Dr. H. W. Rauhut
Research Scientist, Dexter Electronic Materials Division
Technical Paper, April 1996
A SPECIAL POSITION
Epoxy molding compounds hold a special position in electrical and electronic applications, primarily because of their unusually low melt viscosity in combination with excellent mechanical and electrical properties. Besides electrical insulation, they feature electronic compatibility. They are easily transfer molded at low pressure, typically at 1 Kpsi (70.3 Kg/sqcm) and 160–180°C. Therefore, their charter has been the encapsulation of electrical and electronic components. Among other thermosets, silicones still hold a niche in electronic encapsulation. In the early history of transistor encapsulation, phenolics were also commercially used. But in general, phenolics and other thermosets require transfer pressures at or above 10 Kpsi. One can imagine what such pressure would do to wire sweep in integrated circuits.
Considering thermoplastics, they are mostly processed by injection molding at 200–375°C and pressures of 8–30 Kpsi. A few allow processing at 180–300°C, with transfer molding at high pressure. There have been injection overmolding applications with thermoplastics, where transistor wire bonds were protected by an epoxy powder coating, and of course thermoplastics have served many electrical applications other than encapsulation. In the inorganic field, we find that high performance ceramic packages for electronic components are very expensive. Thus, for the best balance of economics and performance, the plastics package is synonymous with epoxies.
Electrical performance contributed to the epoxies’ commercial success. The recent decade has seen a major proliferation of electronic device types, combined with greater quality awareness and expectation. Accordingly, epoxy compositions have changed, too. It is in order to look again at electrical data under varied conditions, even to revisit electrical ASTM test results. In addition, dielectric analyses (DEA) have helped shed new light into epoxy compounds and applications.
Basically, epoxy molding compounds consist of organic and inorganic materials. Most compounds are fused silica (FS) filled. Some contain crystalline silica or other filler selections. By hardener, epoxies can be divided into phenolic novolac (PN) and anhydride cured compounds, the latter for special, relatively lower volume applications. A general
Description of epoxy compounds is shown in Figure 1, with fillers, epoxy resin and phenolic hardener as major weight (and volume) fractions. These and other materials are main factors in electrical insulation and electronic performance.
Epoxy compounds have been 70- 74 % FS filled. Newer types contain a higher (up to 85%) fused silica level, and/or special epoxy resins (1). Most compounds are still based on ECN (Epoxy Cresolic Novolac). For some advanced applications, biphenyl epoxy, Tris-epoxy, or other epoxy resins are employed. Our performance study below focuses on highly filled fused silica systems with varied resins, all phenolic novolac cured and postcured 4 hours @ 175°C, unless indicated otherwise.
ELECTRICAL AND DIELECTRIC PERFORMANCE BY ASTM TESTS
Electrical insulation features may be described by such diverse parameters as Dielectric Strength (DS), Arc Resistance (AR), Volume (VR) and Surface Resistivity (SR), Dielectric Constant (K) and Dissipation Factor (DF), reference (2).
Dielectric Strength and Arc Resistance
The high-voltage dielectric strength test follows ASTM D149, by our testing at a thickness of about 20 mils. Test results significantly decrease with increasing specimen thickness, for instance by about 70% at 200 mils (5 mm). OS per mil at any thickness can be calculated with reasonable accuracy from a measured OS value and specimen thickness, according to reference (3) and the following formula:
In other words, calculated OS (y) equals the measured DS, times the square root of its specimen thickness, divided by the square root of a different thickness T (y) in mils. DS decreases with increasing temperature (for instance by 25% between 25 and 150°C). It also decreases by chemical, physical, long-time electrical stress, and specimen voids.
Figure 2 shows dielectric strength data @ 23°C on various ECN compounds over a spread of 15% filler, up to about 85% FS. There is an increasing OS trend with increasing FS level, which appears reasonable considering FS’s very high dielectric strength of 20 KVolts per mil. Figure 2 also shows a similar arc resistance trend line. AR is measured by ASTM 0495. Evidently, an increased FS level does not hurt an epoxy’s resistance to a high voltage, low-current arc, which may form a conducting surface path.
Volume and Surface Resistivity
Volume resistivity is an epoxy’s most important insulating characteristic. Both VR and SR are measured by ASTM 0257, at 500 Volts by our testing. VR and SR at room temperature and 100 Volts tend to be slightly lower than man 500 V data.
Figure 3 shows similar volume resistivities for varied resin systems after post-cure 6 hours @ 175°C. At 100°c, VR is typically one decade lower than at RT. The very high VR of FS filler contributes to VR data of highly filled compounds. ECN resin systems at RT without post-cure showed VR data up to one decade lower than with postcure, and also lower SR data. No significant VR and SR changes were observed after extended cure times (up to 3 minutes) in the mold. Through moisture exposure, VR of the highly filled ECN and Biphenyl resin compounds declined less than that of previous reference compounds, Figure 4. An anhydride-cured compound’s VR showed the greatest moisture sensitivity.
