A method for removing flux residue from a printed circuit board hole using plasma

Abstract

This invention relates to a plasma adhesive removal method for holes in printed circuit boards. While maintaining a constant vacuum rate and the flow rate of a first process gas, the method involves activating an RF power supply for a first set time. During this time, the electrode plate dissociates the first process gas into plasma, which envelops the printed circuit board. During this period, the plasma reacts with a portion of the adhesive residue inside the holes to generate byproducts. Then, the RF power supply is turned off for a second set time. During this second time, the plasma adhering to the surface of the printed circuit board is replaced by the first process gas, and the byproducts inside the holes escape and are carried away under vacuum. Afterward, the RF power supply is activated again for the first set time, and newly generated plasma enters the center of the holes for etching. This process is repeated until the set total time is reached, completing the adhesive removal step-by-step, thereby improving the etching uniformity of high aspect ratio printed circuit boards.

Description

Technical Field

[0001] This invention relates to the field of printed circuit board manufacturing, and in particular to a method for plasma adhesive removal inside holes of printed circuit boards. Background Technology

[0002] During PCB drilling, the high temperature generated by the friction between the drill bit and the substrate dielectric layer exceeds the Tg temperature (glass transition temperature) of the substrate dielectric resin. The resin around the via melts, and after cooling and solidifying, it becomes adhesive residue that adheres to the copper surface, the inner copper cross-section of the hole wall, or the copper surface at the bottom of the hole. Ultimately, this can cause an open circuit between the copper layer and the traces inside the PCB hole, rendering it unusable. Therefore, plasma etching is necessary to remove the adhesive residue remaining after drilling.

[0003] The existing plasma etching process involves placing the printed circuit board between electrode plates in a vacuum chamber, evacuating the chamber and introducing a first process gas. When the electrode plates are energized, the first process gas is dissociated into plasma. The plasma penetrates the through holes drilled into the printed circuit board and reacts with the adhesive residue inside the holes. The generated byproducts are extracted with the vacuum. After maintaining the ventilation, vacuuming, and energizing for a certain period of time, the plasma adhesive removal inside the holes of the printed circuit board can be completed.

[0004] However, while the above methods are effective for etching printed circuit boards with low aspect ratios (the ratio of PCB thickness to hole diameter), they are less effective for PCBs with large aspect ratios, such as those with an aspect ratio greater than or equal to 10:1. The adhesive residue and plasma reaction byproducts near the outer edges of the hole are easily removed, but those in the middle of the hole are difficult to remove. This results in fresh plasma not continuously reaching the middle of the hole, leading to a weaker adhesive removal effect in the middle and a lower etching efficiency in the middle compared to the edges, resulting in poor etching uniformity within the hole. For high-density interconnected PCBs, poor uniformity in plasma etching within the holes can lead to localized decreases in insulation and even defects such as micro-conductivity. Therefore, a method to improve the uniformity of plasma etching within holes is needed. Summary of the Invention

[0005] To overcome the above problems, this invention provides a plasma adhesive removal method for holes in printed circuit boards. The technical solution adopted by this invention to solve its technical problems is as follows:

[0006] A method for plasma adhesive removal inside holes of a printed circuit board includes the following steps:

[0007] Step S1: Inside the vacuum chamber, at least two parallel electrode plates are installed, and the electrode plates are connected to the radio frequency power supply.

[0008] Step S2: A printed circuit board is placed between every two electrode plates;

[0009] Step S3: Continuously evacuate the vacuum chamber to keep the vacuum pressure in the vacuum chamber always less than or equal to the set vacuum pressure.

[0010] Step S4: Continuously introduce the first process gas into the vacuum chamber, and detect and control the flow rate of the first process gas in the vacuum chamber;

[0011] Step S5: When the flow rate of the first process gas in the vacuum chamber reaches the set value, the radio frequency power supply is started to dissociate the first process gas into plasma.

[0012] Step S6: After starting the RF power supply for the first set time, turn off the RF power supply while keeping the vacuuming rate and the flow rate of the first process gas constant.

[0013] Step S7: After turning off the RF power supply for the second set time, turn the RF power supply back on to keep the vacuuming rate and the flow rate of the first process gas constant.

[0014] Step S8: Repeat steps S6 and S7 until the set total time is reached to complete the plasma adhesive removal inside the holes of the printed circuit board.

[0015] Furthermore, between steps S3 and S4, step S301 is also included: introducing the second process gas and turning on the radio frequency power supply. When the temperature inside the vacuum chamber rises to the set temperature, the introduction of the second process gas is stopped and the radio frequency power supply is turned off.

