The lamination process is a critical manufacturing step that bonds together the layers that make up a multilayer PCB. Lamination presses layers of conductive copper foil and insulating dielectric material under heat and pressure to create a unified circuit board. This process connects the embedded inner layer circuitry with outer layer traces, enabling complex routing in a compact form factor.

This article will provide an in-depth overview of PCB lamination covering:

• The role of lamination in PCB fabrication
• Materials laminated in the PCB stackup
• Process steps for lamination
• Key lamination parameters
• Advanced lamination methods
• Quality control considerations
• Recent innovations in lamination

Understanding the PCB lamination process provides useful insight into this crucial transformation step that allows translating a PCB design into the multilayer interconnected reality of a finished circuit board.

The Role of Lamination in PCB Fabrication

Lamination is the process of permanently bonding together the layers of a multilayer PCB stackup under heat and pressure. It serves several important purposes:

• Joins insulating dielectric substrate cores with conductive copper foils
• Bonds adjacent copper layers with insulating prepreg material
• Creates continuous insulation between conductors on different layers
• Allows inner layer circuits to connect via plated through holes
• Provides mechanical structure and rigidity to the PCB

Without lamination, high-density routing of traces on closely spaced layers would not be feasible. Lamination creates a unified board with embedded circuitry insulated by alternating dielectric films.

The individual layers laminated together include:

• Base dielectric substrates (cores)
• Copper foil conductive layers
• Prepreg (resin-impregnated fiberglass)
• Metal sustaining foils

The sequence of layers, properties of materials, and lamination process parameters all determine the characteristics and performance of the finished circuit board.

PCB Layer Stackup

A multilayer PCB is composed of a stack of conductive and insulating layers which are laminated together:

1. Core Substrate

The core substrate forms the foundational layer. Common materials are:

• FR-4 glass reinforced epoxy
• High-Tg epoxy blends for high temperature rating
• Polyimide for maximum thermal and mechanical stability
• Composite epoxies or cyanate esters for high frequency applications

2. Copper Foil

Very thin rolled copper foil, around 1⁄2 oz per square foot (18 μm), is laminated to the core substrate. This forms the conductive layer for traces.

3. Prepreg Material

Prepreg is fibreglass cloth pre-impregnated with partially cured epoxy resin. Layers of prepreg are sandwiched between copper foil to insulate conductive layers from each other.

4. Metal Sustaining Foil

A thin aluminum or copper foil on the outside helps prevent warping and wrinkling of the stack during lamination at elevated temperature.

PCB Lamination Process Steps

Printed circuit board lamination involves the following general steps:

1. Layer Preparation

The individual layers are prepared prior to lamination:

• Core substrates are cleaned then treated to improve adhesion
• Copper foil is applied to core dielectric layers
• Foil-coated cores and prepreg sheets are cut to size
• Adhesion promoters can be selectively applied between layers
• Alignment pins or films help align layers

2. Layup

Layers are stacked in the precise sequence defined by the layer stackup documentation:

• Core layers alternate with prepreg and foil
• Buildup sequence ensures copper and dielectric layers align
• Layup is symmetric to avoid curling or warping
• Layup Considerations for HDI PCBs

HDI (high density interconnect) PCBs require special layup considerations:

• Very thin dielectric prepregs (25-50 μm)
• Thin cores (as low as 100 μm)
• Tight lamination pressure control
• Use of low-flow prepregs
• Filler-free dielectric materials

3. Vacuum Pressing

The layer stack is sealed in a lamination press vacuum bag:

• Layers rest on bottom caul plate covered by release film
• Breather and bleeder materials sit above stack
• Layers sealed in the bag under vacuum
• Vacuum removes air pockets and volatiles

4. Autoclave Lamination

The layer stack undergoes high temperature and pressure in the lamination press:

• Temperature ramps up and is held, typically 180-200°C
• Pressure up to 100 PSI is applied
• Prepregs flow and partially cure to bond layers
• Pressure is maintained during cool down
• Multiple press openings may be required for thicker boards

5. Degassing

Post-lamination degassing removes trapped solvents or air bubbles:

• Board is heated in an oven under vacuum
• Volatiles diffuse out of the laminate
• Prevents unwanted outgassing and delamination later

Proper lamination processing is critical to produce flat, well-bonded circuit boards free of wrinkles, pits, or other defects between layers.

