PCB warping correction method based on hot pressing and cold pressing coordinated regulation and control

Abstract

The invention discloses a PCB warpage correction method based on hot pressing and cold pressing coordinated regulation and control, which comprises the following steps that multiple layers of circuit boards are stacked and then placed in a hot pressing furnace, and the lamination height of the circuit boards is smaller than or equal to 90 mm; hot pressing is conducted, the hot pressing temperature is set to be 155-160 DEG C, the hot pressing time is set to be larger than or equal to 120 min, the pressure is set to be 0.5 + / -0.1 Mpa, and after the temperature of the lamination center position reaches TG + 10 DEG C, the pressing plate warping time is set to be larger than or equal to 30 min; and cold pressing, wherein the cold pressing time is set to be longer than or equal to 120 min. According to the PCB warping correction method based on hot pressing and cold pressing coordinated regulation, the temperature and time parameters of pressing plate warping are optimized, so that the circuit board in the lamination center meets the requirement of pressing plate warping.

Description

Technical Field

[0001] This invention relates to the field of circuit board manufacturing technology, specifically to a PCB warpage correction method based on the coordinated control of hot and cold pressing. Background Technology

[0002] Board warping is a bending deformation phenomenon caused by uneven stress during the processing or use of a circuit board. Poor board warping is a bending deformation phenomenon that occurs during the processing of the circuit board. When poor board warping occurs, it can lead to a series of problems such as poor contact and component detachment. The main causes of board warping include:

[0003] 1. Differences in the coefficient of thermal expansion (CTE) of materials: The substrate (such as FR-4), copper foil, resin and other materials have different expansion / contraction rates when heated or cooled, which leads to the accumulation of internal stress;

[0004] 2. Influence of TG value (glass transition temperature): When the temperature exceeds the TG value of the substrate, the material changes from a rigid state to an elastic state. If the cooling rate is uneven or the stress is not fully released, it is easy to cause permanent deformation.

[0005] 3. Improper pressing process parameters: Insufficient hot pressing temperature or time: The temperature at the center of the laminate has not reached the threshold for effective stress release (usually ≥TG+10℃ and maintained for a sufficient time);

[0006] 4. Excessive cold pressing rate: Rapid cooling exacerbates the difference in shrinkage between the inner and outer layers, leading to warping;

[0007] 5. Layer stacking design issues: Asymmetrical structures in multilayer boards (such as uneven copper layer distribution) or improper stacking methods (such as mismatched arched / concave surfaces) can amplify stress.

[0008] The circuit board lamination warping process involves stacking and laminating multiple layers of circuit boards. Testing revealed that lamination temperature and lamination time are key factors causing warping defects. When the lamination temperature and time are inappropriate, insufficient temperature at the center of the stack and inadequate lamination time result in warping defects.

[0009] Therefore, the purpose of this invention is to provide an optimized process to solve the problem of board warping defects. Summary of the Invention

[0010] The technical problem to be solved by the present invention is to provide a PCB warpage correction method based on the coordinated control of hot and cold pressing. By optimizing the temperature and time parameters of the pressure plate warpage, the circuit board at the center of the stack meets the requirements of the pressure plate warpage.

[0011] The technical solution of the present invention is as follows:

[0012] A PCB warpage correction method based on the coordinated control of hot and cold pressing includes the following steps:

[0013] Step S1: Stack the multi-layer circuit boards and place them in a hot press oven. The stacking height of the circuit boards is ≤90mm.

[0014] Step S2, hot pressing: set the hot pressing temperature to 155-160℃, the hot pressing time to ≥120min, and the pressure to 0.5±0.1Mpa. After the temperature at the center of the laminate reaches TG+10℃, the pressure plate warping time is ≥30min.

[0015] Step S3, cold pressing, set the cold pressing time to ≥120min;

[0016] Step S4: Remove the circuit board.

[0017] Furthermore, warpage correction is performed on the multi-layer circuit boards, and the number of unit boards in each layer is 50-60 when the circuit boards are stacked.

[0018] Furthermore, during the pressing process, the circuit boards are symmetrically placed at the center of the pressure cover, and the spacing between the stacked boards is ≥10mm, with each stack having the same height.

[0019] Furthermore, when stacking circuit boards, the curved surfaces of the warp-shaped circuit boards are placed facing each other.

[0020] Furthermore, the arched surfaces of the warped road slabs fit together, or the concave surfaces face each other.

[0021] Furthermore, in step S4, when the board surface temperature drops to 40-50℃, the circuit board is removed.