[0016] Furthermore, after step S8, step S9 is also included: turning off the radio frequency power supply and stopping the supply of the first process gas, continuously supplying the second process gas, maintaining vacuum, and stopping the supply of the second process gas after the set time is reached.

[0017] Furthermore, after completing step S9, step S10 is also included: stop vacuuming and introduce air or nitrogen into the vacuum chamber until the air pressure inside and outside the vacuum chamber is balanced, open the vacuum chamber, and take out the printed circuit board that has completed plasma de-adhesive removal from the hole.

[0018] Furthermore, the area of ​​the electrode plate is larger than the area of ​​the printed circuit board.

[0019] Furthermore, the first process gases include O2, N2, and CF4 gases.

[0020] Furthermore, the first set time range is 30-300 seconds, and the second set time is 10-120 seconds.

[0021] Furthermore, the flow rate of the first process gas ranges from 0.5 to 4 slm.

[0022] Furthermore, the vacuum chamber is equipped with a temperature detector and a gas flow detector, both of which are electrically connected to the central controller. The radio frequency power supply is also electrically connected to the central controller, which includes a timing module.

[0023] The beneficial effects of this invention are:

[0024] This method, while maintaining a constant vacuum rate and flow rate of the first process gas, starts the RF power supply for a first set time. The electrode plate dissociates the first process gas into plasma, which envelops the printed circuit board. During this time, the plasma reacts with some of the adhesive residue inside the holes to generate byproducts. Then, the RF power supply is turned off for a second set time. During this time, the plasma is extinguished, and the byproducts inside the holes of the printed circuit board escape and are carried away under the vacuum. At the same time, new first process gas is replenished. Then, the RF power supply is restarted for the first set time, and newly generated plasma enters the center of the hole for etching. The above steps are repeated until the set total time is reached to complete the plasma adhesive removal. This method, by controlling the on and off of the RF power supply, removes the byproducts in the middle of the hole in stages and gradually etches in segments, making the etching rate equal at the center and both ends of the hole of the high aspect ratio printed circuit board, thus improving the etching uniformity inside the hole. Attached Figure Description

[0025] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, wherein:

[0026] Figure 1 This is a flowchart illustrating the steps of this method. Detailed Implementation

[0027] To better understand the purpose, structure, and function of this invention, the following detailed description of specific embodiments of the invention, "A method for plasma adhesive removal in holes of printed circuit boards," is provided in conjunction with the accompanying drawings.

[0028] See Figure 1 In this embodiment, the plasma adhesive removal method for the holes of the printed circuit board includes the following steps:

[0029] Step S1: Inside the vacuum chamber, at least two parallel electrode plates are set up. The electrode plates are preferably set vertically and connected to the radio frequency power supply.

[0030] Step S2: A printed circuit board is placed between every two electrode plates. The printed circuit board is parallel to the electrode plate, and the area of ​​the electrode plate is larger than the area of ​​the printed circuit board, so as to ensure that the plasma generated between the electrode plates can completely envelop the printed circuit board.

[0031] Step S3: Continuously evacuate the vacuum chamber to keep the vacuum pressure in the vacuum chamber always less than or equal to the set vacuum pressure. The set vacuum pressure can be manually set according to different products.

[0032] Step S4: Continuously introduce the first process gas into the vacuum chamber, and detect and control the flow rate of the first process gas in the vacuum chamber;

[0033] Step S5: When the flow rate of the first process gas in the vacuum chamber reaches the set value, the radio frequency power supply is turned on. After the electrode plate is powered on, the first process gas between the electrode plates can be immediately dissociated into plasma, so that the printed circuit board is surrounded by plasma.

[0034] Step S6: After starting the RF power supply for the first set time, turn off the RF power supply while keeping the vacuuming rate and the flow rate of the first process gas constant. It should be noted that during the first set time of starting the RF power supply, the plasma reacts with the adhesive residue in the printed circuit board hole to generate byproducts, and some of the plasma is vacuumed away. At the same time, the continuously introduced first process gas will replenish and be dissociated into new plasma to replenish the plasma that has been removed. During the process of plasma removal, the byproducts on the epitaxial surface of the hole will also be removed along with the plasma. However, the byproducts at the center of the hole are blocked by the continuous replenishment of plasma at both ends and the reaction of adhesive residue in the hole to generate byproducts, and cannot be removed.

[0035] Step S7: After turning off the RF power supply for the second set time, turn the RF power supply back on, keeping the vacuuming rate and the flow rate of the first process gas constant. It should be noted that during the second set time when the RF power supply is turned off, the continuously replenished first process gas will not be dissociated into plasma by the electrode plate. Therefore, as the vacuuming and the first process gas are continuously introduced, the plasma that previously wrapped the printed circuit board will be replaced by the first process gas. There will be no plasma and adhesive residue reaction in the hole to continuously generate byproducts. The byproducts in the hole of the printed circuit board will escape and be carried away under the action of vacuuming, opening the center of the hole and facilitating the newly generated plasma to continue to enter the center of the hole to etch the adhesive residue.

