Novel Termination Structure For Polymer Thick Film Embedded Resistors
Original Publication Date: 2002-Feb-21
Included in the Prior Art Database: 2002-Feb-21
Gregory J. Dunn: AUTHOR [+2]
A novel termination structure for screen-printed polymer thick film (PTF) embedded resistors is demonstrated. The number of process steps is equivalent to current practice, but improved linearity of resistor value both as-printed and during environmental stress vs. resistor length is obtained, allowing improved tolerances, especially in resistor ratios. The new termination structure also offers superior resiliency in thermal shock.
Novel Termination Structure for
Polymer Thick Film Embedded Resistors
Abstract—A novel termination structure for screen-printed polymer thick film (PTF) embedded resistors is demonstrated. The number of process steps is equivalent to current practice, but improved linearity of resistor value – both as-printed and during environmental stress – vs. resistor length is obtained, allowing improved tolerances, especially in resistor ratios. The new termination structure also offers superior resiliency in thermal shock.
Conventionally, polymer thick film (PTF) embedded resistors are printed either directly on etched copper terminations or on screened silver paste terminations. Terminating directly on copper results in relatively poor resistor stability under environmental stress. This can be ameliorated by printing at fairly large resistor dimensions (widths of several mm), but Motorola products require sub-mm resistor widths to provide significant space savings vs. discrete surface mount chip resistors. Interposing screened polymer thick film silver paste between the copper and resistor results in highly reliable and stable resistors because it reduces corrosion at the copper/carbon ink interface. However, this improved reliability is accompanied by a loss of precision and space efficiency. Resistor length is determined by the silver paste screening step rather than photolithographic patterning of the copper, and space must be allowed for misalignment of the silver paste with the copper, resulting in larger resistor structures.
We have demonstrated that applying immersion silver or electroless palladium to the copper terminations eliminates corrosion of the termination/resistor interface under environmental stress, while preserving the precise edge definition and density advantages of photolithographically patterned copper. Commercially available plating processes that work well include Alpha LevelÔ immersion silver from Alpha Metals, SterlingÔ immersion silver from MacDermid, and STÔ immersion silver and PallatectÔ electroless palladium from Atotech. Replacing the silver paste with a plated finish allows for better control of resistor length, thereby improving the precision of the resistor printing and consequently the tolerance. Additionally, it eliminates extra printing and curing cycles, and allows for more compact resistor designs.
Novel Termination Process
This paper reports on a novel process for forming terminations for PTF resistors that provide optimal resistor stability. This new approach builds on our prior findings regarding the benefits of silver and palladium finishes and thin terminations. Thin metal terminations allow for improved linearity of resistor value vs. length. This is due to two factors. First, the resistor ink tends to pool at the termination step – that is, at the edge of the patterned copper, which is typically 10-30 mm thick. Second, thick copper will influence the temperatu...