Electronics Guide

Laminate Materials and Construction

Organic substrates are constructed from layers of copper foil laminated with organic dielectric materials, primarily epoxy-based resins reinforced with glass fibers or other strengthening materials. The most common materials include FR-4 for standard applications, BT (bismaleimide triazine) resin for enhanced thermal and electrical performance, and ABF (Ajinomoto Build-up Film) for high-density build-up layers.

The lamination process involves stacking multiple layers of prepreg (pre-impregnated fiber sheets) with patterned copper foils, then applying heat and pressure to cure the resin and bond the layers. Through-holes and microvias connect layers, creating three-dimensional routing networks. The mechanical drilling of through-holes and laser drilling of microvias enable increasingly fine interconnect features.

Electrical Considerations

Organic dielectrics exhibit frequency-dependent properties that become increasingly important at higher operating speeds. The dielectric constant (Dk) and dissipation factor (Df) determine signal propagation velocity and losses. Low-loss materials with Df values below 0.005 are essential for high-frequency applications such as 5G communications and high-speed data centers. Material selection must also consider dielectric constant stability across temperature and frequency ranges.

Signal integrity in organic substrates requires careful attention to impedance control, achieved through precise management of trace width, dielectric thickness, and copper weight. Differential pair routing, ground plane design, and via stub management minimize crosstalk and reflections in high-speed designs.

Thermal and Mechanical Properties

The coefficient of thermal expansion (CTE) mismatch between organic substrates (typically 15 to 18 parts per million per degree Celsius) and silicon die (approximately 3 parts per million per degree Celsius) creates stress during thermal cycling. This mismatch drives the need for careful design of underfill materials and bump structures in flip-chip packages to ensure long-term reliability.

Glass transition temperature (Tg) defines the upper operating limit where the polymer transitions from rigid to rubbery behavior. High-Tg materials exceeding 180 degrees Celsius are required for lead-free soldering processes and demanding thermal environments. Moisture absorption can reduce Tg and cause delamination, making moisture management critical during manufacturing and storage.

Material Advantages

Glass provides exceptional dimensional stability with near-zero moisture absorption and low thermal expansion coefficients that can be tailored to match silicon. This stability enables tighter tolerances in high-density interconnects and reduces warpage issues that plague organic substrates. The smooth surface finish achievable with glass supports finer lithographic patterning than the rough surfaces of woven glass reinforcement in organic laminates.

Electrical properties of glass are excellent for high-frequency applications, with low and stable dielectric constants and extremely low loss tangents. The absence of woven reinforcement eliminates the fiber weave effect that causes impedance variations in organic substrates, improving signal integrity consistency.

Through-Glass Vias

Through-glass vias (TGVs) represent a key enabling technology for glass substrates, providing vertical interconnections with superior electrical characteristics to through-silicon vias. TGVs can be formed using laser drilling, sandblasting, or photosensitive glass processes, then metallized with copper or other conductors. The insulating nature of glass eliminates the need for dielectric liners required in through-silicon vias, simplifying fabrication and reducing parasitic capacitance.

High aspect ratio TGVs with depths exceeding 300 micrometers and pitches below 100 micrometers are achievable, supporting high-density interconnects in glass interposers. The mechanical strength of glass enables thin substrates below 100 micrometers while maintaining structural integrity.

Manufacturing Challenges

Glass substrate fabrication requires specialized equipment and processes different from organic substrate manufacturing. Handling brittle glass panels without breakage demands careful attention to edge finishing and support during processing. Metallization adhesion requires surface treatment and specialized seed layer deposition. Despite these challenges, manufacturing infrastructure is developing rapidly as major suppliers invest in glass substrate production.

Alumina Ceramics

Alumina (aluminum oxide, Al2O3) substrates represent the most widely used ceramic substrate technology, available in purities ranging from 92 to 99.6 percent. Higher purity alumina provides better thermal conductivity (25 to 30 watts per meter-kelvin) and lower dielectric loss, while lower purity grades offer cost advantages for less demanding applications.

Thick-film and thin-film metallization processes create conductors on alumina surfaces. Thick-film techniques screen-print conductive pastes followed by firing at high temperatures, while thin-film processes deposit and pattern metals using semiconductor-style photolithography. Thin-film achieves finer features and better line definition but at higher cost.

