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18 PCB007 MAGAZINE I OCTOBER 2023 display higher values (~55 ppm/°C). e quantitative differences in CTE between cop- per and organic materials can better explain why delamination may occur: Some types of materials just expand and contract more easily than others, leading to stresses and ultimately cracks during reliability testing. But why do some materials have higher CTE values than others? Why do organic materials like epoxy and polyimide have higher CTE val- ues compared to metals and ceramics? e CTE is contingent on the bond strength between the atoms that make up that material. Covalently bonded materials exhibit strong shared bonds between the individual atoms, resulting in very low CTEs. For this reason, dielectric layers in PCBs are oen reinforced with fillers like fused SiO 2 , in which silicon is covalently bonded to oxygen (SiO 2 CTE = 0.5 ppm/°C). Other popular fillers include ceramics such as Al 2 O 3 (CTE = 8 ppm/°C) or TiO 2 (CTE =10 ppm/°C). Ceramic mate- rials exhibit ionic bonding, where the oxygen atom completely transfers its valence electrons to a metal atom, generating two oppositely charged ions. e unequal sharing of electrons yields a slightly weaker bond and contributes to a higher overall CTE (compared to materi- als that are purely covalently bonded). What about metals? In the early 1900s, Paul Drude described the bonding between metal- lic atoms as a "sea of electrons," in which met- als can be modeled as a lattice of positively charged metallic cores, all sharing a vat of com- munal valence electrons. e electron delocal- ization (or electron sharing) in metals results in an even lower bonding energy than covalent or ionic bonding, which may contribute to why pure metals like copper typically have higher CTE values than ceramic fillers. Finally, we arrive at the metaphorical glue holding together the PCB dielectric layer: the polymer. e CTE values of typical poly- mers can range from 20–100 ppm/°C (from 20°C–25°C), which are much higher than ceramics or metals. Polymers are defined as chemical compounds in which individual mol- ecules are bonded together in long, repeating chains. (Imagine pearl beads bound together to make a necklace.) e low energetic bar- rier of polymer chains to move and undergo conformational changes can help explain their higher CTE values. Bearing all this in mind, it is the task of for- mulation scientists to develop dielectric lami- nates, prepregs, and bond plies that will mit- igate the impact of thermal expansion within a PCB and ultimately pass thermal reliabil- ity testing. By considering the intrinsic mate- rial properties of all components (and combin- ing them in appropriate quantities), it is pos- sible to create new materials that can meet, or even surpass, thermal reliability perfor- mance requirements. For example, fastRise™ TC (from AGC Multi Material America) is an example of a non-reinforced (pure resin, fiberglass-free) bond ply that exhibits excel- lent thermal reliability. By matching the CTE of copper (18 ppm/°C), this material is able to expand and contract at the same rate as the copper, mitigating stresses formed in the PCB during thermal reliability testing. In following IPC-TM-650 2.5.27, 24 panels of test coupons made from this material underwent 200 ther- mal cycles from 25°C to 260°C without exhib- iting any failures. is is especially remark- able, considering that most dielectric materi- als on the market contain fairly large amounts of polymeric components. Breaking through barriers like this illustrates the challenges and fulfillments of formulation science: pushing past intrinsic physical limita- tions to create something greater than the sum of its parts. It is what first drew me to become a materials scientist, and what will keep me grounded in this field for years to come. PCB007 Preeya Kuray, PhD, is a material scientist at AGC Multi Material America. To read previous columns, click here.