Last year, not only were optical modules in high demand but DAC (Direct Attach Copper) and AEC (Active Electrical Cables) were also highly sought after. NVIDIA extensively used DACs, essentially referring to direct connection copper cables.
These are dual-core copper cables, which may or may not include signal return GND wires. Some cables utilize shielding metal foil connected to GND for signal return instead of GND conductors.
For high-frequency signals, maintaining impedance continuity is crucial. This parameter is influenced by the material of the conductor, wire diameter, spacing between wires, and the dielectric constant of the insulating medium.
Manufacturing cables to meet these stringent requirements is challenging. For instance, fluorine-containing insulating materials like Teflon, which become soft when foamed, can cause structural deformations during application, leading to impedance changes and signal distortion.
Additionally, different hardness levels of insulating medium fillings need to be considered for balance characteristics. The wave velocity of the signal correlates with the dielectric constant, and an imbalance in the dielectric constant of differential signals can lead to common-mode noise caused by delays, thereby degrading signal quality.
High-frequency signal cables are difficult to manufacture, and assembling these cables also requires consideration of solder joint impedance mismatches, wire terminal tail effects, etc.
Here is an example of a FiberMall DAC with OSFP housing, commonly seen in optical modules.
Internally, DACs are simple, with few electronic chips. One would first inspect the outer shell and the internal PCB hot-swap golden fingers, along with the cable soldering points.
The PCB solder joints must be as small as possible to avoid resonance caused by parasitic parameters L, C, and R. FiberMall uses surface mount pads to ensure signal integrity after assembly.
Additionally, the PCB includes a cable clamp; the clamp and PCB are soldered traditionally, while the cable and clamp are joined using laser welding to avoid distortion.
The PCB’s top surface contains 8 pairs of differential cable clamps, with another 8 pairs on the bottom.
Upon dismantling the cable clamp, the internal structure reveals the signal line terminals.
The rear end features two U-shaped slots to secure the wire cores, which are then fixed by pressing two metal pieces and laser welding.
Moreover, a metal piece is used for GND connection; the front end is soldered to the PCB, and the rear end uses a spring piece for elastic contact with the cable shield layer, forming a complete signal return path.
A side view shows the cable aligning and pressing into the cable clamp.
Laser welding the signal lines ensures metallurgical bonding and removes excess terminals, avoiding noise from electromagnetic wave reflections caused by tail stub effects.
Cable clamps can come in various types.