Aluminum Nitride

Aluminum nitride (AlN) ceramics provide thermal conductivity exceeding 170 watts per meter-kelvin, approaching copper while maintaining electrical insulation. This exceptional thermal performance makes aluminum nitride ideal for power semiconductor packages where efficient heat removal is critical. The thermal expansion coefficient of aluminum nitride closely matches silicon, reducing thermal stress in die attach applications.

Manufacturing aluminum nitride substrates requires careful control of sintering atmosphere to prevent oxidation and achieve maximum thermal conductivity. Cost premiums over alumina limit aluminum nitride to applications where its superior thermal performance justifies the expense.

Low-Temperature Co-fired Ceramics

Low-temperature co-fired ceramics (LTCC) enable multilayer ceramic substrates with embedded passive components and complex three-dimensional routing. The low firing temperature (below 900 degrees Celsius) permits co-firing with silver or gold conductors, which would melt at the higher temperatures required for alumina. This technology supports integration of capacitors, inductors, and resistors within the substrate, reducing component count and board area.

LTCC modules find extensive use in RF and microwave applications where the excellent high-frequency properties of ceramic dielectrics and the ability to create three-dimensional structures with controlled impedance lines provide performance advantages. Cavity structures within LTCC packages protect sensitive die while maintaining excellent electrical isolation.

High-Temperature Co-fired Ceramics

High-temperature co-fired ceramics (HTCC) based on alumina fired at temperatures exceeding 1500 degrees Celsius provide superior thermal performance compared to LTCC but require refractory metallization (tungsten or molybdenum) that has higher resistance than precious metals. HTCC remains important for high-reliability applications in aerospace and military systems where thermal performance and long-term stability take precedence over cost.

Polyimide Films

Polyimide films, particularly Kapton and similar materials, dominate flexible substrate applications due to their exceptional combination of flexibility, thermal stability, and chemical resistance. These films maintain their properties from cryogenic temperatures to above 300 degrees Celsius, enabling use in extreme environments from space to jet engines.

Copper-clad polyimide laminates form the basis of flexible printed circuits, with copper foils bonded to polyimide using adhesive layers or directly deposited without adhesive for enhanced flexibility and reliability. The thin profile of flexible circuits (typically 25 to 125 micrometers total thickness) contributes to weight savings critical in portable electronics and aerospace applications.

Liquid Crystal Polymer Substrates

Liquid crystal polymer (LCP) substrates offer lower moisture absorption than polyimide along with excellent high-frequency electrical properties. The low water uptake (less than 0.04 percent) maintains stable dielectric properties in humid environments where polyimide's higher absorption (2 to 3 percent) can cause dimensional and electrical changes. LCP's low loss tangent (around 0.002) supports millimeter-wave applications including 5G antenna modules and automotive radar.

Stretchable Substrates

Stretchable electronics extend beyond flexibility to enable substrates that elongate under strain while maintaining electrical connectivity. Elastomeric substrates based on silicone (PDMS) or thermoplastic polyurethane accommodate strains exceeding 100 percent through careful design of conductor patterns. Serpentine, mesh, and fractal conductor geometries distribute strain to prevent conductor failure during stretching.

Applications for stretchable substrates include skin-mounted biomedical sensors, conformable electronics for robotics, and stretchable displays. Challenges include maintaining consistent electrical performance under repeated strain cycles and integrating rigid components such as integrated circuits with stretchable interconnects.

Process Technology

The ETS process begins with creating copper traces on a carrier using conventional patterning. A dielectric layer is then laminated over the traces, embedding them within the material. After carrier removal, the exposed trace surfaces provide flat, coplanar bond pads ideal for fine-pitch flip-chip assembly. The process eliminates the trace height variations that can cause yield issues in conventional substrates.

Performance Benefits

The flat surface topology of ETS improves bump coplanarity control during flip-chip assembly, reducing defects related to non-contact opens and shorts. Fine-pitch capability extends to bump pitches below 100 micrometers, supporting advanced package designs. The embedded traces also benefit from reduced high-frequency losses due to smoother conductor surfaces compared to etched traces.

Applications

High-performance processors, graphics units, and application processors increasingly use ETS technology to enable the fine-pitch bumps required for growing die sizes and I/O counts. Mobile device packages particularly benefit from ETS as the technology supports the thin package profiles demanded by smartphone designs.

