Printed wiring materials, thermal

Thermal vias have been used to improve thermal conductivity of packages and modules, as well as to provide a design approach to balancing junction temperatures in multichip modules [25,26]. Testing has shown the thermal conductivity of new low-loss LTCC materials to be 20 times that of printed wiring materials [27]. 

Liquid encapsulation and potting compounds, e.g., for automotive parts Thermal and RT-curing adhesives, underfiller materials Composites, fiber-reinforced, e.g., for transportation or in printed wiring boards PCB build-up materials... 

Scanning acoustic microscopy (SAM) is an ideal nondestructive method for revealing internal flaws within materials or between material interfaces. SAM is extensively used in detecting voids, delamination, and other separations that can occur in adhesive-attached parts, especially after thermal cycling. SAM is particularly useful in the analysis or evaluation of many types of electronic parts, including ceramic and plastic-encapsulated ICs, plastic-encapsulated microcircuits (PEMs), hybrid micro-circuits, CSPs, PBGAs, and printed-wiring boards. 

Saytex CP-2000 is widely used for printed wiring boards laminates made from FR4 epoxies. It is reacted into the epoxy, so that there is no potential for leaching out of the resin. The FR meets the German Dioxin Rule. It is environmentally acceptable, thermally stable at high temperatures and does not disrupt resin properties when soldered. It can be used as an additive in ABS and also as a raw material in brominated epoxy oligomers in ABS and high impact polystyrene (HIPS). 

Hitachi has developed a heat-resistant material for printed wiring boards, consisting of an epoxy polymer containing nanosized silica, evenly dispersed by means of a sol-gel reaction. At 260 °C, the nanocomposite s elasticity was said to be about ten times higher than that of epoxy, and its thermal expansion coefficient was about one-third. 

The conditions under which the device is thermally characterized have to be clearly described. In fact, thermal resistance is made up of constant terms that are related to device and package materials and geometry in series with a number of variable terms. These terms are related to the heat paths from the package boundary to some point in the system that serves as the reference temperature and are determined by the method of mounting, the printed-wiring-board (PWB) if used, other heat generating components... 

Coefficient of thermal expansion Tee.—Surface-mount assembly process subjects the printed wiring assembly to more numerons temperature shocks than typical through-hole processes. At the same time, the increase in lead density has caused the designer to nse more and more layers, making the board more susceptible to problems concerned with the base material s coefficient of thermal expansion Tce- This can be a particular problem with regard to the z-axis expansion of the material, as this induces stresses in the copper-plated hole, and becomes a rehabihty concern. Figure 13.12 shows typical z-axis expansion for a variety of printed circuit base laminate materials. 

A basic understanding of material performance is necessary for both the designer and fabricator of the printed wiring board (PWB). Understanding the materials available is one of the primary tasks for designing for manufacturability and performance. It is necessary to match the product end-use performance requirement as well as the environmental exposure the board experiences in the fabrication and circuit card assembly (CCA) processes with the capabilities of the material. It is common for boards to experience as many as five thermal excursions in CCA. 

In addition to providing circuit interconnection, a multilayer printed wiring board (ML-PWB) provides the electrical and mechanical platform for the system. This means that the electrical and thermal properties of the ML-PWB material are very important for the proper functioning of the system. Among the properties of importance are dielectric constant, Du (also known as Er) dielectric loss, Df (or tan 8) glass transition temperature, Tg time to delamination,Txxx thermal decomposition temperature.Ta coefficient of thermal expansion, CTE and moisture absorption. The following sections discuss the importance of these properties to an ML-PWB snbstrate. 

Early ovens utilized focused and unfocused IR lamps, mounted in the reflow oven tunnel. These bathe the solder paste-coated circuit boards and associated components with a broad spectrum of photonic energy heavily weighted to the IR end of the electromagnetic spectrum. The radiant energy absorbed by the printed wiring assembly (PWA) and related materials brings about the thermal increase needed to reflow solder. As the board travels beyond the last reflow heaters at the exit end of the oven, the absorbed heat is lost to the environment or the board is actively cooled by fans. The cooling results in resolidification of the molten solder and solder-joint formation. 

High temperature thermoplastics such as polysulphones and polyetherimides are newcomers to the printed wiring substrate arena and offer the potential for the production of relatively low-cost moulded substrates with excellent electrical characteristics and thermal stability. The low dissipation factors characteristic of these materials have generated considerable interest in their potential for use in... 

Polymer dielectric materials have been in widespread use for several decades as insulators in printed wiring boards and as encapsulates in packaging. Currently, polymers are being used as the interlevel insulation materials in solid-state devices. Polyimides and fiuorinated polymers have ceptional insulation capability and the required thermal and environmental stability to survive both the rigors of the manufacturing process and the thermd cycling of long-term use. This book presents a substantial bo(ty of new information on advanced polymer systems for these applications. 

Electrical and electronic devices are made utilizing several various types of plastic materials, thus when discarded their waste is difficult to recycle. The plastics employed in housing and other appliances are more or less homogeneous materials (among others PP, PVC, PS, HIPS, ABS, SAN, Nylon 6,6, the pyrolysis liquids of which have been discussed above). However, metals are embedded in printed circuit boards, switches, junctions and insulated wires, moreover these parts contain fire retardants in addition to support and filler materials. Pyrolysis is a suitable way to remove plastics smoothly from embedded metals in electrical and electronic waste (EEW), in addition the thermal decomposition products of the plastics may serve as feedstock or fuel. PVC, PBT, Nylon 6,6, polycarbonate (PC), polyphenylene ether (PPO), epoxy and phenolic resins occur in these metal-containing parts of EEW. 

Aromatic polyimides have been the subject of much attention as a material for use in the electronics and other leading-edge industries. Films are used in flexible printed circuit boards, as wire and cable wrap, motor-slot liners, transformers and capacitors. Molded parts are used in applications requiring resistance to thermally harsh environments such as automotive transmission parts, thermal and electrical insulators, valve seats, rotary seal rings, thrust washers and discs, bushings. 

---