[0022] Compared with existing technologies, the PCB warpage correction method based on the coordinated control of hot and cold pressing provided by this invention has the following advantages:

[0023] Circuit boards (PCBs) are composed of substrates, copper foil, and epoxy resin, among other materials. Due to the different coefficients of thermal expansion of these materials, their expansion and contraction during heating or cooling are inconsistent, leading to stress accumulation and warping. This invention provides a PCB warping correction method based on the coordinated control of hot and cold pressing. The hot pressing temperature is set at 155-160℃, and the hot pressing time is 2 hours. Under these conditions, the actual hot pressing temperature at the PCB center of the stack exceeds 140℃ (TG+10℃) for 54 minutes, which is greater than the effective time for warping (≥30 minutes). The cold pressing process is set to a cold pressing time of 120 minutes, and the actual temperature at the stack center can be reduced to 90℃ after 73 minutes of cold pressing. By optimizing the temperature and time for warping, the process meets the warping requirements, significantly reducing the board warping defect rate to ≤0.5%. Detailed Implementation

[0024] To enable those skilled in the art to better understand the technical solutions in the embodiments of the present invention, and to make the above-mentioned objectives, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be further described below.

[0025] It should be noted that the descriptions of these embodiments are for the purpose of aiding understanding the present invention, but do not constitute a limitation thereof. Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0026] The principle of the circuit board pressure plate warping (hot pressing leveling) of the present invention:

[0027] PCB warping (also known as thermo-pressing or stress-relief pressing) is a technique that uses a combination of heating, pressure, and cooling to restore a warped PCB to a flat surface. Its core principle is based on the stress relaxation of thermoplastic materials and the synergistic effect of temperature and pressure, specifically encompassing the following key mechanisms:

[0028] 1. Stress relief of thermoplastic materials (core principle) The circuit board substrate (such as FR-4) is mainly composed of epoxy resin + glass fiber cloth + copper foil. The resin is a thermoplastic material, which will soften and reflow at high temperature.

[0029] When the temperature is ≥TG (glass transition temperature): the resin changes from a rigid state to a highly elastic state, the molecular chain segments can rearrange, and the internal stress (such as thermal stress and mechanical stress) can be released;

[0030] When the temperature is ≥TG+10℃: the resin fluidity increases, and it can redistribute under pressure, allowing the warped PCB to gradually return to flatness. Key factors:

[0031] The temperature must be high enough (e.g., 155°C) to ensure that the center of the laminate also reaches the effective temperature (e.g., above 140°C); the time must be long enough (e.g., 120 min) to allow the stress to be fully released.

[0032] 2. Pressure effect: forced orthodontic treatment + interlayer combination

[0033] 1) Mechanical pressure (such as the pressure of a hot press):

[0034] This process forces the warped PCB to flatten under high temperature, reducing deformation. It also promotes resin redistribution, filling the micro-voids caused by warping.

[0035] 2) Interlayer bonding (for multilayer boards):

[0036] If the PCB warping is caused by uneven lamination, hot pressing can promote secondary resin flow and improve interlayer bonding.

[0037] 3. Cold pressing process: Locking in a flat state

[0038] 1) Slow cooling (e.g., 120 min): Avoid rapid cooling which can cause new thermal stress (different materials have different CTEs, and sudden cooling will cause warping again). Allow the resin to gradually solidify during the cooling process, locking in a flat state.

[0039] 2) Cool to below 90℃: Ensure the PCB temperature is below the TG value so that the resin regains its rigidity and prevents springback deformation.

[0040] 4. The impact of layered design

[0041] 1) Stacking method (arched surfaces facing each other or concave surfaces facing each other): Reduce opposing stress and avoid superimposed warping.

[0042] 2) Stacking height (≤90mm): Ensure that heat can be evenly transferred to the central layer to avoid insufficient central temperature.

[0043] 5. Key parameters in practical applications

[0044] In summary, the core principle of pressure plate warping is:

[0045] 1. Heating to above TG → resin softens and stress is released.

[0046] 2. Apply pressure → forcefully orthopedic, promote resin flow.

[0047] 3. Slow cooling → locks in a flat state to prevent secondary warping.

[0048] 4. Optimize the stacking method → ​​Ensure uniform temperature and reduce stress resistance.

[0049] This process is suitable for PCB warpage caused by thermal stress, uneven lamination, and processing deformation, and can significantly reduce warpage (e.g., ≤0.5%). However, parameters need to be adjusted according to the substrate TG value, board thickness, etc., and the effectiveness of the process needs to be verified through temperature monitoring.

[0050] The following specific embodiments illustrate the PCB warpage correction method based on the coordinated control of hot and cold pressing of the present invention in detail.

[0051] Example 1

[0052] A PCB warpage correction method based on the coordinated control of hot and cold pressing includes the following steps:

[0053] Step S1: Stack the multi-layer circuit boards and place them in a hot press oven. The stacking height of the circuit boards is ≤90mm.

[0054] Specifically, when pressing the board to prevent warping, white paper is placed on the top and bottom surfaces of the stacked circuit boards to protect the surface and prevent damage during the pressing process. Preferably, warping correction is performed on multiple stacks of circuit boards, and the number of unit boards in each layer is 50-60pcs, which is 60pcs in this embodiment. When pressing, the circuit boards are symmetrically placed in the center of the pressure cover, and the spacing between the stacks is ≥10mm. The height of each stack is consistent. If the height is inconsistent, white paper can be placed at the bottom of the boards to increase the height and make the stack height consistent. More preferably, when stacking the circuit boards, the curved surfaces of the warped circuit boards are placed opposite each other, such as using the method of stacking the arched surfaces of the circuit boards against each other or the concave surfaces against each other, to minimize the resistance force on the boards.