Key Lamination Process Parameters

The lamination pressure, temperature, time, and materials must be tightly controlled:

Temperature

• Determines degree of resin flow and affects bond strength
• Typically 180-200°C for common FR-4 substrates
• Higher temp improves layer bonding but can over-cure resin

Pressure

• Squeezes layers together causing resin to flow and fill gaps
• Around 300 – 500 PSI for rigid PCB lamination
• Higher pressure improves layer-to-layer adhesion

Time

• Extended heating duration improves resin flow and bonding
• But excessive time may over-cure resin before flow occurs
• Pressure is maintained during the cooling ramp

Materials

• Prepreg resin properties influence degree of flow and adhesion
• Low-flow prepregs help prevent excessive squeeze-out

Fine-tuning these parameters for the material set minimizes flaws between layers.

Advanced Lamination Methods

Innovative lamination techniques extend PCB capabilities:

Vacuum Pressing Only

• Uses high temperature and vacuum pressure without applying stack compression
• Reduces inner layer copper deformation for ultra-thin dielectrics

Step Lamination

• Separates core, prepreg, and foil lamination steps
• Permits use of incompatible materials in one board
• Improves control over dielectric thickness

Thermal Pressing

• Applies heat via embedded heating elements instead of external autoclaving
• Facilitates lamination of very thick boards

Laser Drilling

• Laser micromachining can drill microvias in individual layers prior to lamination
• Permits use of low-flow or filled dielectrics unable to be drilled after curing

ALIVH (Any Layer Interstitial Via Hole)

• Blind microvias are drilled between any layer pairs before final lamination
• Eliminates need to align layers precisely for sequential lamination and drilling

Skilled application of these methods expands the design possibilities.

Lamination Quality Control

Careful inspection validates a quality multilayer lamination process:

• Microsectioning confirms proper layer alignment and bonding
• Microscopy checks for resin voids, cracks, inclusion contamination
• Cross-section thickness verifies target dielectric thicknesses
• Peel strength should exceed 1.1 kg/mm between cured prepreg and copper
• Warpage measured on surface table should not exceed 0.75% of panel diagonal
• Hofmann test for inner layer continuity before and after lamination
• Post-lamination baking reveals tendencies for delamination or blistering

Catching flaws early prevents defective boards from progressing further through fabrication.

Recent Lamination Advancements

Ongoing innovations in materials and methods continue to enhance PCB lamination capabilities:

• Nano-engineered resins – Enable thinner dielectrics by reducing resin flow rate and properties like low-loss, low Dk, and lower curing temperature.
• ALIVH – Bonded microvias between any layer pairs prior to final lamination simplifies high density routing.
• Microfluidic facilitation – Micro-channels allowing planarization fluid to flow out during pressing enables smoother surfaces.
• Sequential prepreg lamination – Separate lamination steps for rigid core, then flexible adhesive dielectric allows hybrid flex-rigid boards.
• Rapid curing resins – Fast-curing or snap-cure resins shorten lamination cycles improving throughput.
• Laser direct imaging (LDI) – LDI patterned bond enhancers or adhesives only where needed optimizes layer bonding.
• Embedded components – Capacitors, resistors and other components become integrated into the PCB during lamination as another functional “layer”.
• Thermal pressing – Programmable infrared and convection heating elements apply heat more controllably than external autoclaving.

PCB laminate materials and fabrication processes will continue advancing to provide boards with enhanced performance, customization, and integration.

Conclusion

The PCB lamination process is essential for creating multilayer boards by fusing together insulating and conductive layers into an integrated circuit board. Understanding lamination principles helps PCB designers fully utilize the capabilities of this transformative fabrication step. Tight process controls and material compatibility minimize flaws between layers. With ongoing advances in lamination technology, PCBs will continue gaining complexity, density, and embedded functionality.

PCB Lamination Process FAQs

What is the purpose of lamination in PCB fabrication?

Lamination permanently bonds together the stacked material layers including dielectric substrates, copper foils, and prepregs using heat and pressure to create a multilayer board.

What temperature is used for common FR-4 lamination?

Typical FR-4 lamination temperature is 180-200°C to enable resin flow and curing without over-baking. The temperature is precisely controlled.

How are layers aligned properly prior to lamination?

Alignment pins, fiducials, and/or specialized films help orient layers accurately. Precise layup is critical for layer-to-layer connections.

What are some methods for improving lamination quality?

Careful material selection, use of low-flow prepregs, modifying pressure and temperature profiles, and inspection of cross-sections and peel strengths helps identify and resolve lamination flaws.

What innovations are changing PCB lamination capabilities?

Advances include bonded microvias, thermopresses, laser direct imaging, thinner dielectrics through nano-engineered resins, snap cure materials, and embedding active components. These expand the design possibilities.