode pads. This is because if the package and the printed board are aligned at the center position, the shift between the package and the printed board can be evenly distributed to the outer peripheral portion, and the amount of the shift can be reduced.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a schematic view of a cross section of a package assembly according to a first embodiment of the present invention. Note that the same parts as those in the related art are denoted by the same reference numerals.

Referring to FIG. 1, a package assembly according to a first embodiment of the present invention includes a package (BGA package) 3 having a plurality of outer lead pads 9;
Printed circuit board 1 having a plurality of package pads 15
3 and a plurality of connectors (conductive balls) 5 and 5a for electrically connecting the outer lead pads 9 and the corresponding package pads 15. The conductive balls 5 are connected to the outer lead pads 9. Solder 11 and solder 17 are welded to both package pads 15, respectively, and conductive balls 5a are welded only to outer lead pads 9 by solder 11.

Specifically, first, the semiconductor chip 1 is mounted on the package 3 by a wire bonding method. The semiconductor chip 1 is connected to inner lead pads (not shown) on the package 3 via bonding wires 7. The inner lead pads are connected to outer lead pads 9 on the back surface of the package 3. The size of the outer lead pad 9 is 7
The pitch of the outer lead pads 9 is about 1 mm to 1.27 mm. BGA is attached to such outer lead pad 9
The conductive balls 5 constituting the structure are welded by solder 11.

On the other hand, package pads 15 are also formed on the printed circuit board 13 on which the package 3 is mounted. The conductive balls 5 other than the predetermined portion of the package 3 (here, the outer peripheral portion of the package 3) are welded to the corresponding package pads 15 of the printed circuit board 13 by the solder 17. The size and pitch of the package pad 15 are the same as those of the outer lead pad 9. On the other hand, the conductive balls 5 a on the outer peripheral portion of the package 3 are in electrical contact with the package pads 15 by pressure bonding, but are not welded by the solder 17. That is, since the conductive balls 5 a are not fixed to the package pads 15 by the solder 17, the conductive balls 5 a can move in the horizontal direction while ensuring electrical connection with the package pads 15.

The solders 11 and 17 have a melting point of 100
Low melting point solder or eutectic solder having a temperature of about 200C to about 200C, preferably about 180C may be used. For example, tin (Sn) 6
3% -lead (Pb) 37% eutectic solder, indium (I
n) alloy solder, tin (Sn) -silver (Ag) solder, or the like.

On the other hand, the conductive ball 5 has a melting point of 25%.
A high melting point solder of about 0 ° C. to 360 ° C., preferably about 275 ° C. may be used. For example, a high melting point solder of 10% tin (Sn) -90% lead (Pb) is used. In FIG. 1, conductive balls are used as the connection bodies 5 and 5a, but conductive bumps and conductive pillars (columnar conductive connection bodies) may be used instead. For example, as the conductive bump, a gold (Au) paste, a silver (Ag) paste, or a protrusion such as the above-mentioned high melting point solder may be used.

In FIG. 1, the electrical connection between the semiconductor chip 1 and the package 3 has been described by using the wire bonding method. However, TAB and flip-flops may be used.

In the semiconductor device shown in FIG.
Since the conductive balls 5a disposed on the outer peripheral portion of the package 3 are movable, the mechanical stress due to the difference in the coefficient of thermal expansion between the package 3 and the printed circuit board 13 is fixed to all the conductive balls 5 as in the related art. It will be kept small compared with the case where it was done.

For example, consider a case where the center position of the package 3 and the center position of the package 3 mounting area of the printed circuit board 13 are matched. In this case, even if there is a difference in the amount of thermal expansion between the package 3 and the printed circuit board 13 due to the difference in the coefficient of thermal expansion due to the temperature change, the center position does not shift. Therefore, Δα is the difference between the thermal expansion coefficients of the package 3 and the printed circuit board 13, ΔT is the temperature change, L is the distance between the centers of the outer lead pads 9 farthest apart, and ΔL is the distance between the package 3 and the printed circuit board 13. Assuming that the difference between the thermal expansion amounts is the maximum distance Δd of the positional deviation between the outer lead pads 9 and the corresponding package pads 15, is approximately expressed by the following equation.

