Chip-on-Board (Using Wire Bonding)¶

Chip-on-Board (COB) using wire bonding is often the easiest, cheapest, and fastest packaging option for wafer.space users. This approach involves mounting bare die directly onto a PCB and connecting the chip pads to PCB traces using fine wire bonds, then protecting the assembly with encapsulation.

What is Chip-on-Board?¶

Chip-on-Board assembly consists of several steps:

Die attach - Mounting the bare die onto the PCB substrate
Wire bonding - Connecting chip pads to PCB traces with fine wires
Encapsulation - Protecting the assembly with epoxy or glob-top
Testing - Verifying functionality of the completed assembly

This results in a compact, cost-effective package that’s well-suited for prototypes and low-to-medium volume production.

Advantages of COB¶

Cost Benefits¶

No package costs - Eliminates traditional IC packaging expenses
Simplified supply chain - Direct from wafer.space to PCB assembly
Lower tooling costs - No custom package design needed
Reduced inventory - No packaged parts to stock

Technical Benefits¶

Compact size - Smaller than equivalent packaged parts
Better thermal performance - Direct thermal path to PCB
Reduced parasitics - Shorter wire bonds than package leads
Design flexibility - Custom pad arrangements possible

Time Benefits¶

Faster time-to-market - No packaging lead times
Rapid prototyping - Quick assembly for testing
Easy design changes - Modify PCB without new packages
Simplified logistics - Direct die-to-board flow

COB Assembly Process¶

1. Die Attach¶

Die attach materials:

Conductive epoxy - For electrical and thermal connection
Silver-filled paste - High thermal and electrical conductivity
Eutectic die attach - For high-reliability applications
Thermal interface materials - For heat-sensitive applications

Die attach process:

Dispense adhesive onto PCB die attach area
Pick and place die using vacuum tooling
Align die to PCB reference marks
Cure adhesive (typically 150°C for 1-2 hours)

2. Wire Bonding¶

Wire bonding connects chip pads to PCB traces using:

Gold wire (most common) - 25μm (1 mil) diameter typical
Aluminum wire - Lower cost, requires different bonding parameters
Copper wire - Emerging technology for high-current applications

Bonding process:

Program bonder with chip and PCB pad coordinates
Position first bond on chip pad
Form wire loop to PCB pad
Complete second bond on PCB pad
Repeat for all connections

3. Encapsulation¶

Protection methods:

Glob-top epoxy - Simple, low-cost protection
Silicone potting - Flexible, reworkable protection
Rigid encapsulation - Hard epoxy for harsh environments
Conformal coating - Thin protective layer

Community Resources and Examples¶

The open-source community has developed valuable resources for COB assembly:

Tiny Tapeout COB Work¶

The Tiny Tapeout project has pioneered accessible COB techniques for the maker community:

Reference: Tiny Tapeout COB Guide

This resource provides practical, hands-on guidance for:

DIY COB assembly using readily available equipment
PCB design guidelines optimized for wire bonding
Assembly techniques that work without expensive tooling
Troubleshooting tips from real-world experience
Cost optimization for small volumes

University of Michigan OpenFASoC Work¶

The OpenFASoC team at University of Michigan has developed advanced COB implementations:

Reference: OpenFASoC Tapeouts - GF180MCU Padframe

This repository contains:

GF180MCU-specific designs directly applicable to wafer.space chips
Padframe implementations for COB assembly
PCB design files showing professional COB layouts
Assembly documentation for multi-chip modules
Verification approaches for complex assemblies

Key insights from Mehdi’s OpenFASoC work:

Systematic padframe design for reliable wire bonding
Multi-die assembly techniques for complex systems
Thermal management strategies for high-power applications
Testing methodologies for COB assemblies

PCB Design for COB¶

Die Attach Area Design¶

Size the area to accommodate die plus handling tolerance
Include alignment features like fiducials or hard stops
Plan thermal path from die to heat spreading areas
Consider rework access if die replacement might be needed

Wire Bond Pad Design¶

Pad specifications:

Minimum size - 100μm × 100μm for hand bonding, 75μm × 75μm for auto
Pad spacing - Maintain adequate clearance for bond wires
Metal finish - ENIG (gold) preferred for bondability
Solder mask - Open over bond pads, defined openings

