CSP & WLCSP: Chip Scale Packaging for Miniaturization

The smartphone in your pocket contains dozens of integrated circuits, yet it’s thinner than ever. The smartwatch on your wrist packs more computing power than computers that once filled entire rooms. How? A significant part of the answer lies in chip scale package technology—packaging that’s barely larger than the silicon die itself.

As a PCB engineer, I’ve watched CSP packages transform what’s possible in electronics design. Where we once struggled to fit basic functionality into handheld devices, we now integrate complex systems into spaces measured in millimeters. This guide explains everything you need to know about chip scale packaging and its most advanced variant, WLCSP, from a practical design perspective.

What is a Chip Scale Package (CSP)?

A chip scale package (CSP) is an integrated circuit package where the package area is no more than 1.2 times the area of the semiconductor die inside. This definition comes from the IPC/JEDEC J-STD-012 standard. The key requirement: it must be a single-die, direct surface-mountable package.

Originally, CSP stood for “chip-size package,” implying the package was exactly the size of the chip. Since few packages achieve true 1:1 ratio, the industry adopted “chip-scale package” to reflect the reality that most are slightly larger than the die.

To put this in perspective: a traditional QFP (Quad Flat Package) might have a silicon density of only 10-30%—meaning the package is 3-10 times larger than the actual chip. A CSP package achieves 60-100% silicon density, dramatically reducing the space your component occupies on the PCB.

The concept emerged in 1993, proposed by Junichi Kasai of Fujitsu and Gen Murakami of Hitachi Cable. Mitsubishi Electric demonstrated the first working concept shortly after. By the early 2000s, chip scale packaging had become mainstream, driven by the explosive growth of mobile devices.

How CSP Technology Works

The fundamental innovation in chip scale packaging is eliminating or minimizing the traditional packaging infrastructure—bulky lead frames, long bond wires, and oversized plastic bodies. Instead, CSPs use advanced interconnection methods to connect the die directly to external terminals.

Basic CSP Structure

A typical CSP package consists of:

Die (Silicon Chip): The semiconductor device itself, often thinned to 50-200 micrometers for ultra-low profile packaging.

Redistribution Layer (RDL): A thin metal wiring layer that reroutes I/O connections from the original bond pad locations on the die to an optimized array of external connection points. This is key to converting perimeter-pad designs into area-array configurations.

Under Bump Metallization (UBM): A metal stack that provides a solderable surface on the die or RDL, typically consisting of adhesion, barrier, and wetting layers.

Solder Bumps or Balls: Tiny solder spheres that serve as the external connections, allowing the package to be surface-mounted directly to a PCB.

Protective Coating/Encapsulation: An optional polymer layer or molding compound that protects the die and interconnects from environmental damage.

Types of Chip Scale Packages

The industry has developed over 50 different CSP configurations, each optimized for specific applications. Here are the major categories:

Wafer-Level CSP (WLCSP)

The most compact CSP variant, where all packaging processes occur while the die is still part of the wafer. No interposer or substrate is added—the finished package is literally the die with redistribution layers and solder balls. WLCSP achieves true chip-scale dimensions (1:1 ratio).

Best for: Smartphones, wearables, IoT sensors, power management ICs

Flip-Chip CSP (FCCSP)

Uses flip-chip technology where the die is inverted and mounted face-down on a thin organic substrate using solder bumps. The substrate provides redistribution and mechanical support.

Best for: Medium I/O count devices (up to ~200 I/Os), applications requiring better board-level reliability than WLCSP

Leadframe CSP (LFCSP)

A copper leadframe serves as the substrate, with the die wire-bonded to the frame. Despite using wire bonds, careful design keeps the package within CSP dimensions.

Best for: Cost-sensitive applications, lower I/O counts, better thermal performance

Tape-Based CSP (T-CSP)

Uses flexible polyimide tape as the interconnect substrate. The die is mounted on the tape, wire-bonded, and encapsulated. The tape’s flexibility provides stress relief during thermal cycling.

