CHILLIN' OUT With New Pre-preg Materials

Nov 1, 2001 12:00 PM
By Courtney Furnival, Semiconductor Packaging Solutions, Lake Arrowhead, Calif., and Paul Klouda, Da

To accommodate ever-increasing power density, power products and power circuits require substrates and p. c. boards with lower thermal resistance. At the same time, price pressures — along with mechanical and electrical limitations — prevent ceramic substrate materials like berylia, alumina, and aluminum nitride from filling this need. Conventional boards with FR-4 glass filled resins cannot remove sufficient heat from power components. Furthermore, higher power applications are expanding from single-layer surface mount and chip and wire construction to include power and thermal multilayers.

Isolation or dielectric layers add the highest thermal resistance to these substrates and boards. In insulated metal substrate boards, the thermal resistance through the dielectric layer is the major portion of the resistance between the component pad and the back of the substrate. Newly developed thermal dielectrics offer significant reduction in the dielectric's thermal resistance, while taking advantage of conventional low-cost, standard p. c. board laminating techniques.

Thermal dielectric laminates consist of resins filled with ceramic powders. The fillers provide improved thermal conductivity. You can make these filled resin systems into thinner layers than conventional ceramic substrates. In the late 1970s, the Japanese developed this technology, consisting of applying the ceramic-filled resin system to metal baseplates. Initially used for power audio applications, it quickly expanded into higher power and voltage commercial and industrial applications. By the 1990s, the United States and Europe used this type of resin-ceramic dielectric extensively in high-performance dc-dc converters and other types of power supplies. Today, automotive, motor control, UPS, welding equipment, and computer products use this technology.

You can produce resin-based dielectrics filled with thermally conductive ceramics by a number of processes; however, our focus will be those dielectrics you can laminate and etch with standard p. c. board fabrication techniques. Pre-preg materials are dielectric materials that are laminated to make p. c. boards. The thermally conductive pre-preg materials are T-preg^™. We'll describe some unique high performance T-preg materials that offer exceptionally high thermal conductivity with excellent electrical insulation and mechanical properties. Single layer, double and multilayer p. c. board constructions are possible using these T-preg materials.

Thermal Conductivity

T-preg technology consists of special matrix-filler systems. There are three T-preg materials distinguished by their T_g, or glass transition temperature, above which the pre-preg material loses its flexibility and becomes “glass-like”:

• 1KA T-preg is a mature yet unique material with a boron nitride filler and a conventional low T_g Resin. It provides a thermal conductivity of 3.0 W/m°C.
• 1HTA T-preg is a new material with a mix of alumina thermal fillers enhanced with boron nitride and a premium high T_g resin system. It provides a thermal conductivity of 2.5 W/m°C, which is superior to conventional alumina filled pre-pregs.
• 1HTB T-preg material combines boron nitride fillers and a high T_g resin system. It provides the best of 1KA and 1HTA material properties with high thermal conductivity, high dielectric isolation, low dielectric constant, and superior mechanical, and high temperature properties.

Thermal conductivity of the ceramic-filled resins is lower than that of ceramic substrates, but the filled resin can be produced in much thinner dielectric layers. This reduces the overall thermal resistance of the dielectric layer. The resin-ceramic composite is more flexible than ceramic substrates, which makes it more compatible with heavy copper foil, aluminum and copper baseplates, and surface mount packages. Figs. 1(a), on page 15, and 1(b), on page 16, show the achievable thermal resistance vs. component pad area for a T-preg thickness of 0.004 in., 0.006 in., 0.008 in., and 0.010 in.

In applications where the substrate or board transfers heat to the ambient air, the largest thermal resistance is generally between board surface and the air. Using heavy copper foil planes or an aluminum baseplate, you can effectively minimize the thermal resistance of the board-to-air interface by uniformly distributing the heat from the power component across the full board area. The thermal board or substrate enhances the heat distribution by minimizing the thermal resistance through the dielectric layer to the distribution plane. The resultant thermal resistance of the combined T-preg dielectric and copper foil spreading is designed to be small relative to the thermal resistance between the board surface and the ambient air. Fig. 2(a), on page 18, shows the achievable thermal resistance between the pad and ambient air as a function of board area with natural convection, using various metal and dielectric layers. The thermal resistance is calculated from temperature measurements between a 0.4 in. × 0.4 in. surface mount device and the back of the board.

Fig. 2(b), on page 18, is an example of thermal resistance with forced convection. These simplified examples assume a uniform board temperature and a thermal resistance between the board and air of 50°C/W per square inch in natural convection and of 30°C/W in forced convection, with heat exchange occurring uniformly on both sides of the board. The examples show lower thermal resistance when coupled with the appropriate heat spreading planes and effective heat removal techniques.

