Exposure to extreme processing environments is a major roadblock that stands in the way of using thin, temperature-sensitive materials in the production of printed electronics. Discover the ways in which photonic sintering can clear a path to success.
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By Saad Ahmed
We live in a world surrounded by electronics. Even though the technology for manufacturing devices and components has advanced in leaps and bounds, the basic printed circuit board (PCB) has not changed significantly. The process of applying photo resist to a sheet of copper that is fixed on a rigid board and using a photo-lithographic process with a chemical-etch process to create tracks and pads to form interconnects between components is still the most common way of building PCBs.
Conventional PCB creation is a subtractive, multistep process that requires chemicals, is wasteful of copper, and does not lend itself to a high-speed, low-cost solution. In some special cases, flexible or rigid-flex PCBs are used, but they are based on the same principles of standard PCB manufacture but use flexible plastic substrates instead of rigid materials.
An alternative approach to PCB manufacture is to print the copper traces on the substrate rather than etch away unused copper (Figure 1). This would be a totally additive process and would eliminate the need for lithography and etching. High-speed printing processes could then be used to mass produce circuit boards at a very low cost. Low-cost substrates such as paper or PET could be used to produce circuits that are cheap enough to be disposable; however, we must overcome some significant challenges to achieve that goal.
The most important challenge is that of resistivity. Copper, silver, and gold, which are often used in PCBs, are very good conductors and have very low resistivity. This property allows circuits to run more efficiently. Conduction though metals is possible because electrons are free to move about the metal lattice. Resistance increases when metal clusters are deposited in ink form because the lattice is disjointed. Gaps and voids between metal particles do not allow for the free movement of electrons, which causes increased resistance. Ideally, the metal particles would be melted together or sintered to form a homogenous strip of metal to achieve better resistivity. However, melting points of metals are typically very high. Gold, as an example, melts at 1064°C, and this amount of heat means that low-temperature substrates, such paper and PET, are not useable. But all of this has changed with the advent of nanoparticle-based inks.
An alternative approach to PCB manufacture is to print the copper traces on the substrate rather than etch away unused copper (Figure 1). This would be a totally additive process and would eliminate the need for lithography and etching. High-speed printing processes could then be used to mass produce circuit boards at a very low cost. Low-cost substrates such as paper or PET could be used to produce circuits that are cheap enough to be disposable; however, we must overcome some significant challenges to achieve that goal.
If the application calls for less than 50,000 actuations, polycarbonate is a good choice; otherwise, polyester is the material of choice. Most agree that life-cycle tests show that polyester can be actuated more than 1,000,000 times in a tactile switch without showing signs of wear. For solid tactile feedback in a membrane switch, choose an overlay thickness between 0.006-0.010 in. These thickness ranges offer the durability to meet most requirements.
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