Assembly of electronic components with Optoelectronic Tweezers

In this project we will use a method of controlling electrical forces with light patterns to move and assemble small electrical components into circuits. This is in contrast to the current techniques used where a robotic arm with a vacuum tip on the end is used to pick up and place these components onto printed circuits. We aim to produce a step change in the size of the smallest components that can be handled from the current smallest standard component size of 400x200 microns (0402 metric) e.g. less than half a millimetre across, down to components a few microns across and even nanostructured components (based upon graphene, nanowires or nanotubes, for example). This will be accomplished by developing a radically new assembly strategy based on a touch-less opto-electro-fluidic technique known as optoelectronic tweezers (OET). OET use a photoconductive device to turn patterns of light into patterns of electrical field. By designing the device so that a liquid layer experiences a larger bias across it where it is illuminated, electrical gradients are created which create forces on any particles within the liquid through dielectrophoresis. Changing the light pattern changes the pattern of electrical forces allowing the continuous movement and hence positioning of microparticles.
Proof of concept experiments showing the movement of electronic components 600 microns long have already been demonstrated and during the 18 months of this project the aim is to take this research and to develop the technique to a degree where we are able to incorporate it into an automated process flow.
The electrical components we will be assembling are common in all consumer electronic equipment with, for example, 400 to 500 being present in a typical smart phone. The size of the components is constantly shrinking so that they take up less space and add less weight to portable products such as laptops, cameras and phones. As the components get smaller the reliability of the vacuum tips on robotic arms decreases but it becomes increasingly easier for contactless techniques such as OET to move them.
In its first instance we will assemble electrical components into specific positions within a circuit created by patterning metal wires onto an OET device and then fix them into place by heating the solder which comes on each component. This test system will allow us to assess the speed, positional accuracy and reliability of this methodology whilst demonstrating the flexibility of this approach to assemble multiple components in parallel (something not possible with a robotic arm). By demonstrating the assembly of standard components this project will demonstrate its immediate applicability to industry.
In the next phase of this project we will demonstrate how OET can create a real step change in the assembly industry by placing components smaller than the current smallest standard. We will start with the new range being released in 2013 by muRata which are 250x125 microns and are expected to create a new standard. We will then extend the size range downwards by creating model components of the whole size range from 1000 microns in length down to 1 micron long.
After demonstrating the advantages of using OET to place small components we will then investigate how to do so onto a conventional printed circuit board (PCB). We will also investigate the innovative approach of using the OET device itself to create the wires in the circuit by patterning conductive metallic nanowires into lines in order to connect the discrete components.

The ability to use smaller components in a surface mount technology (SMT) type circuit will bring about impacts through benefits associated with the reduced footprint that the components occupy within a device, the reduced mass that they add and the reduced cost of the assembly process. Each benefit is discussed in detail below:

Reduced space: OET will allow the assembly of smaller components than is possible with conventional pick and place robots which rely on vacuum tips and hence the components will take up less space on the circuit board. This is of great importance in almost all portable electrical items including mass market products such as laptops and cell phones and less common applications such as satellites.

Reduced mass: Smaller components will also weigh less than standard components which again will again be of advantage for portable but will be even more important to aeronautical and satellite applications.

Reduced cost of assembly: The OET technique is an inherently simple and elegant micromanipulation strategy with no moving parts except the component being assembled itself. As such it can be implemented in a relatively small (briefcase size [2]) low cost (from a few thousand pounds) systems with no need for high power light sources or high N.A. optics. State-of-the-art pick and place robots are large, complicated and expensive. Their development is carried out by some of the world's largest companies such as Samsung and Panasonic, the reduced cost of OET systems by contrast offers the possibility of developing competitive process on a reduced budget and offers a path to bringing a new manufacturing process to add to the UKs manufacturing base.

There are three stake holders for who this research will be especially interesting, firstly SMT component manufacturers, secondly SMT assembly equipment manufacturers and thirdly companies that use circuits containing SMT components in their products. We will seek to work with companies involved in all three of these aspects throughout this project, specific examples of which are given below.

Component manufacturers: I have discussed this project with Dr Ivo Koutsaroff, Chief Research Engineer at muRata an industry leading component manufacturer. They recently revealed the world smallest range of capacitors and resistors at 250x125 mincrons length by width which may soon become a new standard for the industry.

SMT assembly machine manufacturers: I have been in discussion about this project with Peter Sundström, Senior Specialist in Systems Design and Board Member at Micronic Mydata AB, and Gustaf Mårtensson, Senior Scientist in R&D - Physics at Micronic Mydata. Micronic Mydata have a reputation in the industry for leading the way in research and development on SMT assembly machines. As they develop SMT assembly machines this project is of great relevance to them and other SMT equipment manufacturers.

End users: The main push for smaller components does not come from the SMT assembly industry but from those that use SMT components in their products. These "end user" manufacturers have various reasons for wanting to reduce the size/weight of their products often for portable electronics but one of the most critical of which is in the weight of satellites. From this point we are in discussions with Allan Colquhoun, University liaison at Selex. Many of Selex's products have critical weight and size considerations and as such we will seek advice and guidance from Selex about the direction of this project and how best for us to demonstrate the advantages of our approach to their products.

1 P. Y. Chiou, A. T. Ohta, and M. C. Wu, "Massively parallel manipulation of single cells and microparticles using optical images," Nature 436, 370-372 (2005).
2 Portable Optoelectronic Tweezers (OET), taking optical micromanipulation out of the optics lab, S.L. Neale, C. Witte, J.M. Cooper, European Optical Society Annual Meeting (EOSAM 2012).