The lamination process is a critical step in the production of printed circuit boards (PCBs). It involves bonding together layers of insulating material and copper foil to create the laminate that serves as the foundation of the PCB. Proper lamination is essential for producing PCBs that are flat, dimensionally stable, and free of defects like delamination or blistering. This article provides an in-depth look at the PCB lamination process, the materials involved, and the key steps manufacturers follow to create quality laminates.

Overview of the Lamination Process

The goal of lamination is to securely bond the layers of a PCB stack together into a solid, unified structure. The basic lamination process involves:

• Preparing the materials – copper foils and insulating prepregs or cores
• Laying up the PCB stack
• Applying heat and pressure to bond the layers together
• Cooling and cutting to size
• Inspecting for defects

The laminate created contains the internal circuitry of the PCB and serves as the foundation upon which other processes like drilling, plating, and etching are performed. The quality and reliability of the final PCB depends heavily on the integrity of the laminate.

Lamination Materials

There are two primary types of materials used in the lamination process:

Copper Foils

Copper foils provide the conductive layers of the PCB. The foils are typically around 1 oz thickness (35 μm) and are bonded to the insulating dielectric layers. Two types of copper foils are used:

• Rolled copper – Produced by rolling and annealing electrolytic tough pitch copper. Lower cost but less consistency.
• Electrodeposited (ED) copper – Produced by electroplating. More consistent thickness and properties. Used for finer line PCBs.

The surface of the foil is also critical. Foils typically have a microscopic “tooth” pattern to help promote adhesion to the dielectric. Foils designated as “reverse treated” have a coarser tooth pattern on the sides bonded to prepregs, while “direct treated” foils have a finer treatment on the sides bonded to cores.

Insulating Dielectric Materials

The insulating dielectric layers provide spacing between conductive copper layers and prevent short circuits. Two types are used:

• Prepregs – Fiberglass cloth impregnated with partially cured epoxy resin. Prepregs are softened and bonded together under heat and pressure.
• Cores – Fiberglass reinforced cores with fully cured resin. Cores provide rigid inner layers and are bonded to prepregs.

Advanced dielectrics like polyimides and fluoropolymers are also sometimes used for their superior thermal and chemical resistance.

PCB Stackup

The materials must be arranged properly in a PCB stackup to create the desired layout of conductive and insulating layers in the final laminate. A few rules govern stackup design:

• Cores are used on the inner layers surrounded by prepregs
• Layer thickness should descend in order from the center to avoid bowing
• Adjacent conductive layers are separated by prepreg
• Foil treatments are matched to the adjacent dielectric

A typical 6-layer PCB stackup may be arranged as:Copy code

Top (Outer) Layer: 1 oz Copper Foil Prepreg 1 oz Copper Foil Core 1 oz Copper Foil Prepreg 1 oz Copper Foil Prepreg Bottom (Outer) Layer: 1 oz Copper

Lamination Process Steps

The main steps carried out during the lamination process are:

Cutting

Sheets of copper foil and dielectric materials are cut to the required sizes and shapes. This is often done using a steel rule die. Proper alignment at this stage helps minimize waste.

Layup

The PCB layer stackup is built up on a lamination plate or press bed. Layers of prepreg and foil are laid up in the pre-determined sequence and alignment. Registration holes or fiducials aid in alignment. Care is taken to avoid wrinkles, folds, and contamination.

Tacking

A soft vacuum pressure or mechanical rollers are applied to tack the layers together before final bonding. This ensures the layers maintain their arrangement and alignment.

Bookbinding

Prepreg layers extending beyond the edge of the stack are folded over to capture and seal in the foils. This “bookbinding” provides a smooth edge and prevents separation during bonding.

Lamination Pressing

The stack is placed in a lamination press between heated metal plates. Hydraulic pressure of up to 300 psi is applied, and the temperature is ramped up to over 350°F. The combination of heat and pressure causes the prepreg resins to flow and cure, bonding the layers together.

Cooling and Cutting

After curing, hydraulics maintain pressure while cooling water is circulated through the plates. The bonded stack is then removed from the press. The laminate is trimmed to final PCB dimensions in a shearing machine.

Post-lamination Processing

Additional processing steps help improve laminate quality:

• Degassing – Heating in a vacuum chamber removes trapped gases and volatiles. Prevents delamination defects.
• Bake cycle – A lower temperature bake improves resin cure.
• Surface preparation – Chemical treatments and abrasion improve adhesion for subsequent processes.