Room temperature surface resistivity of epoxies is 10–100 x 10′ Ohms. Typical SR100 data versus temperature is shown in Figure 5.
Dielectric constant (K) and Dissipation Factor DF (DF)
These parameters are measured according to ASTM 0150. The dielectric constant or permittivity reflects the capacitive nature of an insulator. A material between the power and signal layers of a printed circuit board controls the circuit speed by its dielectric constant (4). In general, an electronic design engineer prefers low dielectric constants (e.g., in the case of epoxy compounds, room temperature K‑values of about 4 @ 1 KHz), and also low dissipation factors (e.g., OF @ 1 KHz and RT of 0.002–0.003). DF is a key insulation parameter, like volume resistivity and dielectric strength. In an alternating electrical field, energy is lost as heat with increasing DF. In our study, various resin systems showed similarly low K, Figure 6, and similar DF within the range of previous ECN reference compounds, Figure 7.
Frequency and Temperature Effects on K and DF
Epoxy dielectric constants slightly decrease with increasing frequency and with decreasing temperature. K and OF changes are caused by dielectric polarization in the material. From highest to lowest frequencies, atomic/ electronic, then molecular dipole, then material inhomogeneity polarizations add up to a maximum K at the lowest frequency. Typical K data by ASTM 0150 is exhibited in Figure 8, and will be reviewed again below on the basis of dielectric analyses. Typical OF data is shown in Figure 9. Both K and OF assume higher values without postcure, as indicated in Figures 8–9 @ 150°C.
Batch-to-Batch Comparison of K and DF
Although epoxy compounds are made by batch processes, they display similar batch-to-batch dielectric constants and dissipation factors, Figures 10–11.
Material Effects on K and DF
Compound raw materials influence K and DF (5). Shown here are only comparisons of fused and crystalline silica (CS), with FS providing lower dielectric constants (K) and generally somewhat lower dissipation factors (DF) than CS, dependent on frequency, Figures 12–13.
Since fused silica plays such a dominating role in microelectronic compounds, its K and DF values are reviewed in Figure 14. Over a wide range, up to gigahertz frequencies, fused silica’s dielectric data remain low and will not cause a rise of K or DF at very high frequencies.
Moisture Effects on K and DF
Both dielectric constants and dissipation factors increase significantly with absorbed moisture, Figures 15–16. Fused silica, with its inherently low K, DF and moisture absorption, shows in its compounds smaller K and DF increases than crystalline silica systems. But moisture effects on K and DF of FS-filled compounds are still significant. One practical consequence relates to dielectric preheating if the moisture content of microelectronic epoxy powder is not controlled. Because of an increased dissipation factor, generated heat may be higher than desired for optimum molding.
DIELECTRIC PERFORMANCE BY DEA TEST
Dielectric analysis (DEA) measures capacitance and conductance versus time, temperature and frequency. The results are correlated to chemistry, rheology and mobility of ions or dipoles in a polymer system. DEA generates dielectric data and their temperature and frequency dependencies in a particularly efficient way, including dielectric constant (generally called permittivity), loss factor (loss index), dissipation factor (Tan Delta), and ionic conductivity. Data below was generated on a DuPont 2970 DEA analyzer using a ceramic parallel plate sensor and is an extension of previously presented DEA information (6).
Dielectric Constant, Loss and Dissipation Factors
Figures 17–20 show data of a highly FS-filled EC /PN compound. As the temperature increases, the various dielectric responses increase at a given frequency. In an alternating electrical field @ 0.3 Hz, dipoles can follow the field and thus contribute to very high dielectric constants (K) at elevated temperatures, shown in Figure 17 only up to 130°C At higher frequencies, the K‑versus temperature data tend to straighten out. However, on scrutinizing 10–100 KHZ data in Figure 18, one observes a K‑peak temperature around 220°C, due to the system’s rheological and dielectric correlation. Figures 19–20 show temperature and frequency effects on the same system’s loss and dissipation factors. Obviously, the loss factor becomes numerically similar to the dissipation factor (DF) at higher frequencies, particularly in the low temperature region.
Dielectric Data by DEA versus ASTM
Figure 21–22 allow a comparison of dielectric DEA and ASTM data on the compound described above. Although just based on available measurements, generally a close correlation was found at low test temperatures, which suggests that DEA may be considered as an alternate test method in place of ASTM D150. Figure 23 shows a projection of dielectric constants by DEA into higher frequency regions, since then confirmed by other means.
DEA Ionic Conductivity
The dielectric loss factor consists of two components, one due to ionic conductivity, the other one due to dipole orientation in the epoxy. At low frequencies, the loss factor is dominated by ionic conductivity. We measured @ 0.3 Hz. Below 150°C, ionic conductivities are very low, even in terms of minute pmho/cm units. There is a significant crosslink effect on ionic conductivity (6), shown by postcure versus no postcure data in Figure 24, since ion mobility is hindered in more viscous or tightly crosslinked (postcured) structures. For the same reason, higher glass transition temperature (TG) means also lower ionic conductivity, figure 25.
