Multilayer PCB consists of multiple conductive copper layers separated by insulating layers, commonly configured with 4, 6, 8 layers and so on. These layers are bonded together using heat and adhesive to form a single, integrated board. The inner layers are made of a substrate material known as the core, and the insulating layers are often referred to as prepreg. Prepreg is a glass-reinforced epoxy material impregnated with resin, and it facilitates the bonding of copper layers. Connections between outer and inner layers are established via plated through holes (PTH) and vias. Multilayer PCB introduces the lamination process, which is where we actually creates the composite PCB from layers of circuitry in a lamination press under high pressure and heat. After pressing, multilayer PCB production is treated almost the same as a traditional double-sided PCB.
The effectiveness and reliability of multilayer PCBs depend on the careful selection and processing of core materials, copper foils, and prepreg. Each component must be optimized to work together, ensuring structural integrity, electrical performance, and manufacturability throughout the production cycle.
Role: Copper foil provides the conductive pathways in the PCB, and its quality directly affects circuit integrity, plating, and lamination.
Types & Sheets: Rolled-annealed or electrolytic copper foils are used, with specifications focusing on thickness, surface finish, and cleanliness.
Processing: Copper foil is laminated onto prepreg or core by stacking and pressing under heat and pressure. The foil’s surface finish and adhesion properties influence reliability, especially during plating and etching.
Critical Aspects: Uniform thickness, smooth surface, and good adhesion to the dielectric layer are critical for high-density circuitry and precise impedance control.
Purpose: Prepreg acts as an adhesive layer, bonding cores and copper foils together in multilayer stackup.
Material Composition: Partially cured resin system (usually epoxy), reinforced with glass fibers, designed to flow and cure during lamination.
Processing: Prepreg is laid up between cores and copper foils, then pressed under controlled heat and pressure, allowing resin flow and bonding. Proper curing ensures mechanical strength and electrical insulation.
Key Features: Its resin content, glass content, and Tg influence the uniformity of bonding, dimensional stability, and thermal performance during manufacturing and operation.
Function: The core provides the foundational dielectric layer in multilayer PCBs, typically consisting of a laminate with dielectric materials bonded to copper foil on both sides.
Material Composition: Usually made from glass-reinforced epoxy resin (like FR-4) cured and stabilized to ensure dimensional stability.
Processing: Cores are manufactured through lamination processes involving controlled temperature, pressure, and resin cure cycles to achieve uniform thickness and stability, critical for maintaining layer alignment and impedance control.
Key Considerations: The core’s physical and electrical consistency influences the overall performance, and its behavior during lamination impacts subsequent layer bonding and the laminate’s dimensional stability.
PTH(Plated-through-hole) technology is used in printed wiring boards (PWBs) to create electrical connections between different layers by metallizing the holes drilled through the boards. This method has been in practice since the mid-1950s and has undergone significant advancements over the decades.
Via is a plated-through hole that connects different layers of a multilayer board, enabling electrical signals to pass between layers. Types include through-hole, blind, and buried vias. They can also be filled or capped for added reliability. The quality and reliability of vias are critical, as defects can cause board failures, especially in high-density or mission-critical applications. Standards like IPC-4761 provide guidelines for via design and protection.
Hole copper plating thicknesses shall meet the requirements of Table 3-10 through Table 3-12, shall be continuous and shall extend or wrap from hole walls onto outer surfaces (Refer to IPC-A-600 for discussion on copper plating thickness for hol walls). Deviations to these requirements shall be AABUS. Each side of the plated hole wall shall be evaluated independently. Measurements of the plating thickness shall be reported as an average thickness of 3 measurements per side of the hole. At least one measurement shall be taken within 40 – 60% of the hole height. Isolated thick or thin sections shall not be used for averaging. Isolated areas of reduced copper thickness due to glass fiber protrusions shall meet the minimum thickness requirements of Table 3-10 through Table 3-12 as measured from the end of the protrusion to the hole wall.
If the internal conductor thickness of the finished printed board is specified by a foil weight, the minimum internal copper foil thickness after processing shall be measured in accordance with Table 3-17 for all classes. When the procurement documentation specifies a minimum copper thickness, the conductor shall meet or exceed that minimum thickness.
The requirements in Table 3-17 are based on minimum copper foil thickness allowances per IPC-4562 followed by two successive scrubbings. Each scrub is expected to remove a specific amount of copper and is represented by a variable processing allowance reduction.
If the internal conductor thickness of the finished printed board is specified by a foil weight, the minimum internal copper foil thickness after processing shall be measured in accordance with Table 3-17 for all classes. When the procurement documentation specifies a minimum copper thickness, the conductor shall meet or exceed that minimum thickness.
The requirements in Table 3-17 are based on minimum copper foil thickness allowances per IPC-4562 followed by two successive scrubbings. Each scrub is expected to remove a specific amount of copper and is represented by a variable processing allowance reduction.
OSPs are exceptionally thin organic coatings specifically formulated to preserve the solderability of copper surfaces on printed circuit boards (PCBs). These coatings work by forming a complex organo-metallic bond with the copper, which is critical for ensuring a reliable solderable finish. OSPs are particularly suitable for a range of applications, including both surface mount and through-hole assembly processes. Additionally, they offer a shelf life that supports effective long-term storage without compromising solderability, making them a practical choice for manufacturers and engineers alike.
