JP3178401B2 - Method of forming and connecting BGA type electrode of package - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bonding between an electronic component package such as an LSI and a printed wiring board, and more particularly to a package or a final product having a BGA type electrode suitable for connection by a BGA (Ball Grid Array) method. And a connection method for obtaining an assembly of a package and a printed wiring board connected by a BGA type.

[0002]

2. Description of the Related Art Conventionally, a package is usually connected to a printed wiring board by inserting a package having pin electrodes formed therein into the printed wiring board. This method is called a PGA (Pin Grid Array) connection method. But,
In the PGA connection method, the pin diameter cannot be reduced too much to maintain the pin strength. Furthermore, the diameter of the pad to which the pin is connected must be increased to some extent in accordance with the diameter of the pin. For this reason, the pin pitch is increased, the mounting density on the printed wiring board is reduced, and it is not possible to cope with the increase in the number of I / O pins accompanying the high performance of the LSI. for that reason,
In recent years, a BGA connection method for connecting between a package and a printed board using minute solder balls instead of pins has attracted attention. In the BGA connection method, the number of connection terminals is significantly improved as compared with the PGA connection method, and high-density mounting on a printed circuit board is possible.

[0003] The BGA type connection is generally performed as follows. Solder balls are placed on the electrode pads on the package surface to melt the solder balls. The solder and the electrode pad are joined by the melting of the solder ball to form a so-called bowl shape. Further, a printed wiring board having electrode pads formed on the surface layer is prepared, and the electrode pads are aligned with the bowl-shaped electrodes of the package. The package and the printed wiring board are connected by remelting the solder balls in that state.

In this method, when the solder balls are re-melted, the distance between the package and the substrate is shortened by the weight of the package. Further, during the melting, if there is unevenness in the heat, the package and the substrate are not connected in parallel. Was causing the failure. For this reason, various improved methods applicable to the BGA type connection have been considered.

Japanese Patent Application Laid-Open No. Hei 5-218134 discloses, as one mode of the above-mentioned improvement method, when the IC chip and the substrate are flip-chip connected, the distance between the two is set to a predetermined distance. It is disclosed that a regulating member for securing is used as a spacer. Japanese Patent Application Laid-Open No. 5-67647 discloses a method in which a plurality of bumps called studs are provided on the outer periphery of solder bumps of an IC chip, the IC chip is connected to a substrate, and then the stud bump portion is removed. Japanese Patent Application Laid-Open No. 8-316628 discloses that high-melting-point solder balls are arranged in at least four places of a large number of pad recesses, reflow-processed, and low-melting-point solder balls are arranged in the remaining pad recesses. A method of processing is disclosed.

[0006]

However, all of the above-mentioned methods have various problems, and in particular, have a difficulty in productivity due to an extremely complicated manufacturing process. Specifically, in the method disclosed in JP-A-5-218134 and JP-A-5-67647, at the time of BGA type connection, another member other than the solder ball for BGA type connection is inserted. Therefore, the number of man-hours and costs are increased. Further, it is necessary to separately secure an area for inserting such a member.
There is a disadvantage that the A-type connection cannot be mounted at high density.

On the other hand, in the method described in Japanese Patent Application Laid-Open No. 8-316628, it is not necessary to secure an area for the members because the above members are not used. However, at least four high-melting-point solder balls are arranged on the package, and a reflow process is performed at a temperature at which the high-melting-point solder balls are melted to secure the connection of the high-melting-point solder balls onto the package. A low-melting-point solder ball is arranged in a portion other than the high-melting-point solder ball, and reflow processing is performed to secure the connection of the low-melting-point solder ball on the package. From the above steps, a package having a BGA type electrode is obtained. Thereafter, cream solder is printed on the electrode pads on the printed wiring board. Align the BGA electrodes on the package with the cream solder on the printed wiring board,
Perform reflow processing.

In this method, since the reflow process is performed three times for convenience, man-hours and costs are increased, and at the same time, the package itself is heated to a high temperature at which the high melting point solder balls are melted. is there. In particular, in recent years, materials used for packages have been shifting from ceramic materials having high-temperature durability to organic materials having poor high-temperature durability. Depending on the materials of the package, heating to high temperatures as described above is not desirable. .

Further, in this method, since the high melting point solder ball on the package is melted, the high melting point solder ball cannot maintain a spherical shape. Therefore, a BGA type electrode composed of a high melting point solder ball can secure a certain distance between the package and the printed wiring board as a spacer, but a BGA electrode composed of a plurality of high melting point solder balls.
It is difficult to form the GA electrodes so that the heights are all the same. This means that the package and the printed wiring board cannot always be connected in parallel.

