What is PCB lamination?

PCB lamination is the process of bonding or fusing multiple layers together to create a multilayer printed circuit board (PCB). It typically involves sandwiching copper foil circuit layers between insulating dielectric substrates using heat, pressure and adhesive resin.

Lamination allows the manufacture of complex multilayer PCBs by bonding together individual copper and substrate layers to form an integral board with electrical connections spanning across all layers.

Why is lamination needed for PCBs?

Lamination provides the following benefits for PCB construction:

• Creates electrical isolation between copper conducting layers by having an insulating dielectric substrate as a barrier. This allows traces to cross over each other on different layers without shorting.
• The insulating dielectric layer also helps minimize stray capacitance between adjacent copper traces. This enables high frequency performance.
• Building up layers of substrates like FR4 gives the PCB structural rigidity and prevents warping or flexing.
• Additional copper circuit layers can be added through lamination to increase circuit density and complexity within the same footprint.
• Lamination allows embedding components like resistors and capacitors into the PCB to save space.

What are the common PCB laminate materials?

Some standard materials used for PCB lamination include:

Substrate dielectric materials

• FR-4 Glass epoxy – Most common rigid PCB substrate
• Polyimide – Used in flexible PCBs requiring bendability
• PTFE/Teflon – High frequency microwave circuit boards
• Ceramic – High temperature stability and thermal performance
• Aluminum – LED light boards requiring heat dissipation

Copper foil

• 1 ounce (35 μm) – For tighter trace spacing
• 2 ounce (70 μm) – Double thickness for higher current capacity
• 3 ounce (105 μm) – Thicker power planes for power supply routing

Bonding adhesive

• Epoxy resin – Popular prepreg bonding material
• Acrylic adhesive – Used for bonding aluminum substrates
• Phenolic butyral – Flexible PCB adhesive

What are the main PCB lamination process steps?

A multilayer PCB goes through the following main lamination steps:

1. Surface preparation

The outer surfaces of the copper foils are cleaned by chemical etching to remove oxidation and surface contaminants. This enhances bonding adhesion.

2. Layer stack up

The individual PCB layers consisting of substrate dielectric and copper foils are precisely aligned and stacked up in the desired sequence. Alignment is critical for connecting matching traces between layers.

3. Book binding

The aligned PCB layer stack is clamped tightly together from the sides to prevent any shifting between layers during lamination.

4. Lamination bonding

The bound stack is loaded into a lamination press machine and goes through a high temperature and pressure bonding cycle:

• Temperature is ramped up to around 180°C to melt the prepreg epoxy resin.
• Pressure up to 1000 PSI squeezes the stack to force intimate bonding between layers.
• After holding at temperature for sufficient time, it is slowly cooled to solidify the epoxy.
• Pressure is maintained during cooling to prevent layer separation or voids.

This heating, cooling and compression permanently fuses the PCB layers into one solid board.

5. Trimming

The fully laminated PCB panel is trimmed to cut off excess material from the sides and give finished board dimensions.

6. Testing

Testing verifies electrical connectivity between layers as per the circuit design to validate lamination quality.

What are the different lamination processes?

There are several PCB lamination techniques:

Standard multilayer lamination

This uses the full lamination bonding cycle with heat, pressure and adhesive to permanently fuse layers together. It provides the strongest and most reliable lamination suitable for most applications.

Oxide bonding lamination

This process relies on chemical bonding achieved by heating the oxidized copper surfaces without applying adhesive between layers. The technique produces PCBs with minimum layer thickness but has lower bond strength.

Thermal bonding lamination

Only heat and pressure are applied between laminate layers to partially melt the substrates like polyimide or thermoplastic dielectric temporarily. It softens the materials to fuse layers together but results in weaker lamination strength.

Adhesive lamination

Adhesive resin films like prepreg are first completely cured onto individual substrates under heat and pressure. The dry adhesive layers are then used to bond metal and dielectric layers together into a multilayer board by applying heat, pressure and vacuum.

Single stage lamination

All laminate materials, metals, resins and additives are stacked together raw into a book bound stack. A single stage lamination process fully cures the adhesive resin while bonding all materials into a finished PCB.

What are the key process parameters for PCB lamination?

Key PCB lamination parameters:

Temperature

• Typically 140°C to 180°C to liquefy resin adhesive
• Higher temperature improves resin flow but can over-cure resin prior to bonding

Pressure

• Around 300 PSI to 1000 PSI
• High pressure squeezes out air bubbles and brings layers in intimate contact

Pressure ramp up/down rate

• 2 PSI to 5 PSI per minute
• Gradual pressure change prevents layer damage

Holding time

• 45 to 90 minutes at curing temperature
• Ensures adhesive is fully liquefied and cured

Cooling rate

• 2°C to 5°C per minute
• Slow cooling without layer shifting until resin solidifies

Copper surface roughness

• Smooth surfaces with Ra under 0.2 um provides best lamination strength
• Rougher surfaces need more resin to bond properly

How is bonding pressure applied during lamination?

There are different ways in which pressure is applied during the multilayer PCB lamination process:

Hydraulic press

This uses heated metal plates compressed together by hydraulic rams. The PCB stack is placed between the hot plates, allowing simultaneous heat and pressure application. Hydraulic pressure is evenly distributed across the panel surface.

Vacuum press

Vacuum pressure brings the hot press plates together instead of hydraulic rams. It eliminates potential leakage of hydraulic oil contaminating PCBs. The surrounding vacuum aids in evenly spreading resin across layers.

