Laser-trimmed Resistors Research Articles

A Current-Feedback Instrumentation Amplifier With 5 $\mu{\hbox{V}}$ Offset for Bidirectional High-Side Current-Sensing

This paper describes an instrumentation amplifier for bidirectional high-side current-sensing applications. It uses a multipath indirect current-feedback topology. To achieve low offset, the amplifier employs a combination of chopping and auto-zeroing in a low frequency path to cancel the offset of a wide-band amplifier in a high frequency path. With a 60 kHz chopper clock and a 30 kHz auto-zero clock, this offset-stabilization scheme results in an offset voltage of less than 5 muV , a CMRR of 143 dB and a common-mode input voltage range from 1.9 to 30 V. The input voltage-to-current (V-I) converters required by the current-feedback topology are implemented with composite transistors, whose transconductance is determined by laser-trimmed resistors. This results in a less than 0.1% gain inaccuracy. The instrumentation amplifier was realized in a 0.8 mum BiCMOS process with high voltage transistors, and has an effective chip area of 2.5 mm2 .

Environmental qualification testing and failure analysis of embedded resistors

Embedding passive components (capacitors, resistors, and inductors) within printed wiring boards (PWBs) is one of a series of technology advances enabling performance increases, size and weight reductions, and potentially economic advantages in electronic systems. This paper explores the reliability testing and subsequent failure analysis for laser-trimmed Gould subtractive nickel chromium and MacDermid additive nickel phosphorous embedded resistor technologies within a PWB. Laser-trimmed resistors that have been "reworked" using an inkjet printing process to add material to their surface to reduce resistance have also been considered. Environmental qualification testing performed included: thermal characterization, stabilization bake, temperature cycling, thermal shock and temperature/humidity aging. In addition, a pre/post-lamination analysis was performed to determine the effects of the board manufacturing process on the embedded resistors. A failure analysis consisting of optical inspection, scanning acoustic microscope (SAM) and environmental scanning electron microscope (ESEM) imaging, and PWB cross-sectioning was employed to determine failure mechanisms. All the embedded resistors were trimmed and the test samples included resistors fabricated both parallel and perpendicular to the weave of the board dielectric material. Material stability assessment and a comparison with discrete resistor technologies was performed.

SEM Studies on Laser Trimmed Resistors

This paper reports the studies conducted using scanning electron microscope (SEM) as a tool for optimizing laser trim parameters leading to reliable trimming. The samples drawn from the regular production lots use DUPONT 1400 BIROX series resistor pastes with compatible conductor and dielectric compositions printed on Alsimag 614 substrates. Samples of the typical size of 5 mm × 5 mm prepared by laser cutting were observed in the secondary electron mode of SEM. Teradyne model W311 Q-switched YAG laser was used in trimming the resistors. A substrate surface removal of 3–5 microns which is necessary for the good quality' cut was confirmed in the cross-sectionally observed samples. Also uniform and well fused kerf edges which are another important aspect of reliable trimming were checked. This dielectric protection of kerf edges ensures the non-exposure of disturbed resistor material. Microcracks were found to be present at the end of the cuts but they seemed to have insignificant effect on the thermal stability of the trimmed resistors; the storage at 150°C for 1000 hrs. has brought about the drift (ΔR/R × 100) of less than 0.2% in case of L cut and less than 0.35% in case of double plunge cut. Further efforts are being made to find out the depth of these microcracks using energy dispersive scan.