Advances in the testing of flexible circuits can improve feature inspection.

Timothy Weber, Buehler

Flexible circuits may seem to be a recent development, but they date back to German inventor Albert Hanson in 1898. He described the production of flat conductors on a paraffin-coated sheet to improve telephone exchange boards

What the waxed paper provided in flexibility it lacked in durability. Today’s product is sandwiched between a polymer cover layer and base film of either polyimide or polyester. Designers favor polyimide where soldering of the assembly is required, and it is the material of choice for nearly all chip-scale packages and flex ball grid arrays.

The wider application of flexible circuits took the recent development of new materials and manufacturing techniques, enabling the design to recently become one of the fastest-growing segments of the global printed circuit industry. New applications, usually involving smaller, more mobile devices, have pushed this expanded use along, particularly in the medical field.

Circuitry in medical devices — a tight squeeze

Medical device product specifications can limit the amount of available for circuitry. A multilayer flexible circuit board can handle all the control functions of a complex medical device and provide for greater functionality.

Evolving design can pose manufacturing and testing challenges. PCB manufacturers are finding that thinner material leads to smaller vias, reduced aspect ratio of vias, smaller lines and spaces, less copper plating and better fill grades, but it can also increase handling complexity.

Flexible circuits’ quality assurance needs differ from rigid PCBs. In response, testing systems manufacturers are developing new products and techniques to inspect features such as vias and through-holes. These approaches meet the unique needs of increasingly smaller and lighter flexible circuits demanded by the medical market

Metallography plays an integral role in analyzing and understanding the effects of these improvements on the PCB internal structure. Quality control benefits from evaluating the product at various stages of a new production process. Cross-sectioning samples can reveal vital details, including cracks, voids, conducting layers and interconnections.

Isolating problems and finding improvements

In other cases, a more targeted approach involves cross-sectioning the center-line of through-holes or progressively thinning the specimen and examining each layer. The more information gathered during an analysis, the higher the probability that the product can be further improved.

Labs cut these boards using a precision saw to minimize the potential for damage to the board. Cutting and handling these boards inappropriately can cause defects such as delamination that may be incorrectly attributed to the manufacturing process.

Testing equipment manufacturers offer precision saws with table attachments. These saws enable the testing lab to quickly cut flex and hybrid boards without damaging the area of interest while providing a higher degree of safety for operators than band saws or hand tools.

Flex circuits and the hybrid rigid-flex circuits in medical devices present a more significant challenge in targeting regions of interest. The same properties that make these board materials (often PEEK or polyamide) useful also introduce the potential for inaccuracies.

Dealing with small feature size

Note that these features are, in many cases, quite small, 4 mil (~100 microns), or even less. Pin placement variability (when pinning boards for processing) or positioning of board coupons in the accessory when mounting must be avoided.

These boards are often very light compared with rigid PCBs. Care is needed to prevent coupons from “floating” in media during mounting, which can also lead to inaccuracy in targeting.

Reduced feature size has been the trend along with the miniaturization of medical devices. Smaller feature size enables flexible boards to have multiple rows of features, often at varying dimensions within the same board. These boards have targeted features of varying sizes and features placed at different distances from a reference within the board.

Testing equipment manufacturers have developed accessory kits for semi-automatic grinder-polisher equipment that can target these smaller features. These latest kits can target such features in relatively high production volume, with up to 18 PCBs able to be processed in less than 30 minutes. These newer accessory kits also have a significantly reduced cost.

They can also handle tighter tolerances that have been specified on small features, the use of polycrystalline diamond stops that resist wearing when compared to mono-crystalline or carbide stops, and the reduction of the span of the reference pins used in the kit, thus ensuring these features are presented in a plane perpendicular to the view under the microscope. Kit manufacturers can provide technical expertise specifically for users working with small (<= 4 mil) feature sizes and aspects of preparation to pay particular attention, which enhances the capability of success in targeting.

Happening at the same time with these developments in PCB production, preparation, and inspection, there has been an increased demand for reliability. Accordingly, addendums to IPC standards have been published not only for medical devices but other industry segments such as aerospace and automotive.
Proper preparation provides the capability to verify that these reliability standards are met. Testing equipment manufacturers continue to work with many PCB manufacturers to ensure compliance with IPC specifications.

Timothy Weber is senior metallurgical engineer for Buehler, focused on metallographic materials preparation, image analysis and materials characterization. Prior to joining Buehler, he conducted research in fabrication and characterization of thin films and coatings at Argonne National Laboratory.

The opinions expressed in this blog post are the author’s only and do not necessarily reflect those of Medical Design and Outsourcing or its employees.