An object of the present invention is to provide a method of forming a BGA-type electrode capable of realizing a high-density BGA connection portion of a package by a simple method without increasing man-hours and cost, and connecting a package and a printed wiring board in parallel. It is an object of the present invention to provide a method of connecting a package and a printed wiring board, and a package having a BGA type electrode manufactured by these methods and an assembly having a connection portion.

[0011]

The inventor of the present invention prints cream solder on an electrode pad provided on a package,
By placing two types of solder balls having different melting points on the cream solder and performing reflow treatment on at least the cream solder, it was found that a BGA type electrode capable of realizing a high-density electrode can be formed without adding man-hours and cost. The present invention has been reached.

Here, the present invention relates to a BGA on a package.
In the method of forming a mold electrode, cream solder is printed on an electrode pad provided on the package, and at least one solder paste is printed.
A high melting point ball was placed on the cream solder printed on the electrode pads of the above, and a low melting point solder ball having a lower melting point than the high melting point balls was placed on the cream solder printed on the remaining electrode pads, and then at least obtained. A method of forming a BGA-type electrode on a package, characterized by performing a solder reflow process on the cream solder on the package. At this time, after both balls are placed on the cream solder, the positions may be fixed by applying pressure, and then reflow processing may be performed.

Further, by implementing the above-mentioned method of forming a BGA type electrode, a package in which at least one BGA type electrode is made of a substantially spherical metal can be obtained. further,
By performing a reflow process on at least the cream solder and the low-melting-point solder ball, a package is obtained in which BGA-type electrodes other than spherical BGA-type electrodes are substantially bowl-shaped solder.

On the other hand, the present inventors use the package having the BGA type electrode and connect the package and the printed wiring board in the same manner, so that the package and the printed wiring board keep a constant distance and are parallel. I found that I can connect to.

According to another aspect of the present invention, there is provided a method for connecting a package and a printed wiring board, the method comprising:
Prepare a package obtained by the method of forming an A-type electrode, print cream solder on an electrode pad provided on a printed wiring board, and align the BGA type electrode of the package with the cream solder on the printed wiring board. Then, a solder reflow process is performed on at least the cream solder of the obtained assembly on the printed circuit board, thereby connecting the package and the printed wiring board.

Further, by manufacturing an assembly of a package and a printed wiring board by the above connection method, an assembly in which a predetermined connection portion is made of a substantially spherical metal can be obtained. Further, by performing a reflow process on at least the cream solder and the low melting point solder ball, an assembly in which another predetermined connection portion is substantially made of a drum-shaped solder is obtained.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention provides a method of forming a BGA type electrode capable of realizing a high density of BGA connection portions in a package by a simple method without increasing man-hour and cost. Further, from another aspect, the present invention provides a method for connecting a package and a printed wiring board, which enables parallel connection between the package and the printed wiring board. Further, the present invention is a package having a BGA type electrode manufactured by the above method, and an assembly having a connection portion manufactured by the method.

In the method according to the present invention, the cream solder plays a role of connecting the high melting point ball and the electrode pad. Since the high melting point ball and the electrode pad are connected by the cream solder, the high-temperature reflow process step required by the method described in Japanese Patent Application Laid-Open No. 8-316628 can be omitted.

A cream solder (also called a paste solder) is a mixture having a viscosity suitable for application or printing, which is generally prepared by kneading a viscous liquid flux and a granular solder. It is preferable that the flux be easily evaporated by a reflow treatment performed later.
The melting point of the solder is, of course, lower than the melting point of the metal constituting the high melting point ball.

The cream solder is printed on the electrode pads provided on the package. Here, the formation of the electrode pad
The conductor paste that is to be fired to become an electrode pad is printed on a green sheet, and the green sheet is fired at the same time as the conductor paste is fired to form an electrode pad. Printing and baking again to form an electrode pad. Moreover,
It may be an electrode pad formed on an organic package.

A high melting point ball is placed on the cream solder printed on at least one electrode pad, and a low melting point solder ball is placed on the remaining cream solder. The reason why the two kinds of solder balls having different melting points are used is that when performing the reflow treatment performed in the subsequent step, the high melting point ball is not melted, its spherical shape is maintained, and the ball functions as a spacer. High melting point balls may be placed on the cream solder printed on all the electrode pads. Preferably, the high melting point balls are placed on the cream solder printed on at least four electrode pads. More preferably, the high melting point balls are arranged at the four corners of the package or in the vicinity thereof.

