CN111629533B

CN111629533B - Warpage improving method of PCB

Info

Publication number: CN111629533B
Application number: CN202010616336.6A
Authority: CN
Inventors: 杜红兵; 刘梦茹; 傅宝林; 王小平
Original assignee: Shengyi Electronics Co Ltd
Current assignee: Shengyi Electronics Co Ltd
Priority date: 2020-06-30
Filing date: 2020-06-30
Publication date: 2021-06-18
Anticipated expiration: 2040-06-30
Also published as: CN111629533A

Abstract

The invention relates to the technical field of PCBs, and discloses a method for improving the warpage of PCBs. The direction of the part pointing to the lower copper density part is the same as the warping direction of the carrier board; or, the copper density of the intermediate layer of the designated area of the PCB is greater than the preset value; wherein, the designated area is the vertical direction of the preset welding area of the PCB surface Projection area; mount the chip on the preset welding area and then solder it. In the present invention, the copper density of the upper and lower parts of the PCB along the board thickness direction is asymmetrically designed, so that the PCB can present the same warping direction as the chip after soldering and cooling, or by increasing the copper density of the middle layer of the PCB, the PCB can be The overall shrinkage rate becomes smaller, which can reduce the warpage difference between the PCB and the chip, effectively improve the closeness of the two, and ensure the product yield.

Description

Warpage improving method of PCB

Technical Field

The invention relates to the technical field of Printed Circuit Boards (PCBs), in particular to a method for improving warping of a PCB.

Background

As chip processing capacity of information-enabled electronic products increases, the size of the integrated circuit Die (Die) increases rapidly, and the warpage of the entire chip formed by packaging also increases. As the size of the chip increases, the CTE (coefficient of thermal expansion) of the PCB also increases, eventually resulting in increased deformation and complicated orientation and morphology of the PCB and the soldered chip.

Referring to the PCB 20 and the chip 10 shown in fig. 1, the chip 10 is usually attached to the surface of the PCB 20, and then the two are connected by high temperature reflow soldering. After the soldering is cooled, the warpage directions of the PCB 20 and the chip 10 are opposite, or the warpage degrees differ too much though the warpage directions are the same, which may cause the bonding degree between the chip 10 and the BGA (Ball Grid Array) pad on the PCB 20 to decrease, which may have a large adverse effect on the yield of the assembly, and may even cause the solder joint to break and the product may be discarded.

Disclosure of Invention

The invention aims to provide a method for improving the warpage of a PCB (printed circuit board), which aims to solve the problem that the bonding degree of a chip and the PCB is easy to reduce due to the warpage problem after a high-temperature reflow soldering process in the prior art.

In order to achieve the purpose, the invention adopts the following technical scheme:

a warpage improving method of a PCB, the chip comprising a die and a carrier board for carrying the die, comprising:

manufacturing a PCB, so that the PCB is divided into two parts along the thickness direction of the PCB, the two parts have different copper densities in the same appointed area, the direction of the part with the higher copper density pointing to the part with the lower copper density is the same as the warping direction of the carrier plate relative to the PCB, and the two parts have the same thickness in the same non-appointed area; or enabling the copper density of the middle layer of the designated area of the PCB to be larger than a preset value;

the designated area is a vertical projection area of a preset welding area on the surface of the PCB;

and attaching the chip to a preset welding area on the surface of the PCB, and then welding.

Optionally, the manufacturing the PCB such that the PCB is divided into two parts along a board thickness direction, the two parts have different copper densities in a same designated area, a direction in which a part with a higher copper density points to a part with a lower copper density is the same as a warping direction of the carrier board relative to the PCB, and the two parts have equal thicknesses in a same non-designated area, includes:

performing local thick copper plating operation on at least one core plate which forms the part with the larger copper density in a specified area of a single plate surface or two opposite plate surfaces of the core plate;

and applying the core board, and laminating the boards to manufacture the PCB.

Optionally, if the local thick copper plating operation is performed on the designated area of the single board surface of the core board, the increased thickness of copper is less than or equal to the thickness of the adjacent prepreg-the thickness of the glass fiber cloth of the adjacent prepreg.

Optionally, if the local thick copper plating operation is performed on the designated areas of the two opposite plate surfaces of the core plate, the thickness of the copper thickness added on each plate surface is less than or equal to (the thickness of the adjacent prepreg-the thickness of the glass fiber cloth of the adjacent prepreg)/2.

performing local copper reduction operation on at least one core plate forming the part with low copper density in a specified area of a single plate surface or two opposite plate surfaces of the core plate;

and applying the core board, and laminating the boards to manufacture the PCB.

sequentially stacking the core boards and the prepregs which form the PCB, and additionally stacking a preset number of copper foils at the inner layer position of the part with higher copper density;

and pressing at high temperature to manufacture the PCB.

