Zulki’s PCB Nuggets: What’s Different Between C2 and C4 for PCB Microelectronics Assembly?

In my last column, I talked about flip-chip ball grid array (BGA), or FCBGA, making its grand entrance into PCB microelectronics assembly. But that subject requires a lot more digging to get the full story for OEMs planning highly advanced products that demand PCB microelectronics assembly.

In that regard, C4 and C2 bumps for flip-chip assemblies are among the top techniques that require close attention. Those two are flip-chip (FC) bump connection techniques and are crucial for efficient and cost-effective microelectronics assembly. In short, C4 and C2 are interposers that connect a small die in an FCBGA, using flip-chip on one side and BGA on the other side. This connection technique significantly reduces a board or substrate real estate by making the same amount of connections in a traditional BGA using these FCBGA connections.

In the C4 technique (Figure 1), solder bumps are deposited on chip pads situated on the top side of a silicon wafer during the final wafer processing step. A chip or die is then mounted to external circuitry through a substrate, which can be another organic material circuit board. It is then flipped over. The top part faces down and is aligned to matching pads on that substrate. Then, solder is reflowed to complete the interconnecting process, and the circuitry is joined together.

zulki_fig1.jpg

C4 is different than wire bonding where a chip is sitting face up. In this case, die pads are connected to the pads of an interconnect circuit, like a substrate, BGA, or glass or ceramic. This connects to outside circuitry, using gold, aluminum, or copper wires.

C4 is an abbreviation for the controlled collapse of chip connection and has long been associated with the BGA packaging process. The “collapse” part of C4 happens when the BGA balls undergo IR reflow. When the C4 balls collapse, they melt and solder into the circuit board pads to make the assembly connections, completing the joint formation.

The controlled portion of the collapse comes from the fact that it is controlled by the type of ball and reflow thermal profile being performed during the BGA connection or C4 connection. It’s also important to emphasize that C4 is well-proven, has been a commodity known technology for decades, has been perfected, and isn’t very costly, unless fine pitch is involved.

That’s where C2 comes in. A relatively new technology, C2 stands for chip connections and is also known as copper pillars assemblies. It differs from C4 in that C4 has a controlled collapse of balls or chip connections. However, C2 doesn’t have that. There’s no ball collapse like in C4 because copper pillars cannot be melted like the solder on a BGA ball.

C2 is typically used for finer-pitch devices when the BGA pad has a pitch of 180 µm or less, which is about seven mils or less pitch pad of a BGA. If the BGA ball pad pitch is 200 µm or eight mils or higher, then C4 is used. In short, C4 effectively deals with a 200 µm or greater pad pitch. However, 150- or 180-µm pitch devices cannot typically be assembled properly on an organic substrate using a C4 technology; instead, they must use C2.

Advantages and Disadvantages

Aside from these differences, C2 and C4 have advantages and disadvantages. I already said C4 is well-established and relatively inexpensive. Moreover, when BGA balls are melted using the C4 process, they realign to the BGA pads on PCB (substrate) surface to create perfect solder joints. On the other hand, with C2 and no collapse, the PCB microelectronics assembly house must be highly experienced at precise placement tolerance. That’s critical so that C2 copper pillars align perfectly with pads on a substrate’s surface pads for optimal assemblies.

Also, the use of thermal compression adds expense to C2 assembly because you need a specialized tool for this added step. But that methodology isn’t required for C4 assembly. However, C2 has a distinct advantage: copper pillars. Since they are a certain length and copper, they act as an extremely efficient heat conductor, providing excellent heat sinking. If you have a chip that’s creating a lot of power and heat and you’re using C2 copper pillars, the device can radiate the heat through these Cu pillars. This makes the chip cooler more quickly compared to C4. C4 doesn’t have this luxury of heat sinking or heat dissipation.

C2 and C4 bumps can be used either on organic or inorganic substrates. Organic substrates are based on FR-4, Rogers, or polyimide inorganic substrates are alumina (AI203), aluminum nitride (AIN), and gallium arsenide (GaAs).

