Li Qingchun, Hu Yuchun, Qiu Xiaohua

(Huizhou Zhongjing Electronic Technology Co., Ltd., Huizhou 516029, Guangdong)

0 Preface

Optical module based on photoelectric signal conversion plays an increasingly important role in modern high-speed communication system. As the PCB main board of its electrical connection function, due to the requirements of large data high-speed transmission, heat dissipation, surface mounting and hot plugging, its design process is different from that of ordinary PCBs. High-speed materials are required for materials, gold plating is required for hot plugging and plugging, and gold or nickel palladium is required for other mounting positions. For thermal management requirements, PCB heat dissipation is generally designed with copper paste, copper block, electroplated through-hole, or ELIC electroplating. Optical module packaging uses high thermal conductivity materials to assist heat dissipation (see Figure 1).

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1 Design and manufacturing process of optical module PCB lamination

Since the development of optical modules, PCB has basically adopted HDI structure, whether it is mechanical blind hole HDI, laser blind hole HDI, or soft and hard combination board+HDI. According to the requirements of its standard size interface, the thickness of optical module PCB is basically 1.0 ± 0.1 mm, and most optical module PCBs are basically ≤ 12 layers. There are roughly two different manufacturing processes for mechanical blind hole lamination and conventional HDI structure, as follows.

(1) The mechanical blind hole structure is generally 2+2, 4+4, 2+2+2, 4+2+2, and a few asymmetric structures, such as 2+4 or 2+6, are as follows (see Figure 2).

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2+2: cutting → burying drilling → electroplating → resin plug hole → line → pressing → drilling → electroplating (part with resin plug hole+cap electroplating) → line → anti-welding → text → surface treatment → molding="test → FQC;

4+4: cutting → inner layer → pressing → burying drilling → electroplating → resin plug hole → line → pressing → drilling → electroplating → (some resin plug holes+CAP electroplating) → grinding and polishing → line → anti-welding → text → surface treatment → post-process.

(2) Conventional HDI, generally 1-4 HDI, with 4-10 layers. The basic process is as follows. Material cutting → burying drilling → electroplating → resin plug hole or copper slurry plug hole → CAP electroplating → line → pressing → laser → hole filling → line (pressing, laser, hole filling, line process multiple cycles to complete the layer increase) → pressing → laser → mechanical drilling → hole filling → grinding and polishing → line → anti-welding → text → surface treatment → post-process;

(3) In addition, due to the different design and thermal management methods of the golden finger, the manufacturing process of the optical module PCB will be different. Buried copper block, copper slurry and through-hole filling are all used to solve the heat dissipation problem of optical module, with different implementation methods. At present, there are two main types of optical modules: long and short gold fingers and graded gold fingers. The production process of printed plug includes five graphic transfers: one time for outer circuit, one time for soldering exposure, one time for plating printed plug, one time for etching lead, and one time for selective gold or nickel palladium. There are many details of the process that will not be discussed in detail here. The production methods of different companies are similar.

2 Optical module gold finger design

The optical module mostly uses long and short gold fingers and graded printed plugs, but rarely uses the traditional equal-length printed plug design. The main reason is that the gold finger is used as the electrical interface of high-speed signal, with two different definitions of signal and power supply pins. The power supply pin is longer than the data pin, which ensures that the power is on before the data, and the data is off before the power is off. If the power supply is not powered on, the IO is connected first, and there is data coming in at the same time, which may cause damage to the logic chip or the motherboard interface chip.

The PCB electrical interfaces corresponding to 100G to 400G optical modules are long and short printed plugs or graded printed plugs. Take the early CFFP as an example, the PIN pin of the electrical part of the CDFP is arranged as follows. There are 8 differential pairs on the upper and lower layers, and a total of 16 differential pairs. Each differential pair supports 25 G NRZ signals, and can transmit up to 400 G bandwidth. The longer gold fingers are both power supply and stratum (see Figure 3).

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There is no graded printed plug in the actual definition of electrical interface. As an option of electrical interface, graded printed plug has certain advantages for mechanical plugging and wear resistance (see Figure 4).

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On the other hand, in order to prevent the capacitance effect, especially the high-speed electrical interface, whether for high-order graphics cards, GPU accelerators, or optical module products, all the inner layers of the corresponding positions of the printed plug are basically cut copper (digging under the fingers), which can reduce the impedance difference between the printed plug and the impedance line, and is also beneficial to ESD. For the optical module products shown in the figure below, only two outer layers of the printed plug have copper, and the inner layer has no copper distribution (see Figure 5).

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3 Surface treatment

The surface treatment of optical modules has many changes. According to different packaging methods, the mainstream is nickel palladium+printed plug or sunk gold+printed plug. The surface treatment process is divided into two types according to whether there is a solder-proof partition between the printed plug and the nickel palladium (or gold deposit) position. The first kind of gold finger has no boundary with nickel palladium (or gold deposit). It is required to make nickel palladium (or gold deposit) first, and then directly gild the nickel palladium (or gold deposit) base. Of course, the exposed high-speed transmission line can also be covered with ink according to the actual situation. Another kind of nickel palladium (or sunk gold) and gold fingers are separated by resistance welding. Generally, gold fingers are made first, and then nickel palladium (or sunk gold). The first manufacturing method can effectively shorten the process and reduce the complexity of the process, which has been adopted by PCB manufacturers in the industry. However, due to the relatively thin gold thickness of gold and nickel palladium, the requirements for process management are relatively strict (see Figure 6).

