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

In my last column, I talked about protecting both the bare die and associated wire bonding as a major part of today’s PCB microelectronics assembly. I also discussed two die protection methodologies—glob top and lid and cover—as well as a glob top sub-method known as dam and fill. This column will further describe how you can get a handle on glob top epoxies.

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.

However, one has to keep in mind that electronic components continue to be increasingly minuscule, and product reliability is at a premium. Therefore, it’s prudent on the OEM customer side to gain a basic understanding of the key differences among these epoxies because they play a vital role in the levels of die protection integrity and reliability that customers expect.

Some of the key aspects to be aware of include:

  • Epoxy characteristics
  • Curing conditions
  • Viscosity
  • Glass-transition temperature (Tg)

Epoxy is composed of different characteristics or elements. Some may have adhesive characteristics; others may have conductive thermal characteristics or non-conductive electrical characteristics, depending on the applications that are being used.

Moreover, a general-purpose epoxy should be able to adapt to a variety of different temperature ranges to work with different substrates and surface finishes. A good epoxy should display low thermal stress during temperature cycling. This means taking a board or substrate to very high and low levels of temperature to ensure they are reliable at a wide range of temperature cycles.

Curing depends on an epoxy's application. Curing means the epoxy goes through a cycle of a specific temperature range, which is of a certain time and at a certain interval. Some epoxies are cured at lower temperatures and others at higher temperatures. Some are cured in minutes, while others take 1.5–2 hours. For example, some epoxies take 20 minutes to cure at 120°C while others take five seconds at 150°C. Again, epoxy curing depends on the die-attach application as well as a particular epoxy’s characteristics.

Viscosity comes into play for multiple reasons regarding die attach or wire bonding (not so much for wire bonding, but definitely for die attach). Process engineering on a PCB microelectronics assembly line should closely inspect viscosity. It is one of the most important epoxy characteristics that will either make your project successful or a failure.

Different applications require various viscosities, so viscosity is very important. If the viscosity isn't right for the application, it could cause problems. For instance, if it's too thin, it could leach out to the outside periphery of the die, creating unstable die-attach joints before wire bonding can be performed. If it's too thick, it might create non-optimal flashpoint with the surface. This means it's overly lifted from the surface of the substrate on a PCB to the die itself, creating possibilities for voids within the epoxy.

Dispensing the epoxy is yet another factor to keep in mind. The accuracy of the epoxy lines a dispenser makes underneath or around the die that's to be attached plays an important role. Dispensing methods are equally as important. There are different dispensing pumps, needles, and syringes, and each methodology should be considered carefully to assure the die-attach application.

Also, consider tack time. Die-attach epoxy can get dried up unless properly applied. When epoxy is dispensed, there is a distinct amount of time allowed to attach a die or else it will dry too quickly, resulting in a non-optimal die attach for a given application, thereby creating field failures.

As for Tg, it is important to know that high glass transition epoxy compounds are critical to the adhesive selection for higher-temperature applications. These products have superior mechanical, thermal, and electrical properties than lower Tg materials at high temperatures. Thus, you also have to consider Tg as a major factor when choosing the right epoxy for your given application.

For example, an application may require a flexible epoxy; therefore, a low Tg or a level below 0°C is acceptable. Other applications may require the strongest possible epoxy physical properties. In cases like these, a high Tg is fine. In turn, the high Tg uses high strength, compression, and tension epoxy properties.

One key point to make when selecting epoxies for microelectronics is that Tg should be carefully considered for the substrate temperature operating levels versus the die in terms of the ranges at which these epoxies can be cured. OEM customers should get a handle on the various types of die-attach epoxies available so they can ensure it’s a highly reliable product as it goes through an effective PCB microelectronics assembly line.

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

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2019

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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