The low melting point solder ball is preferably made of the same material as the granular solder used for the cream solder. If the two materials are different, an intermetallic compound is formed during reflow and becomes brittle, which may cause poor connection at the interface between the two materials. However, for example, the formation of intermetallic compounds is substantially achieved by using a solder alloy having the same metal type as the alloy constituting the granular solder but having a different composition (atomic ratio) for the low melting point solder balls. It is also possible to use different alloys while avoiding.

The high melting point ball preferably has a solidus temperature that is at least 40 ° C. higher than the liquidus temperature of the low melting point solder ball and the cream solder granular solder so as not to melt during reflow. If the high-melting point ball is partially melted at the time of reflow, it may be difficult to keep the distance between the package and the printed wiring board constant and connect them in parallel in the finally obtained assembly. .
The high melting point ball may be made of a solder alloy having a higher melting point than the low melting point solder ball, or may be made of a high melting point metal (including an alloy) that does not function as a solder (eg, copper).

The solder balls placed on the cream solder are:
Since the cream solder has viscosity, its position is rarely changed. However, if various processes are performed before reflow, the solder balls may change their positions during the process and may be displaced from the cream solder. To prevent this, after placing the solder ball on the cream solder,
It is preferable to apply pressure to the solder ball before reflow to fix the position. In this case, the solder ball slightly sinks in the cream solder and the position thereof is less likely to change, thereby improving the yield.

In the reflow treatment step, at least the cream solder is subjected to a solder reflow treatment. That is, only the cream solder or both the cream solder and the low melting point solder ball are reflowed. By performing such reflow treatment, at least the high melting point ball maintains its shape and plays a role as a spacer.

When only the cream solder is reflowed, all the BGA electrodes become BGA electrodes made of substantially spherical metal. In other words, both the low-melting-point solder ball and the high-melting-point ball maintain their spherical shape, and since these spherical metals are usually buried in the melted cream solder so as to be shallow, all the BGA electrodes are connected to the joint. It has a shape with a neck (neck). Since the joining area is very small in this shape, the mechanical strength of the joining portion may not be sufficiently maintained in some cases. Even without the use of low melting point solder balls, all BGA electrodes consist of substantially spherical metal. In such a case, the low melting point solder ball and the high melting point ball are balls having substantially the same diameter.

In order to increase the mechanical strength of the joint, it is preferable to reflow solder balls having a low melting point in addition to the cream solder. If the granular solder of the cream solder and the low-melting-point solder ball are made of the same material, the low-melting-point solder ball is also reflowed together with the cream solder.

When the low-melting-point solder balls are also melted together by reflow, the BGA type electrode has a substantially bowl-like shape without constriction (a shape obtained by cutting a part of a sphere in a plane, ie, a truncated sphere). Will have. For this reason, the bonding area increases, and the mechanical strength of the bonding portion is significantly improved. On the other hand, the high melting point ball keeps a substantially spherical shape and plays a role of a spacer. By mixing the bowl-shaped and spherical electrodes, a BGA-type electrode having sufficient mechanical strength and also serving as a spacer can be obtained. In order to ensure mechanical strength, the number of high melting point balls should be as small as possible to ensure the function of the spacer (eg, one at each corner), and most of the balls should be low melting point solder balls. preferable. Also in this case, it is preferable that the diameters of the low-melting point solder ball and the high-melting point ball are substantially the same. However, since the low-melting point solder ball is subjected to a reflow treatment, some difference in diameter may be acceptable.

The temperature at which the reflow treatment is performed is preferably a temperature about 20 ° C. higher than the melting point (liquidus temperature) of the low melting point solder ball. In order to prevent the melting point of the high-melting point ball, it is preferable that this temperature is lower than the melting point of the high-melting point ball (solidus temperature) by 20 ° C. or more.

In the package having the BGA electrodes manufactured as described above, at least one of the BGA electrodes is made of substantially spherical solder, and the remaining BGA electrodes are made of substantially bowl-shaped solder. It has features in terms of shape. Here, the fact that the BGA type electrode is made of a substantially spherical solder means that the spherical solder ball maintains its shape, is connected to the electrode pad by the solder component of the cream solder, and has a constriction in the BGA type electrode. Means that. Further, the fact that the BGA type electrode is substantially made of a bowl-shaped solder means that not only the cream solder but also the low-melting-point solder ball is reflow-treated and is typically formed into a hemispherical shape or a shape close to it by the surface tension. Means that.

In order to connect a package having such BGA type electrodes to a printed circuit board, cream solder is printed on electrode pads provided on the printed circuit board, and the positions of the BGA type electrodes on the package and the cream solder on the printed circuit board are determined. It is sufficient to perform the alignment and at least perform the reflow treatment of the cream solder.

