An ultra-thin single side printed circuit board (PCB) refers to a highly miniaturized single-layer PCB with reduced thickness specially designed for applications with tight space constraints. These boards offer routing and component mounting on only one side while achieving extremely slim profiles.

Advancements in fabrication techniques and materials have enabled development of ultra-thin single side boards with thicknesses of just 0.1mm to 0.3mm compared to 1.6mm thickness of standard PCBs. The ultra-thin dimension allows fitting these boards in compact devices and packages.

This article provides an in-depth overview of ultra-thin single side PCB technology, manufacturing processes, design considerations, applications and advancements.

Need for Ultra-Thin PCBs

Many modern electronic devices demand ever-shrinking PCBs to accommodate more functionality in compact form factors. Some key needs driving adoption of ultra-thin PCBs include:

• Reducing weight and size of wearable devices like smart bands, hearables.
• Achieving sleeker industrial designs for smartphones and tablets.
• Enabling flexible/bendable PCBs for folding displays and devices.
• Packing more layers and components in stacked PCB assemblies.
• Developing thinner medical implants and ingestible devices.
• Increasing portability of IoT sensors, consumer gadgets.
• Adding electronic intelligence in credit cards, ID cards.
• Improving aerodynamics in drones by minimizing protrusions.

Ultra-thin PCBs meet these demanding requirements through extreme miniaturization not feasible with conventional PCBs.

Typical Thickness Range

Ultra-thin printed circuit boards cover a wide range of slimmed down thicknesses:

• Thick ultra-thin PCBs: 0.8mm to 0.4mm
• Midrange ultra-thin PCBs: 0.4mm to 0.2mm
• Super thin PCBs: 0.2mm to 0.1mm
• Extremely thin flex PCBs: <0.1mm

PCB thicknesses below 0.2mm are considered ultra-thin, with high complexity boards reaching down to 50-100 microns (0.05mm to 0.1mm).

In comparison, standard rigid PCBs are typically 1.6mm, while flex PCBs are around 0.25mm to 0.3mm thick. Ceramic substrates can also be thinned down to 0.3-0.5mm for ultrathin boards.

Construction of Ultra-Thin PCBs

Ultra-thin PCBs can be constructed as single sided, double sided or multilayer boards. Here are some typical construction methods:

Substrate Materials

• Polyimide – Most common is Kapton polyimide which can be produced in 12.5 to 75 micron sheets. Enables flexible circuits.
• LCP (Liquid Crystal Polymer) – Used as an ultra-thin rigid substrate with low moisture absorption. Builds up to 4 metal layers are possible.
• Modified FR4 – Glass fabric in the core dielectric is replaced with thinner woven glass or non-woven aramid fibers.
• Ceramics – Alumina or aluminum nitride cofired ceramic substrates thinned to below 0.5mm.

Layer Count

• Single sided – Only one layer of copper tracings, lowest cost.
• Double sided – Tracings on both sides but no inner layers. Via interconnects tracks.
• Multilayer – 4-6 conductive layers interleaved using advanced lamination.

Tracking and Spacing

• Tight trace/space – Below 100 micron lines/spaces.
• Laser etched microvias – To interconnect layers with small pad diameters.
• Thin coverlay- As low as 12.5 microns to cover surface tracks.

Surface Finishes

• Immersion silver or gold – Provide excellent solderability on exposed pads.
• Organic solderability preservatives – Help temporary protection of traces.

Coatings

• Epoxy or acrylic coating – Help reduce flexing stress and prevent cracking.
• Nano-coatings – Can be applied to strengthen surface and extend flex life.

Fabrication Process for Ultra-Thin PCBs

Producing ultra-thin printed circuit boards requires specialized fabrication techniques:

Substrate Preparation

• Bonding thin sheets like 12.5 micron Kapton using adhesive.
• Press curing laminate stack under high temperature and pressure.

Drilling

• Using small diameter drills and high drill speeds.
• Employing novel techniques like laser and plasma drilling.

Metallization

• Sputtering deposition to build up copper thickness.
• Electroless plating of thin copper without electrodes.

