Key Technology of Optical Module PCB

As a medium for converting signals between optical fiber and cable transmission, optical modules are widely used in modern communication and network construction. As data transmission speeds and communication needs continue to improve, the design requirements for optical modules are also gradually improving. To ensure stable transmission of high-speed signals, PCB designs for optical modules require high-density wiring technology and solutions for heat dissipation and reliability. As technology advances, providing powerful functions and performance in limited spaces has become a major challenge in optical module PCB design.

What is Optical Module PCB?

It consists of a photoelectric converter, driver circuit, receiver circuit, and control circuit. These components work together to efficiently convert and precisely transmit optical and electrical signals. Since they are used to interconnect electronic devices, optical module PCBs are designed to meet several requirements, such as supporting high-speed data transmission, dissipating heat, and enabling hot-swapping. The technical characteristics of optical module PCBs are therefore mainly reflected in gold finger processing technology, high-speed material selection, and critical thermal management design. This article will focus on these three points.

Gold Finger of Optical Module PCB

Optical modules are typically designed with long and short gold fingers, as well as graded plugs. Traditional equal-length plug designs are rarely used. The main reason for this is that the gold fingers serve as an electrical interface for high-speed signals and need to transmit signals and provide power simultaneously. To ensure the power supply is energized before the data transmission, the power pins are usually longer than the data pins. This ensures that the power supply turns on first and the data transmission begins.

Similarly, a power failure cuts off the data transmission before disconnecting the power supply. If the power is not turned on but the data pins are connected and data is still coming in, it can damage the logic chips in the optical module or even the interface chips on the motherboard.

Additionally, to avoid capacitive effects in high-speed electrical interfaces, especially in high-speed devices such as optical modules, the inner layer at the plug location typically has no copper. This design reduces the impedance difference between the plug and the impedance line, and it helps suppress electrostatic discharge (ESD). In optical module designs, only the outer layer has a copper layer; the inner layer does not. This design effectively optimizes signal transmission performance.

The gold finger part of the PCB of the optical module is an important interface area connecting the optical module with external devices, so its manufacturing requirements are also very strict.

• Dimensional tolerance control: board thickness tolerance 1.0±0.07mm, shape tolerance ±2mil, so as to ensure the stability of signal connection.
• Wear-resistant design: Hard gold plating ensures 10,000 times insertion and removal life.
• Hot plugging and unplugging: Adopt long and short gold finger structure to realize the timing protection of “power on first and then communicate the signal”.
• Environmental resistance: Exposed gold fingers need to be corrosion-resistant to adapt to industrial environments.
• Zero defects in appearance: contact resistance of optical modules, no scratches/pits on the surface to meet the terminal appearance standards.

Differential Line Layout of Optical Module PCB

The layout of the differential lines on the PCB board determines whether the electrical signals of the optical module can be transmitted at high speed.
1.High-density wiring: Optical modules are moving toward multi-channel development and therefore require tighter control of line widths, line spacing, and impedance tolerances.

Optical Module PCB	100G products 4-channel	200G products 8-channel	400G products 16-channel
Line width and line spacing	4/4mil	3.5/3.5mil	3/3mil
Line Width Accuracy	±25μm	±15μm	±15μm
Impedance tolerance	±10%	±7%	±5%

2.High-speed materials: Optical modules have a single-channel rate of up to 56Gb/s, while the conventional rates are 25Gb/s, 10Gb/s, and 2.5Gb/s. Therefore, it is recommended that higher-rate boards be used in order to comply with the requirements. The following is FastlinkPCB commonly used high-speed PCB materials.

1-5G (Mid-Loss)	5-10G (Low-Loss)	10-25G (Very-Low-Loss)	>25G (Ultra-Low-Loss)
TU-862 HF: Dk 4.4, Df 0.0100	TU-872 SLK: Dk 3.8, Df 0.0090	R-5775G: Dk 3.6, Df 0.0040	R-5785N: Dk 3.3, Df 0.0020
IT-1706RA1: Dk 4.4, Df 0.0080	R-5725/M4: Dk 3.8, Df 0.0070	IT-968: Dk 3.7, Df 0.0047	IT-988G: Dk 3.2, Df 0.0025
S7038: Dk 3.8, Df 0.0070	EM-888: Dk 3.6, Df 0.0080	EM-891: Dk 3.6, Df 0.0050	TU-933: Dk 3.4, Df 0.0025
TU-863: Dk 3.9, Df 0.0070	TU-872: Dk 3.6, Df 0.0080	S7338: Dk 3.9, Df 0.0049	S7335: Dk 3.4, Df 0.0025
EM-822G: Dk 3.8, Df 0.0110	S7493: Dk 4.1, Df 0.0068	Meteorwave2000: Dk 3.4, Df 0.0040	Meteorwave4000: Dk 3.5, Df 0.0028

 

Thermal Management of Optical Module PCB

A large amount of heat is generated near the chips and optical devices (TOSA and ROSA) during high-speed data transmission. Effectively managing this heat is a core technology in the design of the PCB. PCB thermal management generally involves consumption reduction, thermal conductivity, and layout. Consumption reduction involves reducing the temperature by reducing the heat generated. Thermal conductivity involves effectively guiding the heat away to avoid its impact on the surrounding sensitive components. A reasonable layout involves precisely designing to isolate or guide the heat to non-critical areas.