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It can also make nickel and palladium at all window openings to shorten the complexity of the overall process, improve the appearance yield and reduce costs. Generally, the gold plating of printed plug is mainly oxidation resistance and plugging resistance. The purity of gold-plated hand gold is 99.5%~99.7%, and the hardness of the printed plug is 150~200 HV. In the actual gold-plating process, a small amount of high-hardness wear-resistant metals such as cobalt and nickel are added to improve the wear resistance of the printed plug (metal hardness ranking: cobalt Co>chromium Cr>nickel Ni>copper Cu>zinc Zn>aluminum Al>gold Au).

The following is the Vickers hardness value of each component of the coating provided by the nickel and palladium manufacturers. The hardness of nickel and palladium is much higher than that of gold, which has a certain effect as the resistance to plugging for more times. The printed plug of optical module can use the whole plate of nickel and palladium instead of the surface treatment of gold-plated fingers+nickel and palladium, which is very beneficial to reduce the overall cost (see Table 1).

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4. Extend corresponding technical process according to optical module design

Optical module products basically adopt high-speed pure or mixed compression, and the materials are basically M4, M6, M7 and corresponding grade materials. Optical modules designed with pure high Tg plates are relatively low-end, and have been rarely used at present. Skip via can make the secondary HDI by one-shot blind hole electroplating, which mainly solves the cost problem. The disadvantages are the high aspect ratio of the blind hole, the great difficulty of hole filling, and the poor hole type, which is easy to produce crab feet, bubbles and other problems. When the resin plug hole+surface electroplating technology is used to solve the apparent flatness problem of optical module skip via, the increased plug hole and electroplating process will increase the cost. If the large blind hole (6~8mil) is filled twice to meet the requirements of the flatness of the blind hole, the capacity of the filling liquid is relatively high, the cost is also relatively high, and there will be a small amount of bubbles, which has a certain impact on the reliability. Therefore, skip via design is generally not recommended, but second-order HDI (see Figure 7).

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5 Thermal management of optical module

The optical module device block has a very high thermal management level. The heat source is mainly near the chip and optical device (TOSA and ROSA). See Figure 8 (the white circle indicates the heat dissipation position). Generally, thermal management is solved through three ways: consumption reduction, heat conduction and layout. Consumption reduction is to reduce heat generation; Heat conduction is to conduct heat away without any influence; The layout is that the heat is not dissipated, but some measures are taken to isolate the thermal sensitive devices. The space of the optical module is small, so it can not be cooled by strong convection. It mainly adopts heat conduction, including external heat conduction and internal heat conduction. The internal heat conduction is a solution from the optical device packaging materials and PCB materials. The overall power consumption of chip update iteration has been greatly improved. The heat dissipation optimization direction of optical module is to improve the thermal conductivity, increase the heat dissipation area, reduce the roughness of the contact surface, improve the flatness, and reduce the thickness of the heat transfer path.

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The main difficulty of the chip heat dissipation on the motherboard is that when the sub-motherboard or single board is used, the components with high heat generation are at the bottom, and the chip heat cannot be transferred to the main heat dissipation surface in time. To solve the heat dissipation problem of the optical module, both heat conduction and heat dissipation must meet the conditions. The following table shows the temperature test results of chip balance under different heat treatment conditions (see Figure 9).

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The heat dissipation of the optical module chip mainly uses soft and compressible high thermal conductivity materials, such as thermal conductivity silicon film, which is suitable for the optical module heat dissipation solution with its high thermal conductivity, low pressure deflection and low contact resistance. The thermal conductivity of common materials is as follows. Most of the heat dissipation of optical module PCB is based on the heat dissipation of material technology. Copper has the advantages of high thermal conductivity, relatively low cost, and compatibility with PCB, so copper is preferred for heat dissipation of optical module PCB. The thermal conductivity of red copper is 400 W/m.K. Generally, the copper plating is close to 350 W/m.K. The thermal conductivity of copper slurry is 8-10 W/m.K (the copper slurry contains many resin components). The following is the thermal conductivity of common materials (see Figure 10).

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PCB heat dissipation technology is based on the characteristics of copper itself. At present, copper slurry plug holes, buried copper blocks, electroplated through holes or PCB is designed as ELIC, and each layer of blind holes is stacked into a column to dissipate heat.