Patterning

• Using ultra-fine line lithography and etching processes.
• Laser direct imaging (LDI) avoids use of artwork films.

Plating

• High precision electrolytic plating to build copper thickness.
• Tight process controls to achieve uniformity.

Testing

• Specialized micro-sectioning and AOI techniques to verify layer alignment.
• X-ray imaging to check ultra-thin inner layer continuity.

The combination of specialized materials, fabrication tools, and techniques enables reliable volume production of ultra-thin PCBs.

Design Considerations for Ultra-Thin PCBs

Designing ultra-thin PCBs require attention to:

Component Selection

• Prefer small outline components like 0201 or 01005 chips.
• Choose components rated for reflow temperatures compatible with the thin laminate.
• Utilize bare die or flip chips for high density.

Layout Optimizations

• Use gridded layout with components placed in rows/columns.
• Optimize trace widths and clearances to achieve high routing density.
• Use blind/buried vias to transition between layers.
• Eliminate non-critical connections to reduce layer count.

Board Stiffening

• Incorporate thin stiffeners made of carbon fiber or aluminum.
• Use edge plating techniques to strengthen periphery.
• Specify board chamfers or rounded corners to eliminate stress points.

Warpage Control

• Minimize asymmetric layouts and uneven copper densities.
• Use techniques like balanced metal layers and filled microvias.
• Incorporate strain relief flex points on large boards.

Testability

• Include probe/test points for validating inner layers.
• Allow edge access to critical signals.
• Ensure coverlay openings for loaded components.

Assembly Considerations

• Specify minimal soldermask web thickness to enable fine pitch SMT.
• Use adhesive staking to strengthen component hold down.
• Plan for specialized component rework methods.

By proactively addressing these design aspects, reliable and manufacturable ultra-thin PCBs can be developed.

Applications of Ultra-Thin PCBs

Some common application areas for ultra-thin printed circuit boards include:

Wearable Electronics

• Smart watches, bands
• Fitness trackers
• Health monitoring patches

Mobile Devices

• Smartphones
• Laptops, tablets and ultrabooks
• Foldable/bendable devices

Medical Devices

• Implantable devices
• Ingestible diagnostic capsules
• Wearable health aids

IoT and Consumer Electronics

• Smart cards
• Activity trackers and tags
• Low profile consumer gadgets

Industrial Electronics

• Surveillance cameras
• Inspection equipment
• Machine vision systems

Automotive Electronics

• Automotive displays and touchscreens
• Body controllers
• Vehicle tracking and toll transponders

Aerospace and Aviation

• Onboard avionics
• UAV electronics

Military and Defense

• Soldier-worn systems
• Missile and ammunition guidance
• Compact radar and communication hardware

The combination of small footprint and light weight of ultra-thin boards allows packing more intelligence and functionality in miniaturized next-gen electronic products across these industries.

Advanced Technologies for Ultra-Thin PCBs

A range of advanced manufacturing technologies are leveraged to improve capabilities and densities of ultra-thin PCBs:

Laser Drilling

Allows drilling micro vias with diameters below 50 microns in multilayer boards. Eliminates smear and debris issues.

Conductive Ink Printing

Printing tracks and fills using novel silver/copper inks allows simpler and lower cost ultra-thin PCB fabrication.

Embedded Actives and Passives

Components like capacitors and resistors can be formed within the board layers by techniques like thin film deposition.

Sequential Buildup

Building interconnect layers sequentially by additive plating avoids using laminated cores. Enables high density boards below 100 microns thickness.

Flex-Rigid Integration

Combining rigid sections for components along with flexible substrate regions. Allows folding and 3D packaging.

Direct Chip Attach

Attaching bare semiconductor dies directly onto boards avoids bulky packaging. Provides ultrathin profiles.

Stretchable Substrates

Use of novel stretchable materials allows elastic interconnections for wearable devices.

Chip Embedding

Techniques like laser micromachining allow complete embedding of active and passive chips inside ultra-thin boards.

3D Printing

Additive manufacturing methods like inkjet and aerosol are maturing for direct printing of dielectric and conductive materials to form boards.

