1. Material selection for PCBs primarily revolves around plate thickness and copper thickness:
For sheet materials thicker than 0.8mm, standard thicknesses include 1.0mm, 1.2mm, 1.6mm, 2.0mm, and 3.2mm. Materials thinner than 0.8mm do not adhere to the standard series and can be customized based on requirements. Common thickness options include 0.1mm, 0.15mm, 0.2mm, 0.3mm, 0.4mm, and 0.6mm, typically used for inner layers of multilayer boards.
During the production of inner layers, the thickness post lamination can be adjusted using prepreg (PP) thickness and structural configuration. Core board selection offers flexibility; for instance, if the finished board requires 1.6mm thickness, core boards of 1.2mm or 1.0mm can be chosen, ensuring the laminated board thickness remains within specified limits to meet final thickness requirements.
When designing the outer layers of PCBs, attention must be paid to board thickness. Production processes necessitate increased copper plating thickness, solder mask thickness, surface treatment (such as tin spraying or gold plating), as well as the thickness of characters and carbon oil. Actual sheet metal production typically increases board thickness by 0.05mm to 0.1mm, while tin plating can add 0.075mm to 0.15mm. For example, a design specifying 2.0mm thickness will result in a finished product thickness between 2.1mm and 2.3mm, accounting for sheet tolerance and processing variations. If strict adherence to a 2.0mm thickness is required, selecting non-standard 1.9mm sheet material may necessitate temporary orders from plate manufacturers, potentially elongating delivery cycles for PCB processing plants.
The other concern is PCB thickness tolerance. PCB designers should account for product assembly tolerance when considering post-processing PCB thickness tolerance. Three main factors affect finished product tolerance: sheet material tolerance, laminate tolerance, and outer layer thickness tolerance. Several conventional sheet tolerances are provided for reference: (0.8-1.0) ±0.1, (1.2-1.6) ±0.13, 2.0 ±0.18, 3.0 ±0.23. Laminating tolerances are controlled within ±(0.05-0.1) MM depending on layers and thicknesses. Especially for boards with edge connectors (like printed plugs), board thickness and tolerance must match connector requirements.
Another critical issue is copper thickness on PCB surfaces. During chemical and electroplating of hole copper, surface copper thickness increases unless specially treated. According to IPC-A-600G, minimum copper plating is 20um for levels 1 and 2, and 25um for level 3. Hence, in PCB production, achieving 1OZ (minimum 30.9um) copper thickness sometimes involves choosing HOZ (minimum 15.4um) based on line width and spacing, allowing for 2-3um tolerance, potentially reaching 33.4um. Opting for 1OZ results in a minimum finished copper thickness of 47.9um, with other thicknesses calculated similarly.
PCB drilling primarily considers hole size tolerance, pre-drill enlargement, edge processing, non-metalized holes, and positioning hole design:
Mechanical drilling uses a minimum 0.2mm bit, but aperture designs require enlargement due to hole wall copper thickness and protective layer considerations. Tin plating adds 0.15mm and gold plating 0.1mm. Enlarged hole diameters must still meet spacing requirements relative to circuits and copper skin. For instance, a 0.2mm via hole design with 0.35mm pad theoretically allows 0.075mm solder ring clearance, but tin plating may erase this. CAM engineers may struggle to enlarge pads if spacing is tight, halting production.
Aperture tolerance: Most domestic drills allow ±0.05mm, with metalized hole tolerance ±0.075mm and non-metalized hole tolerance ±0.05mm.
Overlooked issues include isolation distances between drilled holes and inner layer copper/wires in multilayer boards. With drilling tolerance ±0.075mm and pattern expansion/contraction ±0.1mm during lamination, 4-layer boards require ≥0.15mm hole-edge distance; 6/8-layer boards need ≥0.2mm for smoother production.
Non-metalized holes use dry film sealing or rubber plugging, ensuring copper-free areas ≤0.2mm during etching. Dry film seals holes ≤6.0mm, rubber plugs ≥11.5mm. Secondary drilling can also make non-metalized holes.
Positioning hole design is crucial for processing: board testing, shape punching, electric milling need holes ≥1.5mm. Triangular hole distribution on three board corners is optimal.
These edits aim to clarify technical details and ensure consistency in terminology, enhancing the text’s readability and precision.