Best for: Applications requiring good thermal fatigue resistance

Rigid Substrate CSP

Uses a thin laminate or ceramic substrate for redistribution. More robust than WLCSP but slightly larger.

Best for: Higher I/O counts, applications requiring additional routing layers

CSP Package Type Comparison

Understanding WLCSP in Detail

Wafer-Level Chip Scale Package (WLCSP) deserves special attention because it represents the ultimate in miniaturization. The term “wafer-level” indicates that all packaging steps—redistribution, bump formation, and testing—happen while the chips are still together on the silicon wafer. Only after packaging is complete is the wafer diced into individual units.

WLCSP Construction

A typical WLCSP structure, from bottom to top:

1. Silicon die with original aluminum bond pads
2. First polymer layer (P1) providing passivation and stress relief
3. Copper redistribution layer (RDL) routing signals from perimeter pads to area-array configuration
4. Second polymer layer (P2) insulating the RDL except at bump locations
5. Under bump metallization (UBM) providing solderable surface
6. Solder balls (typically lead-free SAC alloy) for PCB connection
7. Optional backside laminate protecting the die back surface

WLCSP Advantages

Smallest possible footprint: The package is exactly the die size—nothing smaller exists short of bare die assembly.

Lowest inductance: No bond wires or long substrate traces. The signal path from die to PCB is minimal, making WLCSP excellent for RF and high-speed applications.

Best electrical performance: Short interconnects mean lower resistance, lower inductance, and better signal integrity.

Good thermal performance: Heat transfers efficiently from the die through the solder balls directly to the PCB.

Cost-effective at volume: Wafer-level processing is highly parallel—thousands of packages are processed simultaneously.

WLCSP Limitations

Board-level reliability concerns: Without underfill, WLCSP solder joints are vulnerable to thermal cycling and mechanical stress. The coefficient of thermal expansion (CTE) mismatch between silicon (2.6 ppm/°C) and FR-4 (~16 ppm/°C) stresses solder joints during temperature changes.

Limited I/O count: Practical WLCSP devices typically have fewer than 200 balls. Higher counts require impractically small ball pitches.

PCB complexity: Fine-pitch WLCSP (0.4mm or less) requires HDI PCB technology with microvias.

Handling sensitivity: The bare die backside is exposed and relatively fragile.

CSP Ball Pitch and PCB Requirements

Ball pitch significantly impacts PCB design complexity:

For programmable devices like Altera FPGA products, CSP options provide compact alternatives to larger BGA packages for space-constrained designs while maintaining the necessary I/O density.

PCB Design Guidelines for CSP Packages

Successfully implementing CSP devices requires attention to several critical design factors:

Land Pattern Design

Pad type selection: Choose between Solder Mask Defined (SMD) and Non-Solder Mask Defined (NSMD) pads:

• NSMD provides better solder joint reliability for thermal cycling (recommended by most manufacturers)
• SMD offers better drop test performance due to the solder mask mechanically anchoring the pad

Pad sizing: Typically, pad diameter should match the solder ball diameter within ±5%. Consult the component datasheet for specific recommendations.

Solder mask opening: For NSMD pads, the mask opening should be 75-100 micrometers larger than the copper pad diameter.

Via-in-Pad Considerations

Fine-pitch CSPs often require via-in-pad to escape route inner balls:

• Vias must be filled and planarized (copper-filled preferred)
• Unfilled vias cause solder wicking and voiding
• Via diameter typically 100-150 micrometers for microvias
• VIPPO (Via-In-Pad Plated Over) is the standard approach

Routing Strategy

• Route outer balls on the top layer when possible
• Use microvias to access inner ball rows
• Maintain controlled impedance for high-speed signals
• Provide adequate ground vias for return paths
• Keep trace lengths short and balanced for differential pairs