Hot air solder level (HASL or HAL) is the process of immersing a circuit board into molten solder, immediately followed by blowing the excess molten solder out of the holes using hot air under high pressure. For the vast majority of HASL applications, the solder alloy is tin-lead. For Pb-free applications, the main alloys include tin-copper, tin-silver-copper, tin-coppernickel, and tin-copper-nickel-germanium alloys.
ENIG is a versatile surface finish consisting of a nickel layer topped with immersion gold. This configuration protects the nickel from oxidation and serves various functions, including soldering and as a bonding surface. The nickel layer acts as a diffusion barrier, preventing copper from leaching into the gold, and enhances the strength of thru holes. Gold, being a stable element that does not oxidize, ensures excellent wettability in soldering processes, contributing to the overall reliability of PCBs.
Immersion Silver (IAg) is a surface finish that forms a thin layer over copper, suitable for soldering, press-fit connections, and potentially for aluminum wire bonding. This finish protects the copper from oxidation during its shelf life and is applied using a galvanic displacement process with thicknesses ranging from 0.1 to 0.4 μm. Although the fabrication process is straightforward, the layer can tarnish when exposed post-assembly, which may lead to corrosion and functional degradation over time.
ISn is a protective layer of pure tin, applied using a galvanic displacement process. This coating safeguards the underlying copper from oxidation while ensuring excellent solderability and compliant pin connector functionality. The IPC-4554 specification mandates a minimum thickness of 1 μm to guarantee sufficient virgin tin for soldering, considering that an intermetallic layer forms over time between the tin and copper. Solderability tests and stress testing conditions are also outlined in the specification.
ENEPlG is a relatively new surface finish. This surface finish is much more expensivethan ENlG but is the only finish that is solderable without creating a brittle solderjoint and wire bondable. This is usually used in applications requiring both thesefeatures due to its costs.
Most PCBs use copper as their primary conductor. If the copper is left unprotected after finishing, it can oxidize, making soldering difficult. An ideal surface finish would possess good solderability, a flat coplanar surface, cost-effectiveness, the ability to withstand unlimited heat cycles, and minimal health and safety concerns. There are various final finishes available, each with its own advantages and disadvantages. Different finishes are more suitable for specific applications.
In addition to standard FR-4, double-sided PCBs include metal-core (MCPCB) for high-power LED/automotive applications, flexible PCBs for bendable electronics, and high-frequency laminates (like Rogers) for RF/microwave circuits etc. These offer superior thermal management, flexibility, or signal integrity for specialized designs.
Double-sided aluminum-based PCBs are printed circuit boards featuring two conductive copper layers on either side of an electrically insulating dielectric, which is itself bonded to an aluminum base. This structure allows for complex circuitry and component mounting on both sides of the board, enhancing design flexibility and circuit density. The aluminum core offers excellent heat dissipation, making these PCBs ideal for applications with high power or thermal demands, such as LED lighting, automotive, and power electronics. They also provide improved mechanical durability and better EMI shielding compared to standard FR4 boards.
Double-sided copper-based PCBs are printed circuit boards featuring conductive copper layers on both sides, bonded to a thermally conductive substrate, typically aluminum or copper. These boards excel in heat dissipation, making them ideal for high-power applications like LED lighting, power supplies, and automotive electronics. The dual copper layers allow for more complex circuit designs and higher component density compared to single-sided versions. Vias (plated-through holes) enable electrical connections between layers, enhancing functionality. Their robust thermal management ensures reliability in demanding environments. Double-sided copper PCBs offer a balance of performance, durability, and design flexibility, catering to advanced electronic and industrial applications.
Double-sided flexible printed circuit boards (PCBs) are composed of two conductive layers separated by a flexible insulating substrate, allowing for circuit traces on both sides. These PCBs enable more complex designs compared to single-sided versions and can incorporate plated through-holes, enabling electrical connections between the two sides. Their flexibility makes them ideal for applications requiring dynamic bending or where space and weight constraints are critical, such as in medical devices, wearable electronics, and aerospace equipment. Overall, double-sided flexible PCBs offer improved design versatility, increased circuit density, and reliable performance in challenging mechanical environments.
Common FAQ topics. If you have specific questions in mind, feel free to ask!
Essential information includes part numbers, fabrication and drill drawings, manufacturing notes, board details, material specifications, conductor patterns, artwork data for both sides, aperture lists, drill data, netlist data, and any special requirements.
Issues like poorly documented designs, misaligned layers, inadequate drill-to-copper clearance, and incorrect file formats can lead to manufacturing delays or defects. Most of these problems should be addressed during the design phase and by using modern formats and clear documentation.
Common substrates include glass fiber (like FR-4) combined with epoxy resin, and copper is used for both top and bottom conductive traces, often specified by weight (e.g., 1 oz or 0.5 oz copper).
Quality control includes electrical testing, comparison of netlists, automated optical inspection (AOI), and conformity with IPC standards to ensure the finished PCB matches the intended design and performance criteria.
Preferred formats include Gerber RS-274X, ODB++, intelligent CAD/CAM outputs (such as GenCAM or DirectCAM), and IPC-D-356 netlist formats for ease of manufacturing and minimizing errors.
The CAM-tooling process analyzes design data, checks design rules, manages manufacturability, edits for single images, panelizes boards, and extracts fabrication parameters to prepare the design for successful production.
Accurate drill and route files are crucial to ensure proper hole placement for through-hole vias and components, which connect both sides of the board. Drill bits (usually tungsten carbide) and careful preparation are essential to avoid damage and maintain high-quality hole walls.
AOI files are used by inspection systems to verify that the manufactured features (on both sides) match the design, helping detect errors like missing or misaligned traces and components.
