The PCB Substrate Materials
This article aims to introduce the different types of materials used in the printed circuit board(PCB) substrate. The selected substrate material must be suitable for the designer’s application. The packaging substrate is an essential part of electronic packaging and a bridge between the chip and the external circuit. The substrate plays the following roles in the packaging:
- Realize the transmission of current and signal between the chip and the external circuit
- Mechanically protect and support the chip
- It is the main way for the chip to dissipate heat to the outside world
From the material point of view, the commonly used substrates include the organic substrate (FR-4), ceramic substrate, and metal substrate.
FR-4 material is generally considered the standard material for printed circuit boards (PCB). The cost of FR-4 material is affordable, which makes FR-4 material as a standard option for PCB. Compared to FR-1, FR-2 and FR-3 materials, FR-4 material is the best selection to make single-side to multi-layer PCB. FR is a code for the grade of fire-resistant materials and 4 represents the woven glass-reinforced epoxy resin. There are various types of FR-4 grade material used in PCB, and most of them are composite materials made of tera-function epoxy resin, filler and glass fiber. The main technical features of FR-4 material are stable electrical insulation, excellent surface flatness, and standard thickness tolerances, all of which make FR-4 material suitable for products with high-performance electronic insulation requirements. FR-4 material maintains high mechanical strength and good insulating ability in dry and humid environments because of its low moisture absorption. However, FR-4 material is not suitable for high-frequency PCB because FR-4 material has a higher dissipation factor (Df), which means that as the signal frequencies increase, more signals will be lost. Furthermore, FR-4 material is not suitable for use in high-temperature environments such as aerospace applications.
Examples of FR-4 material:
- Standard FR-4: the glass transition temperature (Tg) of this type of FR-4 is between 130 to 140
- Mid Tg FR-4: the glass transition temperature (Tg) of this type of FR-4 is between 150 to 160
- High Tg FR-4: the glass transition temperature (Tg) of this type of FR-4 is greater than 170
- High speed and very low loss HDI material: an example of this FR-4 material is the Isola I-speed
The Ceramic board is made of thermally conductive ceramic powder and organic adhesive. Similar to the term FR-4, ceramic refers to a series of materials with similar chemical structures and physical properties. The ceramic substrates of printed circuit boards are mostly aluminum oxide, aluminum nitride, and beryllium oxide. As for silicon carbide and boron nitride, they are also prevalent ceramics with similar properties. Compared with the FR-4 substrate, the ceramic board has a higher thermal conductivity ranging from 9 to 20 W/mk and has some advantages in certain applications. In other words, ceramic is an ideal material for a new generation of large-scale integrated circuits and power electronic modules. For instance, in the field of high-power LED lighting, ceramics and metal with excellent heat dissipation properties are usually used to prepare substrates. Other advantages of the ceramic board are:
- Closer thermal expansion coefficient to the values for their conductor structure
- The metal film with lower resistance
- The excellent solderability of the substrate and the high-use temperature
- The low high-frequency loss
- Good insulation
- The high-density assembly
- The long-term usage in a reducing environment
- The high reliability in the aerospace field
- Desirable mechanical strength
The metal substrate in PCB means that the core material is metal, which can quickly dissipate a large amount of heat to prevent component damage. At present, the most commonly used metals in the manufacture of the metal substrate are copper and aluminum. Both of them have excellent ability of heat transfer. Compared with aluminum, copper is more costly, heavier, and less rigid. Therefore, aluminum is a more economic option in the metal substrate selection. The applications of the metal substrate include LED lights, industrial power equipment, input-output amplifier, semiconductor refrigeration device, etc.