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

In my last column, I cited important aspects of glob top epoxies, calling attention to the fact there are different epoxy manufacturers. Consequently, those epoxies have different compositions and characteristics. I discussed several key epoxy aspects earlier; however, there are others as well. Getting a handle on these additional factors are important, considering that die attach and associated wire bonding must be effectively protected and is a major step in PCB microelectronics assembly and manufacturing.

In this column, I will continue to emphasize six other important factors of glob top epoxies, including:

  1. Chemical composition
  2. Epoxy dispensing and temperature ranges
  3. Viscosity measurement
  4. Color
  5. Pot life
  6. Shelf life

Important Factors

Chemical composition is important for a number of reasons. It ensures that the selected epoxy has the right chemical mix for excellent adhesion to the die and substrate, has a high resistance to corrosion, and maintains low shrinkage during epoxy curing, among others.

Epoxies covering a die attach and its wire bonding are dispensed at certain temperature ranges. Some range from -40°C to 150°C while others range from 60°C to 100°C or 40°C to 100°C. The manner and temperature that epoxies are dispensed are highly critical when it comes to a PCB microelectronics application. EMS companies have to match the right epoxies with substrates, depending on epoxy characteristics to create reliable and optimal joints.

Dispensing an epoxy with a relatively low viscosity would be rather easy compared to an epoxy that has high or thick viscosity. But regardless of whether a low or high viscosity epoxy is used, the application must be very precise. In some cases, the dispensing tolerance is in microns.

Viscosity is measured in terms of centipoise (CPS) to ensure the proper epoxy is used for a given application. CPS is a unit of dynamic viscosity and is defined as the amount of force required to move a layer of liquid in relation to another liquid. Viscosity can be as low as 9,000 CPS to over 500,000 CPS, and in some cases, 750,000 CPS. The latter is considerably more viscous and closer to a semi-solid compared to an epoxy viscosity around 9,000–15,000 CPS.

But what about applying the wrong viscosity epoxy to an OEM product undergoing PCB microelectronics assembly and manufacturing? Let’s say too thin of a viscosity is used. In effect, it will ooze out at the outer edges of the die, thereby not creating a solid, sturdy joint. It also creates bleeding at the edge of the die, which will require cleaning. On the other hand, if too thick of a viscosity is used, it may harden too soon or create a non-uniform height throughout the die. In turn, that might tilt the die, not creating a perfect integral joint, which is at the heart of the die-attach process.

Color It Right

Epoxy color comes in clear, white, and various shades of gray or black. But why is color important? Because some applications require that the epoxy be transparent enough to see that microelectronic joints are indeed intact. A clear epoxy allows easy visibility versus a black or gray epoxy where you cannot see through the epoxy.

Further, pot life refers to the amount of time taken to apply the epoxy before it dries up. Pot life can be anywhere from 60 seconds to a couple of hours. Epoxy’s hardness and shear strength are two other factors relating to pot life. Shear strength is measured in pounds per inch (psi). Some epoxies have relatively low shear strength in the range of a few 100 psi to 1,000 psi. On the higher side, it could be a 2,500–4,500, and in some cases, 5,000 psi.

As far as shelf life, epoxies left on the shelf for too long can become dry and brittle. As you can imagine, using these outdated/expired epoxies can create problems during assembly and manufacturing or in the field. The inevitable can happen, putting your PCB microelectronics project in jeopardy.

Epoxy date expirations generally reduce the shear strength originally specified in the product’s datasheet. For example, if the shear strength was originally specified at 2,500 psi, with shelf life expired, the leftover epoxy on the shelf may only provide a shear strength of 1,500 or 1,800 psi. Nevertheless, that expired epoxy will certainly not deliver the original 2,500 psi.

Conclusion

There are various factors associated with epoxies used in the glob top approach for die-attach/wire-bond protection. Each one merits close attention and a good understanding. By having a good grasp of these factors, you’ll be in a better position to ensure that your next project undergoing PCB microelectronics assembly and manufacturing will be successful.

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

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2019

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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PCB Hybrid Manufacturing: Coming Your Way

05-15-2019

Today, PCB manufacturing is rapidly evolving to include SMT as well as microelectronics, such as chip-on-board (CoB) installation, flip-chip assembly, wire bonding, and die attach. SMT merged with microelectronics is also known as hybrid manufacturing. This is occurring because PCBs have begun shrinking at a faster pace in recent years due to the introduction of wearables, IoT devices, and portables demanding smaller circuit boards.

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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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