KR102364739B1 - PCB for stress reduction - Google Patents

Abstract

The present invention relates to a substrate for stress reduction, and more particularly, by minimizing the stress generated in a part connecting the chip and the substrate in a substrate on which a chip installed in a place where a lot of vibration occurs, such as in a car, is mounted, It relates to a substrate for stress reduction designed to prevent damage to the connection part.
In the present invention for achieving the above object, in a substrate for reducing stress on which a plastic ball grid array (PBGA) chip is mounted, the substrate is formed of a material having anisotropic properties, and the substrate has a vertical axis forming anisotropic properties. The modulus of elasticity is 38 ~ 42 MPa, and the elastic modulus of the horizontal axis in the plane direction is 8 ~ 12 MPa.

Description

A substrate for stress reduction {PCB for stress reduction}

The present invention relates to a substrate (PCB, Printed Circuit Board) for stress reduction, and more particularly, a chip installed in a place where a lot of vibration occurs, such as an automobile, is mounted on a board that connects the chip and the board. It relates to a substrate for reducing stress, which is designed to minimize stress to prevent damage to a connection part with a chip.

The substrate is generally manufactured by reinforcing fiberglass fibers in a polymer resin, and in particular, processes such as lamination and processing are performed to form a circuit of a multilayer substrate.

Basically, glass fiber forms a fabric structure, and it is made in the form of a composite material by impregnating the fiber glass fiber with a polymer material such as epoxy.

In general, the fabric structure has the same physical properties in the longitudinal and transverse directions, and thus, actually behaves as an isotropic material in the planar direction.

The board is the most important basic component that allows the electrical components to be mounted and the electrical circuits that connect them are built-in so that the electrical components work properly and also forms the basic structure of the module to form a single electrical component.

In accordance with the recent advent of electric vehicles, as the substrate vibrates due to shock or vibration of electric components of the vehicle, the mounted parts are exposed to an environment where damage is easy to occur.

Therefore, the importance of mechanical reliability for evaluating failure due to stress load is rising. In addition, in the case of an electric device mounted on a vehicle, the vibration does not occur at one frequency, but as a random vibration within several frequency ranges.

In this random vibration, the board on which the chip is mounted vibrates, and as a result, the electronic components mounted thereon, especially the solder connecting the electronic components, are fatally damaged as cracks occur.

In order to prevent such damage, it is urgent to find a method to minimize the stress generated by controlling the vibration of the substrate even when vibration occurs.

Korean Patent Publication No. 10-2015-0000653 Korean Patent Laid-Open Patent 10-2019-0061662

The present invention has been devised to solve the above problems, and an object of the present invention is to control the vibration behavior of the substrate by forming the material of the substrate on which the PBGA chip is mounted with a material having anisotropic properties, thereby connecting the PBGA chip and the substrate. An object of the present invention is to provide a substrate for reducing stress that can reduce the stress generated in the solder ball portion and prevent the connection portion from being damaged or dropped.

The present invention for solving these problems;

A substrate for reducing stress on which a plastic ball grid array (PBGA) chip is mounted, wherein the substrate is formed of a material having anisotropic properties.

Here, the substrate has an elastic modulus of 38 to 42 MPa in a vertical axis forming anisotropic properties, and an elastic modulus in a horizontal axis in the plane direction is 8 to 12 MPa.

In addition, the substrate is characterized in that it is formed by rotating a vertical axis forming anisotropic properties at an angle of 55 to 65° in a plane direction and a horizontal axis direction.

According to the present invention having the above configuration, the substrate on which the PBGA chip is mounted is formed of an anisotropic material to control the vibration behavior of the substrate, thereby reducing the stress generated in the solder ball portion connecting the PBGA chip and the substrate. It has the effect of preventing the part from being damaged or falling.

