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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.