Organic substrates like an FR-4, Rogers, or polyimide have significant challenges when it comes to creating trace widths and trace or air gaps in between BGA pads when it reaches below three mils for 75 µm. Based on the available technology right now, some high-end PCB fabrication shops can create three-mil traces and three-mil air gaps. But in some cases, they can also do two-mil lines or traces and two-mil air gaps. But that’s the absolute limit when it comes to organic substrates using C2 or C4 interposers.

However, with inorganic substrates like alumina, these limitations are eliminated. Trace spaces and air gaps can be in microns rather than mils. If you have an organic substrate, you can design a board with one micron of line and one micron of space, which is sub-mil. For example, 0.4-mil traces and 0.4-mil gaps can easily be designed and fabricated using an inorganic substrate.

But the cost of an inorganic substrate is the main problem. Fabrication is approximately 10 times more expensive compared to organic substrates like FR-4, Rogers, or polyimide material. Most OEMs are trying to switch to the organic substrate because of the cost reduction that is involved in the whole process.

Considering the various C4 and C2 aspects and differences, it’s always best for OEMs to first collaborate with their PCB microelectronics assembly houses to get a solid foundation of the pros and cons before launching into an advanced product.

Zulki Khan is the president and founder of NexLogic Technologies Inc.

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2020

Zulki’s PCB Nuggets: What’s Different Between C2 and C4 for PCB Microelectronics Assembly?

10-21-2020

In Zulki Khan's last column he talked about flip-chip ball grid array (BGA), or FCBGA, making its grand entrance into PCB microelectronics assembly. But that subject requires a lot more digging to get the full story for OEMs planning highly advanced products that demand PCB microelectronics assembly. In that regard, C4 and C2 bumps for flip-chip assemblies are among the top techniques that require close attention.

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Zulki’s PCB Nuggets: FCBGA Packaging Enters PCB Microelectronics Assembly

09-09-2020

The demand for smaller circuitry and packaging, as well as ever-shrinking PCB real estate, have continually pushed PCB assembly and manufacturing protocols. Part of these technological advances involves a combination of flip-chip and BGA (FCBGA) packaging. Zulki Khan explains the importance of FCBGAs.

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Zulki’s PCB Nuggets: DOEs on Call for New Wearable Medical Devices

08-05-2020

Zulki Khan explores how biosensors for human-machine interfaces (HMIs) and new, flexible electrodes are leading the way, are among the most recent developments and promise more sophisticated medical wearable devices for health monitoring.

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Zulki’s PCB Nuggets: Soft Electronics Pose PCB Microelectronics Assembly Challenges

07-08-2020

Zulki Khan explains how PCBs have moved from traditional large rigid boards to considerably smaller rigid and combinations of rigid and flex circuit boards, even to the point that bare chips and wire bonding are used during the PCB microelectronics assembly of these tiny boards.

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Zulki’s PCB Nuggets: Medical Miniaturization and PCB Microelectronics Assembly

06-24-2020

Medical electronics continue to be a gamechanger, with miniaturization being foremost today in the minds of medical OEMs. Zulki Khan discusses how there is a growing demand for even greater device and component miniaturization that plays a major role in the PCB microelectronics assembly of these medical devices today.

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Zulki’s PCB Nuggets: Add Hi-rel to ISO 13485 for More Robust Ventilator PCBs

05-13-2020

It's important to meet FDA and ISO 13485 standard quality and reliability requirements for ventilators and other medical equipment. Zulki Khan explains how there’s still more that ventilator OEMs need to put into practice, specifically in the high-reliability or “high-rel” area to further add to ISO 13485.

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Zulki’s PCB Nuggets: Urgent Call for Ventilators—PCB Technology at the Ready

04-15-2020

An urgent call is out to medical equipment makers that thousands—even millions—of ventilators are in the greatest demand in our history due to the worldwide COVID-19 outbreak. Zulki Khan explains how new ventilator makers—as well as traditional ones—must weigh a number of key PCB design, assembly, and manufacturing factors.

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Zulki’s PCB Nuggets: Putting the Heat on for Thermal Profiling

03-11-2020

A unique thermal profile is designed for each PCB job undergoing conventional SMT assembly, as virtually every PCB assembly professional knows. But what about a PCB assembly project involving both conventional rigid board and an extraordinarily small rigid or rigid-flex circuit undergoing microelectronics assembly? Zulki Khan covers PCB hybrid assembly, which requires two separate, unique, and distinctly different thermal profiles.