1 is a perspective view of a conventional substrate on which a PBGA chip is mounted.
2 is a perspective view of a solder ball for installing a PBGA chip on a substrate.
3 is a conceptual diagram illustrating an arrangement of solders provided on a lower surface for mounting a PBGA chip.
4 is a graph showing Power Spectrum Density (PSD) indicating the magnitude of random vibration applied to a substrate on which a PBGA chip is mounted.
5 is a state diagram illustrating a solder ball to which the maximum stress is applied by calculating the stress applied to the solder ball in an arbitrary vibration state with respect to one embodiment of a substrate on which a PBGA chip is mounted.
6 is a graph showing values of stress generated in a solder ball of a chip mounted on a substrate having isotropic and anisotropic properties when the aspect ratio is 1:1.
7 is a graph showing values of stress generated in a solder ball of a chip mounted on a substrate having isotropic and anisotropic properties when the aspect ratio is 1:1.2.
8 is a graph showing values of stress generated in a solder ball of a chip mounted on a substrate having isotropic and anisotropic properties when the aspect ratio is 1:1.5.
9 is a graph showing values of stress generated in a solder ball of a chip mounted on a substrate having isotropic and anisotropic properties when the aspect ratio is 1:1.8.
10 is a perspective view of a fiberglass fiber polymer composite material of a general PCB.
11 is a more simplified perspective view of a textile glass fiber.
12 is a perspective view showing a unidirectional glass fiber.
13 is a perspective view showing a method of controlling physical properties in each direction by laminating a unidirectional glass fiber filler.

Hereinafter, preferred embodiments of the present invention will be described in more detail with reference to the accompanying drawings. The same reference numerals are used for the same components in the drawings, and repeated descriptions of the same components are omitted. And, it should be understood that the present invention may be implemented in many different forms and is not limited to the described embodiments.

1 is a perspective view of a conventional substrate on which a PBGA chip is mounted, FIG. 2 is a perspective view of a solder ball for installing the PBGA chip on the substrate, and FIG. 3 is a conceptual diagram showing an arrangement of solder provided on the lower surface for mounting the PBGA chip , and FIG. 4 is a graph showing Power Spectrum Density (PSD) indicating the magnitude of random vibration applied to a substrate on which a PBGA chip is mounted, and FIG. It is a state diagram showing the solder ball to which the maximum stress is applied by calculating the stress applied to the solder ball. 7 is a graph showing values of stress generated in solder balls of a chip mounted on a substrate having isotropic and anisotropic properties when the aspect ratio is 1:1.2, and FIG. 8 is isotropic and anisotropic properties when the aspect ratio is 1:1.5 is a graph showing the value of the stress generated in the solder ball of the chip mounted on the substrate of , FIG. 10 is a perspective view of a fiberglass fiber polymer composite material of a typical PCB, FIG. 11 is a perspective view showing a fiberglass fiber more briefly, FIG. 12 is a perspective view showing a unidirectional glass fiber, and FIG. 13 is a unidirectional glass It is a perspective view showing a method of controlling the physical properties in each direction by laminating the fibers.

The present invention relates to a printed circuit board (PCB) substrate for reducing stress. First, as shown in FIG. 10, the substrate is generally represented by a polymer composite material impregnated with a polymer resin with a fabric glass fiber structure. is treated as an isotropic material because the physical properties are almost similar in the planar direction, and Fig. 11 schematically shows the woven glass fiber.

Here, FIG. 12 is a representative anisotropic material in which glass fibers are arranged in one direction, unlike the fabric, maintaining strong physical properties in the glass fiber direction and showing weak physical properties in the direction perpendicular to the glass fiber. By arranging them in a vertical position and impregnating them with a polymer resin, a polymer composite material with strong physical properties in the direction of glass fibers can be produced.

At this time, FIG. 13 shows a laminate in which unidirectional glass fibers are laminated in different directions for each layer to obtain desired properties in a desired direction.

