Zulki’s PCB Nuggets: Thermal Compression Bonding for Extra Fine Pitch Flip Chips

Medical OEMs and others moving toward developing and producing small, portable products may want to investigate the emerging technology of thermal compression bonding or TCB. TCB is ideally suited for miniature products that use fine pitch flip chip devices undergoing PCB microelectronics assembly.  

Certainly today, regular pitch flip chips can be bonded using die attach and wire bonding. However, there is another way for bonding miniature designs that call for fine pitch flip chips. In this case, fine pitch pad size is defined as 50 to 75 microns. If that is so for your application, then it’s worth looking into thermal compression bonding.

Overview
Three factors are involved in thermal compression bonding—time, temperature, and force. This process is relatively quick-and-easy and is time tested. 

There are certain advantages to flip chip bonding using thermal compression methodology, which is a diffusion process. Performing TCB on one machine or station is one of its top advantages, contrasted to a typical SMT reflow, where you have to take the PCB from pick-and-place to the reflow oven, thereby moving the product from one machine to another. 

Whether that movement is a few inches, a few feet, or maybe across the hall, physical movement of the boards any distance can possibly jeopardize placement accuracy. With TCB, you basically use one machine, one station. That means placement accuracy will not be challenged since you are eliminating travel distance from one machine to another.  

Moreover, TCB has excellent connection properties which improve solder joints, especially if you are talking about RF based applications that demand extremely accurate connections with low signal-to-noise ratios (SNR). 

TCB can also be used for proof-of-concept application because it can be developed with relative ease by experienced process engineers, using the factors of force, time, and temperature. Moreover, for TCB, wire diameter is important, which is in the range of typically 25-micron.  

Seasoned process engineers working with TCB review an application to define and improve process parameters. They then define and design the temperature range the process requires for the bonding. At the same time, they define and design the level of force that needs to be applied and the necessary time needed to maintain that temperature for a perfect bond. 

Disadvantages
TCB has challenges and disadvantages, as well. This type of bonding requires high placement accuracy, meaning two to three microns. Moreover, it needs to have some flexibility and wiggle room that allow process engineers to define placement accuracy according to the machine being used for TCB process. 1-2 micron wiggle room is plenty to work with, if you have a sub-micron placement capabilities  

Further, according to die size, which is being flip chip bonded, there is a disadvantage due to the high placement accuracy required. If you are using a large size die, then certain machines would not be able to perform TCB because of field-of-view (FOV) limitations. A limited field of view is a direct consequence to that high placement accuracy. If placement is extremely accurate, then you cannot flip chip a bigger size die, let’s say more than half an inch in size with a typical TCB machine.  

Another disadvantage is high equipment cost. Not only does TCB require a machine that has top and bottom heaters for heating from both sides, but it also needs a very high accurate placement tolerance, which doesn't come easy without spending a lot of money. Extra cost is associated with such other items as tools and fixtures, which could differ from project to project.  

Also, one has to keep in mind that a conventional substrate like FR-4 must stay below certain temperatures. The extra costs here go toward using a non-organic substrate, like alumina or aluminum nitrate.  Such a substrate provides better results because it is considerably more co-planar and can handle higher temperature levels.  

Specifics: Time, Temperature and Force
Typical time is from 30 to 60 seconds at 275–300°C for the TCB to be accurately performed.  Keep in mind that the amount of time required to bond using the TCB process depends on ball size of a flip chip device, size of the die, as well as its thickness. 

Temperature
TCB requires a high temperature such as 300°C. In this case you need a heating tool for device placement and a sophisticated automatic bonder for performing the thermal compression bonding. For TCB to be properly performed, heating comes from two sides, from the top to the bottom of the bonding machine. The higher temperature is more from the top side than bottom side. Typically, from the top side of the machine it is 250–300°C and on the bottom side, 90°C to about 150°C. But, again, it must have the heating temperature from both sides. 

Force
As for bond force, the lowest you can go is 0.05 Newton, and the highest is typically 1,000 Newton. The force range is going to be defined by the die size. The bigger the die size, the bigger the bond force that will be required. Bond force holds the die on top of the substrate. The required load force is dependent on die size. Also, during this TCB process, melting of device package balls requires a small period of time, 30 to 60 seconds. But the process engineer needs to make calculations for intricate details and adjust parameters for that.   

Verify
Lastly, like any other process, you need to verify the bonding performed by three different methodologies. Those are visual, X-ray, and shear pull test verification inspections. And when you're doing that, then you have to assure that you are performing the pull test as per the parameters defined by the customer or the process application. 