Quality Control

Throughout the lamination process, inspectors visually examine the materials and laminate for defects. Some common defects checked for include:

• Wrinkles/folds in materials or stackup
• Registration errors between layers
• Resin curing issues leading to delamination, blistering, or handling cracks
• Contamination between layers
• Voids or trapped air pockets

Automated optical inspection systems are also commonly used to detect defects and ensure a quality lamination. Parameters like lamination pressure and temperature are tightly controlled with monitoring systems.

Advantages of Lamination

The PCB lamination process provides some key benefits:

• Bonds layers together into an integral structure
• Allows complex, multi-layer PCB designs
• Insulates conductive layers from each other
• Provides a flat, rigid substrate for subsequent PCB processing
• Heat and pressure improves resin cure
• Encapsulates and seals in the inner copper layers

With careful process controls and inspections, PCB manufacturers can produce laminates with excellent layer-to-layer registration, dimensional stability, and bond integrity. This leads to more reliable PCBs able to withstand stresses over the product lifetime.

Recent Trends in Lamination

Some newer developments seek to improve certain aspects of the lamination process:

• Low-flow Prepregs – Require lower lamination pressure while reducing resin bleed.
• Flexible Laminates – Allow flexible PCBs using polyimide films or LCP.
• Green Laminates – Environmentally-friendly materials like ceramic-filled PTFE composites.
• High-Speed Pressing – Presses applying faster heating and cooling cycles to increase throughput.
• directMET Subtractive Process – Removes need for inner layer lamination and improves registration accuracy.

Even with these advances, the fundamental concept of using heat and pressure to bond copper and dielectric layers persists as a proven, reliable PCB production method.

Frequently Asked Questions

What is the difference between pressed and bonded inner layers during multilayer PCB lamination?

The main difference lies in how the inner layer core is prepared before lamination:

• Pressed cores: The inner layer circuitry is etched on a fully cured dielectric core layer. This etched core is then pressed between prepreg layers during lamination.
• Bonded cores: The inner layer circuitry is etched on a partial-cure prepreg layer. Multiple prepreg layers are stacked and bonded together in one lamination cycle, creating the inner core and outer layers simultaneously.

Pressed cores provide a more stable and rigid inner foundation. But bonded cores avoid the separate core preparation steps needed for pressed cores. The choice depends on PCB performance requirements vs production time and cost.

What causes delamination defects in PCBs?

Delamination refers to separation of the dielectric layers from the copper foils. Some common causes are:

• Inadequate resin flow and cure during lamination, leading to weak polymer-to-foil bonds
• Contamination on foil or prepreg surfaces inhibiting adhesion
• Foil treatment not properly matched to the prepreg resin system
• Excessive moisture absorption post-lamination leading to interface weaknesses
• Mechanical stress exceeding the interface bond strength

Good process controls and testing during lamination as well as proper handling post-lamination helps prevent delamination issues.

How are different PCB materials laminated together?

When laminating dissimilar materials like FR-4 and polyimide, some special considerations are required:

• Adhesion between materials must be ensured through compatible resin systems or bonding films
• Stacks must be designed so materials with similar CTEs are paired together
• Lamination pressures and temperatures must suit the requirements of each material type
• Warpage stresses from CTE mismatch during cooling must be managed

In some cases, an intermediate bonding ply is used between two dissimilar materials to ease processing and improve bonding. With care in stackup design and process settings, a wide range of material combinations can be reliably laminated.

What are steps can be taken to minimize warp and twist in multilayer PCBs?

Some best practices to reduce warp and twist include:

• Symmetrical stackups with same thickness materials paired top-to-bottom
• Bonding together laminate layers of similar CTEs
• Balanced construction techniques with same amount of copper on each layer
• Minimal ramp rates during heating and cooling cycles
• Annealing after lamination to relax stresses
• Enlarging panel sizes to allow warpage to distribute over larger area
• Improving resin cure to minimize dimensional changes post-lamination

Careful modeling, lamination process design, and both process and product testing are key to controlling warp and twist.

How are very small multilayer PCBs laminated reliably?

For PCBs with fewer than 6-8 layers and/or very small sizes under 25 x 25 mm, some special provisions must be made:

• Using press plates with customized cutouts or recesses to support the stackup
• Adhesive films to help tack layers in place during assembly
• Fine tip vacuum placement tools aid layup alignment
• Low pressure lamination process with smooth, even ram motion
• Small multi-opening presses to laminate multiple small boards simultaneously
• Warp correction plates applied post-lamination

With the right fixturing and process optimizations, even small complex PCBs can be laminated with minimal defects.