In the assembly of the package and the printed wiring board manufactured as described above, since the high melting point ball functions as a spacer, a predetermined distance is maintained between the package and the board, and the package is connected in parallel with the board. . This assembly is characterized in that at least one of the connections connected by the BGA method comprises a substantially spherical solder, and the remaining connections comprise a substantially drum-shaped solder. . Such a connection portion also has a large bonding area for the same reason as described above, so that the mechanical strength is improved.

Here, the fact that the connection portion is made of a substantially spherical solder means that the spherical solder ball maintains its shape similarly to the above-mentioned BGA type electrode, and the package and the printed wiring board are formed by the solder component of the cream solder. Means that there are two constrictions at the joint (one each at the package side and the substrate side). The fact that the joint is made of a substantially drum-shaped solder ball means that both ends of the joint are larger in diameter than the center and have a constricted shape at the center. However, the constriction at the center does not necessarily have to be plane-symmetric about the center, and the constriction may be deviated to either the package side or the printed wiring board side.

[0034]

EXAMPLE In this example, an alumina ceramic package was used for the package, and an epoxy resin substrate was used for the printed wiring board. 225 pcs (0.9 mm diameter, 1.5 mm pitch) on each joint surface of the package and printed circuit board (15
× 15) were provided, and the electrode pads of the package were made of tungsten, and the electrode pads of the printed wiring board were made of copper. These electrode pads were plated with Ni (2.5 μm) / Au (0.1 μm) in order to improve the wettability of the solder and prevent the solder from diffusing into the electrode pads.

An embodiment will be described with reference to FIGS. As shown in FIGS. 1A to 1D, BGA electrodes were formed on the electrode pads 12 of the package 10 (installed on the vias 13). First, a cream solder 16 in which Pb-37wt% Sn (melting point: 183 ° C.) granular solder was mixed in a flux was selectively printed on the electrode pad 12 of the package using a screen printing method [FIG. 1 (b)]. ]. Next, a high-melting ball 18 and a low-melting solder ball 20 each having a ball diameter of 0.9 mm were arranged on the printed cream solder 16 [FIG. 1 (c)]. Used high melting point ball 18, low melting point solder ball
The 20 materials are as shown in Table 1. The high melting point ball 18
The electrodes were arranged at the four corners of 15 × 15 electrode pads (that is, a total of four) (the high-melting point ball 18 is shown at the center in FIG. 1 for explanation). In Example 2 of Table 1, after each ball was placed on the cream solder 16, pressure was applied to all the balls from above to fix the positions.

After the balls were placed, they were placed in a reflow furnace maintained at about 230 ° C. for one minute to fuse and connect the balls to a package, thereby forming BGA electrodes 22 and 24 [FIG. 1 (d)]. On the other hand, as a comparative example, a package in which only a low-melting-point solder ball was placed on all the cream solder without using the high-melting-point ball 18 and subjected to reflow treatment to form a BGA-type electrode was also manufactured.

By reflowing at about 230 ° C., the cream solder 16 and the low melting point solder ball 20 melted, but the high melting point ball 18 did not melt. Observing the BGA electrodes 22, 24 with an optical microscope for this point,
As schematically shown in FIG. 1 (d), the electrode 24 using the high melting point ball 24 was formed in a substantially spherical shape, and the electrode 22 using the low melting point solder ball 20 was formed in a bowl shape. Therefore, in the comparative example, all the electrodes had bowl-shaped shapes.

Next, as shown in FIG. 2, the package having the BGA type electrodes formed as described above was connected to the printed wiring board 30. The electrode pads of the printed wiring board 30 are formed in the same manner as the BGA type electrodes of the package are manufactured.
Screen printing of cream solder 34 was selectively performed on 32 [FIG. 2 (a)]. The cream solder 34 used was the same as that printed on the electrode pads of the package.
With the surface on which the electrode pads 32 of the printed printed wiring board 30 are arranged facing up, and the electrode surface of the above-mentioned package facing down, the BGA type electrodes 22, 24 and the electrode pad surfaces
The alignment of 32 was performed [FIG. 2 (b)]. This is again 240 ° C
And the fusion splicing was performed in the reflow furnace [Fig. 2 (c)].

Thereafter, the connection was observed again with an optical microscope. In addition, the distance between the package and the printed wiring board was measured at a plurality of positions using an optical microscope with a scale. Table 1 also shows the measurement results.

