JPH02143583A - Memory module - Google Patents

Abstract

PURPOSE:To obtain a highly accurate memory module which is protected against warpage when it is completed by a method wherein the cross-sectional structure of the inner circuit of a multilayer printed wiring board is made asymmetrical about a vertical direction so as to make a side where a semiconductor memory element is mounted high in density. CONSTITUTION:In a printed wiring board, a power source layer 6d formed of a thick electrolytic copper foil large in area as compared with other inner layers is arranged near the surface on which semiconductor memories 1 are mounted, whereby the printed wiring board is made assymetrical in a cross-sectional structure. In result, in a solder reflow process, solder is fused as the printed wiring board is kept warped due to the linear expansion coefficient difference between a glass epoxy laminated board 5 and the inner layers when the printed wiring board is expanded due to heat applied, so that the warpage of a memory module is offset as a whole when the module is cooled down to a room temperature even if the shrinkage of the printed wiring board on a semiconductor memory mounted side is prevented due to the solidification of the solder, and consequently a flat memory module can be offered.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a memory module in which a plurality of semiconductor memory elements are mounted only on one surface of a single printed circuit board, and particularly to a memory module having a multilayer structure. This relates to a device using a printed wiring board.

[Conventional technology]

Memory modules were developed to realize Takada mounting, a mounting method that mounts as many components as possible in a limited space on the main printed wiring board (motherboard). It is a type of memory board in which multiple semiconductor memories are mounted on a board (daughter board). Memory modules can be mounted by inserting them into sockets provided on the motherboard, or solder directly into through-holes on the motherboard. In any mounting method, smooth mounting cannot be expected if the memory module has dimensional errors such as warping or distortion, so it is required that the finish be as precise as possible.

Conventional memory modules are for example Sem1con Ne
ws triple A (issued by Mitsubishi Electric, !1h19.198
There are two-sided mounting methods, in which multiple semiconductor memory elements are mounted on both the front and back sides of a single printed wiring board, and two-sided mounting methods, in which multiple semiconductor memory elements are mounted on both the front and back surfaces of a single printed wiring board, and those with specifications as described on pages 6 and 7 (March 2008 issue). There are two types of single-sided mounting, in which memory modules are mounted only on one side. Among these, single-sided mounting memory modules have a cross-sectional structure as shown in Figure 4.
5 is a glass cloth base epoxy resin layer (hereinafter referred to as glass epoxy layer); 6a is a signal layer on the front side of the printed wiring board; 6b is a signal layer on the back side; 6C is a signal layer, 6d is a power layer, 6e is an earth layer, 6f is a signal layer, 7 is a copper-plated through hole, and 10 is a multilayer printed wiring board made up of the above.

Such a memory module is, for example, as shown in Figure 5 fa) to (e) as described in Printed Circuit Technology Handbook (Japan Printed Circuit Industry Gold Wire, Nikkan Kogyo Shimbun, published February 1986) P2S5. It is manufactured through the following steps. FIG. 5 Ta) shows a multilayer printed wiring board 11 on which a semiconductor memory is mounted, FIG. FIG. 5(d) shows a state in which a surface mount type semiconductor memory 1 is mounted, and vapor phase soldering is a solder reflow process using the device 15, and 2a indicates molten solder.

Further, FIG. 5(e) shows the completed memory module 20, and 2b is solidified solder.

[Problem to be solved by the invention]

Conventional memory modules are manufactured through the manufacturing process described above, and the printed wiring board 11 on which the semiconductor memory 1 is mounted has an almost symmetrical structure in its cross-sectional inner layer circuit, as shown in FIG. FIG. 5 (d+ solder reflow process, the solder 12 melts when the printed wiring board 11 expands due to heat, and the temperature at which the printed wiring board 11 does not sufficiently contract in the subsequent cooling process (usually 180°C or higher) When the solder solidifies, the contraction of the side on which the semiconductor memory is mounted is mechanically prevented by the wedge effect between the solidified solder 2b and the semiconductor memory 1.
When cooled to room temperature, as shown in Figure 2 (el),
There is a drawback that the memory module 20 is warped, making it difficult to mount the memory module 20 on a motherboard. These drawbacks become more and more noticeable as the capacity of the semiconductor memory 1 mounted on the memory module 20 increases, and as the area of the printed wiring board 11 tends to increase at the same time.

The present invention was made to eliminate the structural defects of such conventional memory modules, and it is possible to eliminate warping when completed without changing the conventional manufacturing process.
The purpose is to provide high-precision memory modules.

[Means to solve the problem]

In the memory module according to the present invention, the cross-sectional structure of the inner layer circuit of the multilayer printed wiring board is asymmetrical in the vertical direction so that the side on which the semiconductor memory element is mounted has a higher density.

[Effect]

In the present invention, the cross-sectional structure of the inner layer board of the multilayer printed wiring board for memory modules is made vertically asymmetrical so that the side where semiconductor memory is mounted has a higher density. A concave warp occurs in the opposite direction to the convex warp that occurred during the process, and the warp that occurs during the subsequent cooling process can be offset.
It is possible to suppress warpage and distortion at the time of completion.