Optionally, the manufacturing the PCB so that the copper density of the intermediate layer in the designated area of the PCB is greater than a preset value includes:

aiming at a core board which forms the PCB and is positioned at the innermost layer, local thick copper plating operation is simultaneously carried out on specified areas of two opposite board surfaces of the core board;

and applying the core board, and laminating the boards to manufacture the PCB.

sequentially stacking the core boards and the prepregs which form the PCB, and stacking a middle board at the most middle position; copper layers are reserved in the designated areas of the two opposite plate surfaces of the middle plate, and the copper layers outside the designated areas of the two opposite plate surfaces are etched and removed;

and pressing at high temperature to manufacture the PCB.

sequentially stacking the core boards and the prepregs which form the PCB, and stacking a preset number of copper foils at the middle position;

and pressing at high temperature to manufacture the PCB.

Optionally, the welding conditions are as follows: peak temperature at 260 ℃, 20-30 seconds above 255 ℃, and 150 seconds above 217 ℃.

Compared with the prior art, the invention has the beneficial effects that:

according to the embodiment of the invention, the copper density of the upper and lower equal-thickness parts of the PCB along the plate thickness direction is asymmetrically designed, so that the PCB can present the same warping direction as the chip after welding and cooling, or the overall shrinkage rate of the PCB is reduced by increasing the copper density of the middle layer of the PCB, the warping difference between the PCB and the chip can be reduced, the tightness of the PCB and the chip can be effectively improved, and the product yield can be ensured.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.

Fig. 1 is a schematic structural diagram of a chip and a PCB before soldering.

Fig. 2 is a flowchart of a method for improving warpage of a PCB according to an embodiment of the present invention.

Fig. 3 is a schematic structural diagram of a PCB after local copper thickness is achieved through addition according to an embodiment of the present invention.

Fig. 4 is a schematic structural diagram of a PCB after local copper thickness is achieved by subtraction according to an embodiment of the present invention.

Fig. 5 is a flowchart of another method for improving warpage of a PCB according to an embodiment of the present invention.

Fig. 6 is a schematic structural diagram of a PCB after increasing the copper density of the intermediate layer according to an embodiment of the present invention.

Description of the figure numbers: chip 10, PCB 20, bare chip 11, carrier board 12, intermediate board 21.

Detailed Description

In order to make the objects, features and advantages of the present invention more obvious and understandable, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention, and it is obvious that the embodiments described below are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

In order to solve the problem that after the chip 10 and the PCB 20 are welded and cooled, the PCB 20 and the carrier plate 12 of the chip 10 are easy to generate different warping directions, so that the tightness of the PCB 20 and the carrier plate 12 is poor, and even a welding point is broken, the invention provides a PCB warping improvement scheme, wherein the PCB 20 can present the same warping direction as the chip 10 after being cooled by carrying out asymmetric design on the copper density of the upper part and the lower part of the PCB 20 along the plate thickness direction; or the copper density of the middle layer of the PCB 20 is increased, so that the overall shrinkage rate of the PCB 20 is reduced, the warpage difference between the PCB 20 and the chip 10 is reduced, and the tightness between the two is improved.

Example one

Referring to fig. 2, the method for improving warpage of the PCB 20 provided in the present embodiment specifically includes the steps of:

step 101, manufacturing the PCB 20, so that the PCB 20 is divided into two parts along the board thickness direction, the two parts have different copper densities in the same designated area, and the direction in which the part with the higher copper density points to the part with the lower copper density is the same as the warping direction of the carrier board 12 relative to the PCB 20, and the two parts have the same thickness in the same non-designated area.

In the present embodiment, the warping direction of the carrier 12 relative to the PCB 20 refers to a moving direction of the edge portion of the carrier 12 relative to the central portion when the carrier 12 and the PCB 20 are deformed in a relative position state during soldering. Taking fig. 3 as an example, since the peripheral portion of the carrier plate 12 moves upward relative to the central portion when the deformation occurs (as shown by the arrow in fig. 3), the warpage direction of the carrier plate 12 is referred to as an upward direction in this embodiment, and the carrier plate 12 after the deformation has a concave structure as a whole; on the contrary, if the peripheral portion of the carrier 12 moves downward relative to the central portion when the deformation occurs, the warpage direction of the carrier 12 is referred to as a downward direction in this embodiment, and the carrier 12 takes on a convex structure after the deformation.

Wherein the designated area is a vertical projection area of a preset soldering area on the surface of the PCB 20.