[0036] Step S8: Repeat steps S6 and S7 above until the set total time is reached to complete the plasma desmearing inside the holes of the printed circuit board. The set time is set according to the different thickness-to-diameter ratios of the printed circuit board. The higher the thickness-to-diameter ratio of the printed circuit board, the longer the set total time is, in order to improve the etching uniformity inside the holes to a qualified level.

[0037] In summary, this plasma-based adhesive removal method for printed circuit board (PCB) holes primarily aims to address the issue of uneven etching within the holes of high aspect ratio PCBs under traditional plasma-based adhesive removal methods. This method, while maintaining a constant vacuum rate and the flow rate of the first process gas, modifies the RF power supply to intermittent operation. First, the RF power supply is activated for a first set time. During this period, the electrode plate dissociates the first process gas into plasma. The plasma encapsulates the PCB, and a portion of the adhesive residue within the holes reacts with the plasma, generating byproducts. Then, the RF power supply is turned off for a second set time. During this second time, the plasma adhering to the PCB surface is replaced by the first process gas, and the byproducts within the PCB holes escape and are carried away under vacuum. It should be noted that due to the previous first set time… Within a short time, the plasma reacts with a portion of the adhesive residue inside the hole, generating only a small amount of byproducts that do not clog the hole. Under vacuum, these byproducts are more easily escaped and carried away. After that, the RF power supply is restarted for the first set time, allowing the newly generated plasma to smoothly enter the center of the hole for etching. The above steps are repeated until the set total time is reached, completing the plasma adhesive removal of the high aspect ratio printed circuit board. This method controls the on / off state of the RF power supply to remove byproducts from the center of the hole in stages, gradually etching in segments to ensure that the etching rate is equal at the center and both ends of the hole in the high aspect ratio printed circuit board, thus improving the etching uniformity inside the hole.

[0038] Preferably, between steps S3 and S4, step S301 is also included: introducing a second process gas and turning on the RF power supply. When the temperature inside the vacuum chamber rises to the set temperature, the introduction of the second process gas is stopped and the RF power supply is turned off. The second process gas includes an inert gas. Introducing the second process gas can clean the chamber and remove impurities from the inside. Turning on the RF power supply can preheat the electrode plate so that the temperature inside the vacuum chamber reaches a suitable etching temperature. It can also dry and dehumidify the printed circuit board, preparing it for subsequent etching.

[0039] Furthermore, in this embodiment, after step S8, step S9 is included: turning off the RF power supply and stopping the introduction of the first process gas, continuously introducing the second process gas to maintain vacuum, and stopping the introduction of the second process gas after a set time; the second process gas includes an inert gas, and introducing the second process gas can remove the remaining active first process gas in the chamber, and can further remove by-products that may be attached to the chamber walls and the surface of the printed circuit board, playing a finishing cleaning role. And after completing step S9, step S10 is included: stopping the vacuuming, and introducing air or nitrogen into the vacuum chamber until the gas pressure inside and outside the vacuum chamber is balanced, opening the vacuum chamber, and taking out the printed circuit board that has completed the plasma de-adhesive removal in the hole.

[0040] Preferably, in this embodiment, the first process gas includes O2, N2, and CF4 gas; the first set time ranges from 30 to 300 seconds, and the second set time ranges from 10 to 120 seconds, ensuring that the plasma can react with the slag inside the pore and that the slag inside the pore has sufficient time to be discharged; the flow rate of the first process gas ranges from 0.5 to 4 slm, maintaining the vacuum pressure balance inside the chamber, continuously replenishing the plasma, and having sufficient flow to carry out the byproducts inside the pore.

[0041] Furthermore, in this embodiment, a temperature detector and a gas flow detector are installed inside the vacuum chamber. Both the temperature detector and the gas flow detector are electrically connected to the central controller, and the radio frequency power supply is also electrically connected to the central controller. The central controller includes a timing module so that the entire adhesive removal process can be completed automatically. The operator only needs to set the specific parameters through the central controller to achieve automation.