[0055] In this embodiment, the TG value of the circuit board is 130°C.

[0056] Step S2, hot pressing, set the hot pressing temperature to 155℃, the hot pressing time to 120min, and the pressure to 0.5±0.1Mpa;

[0057] Step S3, cold pressing: While maintaining the pressure plate, air is sent into the oven for cooling. The cold pressing time is set to 120 minutes.

[0058] Step S4: When the board surface temperature drops to 40-50℃, remove the circuit board.

[0059] The PCB warpage correction method based on the coordinated control of hot and cold pressing in this embodiment was tested, and the method is as follows:

[0060] Cut grooves in scrap boards, bring the temperature sensing wire into contact with the board, place the board (60pcs / layer, stack ≤90mm), bury the temperature sensing wire in the center of the upper, middle and lower layers, set the hot pressing temperature to 155℃, and hot pressing and cold pressing for 120min each.

[0061] The temperature sensing wire test data is as follows:

[0062]

[0063] The data above shows that when the hot-pressing temperature is set to 155℃, the temperature at the center of the laminate reaches 150℃, and the hot-pressing time is set to 120 minutes, the actual pressure plate warping time at the center of the laminate is 54 minutes, which meets the pressure plate warping requirement. Meeting the pressure plate warping requirement means that after the actual temperature at the center of the laminate reaches (TG+10℃), the pressure plate warping time is ≥30 minutes.

[0064] During the cold pressing process, the actual temperature at the center of the laminate decreased to 90℃ after 73 minutes. When the cold pressing time was set to 120 minutes, the surface temperature and center temperature of the board could be reduced to 40-50℃.

[0065] In this invention, the temperature and time control of the pressure plate warping hot pressing simultaneously take into account the temperature of the board surface, bottom surface and middle surface. The circuit board can release thermal stress when the pressure plate warps, thereby improving the board warping problem and making the circuit board warping ≤0.5%.

[0066] Comparative Example 1

[0067] A method for warping a pressure plate is basically the same as that in Example 1, except that the hot pressing temperature is set to 150°C, the hot pressing time is 90 min, and the cold pressing time is 90 min.

[0068] Using the same testing process, the pressure plate warping method of Comparative Example 1 was tested, and the test results are as follows:

[0069]

[0070] This shows that when the hot-pressing temperature is set to 150℃, the temperature at the center of the laminate fails to reach 150℃, and the highest temperature is only 147℃. When the hot-pressing time is set to 90 minutes, the actual pressing time at the center of the laminate exceeding 140℃ is 22 minutes. At this time, the thermal stress of the circuit board cannot be completely released, failing to meet the requirements for pressure plate warping.

[0071] During the cold pressing process, the actual temperature at the center of the laminate drops to 90°C in 62 minutes. If the cold pressing time is 90 minutes, the thermal stress of the circuit board cannot be completely released, and the problem of circuit board warping cannot be solved.

[0072] The embodiments of the present invention have been described in detail above, but the present invention is not limited to the described embodiments. For those skilled in the art, various changes, modifications, substitutions, and variations made to these embodiments without departing from the principles and spirit of the present invention still fall within the protection scope of the present invention.

Claims

1. A PCB warpage correction method based on the coordinated control of hot and cold pressing, characterized in that, Includes the following steps: Step S1: Stack the multi-layer circuit boards and place them in a hot press oven. The stacking height of the circuit boards is ≤90mm. Step S2, hot pressing: set the hot pressing temperature to 155-160℃, the hot pressing time to ≥120min, and the pressure to 0.5±0.1Mpa. After the temperature at the center of the laminate reaches TG+10℃, the pressure plate warping time is ≥30min. Step S3, cold pressing, set the cold pressing time to ≥120min; Step S4: Remove the circuit board.

2. The PCB warpage correction method based on the coordinated control of hot and cold pressing according to claim 1, characterized in that, Warp correction is performed on multi-layer circuit boards, and the number of unit boards in each layer is 50-60 when the circuit boards are stacked.

3. The PCB warpage correction method based on the coordinated control of hot and cold pressing according to claim 2, characterized in that, When pressing the board, the circuit board is placed symmetrically in the center of the pressure cover, and the spacing between the stacked boards is ≥10mm, and the height of each stack is consistent.

4. The PCB warpage correction method based on the coordinated control of hot and cold pressing according to claim 3, characterized in that, When stacking circuit boards, the curved sides of the warp boards should be placed facing each other.

5. The PCB warpage correction method based on the coordinated control of hot and cold pressing according to claim 4, characterized in that, The arched surfaces of the curved road slabs fit together, or the concave surfaces face each other.

6. The PCB warpage correction method based on the coordinated control of hot and cold pressing according to claim 1, characterized in that, In step S4, when the board surface temperature drops to 40-50℃, the circuit board is removed.

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