Layout guidelines:

Keep bonds short - Minimize wire length for better electrical performance
Avoid crossing wires - Plan routing to prevent wire-to-wire contact
Group by function - Place related signals together
Include test points - For post-assembly verification

Routing Considerations¶

Trace impedance - Control for high-speed signals
Power distribution - Adequate copper for current requirements
Ground planes - Provide good ground return paths
EMI considerations - Shielding and filtering as needed

Assembly Equipment Options¶

Manual Assembly (Low Volume)¶

Basic setup:

Stereo microscope (10x-40x magnification)
Hot plate or small oven for die attach cure
Manual wire bonder or wedge bonder
ESD-safe workstation
Basic dispensing equipment

Suitable for:

Prototypes and development
Very low volumes (< 100 units)
Educational purposes
Proof-of-concept assemblies

Semi-Automated Assembly (Medium Volume)¶

Equipment:

Pick-and-place machine for die attachment
Automatic wire bonder
Dispensing robot for adhesives
Convection oven for curing
Vision system for alignment

Suitable for:

Pre-production builds
Medium volumes (100-10,000 units)
Pilot production
Cost-sensitive applications

Fully Automated Assembly (High Volume)¶

Production line:

Automated die attach systems
High-speed wire bonders
Inline curing systems
Automatic encapsulation
100% electrical test

Suitable for:

Production volumes (>10,000 units)
High-reliability applications
Cost-optimized manufacturing
Consistent quality requirements

Quality and Reliability¶

Common Issues and Solutions¶

Bond failures:

Poor adhesion - Clean surfaces, proper bonding parameters
Wire breaks - Optimize loop height and wire tension
Contamination - Maintain clean assembly environment

Die attach problems:

Voids in adhesive - Proper dispense pattern and cure profile
Die tilt - Ensure flat, clean mounting surface
Thermal resistance - Optimize adhesive thickness and coverage

Testing and Validation¶

Assembly verification:

Continuity testing - Verify all bonds are intact
Functional testing - Confirm circuit operation
Thermal testing - Validate thermal performance
Reliability testing - Accelerated aging and cycling

Cost Optimization¶

Design for Low-Cost COB¶

Minimize bond count - Reduce assembly time and cost
Standardize processes - Use common materials and procedures
Design for test - Enable efficient post-assembly verification
Plan for yield - Anticipate and design around common failure modes

Volume Considerations¶

Volume Range	Assembly Approach	Cost per Unit*	Setup Cost	Timeline
1-100	Manual assembly	$25-50	$500-2K	1-2 weeks
100-1,000	Semi-automated	$8-15	$5K-20K	2-4 weeks
1,000-10,000	Automated line	$3-8	$50K-200K	4-8 weeks
>10,000	Optimized production	$1-3	$200K+	8-16 weeks

*Costs are approximate and include die attach, wire bonding, and encapsulation. Actual costs vary based on die size, pin count, and complexity.

Getting Started with COB¶

Step 1: Design Preparation¶

Review the Tiny Tapeout COB guide for basic techniques
Study the OpenFASoC padframe designs for professional approaches
Design your PCB with COB-friendly features
Plan your assembly process and equipment needs

Step 2: Prototype Assembly¶

Start with manual assembly for first prototypes
Use simple glob-top protection initially
Focus on functionality before optimizing for production
Document your process for repeatability

Step 3: Scale Considerations¶

Evaluate volume requirements and timelines
Consider assembly service providers for higher volumes
Plan for quality control and testing
Investigate automation options as volumes grow

Assembly Service Providers¶

Many PCB assembly houses offer COB services:

Local assemblers - For prototypes and small volumes
Specialized COB houses - For medium to high volumes
Turnkey providers - Full service from PCB to test
Academic partnerships - University fabrication facilities

Next Steps¶

Ready to implement COB for your wafer.space chips?

Review the referenced resources - Study both Tiny Tapeout and OpenFASoC approaches
Design your PCB with COB requirements in mind
Plan your assembly strategy based on volume and timeline
Start with prototypes to validate your approach
Scale gradually as you gain experience and volume

Chip-on-Board assembly offers an excellent balance of cost, performance, and time-to-market for wafer.space users. Leverage the community’s experience to accelerate your own COB success.