Fibre-Optical Module PCB

Fibre-optical module is a kind of electronic components for photoelectric conversion. Simply speaking, optical signal is converted into electrical signal, and electrical signal is converted into optical signal, including transmitter, receiver and electronic functional circuit. Therefore, as long as there is optical signal, there will be application of optical module. According to the function, the optical fiber module is divided into optical receiving module, optical transmitting module, optical transceiver integrated module and optical forwarding module.

Design method of Fibre-optical module PCB

1. Panel method

Design of symmetrical panel with center mirror image recommended for miniaturized optical module PCB

2. Mark point design

It is recommended to place two mark points on the unit board. If the unit board can not place two mark points, but at least one mark point can be placed, and the mark point without copper ring can be used. At the same time, a mark point should be added on the auxiliary edge for coordination

3. PCB connection mode

Milling slot + stamp hole design: the width of milling slot is 2mm, the stamp hole is non-metallic, and the recommended hole spacing is 1.0mm and the hole diameter is 0.5mm,

The center of stamp hole should move towards the PCB of optical module. It is suggested that the distance between the center of stamp hole and the edge of milling groove should be no less than 0.6mm, which can reduce the burr on the edge of optical module PCB and avoid interference with the shell,

V-CUT connection: refer to general requirements of PCB process design specification.

Milling groove + solid connection: the width of milling groove is 2 mm, the width of real connection is 2-10 mm, and the cloth forbidden area is 1 mm,

Stamp hole + milling groove is preferred for panel and auxiliary block connection, followed by real connection + milling groove, and V-CUT connection is not recommended.

Note: in V-CUT connection mode, milling cutter splitting machine is not allowed to split plate, but hob splitting machine is used for plate splitting, which has large stress and optical module layout can not meet the cloth prohibition requirements.

4. PCB process layout requirements

The layout distance between components can be designed according to the table below.

In the actual PCB processing, the micro devices with lower body and non chip devices with higher body can be assembled well;

All the above distances are taken as the smallest of pad to pad, pad to device body, and device body to device body.

No device is allowed in the area of PCB and FPC welding surface

Hotbar process for optical device welding: the hot pressing area is 0.5mm larger than the pressing operation area of hot pressing head, and the hot pressing area is equal to or greater than the length of assembly area + 4mm,

For optoelectronic devices fixed with screws: it is forbidden to cloth any device and non ground via within 1.5mm around the body and at the bottom, and 1.5mm around the optical fiber sheath. If it is necessary to arrange the non ground via, the plug hole must be used to increase the character oil insulation, but the non ground through hole must be prohibited at the bottom of silk screen printing.

There is a gap of about 0.25 between the screw hole and the screw of the photoelectric device, and the tolerance of the device itself is ± 0.25. When the tolerance of the device chip is added, the 1.5mm cloth ban is required.

It is forbidden to place vias, test panels and devices around 1.0 mm pad of manually welded optical devices. If the through hole must be arranged, the through hole must be fully plugged with green oil.

The bright copper area cannot be directly connected with the pad, and there must be a solder mask in the middle. The minimum width of the solder mask is 3mil,

The layout distance between optical devices and between optical devices and electrical components shall meet the operation space requirements for manual welding and maintenance,

The pins of optical devices are basically welded by hand, so the layout must meet the requirements of manual welding, otherwise it will be easy to hit parts and cause difficulties in welding and repair.

In the layout of optical devices, the fiber outlet part can not enter into the insertion and extraction forbidden area of fiber connector, so as to prevent breaking the fiber outlet part of optical module tail fiber when inserting and plugging the connector.

5. PCB routing design requirements

For the wiring connected with PCB hot bar reflow pad, the recommended line width is 5-10mil; when large area grounding is required, the lead length d ≥ 50mil,

Note: according to the test results, if the lead length is too small for large area grounding, the heat transfer is too fast during hot pressing, and the process parameters are not easy to control, resulting in poor welding. Therefore, it is recommended that the lead length be greater than 50mil. Thick and long wiring, connecting via, large area copper foil heat dissipation fast, resulting in uneven temperature and inconsistent welding reliability.

6. Pad design

It is forbidden to design through-hole for non radiating pad of device.

7. Surface treatment

ENIG surface treatment is preferred. For hotbar pads, OSP surface treatment is prohibited.

8. PCB thickness design

The PCB thickness of the SFP and XFP optical modules must be 1.0 mm. (the required thickness of 1.0 mm for SFP and XFP is stipulated by MSA agreement.)