[0023]

Δd = ΔL / 2 = Δα × ΔT × (L / 2) (1) Here, in the semiconductor device shown in FIG. 1, the conductive balls 5 a arranged on the outer peripheral portion of the package 3 Is not fixed to the package pad 15 and is movable, so that mechanical stress due to the difference in thermal expansion coefficient is not applied to the connection portion by the conductive ball 5a. The mechanical stress is applied only to the connection portion by the immovable conductive ball 5 fixed to the package pad 15. In the case of FIG. 1, L in the above equation (1) actually corresponds to the center-to-center distance between the farthest pads among the outer lead pads 9 excluding the outer peripheral portion.
Therefore, the value of L will be small. From the above equation (1), it is clear that the value of Δd decreases as the value of L decreases. That is, mechanical stress is applied to the connection portion by the non-movable conductive ball 5, but as described above, the mechanical stress itself is reduced as compared with the related art, and as a result, the mounting reliability is improved. Will be planned.

In the mounting method shown in FIG. 1, for example, first, the conductive balls 5 and 5a are welded to the outer lead pads 9 with solder 11, and then the package pads 15 and the conductive balls 5 are welded with solder 17. Is realized. Normal,
The first welding of the outer lead pad 9 and the conductive balls 5 and 5a is a temporary fixing, and the final fixing is performed at an accurate position when the package pad 15 and the conductive ball 5 are welded later. At the time of this final fixing, alignment is performed in a self-aligned manner at the connection portion where the conductive ball 5 and the package pad 15 are welded by the solder 17. Therefore, the package 3 can be easily and accurately mounted on the printed circuit board 13.

In the semiconductor device shown in FIG. 1, it is desirable that the solder 11 has a higher melting point than the solder 17. If the melting point of the solder 11 is lower than or similar to that of the solder 17, the solder 11 will be melted again when the conductive balls 5 and the package pads 15 are welded. This is because if the solder 11 melts in the conductive ball 5a that is not welded by the package pad 15 and the solder 17, the solder 11 may run around to the package pad 15 side.

In FIG. 1, the conductive balls 5 arranged near the center of the package 3 except for the outer peripheral portion are welded to the package pads 15. This is package 3
By aligning the center position of the package 3 with the center position of the mounting area of the printed board 13, the shift between the package 3 and the printed board 13 is evenly distributed to the outer peripheral portion, and the shift amount is reduced. For example, the package pads 15 may be arranged as shown in FIG. Here, FIG. 2 is a plan view of FIG. 1, in which 19 indicates a package pad 15 corresponding to the conductive ball 5a, and 21 indicates a package pad 15 corresponding to the conductive ball 5. Also,
It is also possible to increase the number of conductive balls 5a further from the center than in FIG. Basically, as the number of the conductive balls 5a increases, the mechanical stress is reduced as described above. However, since the package 3 is fixed to the printed circuit board 13 by the conductive balls 5 welded to both the outer lead pads 9 and the package pads 15, the number of the conductive balls 5a cannot be increased without limitation. On the other hand, in FIG. 2, the package pads 15 corresponding to the conductive balls 5 are arranged in a circular shape, but the present invention is not limited to this.
As long as the center position of the mounting area 3 matches, any arrangement may be used as long as it is securely fixed.

In FIG. 1, the conductive ball 5a is in point contact with the electrode pad 15, and its electrical resistance is a concern. However, during mounting, some pressure is applied to the conductive ball 5a. In addition to the conductive balls 5a, the conductive balls 5a are slightly crushed, that is, are pressed, so that a sufficient contact area is ensured. Therefore, the electrical resistance is actually at a level that does not matter.

Second Embodiment This embodiment is used for the purpose of reinforcing the first embodiment. That is, the electrical connection between the conductive ball 5a and the package pad 15 shown in FIG. 1 is further ensured. FIG. 3 is a schematic diagram of a cross section of a package assembly according to a second embodiment of the present invention. The same parts as those in the related art are denoted by the same reference numerals. As shown in FIG. 3, in the present embodiment, instead of conductive ball 5 a in FIG. 1, conductive ball 5 in which at least the surface in contact with electrode pad 15 is covered with gold (Au) 23
b. In the case of FIG. 1 as well, the electrical connection between the conductive balls 5a and the package pads 15 is sufficiently ensured at the beginning, but if the surface of the conductive balls 5a is oxidized, the contact resistance will be long-term. May be increased. Therefore, by covering at least the surface in contact with the package pad 15 with gold, oxidation can be prevented, and an increase in contact resistance can be prevented. Alternatively, a conductive ball whose entire surface is covered with gold, that is, a gold-plated conductive ball may be used. Of course, the conductive ball itself may be gold.