Typically, a BGA pad is disposed on the surface of the PCB 20 in a predetermined bonding area for bonding with the bonding surface of the carrier 12 of the chip 10.

Step 102, the chip 10 is firstly attached to a preset welding area on the surface of the PCB 20, and then welding is carried out.

Specifically, the method further comprises the following steps: the method comprises the steps of firstly carrying out silk-screen printing or dispensing on solder paste on a BGA bonding pad on the outer layer of a PCB 20, then adopting a chip mounter to mount a carrier plate 12 supporting an integrated circuit bare chip 11 on the BGA bonding pad, and then passing through a reflow oven to carry out soldering under reflow soldering conditions (peak temperature at 260 ℃, 20-30 seconds above 255 ℃, and 120-150 seconds above 217 ℃).

Since the shrinkage rate decreases as the copper density increases and the shrinkage rates of the object are different on opposite sides thereof, a warp direction from the side having a small shrinkage rate toward the side having a large shrinkage rate occurs. Therefore, the PCB 20 is divided into two equal-thickness portions along the board thickness direction at a theoretical angle, and the copper density of the two portions is differentially designed according to the warping direction of the chip 10 relative to the PCB 20, so that the PCB 20 can have the same warping direction as the chip 10 after soldering cooling. Because the warping directions of the two are the same, the welding point between the BGA pad of the PCB 20 and the chip 10 is not broken by stress, thereby ensuring that the two are always kept in high tightness.

Illustratively, the specific implementation method of step 101 includes the following three types:

first, an additive solution concept for local copper thickness includes: for at least one core board with a large copper density, performing local thick copper plating operation on a single board surface or specified areas of two opposite board surfaces of the core board, as shown in fig. 3; then, the core board stack is pressed to form the PCB 20.

Taking fig. 3 as an example, there is an upward warping direction of the carrier 12 of the chip 10, and the bottom surface of the PCB 20 away from the chip 10 is partially increased in copper thickness, so that the copper density of the bottom portion of the PCB 20 is higher than that of the top portion, and after soldering and cooling, the PCB 20 will generate an upward warping direction, which is consistent with the warping direction of the carrier 12.

Further, the operation of locally plating thick copper may include: pasting a film on the core board, exposing and developing to remove the dry film in the designated area (namely the vertical projection area of the preset welding area) on one or two surfaces, and reserving the dry films at other positions; copper plating is carried out on the core plate, so that the copper thickness of the dry film windowing area is increased; and stripping to form the local thick copper core plate.

The core plates for local thick copper plating operation are only required to belong to the part with higher copper density, the number of the core plates is not limited, and the number of the core plates can be controlled according to the warping degree required in practice.

Preferably, if local thick copper plating operation is carried out on the specified area of the single board surface of the core board, the thickness of the added copper is less than or equal to the thickness of the adjacent prepreg-the thickness of the glass fiber cloth of the adjacent prepreg; if the local thick copper plating operation is carried out on the specified areas of the two opposite plate surfaces of the core plate, the thickness of the copper thickness increased by each plate surface is less than or equal to (the thickness of the adjacent prepreg-the thickness of the glass fiber cloth of the adjacent prepreg)/2. Wherein adjacent prepreg refers to prepreg adjacent to the core board.

Second, a solution for local copper thickness subtraction comprises: for at least one core board forming a part with a small copper density, performing a local copper reduction operation on a single board surface or a designated area of two opposite board surfaces of the core board, as shown in fig. 4; the PCB 20 is manufactured by laminating the core boards.

The third, directly increase the solution thinking of copper foil, include: sequentially stacking the core boards and the prepregs which form the PCB 20, and additionally stacking a preset number of copper foils at the inner layer position of the part with higher copper density; and (4) pressing at high temperature to manufacture the PCB 20.

In fact, the first and second solutions are: in the PCB 20, the two portions having the same thickness are theoretically divided in the plate thickness direction, and the copper density of one portion is increased or decreased on the premise that the copper density of the other portion is not changed, so that the copper density of one portion is smaller or larger than that of the other portion. The third solution is to add copper foil directly to the part where the copper density needs to be increased. In practical applications, the three schemes can be applied independently or in various combination modes, and the invention is not limited.

Example two

Referring to fig. 5, the method for improving warpage of the PCB 20 provided in the present embodiment includes the following steps:

step 201, manufacturing the PCB 20, so that the copper density of the middle layer of the designated area of the PCB 20 is greater than a preset value.

The designated area is a vertical projection area of a preset soldering area on the surface of the PCB 20.

The copper density of the intermediate layer refers to the copper density of the intermediate layer of the PCB 20. The intermediate layer may be one or a plurality of successive layers in the order of lamination at the most intermediate lamination position.