(1) For the optical module PCB designed with embedded copper block, the PP and base plate are windowed at the corresponding level, and the copper block is pressed and embedded into the PCB. The copper block embedded in the inner layer is generally regular round or square copper, and the copper block embedded in the outer layer is generally "T" type copper, and the copper block is generally located at the bottom of TOSA and ROSA chips to assist chip heat dissipation. When the copper PCB is embedded, the copper block will be densely laser blind holes and filled with holes to quickly and effectively transmit heat. Due to the small size of the optical module itself and the small corresponding position of the photoelectric conversion chip, the size of the optical module that needs to be embedded is smaller. This kind of small copper block is easy to shake during the pressing and embedding process, and the pressing operation is difficult, affecting the operation efficiency. At the same time, the relative position of the copper block is skewed, which will affect the flow filling effect of the pressed semi-cured sheet, and finally show the thermal stress is poor. In addition, the design of buried copper block, corresponding to the outer layer, is the substrate. During pressing, the browning surface of the substrate is easy to bond with the dust of the prepreg. After pressing, the copper surface prepreg remains, which ultimately affects the appearance yield of the optical module (see Figure 11).

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(2) The copper slurry plug hole is also a major means to solve the heat dissipation problem. The copper slurry material has been developed for many years. At present, compared with the buried copper block, the cost is lower, the processability and reliability are better, and the thermal conductivity is much lower than that of the buried copper block. However, the main bottleneck of PCB heat dissipation is the resin, and the thermal conductivity of the plug hole copper slurry reaches more than 8 W/m.K. Generally, it is recommended to use copper slurry to plug holes. The process is similar to that of vacuum resin plug holes, and the use of copper slurry has no impact on the production capacity of pressing, which is more conducive to industrialization than burying copper blocks (see Figure 12).

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(3) According to the economy of the design, the optical module itself is designed as ELIC, which can lay a large piece of copper at the position of the PCB chip and connect each layer through blind holes. The ELIC structure is equivalent to the copper column of pure copper in space, and the large copper surface forms a heat dissipation channel from top to bottom through the copper column, similar to Figure 13. This kind of design has high cost and long manufacturing cycle. Unless the wiring density is very high, so that the PCB must adopt ELIC design, the heat dissipation area can adopt similar ELIC structure to solve the heat dissipation problem, which is generally not recommended.

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(4) Through-hole filling heat dissipation: This technology was born in the semiconductor electroplating industry. After improvement, this technology was applied to PCBs with high heat dissipation and high reliability requirements, compared with laser micro-blind-hole or laser X-through-hole electroplating in the semiconductor industry. The X-through-hole is as follows, and the general aspect ratio is about 2:1. Because the X-hole has a relatively small funnel hole at the hole center, the electroplating copper is easy to deposit at this position with the aid of additives, and close to form the upper and lower blind holes. After forming the blind hole, the plating is the same as the blind hole filling (see Figure 14).

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The mechanism of through-hole filling is roughly the same as that of blind hole filling, that is, inhibiting the precipitation of the coating on the surface layer (high potential part) and promoting the precipitation of the coating at low potential to complete the filling. The low potential part of the so-called through hole is equivalent to the central position inside the through hole. Generally, there are more inhibitors than accelerants in through-hole filled copper plating bath. Since the concentration of additives in the surface layer of the substrate is higher than that in the through-hole, the coating will be preferentially precipitated from the central part (low potential part) of the through-hole using this principle. If the plating is preferentially precipitated from the center of the through-hole, the plating surface from the center of the hole will collide and extend to form two blind holes above and below the through-hole. That is, the so-called electroplating bridging, and then fill the blind hole under high current and high Cu2SO4 density. The biggest disadvantage of through-hole filling is that the parameter conditions are harsh and the efficiency is too low. It usually takes 6~8 hours or longer to complete. The pictures of PCB through-hole filling steps are as follows (see Figure 15).

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The plating bridging conditions of Amete through-hole filling are as follows: after testing, the small hole is ≤ 0.25 mm, and the aspect ratio is 2~4, which is easy to achieve through-hole filling. On the contrary, due to the high efficiency of liquid medicine exchange, the large hole and low aspect ratio can not form a perfect flatness at the orifice position, and the depression is deep, and even can not form a bridging effect. The effect of through-hole filling is closely related to the aspect ratio, the combination of additives and the exchange rate of liquid medicine. The best liquid medicine conditions (see Figure 16).

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6 Summary

With the development of optical module towards miniaturization, low cost, low power consumption, high speed, long distance and hot plugging, the corresponding PCB has higher integration. Because the optical module PCB is based on copper signal transmission, the maximum speed is limited, so there are multiple channels in the big data conversion, and each channel corresponds to a differential signal pair of the optical module PCB. Higher transmission rate corresponds to more differential signal pairs, that is, under the premise of miniaturization of optical module, the distribution density of blind holes and lines is higher. Such high wiring density and transmission rate mean that the optical module generates more heat during its working process. The spectral thermal drift effect of the optical module laser directly determines whether it works effectively. The heat dissipation problem directly affects the working state of the core components of the optical module, and the solution direction:

(1) Improve chip manufacturing technology and save energy consumption from chips;

(2) PCB adopts high thermal conductivity materials and designs efficient heat dissipation path;

(3) Solve the heat dissipation problems from the optical module packaging technology, such as heat dissipation paste, heat dissipation channel, shell heat dissipation design, etc. As an optical module component, PCB's heat dissipation technology will be used more and more in optical modules above 100G.