What is Multilayer PCB: Manufacturing, Design and Cost ?
Manufacturing multilayer PCB up to 56 layer, IPC III Standard, Multilayer Rigid PCB, Multilayer Flex PCB, Rigid-flex Board, hybrid PCB…
Introduction
A multilayer printed circuit board (PCB) contains two or more conductive copper layers separated by dielectric material layers such as FR-4. Multilayer PCBs enable increased component density by allowing placement on both sides as well as internally buried layers. They are commonly used in complex digital circuits, RF/microwave systems, high speed computing and other applications requiring high interconnectivity.
This article provides an in-depth overview of multilayer PCB technology covering topics such as:
- Multilayer PCB manufacturing processes
- Materials and construction
- Key design considerations
- Modeling and analysis
- Thermal management
- Signal integrity
- Cost tradeoffs
- Reliability factors
Understanding multilayer PCB capabilities and design best practices is essential to leveraging the benefits in electronic product development.
Multilayer PCB Manufacturing Process
Fabricating a multilayer PCB requires specialized sequential lamination processes to bond multiple double-sided circuit layers together into a consolidated board. Here are the typical manufacturing steps:
1. Inner Layer Formation
- Photolithographic patterning to define circuit traces on treated copper clad laminate
- Etching to remove unwanted copper leaving behind traces
- Layer registration targets, tooling holes added
- Electrical testing of traces
- Surface preparation for lamination
2. Layer Lamination
- Sheet lamination presses used to bond layers together
- Stackup arrangement of cores, prepregs, copper and dielectrics
- Curing under temperature and pressure to form laminate
3. Drill Holes
- High precision drilling to create tooling and via holes
- Accurate registration to drill each layer precisely
4. Hole Plating
- Electroless copper plated as thin conductive layer
- Electrolytic copper plating to desired thickness
- Copper builds up on innerlayers and drilled holes
5. Outer Layer Processing
- Apply liquid photoimageable (LPI) solder mask
- Print legend with identification markings
- Pattern and etch outer layer circuitry
- Panel rout into individual PCBs
6. Testing and QA
- Automated optical inspection (AOI)
- Net connectivity testing
- Impedance, high voltage and functional testing
- Dimensional quality control
Multilayer PCB Materials and Construction
Multilayer boards consist of conductive copper separated by dielectric prepreg and core layers. Here are some typical material options:
Conductive Layers
- Rolled annealed copper foil, 1⁄2 to 3 oz thickness
- Electrodeposited copper foil
- Silver, gold or nickel plating finishes
Dielectric Layers
- FR-4 – glass reinforced epoxy
- High Tg epoxy for high temp stability
- PTFE (Teflon) – for RF/wireless boards
- Polyimide – for flexible PCBs
- Cyanate ester – for radar, defense
Bonding Layers
- FR-4 prepregs (partially cured resin)
- Rogers prepregs
- Fluoropolymer adhesive films
- Reinforced and non-reinforced options
Common Multilayer Constructions
Multilayer PCB Design Considerations
esigning multilayer PCBs presents challenges due to higher interconnect density, thermal characteristics, fabrication constraints and signal integrity factors. Here are key multilayer PCB design guidelines:
Board Stackup
- Choose dielectric materials based on electrical, thermal, CTE properties
- Model performance with different laminate combinations
- Use symmetric construction when possible
- Incorporate reference planes for every signal layer
- Assign plane layers judiciously – ground, power, signal
High Speed Routing
- Use impedance controlled routing for high speed nets
- Place sensitive traces between ground/power planes
- Match trace widths, spacing to differential pairs
- Minimize abrupt bends, stubs, length matching
- Simulate performance in applied environment
Thermal Design
- Model hot spots and heat spreading layers
- Incorporate thermal vias for heat conduction
- Use thicker copper planes for heat spreader
- Select dielectrics for good thermal conductivity
- Ensure component spacing and airflow
Signal Integrity
- Optimize layer stackup to isolate noise coupling
- Assign return paths for high speed traces
- Use plate-through holes for consistent returns
- Incorporate passive components – capacitors, resistors
- Include provisions for decoupling and terminations
Power Distribution
- Provide adequate pinout for number of supplies
- Use separate regulator areas for analog and digital
- Provide bulk decoupling near supply sources
- Distribute power planes to minimize branch lengths
Component Layout
- Place components on both sides for density
- Group components by type to simplify routing
- Ensure components fit within board outline
- Provide access spaced for rework when needed
- Allow clearance for routing channels
Layer Transitions
- Minimize changes between layers when possible
- Use blind/buried vias when changing layers
- Taper trace widths when changing layers
- Ensure smooth impedance transitions
Testability
- Include test points, ports and potential probes
- Provide grid of test pads for bed of nails
- Add built-in test structures when possible
- Facilitate access to debug interface ports
Modeling and Analysis
Given the multiple interactions between material properties, stackup, component layout and routing density, modeling tools are essential for effective multilayer PCB development:
Electromagnetic Simulation
- Model radiation, coupling, resonances, etc.