Thermal Management

• Place thermal vias under thermal pads (if present)
• Connect ground balls to solid ground planes
• Consider copper pours for heat spreading
• For high-power devices, ensure adequate copper area on inner layers

CSP Assembly Process

Assembling CSP packages follows standard SMT processes with some specific requirements:

Stencil Design

• Stencil thickness: 0.1mm (4 mil) to 0.125mm (5 mil) typical
• Aperture ratio: Greater than 0.66 for good paste release
• Square apertures with corner radius (25 μm) recommended for fine pitch
• 1:1 aperture-to-pad ratio or slightly reduced

Solder Paste

• Type 4 or finer powder for pitches below 0.5mm
• No-clean flux recommended to avoid cleaning challenges
• Controlled paste volume critical for consistent joints

Placement

• Placement accuracy: ±25-50 micrometers depending on pitch
• Component height sensing important due to thin packages
• Proper nozzle selection to avoid die damage

Reflow

• Standard lead-free profile (peak 235-250°C)
• Controlled ramp rates to avoid thermal shock
• Nitrogen atmosphere recommended for improved wetting
• Avoid exceeding 260°C to prevent package damage

Inspection

• X-ray inspection recommended for solder joint verification
• Void percentage should be less than 25% per IPC standards
• Visual inspection limited due to hidden joints

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CSP vs Other Package Types

Understanding when to choose CSP over alternatives helps optimize your design:

CSP vs BGA Comparison

CSP vs QFN Comparison

When to Choose CSP

• Space is the primary constraint
• Low-to-medium I/O count (under 500)
• Electrical performance is critical (RF, high-speed)
• Volume production justifies HDI PCB costs
• Weight reduction matters (mobile, aerospace)

When to Choose Alternatives

• High I/O count requirements (>500): Standard BGA
• Cost-sensitive, moderate density: QFN
• Thermal performance priority: QFN or large BGA
• Ease of inspection and rework: QFN or leaded packages
• Extended temperature range: Consider package reliability data

Common CSP Applications

Chip scale packaging enables countless modern electronic products:

Consumer Electronics

Smartphones use CSPs extensively for processors, memory, power management ICs, RF front-ends, sensors, and audio codecs. A single smartphone may contain 50+ CSP devices.

Wearables

Smartwatches, fitness trackers, and wireless earbuds depend on CSP miniaturization. The ultra-thin form factors of these devices would be impossible with conventional packaging.

IoT and Sensors

MEMS sensors, environmental sensors, and wireless connectivity modules commonly use WLCSP for minimal footprint in distributed sensor networks.

Medical Devices

Implantable devices, hearing aids, and portable diagnostic equipment leverage CSP technology for life-critical miniaturization.

Automotive

Advanced driver assistance systems (ADAS), infotainment, and vehicle networking increasingly adopt CSP for space and weight savings.

Computing

DDR memory, solid-state drive controllers, and laptop components use CSP variants for space efficiency in thin-and-light designs.

CSP Reliability Considerations

Designing for reliability requires understanding CSP-specific failure modes:

Solder Joint Fatigue

The primary reliability concern. CTE mismatch between silicon and FR-4 causes cyclic stress on solder joints during thermal excursions. Mitigation strategies:

• Use underfill for critical applications
• Select PCB materials with lower CTE
• Design adequate solder joint standoff height
• Avoid placing CSPs at board edges where flex stress concentrates

Drop and Mechanical Shock

WLCSP solder joints can crack under mechanical shock. Solutions:

• Apply underfill or corner staking
• Use SMD (solder mask defined) pads
• Strengthen mounting with adequate copper area
• Avoid placing large CSPs near board mounting points

Moisture Sensitivity

CSPs typically achieve MSL-1 rating (unlimited floor life at <30°C/85% RH), but verify for your specific device. Baking may be required if packages have been improperly stored.

