By definition, the glass transition temperature (Tg) of an epoxy molding compound (EMC) is the temperature at which the EMC changes from a hard, glassy substance to a soft rubbery one. Physical material properties such as dimension, volume, and coefficient of thermal expansion (CTE) increase after this transition region. CTE will be discussed later. Other properties such as modulus and hardness decrease beyond this region.
A good analogy to this behavior (though not entirely correct) is the dashboard in your car. At 6:00 am the morning of a hot summer day, your dashboard will be stiff and hard. By midday, after the sun has beat down on it, it will have become softer, more flexible and have expanded somewhat. However, by 9:00 p.m. that evening, the dashboard has returned to a stiff, hard material. So as long as the heat does not push the dashboard beyond its elastic limits, this cycle is repeatable and non-destructive. The same holds true with thermoset plastics epoxy
What the numbers don’t tell you
Unfortunately, there’s a lot that the numerical value for Tg does not tell you. As you can see in Figure 1, this temperature is more accurately defined as a region and not a single point. Depending on the tests used and the reporting method, the number reported can vary widely. Typically, the number is given as the midpoint of the transition region, but what the value doesn’t tell you is how wide a region this actually is.
For example, say the Tg region of Epoxy Mold Compound A spans from 95°C to 115°C while that of Epoxy Mold Compound B spans from 98°C to 102°C, the Tg values associated with them would be reported as 105°C and 100°C respectively if the midpoint rule were followed. Looking strictly at the reported midpoint values, one would be inclined to select Epoxy Mold Compound A with a Tg of 105°C, but it likely that the 100°C Tg of Epoxy Mold Compound B would make a better candidate based on the narrower transition window.
Also, as was mentioned earlier, the Tg of a material is typically reported as the midpoint of the transition region, though this neither is a hard and fast rule. Truly of interest to a designer is the point at which material properties begin to change, not halfway through this transition. In this case, the designer would most likely prefer to have the Tg reported at the beginning of the transition region.
Taking the same example above, Epoxy Mold Compound A’s transition region spans from 95°C to 115°C and the supplier decides to list the material’s Tg as the midpoint of the transition region, 105°C. Epoxy Mold Compound B’s supplier lists the Tg as the beginning of the transition phase, 98°C. Again, Epoxy Mold Compound A would likely win out in the eyes of an untrained designer.
Conversely, two suppliers could provide the same Epoxy Mold Compound A, the first reporting the endpoint of the transition region, 115°C and the second reporting the beginning of the transition region, 95°C. Again, an untrained designer would be inclined to select the first supplier, even though the materials are identical.
DSC vs. DMA vs. TMA
Equally as important as the width of the glass transition region is the test used to determine the region. The three most popular ways of measuring this temperature are Differential Scanning Calorimetry (DSC), Dynamic Mechanical Analysis (DMA) and Thermo Mechanical Analysis (TMA).
Differential Scanning Calorimetry (DSC) is the fastest and easiest way of determining the Tg of a material. It involves taking a small sample of Epoxy Molding Compound, usually 10–20 mg and heating it in a pan at a constant rate of 10°C/min. The amount of heat absorbed is then measured and plotted as a graph of heat flow vs. temperature (Figure 2). The figure is then analyzed, and the first shift in baseline heat flow is defined as the DSC onset, the peak of this curve is defined as the DSC peak and (typically) the midpoint temperature of this transition is defined as the Tg.
Unfortunately, this method of analysis is not very reliable and results obtained by this method compared to those obtained by TMA and DMA often differ by 10–15°C or more.
Thermo Mechanical Analysis (TMA) uses the change in
TMA also takes additional preparation time. All the samples must be as close in size as possible and all readings need to be taken in the same place. This is because larger samples will have different cure stresses than smaller ones, and for the same reason, readings taken at the center can be different from readings taken towards an edge. Assuming that measurements are always taken in the same place, this method produces more repeatable results. Thermomechanical Analysis is also the way most Epoxy Mold Compound Manufacturers report the Tg of their EMCs.
