Comparative flow characteristics of selected dental materials with emphasis on thixotropic behavior.
Abstract Flow behavior of selected impression materials, dental cement liquids and fluoride gels were measured using a unique cup and bod viscometer. Programmable functions of the instrument allowed shear stress measurement during increasing and decreasing shear rates. Shear-dependent and time-dependent properties of plastic, pseudoplastic and thixotropic behavior were evaluated for these materials. The thixotropic index was also used to compare these materials. The impression materials tested showed pseudoplastic and thixotropic behavior, while the dental cements showed very little pseudoplastic or thixotropic behavior. The fluoride gels showed essentially plastic yield with minimal thixotropy.
Modifying the Setting Rate of an Addition-Type Silicone Impression Material
Two methods for controlling the working time of an addition-type silicone impression material are presented. The progress of the polymerization reaction of this modified system was studied using viscosity and a new "compression set" technique. The physical properties are presented and compared to those typical of condensation silicones, and a method of clinical use is suggested.
Failure analysis for poor solderability of Au surface of PCB
Nonwetting is one of the common problems and is always rejected in electronic assemblies. ENIG pads of a PCBA exhibited nonwetting after lead-free reflow soldering. With the aid of stereomicroscope, SEM&EDS and XRF, a series of analysis was performed. And after visual inspection, surface and cross section analysis, thickness measurement and solderability test, it can be inferred that oxidation of plating surface was the main reason for nonwetting, and corrosion in Ni plating caused an adverse impact on solderalibity.