Useful Resources for CSP Design

Standards and Specifications

• IPC/JEDEC J-STD-012: Implementation of Flip Chip and Chip Scale Technology
• IPC-7351: Generic Requirements for Surface Mount Design
• IPC-A-610: Acceptability of Electronic Assemblies
• JEDEC JEP95: Design Guide for Ball Grid Array Packages

Manufacturer Application Notes

• NXP/Freescale AN3846: WLCSP implementation guidelines
• Analog Devices AN-617: WLCSP design and assembly
• Texas Instruments Application Reports: CSP package-specific guidelines
• Infineon/Cypress AN69061: WLCSP handling and assembly

Design Tools

• IPC-7351 Land Pattern Calculator: Generate CSP footprints
• Cadence Allegro/OrCAD: Advanced CSP routing capabilities
• Altium Designer: Comprehensive CSP library support

Frequently Asked Questions

What is the difference between CSP and WLCSP?

CSP (Chip Scale Package) is a category of packages where the package area is no more than 1.2 times the die area. WLCSP (Wafer-Level CSP) is a specific type of CSP where packaging is performed at the wafer level before dicing, resulting in a package that is exactly the same size as the die (1:1 ratio). All WLCSPs are CSPs, but not all CSPs are WLCSPs. Other CSP types like FCCSP and LFCSP use interposers or leadframes and are slightly larger than the die.

Do CSP packages require special PCB technology?

It depends on the ball pitch. CSPs with 0.65mm pitch or larger can often use conventional PCB technology. Finer pitches (0.5mm and below) typically require HDI (High-Density Interconnect) technology with microvias for escape routing. Via-in-pad with filled and planarized vias becomes necessary for fine-pitch designs. Always consult your PCB fabricator early in the design process to confirm manufacturing capability.

How do I inspect CSP solder joints after assembly?

X-ray inspection is the primary method for CSP solder joint verification. Unlike perimeter packages where joints are partially visible, CSP joints are completely hidden under the package. Automated X-ray inspection (AXI) can detect voids, bridges, missing balls, and misalignment. Boundary scan (JTAG) testing verifies electrical connectivity. Destructive analysis using dye-and-pry or cross-sectioning is used for failure analysis and process qualification.

When should I use underfill with WLCSP?

Underfill is recommended when the application involves thermal cycling, mechanical shock, or vibration. Consumer electronics with short product lifecycles may not require underfill, while automotive, industrial, and medical applications typically do. The decision depends on reliability requirements, package size (larger packages stress joints more), and operating environment. Capillary underfill or corner staking are common options. Consult the component manufacturer’s reliability data for your specific use case.

Can I rework a CSP package?

Yes, but CSP rework is more challenging than larger packages. It requires specialized rework equipment with precise temperature control and small nozzles. The process involves locally heating the package to reflow temperature, removing it with vacuum pickup, cleaning the site, applying new solder paste, and placing the replacement component. For WLCSP, the fragile die requires careful handling. Success rates are lower than with larger packages, so design for first-pass assembly success whenever possible.

Conclusion

Chip scale package technology has fundamentally transformed electronics design, enabling the compact, powerful devices we rely on daily. From WLCSP’s ultimate miniaturization to FCCSP’s balance of size and reliability, the CSP package family provides solutions for virtually every space-constrained application.

Key takeaways for successful CSP implementation:

• Understand the tradeoffs between CSP variants (WLCSP, FCCSP, LFCSP)
• Match ball pitch to your PCB technology capability
• Use NSMD pads for thermal cycling reliability, SMD for drop performance
• Plan for via-in-pad with filled vias on fine-pitch designs
• Consider underfill for applications with harsh environmental requirements
• Invest in X-ray inspection capability or partner with assemblers who have it
• Consult component datasheets and manufacturer application notes early

As semiconductor devices continue shrinking while functionality increases, chip scale packaging will remain at the forefront of electronics miniaturization. Whether you’re designing the next generation smartphone, a life-saving medical implant, or an industrial IoT sensor, understanding CSP technology is essential for competitive product development.