Effects of Solder Mask Variability on the Electrical Response of Commercially Manufactured Interdigitated Circuits

The use of electronics substrates, such as printed circuit boards (PCBs) in modern technology has become nearly ubiquitous. As PCBs become smaller, denser and mass produced, printed, interdigitated circuit (IDC) sensors are increasingly utilized to qualify the geometric, material and process decisions for manufacturing electronics assemblies. Despite this, the accuracy in determining reproducibility and reliability of printed circuit designs for these applications is not well studied. In this article we report on the usage of small signal ac impedance spectroscopy to determine measurement repeatability and manufactured board reproducibility as a function of frequency, humidity and solder mask coverage for a single IDC design. These measurements allowed detection of systematic changes in the electrical response as the frequency (10MHz-0.1Hz) and humidity were varied (96%-10%RH). Our ac impedance results indicate that the measurement repeatability error is better than 0.6% while circuit or board reproducibility ranges from 2.5%-5.2%. Detailed surface analysis of the circuit structures indicated that differences observed were primarily due to porosity in the solder mask as well as differences in solder coating thickness and coverage between the interdigitated combs. Results are explained by a model that considers water surface adsorption, then infusion into the pore space and finally diffusion through the solder mask as the humidity of the ambient increased. These effects were most easily detected using imaginary electric modulus M" vs log frequency plots. It is anticipated that this methodology will have application to other circuit designs, solder mask or contamination variability.