Glass Transition Temperature

The optimum electrical, mechanical, and reliability performance generally occurs when the dielectric's T_g is higher than the maximum continuous operating temperature of the thermal substrate or board. In applications where these factors are not stressed, a lower T_g dielectric is usually adequate. The coefficient of thermal expansion (CTE) of T-pregs increases above the T_g, and both mechanical and electrical properties change. These factors may create reliability problems when operating the board above the T_g and in severe temperature cycling conditions.

For most dielectric materials, the mobility of natural ionic impurities in the dielectric increases with higher temperature. The mobility increase is significantly greater above the T_g, which results in a decrease in insulation resistance or volume resistivity above the glass transition temperature. Fig. 3 shows how the volume resistivity changes with temperature for 1KA and 1HTA T-pregs. The 1KA has a T_g of 105°C and the 1HTA's T_g is 165°C. At very high dc voltages there can be potential long-term reliability issues with T-preg boards operated continuously above their T_g. There are differences between T-preg materials, but as a rule of thumb applications with continuous operating potentials above 125 V/mil require a T-preg with a T_g above the maximum operating temperature.

1HTA T-preg is specifically formulated for high voltages and operating temperatures of up to 150°C. It is ideal for applications such as industrial motor controls, ballast lighting, UPS, welders and other products that operate on 220 and 440 Vac lines with associated high dc bus voltages. The T_g of 165°C also makes it suitable for automotive under-hood applications where operating temperatures may reach 140°C or higher.

Dielectric Properties

The type and percentage of ceramic fillers influence the dielectric constants and dissipation factors of T-preg materials. Alumina filled T-preg materials typically have dielectric constants of 6.0 to 7.0 at 1 kHz, while boron nitride filled materials typically have dielectric constants of 4.0 to 4.5 at 1 kHz. Fig. 4 shows the dielectric constant for the boron nitride filled 1HTB and the 1HTA alumina filled T-pregs as a function of frequency. Although dielectric constants vary with filler loading, the examples are generally representative of these types of materials.

1HTA T-preg has a dielectric constant similar to conventional thermal T-pregs, so it is directly interchangeable with boards and substrate layouts designed for conventional alumina-filled materials. Boron nitride filled T-pregs, with a lower dielectric constant, are advantageous in some higher frequency applications. These advantages can include lower ground currents, lower losses and less capacitive coupling.

Dissipation factor is a measure of electrical losses with applied ac voltage, and includes both capacitive and resistive losses. Capacitive losses generally increase with higher dielectric constant and higher frequency. Resistive losses correspond to the dc insulation resistance or volume resistivity, and do not change with the dielectric constant. Fig. 5, on page 24, shows the dissipation factor as a function of frequency for 1KA and 1HTA T-preg.

AC reliability is generally not influenced by dielectric constant or dissipation factor. If operated at extremely high voltages where there is partial discharge within the dielectric, ac voltages can degrade the dielectrics resin matrix. The voltage level at which partial discharge occurs is called the “corona inception level” (CIL). The CIL for T-pregs are generally greater than 4 kV_p for 4 mil dielectrics and greater than 6 kV_p for 8 mil dielectrics. Continuous recurring peak voltages should be kept well below the CIL of the T-preg layers. When using T-preg materials in very high ac voltage applications, it is recommended that partial discharge testing be performed on the actual board or substrate as a part of the design qualification.

Mechanical Properties

Since conventional thermal T-pregs are alumina filled, they require very high filler loading to achieve adequately high thermal conductivity. This can result in rigid, brittle, and abrasive dielectric layers. The addition of boron nitride to the 1HTA mixed-filler system reduces the rigidity, and the exclusive use of boron nitride (BN) in the 1KA T-preg effectively eliminates excessive rigidity. This creates a much more flexible dielectric layer that is much more immune to thermal expansion mismatch. This expansion mismatch tolerance makes the boron nitride filled materials more compatible with heavy copper, large thermal vias, and a broader range of surface mount components and materials. These advantages are best demonstrated by temperature cycle testing of finished substrate and board products. In addition, T-preg materials containing BN are easier to fabricate into etched boards and substrates. Conventional thermal dielectrics with highly filled alumina are very abrasive to the punching and cutting tools used to singulate parts and drill holes.

The T-pregs with a BN additive not only have a lower loading of ceramic, but the BN powder acts as a lubricant as a result of its soft, graphitic crystal structure. The table provides important thermal, electrical, and mechanical parameters of three types of thermal pre-preg dielectric materials discussed in this article.

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