In the past years, following the increasing functionality of a variety of equipment such as smartphones and tablet PCs, there has been an evidently growing need for thinner, more compact form factors at the component level that provide greater precision and reliability. The recent advent of wearable electronics e.g. for consumer and healthcare purposes, has also spurred demand for smaller, thinner products. also sports and healthcare devices.
Since 2004, it has proven impossible to produce chip resistors smaller than the 0402 size (0.4x0.2mm) using existing manufacturing technology, due in large part to the large package dimensional tolerance of up to ±20um during the cutting process, chip loss, and a variety of other factors.
In response, ROHM leveraged market-proven proprietary technology to advance technological innovations in the field of IC component miniaturization that has been transferred to discrete and passive devices. This made it finally possible to offer the smallest products in the world, from resistors, transistors and diodes to tantalum capacitors and LEDs, featuring unprecedented dimensional accuracy (±10μm).. Miniaturization was achieved using pioneering new manufacturing techniques that break completely from convention, resulting in the RASMID (ROHM Advanced Smart Micro Device) series of ultra-compact components.
In 2011 ROHM was able to successfully develop the world’s smallest chip resistor in the 03015 size (0.3mm × 0.15mm), reducing footprint by 44% compared to the conventional 0402 size, and in 2012 produced Zener diodes in the 0402 size that represented the industry’s smallest semiconductors. In 2013 0402-size Schottky barrier diodes (SBD) were added to the lineup, while the 03015-size chip resistors ultimately entered mass production RASMID™ technology involves not only product miniaturization, but methods and techniques (i.e. in the mounting process) that can enable practical realization and commercialization as well.
Factors in this development
In addition to developing manufacturing systems utilizing proprietary miniaturization technology, dicing methods have been revamped to greatly limit the package dimensional tolerance in the cutting process to ±10μm. As a result, burrs and cracks are eliminated, minimizing adhesion errors while increasing mounting accuracy.
As to what causes the components to stand erect, a variety of factors come into play, such as the surface tension of the solder and the timing of temperature rise, but a large role is also played by the lateral balance of electrodes. The contact area difference of the electrode and the solder is increased. If there is dispersion in the electrode size and print position of the solder, so when irregularities exist in the print position of the solder and electrode size (as in the figure), pulling occurs where the contact area is larger, making tombstoning more likely to occur (particularly with compact chips). In contrast, with bottom electrode devices without any electrode on the side, pulling occurs only in a downward direction, virtually eliminating tombstoning.
Regarding RASMID products, lateral irregularities of the electrode are further reduced through improved dimensional accuracy and by adopting a gold electrode surface that provides excellent corrosion resistance for improved solder wettability. General Pb free solder has been used for the evaluation test.
High-precision high-density mounting through joint development of respective technology
ROHM developed the RASMIDTM series not simply to pursue product miniaturization, but also to cultivate technology to enable practical realization that extends to mounting technology. With respect to the new 03015 size, numerous discussions were conducted with the main mounter manufacturers and joint efforts were made to improve the pick-up rate by the assembly machines. In consequence, the chip configuration, taping material and taping dimension etc. have been optimized to finally result in a zero defect rate during mounting evaluations - even when using large sample quantities.
In addition, in the case of a multi-faceted electrode, a large amount of board area (land pattern) is required to allow space for the solder to wrap around the side and top surfaces (fillet). However, through greater precision and by adopting bottom electrodes, ROHM was able to minimize the required solder area, reducing footprint by 40%. Moreover this leads to a reduction of requested solder material.
By reducing size and weight, excellent shock properties and bonding reliability could be achieved
There were concerns that reducing product size and adopting an underside electrode configuration would reduce the size of the ground contact area of the solder, weakening shock resistance, and robustness but by reviewing materials and reducing the weight of mass compared with the conventional 0402 size, resistance to shock force applied to the substrate when dropped has, on the contrary, improved. Multiple tests in terms of adsorption and accuracy underlined the result that characteristics were not impacted disadvantageously. Additionally, mechanical stress tests showed that RASMID devices performed identically to items based on mold packaging when it came to mounting force, PCB bend stress, solder adherence and pressure.
Minimizing packaging usually leads to die shrink in the mold process which negatively impacts electrical performance. In case of RASMID, the die size is identical with the packaging so that electrical performance can be kept high. Nevertheless, size is reduced by 82% compared to the conventional 0603 size (0.6mm × 0.3mm)
By breaking this relationship and adopting proprietary chip device structures as well as using ultra-precise machining technology, key electrical characteristics such as forward voltage are maintained while reducing size and thickness compared to conventional 0603-size products. Mounting force, PCB bend stress, solder adherence and pressure tests lead to the same result as MOLD PKG based products.