These technologies will enable easier fabrication along with expanded capabilities for ultra-thin PCBs.

Ultra-Thin PCB Characteristics Comparison

Here is a comparison of key characteristics for standard PCBs versus ultra-thin PCBs:

Parameter	Standard PCB	Ultra-Thin PCB
Board Thickness	1.6mm typical	<0.2mm
Layers	Up to 32+	Typically single or dual sided
Line Width/Space	100 microns typical	50 microns or below
Materials	FR4 glass epoxy	Polyimide, LCP, ceramic
Fabrication	Multi-step lithography	Laser drilling, additive buildup
Processes	High accuracy required	Specialized micro-machining
Density	Component spacing driven by thickness	Extremely dense assemblies
Assembly	Reflow soldering	Adhesive mounting, direct chip attach
Rework	Standard methods	Challenging, micro-rework
Applications	General purpose	Compact, portable devices
Cost	Low to moderate	Higher

In summary, specialized materials, processes and design rules are required to achieve miniaturized PCBs using ultra-thin technology compared to standard boards while enabling advanced applications.

Future Outlook for Ultra-Thin PCBs

Emerging and future trends shaping ultra-thin PCB technology development include:

• Expanding into medical implants and ingestible electronics.
• Allowing increased functionality in wearable devices through multilayer ultra-thin boards.
• Achieving bending radii below 1mm by using novel flexible and foldable materials.
• Scaling up fabrication using roll-to-roll processing and large panel sizes.
• Increasing adoption of printed electronics methods beyond conventional lithography processes.
• Integrating more passives and actives inside the ultra-thin board substrate.
• Embedding bare semiconductor dies and stacks in cavities in the board.
• Combining with flexible hybrid electronics manufacturing allowing stretchable circuits.
• Leveraging micro-dispensing and direct digital manufacturing techniques.
• Expanding applications in consumer electronics like mobiles, laptops, and IoT devices.

With demands for miniaturized and wearable devices increasing, ultra-thin PCB technology will continue advancing to meet the needs of next-generation electronics across industries.

Conclusion

In conclusion, ultra-thin single side PCBs with thicknesses below 0.2mm provide transformative capabilities for electronics miniaturization. Polyimide is the most commonly used substrate allowing flexible thin circuits. Several additive manufacturing technologies are maturing to fabricate multilayer thin boards.

Demand from applications like wearables, medical devices, cards, and avionics drives ultra-thin PCB development. While fabrication complexity and costs are higher, the benefits like ultralight construction, space savings, and flexibility outweigh the challenges for advanced electronics products.

With progress in processes like inkjet printing, laser machining, and chip embedding, ultra-thin PCBs will become mainstream in the future. They promise to be a disruptive platform enabling design innovation across several industries.

Frequently Asked Questions

Q1. Can passive components be incorporated in ultra-thin PCBs?

Yes, techniques like thin film resistor deposition and parallel plate capacitor formation allow fabrication of embedded passives like resistors and capacitors as part of the ultra-thin PCB stackup.

Q2. What are some key design software tools used for ultra-thin PCBs?

Specialised PCB layout tools like Mentor Expedition, Cadence Allegro, and Altium Designer offer extensive capabilities to design and validate ultra-thin boards addressing factors like thermal dissipation, manufacturability and reliability.

Q3. What are some limitations of ultra-thin PCBs?

Limitations include reduced current carrying capacity, challenges in dissipating heat from high powered components, lower tolerance to vibration/shock, susceptibility to warpage, constraints on component types and reduced reworkability.

Q4. How are components assembled on ultra-thin PCBs?

Reflow soldering of conventional SMT packages can be used for thicker boards above 0.2mm. For lower thicknesses, flex assembly processes like soldering, conductive epoxy adhesive and direct chip attach are employed.

Q5. What types of materials are used for reinforcing ultra-thin PCBs?

Polymer composite materials impregnated with fiberglass strands or carbon fiber cloth can be integrated to impart strength. In some cases, thin aluminum or stainless steel sheets are bonded to the boards to enhance rigidity