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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 (T_g)

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 T_g, 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 T_g materials at high temperatures. Thus, you also have to consider T_g 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 T_g is fine. In turn, the high T_g uses high strength, compression, and tension epoxy properties.

One key point to make when selecting epoxies for microelectronics is that T_g 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.