What is the process of press-fit in PCB? Detailed description

PCB (Printed Circuit Board) press-fit process is an important part of multilayer PCB manufacturing, which involves bonding different material layers together through high temperature and pressure to form a strong multilayer board structure. The following is a detailed description of the press-fit process:

Material Preparation:

Firstly, the various materials required need to be prepared, including the copper foil layers, prepreg and possibly the inner copper foil board (Core Board).

Prepreg is a special material that has been soaked in resin and usually comes with one or more layers of glass fibre or other reinforcing material.

The copper-clad layers are used for circuit alignment and connections and are usually on the top and bottom sides of the prepreg.

Layering Sequence:

The copper-clad layers and prepreg are arranged in a specific stacking sequence according to the design requirements. Typically, the copper-clad layers are placed above and below the prepreg, while the inner copper-clad sheet is placed in the centre.

The layer stacking sequence needs to ensure that all materials are aligned, especially the position of the holes, for the subsequent drilling and graphic transfer process.

Preheating:

Preheating of the material prior to lamination is often required to remove internal moisture and preheat the material to reduce thermal stress and distortion during the lamination process.

The temperature and duration of preheating needs to be adjusted according to the material properties and manufacturer's recommendations.

Press-fitting:

The preheated materials are placed in a press-fit machine, where they are bonded together by high temperature and pressure.

The temperature and pressure during the pressing process need to be tightly controlled to ensure good bonding between the materials and the quality of the PCB.

The press cycle may include a heat-up phase, a holding phase and a cool-down phase, each of which requires precise control of temperature and time.

Cooling and Curing:

Once the press-fit is complete, the PCB needs to be cooled slowly to avoid internal stresses due to rapid temperature changes.

During the cooling process, the resin cures to form a strong multilayer structure.

Post-processing:

After cooling, the PCB may require post-processing, such as removing excess copper foil, levelling, removing copper foil from drilled holes, etc.

These steps help to improve the PCB's appearance quality and electrical performance.

Quality Inspection:

PCBs after lamination need to be quality checked, including checking the integrity of the laminated structure, the quality of the holes, and the strength of the interlayer bonding.

Quality inspection can be carried out through visual inspection, X-ray inspection, ultrasonic inspection, etc.

Subsequent Processes:

After the press-fit is complete, the PCB requires subsequent manufacturing processes such as drilling, graphics transfer, plating, etching, silk-screening, etc. to complete the circuit board.

Material Matching:

During the lamination process, there is a need to ensure that the coefficients of thermal expansion (TCE) between the materials used (e.g., prepregs and copper foils) are matched to minimise internal stresses and potential layer separation due to temperature changes.

Pressure Uniformity:

The laminator must be able to provide uniform pressure to ensure proper bonding across all areas of the PCB. Uneven pressure may result in inadequate interlayer bonding or localised overheating.

Temperature Control:

In addition to controlling the temperature during the lamination process, the preheating temperature and cooling rate of the material need to be considered. These factors affect the curing of the resin and the final laminate.

Environmental Control:

Environmental conditions (e.g., humidity) during the lamination process also need to be controlled, as humidity may affect material properties and lamination quality.

Material Storage:

Prepregs and other materials need to be stored under appropriate conditions prior to lamination to prevent moisture or degradation of material properties.

Laminate Flatness:

After pressing, the laminate needs to be levelled to ensure accuracy for subsequent processes (e.g. drilling, graphic transfer).

Curing of Adhesives:

Adhesives (usually resins) used in the press-lamination process need to be fully cured to ensure long-term bond strength between layers.

Process Parameter Recording:

All critical parameters (temperature, pressure, time, etc.) of the lamination process should be recorded and monitored in detail so that they can be traced and adjusted if problems are detected.

Laminate Handling and Transport:

Laminates need to be handled and transported with care to prevent mechanical damage or ply separation.

Quality Standards and Certification:

The lamination process and final product need to comply with industry standards and quality certification requirements, such as IPC standards or ISO certification.

Continuous Improvement:

By continuously monitoring and analysing data from the lamination process, manufacturers can continually improve the process and increase product quality and productivity.

Safety and Health:

Throughout the lamination process, safety protocols and health standards need to be adhered to, ensuring the safety of operators and a harmless working environment.

The lamination process is a critical step in the manufacture of multilayer PCBs, which directly affects the structural strength, electrical properties and reliability of the PCB. Therefore, throughout the press-fit process, various parameters need to be strictly controlled to ensure that each step meets design and quality requirements.