[0042] It should be noted that in existing technologies, there are also intermittent methods for removing adhesive residue from the vias. These typically involve keeping the RF power supply constantly on, stopping the vacuum process when releasing the first process gas, allowing the first process gas to convert into plasma and react with the adhesive residue in the vias for a period of time, then stopping the first process gas supply and resuming the vacuum process to remove the plasma and byproducts from the chamber. This process is then repeated until etching is complete. However, this existing method has significant problems. First, the RF power supply is continuously on. When the vacuum process stops and the first process gas is continuously introduced, the vacuum pressure inside the chamber changes considerably. This causes a change in the impedance of the load inside the chamber, resulting in a large reflected power from the RF power supply. High reflected power can trigger an excessive reflected power alarm or even burn out the RF power supply. Secondly, when the first process gas is stopped and vacuum is activated to remove the plasma and byproducts from the chamber, there is essentially no first process gas left. When the vacuum is shut off and the first process gas is introduced a second time, the time required for the first process gas is significantly extended, resulting in a long overall etching time and low efficiency. The resist removal method of this invention perfectly solves these problems of the prior art. This method maintains a stable vacuum pressure within the chamber throughout the entire cycle, and the plasma / first process gas removed by the vacuum is replenished by subsequent first process gas injections. Furthermore, the first process gas is continuously introduced throughout the process, maintaining a sufficient amount of first process gas within the chamber during etching, eliminating the need for prolonged replenishment.

[0043] It is understood that the present invention has been described through some embodiments, and those skilled in the art will recognize that various changes or equivalent substitutions can be made to these features and embodiments without departing from the spirit and scope of the invention. Furthermore, under the teachings of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the invention. Therefore, the present invention is not limited to the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application are within the protection scope of the present invention.

[0044] In the description of this invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. In the description of this application, "multiple" and "several" are understood as "at least two." "And / or" describes the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. A connected to B can represent: A and B directly connected, and A and B connected through C. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

Claims

1. A method for plasma adhesive removal inside holes of a printed circuit board, comprising the following steps: Step S1: Inside the vacuum chamber, at least two parallel electrode plates are installed, and the electrode plates are connected to the radio frequency power supply. Step S2: A printed circuit board is placed between every two electrode plates; Step S3: Continuously evacuate the vacuum chamber to keep the vacuum pressure of the vacuum chamber always less than or equal to the set vacuum pressure; Its features are, It also includes the following steps: Step S4: Continuously introduce the first process gas into the vacuum chamber, and detect and control the flow rate of the first process gas in the vacuum chamber; Step S5: When the flow rate of the first process gas in the vacuum chamber reaches the set value, the radio frequency power supply is activated to dissociate the first process gas into plasma. Step S6: After starting the radio frequency power supply for a first set time, turn off the radio frequency power supply while keeping the vacuuming rate and the flow rate of the first process gas constant. Step S7: After turning off the RF power supply for a second set time, turn the RF power supply back on to keep the vacuuming rate and the flow rate of the first process gas constant. Step S8: Repeat steps S6 and S7 above until the set total time is reached to complete the plasma adhesive removal inside the holes of the printed circuit board.

2. The method for plasma adhesive removal inside holes of a printed circuit board according to claim 1, characterized in that, Between steps S3 and S4, there is also step S301: introduce the second process gas and turn on the radio frequency power supply. When the temperature inside the vacuum chamber rises to the set temperature, stop introducing the second process gas and turn off the radio frequency power supply.

3. The method for plasma adhesive removal inside holes of a printed circuit board according to claim 1, characterized in that, After step S8, step S9 is also included: turn off the radio frequency power supply and stop the supply of the first process gas, continue to supply the second process gas, maintain vacuum, and stop supplying the second process gas after the set time is reached.

4. The method for plasma adhesive removal inside holes of a printed circuit board according to claim 3, characterized in that, After completing step S9, the process further includes step S10: stopping the vacuuming process and introducing air or nitrogen into the vacuum chamber until the air pressure inside and outside the vacuum chamber is balanced, opening the vacuum chamber, and taking out the printed circuit board that has undergone plasma debonding in the hole.

5. The method for plasma adhesive removal inside holes of a printed circuit board according to claim 1, characterized in that, The area of ​​the electrode plate is larger than the area of ​​the printed circuit board.

6. The method for plasma adhesive removal inside holes of a printed circuit board according to claim 1, characterized in that, The first process gas includes O2, N2 and CF4 gases.

7. The method for plasma adhesive removal inside holes of a printed circuit board according to claim 1, characterized in that, The first set time range is 30-300 seconds, and the second set time ranges 10-120 seconds.

8. The method for plasma adhesive removal inside holes of a printed circuit board according to claim 1, characterized in that, The flow rate of the first process gas is in the range of 0.5-4 slm.

9. A method for plasma adhesive removal inside holes of a printed circuit board according to claim 2, characterized in that, The vacuum chamber is equipped with a temperature detector and a gas flow detector, both of which are electrically connected to the central controller. The radio frequency power supply is also electrically connected to the central controller, which includes a timing module.

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