What is a printed circuit board?

A printed circuit board (PCB) is the foundation of an electronic device. It provides the mechanical structure to mount and interconnect electronic components using conductive pathways or traces etched from copper sheets laminated onto a non-conductive substrate. PCBs act as the wiring framework to connect various components like integrated circuits, resistors, capacitors, etc. together to perform the intended functions.

PCBs went through several design changes over the years, starting from single-sided to double-sided and eventually multilayer boards. But the concept of laminating conductive copper onto insulating substrates remained constant in PCB technology.

What is lamination in PCB context?

Lamination is the technique of bonding or fusing multiple layers together by applying heat, pressure, and adhesives. The lamination process allows the creation of multilayer PCBs by adhering copper foil onto insulating substrate materials.

The lamination process entails three main steps:

1. Layer preparation

The individual layers consisting of copper foils and substrate materials are prepared first.

The substrate materials are typically fiberglass-reinforced plastic known as FR-4 in most PCBs. Other substrates like polyimide and ceramic are also used for flexible PCBs and high-frequency PCBs respectively.

The copper foils come with adhesive backing to facilitate bonding with the substrates. A common adhesive is epoxy resin prepreg, which is a fibreglass sheet pre-impregnated with adhesive epoxy.

2. Layer stacking

The prepared layers are precisely aligned and stacked together in a sandwiches structure, with copper layers alternating between substrates. The layer stack aligns the copper traces between matching connection points across layers.

3. Lamination bonding

The layer stack goes through a lamination press, where it undergoes high temperature and pressure. The combination of heat, pressure and adhesive resin liquifies to bond the layers together permanently after it solidifies during cooling.

The resulting multi-layered PCB allows conductive pathways spanning across all layers, enabling complex circuit networks within a compact PCB area.

Why laminate PCBs?

Laminating sheets of copper onto insulating substrates to create PCBs has several advantages:

Electrical isolation

Lamination provides electrical isolation between copper layers by using insulating substrate as a dielectric barrier. This allows traces to pass over each other on different layers without shorting. Discrete signals can crossover each other to minimize layer usage.

Structural rigidity

Layering substrates like FR4 reinforce the PCB to prevent warping or flexing during manufacturing or operation. A robust PCB structure allows reliable functionality for components mounted over time. The layering creates sufficient stiffness for most PCBs.

Multi-layer circuits

Laminating additional copper layers multiplies circuit capacity within the same footprint area. Large complex circuit networks can be designed by routing traces over multiple layers for high-density PCBs. Components placement also becomes flexible with more routing layers.

Shielding ability

The insulating substrates act as shielding barriers between layers. It allows separation of analog and digital circuits to prevent noise interference when routing on different layers. Sensitive circuits can also be shielded by a copper ground plane around them.

Thermal performance

The dielectric substrate helps conduct heat away from hot components mounted on the surface layers. Inner power planes also facilitate heat dissipation in multilayer PCBs. This allows better thermal management of heat generating parts.

Cost effectiveness

The lamination process provides an affordable way to manufacture PCBs compared to exotic PCB types. Lamination techniques have also matured over decades to be mass production capable for economical PCB fabrication.

Reliability

Mature lamination processes result in robust PCBs with stable long-term performance. The high durability allows PCBs to function reliably in demanding operating conditions over time. Stress testing during manufacturing also qualifies the lamination quality.

What are the multilayer PCB lamination process steps?

1. Surface preparation

The outer surfaces of the stacked layer pack are prepared by cleaning off any oxidation or contaminants. For multilayer PCBs, this is done by chemical etching of the outermost copper foils. This cleans the surfaces and improves bonding adhesion.

2. Layup stacking

The individual PCB layers consisting of substrates like FR4 and copper foils are precisely aligned and stacked up in the desired sequence. Alignment is critical for connecting matching traces between layers during lamination.

3. Book binding

The aligned PCB layer stack is clamped together carefully from the sides to hold everything in place. This prevents any shifting between layers during lamination. It prepares the layer pack for insertion into the lamination press.