Cavity Formation Methods

Cavities in organic substrates can be formed through selective material removal using routing, laser machining, or chemical etching. Alternatively, build-up processes can create cavities by selectively omitting material in certain regions during layer fabrication. Each method offers tradeoffs in cavity wall quality, dimensional control, and cost.

Component Integration

Embedding components within substrate cavities reduces overall package height while providing mechanical protection and improved thermal pathways. Passive components such as capacitors and inductors can be integrated close to the die, reducing parasitic inductance and improving power delivery. Active devices embedded in cavities benefit from shorter interconnect paths and potential performance improvements.

Die Stacking Applications

Cavity substrates enable innovative die stacking configurations where dies mount at multiple levels within the package. The cavity provides clearance for wire bonds or allows face-to-face die mounting with reduced package height. Multi-die packages using cavity substrates can achieve higher integration density than side-by-side die placement.

Construction and Materials

Metal core printed circuit boards (MCPCBs) consist of a metal base layer (typically 0.5 to 3.0 millimeters thick) bonded to a thin dielectric layer and copper circuitry. The metal core conducts heat laterally to spreading regions and vertically to heat sinks. Aluminum cores offer good thermal performance at low cost, while copper cores provide approximately twice the thermal conductivity for demanding applications.

The dielectric layer between the metal core and circuitry must provide electrical isolation while minimizing thermal resistance. Thermally conductive dielectrics using ceramic fillers achieve thermal conductivities of 1 to 3 watts per meter-kelvin, significantly better than standard FR-4 at 0.3 watts per meter-kelvin.

Thermal Management Advantages

Metal core substrates can reduce junction-to-ambient thermal resistance by 50 percent or more compared to conventional FR-4 substrates. This improvement enables higher power dissipation from components, longer lifetime through reduced operating temperatures, or smaller heat sink requirements. The excellent thermal spreading in the metal core also improves temperature uniformity across the substrate.

LED Lighting Applications

High-brightness LED modules extensively use metal core substrates to manage the significant heat generated by power LEDs. The substrate provides both the electrical interconnect and the primary thermal path from LED junctions to heat sinks. White LED efficacy and lifetime depend strongly on junction temperature, making effective thermal management through metal core substrates critical for lighting product performance.

Power Electronics Applications

Power conversion modules, motor drivers, and power supplies use metal core substrates when power dissipation exceeds the capability of conventional organic substrates. Direct bonded copper (DBC) on ceramic substrates provides even higher thermal performance for the most demanding power electronics, but metal core substrates offer a cost-effective solution for moderate power levels.

Build-up Film Materials

Ajinomoto Build-up Film (ABF) dominates the high-performance organic substrate market, providing thin dielectric layers (10 to 40 micrometers) that support laser-drilled microvias with diameters below 50 micrometers. The unreinforced resin film achieves smooth surfaces suitable for fine-line patterning and low loss at high frequencies. Alternative build-up materials include photo-imageable dielectrics and spin-on materials for specific applications.

Semi-additive Patterning

Semi-additive processing (SAP) enables the fine lines and spaces (below 15 micrometers) required for advanced substrates. The process deposits a thin seed layer, patterns photoresist, electroplates copper in the exposed areas, strips the resist, and removes the seed layer by flash etching. This approach creates conductors with controlled geometry and smooth surfaces, unlike subtractive etching where undercut limits feature resolution.

Modified semi-additive processing (mSAP) achieves even finer features by using ultra-thin copper foil (less than 5 micrometers) as the seed layer. The reduced etching required to remove the thinner seed layer minimizes undercutting and line loss, enabling lines and spaces below 10 micrometers.

Layer Count Considerations

Advanced processor packages may include 12 or more build-up layers on each side of a core, creating substrates with 20 or more total layers. Each additional layer adds cost and potential yield loss, driving careful optimization of layer count versus routing density. Advanced design tools optimize layer assignments and via utilization to minimize layer count while meeting electrical requirements.

Microvia Technology

Microvias with diameters of 75 micrometers or less form the essential vertical interconnects in HDI substrates. CO2 laser drilling creates blind vias in organic dielectrics efficiently, while UV lasers achieve smaller diameters for the finest pitches. Stacked microvias, where vias in successive layers align directly above one another, maximize routing density but require careful copper filling to ensure structural integrity.