[0040]

[Table 1]

In Examples 1 to 3, as schematically shown in FIG. 2C, the electrode 24 using the high melting point ball 18 maintains a spherical shape, and a substantially spherical connection portion is formed. I was
On the other hand, in the electrode 28 using the low melting point solder ball 20, a drum-shaped connection portion was formed. As shown in Table 1, the package 24 and the printed wiring board 30 are connected in parallel (the separation distance is constant) as a result of the electrode 24 using the high melting point ball 18 functioning as a spacer. Was not melted, so that a sufficient separation distance of 1.0 mm or more was obtained.

On the other hand, in Comparative Example 1 in which the electrodes were formed on the package using only the low-melting-point solder balls 20, the electrodes were crushed during the reflow by the weight of the package itself, so that a flat spherical shape like a sphere was obtained. It became a connection part, and the separation distance was about 0.3 mm, which was about ４ of the example. In addition, the variation in the separation distance depending on the position was large, and the package 10 and the printed wiring board 30 were not connected in parallel.

[0043]

According to the present invention, a BGA-type electrode can be manufactured more easily than in the conventional method, so that man-hour and cost are not required. In addition, since the reflow temperature does not need to be raised as much as the high melting point ball is melted, a package having less damage to the package and having BGA type electrodes arranged at high density can be formed. In particular, since the packaging material is currently changing from ceramics to plastic materials that are weak to high temperatures, it is very useful to be able to form a BGA type electrode only by performing reflow processing at a relatively low temperature. In addition, when the package having the BGA-type electrode obtained here is connected to a printed wiring board, the package and the printed wiring board can be connected substantially in parallel, so that an assembly of the package and the printed wiring board with less defective products and high yield. Is obtained.

[Brief description of the drawings]

1 (a) to 1 (d) show a BGA according to an embodiment of the present invention.
It is explanatory drawing of a series of manufacturing processes of an electrode.

FIGS. 2 (a) to 2 (c) are explanatory views of a series of manufacturing steps of an assembly according to an embodiment of the present invention.

[Explanation of symbols]

 10… Package, 12,32… Electrode pad, 13… Via, 16,34… Cream solder, 18… High melting point ball, 20… Low melting point solder ball, 22… BGA type electrode made of bowl-shaped solder, 24… Spherical BGA type electrode made of solder, 26 ... connection part made of spherical solder, 28 ... connection part made of drum-shaped solder, 30 ... printed wiring board

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-11-186454 (JP, A) JP-A-8-316628 (JP, A) JP-A 8-288640 (JP, A) JP-A 8- 111581 (JP, A) JP-A-9-260438 (JP, A) JP-A-3-201546 (JP, A) JP-A-1-264245 (JP, A) JP-A 10-335533 (JP, A) JP-A-9-45807 (JP, A) JP-A-8-70062 (JP, A) JP-A-3-217024 (JP, A) JP-A-2-43748 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) H01L 21/60 H01L 23/12 H05K 3/34 H05K 3/36

Claims (7)

(57) [Claims] 1. A method for forming a BGA type electrode on a package, wherein cream solder is printed on an electrode pad provided on the package, and a high melting point ball is formed on the cream solder printed on at least one electrode pad. Placing the low melting point solder ball having a lower melting point than the high melting point ball on the cream solder printed on the remaining electrode pads, and then performing a solder reflow process on at least the cream solder on the obtained package. A method for forming a BGA type electrode of a package. 2. The method for forming a BGA type electrode of a package according to claim 1, wherein said high melting point ball and said low melting point solder ball are placed on cream solder, and then pressed to fix the position, and then reflowed. A method for forming a BGA-type electrode, comprising performing a treatment. 3. A package comprising a BGA-type electrode, wherein at least one of the BGA-type electrodes comprises a substantially spherical metal. 4. The package according to claim 3, wherein in the package having BGA type electrodes, the remaining BGA type electrodes are made of substantially bowl-shaped solder. 5. A connection method for connecting a package and a printed wiring board, wherein the BG obtained in claim 1 or 2 is provided.
A package having an A-type electrode was prepared, cream solder was printed on an electrode pad provided on a printed wiring board, the BGA type electrode of the package was aligned with the cream solder of the printed wiring board, and then the obtained solder was obtained. A method for connecting a package and a printed wiring board, wherein a solder reflow process is performed on at least the cream solder on the printed board of the assembly. 6. An assembly of a package and a printed wiring board connected by the BGA method, wherein at least one of the connection parts connected by the BGA method is made of a substantially spherical metal. Assembly. 7. An assembly of a package and a printed wiring board connected by the BGA method, wherein the remaining connection parts of the connection parts connected by the BGA method are substantially made of a drum-shaped solder. 7. The assembly of claim 6, wherein: