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58 The PCB Magazine • March 2017 evated temperatures, resulting in the material becoming increasingly rubbery as the temper- ature climbs, and ultimately becoming plastic above the melt temperature (Tm)—and thus the term thermoplastic. The fundamental polymer structure of these materials does not change with temperature, only the degree of molecular entanglement. Nor is the basic polymer com- position altered by repeated melting and cool- ing cycles. Some common thermoplastic res- ins used in electronic manufacturing include polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polycarbonate, polyamide, polyetheretherketone (PEEK) and polyvinyl chloride (PVC). Additionally, thermoplastic materials can be categorized into two subclasses based on their sub-Tg molecular arrangement: amorphous and semi-crystalline. In amorphous thermoplastic polymer systems, the molecule chains are com- pletely randomly arranged and tangled with each other. The molecules of semi-crystalline thermoplastic polymers form a crystalline struc- ture in some regions of the polymer matrix. (A crystalline structure is any structure of ions, molecules, or atoms that are held together in an ordered, three-dimensional arrangement.) In addition to other performance differences, the pockets of ordered polymers in semi-crystalline thermoplastics impart a degree of toughness in the transition region between the Tg and the Tm as compared to amorphous thermoplastic materials. Thermosetting polymers undergo a non-re- versible chemical reaction during curing. Lower molecular weight compounds like monomers and oligomers are chemically cross-linked and form high density, three-dimensional polymer networks. Material formulations based on ther- mosetting polymers often require energy input such as heat or high intensity light to initiate, accelerate and complete the polymerization process. Some of the more common thermo- setting polymers used in electronic assembly applications include epoxies, silicones and urethanes. Generally, thermosetting materials exhibit lower coefficient of thermal expansion (CTE), better temperature tolerance and chemi- cal resistance than thermoplastics. Thermosets have dominated the circuit board materials and electronic assembly markets. Epoxies have been the primary chemistry for circuit board lami- nates, solder masks, adhesives and potting com- pounds. Polyimide films are a mainstay for the copper-clad flexible substrate market, urethanes are used for many types of conformal coatings, and silicones are a binder common in thermal interface material (TIM) formulations. Rigid PCBs—The Industry Workhorse Circuit boards have been around for decades. In their oldest, and still most common, embodi- ment, a PCB consists of copper circuits formed on rigid thermoset polymeric substrates. The cir- cuits are fashioned subtractively by employing photolithographic resists to create a pattern on the surface of the copper-clad laminate. When the imaged copper is etched and the resist is re- moved, the basic printed circuit construction is formed. Copper circuitry works well for a vari- ety of reasons including the fact that it's very conductive (1.72E-08 ohm-m), solderable and a comparatively abundant element. Rigid circuit board laminates are commonly composed of woven glass fibers impregnated with a thermo- set resin (typically epoxy). This combination of cured thermoset resin and glass cloth creates a stiff and stable substrate which is relatively easy to handle in fabrication and assembly process- es. This type of construction is also conducive as a rigid substrate for mounting components and can withstand the temperatures required for forming soldered connections. Circuit board technology has progressed far beyond this very basic conductive copper circuit on an epoxy glass substrate model and STRETCHING BEYOND FLEX " In amorphous thermoplastic polymer systems, the molecule chains are completely randomly arranged and tangled with each other. "