- Detect issues with stackup, trace geometry, splits
- Identify needed shielding, ground planes, etc.
Signal Integrity
- Analyze impedance discontinuities, reflections, etc.
- Verify termination strategies, transmission line effects
- Account for losses, noise, coupling
Power Integrity
- Model DC and AC supply characteristics
- Confirm power delivery network design
- Include effects of decoupling, PDN, etc.
Thermal Analysis
- Predict temperature profiles under load
- Identify hot spots in component layout
- Model impacts of heat sinking, spreading, etc.
Vibration/Shock
- Assess vibration modes, resonances
- Identify mechanical reinforcement needed
- Verify component mounting, solder joint reliability
DFX Analysis
- Check design for fabrication, assembly, test
- Assess serviceability, rework considerations
- Improve manufacturability, yield, cost
Thermal Management
Managing heat buildup is crucial for multilayer PCB reliability and performance. Key techniques include:
- Incorporating thermal vias/pads connected to inner plane layers
- Using copper planes as heat spreaders
- Adding thicker copper weights in outer layers
- Applying solder mask over bare copper (SMOBC)
- Providing adequate airflow paths around components
- Using thermally conductive dielectric materials
- Adding separate heat sinks and thermal adhesives
- Employing interposers to route heat away from components
- Considering liquid cooling for very high power densities
Signal Integrity Considerations
Maintaining signal integrity is more challenging with multilayer boards due to increased crosstalk and coupling across parallel layers. Strategies include:
- Assigning continuous reference planes adjacent to signals
- Using lower loss dielectric materials
- Controlling trace impedances through all layer transitions
- Applying ground guard traces around sensitive signals
- Incorporating shielding ground planes between critical nets
- Providing proper decoupling for power distribution
- Minimizing abrupt bends, stubs, length mismatches
- Terminating traces correctly at endpoints
Cost Considerations
Increasing multilayer PCB complexity results in higher fabrication costs. Key cost factors include:
Layer Count
- Each additional conductive layer adds cost
- More process steps for imaging, plating, lamination
Board Size
- Larger boards require larger equipment capacity
- Maximum panel sizes range 24”x36” to 28”x44”
Higher Density
- Tighter tracing, spacing and hole size tolerances
- Thinner materials enable more layers
- Complex panelization and breakouts
Advanced Materials
- Low loss laminates have higher material cost
- Thin core/prepregs enable more layers
- Thicker copper, stacked microvias add cost
- Non-recurring engineering charges
- Lower volume orders cannot leverage economies of scale
Reliability Considerations
Key factors affecting multilayer PCB reliability include:
Fabrication Process Control
- Exceptional drill registration accuracy
- Stringent lamination quality – lack of voids
- Plating integrity – continuity, adhesion
Laminate Quality
- Glass transition temperature (Tg)
- Coefficient of thermal expansion (CTE)
- Moisture absorption characteristics
Design Factors
- Internal layer heat dissipation
- Mechanical reinforcement and stiffness
- Component layout and density
Testing and Inspection
Conclusion
Multilayer PCB technology enables denser, higher performance electronic designs by utilizing stacked circuit layers interconnected through plated holes and vias. Realizing the benefits requires expertise in specialized materials, modeling tools, thermal and signal integrity techniques, reliability testing and advanced manufacturing processes. This article provided a comprehensive overview of multilayer PCB capabilities to help engineers successfully design, analyze and produce robust multilayer boards.
Frequently Asked Questions
Here are some common questions about multilayer PCBs:
Q: What are the main benefits of using multilayer PCBs?
Multilayer PCBs allow higher component density, improved electrical performance, smaller product size, and higher functionality in complex electronic designs.
Q: How many layers are typical in multilayer PCBs?
4-6 layers meets many needs. High complexity boards use up to 20 layers, HDI technology enables 60+ layers.
Q: What design challenges are unique to multilayer PCBs?
Thermal management, signal integrity and power distribution complexity, modeling interactions between layers, and fabrication/assembly process factors.
Q: What types of testing are used to ensure multilayer PCB quality?
Testing includes automated optical inspection, net connectivity testing, x-ray imaging, cross-sectioning, environmental stress testing, and extensive electrical/functional tests.
Q: What causes multilayer PCBs to cost more than double layer boards?
Additional materials, more fabrication processing steps, lower production volumes, high complexity of design and production processes all increase multilayer PCB costs.