And, as shown in FIG. 1, a PBGA (plastic ball grid array) chip is mounted on the substrate, and a solder ball as shown in FIG. 2 is used to connect the part for connecting the chip and the substrate, and the solder ball is shown in FIG. 3 As shown in the figure, it is provided in a portion for connecting the chip and the substrate to electrically firmly connect the chip and the substrate.

However, although solder balls are used to connect the PBGA chip and the board with high bonding force, the expansion and contraction caused by heat generated during operation as well as vibrations generated in the environment where the board is installed add stress to the solder balls and cause cracks in the solder balls. This causes a fatal problem in that the connection part is eventually cut.

Vibration generated in such a usage environment is transmitted to the solder ball through the board. Controlling the deformation of the board caused by the vibration on the board reduces the stress applied to the solder ball and prevents cracks in the solder ball. do.

Therefore, in the present invention, the substrate has anisotropic properties using unidirectional glass fiber and by controlling the vibration characteristics accordingly, the vibration generated in the environment is reduced, thereby reducing the stress applied to the solder ball part connecting the PBGA chip and the substrate. will give

And, in the case of forming a substrate having anisotropic properties and a general substrate having isotropic properties, when vibration is applied to the substrate, the degree of stress applied to the connection part of the chip and the substrate was checked through the finite element analysis method.

Here, in order to apply the finite element analysis method, calculations were performed using ABAZUS v.6.2, a commercially used finite element program.

First, the substrate, the PBGA chip, and the solder ball were modeled as shown in FIGS. 1 to 3, and the PBGA chip was mounted on the substrate using solder balls having a horizontal and vertical size of 17 mm × 17 mm and a diameter of 0.4 mm.

Here, the volume ratio of the glass fiber is 60%, and the volume ratio of the base material (epoxy) is 40%.

In addition, looking at the physical properties of the material forming the substrate, the Y axis, which is the vertical axis for forming the anisotropic properties, is the direction of the unidirectional glass fiber, and the horizontal axis in the plane direction is the X axis. As described in Table 1 below, the anisotropic and isotropic properties are to form

Properties heathen isotropic Ex(Mpa) 10.400 24.000 Ey(Mpa) 41.000 24.000 Ez(Mpa) 10.400 11.000 Gxy(Mpa) 4.300 4.700 Gxz(Mpa) 3.500 2.600 Gyz(Mpa) 4.300 3.600 Vxy 0.28 0.11 Vxz 0.5 0.15 Vyz 0.28 0.2

(E: modulus of elasticity, G: shear modulus, V: Poisson's ratio)

Here, the power spectrum density (PSD) indicating the magnitude of random vibrations occurring in the environment of use was applied to a vibration condition of up to 0.5g ² /Hz from a frequency of 100Hz to 1000Hz, as shown in FIG. 4 , and the four sides of the PCB were fixed. boundary conditions were given.

And, in order to examine the effect of the shape and anisotropic properties of the substrate on the stress generation of the solder using the PSD (power spectrum density), the area of the substrate is kept constant at 80.300 cm ² , and the aspect ratio is 1: 1, 1:1.2, 1:1.5, 1:1.8 were transformed.

Here, in order to additionally introduce anisotropic properties to the substrate, as shown in FIG. 1 , the Y-axis, which is the longitudinal direction of the anisotropic properties, was rotated in the X-axis direction (clockwise), which is the horizontal direction of the plane, and the stress applied to the solder ball was calculated. .

At this time, the above-described random vibration frequency of 100 to 1000 Hz is calculated through the finite element analysis method. Referring to FIG. 5 as an example among the plurality of examples described above, it is located at the edge corner of the solder ball array connecting the PBGA chip. It can be seen that the maximum stress occurs in the solder ball.

After completing the calculations according to each example in this way, the solder ball generating the maximum stress in each case was found and the values were compared.