〔Example〕

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

FIG. 1 is a diagram showing a cross-sectional structure of a memory module according to an embodiment of the present invention, and in particular, shows the main parts of a printed wiring board by taking a six-layer wiring board as an example. Also, the second
Figures (al to (el) are diagrams showing the manufacturing method of the memory module in Figure 1. In both figures, l is a semiconductor memory,
2 is solder, 2a is molten solder, 2b is solidified solder, 5 is glass epoxy layer, 6a is a signal layer on the front side of the printed wiring board, 6b is a signal layer on the back side, 6c is a signal layer, 6d
6e is a power layer, 6e is a ground layer, 6f is a signal layer, 7 is a copper-plated through hole, 10 is a multilayer printed wiring board consisting of the above, 12 is a solder cream, 15 is a vapor phase soldering device, 20 is a memory It is a module. In such a structure, the power supply layer 6d is a thick layer using electrolytic copper foil with a thickness of 70 μm, for example, and the other layers 1i6a, 6b, 6c,
6e and 6f are layers using electrolytic f1 foil having a thickness of 35 μm, for example.

The printed wiring board 11 according to the present invention uses thick electrolytic copper foil for the power supply layer 6d, which has a large area among the inner layers, and is arranged near the surface on the side where the semiconductor memory 1 is mounted, thereby making the printed wiring board 11 asymmetrical. It has a cross-sectional structure, and as a result, the following effects can be obtained. That is, in this embodiment, among the conventional steps shown in FIG. 5, the steps (d) and tel in particular become the steps (d) and (e) shown in FIG. 2. That is, Fig. 2 fa) ~
In the solder reflow process shown in (e), when the printed wiring board expands due to heat, the printed wiring board 11
As the solder melts while warping due to the difference in linear expansion coefficient due to the asymmetry between the glass epoxy laminate 5 and the inner layer, the printed wiring on the side where the semiconductor memory is mounted is caused by the solder solidifying in the subsequent cooling process. Even if the shrinkage of the plate is prevented, the condition shown in Figure 2 (
As shown in (ill), the warpage of the memory module as a whole is canceled out, and a flat memory module can be provided.

In the above embodiment, an example was shown in which the power source N6d of the inner layer board was thickened, but the ground Ji6e, which has a large area, may be made thicker in the inner layer board, similar to the tB layer 6d. However, in this case, the positions of the power layer 6d and the earth layer 6e need to be reversed from those in the above embodiment.

Furthermore, although the above embodiment shows an example in which a thick copper foil is used for the inner layer plate, both the power supply layer 6d and the ground N6e can be connected as shown in FIG. 3 of another embodiment of the present invention without using a particularly thick inner layer plate. They may be arranged in a cross-sectional structure as shown, and furthermore, the power layer 6d and the earth j! It is also possible to use thick copper foil on both sides of i 6 e and arrange them so as to have a cross-sectional structure as shown in FIG. 3, and in either case, the same effects as in the above embodiment can be achieved.

〔Effect of the invention〕

As described above, in the present invention, the cross-sectional structure of the inner layer circuit of the multilayer printed wiring board is asymmetrical in the vertical direction, and the upper side where the semiconductor memory element is mounted has a high density, so that concave warpage occurs during solder reflow. This can offset the dimensional change caused by the warpage of the convex shape that occurs during cooling, which has been a problem in the past. Therefore, it is possible to obtain a flat, high-precision memory module without warping, and furthermore, it is possible to improve the yield when mounting the memory module.

[Brief explanation of the drawing]

1 is a diagram showing a cross-sectional structure of a memory module according to an embodiment of the present invention, FIGS. 2(a) to (ill1) are diagrams showing the manufacturing process of the memory module of FIG. 1, and FIG. A diagram showing a cross-sectional structure of a memory module according to another embodiment, FIG. 4 is a diagram showing a cross-sectional structure of a conventional memory module, and FIGS. In the figure, 1 is a semiconductor memory, 2 is solder, 2a is molten solder, 2b is solidified solder, 5 is a glass epoxy layer, 6a is a signal layer on the front side of the printed wiring board, and 6b is a signal layer on the back side. , 6c is a signal layer, 6d is a power layer, 6e is a ground layer, 6f is a signal layer, 7 is a copper plated through hole, 10
1 is a multilayer printed wiring board, 12 is a solder cream, 15 is a vapor phase soldering device, and 20 is a memory module. Note that the same reference numerals in the figures indicate the same or equivalent parts.

Claims (1)

[Claims] (1) In a memory module in which a plurality of semiconductor memory elements are mounted only on one surface of a multilayer printed wiring board, the cross-sectional structure of the inner layer circuit of the multilayer printed wiring board has a high density on the side where the semiconductor memory elements are mounted. A memory module characterized by being asymmetrical in the vertical direction so that