In summary, TCB may be a niche for the near term. However, with the increasing interest in smaller, miniature medical products, the likelihood is greater that TCB becomes more prominent in PCB microelectronics assembly. So, it’s a good tool for EMS provides to have in their arsenals.  

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

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2021

Zulki’s PCB Nuggets: Thermal Compression Bonding for Extra Fine Pitch Flip Chips

10-20-2021

Medical OEMs and others moving toward developing and producing small, portable products may want to investigate the emerging technology of thermal compression bonding or TCB. TCB is ideally suited for miniature products that use fine pitch flip chip devices undergoing PCB microelectronics assembly.

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Zulki’s PCB Nuggets: WBAN Steps Into PCB Microelectronics Assembly

09-15-2021

A new miniature wireless network is stepping out of academic journals, into micro-miniature medical device applications, and subsequently onto the PCB microelectronics assembly floor.

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Zulki’s PCB Nuggets: Photonics Comes to PCB Microelectronics Assembly

08-11-2021

Photonics has been around for many years. Today, that technology is beginning to make its presence known in PCB microelectronics assembly and in a variety of new applications. But we need to step back a bit and revisit reasons why photonics is so vital to meet today’s demands for increasingly higher speeds in advanced systems.

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Zulki’s PCB Nuggets: Sub-Micron Die Bonding—Don’t Be Misled by Five Micron Placement Standard

07-14-2021

It is critical for an EMS company or microelectronics service provider to have the capabilities to design, define, implement, and assemble sensor, biosensor, and MEMS device placement with sub-micron accuracy. Not only should they be able to do that with high accuracy, but also, they should be able to quantitatively verify that accuracy.

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Zulki’s PCB Nuggets: Meet the New Player in PCB Microelectronics Assembly—Interposers

06-09-2021

Today, everything is shrinking and getting smaller, as I have written about in earlier columns. Now, a new player is coming in to help make PCB real estate more available on small flex and flex-rigid circuits undergoing microelectronics assembly. That new entrant is an interposer, which is a type of connector that takes connections from one side of the substrate or board or material to the other side.

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Zulki’s PCB Nuggets: How Effective Is Your PCB Microelectronics Assembly?

05-05-2021

There is a good chance that all PCB microelectronics assembly and manufacturing processes are not created equal. In today’s day and age, that can be a major issue for medical OEMs who are now in the midst of developing or producing their newly advanced miniature medical products.

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Zulki’s PCB Nuggets: Five Key Benefits for Onshoring PCB Microelectronics Assembly

04-06-2021

There are five key benefits to U. S. OEMs for onshoring PCB microelectronics assembly and manufacturing, but this discussion is prefaced by this fact: the future lies in PCB microelectronics assemblies compared to traditional SMT assemblies.

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Zulki's PCB Nuggets: Onshoring for PCB Microelectronics Production?

03-04-2021

Many OEMs may not be aware that here in the U. S., microelectronics assembly and manufacturing is leading in technology advances and provides OEMs a relatively easier way to bring that production back to the U. S. compared to on shoring PCB SMT production.

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Zulki's PCB Nuggets: Take a Deep Dive Into U.S. Medical Device Production

01-27-2021

There are new angles, new thinking emerging for medical OEMs to consider in order to keep production in the U. S. versus overseas. The foremost thinking (biggest challenge) associated with those angles focuses on whether the product can be produced cost effectively in a timely fashion, so it’s distributed in the supply chain for a given medical device OEM.

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Zulki’s PCB Nuggets: Is U.S. Production Ready for Advanced Medical Devices?

01-06-2021

Medical electronics OEMs may be able to move forward with their prototypes into pilot, medium, and high runs right here on American soil. And it may be sooner than later due in large part to the advances in PCB microelectronics assembly, and perhaps, a helping hand from the U.S. government.

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2020

Zulki’s PCB Nuggets: Wire Bonding and CoB for PCB Microelectronics Assembly

12-09-2020

Real estate continues to be a precious commodity for substrates, package sizes, dies, and PCBs. But now, the industry is taking another stab at further reducing PCB real estate. Zulki Khan examines wire bonding and chip on board (CoB) to point out what might be best for your medical electronics device while undergoing PCB microelectronics assembly.

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Zulki’s PCB Nuggets: Consider Low-Temp Solder for PCB Microelectronics Assembly

11-11-2020

With newer, smaller devices coming on the market, EMS providers and contract manufacturers (CMs) must adjust their PCB assembly technologies to comply with these demands. Zulki Khan details how thermal profiling and the use of the correct solder paste become an even more critical step than before in conventional SMT and the newer microelectronics assembly.

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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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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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