4. Lamination press cycle

The bound layer pack is loaded into a lamination press machine and undergoes high temperature and pressure:

• Temperature is increased to around 180°C to liquify the epoxy resin and other bonding materials within the stack.
• Pressure of up to 1000 PSI squeezes the stack tightly to force bonding between layers.
• The temperature is held for time sufficient to allow cured bonding throughout the layer interfaces.
• It is slowly cooled to solidify the bonding material back into place.
• Pressure is maintained during cooling to ensure no separation or voids.

This heating, cooling and compression cycle permanently laminates the PCB layer stack into a unified board.

5. Trimming

The fully laminated PCB panel is trimmed to cut off any excess material from the sides to give it desired dimensions. Additional drilling and routing can be done to form finished PCB shape.

6. Testing

The laminated PCB panel undergoes electrical testing to verify connectivity between layers as per the design. This qualifies the lamination quality before further PCB fabrication steps.

What are the different types of PCB laminates?

PCB laminates can be categorized based on:

Substrate material

Popular substrates include:

• FR-4 – Most common rigid PCB material with good electrical and mechanical properties. FR-4 is a glass-reinforced epoxy laminate material.
• Polyimide – Used for flexible PCBs. Can withstand bending and flexing stresses.
• PTFE – High frequency microwave PCBs benefit from PTFE’s low dielectric loss.
• Ceramic – Provides thermal stability and performance needed by PCBs for extreme environments.
• Aluminum – Dissipates heat efficiently away from electronics. Used for LED light boards.

Copper thickness

Common copper foil thickness in multilayer PCBs:

• 1oz – Smallest at 35um. Suitable for tighter trace spacing and clearance.
• 2oz – Twice the thickness at 70um for higher current capacity.
• 3oz – 105um thick for power planes to handle high amperage.

Thicker copper requires wider spacing between traces and holes.

Layer count

Number of conductive copper and dielectric substrate layers:

• Two-layer – Most basic with only top and bottom copper layers sandwiching a substrate.
• Four-layer – Inner power and ground planes added for better performance.
• Six or more layers – Provides high density interconnection for complex circuit boards .

More layers accommodate larger and complex circuits within the same area.

What are the common PCB laminate substrates?

FR-4

Flame retardant grade 4 glass epoxy is the most popular and cost-effective rigid PCB substrate. The features of FR-4 include:

• Woven fiberglass cloth strengthening for structural rigidity. Varying cloth weaves (loose or tight) provide different degree of stiffness.
• Flame resistant epoxy resin binds everything together providing mechanical and electrical integrity.
• High thermal stability retains integrity through PCB assembly soldering steps.
• Suitable dielectric properties support high-speed signals. Low moisture absorption.
• Good thermal conductivity helps dissipate heat from components mounted on board.
• Well controlled thickness across industry standards enables competitive fabrication.
• Mature industry adoption guarantees availability and affordability.

Polyimide

Polyimide laminates are used for flexible PCBs which require bending and flexing capability. Notable characteristics are:

• Flexible substrate can wrap around objects and survive repeated bending stresses.
• Polyimide material has high temperature tolerance allowing soldering.
• Lightweight structure desired for small flexible circuits.
• Variants like Kapton provide excellent electrical insulation properties.
• Cost is higher than FR-4 but offset by design benefits.
• Used in consumer electronics, medical and industrial sensors needing flexible construction.

PTFE (Teflon)

Polytetrafluoroethylene (PTFE) substrates find application in high frequency analog PCBs due to attributes:

• Extremely low dielectric constant provides low capacitance between traces.
• Low dielectric losses or dissipation factor proves vital for high frequency boards above 500MHz.
• Excellent electrical and temperature insulation properties.
• PTFE laminates withstand soldering heat during component assembly.
• Chemical resistance to acids, bases and solvents. Easy cleaning.
• Downside is higher material cost but important for RF microwave and telecom PCBs.