Looking at the results, as shown in the graphs shown in FIGS. 6 to 9, the solder stress of the chip mounted on the isotropic substrate is calculated at an arbitrary vibration frequency of 100 to 1000 Hz, and the anisotropic substrate uses the Y axis as the vertical axis as the horizontal axis in the plane direction. The state of rotation at 0°, 30°, and 60° in the X-axis direction, that is, the state of converting the physical properties of unidirectional glass fiber into the above angles, shows the stress state of the solder ball where the maximum stress occurs in a graph. , and the results are summarized in a table as shown in Table 2 below.

aspect ratio Isotropic PCB Anisotropic PCB 0° 30° 60° 1:1 56.2 42.1 50.9 50.9 1:1.2 56.4 45.6 53.4 47.9 1:1.5 52.9 51.2 53.1 41.9 1:1.8 51.8 53.4 52.0 39.4

(Unit: MPa)

As calculated through the above-described finite element analysis method, the maximum stress generated in the solder ball of the anisotropic board was calculated to be lower than the maximum stress generated in the solder ball of the isotropic board. It can be seen that the maximum stress was reduced by up to %.

Therefore, it can be seen that when the substrate on which the PBGA chip is mounted is installed in an environment where vibration occurs, the stress applied to the solder ball for mounting the chip can be reduced if the material of the substrate is formed of a material having anisotropic properties.

Furthermore, when the vertical axis (Y axis) forming the anisotropic properties is rotated in the plane direction horizontal axis (X axis) direction, it can be seen that the closer to 60°, the greater the reduction in stress.

In addition, although not shown in the graph, when the vertical axis forming the anisotropic property of the substrate is rotated in the plane direction and the horizontal axis direction, when the angle is smaller than 55° or larger than 65°, a result similar to 0° is derived do.

Therefore, in cases other than when the aspect ratio of the substrate is 1:1, when the vertical axis forming the anisotropic properties is rotated at an angle of 55° to 65° in the plane direction and the horizontal axis direction, the stress applied to the solder ball is will decrease to the maximum.

In addition, although not shown separately in the drawing, the calculation was performed while changing the elastic modulus of the vertical axis and the elastic modulus of the horizontal axis in the plane direction, which are the biggest characteristics of the anisotropic properties of the substrate, respectively. When the modulus of elasticity of is larger than 12 MPa, the properties similar to the properties of the isotropic substrate are exhibited, and the effect of reducing stress is significantly reduced. In this case, the difference in anisotropy characteristics becomes too large, and the effect of reducing stress is greatly reduced.

Therefore, it is preferable that the elastic modulus of the vertical axis forming the anisotropic properties of the substrate is 38 to 42 MPa, and the elastic modulus of the horizontal axis in the plane direction is 8 to 12 MPa.

Although preferred embodiments of the present invention have been described above, the scope of the present invention is not limited thereto, and the scope of the present invention extends to those substantially equivalent to the embodiments of the present invention. Various modifications are possible by those of ordinary skill in the art to which the invention pertains without departing from the scope of the invention.

The present invention relates to a substrate for stress reduction, and more particularly, by minimizing the stress generated in a part connecting the chip and the substrate in a substrate on which a chip installed in a place where a lot of vibration occurs, such as in a car, is mounted, It relates to a substrate for stress reduction designed to prevent damage to the connection part.

Claims (3)

In the substrate for stress reduction on which a PBGA (plastic ball grid array) chip is mounted,
The substrate is formed of a material having anisotropic properties, and the elastic modulus of the vertical axis and the horizontal axis of the substrate is formed differently,
The substrate is a substrate for stress reduction, characterized in that it is formed by rotating the longitudinal axis at an angle of 55 to 65° in the horizontal direction, which is the plane direction.
The method of claim 1,
The substrate is a substrate for stress reduction, characterized in that the elastic modulus of the vertical axis forming anisotropic properties is 38 ~ 42 MPa, and the elastic modulus of the horizontal axis is 8 ~ 12 MPa.
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