Design Variables of Thermoforming Process on Printed Electronic Traces

Gill M. , Gruner A., Ghalib N., Sussman M., Avuthu S., Wable G., Richstein J.

One specific market space of interest to emerging printed electronics is In Mold Label (IML) technology. IML is used in many consumer products and white good applications. When combined with electronics, the In Mold Electronics (IME) adds compelling new product functionality. Many of these products have multi-dimensional features and therefore require thermoforming processes in order to prepare the labels before they are in-molded. While thermoforming is not a novel technique for IML, the addition of printed electronic functional traces is not well documented. There is little or no published work on printed circuit performance and design interactions in the thermoforming process that could inform improved IME product designs.

A general full factorial Design of Experiments (DOE) was used to analyze the electrical performance of the conductive silver ink trace/polycarbonate substrate system. Variables of interest include trace width, height of draw, and radii of both top and bottom curvatures in the draw area. Thermoforming tooling inserts were fabricated for eight treatment combinations of these variables. Each sample has one control and two formed strips. Electrical measurements were taken of the printed traces on the polymer sheets pre- and post- forming with a custom fixture to evaluate the effect on resistance. The design parameters found to be significant were draw height and bottom radius, with increased draw and smaller bottom curvature radii both contributing to the circuits' resistance degradation.

Over the ranges evaluated, the top curvature radii had no effect on circuit resistance. Interactions were present, demonstrating that circuit and thermoforming design parameters need to be studied as a system. While significant insight impacting product development was captured further work will be executed to evaluate different ink and substrate material sets, process variables, and their role in IME.

As expected, there is a parabolic mean variation of resistance as a function of trace width. This study did not consider variability and shape of the ink deposit, which could introduce other variance.

The wider the traces, the less the draw height impacts the electrical performance.

The sharper the bottom radius, the more stress is induced with an increased probability of trace discontinuities.

The top radius did not have an impact on the resistance changes after forming in the range studied as a main factor.

There were numerous design interactions between the design variables. At the lower draw height, the radius did not have an effect. However, as the draw height increased, it caused a large difference in resistance. This translates into product design and electrical operating limits.

This study covered a limited range of varying radii, however, additional levels in these factors will be included in later studies. Future investigations will address interactions in more detail to determine potential design trade-offs for product design and functionality.