The printed circuit board industry makes copper-clad circuits in four different classes, the traditional rigid circuits, flexible circuits, circuits meant for high speed and high frequency, and High Density Interconnect or HDI. Traditional flex circuits are popular as they are primarily of low layer-count, are highly flexible, and used in static as well as dynamic motion applications.
There are also the rigid-flex circuits with the ability to join flexible circuits with multilayer rigid PCBs. Conceptually, this is like multilayer PCBs with built-in flex circuit layers. Typically, FR-4 type materials form the rigid areas, and polyimide-based materials make up the flex layers. Although consumers find the rigid-flex circuits highly useful, manufacturers find them problematic because of reliability and manufacturing issues. However, manufacturers have now fine-tuned their rigid-flex technology and are better equipped to handle the reliability and manufacturing issues. As a result, they can now make rigid-flex effectively and with excellent reliability.
Difference in Material and Construction of Rigid and Flex Circuits
Properties of materials used for flexible circuits are great for typical end-use applications. However, just as all materials do, those for flex circuits also have potentially limiting properties such as high coefficient of thermal expansion (CTE), poor thermal conductivity, relatively high moisture absorption, and poor dissipation factor. Manufacturers are addressing these issues by using specific combinations of a wide range of adhesives and polyimide films for constructing flex circuits.
Customary flexible circuit materials suffer from one major issue with performance at high frequencies. There are two basic reasons for this. First, unlike rigid circuits, flexible circuits do not have glass reinforcement in their base dielectric. Rather, they contain different grades of polyimide as the dielectric, providing both flexibility and mechanical integrity.
The second reason is flex circuits do not use soldermask to cover the outer layers. Soldermask is a brittle substance that cracks when flexed. Instead, the sides of a flex circuit are covered with an adhesive as a conformal coating and referred to as coverlay. Additionally, copper clad flex cores are bonded together on both sides of a polyimide layer referred to as bondply.
All the above not only make the dielectrics of flex circuits different, the processes involved in building up the base dielectrics are also different from those used for rigid PCBs. While manufacturing, the dielectrics are made in large rolls of coated films and the lamination to copper takes place as a separate step. The process allows very consistent thicknesses of these cast films in the range of 100 µm.