Ceramic

Ceramic makes a niche PCB substrate where the utmost in thermal and dimensional stability is needed:

• Alumina ceramic withstands extremely high temperatures beyond soldering. Zero water absorption.
• Does not deteriorate or deform under wide temperature fluctuations required in some electronics.
• Excellent thermal conductivity to dissipate heat from high power components.
• Dense structure provides outstanding dimensional stability crucial for precision boards.
• Insulating dielectric properties support high frequency and high voltage operation.
• Costs significantly higher than conventional laminates but vital for specific applications.
• Used in aviation, space, medical and oil/gas level electronics requiring sturdy performance.

Aluminum core

Aluminum substrates are unique for LED lighting PCBs which require heat dissipation without using cooling fans:

• Aluminum core conducts heat excellently away from densely packed LEDs generating significant heat.
• LED lamps can be packed tightly and driven strongly without overheating.
• Efficient heat dissipation allows higher lumen output from LED lamps.
• Aluminum laminate retains rigidity from the metal core instead of dependence on fiberglass.
• Insulating coatings like Aluminum oxide provide dielectric isolation over aluminum.
• Downside is higher cost but important for thermal operating needs.

What are the key PCB laminate properties?

Some key laminate parameters:

Dielectric constant (Dk)

It indicates the insulating properties of the laminate material. It determines how tightly electric flux lines are bound in the substrate. Lower Dk allows faster propagation of signals by reducing capacitive coupling between traces. It improves high frequency performance. Typical DK values are 4.2 for FR4; 3.4 for polyimide; and 2.1 for PTFE.

Dissipation factor (Df)

It measures the inherent dielectric losses due to material resistance. Lower Df reduces signal losses especially at high frequencies for better performance. FR4 has Df of 0.02 while PTFE has 0.002 making it advantageous for microwave PCBs.

Thermal conductivity

Ability of the substrate to conduct heat away from components. Higher thermal conductivity allows better heat dissipation from hot parts. Aluminum has the best thermal conductivity followed by ceramic for high power PCBs. FR4 is poorer by comparison.

CTE – Coefficient of thermal expansion

Amount of expansion/contraction exhibited by material with temperature changes. Close CTE match between laminate, copper and mounted components prevents warping under temperature cycling. Aluminum or Invar clad core boards provide excellent CTE stability.

Tg – Glass transition temperature

It is the temperature where laminate transitions from rigid to rubbery state impacting construction. PCB laminates are chosen with Tg higher than soldering temperature to withstand assembly heat without deforming.

Moisture absorption

Tendency of material to absorb atmospheric moisture affecting electrical and mechanical properties. Lower moisture absorption ensures stable PCB performance in humid conditions. Epoxy resin in FR4 absorbs more moisture compared to polyimide or PTFE.

What are the different types of multilayer PCBs by lamination technology?

PTH multilayer PCBs

PTH or plated-through-hole multilayer PCBs have been the mainstay of circuit boards for decades owing to their robustness and reliability.

• Holes are drilled through the entire PCB layer stack.
• Plated copper coats the hole barrels to connect individual layers together.
• Solder mask lacquer coats the PCB providing insulation and markings.
• Components are leaded and mount through the plated holes onto the board.
• Mature laminating and plating technology provides stable, long-lasting PCBs.
• Cost effective volume manufacturing with tight tolerances makes them ideal for commercial electronics.

Thin core multilayer PCBs

These use new lamination techniques to sandwich ultra-thin core substrate between copper layers.

• Core substrate like 25um thick polyimide dielectric supports very high layer counts.
• Allows packing more copper circuit layers within the same PCB thickness.
• Enables finer line, space and hole dimensions for higher routing density.
• Thinner dielectric provides improved electrical performance for high-speed signals.
• More complex boards can be designed within smaller footprint area.
• Suits the increasing interconnection needs of advanced electronics.

Any-layer PCBs

Any-layer PCBs can integrate components into inner layers instead of just relying on surface mount.

