Optoelectronic Rigid PCB: Boards for Light-Based Electronics

What an Optoelectronic Board Has to Do

An optoelectronic board sits at the boundary between electricity and light. It drives an emitter such as an LED, a laser diode or a vertical cavity laser, it receives light on a photodiode or an image sensor, or it does both in a transceiver. The electrical part of the design is ordinary; the difficulty comes from the two requirements that only appear when light is involved.

The first is thermal, because an emitter converts a large part of its input into heat and its efficiency, wavelength and life all depend on the junction temperature. The second is optical, because an optical component has to be aligned to something outside the board, and stray light has to be kept away from the detectors that share the assembly.

The Main Applications

• LED arrays. Illumination, display backlights and lighting products, where the board is the thermal path as much as the electrical one.
• Laser diode modules. Distance measurement, barcode scanning, material processing and optical storage, where the diode needs a controlled current and a controlled temperature because its wavelength drifts with heat.
• Optical transceivers. Data centre and telecom modules, where a laser or a photodiode is driven at several gigabits per second on a board that also carries a serialiser and a power supply.
• Photodetector boards. Instrumentation, medical and industrial sensing, where the signal currents are small and the board has to protect them from noise and from stray light.
• Sensor modules. Proximity, ambient light and spectroscopy devices, where an emitter and a detector sit side by side and must not interfere.

Thermal Design of an Emitter Board

An LED or a laser diode is a small source of significant heat, and the board is the first stage of its thermal path. Three techniques are used together.

Copper spreading. A large copper area under and around the emitter spreads the heat before it has to pass through the laminate. Heavy copper, two to four ounces, both spreads the heat and carries the current with a lower loss.

Thermal vias. A matrix of vias under the emitter pad carries the heat through the board to a copper plane on the other side. Where the emitter is reflowed, the vias must be filled and plated over or the solder will wick into them.

A metal base. Where the package has a thermal pad that must be soldered, a metal core board puts the die directly onto an aluminium base with only a thin dielectric in between. This is the construction used for high power LED arrays and for laser modules.

Wavelength stability is the reason temperature control matters as much as it does. A laser diode shifts its emission wavelength with temperature, typically by a fraction of a nanometre per degree, which is enough to move it out of the passband of a filter or out of the absorption line of a measurement. Where the wavelength matters, the diode is mounted on a thermally controlled substrate and a thermistor is placed next to it. Our notes on PCB design and layout cover the layout of the thermal path, and our notes on PCB manufacturing describe the metal core and heavy copper constructions.

Optical and Electrical Isolation

An optoelectronic assembly often contains a sensitive detector and a strong emitter on the same board or in the same housing, and the design has to stop the emitter from being seen by the detector.

Physical separation. Keep the emitter and the detector as far apart as the mechanical design allows, and place the detector so that its field of view does not include the emitter.

An optical barrier. A wall of opaque material, a slot in the board or a shield prevents light from travelling along the surface. A black solder mask or a matte coating reduces reflection, because a glossy surface bounces light between the two.

Electrical separation. The emitter is driven by a switching current that produces a magnetic field and a voltage transient. The detector amplifies a very small current, so the emitter driver is placed as far away as possible, its loop is kept small, and the detector input is guarded by a ground ring and, where the current is really small, by a guard trace at the same potential.

Leakage control. A photodiode amplifier measures femtoamperes on some designs, and at that level the surface leakage of the laminate and the flux residue matter. The input node is kept short, guarded and, where necessary, the board is cleaned and coated to keep the surface resistance high. The cleanliness controls matter as much as the circuit design.

Materials and Finish

The substrate follows the thermal requirement. FR-4 with heavy copper and thermal vias is adequate for medium power LED boards; a thermally conductive laminate is used where the board is hot but multilayer routing is still needed; a metal core board is used where the emitter must be soldered directly to a good thermal path; and a ceramic substrate is used for the highest power density and for laser modules where the expansion has to match the die.

The surface finish is chosen for the assembly and for the optical requirement. Electroless nickel immersion gold is the usual choice for a fine pitch reflow and for wire bonding, with a controlled gold thickness for the bonding process. A matte black solder mask is often used on optical assemblies to reduce stray reflections, and where the emitter or the detector is wire bonded, the finish and the bond pad cleanliness are specified rather than left to the default.

Where a board carries both optical and ordinary digital circuits, as in a transceiver, the stack is often hybrid: a high frequency or low loss material for the radio frequency and the high speed lines, and ordinary laminate elsewhere, with the optical components on the section that has the best thermal path.

Assembly and Alignment

Optoelectronic assembly is a precision process. The emitter or detector is placed with an accuracy measured in tens of micrometres, reflowed with a profile that suits the package and the thermal mass of the board, and where the light has to be coupled to a fibre or a lens, the optical element is actively aligned: the device is powered, the coupling is measured, and the position is adjusted until the output peaks and then fixed.

That active alignment step is what makes the difference between a component that works in the laboratory and one that works in the field, and it means the mechanical design of the board has to leave room for the adjustment and for the adhesive that holds it. Substrates for this class of assembly are often ceramic or metal core, because they hold their dimensions through the process.

Testing

Three measurements matter. The optical output power and the wavelength are measured on the assembled emitter, usually through a calibrated integrating sphere or a fibre coupled measurement. The electrical characteristics are checked against the specification, including the forward voltage and the threshold current. And for a detector, the responsivity and the dark current are measured at the operating temperature, because the dark current doubles roughly every eight to ten degrees and can dominate the signal if the board runs warm.

Where the assembly includes a thermistor or a temperature controller, the loop is verified as part of the test, since a mis-set control loop produces a wavelength error that no optical measurement will explain. Our notes on PCBA testing describe the test methods, and our notes on energy PCBA cover the driver circuits these boards use.

Design Practice

• Design the thermal path before the electrical one. The emitter temperature determines the performance, so the copper, the vias and the base come first.
• Keep the emitter loop small. The current path through the driver, the switch and the emitter sets the noise and the efficiency.
• Separate the light paths. Distance, an optical barrier and a matte finish prevent the emitter from reaching the detector.
• Guard the sensitive input. A ground ring, a short input trace and a clean, coated board protect a photodiode amplifier.
• Allow for the alignment. Leave clearance and a defined adhesive area for the optical element rather than assuming the placement is final.
• Specify the finish and the cleanliness. Bond pad quality and surface leakage are part of the optical performance. Our notes on PCB assembly describe the process.

FAQ

What substrate is used for a laser diode board? Usually a metal core, ceramic or aluminium nitride substrate, because the die has to be soldered to a low thermal resistance path and the expansion has to be close to the die.

Why does a laser diode need temperature control? Because its wavelength and threshold current change with temperature, and a wavelength shift of a fraction of a nanometre can move it out of a filter passband.

How is stray light controlled? By physical separation of the emitter and detector, an opaque barrier between them, a matte or black surface, and by keeping the optical surfaces clean.

Why is a photodiode board cleaned so carefully? Because the amplifier measures very small currents, and flux residue and surface contamination create leakage paths that add to the signal.

Conclusion

An optoelectronic board is an ordinary circuit with a thermal and an optical requirement attached. Put the emitter on a thermal path designed from the die outward, keep the wavelength stable where it matters, separate the light paths physically and electrically, and treat cleanliness and finish as part of the optical design. The electrical design is the easy part; the board earns its place by managing the heat and the light.