Estimation of through-Plastic Via (TPV) Filling through Computed Tomography for Dielectrics and Conductive Inks in Flexible Printed Electronics
Authors: Sondhi Kartik , Avuthu Sai Guruva Reddy , Fan Hugh , Nishida Toshikazu 
Published: February 2020

Computed Tomography (CT) is an advanced optical imaging tool for non-destructive 3D analysis of test structures compared to the conventional imaging tool Scanning Electron Microscopy (SEM). SEM has been used extensively for analyzing the via filling1, however as per the available literature the CT-scanner has not been employed for via filling analysis of through plastic vias (TPV). In this study, we have employed both the techniques for investigating the via filling and calculating the fill ratios in TPV. A non-destructive 3D imaging technique is essential for estimating the fill ratio while overcoming the loss of material that can be accrued in sectioning samples for SEM. We show that CT-Scanning achieves a better estimation of via-filling. This method can be useful for PCB/circuit designers in obtaining a higher accuracy of via-resistance (for conductive inks) and a better 3D packaging reliability (for dielectric polymers2). This technique also shows an additional advantage for scanning dielectric materials as it doesn’t require pre-processing step of metal deposition. Images from CT-Scanning and SEM have been presented in Fig. 1 (a) and (b) respectively.
Vias in different samples were prepared by laser ablation using a pico-second Nd:YAG laser3. Via-filling was accomplished by screen-printing on both sides of polyethylene terephthalate (PET) using a 325 stainless-steel mesh screen and CI-1036 (silver ink) 4,5 individually and separately with DI-7540 (dielectric). Curing of all the printed inks were done at 1220C for 12 minutes individually. No treatment of the substrates was carried out before printing. A 7-mil (177 µm) thick PET with a surface energy of 28.3 mN/m was used as a substrate for this study. Samples were first freeze-dried and cut before being used for SEM whereas samples used for CT-scanning did not require any pre-processing.
Three samples for each diameter were investigated through SEM and CT-Scanning. Fill ratios from SEM images was quantified using the method outlined in other studies for through silicon vias (TSV)6,7 (Fig. 1(c)). Two sections (the center and bottom of the via) of the scanned samples were inspected for measuring the thickness of deposited material that is filled in the walls of via, as illustrated in Fig. 1 (d). For the CT-scanned images the process for calculating the fill ratio was as follows. First, the image processing toolbox of MATLAB was used to mark different points on the curvature (Fig. 1(c)) and then these points were converted into cartesian coordinates. These coordinates were then fitted into a quadratic equation of second degree to obtain an equation in f(x) (where x is parallel to cross section of the via). The volume of the cured silver was then calculated by using Eq. 1 and the fill ratio was calculated using Eq. 2.
Vfill= Π∫(f(x))2 dx (1)
Fill ratio=Vfill/Πr2l (2)
where r is the radius of the via and l is the length of the via.
The fill ratios of different diameter vias are presented in Fig. 1 (e) (only CI-1036) & Fig. 1(f) (CI-1036 and DI-7540). A mean difference of 21.11% for CI-1036 and 16.44% for DI-7540 in fill ratios was observed between CT and SEM. The work aims to address the uncertainty that may occur due to sample pre-processing required for SEM. It also discusses a rigorous approach for estimation of the via fill ratio of liquid phase materials such as dielectric or conductive inks in TPVs.
*Acknowledgement
This work was supported in part by the National Science Foundation (IIP-1439644) through the Multi-Functional Integrated Systems Technology (MIST).

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