Since the space in an optical module is very narrow, traditional strong convection cooling methods are not applicable. Additionally, the power consumption of the chip has greatly improved. Therefore, PCB thermal management for optical modules mainly involves heat conduction.

1.Copper Block Design and Buried Copper Technology

To improve thermal management efficiency, the PCB design of optical modules often uses buried copper blocks and copper paste plug holes. The buried copper block is usually placed at the bottom of the TOSA and ROSA chips. Burying the copper block inside the PCB enhances the heat conduction ability. Additionally, copper paste vias are widely used and have higher industrialization potential than buried copper blocks due to their lower cost and better processability. Despite its low thermal conductivity, copper paste provides effective thermal management for most optical module PCBs’ heat dissipation needs.

2.ELIC Structural Design

ELIC (electro-thermal combination design) is a complex heat dissipation technology for optical modules with high heat dissipation requirements. In this design, the PCB chip position will have a large number of copper connections through blind holes to each layer. This design is equivalent to forming multiple copper columns inside the PCB, thus improving heat dissipation performance. But it is usually only used in cases of very high wiring density and strong heat dissipation needs due to the high cost and long production cycle.

3.Through-Hole Filling Heat Dissipation

Additionally, the optical module PCB can use through-hole filling technology to improve thermal efficiency. This technology involves drilling through holes in the PCB and using special plating methods to fill these holes, forming an effective heat dissipation channel. The through-hole filling process is similar to the blind hole filling process, mainly using a plating solution with different chemicals that prompt the copper layer to be deposited preferentially from the center of the through-hole.

This type of filling ensures the copper layer uniformly covers the inner wall of the hole, forming a complete copper column structure that aids in heat dissipation. Since copper precipitates more readily from the center of the hole at lower potentials, the copper layer formed during the plating process extends and eventually joins the upper and lower ends of the hole. This creates an effect similar to that of a blind hole.

However, through-hole filling presents some challenges. It is less efficient, and the plating process typically takes six to eight hours or more to complete. Nevertheless, through-hole filling technology can effectively dissipate heat where high heat dissipation is required and is particularly suitable for high-power applications, especially optical modules.

Fastlink’s Optical Module PCB Advantages

With the rapid evolution of high-speed communication, cloud computing and AI arithmetic, the role of optical modules in data transmission links is becoming more and more prominent, which puts forward higher requirements for their supporting PCBs: high speed rate, high density, low loss, strong heat dissipation and high reliability. Only companies with advanced process capabilities and high-speed electrical performance control experience can truly participate in the core competition in this high threshold market.

Fastlink specializes in high-speed and high-frequency PCB R&D and manufacturing, and has been providing technical support and volume delivery services for domestic and international mainstream optical module customers for a long time. We support 400G/800G optical module high-speed board (such as RO4003C, Megtron6, etc.) processing experience, with HDI structure, back drilling process, gold finger processing and blind buried hole control capabilities.

FastlinkPCB Manufacturing Capabilities
Items	Mass Production	Sample
Layer count	18	22
Board Thickness	0.1mm~1.6mm	0.08mm~1.6mm
Board Thickness Tolerance	±10%	±8%
Max Delivery Panel	20”x 24”	21”x 24”
Max Copper Thickness Inner/Outer	2oz/2oz	3oz/3oz
Width/Space – Inner Layer	30um/30um (Hoz)	20um/20um (Hoz)
Width/Space – Outer Layer	30um/30um (Hoz)	20um/20um (Hoz)
Min Through Hole Diameter	0.10mm	0.10mm
Max Through Hole Diameter	6.3mm	6.5mm
Hole Position Tolerance	±3mil	±2mil
PTH Diameter Tolerance	±3mil	±2mil
Press Fit Hole Tolerance	±2mil	±2mil
NPTH Diameter Tolerance	±2mil	+2/-0mil
Min Distance Between Hole and Circuit	6mil	5mil
Deep Plating Capability	10:01	12:01
Hole Fill Aperture Ratio	8:01	10:01
Back Drill	Yes (Via >= 0.35mm)	Yes (Via = 0.30mm)
Edge Plating	yes	yes
Contour Tolerance	(+/-0.075mm)	(+/-0.05mm)
Width Tolerance for Gold Finger	(+/-0.03mm)	(+/-0.025mm)
Min Distance Between Gold Finger	5.0mil	4.0mil
Alignment Accuracy for layers	(+/-1mil)	(+/-0.8mil)
Impedance Tolerance (≥50ohm)	±8%	±5%
Surface Finish (In House)	Soft Gold, Hard Gold, Electroplate Nickel, Electroplate Silver, ENSG, ENIG, NiPdAu, OSP

If you have any other questions about Optical Module PCB, please feel free to contact us in the lower right corner.