Industrial Epoxy Mold Compounds

To add insult to injury, engineers often mistake a material’s glass transition temperature (Tg) for its maximum operating temperature. While it is true that a materials’ physical properties - including its coefficient of thermal expansion (CTE) and Flexural Modulus (E) – do change at a material’s Tg, these are not signs of material degradation. In many cases, these properties go through a perfect hysteresis meaning that the material’s original CTE and modulus are reverted once the temperature is returned below its Tg.

But don’t take our word for it, the proof is in the pudding as they say. The Underwriter’s Laboratory (UL) has designed a UL-Approved Electrical Insulation system (EIS) class which are based on the maximum "hot spot" in the system. (See Table 1) . Manufacturers have been formulating two anhydride chemistries, MH6-0504 and MH6-0510 for over forty years that are in use on applications that have a Class H rating, meaning they can be used in applications that can see temperatures up to 180°C.

In addition to this, though anhydride chemistries may not be able to achieve the same higher maximum electrical insulation class (Class N) as some of its novolac counterparts, the temperatures at which it is stable (up to 180C), it is very stable. As you can see in Figure 1 below, tests were carried out with a “green color” version of epoxy molding compounds in both an anhydride chemistry and a novolac chemistry. The green colored novolac chemistry candidate showed a much worse temperature stability than the same color mold compound using an anhydride chemistry.

Figure 1: Anhydride chemistry is much more color stable at 180°C then novolac chemistry

 

Concern 3: Anhydrides have a higher moisture absorption than other chemistries

Moisture absorption tests measure the amount of moisture that epoxy mold compounds absorb after post mold curing and exposed to high moisture environments. Three standard tests for this include:

• 1 hour water boil @ 100°C
• 24 hour water soak @ 23°C
• 168 hour moisture exposure at 85°C and 85% relative humidity (RH)

In all three of these tests, anhydride chemistries have a distinct disadvantage against other epoxy mold compound chemistries. They all reveal that the moisture absorption of anhydride chemistries is greater than their counterparts. So why does this matter to us?

A significant amount of epoxy mold compounds are used in the manufacture of semiconductor devices. In semiconductor applications, epoxy is used to encapsulate the semiconductor chip, thus providing both mechanical and environmental protection.

Early in the development of epoxy mold compounds for semiconductors, Joint Electron Device Engineering Council (JEDEC), an independent semiconductor engineering trade organization and standardization body, developed a series of moisture sensitivity level (MSL) tests that correlated the MSL level of semiconductor devices to the floor life of these same devices before these same devices needed to be “prebaked” again. See Table 2.

“Prebaking” is a process in the printed circuit board (PCB) assembly process that literally bakes the semiconductor devices in an oven for 24 hours @ 100°C to drive out moisture that may have accumulated in the device as the device sat on the production floor waiting to be mounted onto the printed circuit board. Because the printed circuit board process always involves the soldering of components onto the PCB in a solder reflow oven, a process that exposes the PCB and all its components to temperatures up to 260°C for up to 10 minutes, it is critical that all the components can withstand this temperature cycle.

What was discovered was that in the absence of this prebaking step, semiconductor devices that contained too much moisture would fail catastrophically during this reflow step as moisture trapped within the epoxy mold compound would expand and cause delamination from the semiconductor device, break critical wirebonds and separate from the leadframe in a process now coined “popcorning”.