Autoclave

The layer stack is placed in a pressure vessel and subjected to heat, pressure and vacuum. The enclosed high pressure promotes excellent resin flow between layers. Overpressure bleeds out excess resin preventing starved bonds. Uniform pressure distribution is achieved.

Thermal laminators

Moving heated rollers apply heat and pressure progressively across a continuously fed PCB panel sandwiched between top and bottom rollers. The mechanical pressure squeezes the layers together as heat from the rollers cure the resin adhesive.

Press plates

These are heated metal plates which clamp together to apply focused pressure and heat. Press plates are typically used for flexible PCB lamination. The precise localized pressure and customizable plate shapes accommodate flexible circuits.

What are the different PCB lamination bond defects?

Some potential bond defects occurring during PCB lamination:

Voids

Air gaps trapped between layers due to insufficient resin or pressure. It leads to localized weak spots prone to delamination under stress.

Wrinkles

Uneven cooling or handling which causes distorted or wrinkled layers. It impacts dimensional stability and layer registration.

Delamination

Separation between bonded layers from resin curing issues or mechanical stresses. It manifests as cracks or lifting at layer interfaces.

Poor resin flow

Inadequate resin distribution observed as dry spots lacking cohesive strength between layers. Starved bonds lead to cracks or crazing.

Bond line shift

Misalignment between layers from shifting during lamination under pressure. It scrambles the circuit layer registration.

Resin seepage

Excess uncured resin bleeding out of the sandwich construction and fouling surfaces. It affects dimensional accuracy and probe testing.

Blisters

Localized delamination spots that bubble out from the board surface when gases get trapped within layers and expand during heating.

How are lamination process parameters optimized?

Lamination process parameters are optimized through:

Trial runs

Performing test lamination builds to assess parameters like temperature, pressure and ramp rates for each PCB design. It establishes the ideal combination for robust bonding strength.

Microsection analysis

Cross-sectioning sample boards after lamination and examining under microscope reveals bonding characteristics between layers. Analysis qualifies resin curing, void defects and layer to layer adhesion.

Peel strength testing

Measuring the force needed to separate laminated layers gives quantification of bond strength. Peel strength indicates resin cohesiveness and adhesion to copper foil.

Thermal stress testing

Subjecting PCB test coupons to repeated temperature cycling identifies any latent defects through delamination or cracking. Thermal stress qualification confirms lamination parameters.

Resin flow analysis

Methodically adjusting temperature, pressure and time while analyzing resin spread gives the optimal lamination viscosity target. Well-cured resin should spread evenly and bleed out slightly without filling vias.

Electrical testing

Checking electrical connectivity across layers verifies insulation resistance provided by the resin while also detecting any internal shorts from insufficient resin fill.

How does the laminate quality affect PCB performance?

High quality lamination is crucial for robust, reliable PCBs that can endure operational stresses over long periods. Following are the performance impacts:

Bond strength

• Strong cohesive laminate bonding prevents delamination induced failures under thermal and mechanical stresses.

Dimensional stability

• Complete resin curing and layer adhesion ensures the PCB maintains precise dimensions without distortion over time.

Signal integrity

• Good lamination minimizes electrical losses and prevents stray coupling between traces for clean high-speed signals.

Current handling

• Eliminating inter-layer voids or cracks maintains continuous current paths in power distribution layers.

Thermal dissipation

• Quality lamination provides effective heat conduction between layers to transfer heat away from components.

Moisture absorption

• Superior moisture resistance prevents electrical leakage and detrimental hygroscopic swelling.

Reliability

• Stringent lamination process control results in robust PCBs able to function flawlessly over decades.

FQA

Q1. Why is lamination an important process for manufacturing multilayer PCBs?

Lamination is critical for multilayer PCB fabrication because:

• It provides electrical isolation between conducting copper layers by sandwiching insulating dielectric substrates in between.
• Layering of materials like FR4 gives the PCB structural rigidity.
• Additional circuit layers can be added through lamination to increase density.
• Embedding of components becomes feasible by laminating them into the PCB.
• Overall it enables construction of complex, reliable multilayer PCBs.

Q2. What are the main defects occurring during PCB lamination?

Typical PCB lamination defects:

• Voids or air gaps between layers due to insufficient resin or pressure
• Wrinkling or distortion of layers from uneven cooling
• Delamination or separation at layer interfaces
• Poor resin fill leading to inadequate bonding
• Misalignment from layer shifting during lamination
• Excess resin seepage affecting accuracy
• Trapped gases causing blisters

Q3. What are the key process parameters controlled during PCB lamination?

Key PCB lamination process parameters:

• Temperature of 140°C to 180°C to liquefy resin
• Pressure between 300 PSI to 1000 PSI
• Pressure ramp up and down rate
• Resin curing time at peak temperature
• Cooling rate after curing
• Copper surface roughness

Q4. How is bonding pressure applied during multilayer PCB lamination?

Bonding pressure is applied using:

• Hydraulic presses with heated plates
• Vacuum presses
• Autoclaves providing vacuum assist
• Thermal laminators with moving heated rollers
• Press plates for flexible PCB lamination

Q5. How is the PCB lamination process optimized and controlled?

PCB lamination process control involves:

• Trial lamination runs to finalize parameters
• Microsection testing to inspect layer adhesion
• Peel strength testing to quantify bond strength
• Thermal stress testing to confirm robustness
• Resin flow analysis for optimum distribution
• Electrical testing to verify insulation resistance