Dynamic Mechanical Analysis (DMA) involves oscillating a load through a rectangular bar of
Though this technique is the most repeatable of the three listed, the heating rate and frequency of oscillation used will greatly influence the resulting Tg measurements. Faster heating rates will result in a higher Tg due to thermal lag, and a higher frequency of oscillation will lead to a higher Tg due to the inherent frequency dependence of viscoelastic materials. Additionally, the data obtained can be interpreted in several ways. The summary of the considerations of each of these tests is listed in Table 1.
|Method||Typical Run Time||Sample Preparation||Repeatability||Dependability||Comments|
|DSC||20 minutes||Easy||Good||Marginal||Many materials do not show a clear transition|
|TMA||40 minutes||Medium||Fair||Good||Dependent on sample preparation|
|DMA||120 minutes||Difficult||Excellent||Good||Rate of heating, oscillation and interpretation can lead to different values|
Now that it has been established that the Tg is a range, and not just a numerical
The chemistry of epoxies is known as addition chemistry. This means that in order to obtain specific material properties, part A (known as the resin) is mixed with part B (known as the hardener). The resultant mixed chemistry has certain properties including viscosity, hardness, CTE and Tg.
With this type of chemistry, it is very difficult to obtain a Tg that is much higher than the cure temperature, especially with short cure times. For example, if the epoxy mold compound is cured at 80°C, the Tg of the EMC will not typically be much higher than 85°C – 90°C. An epoxy matrix will gel rapidly until the Tg reaches the cure temperature, then the cure rate slows considerably. In the case of many high-temperature cure epoxy mold compounds with multiple cure schedules such as 5 minutes at 165°C, the product will appear cured in approximately 2–3 hours at 100°C, but will not reach the high Tg (typically 130°C) stated on the technical data sheet. So although there are longer, lower-temperature cure cycles for these high-temperature cure EMCs, the high Tg listed on the technical data sheet will not be reached by these cycles. Higher temperature cures are required to achieve the high Tg listed by high Tg epoxy molding compounds.
But simply exposing these epoxies to higher temperatures is also not sufficient. Time must be allowed for a heat cure epoxy to “soak” at these temperatures. This additional soaking time allows the matrix to complete its cure. A case study was performed on a heat cure epoxy with an ultimate Tg listed on its technical datasheet of 125°C. Following the recommended cure temperature of 150°C yielded the ultimate Tg in 2 hours. A lower cure temperature of 120°C yielded a Tg of 100°C in 1 hour, but an additional 2.5 hours was required to reach the ultimate Tg of 125°C. Further reducing the cure temperature to 100°C yielded a Tg of 100°C in 2.5 hours, but a full 8 hours was required to reach the Tg of 125°C listed on the technical data sheet. These results are summarized in Table 2 and a more detailed explanation can be found in our article, “How Epoxy Mold Compounds Cure”.
|Cure Schedule||Ultimate Tg|
|2 hours at 150°C||125°C|
|1 hour at 120°C||100°C|
|3.5 hours at 120°C||125°C|
|2.5 hours at 100°C||100°C|
|8 hours at 100°C||125°C|
With the above understanding, let’s examine a common example where an untrained designer might come up with some erroneous conclusions. For demonstration’s sake, let’s say that the designer selects the above-mentioned epoxy mold compound with a Tg listed on its data sheet of 125°C. In order to cure this epoxy, he selects the alternate cure schedule of 2.5 hours at 100°C. Then, satisfied that he has thoroughly cured the epoxy, he runs a DSC test to verify his conclusions. As explained above, a typical DSC test takes a sample of EMC and heats it a constant rate of 10°C/min. Since he is expecting to get a value around 125°C, he will likely run the test to about 150°C. Low and behold, his expectations are met, as the curve indicates that the Tg is around 125°C. What he hasn’t realized is that the test itself has post-cured the epoxy, pushing the sample’s Tg above the 100°C that it would have otherwise achieved.
This example raises two important questions:
- Do the higher cure temperatures needed to increase a material’s Tg need to be performed in a single stage?
- What is the effect of an under-cured epoxy mold compound?
Post-bake cure cycles for Epoxy Mold Compounds
As demonstrated in the above example, a future, secondary-stage cure cycle can be used to increase the Tg of an epoxy mold compound. This means that a primary cure cycle can be used to effectively “gel” the material in place.
In fact, many companies — particularly capacitor manufacturers — do not post mold cure their epoxy mold compounds at all. The in-mold cure gives sufficient mechanical properties, and any increase in performance of EMCs can be obtained during operation.
Effect of an under-cured epoxy mold compound
The short answer is that an under-cured epoxy has not maximized its full potential in terms of environmental resistance, flexural strength and modulus, and glass transition temperature. A better answer will reveal that fully maximized properties may not always be required for all given applications. Although longer, hotter cure schedules will give closer to 100% cure, lower or shorter cure cycles will often give properties that are 90% — 95% those of their fully cured counterparts.
The glass transition temperature, Tg of epoxy molding compounds is better described as a range than a single value on a technical data sheet. An EMCs Tg can be tested in several ways, including DSC, TMA
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