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Zulki’s PCB Nuggets: 7 Steps to Successful Assembly for Medical Devices Using Microelectronics

02-12-2020

Seven major steps need to be taken to achieve successful SMT and microelectronics assembly for medical electronic devices. Zulki Khan explains how these key steps take on special significance for newly emerging implantable and ingestible medical devices and result in medical devices that are robust, smaller, highly reliable, more powerful, and lighter.

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Zulki’s PCB Nuggets: Successful PCB Microelectronics Assembly

01-15-2020

In addition to coverage of PCB microelectronics subjects, Zulki Khan addresses one of the most crucial areas: PCB fabrication that creates the circuit board undergoing microelectronics assembly. The burning question is, “Why is fabrication vitally important when it comes to successful microelectronics assembly?”

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2019

Zulki’s PCB Nuggets: Vital Details for Implantable Medical Devices

12-04-2019

In addition to smart pills and smart cameras, which Zulki Khan covered in a previous column, another segment of the medical electronics devices market is rapidly growing, as well: implantable medical devices, which medical personnel surgically or otherwise insert into various parts of the human body. Zulki explains the extra measures required for these devices.

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Zulki's PCB Nuggets: Multi-tier Wire Bonding—Diving Into PCB Microelectronics

11-07-2019

As the name implies, multi-tier wire bonding involves several levels of wire bonding beyond the single level of wire bonding, which is traditionally used in semiconductor and/or PCB microelectronics assembly. Here, you have two, three, and four levels of wire bonding, in some cases, called stacked wire bonding. Also, multi-tier wire bonding offers OEMs a solution when the number of inputs/outputs (I/Os) are far beyond the traditional ones that are used in the single wire-bonding application.

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Zulki’s PCB Nuggets: Smart Pills & Cameras—The Next Frontier for PCB Microelectronics

10-23-2019

"Take two aspirin and call me in the morning," is the proverbial, jovial, and often-cited elixir that doctors have prescribed over the years for whatever ails you. Today, medical electronics are adopting the same concept but with new technologies. Now, the phrase, "Take two aspirin," takes on new meaning, as medical electronics move into new frontiers of inspecting a human’s gastrointestinal tract with new, revolutionary ingestible smart pills and "pill cams."

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Zulki's PCB Nuggets: A Better Grasp of Glob Top Epoxy Factors

09-25-2019

In my last column, I cited important aspects of glob top epoxies, calling attention to the fact there are different epoxy manufacturers. In this column, I will continue to emphasize six other important factors of glob top epoxies.

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Zulki’s PCB Nuggets: Get a Handle on Glob Top Epoxies

09-12-2019

Most often, glob top is the prevalent method EMS providers use today. However, the most important point to be made about glob top is the fact that multiple manufacturers are producing different glob top epoxies. And within each manufacturer, there are numerous types of epoxies being produced. Another key point is that EMS providers and contract manufacturers generally are the ones deciding on the kind of epoxy to use. This column will further describe how you can get a handle on glob top epoxies.

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Zulki’s PCB Nuggets: Protect the Die and Wire Bonding for Effective PCB Microelectronics Assembly

07-31-2019

Protecting bare dies on a PCB or substrate is a major process of microelectronics assembly. As we’ve said before, microelectronics assembly and manufacturing work in tandem with traditional SMT manufacturing for complete PCB hybrid manufacturing of today’s smaller form factor products, including IoT, wearables, and portable devices.

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Zulki’s PCB Nuggets: PCB Microelectronics—Inspection and Calibration

07-18-2019

Microelectronics manufacturing is the companion of SMT manufacturing and forms PCB hybrid manufacturing. Tools for SMT manufacturing have been around for a long time and have proven their value. Now, with microelectronics, new and different types of high-powered laser microscopes are populating the microelectronics assembly and manufacturing area to provide highly effective inspection and calibration.

