Breaking Through High Thermal PCB Manufacturing Bottlenecks: An In-Depth Analysis of Vacuum Resin Filling for Embedded Copper Blocks

With the rapid development of 5G communications, 人工知能, and high-speed computing technologies, the demand for superior thermal performance in プリント基板 (プリント基板) for electronic equipment is becoming increasingly stringent. According to Prismark, the global market size for PCBs with high thermal dissipation requirements is projected to reach $4.78 10億インチ 2023, with a CAGR exceeding 9.2%. Particularly in the realm of high-frequency and high-speed PCBs, localized overheating has become a critical factor affecting device reliability.

Technical Limitations of Traditional Embedded Copper Block Processes

The current mainstream industry process for embedding copper blocks involves pre-windowing the core board and prepreg (PP) before lamination, placing the copper block during the lamination process, and relying on the flow of the PP resin to complete the embedding and fixation. While this method is widely used, it has two significant limitations:

まず最初に, the PP used in lamination must have sufficient resin content. According to the IPC-4101E standard, high-resin-content PP must have a resin content of 68% ± 5%. If the resin volume is insufficient, voiding occurs around the copper block fill area, forming noticeable gaps.

第二に, the flow of the PP must be precisely controlled. According to the IPC-TM-650 2.3.17 test method, the dynamic viscosity of PP should be controlled within the range of 800-1,500 PA・s (at 180°C). If the flow is too high, excessive resin can flow into the gap areas, causing resin starvation in nearby circuit regions, leading to poor lamination and internal cracks within the board (形 2).

These limitations make traditional methods unsuitable for HDI products manufactured using sequential build-up lamination. To address this industry challenge, the vacuum resin filling method has emerged.

Process Principle and Technical Advantages of Vacuum Resin Filling

The vacuum resin filling method adopts a completely different technical approach: 初め, precision routing is performed on the laminated board to create cavities; copper blocks are then placed and fixed; followed by resin filling under vacuum conditions; after resin curing, the final step is grinding. The complete processing flow is: Panelization → Lamination → Routing → Copper Block Placement → Resin Filling → Grinding → Subsequent Processes.

This method offers significant advantages over the traditional process:

• Applicable to complex structures like HDIボード

• More uniform and reliable filling results

• Capability for rework and repair

• Approximately 30% improvement in production efficiency

• Cost reduction of about 15-20%

Key Parameter Design and Validation for Vacuum Filling Process

Cavity Shape and Size Optimization Design

According to stringent requirements from a major customer, the resin fill size around the copper block needs to be less than 0.254mm. Considering the internal routing machine accuracy of ±0.075mm, the cavity size design must satisfy: 2a + b ≤ 0.179mm (or 2a + b ≤ 0.204mm). その結果, four cavity size schemes were designed:
① a=0.05mm, b=0.050mm
② a=0.05mm, b=0.075mm
③ a=0.05mm, b=0.100mm
④ a=0.075mm, b=0.050mm

Three cavity shape designs were also tested:

• Shape A: Standard rectangle

• Shape B: Rectangle with a protrusion added at the midpoint of each side

• Shape C: Rectangle with a protrusion added 1mm from each corner on all four sides

Using test boards with a thickness of 1.00mm and copper blocks with a thickness of 0.98mm, fixed with high-temperature resistant tape, a resin-less trial filling was conducted on a vacuum filling machine. Results showed that Shape C (rectangle with corner protrusions) provided the best anti-rotation and anti-movement performance, and was selected for subsequent validation.

Copper Block Fixing Film Material Selection

Considering the ease of copper block fixation and removal, as well as thermal stability during baking, the fixing film needs to meet requirements for high-temperature resistance and appropriate tackiness. Two materials were compared: PE film and high-temperature resistant tape.

• PE Film: Insufficient heat resistance (max 150°C), prone to deformation during baking.

• High-Temperature Resistant Tape: Withstands temperatures above 200°C, has moderate tackiness, and leaves no residue upon removal.

Experimental results clearly indicated that high-temperature resistant tape is the optimal choice for the copper block embedding fixing film.

