Glob Top encapsulation
Electronic devices and computers are now ubiquitous in consumer and industrial settings. As more types of boards and chips are exposed to different types of environments, delicate packages require new methods to protect them from environmental and physical stresses. Glob tops provide an excellent way to protect chips at an individual level. While conformal coatings, encapsulation and molding are all options for specific applications, glob topping, or glob-top encapsulation, involves encapsulating a single chip to provide exceptional environmental protection and physical stability.
What are Glob Tops?
Glob tops are usually epoxies that are dispensed to cover a single chip in chip-on- board (COB) applications. They are most frequently used to encapsulate wire-bonded dies, since the wires bonds often need physical strengthening provided by epoxy, and other methods like conformal coatings or underfilling don't provide the same protection. Glob-top epoxies are thixotropic, meaning their viscosity is variable - becoming less viscous under physical agitation and more viscous under static conditions. They provide "controlled flow" - self-leveling without running over the applied area.
"Two-part applications require mixing and have longer cure times, but can be cured at room temperature. "
In addition to physical protection, glob tops provide other benefits. Glob-top epoxies are highly resistant to environmental contaminants, protecting against dust and organic material, as well as preventing corrosion or oxidation by blocking water, air moisture and other chemicals. They also provide some protection from thermal shock, since they connect the chip and board as a single unit, mitigating movement stemming from differences in coefficient of thermal expansion (CTE). Glob-top epoxies are often loaded with a thermally conductive filler like aluminum oxide, which allows them to aid in heat dissipation. Finally, glob tops provide protection from reverse engineering, as they are often opaque and non-removable.
These glob top epoxies can be either one-part, two-part or UV curing, depending on the application needs. One- part formulations are the simplest, but require heat-curing, which may prohibit their use. Two-part applications require mixing and have longer cure times, but can be cured at room temperature. UV-curing adhesives are the quickest option, often used where throughput concerns are paramount, but require additional equipment.
Where are they being used?
Glob tops are used in a range of COB applications. Any wire-bonded chips that require added security or protection from physical and environmental factors can benefit from a glob-top epoxy. Particularly in cases where protection beyond a conformal coating is necessary, or where the particular chip being encapsulated requires extra protection. Although underfilled flip-chips are used frequently in miniaturized and handheld devices, wire-bonded chips still see widespread use. And with board real-estate becoming increasingly more valuable, stacked chip packages with wire bonds are also being used. Modern wire-bonded chips are also highly complex - often using hundreds of extremely fine wires. Glob tops are used more frequently here over other methods of encapsulation like high-pressure molding, since molding can produce "wire-sweep." Wire sweeping can be a problem, especially on complex or delicate chip packages, producing short circuits or physical breakage.
How are they applied?
Glob tops allow for a number of different types of application depending on throughput and other manufacture needs. Because they need to flow freely during dispensing, but rapidly encapsulate a small and specific area, glob-tops are thixotropic. This property comes from the inclusion of silica fibers or other fillers and formulations. During mixing and pre-dispensing, the glob top epoxy is usually churned in an auger, or agitated in another manner, to increase viscosity. Once applied, the epoxy returns to its original, high, static viscosity - restricting the flow to the immediate chip area. There are two methods for applying glob-top epoxies- glob topping, and dam and fill.
- Glob topping - With glob topping, the epoxy is simply dispensed from a syringe directly on top of the die. When dispensed in a precise amount the thixotropic action keeps the epoxy from covering too large (or too small) of an space. With glob topping applications the thixotropy must be finely tuned to allow the proper coverage.
- Dam and fill - With dam and fill applications, a second epoxy or other adhesive compound is placed as a dam around the chip. The glob-top epoxy is then dispensed on top of the chip and flows until it reaches the initial dam adhesive. Usually the adhesives are similar except for rheology and cure. This method allows more precise control over the covered area and finished profile.
With either of these methods one- or two-part epoxies, or UV-curing epoxies can be used. Application and curing setups will vary depending on formulation and can be tailored to use manual or automatic equipment at different stages.