Step 202, the chip 10 is firstly attached to a preset welding area on the surface of the PCB 20, and then welding is performed.

Illustratively, the specific implementation method of step 201 includes the following two methods:

first, a solution idea of adding a middle plate 21 having a special structure at a middle position includes: the core boards and prepregs constituting the PCB 20 are sequentially stacked, and the intermediate board 21 is stacked at the most intermediate position, as shown in fig. 6; copper layers are reserved in the designated areas of the two opposite plate surfaces of the middle plate 21, and the copper layers outside the designated areas of the two opposite plate surfaces are etched and removed; and (4) pressing at high temperature to manufacture the PCB 20.

The manufacturing process of the intermediate plate 21 comprises the following steps: pasting a film on the core plate, exposing and developing to reserve dry films in designated areas (namely vertical projection areas of preset welding areas) on two sides of the core plate and remove the dry films at other positions; etching to remove the exposed copper layer at other positions; the intermediate plate 21 is removed from the film to form a copper layer only in a predetermined region on both surfaces.

The second solution of stacking copper foils in the middle includes: sequentially stacking the core boards and the prepregs which form the PCB 20, and stacking a preset number of copper foils at the middle position; and (4) pressing at high temperature to manufacture the PCB 20.

In the embodiment, the copper thickness is increased at the middle position of the PCB 20 in the plate thickness direction, so that the copper density of the middle layer of the PCB 20 is increased, and the warping degree of the whole PCB 20 is reduced. Therefore, no matter whether the warping directions of the PCB 20 and the chip 10 are the same or not, the warping degree of the whole PCB 20 can be reduced to a certain degree in the embodiment, so that the warping difference between the PCB 20 and the chip 10 is reduced, the tightness between the PCB 20 and the chip 10 is increased, the risk of breakage of the welding points of the BGA pads on the chip 10 and the PCB 20 is reduced, and the product yield is improved.

The above-mentioned embodiments are only used for illustrating the technical solutions of the present invention, and not for limiting the same; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.

Claims

1. A method for improving warpage of a PCB, wherein the PCB is used for mounting a chip, and the chip comprises a bare chip and a carrier plate for carrying the bare chip, characterized in that it comprises:

Make the PCB so that the PCB is divided into two parts along the board thickness direction, the two parts have different copper densities in the same designated area, and the part with higher copper density points in the direction of the part with lower copper density and the direction of the lower copper density part. The warping direction of the carrier board relative to the PCB is the same, and the two parts are of equal thickness in the same non-designated area; or, the copper density of the intermediate layer in the designated area of the PCB is greater than a preset value;

Wherein, the designated area is the vertical projection area of the preset welding area on the PCB surface;

Mounting the chip on the preset welding area on the surface of the PCB, and then welding;

The PCB is fabricated so that the PCB is divided into two parts along the board thickness direction, the two parts have different copper densities in the same designated area, and the part with a higher copper density points in the direction of the part with a lower copper density The same as the warping direction of the carrier board relative to the PCB, and the two parts are of equal thickness in the same non-designated area, including:

For at least one core board that constitutes the lower copper density part, perform a local copper reduction operation on a single board surface of the core board or a designated area relative to two board surfaces;

Using the core board, the laminated board is pressed to form the PCB.

2. The method for improving the warpage of a PCB according to claim 1, wherein the manufacturing of the PCB so that the copper density of the intermediate layer in the designated area of the PCB is greater than a preset value, comprising:

For the core board located in the middlemost layer that constitutes the PCB, local thick copper plating operation is performed simultaneously on the designated areas of the two opposite board surfaces of the core board;

Using the core board, the laminated board is pressed to form the PCB.

3. The method for improving the warpage of the PCB according to claim 1, wherein the manufacturing of the PCB so that the copper density of the intermediate layer in the designated area of the PCB is greater than a preset value, comprising:

The core board and the prepreg that make up the PCB are stacked in sequence, and the middle board is stacked at the most middle position; the copper layer is reserved in the designated area of the opposite two board surfaces of the middle board, and the designated area of the opposite two board surfaces is outside. The copper layer is removed by etching;

The PCB is formed by high temperature pressing.

4. The method for improving the warpage of the PCB according to claim 1, wherein the manufacturing of the PCB so that the copper density of the intermediate layer in the designated area of the PCB is greater than a preset value, comprising:

Stack the core boards and prepregs that make up the PCB in sequence, and stack a preset number of copper foils at the middlemost position;

The PCB is formed by high temperature pressing.

5 . The method for improving PCB warpage according to claim 1 , wherein the soldering conditions are: peak temperature of 260° C., 20-30 seconds above 255° C., and 120-150 seconds above 217° C. 6 .