• Active and passive components like capacitors and resistors are embedded within the board.
• Components occupy empty spaces between copper layers or replace inner core material.
• Provides electrical proximity benefits due to shortest distance to components.
• Eliminates discrete components saving considerable board space.
• Reliability improves from fewer solder joints which may fail over time.
• Enhanced electrical performance from tight integration and elimination of lead inductances.
• Costs higher but vital for packing dense electronics in space constrained applications.

What are some considerations when selecting PCB laminate materials?

Some guidelines for selecting PCB laminate material:

• Operating frequency – High frequency analog boards benefit from specialized microwave laminates like PTFE or ceramic. FR4 suitable for digital below 500MHz.
• Thermal characteristics – High power boards (>10W) need aluminum or ceramic for heat dissipation. FR4 loses thermal conductivity beyond 2W/sq.in.
• Environmental – Hermetic sealed electronics for space or defense may use ceramic for zero moisture absorption. Automotive under-hood electronics can withstand high temperature with polyimide laminates like Kapton.
• Mechanical – Flex PCB applications dictate polyimide or flexible FR4 material. Rigid multilayer boards use FR4 reinforced with tight weave fiberglass.
• Board thickness – Thin cores like 25um polyimide allow very high layer count PCBs. Thick 1.6mm FR4 cores provide rigidity for backplanes.
• Compliance – UL approved FR4 material may be mandated for commercial end products. Some microwave materials require ROHS compliance.
• Reliability – Mature FR4 technology offers proven long term reliability. Ceramic and PTFE gaining acceptance through extensive qualification testing.
• Cost – Affordable FR4 accounts for majority of PCBs. For niche applications, performance merits justify premium pricing of advanced laminates.

FQA

Q1. What are some key advantages of laminating copper and dielectric substrates for PCBs?

Some key advantages are:

• Electrical isolation between copper layers
• Structural rigidity from layering materials like FR4
• Ability to create multilayer PCBs with complex circuitry
• Shielding ability by separating layers with insulating dielectric
• Good thermal performance through layer conduction
• Cost effective manufacturing using mature lamination processes
• High reliability from stable laminated structure

Q2. Why is lamination bonding done under high temperature and pressure?

Lamination bonding involves high temperature and pressure because:

• Heat softens adhesive resin to liquify and evenly coat bonding surfaces.
• Pressure compresses layers tightly together ensuring intimate contact for adhesion.
• Liquified resin is able to penetrate fabric weave and crevices for robust mechanical bonding.
• Bonded adhesive after solidifying back helps provide structural reinforcement.
• Combination of heat, pressure and adhesive permanently fuses layers into an integral unit.

Q3. What are the key considerations when selecting PCB laminate materials?

Key considerations for selecting PCB laminate materials:

• Operating frequency signals
• Thermal characteristics for heat dissipation
• Mechanical rigidity and flexibility needs
• PCB thickness constraints
• Environmental resistance required
• Compliance with standards like UL 94 V-0
• Manufacturing maturity and long-term reliability
• Cost tradeoffs for advanced materials

Q4. Why is FR-4 the most predominantly used PCB laminate material?

Reasons for extensive use of FR-4 laminate include:

• Good electrical properties like dielectric constant suits majority digital circuits
• Reinforced construction provides excellent mechanical rigidity
• High thermal stability to withstand soldering temperatures during assembly
• Strong adoption across industry standards ensures availability
• Very cost effective for volume PCB manufacturing
• Mature technology guarantees long-term reliability in electronics
• Easy to prototype and scale production using established fabrication processes

Q5. What are some emerging PCB laminate materials used today?

Some emerging laminate materials gaining wider adoption are:

• Thin core substrates like 25um polyimide to allow high layer count PCBs
• PTFE microwave composites for low loss RF applications
• Flexible FR4 material for rigid-flex designs
• Ceramic aluminum PCBs for thermal management
• Improved high frequency FR4 laminates
• CAF resistant laminates to prevent conductive anodic filaments
• Halogen free, UL 94 V-0 compliant laminates
• Hybrid laminates