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Effective PCB Substrate Design Practices for IC Package Assembly: A Case Study

In the semiconductor package assembly process, achieving precision in substrate design is a crucial element. The substrate should be designed taking into account the ease of assembly. This applies to die attaching, both flip-chip and traditional die attach for wire bonding. In this article, we will look at a specific case study involving shortcomings of a substrate design, focusing on challenges that were faced during the assembly, die attach, and wire bonding stages.

Case Study Overview For COB Substrate Design

Recently, we faced significant challenges during a Chip-on-Board (COB) bonding job involving a two-die configuration. Au wire bonding the primary die was not a path but rather straightforward and encountered no issues. However, the secondary die presented a unique challenge due to the design of the PCB substrate.

Challenges Faced During Au Wire Bonding

Wire Bonding Parameter Optimization To Bond on ENIG-finished Au Vias for Bond Strength

The second die attach pad, or the die attach surface, was designed with Au vias instead of a continuous Au bonding pad covering the full surface like the primary die attach surface with Electroless Nickel Palladium Immersion Gold (ENEPIG) processed surface. Also, the Au vias were not ENEPIG finish but Electroless Nickel Immersion Gold (ENIG), requiring hours of parameter fine-tuning to achieve acceptable bond strength.

Spatial Constraints in Die Downbonding (GND) on Limited Au Via Surface

The primary challenge was the spatial limitation for downbonding from the die; the die itself covered more than 95% of the Au via surface that was supposed to be used for downbonding. Even with minimal epoxy overflow from all sides of the die, less than 150 micrometers, there was not enough exposed surface for downbonding.

Capillary Tool Risks with Restricted Space

Even with the extremely small capillary, high loop size and changing the PCB position and alignment there was no simply enough space to down bond with out the risk of the capillary tool hitting the side wall of the the die casing side wall chipping or even die crack.

 

Strategic Solutions and Best Practices For PCB substrate design

Ensure Die Attach Pad Is Sufficiently Spacious

Ensure that either the Au vias covered or Full Au surfaced die attaches surface is atleast 750um bigger than the die on all sides for down boding the pads at the edge of the die, and if the down bond is from a die pad from the middle or further away from edge of the die, make sure to leave at least 2 times the length of the bond pad to the edge of the die distance to form proper suitable wire loop size. 

Consider Alternative Grounding Options

Strategically place spare bond pads/vias around the die attach surface to cater to errors caused by the die attach/wire bonding mishaps. It is always better to design the substrate for versatility rather than for small substate size for small volume builds for R&D purposes where the main purpose of the build is for proof of concept or specific function testing.

Implement Design for Manufacturability (DFM)

Before sending the pcbdoc/cad files to the PCB manufacturer for substrate fabrication, always involve the assembly engineer or assembly house in general to make sure that die attach and wire bonding can be done as intended. Their experience can inform the design adjustments that accommodate real-world assembly challenges.

 

PCB with a die attached pad barely larger than the die, showing inadequate space for ground connections and high risk of die damage
Challenges in PCB Die Attach: Limited Space and Bonding Risks during bonding.
PCB with gold grounding wire expertly bonded to vias away from the cramped die attach pad to avoid die damage.

Conclusion

The case study presented demonstrates the importance of thoughtful substrate design in semiconductor package assembly. By adopting these best practices, assembly facilities can improve their capability to handle complex packaging scenarios, thereby enhancing overall reliability and efficiency in semiconductor manufacturing. These strategies help in addressing immediate problems and also serve as proactive measures to prevent future issues, ensuring smoother operations and higher quality outcomes in the assembly process.

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What is Chip On Board Technology (COB)?

COB technology simplifies electronic device assembly by placing the microchip directly onto the circuit board. This direct bonding can be achieved through wire bonding or solder bumps. For a deeper understanding of COB’s basics, Electronics Hub offers a comprehensive guide that breaks down the technology and its application.

A Short History of COB

COB technology emerged in the late 20th century as the electronics manufacturing industry sought compact, efficient circuit integration methods. The evolution from surface mount technology (SMT) and through-hole technology (THT) to COB represents a significant leap in manufacturing. Historical insights and the technological progression are well-documented in IEEE’s electronic library.

The Role of COB in Electronics

From LED displays to automotive applications, COB technology has broadened the horizons of electronics manufacturing. Its influence on LED technology, for example, has led to devices that are not only brighter but also more energy-efficient. ScienceDirect publishes research articles detailing COB technology’s impact on the LED industry and beyond.

The Benefits of COB

COB technology brings several advantages to electronic devices, including reduced size and improved thermal management. For those interested in the technicalities of how COB enhances device performance and longevity, the American Society of Mechanical Engineers (ASME) provides resources and papers on thermal management solutions in electronic packaging.

Conclusion

COB technology is at the forefront of driving the electronics industry towards more integrated, efficient, and sustainable solutions. As we advance, the role of COB in fostering innovations in IoT and AI is undeniable. For future trends and insights into COB technology, keeping an eye on TechCrunch’s hardware section can be immensely helpful.

Understanding Multi-Chip Modules: Making Electronics Better

Understanding Multi-Chip Modules: Key Roles of Die Attach and Wire Bonding

Multi-Chip Modules (MCM) have transformed how electronic devices are built, offering better performance in a tinier package. At the heart of making MCM technology work are two crucial steps: Die Attach and Wire Bonding. These steps are key for making sure the tiny parts inside work well and last long.

What are Multi-Chip Modules?

Multi-Chip Modules bring together several semiconductor devices, like ICs (Integrated Circuits), onto one base or package. This makes devices perform better and do more things. The success of putting these chips together relies a lot on die attach and wire bonding. These processes make sure the chips are not only physically secure but also connected right, so they work as expected.

Die Attach’s Role in MCM

For MCMs, Die Attach is about sticking each chip firmly to the module’s base. This step is critical not just for keeping the chips in place but also for managing heat. Getting rid of heat efficiently is important because it affects how well the module works. Choosing the right materials and methods for die attach can greatly impact the module’s performance. Websites like Semiconductor Engineering delve into the newest approaches and materials used.

Wire Bonding: Connecting Everything Together

After attaching the chips, Wire Bonding is used to link the chips’ contact points to the module’s base or other parts. This needs to be done with great care to ensure the signals are strong and clear, and there’s no unnecessary resistance. The type of wire and how it’s used depend on the module’s use, how it operates, and where it will be used. The International Microelectronics Assembly and Packaging Society (IMAPS) has lots of information on wire bonding and its importance in MCMs.

Why MCMs are Great for Electronics

  • Better Performance: MCMs can do more and work faster by combining several chips.
  • Smaller and Lighter: They help make devices smaller and lighter, which is especially important for things you carry around like phones and wearable tech.
  • More Power-Efficient: MCMs are designed to use power wisely, helping devices last longer on a single charge.

The Challenges of Building MCMs

Putting together MCMs is tricky, especially when it comes to die attach and wire bonding. Making sure everything works together perfectly, without overheating or losing signal, requires a lot of skill and knowledge.

Looking Ahead in MCM Technology

Technology is always moving forward, and so is the way MCMs are made. New techniques in die attach and wire bonding will keep making MCMs even better, helping them meet the growing needs of electronic devices.

Read about Wire Bonding Materials

Read about Substrate Design For Larger Die Sizes