Compression of Multilayer PCBs

Advantages of PCB Multilayer Boards

• High assembly density, small size, and light weight;
• Reduced interconnection between components (including electronic components), which improves reliability;
• Increased flexibility in design by adding wiring layers;
• Ability to create circuits with certain impedances;
• Formation of high-speed transmission circuits;
• Simple installation and high reliability;
• Ability to set up circuits, magnetic shielding layers, and metal core heat-dissipating layers to meet special functional needs such as shielding and heat dissipation.

Exclusive Materials for PCB Multilayer Boards

Thin copper-clad laminates

Thin copper-clad laminates refer to the types of polyimide/glass, BT resin/glass, cyanate ester/glass, epoxy/glass, and other materials used to make multilayer printed circuit boards. Compared with general double-sided boards, they have the following features:

• More stringent thickness tolerance;
• More stringent and higher requirements for size stability, and attention should be paid to the consistency of cutting direction;
• Thin copper-clad laminates have low strength and are easily damaged and broken, so they need to be handled with care during operation and transportation;
• The total surface area of thin-line circuit boards in multilayer boards is large, and their moisture absorption capacity is much larger than that of double-sided boards. Therefore, materials should be strengthened for dehumidification and moisture-proof in storage, lamination, welding, and storage.

Prepreg materials for multilayer boards (commonly known as semi-cured sheets or bonding sheets)

Prepreg materials are sheet materials composed of resin and substrates, and the resin is in the B-phase.

Semi-cured sheets for multilayer boards must have:

• Uniform resin content;
• Very low content of volatile substances;
• Controlled dynamic viscosity of resin;
• Uniform and suitable resin flowability;
• Gelation time that meets regulations.
• Appearance quality: should be flat, free of oil stains, foreign impurities, or other defects, with no excessive resin powder or cracks.

PCB Board Positioning System

The positioning system of the circuit diagram runs through the process steps of multilayer photo film production, pattern transfer, lamination, and drilling, with two types of pin-and-hole positioning and non-pin-and-hole positioning. The positioning accuracy of the entire positioning system should strive to be higher than ±0.05mm, and the positioning principle is: two points determine a line, and three points determine a plane.

The main factors affecting the positioning accuracy between multilayer boards

• The size stability of the photo film;
• The size stability of the substrate;
• The accuracy of the positioning system, the accuracy of the processing equipment, operating conditions (temperature, pressure), and the production environment (temperature and humidity);
• The circuit design structure, the rationality of the layout, such as buried holes, blind holes, through holes, solder mask size, uniformity of wire layout, and setting of the internal layer frame;
• The thermal performance matching of the lamination template and the substrate.

Pin-and-hole positioning method for multilayer boards

• Two-hole positioning-often causes size drift in the Y direction due to restrictions in the X direction;
• One hole and one slot positioning-With a gap left at one end in the X direction to avoid disordered size drift in the Y direction;
• Three-hole (arranged in a triangle) or four-hole (arranged in a cross shape) positioning-to prevent changes in size in the X and Y directions during production, but the tight fit between the pins and holes locks the chip base material in a "locked" state, causing internal stress that can cause warping and curling of the multilayer board;
• Four-slot hole positioning-based on the centerline of the slot hole, the positioning error caused by various factors can be evenly distributed on both sides of the centerline rather than accumulated in one direction.