Here some stardard multilayer stack-up
1): 4 Layer PCB Stack-Up
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2).6 Layer PCB Stack-Up
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3).Typical Stack-Up of 8 Layer PCB
Prepreg specifications
Prepreg specifications
Lay Stack up design guidelines
Multilayer Circuit Board
It is a container filled with high-temperature saturated water vapor, and high-pressure can be applied. The laminated substrate (laminates) sample can be placed in it for a period of time to force moisture into the board, and then take out the sample again. Place it on the surface of high-temperature molten tin and measure its “delamination resistance” characteristics. This word is also synonymous with the pressure cooker, which is commonly used by the industry. In addition, in the multilayer board pressing process, there is a “cabin press method” with high temperature and high-pressure carbon dioxide, which is also similar to this type of Autoclave Press.
It refers to the traditional lamination method of early multilayer PCB boards. At that time, the “outer layer” of MLB was mostly laminated and laminated with a single-sided copper thin substrate. It was not used until the end of 1984 when the output of MLB significantly increased. The current method is the copper-skin type large or mass pressing method (Mss Lam). This early MLB pressing method using a single-sided copper thin substrate is called cap lamination.
The multilayer board pressing often refers to the wrinkles that occur when the copper skin is improperly handled. Such shortcomings are more likely to occur when thin copper skins are below 0.5 oz and laminated in multiple layers.
It refers to the gentle and uniform sag on the copper surface, which may be caused by the partial protrusion of the steel plate used in pressing. If it shows a neat drop of the faulty edge, it is called “dish down.” If these shortcomings are left on the line after copper corrosion, the impedance of the high-speed transmission signal will be unstable, and noise will appear. Therefore, such a defect should be avoided as much as possible on the substrate’s copper surface.
When the multilayer board is pressed, in each opening of the press, there are often many “books” of bulk materials (such as 8-10 sets) of the board to be pressed. Each set of “bulk materials” (Opening) Book) must be separated by a flat, smooth, and hardened stainless steel plate. The mirror stainless steel plate used for this separation is called “caul plate” or “separate plate.” At present, AISI 430 or AISI 630 are commonly used.
Refers to the mass-produced multilayer board, the outer layer of copper foil and film are directly pressed with the inner skin, which becomes the mass lam of the multilayer board. This replaces the early traditional single-sided thin substrate Press legal.
When multilayer boards or substrate boards are laminated, kraft paper is often used as a heat transfer buffer. It is placed between the hot plate (Platern) of the laminator and the steel plate to ease the temperature rise curve closest to the bulk material. Between multiple substrates or multilayer boards to be pressed. Try to minimize the temperature difference of each layer of the sheet; the commonly used specifications are 90 to 150 pounds. Because the fiber in the paper has been crushed after high temperature and high pressure, it is no longer tough and difficult to function, so it must be replaced with a new one. This kind of kraft paper is co-cooked with a mixture of pinewood and various strong alkalis. After the volatiles escape and the acid is removed, it is washed and precipitated. After it becomes pulp, it can be pressed again to become rough and cheap paper material.
When the multilayer board is pressed and the plates are placed and positioned, they will start to heat and be lifted by the hottest layer from the bottom. Afterward, lift with a powerful hydraulic jack (ram) to press each opening (bulk materials in the opening) and are bonded together. At this time, the combined film (prepreg) begins to gradually soften or even flow, so the pressure used for the top extrusion cannot be too large. This is to avoid slippage of the sheet or excessive flow of the glue. This lower pressure (15-50 PSI) initially used is called “kiss pressure.” However, when the resin in the bulk materials of each film is heated to soften and gel and is about to harden. It is necessary to increase to the full pressure (300-500 PSI) so that the bulk materials are tightly combined to form a strong multilayer board.
Before pressing multilayer circuit boards or substrates, various bulk materials such as inner layer boards, films and copper sheets, steel plates, kraft paper pads, etc., need to be aligned, aligned, or registered up and down to prepare. Then it can be carefully fed into the pressing machine for hot pressing. This kind of preparatory work is called Lay Up. To improve the quality of multilayer boards, not only this kind of “stacking” work must be carried out in a clean room with temperature and humidity control, but also for the speed and quality of mass production. Generally, the large-scale press method (Mass Lam) in construction, even “automated” overlapping methods are needed to reduce human error. To save workshops and shared equipment, most factories combine “stacking” and “folding boards” into a comprehensive processing unit, so automation engineering is quite complicated.
Before pressing multilayer circuit boards or substrates, various bulk materials such as inner layer boards, films and copper sheets, steel plates, kraft paper pads, etc., need to be aligned, aligned, or registered up and down to prepare. Then it can be carefully fed into the pressing machine for hot pressing. This kind of preparatory work is called Lay Up. To improve the quality of multilayer boards, not only this kind of “stacking” work must be carried out in a clean room with temperature and humidity control, but also for the speed and quality of mass production. Generally, the large-scale press method (Mass Lam) in construction, even “automated” overlapping methods are needed to reduce human error. To save workshops and shared equipment, most factories combine “stacking” and “folding boards” into a comprehensive processing unit, so automation engineering is quite complicated.