Dielectric Material Effects
DEA data also reflects material effects of epoxy compositions, as discussed earlier (6). Figures 26–27 show other examples of an additive, which helps decrease the dielectric constant, while slightly increasing the ionic conductivity of an epoxy. Figure 28 compares the inherently low ionic conductivity of anhydridecured epoxy with that of two typical novolac-cured compounds. In terms of dielectric constants, these compound types were found more similar. Many other examples exist that recommend DEA as a unique analytical tool in thermoset applications.
In this study, dielectric and electric epoxy compound data were analyzed using ASTM tests and Dielectric Analysis (DEA) to examine various factors such as temperature, frequency, batch-to-batch variations, moisture, materials, and the absence of postcure effects. The results were compared to data obtained from dominant fused silica (FS) filler. The findings indicated that dielectric strength and arc resistance of the new compounds slightly increased with higher levels of FS. The volume resistivities, dielectric constants, and dissipation factors of different resin systems were similar, but decreased with higher temperatures or exposure to moisture. Frequency and temperature had an impact on the dielectric data, with higher values observed in compounds without postcure or with crystalline silica and moist FS fillers compared to reference compounds or conditions. The dielectric properties of ECN-based compounds remained consistent across different batches. Dielectric constants, loss, and dissipation factors increased with temperature, particularly at lower frequencies, and exhibited a peak at around 220°C due to rheological/dielectric correlation. Dielectric constants declined steadily at low temperatures towards the gigahertz range. Ionic conductivities increased with temperature and were influenced by postcure or glass transition temperature. The study concluded that dielectric analyses effectively described material effects of epoxy compositions. The DEA method, which involves efficient temperature and frequency scans, was recommended for generating dielectric epoxy data when ASTM D150 is not specifically required and could be explored further with other thermosets.
CAPLINQ specializes in the distribution and technical expertise of specialty chemicals and materials, including but not limited to thermal interface materials, epoxy molding compounds (EMCs) and coating powders, and adhesives. In particular, CAPLINQ represents EMCs from Hysol Huawei Electronic Co. Ltd. in Europe, America, and Asia while we also manufacture our own line of products.
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- H.W. Rauhut, New Types of Microelectronic Epoxy compounds, SPE, Thermoset RETEC, Chicago 1994
- R. Seeberger, Capacitance and Dissipation Factor Measurements, IEEE Electrical Insulation Magazine, Volume 2, No. 1, January 1986, pp. 27–36
- E.R. Salmon, Dielectric Strength of an Insulation Material ls it a constant?, IEEE Electrical Insulation Magazine, Volume 5, No. 1, January/February 1989, pp. 36–38
- J. Brauer, IBM, Design and Materials, Electronic Materials Handbook, Volume 1, Packaging ASM International, 1989, Section 1, page 1
- H. Lee and K. Neville, Handbook of Epoxy Resins, McGraw-Hill Book Company, New York, 1967
- H.W. Rauhut, Dielectric Analyses of Microelectronic Epoxy Compounds, SPE, ANTEC, Volume 1, San Francisco, 1994, pp. 941–950
List of Definitions
|Arc Resistance||the ability of an insulator surface to withstand conductive bridging (carbonization) by high voltage and low current. Breakdown Unit: Second.|
|Dielectric Analysis (DEA)||measures capacitance and conductance versus time, temperature, frequency. DEA data are correlated to chemistry, rheology and polymer mobility.|
|Dielectric Breakdown||an insulator’s complete failure at high voltage by disruptive electrical discharge. Unit: V/cm.|
|Dielectric Constant||the ratio of the capacitance of an insulator to the capacitance of vacuum at a given electrode configuration.|
|Dielectric Loss||electrical energy dissipated as heat in an insulator, due to ion and dipole motion/ friction in an alternating electrical field.|
|Dielectric Strength||a material’s ability to withstand voltage, i.e., maximum voltage required to break down a certain thickness of insulation. Unit Volts per mil (VPM) or V/mm.|
|Dissipation Factor (DF, loss tangent)||the ratio of the loss index to its permittivity (dieleltric constant).|
|Ionic Conductivity||the sum of ions per insulator volume, times their charges and mobility; determined by loss factor measurements at low frequency.|
|Loss Index or Loss Factor||the product of permittivity and dissipation factor. Ionic conductivity and dipole orientation are the components that contribute to the Loss Factor. At very low frequency, the loss factor is completely dominated by Ionic Conductivity.|
|Permittivity||the ratio of insulator capacitance to the capacitance of vacuum (better term than dielectric constant, which is not a constant). It is due to dipole alignment.|
|Power Factor (PF)||the ratio of energy in watts (dissipated in a material) to the product of effective voltage multiplied with the current, also expressed as cosine of phase angle.|
|Surface Resistivity||a material’s ability to resist passage of an electric current along the surface of an insulator. Unit: Ohm.|
|Volume Resistivity||a material’s ability to resist passage of an electric current through the cross-section of an insulator. nit: Ohm-cm.|