Via-in-pad designs place microvias directly under component pads rather than routing traces to offset vias. This approach reduces trace length and inductance while maximizing component placement density. Filled and planarized microvias provide flat surfaces for reliable component attachment over vias.

Design Rules and Capabilities

Leading HDI substrates achieve line widths and spaces of 8 micrometers or less with microvia pitches below 100 micrometers. These dimensions enable escape routing for flip-chip die with very fine bump pitches and support package-on-package stacking with high interconnect density. Design rules continue tightening as laser and lithography capabilities advance.

Applications in Advanced Packaging

Virtually all high-performance semiconductor packages now use HDI substrate technology. Mobile processor packages, graphics processing units, networking chips, and high-bandwidth memory interfaces all depend on HDI capabilities to connect dense die to the outside world. The substrate has become the limiting factor in many packages, driving ongoing investment in HDI technology advancement.

Technology Transition

SLP technology emerged from the need to route increasingly dense mobile processor packages on main PCBs. Traditional PCB processes with minimum features around 40 micrometers could not efficiently connect advanced packages. By applying substrate-level semi-additive processing to board fabrication, SLPs achieve line widths and spaces of 20 to 30 micrometers, bridging the gap between substrates and conventional PCBs.

Manufacturing Considerations

Producing SLPs requires investment in substrate-like equipment and processes, including thin-film seed layer deposition, advanced photolithography, and precise copper plating. Handling large panels (versus smaller substrate strips) presents additional challenges. Nevertheless, major PCB manufacturers have established SLP production for high-volume smartphone applications.

Design Benefits

SLP technology enables thinner, smaller main boards through higher routing density in fewer layers. Reduced board thickness and layer count directly translate to thinner smartphones and other portable devices. The finer features also improve high-speed signal integrity through tighter impedance control and reduced stub effects.

Electrical Requirements

Operating frequency dictates dielectric material selection, with higher frequencies requiring lower-loss materials to maintain signal integrity. High-speed digital designs demand controlled impedance and low crosstalk achieved through careful geometry control. Power delivery requires sufficient copper cross-section and low-inductance routing. Understanding the electrical environment guides appropriate substrate technology and material selection.

Thermal Requirements

Power dissipation levels determine whether organic substrates suffice or ceramic or metal core solutions are necessary. Thermal expansion matching affects long-term reliability, particularly for large die. Operating temperature range influences material selection, with high-temperature applications requiring polyimide or ceramic over standard organic materials.

Mechanical Requirements

Package form factor and application environment drive mechanical considerations. Flexible substrates enable non-planar applications but add complexity. Dimensional stability requirements affect material and process selection. Reliability under thermal cycling, mechanical shock, and vibration depends on appropriate material matching and design.

Cost and Volume Considerations

Substrate technology selection must balance performance requirements with economic reality. Organic substrates offer the lowest cost for mainstream applications. Ceramic and glass substrates command premiums justified by performance benefits in specific applications. Production volume affects process selection, with high volumes justifying investment in advanced processes that reduce unit cost.

Ultra-fine Line Substrates

Continued scaling demands substrate features approaching 5 micrometers and below. Advanced lithography, semi-additive processing improvements, and new materials enable this evolution. Direct pattern techniques eliminating photoresist may further extend scaling.

Glass Substrate Adoption

Glass substrates are transitioning from development to production for high-performance applications. Superior dimensional stability and electrical properties make glass attractive for chiplet-based systems requiring precise interconnect alignment. Major manufacturers are establishing glass substrate production capabilities.

Heterogeneous Integration

Advanced substrates increasingly serve as platforms for heterogeneous integration, connecting diverse chiplets in 2.5D and 3D configurations. This drives demand for finer features, embedded components, and novel structures such as bridges and embedded silicon. Substrates evolve from simple interconnects to active integration platforms.

Sustainable Substrates

Environmental considerations are influencing substrate development. Halogen-free materials address flame retardant concerns. Reduced copper usage lowers environmental impact. Recyclable and biodegradable materials are under investigation for appropriate applications. The substrate industry is working to reduce its environmental footprint while maintaining performance advancement.