With the rapid development of electronic technology, PCB board technology has advanced significantly. PCB circuit boards have evolved from single-sided and double-sided to multi-layer configurations, with the proportion of multi-layer PCBs increasing annually. The performance demands on PCB multilayers are pushing towards higher precision, density, and finer dimensions.

Lamination is a crucial process in manufacturing PCB multilayer boards. Ensuring high-quality lamination has become increasingly critical. Therefore, a thorough understanding of the lamination process is essential for maintaining quality standards. Based on extensive lamination experience, the following guidelines summarize how to enhance the lamination quality of PCB multilayer boards:

I. Design of Inner Core PCBs Meeting Lamination Requirements

Due to advancements in lamination technology, the hot pressing process has transitioned from non-vacuum to vacuum systems, which are now enclosed and touchless. Consequently, it is imperative to design the inner PCBs appropriately before lamination. Here are some key considerations:

1. Select core PCB thickness based on the total thickness requirements of the multilayer PCB. Ensure uniform core PCB thickness with minimal deviation, particularly crucial for multilayer PCBs exceeding 6 layers. Align the longitudinal and transverse directions of each inner core PCB to prevent unnecessary bending.

2. Maintain a distance between the core PCB shape dimensions and active elements. Optimize space utilization without material waste. Typically, leave over 10mm for four-layer PCBs and over 15mm for six-layer PCBs, scaling with higher layer counts.

3. Design positioning holes to minimize layer-to-layer misalignment. For instance, four-layer PCBs require at least 3 drill positioning holes, whereas PCBs with over 6 layers necessitate over 5 rivet positioning holes and 5 tooling holes. Position these holes closer to the edges for enhanced alignment and production efficiency. Design them in recognizable shapes like circles for automated target recognition.

4. Ensure inner core boards are free from shorts, opens, oxidation, and residual films, with a clean surface.

These guidelines aim to optimize the lamination process and uphold the quality of PCB multilayer boards.

II. Selecting suitable PP and CU foil configurations to meet PCB user requirements is crucial. Customers’ needs for PP primarily focus on several key aspects: dielectric layer thickness, dielectric constant, characteristic impedance, voltage withstand capacity, and surface smoothness of the laminate. Therefore, PP selection should consider the following criteria:

1. The resin’s ability to fill gaps in printed guidance lines during lamination.

2. Efficient removal of air and volatiles between laminates during lamination.

3. Provision of necessary thickness for the media layer in multilayer PCBs.

4. Ensuring bond strength and a smooth surface appearance.

Drawing from extensive production experience, recommended PP configurations for 4-layer laminations include 7628, 7630, or combinations like 7628+1080 and 7628+2116. For multilayer PCBs with 6 or more layers, PP selection typically leans towards 1080 or 2116, while 7628 is preferred to increase dielectric layer thickness. Symmetrical placement of PP is essential to maintain a mirrored effect and prevent PCB bending.

5. CU foil configurations vary based on specific PCB user requirements, ensuring compliance with IPC standards for quality.

III. Processing technology for inner core PCBs involves critical steps in multilayer PCB lamination. The inner core PCB undergoes treatments such as black oxidation and browning.

Black oxidation forms a black oxide film on the inner copper foil, typically with a thickness ranging from 0.25 to 4.50 mg/cm². Browning (horizontal browning) entails forming an organic film on the inner copper foil. These processes serve several essential functions:

1. Enhancing the specific surface area between the inner copper foil and resin to improve bonding strength.

2. Increasing the resin’s effective wetting of the copper foil during flow, ensuring strong adhesion after curing.

3. Shielding the copper surface from high-temperature decomposition effects caused by water in the curing agent dicyandiamide.

4. Enhancing the PCB’s resistance to acids and preventing pink ring formation during wet process operations.

These treatments are critical for optimizing the performance and durability of multilayer PCBs in various operational conditions.

IV. The control of organic matching of lamination parameters primarily involves “temperature, pressure, and time.”

1. Several temperature parameters are crucial during the lamination process. These include the resin’s melting temperature, curing temperature, the set temperature of the hot PCB, actual material temperature, and the rate of temperature increase. The resin starts melting around 70°C, gradually flowing as temperature rises further. Between 70°C to 140°C, resin flowability ensures effective filling and wetting. As temperature continues to rise, resin flowability peaks and then diminishes, ceasing entirely around 160°C to 170°C—this is the curing temperature. Effective control of heating rate, specifying when and how quickly temperatures rise, is vital for achieving optimal resin filling and wetting. Typically, the heating rate is maintained between 2-4°C per minute, though specific PP types and quantities can influence this rate. For instance, 7628PP allows for quicker heating rates of 2-4°C/min, while 1080 and 2116PP are managed at 1.5-2°C/min due to their higher volume. Excessive heating rates can compromise PP wettability, increase resin flow, shorten processing time, and risk delamination, negatively impacting laminate quality. The temperature of the hot PCB largely depends on heat transfer from steel PCBs, leather-based bull papers, etc., generally ranging between 180°C to 200°C.

2. Pressure in multilayer PCB lamination is critical for ensuring resin fills interlayer voids and expels gases and volatiles. Hot presses can be categorized into non-vacuum and vacuum types, with pressure application varying from single-stage to multi-stage compression. Non-vacuum presses typically use single or two-stage pressure settings, whereas vacuum presses employ multi-stage compression, particularly suitable for high-density PCBs. Pressure parameters are usually specified by suppliers, typically ranging from 15-35 kg/cm².

3. Time-related parameters focus on controlling pressure duration, temperature ramp-up time, and gel time. In multi-stage laminates, precise timing of primary pressure application and initial pressure-to-main pressure transition is crucial for ensuring lamination quality. Applying primary pressure too early can result in excess resin extrusion and inadequate curing, leading to defects like laminate voids or thin spots. Conversely, delayed primary pressure application can result in flawed bonding interfaces or air bubbles.

Therefore, determining optimal software parameters for laminate temperature, pressure, and time is critical in PCB multilayer board lamination. These parameters must be carefully tailored to the specific PP combinations, suppliers, models, and characteristics to achieve consistently high-quality laminates, based on extensive lamination experience and rigorous testing.