Optimization of Height Difference Between Copper Block and Board Surface

Two height difference schemes were designed for validation:

• Scheme 1: Copper block 20μm higher than the board surface

• Scheme 2: Copper block 40μm higher than the board surface

Experimental results indicated no mismatch issues with either scheme. しかし, from the perspective of the subsequent grinding process, a 20μm height difference is more conducive to controlling the grinding amount and reducing process time.

Resin Filling Parameter Optimization

Based on the previously designed cavity size and shape, and considering commonly used internal filling screen mesh specifications, a 43T mesh was used for vacuum filling. One-pass and two-pass filling schemes were designed. The resin filling effect in the copper block area was inspected after filling:

• One-pass filling: Fill rate approx. 85-90%, with minor bubbles present.

• Two-pass filling: Fill rate reaches over 98%, with no obvious defects.

明らかに, using a 43T mesh for two-pass resin filling meets the resin volume requirements for the copper block gap area, ensuring reliable filling results.

Comparative Study on Baking Placement Methods

After resin filling, baking for curing is required. Two baking placement methods were available in-house:

• Vertical Placement: On rack carriers

• Horizontal Placement: On stacking trays

Experimental results clearly showed that vertical placement on rack carriers was non-compliant. The primary reason is that the filled resin remains flowable during baking, and under gravity, it flows downward, causing loss of resin from the gaps and resulting in void formation. The horizontal placement on stacking trays showed no abnormalities and is the recommended baking method.

Product Reliability Validation and Test Results

Based on the research conclusions from the key control points above, a batch of embedded copper block PCB products was trial-produced, そして 10 samples were randomly selected for comprehensive reliability testing. Test items included:

Reflow Soldering Test

According to IPC-6012E standard, 6 cycles of lead-free reflow soldering (peak temperature 260°C) were conducted. All samples passed without delamination, blistering, or cracking.

Thermal Stress Test

Following IPC-TM-650 2.6.8 方法, float soldering test at 288°C ± 5°C was performed for 20 秒. All samples showed no abnormalities.

Thermal Cycling Test

According to IPC-9701A standard, 1000 cycles from -55°C to 125°C were conducted. All samples maintained normal electrical performance and structural integrity.

テーブル: Summary of Reliability Test Results

Application Prospects and Commercial Value of Vacuum Filling Method

The vacuum resin filling method for embedding copper blocks not only overcomes the limitations of traditional methods but also brings significant commercial value to the PCB industry:

Translating Technical Advantages into Commercial Value

• Improved Yield: Reduces scrap caused by voiding and cracks, increasing yield by approximately 12-15%.

• コスト削減: Simplifies the process flow, reducing production costs by 15-20%.

• Expanded Applications: Enables the use of embedded copper block heat dissipation technology in HDI products, opening up new market spaces.

Broad Application Areas

This technology is particularly suitable for:

• 5G Base Station Power Amplifier PCBs

• High-Speed Server Motherboards

• Automotive Electronic Control Units (ECUs)

• High-Power LED Lighting Boards

• Industrial Power Modules

Conclusion and Outlook

This article systematically validates the feasibility and reliability of the vacuum resin filling method in embedded copper block PCB technology through experiments. The main conclusions are as follows:

1. Cavity shape using a rectangle with protrusions 1mm from each corner (Shape C) effectively prevents copper block movement and rotation.

2. Using high-temperature resistant tape as the copper block fixing film ensures fixation effectiveness and facilitates subsequent removal.

3. Both 20μm and 40μm height differences between copper block and board thickness design value are feasible, but the 20μm scheme is recommended from a process control perspective.

4. Using a 43T mesh for two-pass resin filling ensures sufficient and consistent filling.

5. Horizontal placement on stacking trays during baking prevents filling defects caused by resin flow.

Compared to the traditional lamination method for embedding copper blocks, the vacuum resin filling method offers significant advantages including higher efficiency, 低コスト, higher reworkability, and suitability for HDI boards. As the requirements for thermal dissipation in electronic equipment continue to increase, this new technology is poised to become a crucial process choice for high thermal dissipation PCB manufacturing.

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