Third Embodiment This embodiment is also used for the purpose of reinforcing the first embodiment, similarly to the second embodiment. That is, the contact area between the conductive ball 5a and the package pad 15 shown in FIG. 1 can be ensured more reliably. FIG. 4 shows a third embodiment of the present invention.
It is a schematic diagram of a section of a package assembly concerning an embodiment. The same parts as those in the related art are denoted by the same reference numerals. In the first embodiment shown in FIG.
The package pads 15 corresponding to a were all of the same size. However, if the displacement between the package 3 and the printed circuit board 13 due to thermal expansion becomes extremely large, specifically, if the displacement of the conductive balls 5a exceeds the size of the corresponding package pad 15, the conductive The contact area between the conductive ball 5a and the package pad 15 is reduced, resulting in an increase in electric resistance. Therefore, in the present embodiment, a configuration is used in which the package pad 15a having a larger size is used instead of the package pad 15 corresponding to the conductive ball 5a in FIG. For example, as described in the section of the problem to be solved by the invention, 35 mm
In the case of the BGA package using FR4 of □, the shift is about 200 μm. Package pad 15
Is about 700 μm to 900 μm, so it is sufficient to further increase the size by 200 μm. The package pad 15a may be larger than the package pad 15 only in the direction in which the conductive balls 5a are shifted.

Fourth Embodiment This embodiment is used for the purpose of reinforcing the first embodiment, as in the second embodiment. That is, even if the conductive ball 5a shown in FIG. 1 moves, it does not come into contact with the adjacent package pad 15. FIG. 5 is a schematic view of a cross section of a package assembly according to a fourth embodiment of the present invention. The same parts as those in the related art are denoted by the same reference numerals.
As shown in FIG. 5, in the present embodiment, instead of package pads 15 corresponding to conductive balls 5a in FIG. 1, package pads 15b having a larger pitch are used.
Is used. For example, as described in the section of the problem to be solved by the invention, a 35 mm square FR
Approximately 200μ for BGA package using
m. Since the pitch of the package pads 15 is about 1 mm to 1.27 mm,
It is only necessary to increase the size by μm. Further, the pitch of the package pads 15b may be increased only in the direction of displacement of the conductive balls 5a.

FIG. 6 is a plan view of the package assembly according to the third embodiment of FIG. 4 and the fourth embodiment of FIG. 5, and reference numeral 23 denotes a package pad 15c corresponding to the conductive ball 5a. 25 indicates a package pad 15 corresponding to the conductive ball 5. In addition,
The package pad 15c is larger in size and pitch than the package pad 15.

In the first to fourth embodiments, the pad for the package on the printed circuit board is provided with a pad which is not welded to the conductive ball. It is also possible to provide a material that is not welded to the conductive balls, and to weld all the conductive balls to the package pad.

[0033]

As described above, according to the present invention,
Mechanical stress caused by a difference in thermal expansion between the package and the printed circuit board can be reduced. This makes it possible to improve the mounting reliability of the package and the printed circuit board.

[Brief description of the drawings]

FIG. 1 is a schematic view of a cross section of a package assembly according to a first embodiment of the present invention.

FIG. 2 is a plan view of FIG.

FIG. 3 is a schematic view of a cross section of a package assembly according to a second embodiment of the present invention.

FIG. 4 is a schematic view of a cross section of a package assembly according to a third embodiment of the present invention.

FIG. 5 is a schematic view of a cross section of a package assembly according to a fourth embodiment of the present invention.

FIG. 6 is a plan view of a package assembly according to a third embodiment of FIG. 4 and a fourth embodiment of FIG. 5;

FIG. 7 is a diagram showing a state of a connection portion when a conventional semiconductor element package (BGA package) is mounted on a printed circuit board.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Semiconductor chip 3 Package 5, 5a, 5b Conductive ball 7 Bonding wire 9 Outer lead pad 11, 17 Solder 15, 19, 21, 23, 25 Package pad

Claims (2)

[Claims] 1. A package having a plurality of outer lead pads, a printed board having a plurality of package pads, and a plurality of connectors electrically connecting the outer lead pads and the corresponding package pads. A package assembly comprising: a part of the connection body is welded to both the outer lead pad and the package pad by soldering; and the remaining connection body is the outer lead pad or the package. A package assembly, wherein only one of the pads is welded by solder. 2. The package according to claim 1, wherein the remaining connection body, which is welded to only one of the outer lead pad and the package pad by soldering, is arranged on an outer peripheral portion of the package. The package assembly according to claim 1.