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Zulki’s PCB Nuggets: Three Die Attach Methods for Microelectronics Manufacturing

06-27-2019

Die attach technology is increasingly being applied in PCB hybrid manufacturing (i.e., combining traditional SMT manufacturing with microelectronics) to comply with the requirements of small PCBs, especially rigid, flex, and combination rigid-flex circuit boards. These smaller boards are used in a variety of IoT, wearable, and portable applications.

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Zulki’s PCB Nuggets: Consider the Integrity of Wire Bonding

06-12-2019

While reliability and integrity can be regarded as synonymous as far as PCB manufacturing with microelectronics assemblies is concerned, the integrity of wire bonding—the methodology of interconnecting the wire to the bond pad—takes on other reliability-associated process qualities. Here are three factors that need to be implemented to create the integrity of wire bonding.

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Zulki’s PCB Nuggets: Avoid PCB Wire-bond Loop Failures

05-30-2019

Today, hybrid PCB manufacturing is making greater inroads into our industry, which is the marriage of traditional SMT manufacturing together with microelectronics and wire bonding. In many cases, the OEM working with EMS providers doesn’t fully understand the nuances of effective wire bonding and related failures.

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2014

Tighter Scrutiny Needed for PCB Cleaning Agents

05-13-2014

PCB cleanliness on the assembly floor is now getting more attention, due to tiny residues and contaminants being left on assemblies after new, advanced assembly processes. Cleaning methodologies, testing, analysis, and special chemistries are being taken to a new level to assure customers of ultraclean boards to avoid costly latent issues.

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Zulki's PCB Nuggets: Tighter Scrutiny Needed for PCB Cleaning Agents

05-13-2014

PCB cleanliness on the assembly floor is now getting more attention, due to tiny residues and contaminants being left on assemblies after new, advanced assembly processes. Cleaning methodologies, testing, analysis, and special chemistries are being taken to a new level to assure customers of ultraclean boards to avoid costly latent issues.

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Uncovering Assembly Problems of High-Speed PCBs

03-12-2014

The high-speed board may be perfect when it comes to BGA assembly. All the balls properly collapse; all the thermal profiles are accurately determined and performed. All soak temperatures, pre-heat, soak, and cool-off periods fall within manufacturer limits and ranges. Yet, this high-speed board fails at high speed at the time of system functional level testing in the system.

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Zulki's PCB Nuggets: Uncovering Assembly Problems of High-Speed PCBs

03-12-2014

The high-speed board may be perfect when it comes to BGA assembly. All the balls properly collapse; all the thermal profiles are accurately determined and performed. All soak temperatures, pre-heat, soak, and cool-off periods fall within manufacturer limits and ranges. Yet, this high-speed board fails at high speed at the time of system functional level testing in the system.

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EMS Discovers Mature IC Technologies

01-14-2014

Columnist Zulki Khan asks, "Did you know that really new, up-to-the-moment PCB technologies are nesting on the doorstep of PCB assemblers?" In fact, he says some of these technologies are very mature, but they're completely new to the assembly side of things.

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Zulki's PCB Nuggets: EMS Discovers Mature IC Technologies

01-14-2014

Columnist Zulki Khan asks, "Did you know that really new, up-to-the-moment PCB technologies are nesting on the doorstep of PCB assemblers?" In fact, he says some of these technologies are very mature, but they're completely new to the assembly side of things.

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2013

Another Look at AOI

11-13-2013

PCB inspection is taking on greater significance as boards and packaging become increasingly smaller, with greater functionality. Automated optical inspection (AOI) and its backup associate, X-ray, team up to catch a variety of board assembly problems. But it's AOI that's at the forefront of this process.

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Zulki's PCB Nuggets: Another Look at AOI

11-13-2013

PCB inspection is taking on greater significance as boards and packaging become increasingly smaller, with greater functionality. Automated optical inspection (AOI) and its backup associate, X-ray, team up to catch a variety of board assembly problems. But it's AOI that's at the forefront of this process.

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Zulki's PCB Nuggets: ECOs Reviewed - The Importance of Accuracy

09-11-2013

Designers can perfectly layout a design and, in theory, follow written specifications to the letter, but when one factors in the practicality of that design, virtually everything associated with it has its limitations--ranging from the material used to make the board to assembly, machine tolerances, and process limitations.

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