Performance assessment of a modular device for micro-sphere singularization
Introduction
The manufacturing of a number of mechatronic products in many industrial fields requires the assembly of components different in shape, material, and size. The strong current trend to miniaturization has led to highly compact products that integrate an increasing number of functions. This requires advanced technologies for the manipulation and assembly of the micro-parts, that are particularly challenging at the micro-scale and are often performed manually or in semi-automatic ways. The automation of the micro-assembly phase would improve its throughput and reliability by means of innovative tools and devices meeting the requirements of e.g. the picking, releasing, and joining operations. Among these, sorting and feeding systems capable of singularizing and delivering microparts are fundamental in various applications. To this purpose, a recent work reported a novel method and mechanism for micro-sphere singularization [1], able to process spheres with diameters ranging from 0.2 to 1 mm, or even wider ranges according to the specific design.
An example of its application in the electronics industry is the reballing operation, that is the rework of Ball Grid Array (BGA) packages to be mounted on a Printed Circuit Board (PCB) in case of presence of defects or to recover the components for their reuse [2]. The most common method to reball a BGA consists in attaching new solder balls onto the area array on the bottom surface of the BGA that provides the electrical connection to the PCB. To accomplish this task automatically, an original tool, namely the “Store-and-Place (S&P) device”, was designed and developed [3] based on the principle presented in Ref. [1]. It integrates a storage of many identical micro-spheres that are singularized and placed onto the circular lands of the package at high speed when required. The tool can be easily mounted on a robot end-effector to ease the integration in automated work-cells for the rework of electronic boards. Its modular design, high throughput, and ease of use make it an effective solution for medium or batch production of different BGA packages.
The paper describes the assessment of the performance of this tool by an extensive experimental analysis for its characterization and validation. Using a structured approach, experimental tests were designed [4] and carried out to highlight the main process parameters influencing the performance in order to optimize them and properly exploit the tool. A set of performance indices: accuracy index, repeatability index, anisotropy index, to quantitatively evaluate the precision of the micro-sphere release was calculated on the basis of the method presented in Ref. [5], together with other indices to evaluate the reliability of the operation and the time needed to execute it.
After a brief review of the literature, Section 3 presents the design and prototype of the novel singularization tool. In Section 4, the experimental analysis for the assessment of the working parameters is reported and the obtained results are discussed, highlighting possible optimization strategies.
Section snippets
Technical background
The need for singularizing identical components from a disordered stock is common to many assembly applications. At the micro-scale, this issue becomes more complex due to the dimensions of the components and the predominance of superficial over volume forces. Many solutions for the singularization of identical micro-components can be found in literature, ranging from scientific publications to patents. For example, in Ref. [6] the simultaneous alignment and ordering of a fixed quantity of
The Store&Place device: design and protype
A new device was designed to perform the reballing task implementing a “store-and-place” (S&P) strategy, that in the current work consists of an up-down movement on the lands of the BGA fluxed bottom surface (see the light-blue dashed arrows in Fig. 1b) [3].
The device integrates a storage of identical micro-spheres to be carried on a robot and was specifically conceived to collect, automatically singularize, and deposit them at high speed, when required.
The working principle is based on the
Performance assessment and optimization by experimental analysis
The goal of the experiments was to optimize the working parameters of the developed S&P device to achieve the best release performance in terms of precision, reliability, and time.
To assess the performance of the device, different repetitions of sphere release cycles on a fluxed substrate were executed varying the working parameters. For each repetition the release accuracy, repeatability, anisotropy, reliability (percentage of release success), and release time (minimum time between the
Conclusions
The paper described the assessment of the performance of the “Store-and-Place (S&P) device”, that is an original tool developed by the authors for micro-sphere singularization and delivery [3] based on the method and mechanism presented in Ref. [1]. Possible applications of the device can be found in the electronics industry and include the automatic reballing of BGA packages: the most common approach to reballing consists in attaching new solder balls on the fluxed bottom surface of the BGA
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank Dr. L. Altissimo, Mr. J.C. Dalberto, Mr. R. Bozzi, Mr. T. Dinon, and Eng. F. Modica (CNR-STIIMA) for their contribution to the development of the prototype.
The work has been partially funded by Regione Lombardia under the Accordo Quadro CNR-Regione Lombardia projects: Fideas (Convention No. 18090) and Cybersort (Decree No. 12054, Nov. 22, 2016).
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