Printed Electronics on Flexible Substrates

Printed electronics deposits a conductive ink onto a substrate and sinters it, rather than etching copper away. The process is additive, it works on flexible films and on paper, and its resolution and conductivity are set by the ink and the sintering rather than by the copper foil.

What the Process Delivers

The deposit is a suspension of metal particles in a carrier, printed through a screen, an inkjet head or a flexographic plate, and then dried and sintered. The sintering fuses the particles into a conductive network, and its completeness decides the resistivity.

The result is thinner and less conductive than a copper foil of the same pattern, which is acceptable for a sensor or an antenna and not for a power path. Our flex materials notes describe the substrates it is printed on.

Silver inks sinter at the lowest temperature and give the highest conductivity, and they are the most expensive. Copper inks are cheaper and oxidise unless the sintering atmosphere is controlled. Carbon inks are the cheapest and the least conductive, and they are used for a resistive element rather than a conductor.

The choice follows the function. A trace that carries a signal and a trace that forms a resistor are different materials even on the same substrate.

Sintering is a time and temperature window, and the substrate limits the top of it. A polymer film that cannot take more than a certain temperature forces a longer dwell at a lower one, and a longer dwell allows the carrier to fully leave.

An incompletely sintered deposit has a higher resistance and it also has a weaker mechanical structure at the particle boundaries, which is where a bend failure starts. Our flex processing notes describe how a flexible circuit is built when the layer is a copper foil instead.

The printed resolution is set by the printing method rather than by an imaging process. Screen printing gives a coarse feature, inkjet gives a fine one at a lower throughput, and flexography sits between them.

Registration is a separate problem, because the ink is printed on a substrate that can move between passes. A multilayer printed circuit therefore has looser registration than an etched one, and its design rules are written accordingly.

A sintered metal deposit is porous compared with a foil, so it cracks at a larger radius than a rolled foil of similar thickness. The design rules for a printed flexible circuit therefore specify a larger bend radius.

The crack appears as a rise in resistance rather than as an open, which makes it detectable by a resistance measurement during a bend test. Our board quality notes describe how that test is arranged.

The applications are the ones where the substrate cannot be etched or where the volume is too low for a foil process: sensors on a film, antennas on a package, electrodes on a medical patch and heaters on a surface.

Where the volume is high and the substrate is a standard laminate, etching a foil is cheaper and better specified. The printed route wins on the substrate and on the shape rather than on the cost per square metre. Our coating notes describe the protection that a printed deposit usually needs.

Process Control and Verification

Reviewing the design before the data is released is cheaper than correcting it after the panel is in the tank, because every step downstream inherits the decision made at the front end. Documenting the assumption is part of the design work, and a short note on the drawing prevents a question that would otherwise arrive a day later and cost a day of schedule.

The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance. A stack-up that is drawn rather than described removes most of the ambiguity from a quotation, and it lets the fabricator price the board against the dielectric and copper weights that will actually be used.

Is printed silver as conductive as copper? Silver is more conductive as a metal and a printed silver deposit is less conductive than a copper foil because it is porous and thin.

Can a printed circuit be soldered? It can where the ink and the sintering tolerate the temperature, and a conductive adhesive or an anisotropic film is often used instead.

What does gopcb provide for printed and flexible circuits? We provide substrate and ink selection against the function, sintering windows that respect the film, design rules written for the printed resolution and registration, bend testing with resistance monitoring, and protection matched to the environment.

Checks Before Release

The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel. Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to.

A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.

Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.