The difference between a rigid printed circuit board and a flexible circuit lies in the dielectric material sets manufacturers use for fabricating them. Most rigid printed circuits are made from glass epoxy, whereas the material of choice for a majority of flexible circuits is Polyimide. Development of several versions of polyimide enables tailoring the material to meet specialty requirements such as in solar arrays, for space applications, and for other unusual environments.
Although it is possible to form glass epoxy in very thin constructions and even bend it for simple applications, polymer films are most suitable for continuous twisting, flexing, and multi-planar folding. Films of polyimide withstand numerous bending cycles without suffering any degradation of their mechanical and electrical properties. Therefore, polyimide films perform reliably in applications where bend cycles of over a million are common. The inherent flexibility of polyimide films offers the electronic packager a wealth of design options. However, a disadvantage of polyimide films is their material dimensional stability is inferior to that of glass epoxy materials.
According to manufacturers, the dimensional stability of polyimide films depends on the residual stresses the manufacturing processes place in the film and its normal coefficient of thermal expansion.
However, the measure of stability represents only the effect of the film alone. The nature of stability grows more complex as the fabricator exposes the film to elevated temperatures and pressures for attaching the copper layers through processing to create an adhesive-less laminate, or through an adhesive lamination cycle. However, the process of creating a laminate and subsequently fabricating a circuit involves two different processing effects, and during each of these fabricating processes, the flexible substrate undergoes dimensional changes.
It is not easy to predict these changes. Raw material variation from batch to batch may cause dimensional changes to vary slightly. Changes also depend on the method of construction and processing conditions, as thin materials are likely to be less stable. Other contributing factors can be the percentage of copper etched, density of copper electroplating, ambient humidity, and material thickness.
Small dimensional changes in the circuitry panel is inevitable as it undergoes processing and exposure to a variety of etching, electroplating, pressures, temperatures, and chemistries. For instance, etch shrink is the stress etching copper releases, but fabricators mistakenly use it as a catch phrase for representing all the dimensional changes that a flexible circuit undergoes during processing.
Fabricators consider compensating for the above changes when setting up the part number for a new flexible circuit. However, accurate prediction of these feature movements requires empirical data from parts they actually produce.
Effects of Material Instability
Lack of stability in the film material is manifest in violation of the minimal annular ring requirement, and in extreme cases, cause full breakout of the hole-to-pad alignment. Another possibility is in the misalignment of the coverlay. For a predicable material change, the operator can adjust either the conductor layout or the drill pattern to re-center the plated-through-hole in the pad.
Dealing with Dimensional Changes
Fabricators deal with dimensional changes by limiting the panel size, and this works very well for cases where the tolerances are extremely tight. In small panel sizes, the effects of dimensional instability issues on registration and alignment are lower, and the handling damages are at a minimum. However, smaller panel sizes may be less efficient for processing as against those for larger panels, since in a circuit factory several costs are based on panel size.
Compensating for Dimensional Changes
It is possible to achieve cost-effective production with suitable panel sizes while compensating for dimensional changes. Fabricators can adopt the following methods to adjust for dimensional changes occurring during circuit fabrication:
Applying Scaling Factors
Where the dimensional changes of the material are predictable, fabricators can apply scaling factors to tooling or secondary layers. The in-process measurements for a given lot can allow fabricators to use scaling factors based on dynamic calculations. For instance, the measured scaling factor of a panel may form the basis of creation of its solder paste stencil. Another instance may be of a final drilling program compensated dimensionally for a multilayer circuit.
Applying Software Compensations
Alignment systems using software controlled operations can use optical fiducials to detect dimensional shifts and compensate for them. Such fabrication machines measure these targets present on the outside corners of the panel and perform a dimensional analysis. Proper alignment is then a process of applying the necessary X, Y, and theta corrections.
Fabricators often divide the panel into smaller arrays for handling dimensional changes. They do this usually after creating the circuit image. As the processing is on subsets of the panel, fabricators effectively gain some of the advantages of small panel alignment, but without compromising the cost advantages of processing a large panel.
Fabricators typically use optical targets on smaller subset panels to compensate for stencil registration commonly. They also use hard tool dies to cut smaller pieces at a time from a multi-piece panel.
Dimensional change is the primary difference between rigid and flexible circuitry, and this requires compensation. Even though material change in flexible circuitry is typically less than one tenth of one percent, it accumulates over a dimension of several units, and can be of a significant nature. For a flexible circuit, this compensation for the expected change becomes a critical part related to panelization. This also serves to balance maximizing process efficiencies and maintaining dimensional tolerances and accuracy.