Communication PCBs are mainly HDI boards. When we design HDI PCB layers, we need to include some vital information, such as:

Complete PCB stack up

PCB stack-up is one of the critical factors in telecommunication PCB design and fabrication. Since the stack-up contains essential information, the PCB manufacturing process is done around the stack. Therefore, a complete telecommunication PCB stack-up includes the following important information:

Layer information

The stack-up includes layer information such as:

• the number of PCB layers
• the thickness of the solder mask layer
• the thickness of the insulating layer between layers,
• the copper thickness of each layer,
• the thickness requirements of the completed board.

Hole location information

We can use the positions of through holes, buried holes, and blind holes to determine the size of the PCB board. We can also design the manufacturing process according to the positions of buried holes, through holes, and blind holes connected between layers.

Impedance related information

The stack should include information such as the theoretical value of impedance line width and line spacing design and the impedance value requirements of the corresponding layer.

Material information

In order to calculate the Er (dielectric constant) value of the material, the PP gauge, thickness, impedance value, etc., should be included in the stack up.

When designing PCB stack-up, considering telecommunication PCBs are mostly high-density, high-frequency, high-speed, and high-heating characteristics, we need to select circuit board materials and optimize circuit board design strictly.

Telecommunication PCB features:

Thin

Since the inner core board is relatively thin, most of them need to use a copper-clad substrate with a thickness of 0.05mm or less; in addition, the thickness of PP used in the stack-up design is relatively thin; we should use 106# and thinner PP material. HDI boards are mostly 8~14-layer boards, and the PCB thickness after manufacturing is usually only 0.6~0.8mm, or even thinner.

High

The intelligent mobile telecommunication PCB is usually an HDI board with any layer interconnection design, which requires a high process production capacity. Since telecommunication PCBs have higher requirements for signal transmission. Therefore, higher standards for impedance consistency.

Dense

High density is an essential feature of HDI boards. High density can shorten the signal transmission distance, reduce the loss caused by capacitance and inductance, save power consumption, and improve the device’s battery life. The finer and denser the PCB circuit design, the smaller the pads and spacing of the corresponding devices, and the more complex the PCB manufacturing.

According to the above telecom PCB characteristics, when we design the PCB, we need to consider the following factors:

Material selection

telecomuncation PCB material hydrocarbon resin

Communication equipment must ensure high frequency, high speed, low transmission line loss and impedance, delay consistency and other characteristics. Telecommunication PCB material requirements are higher than common PCB due to high-frequency requirements. Because the loss increases as the frequency increases, we must choose a high-frequency sheet with low dielectric loss Df to ensure a faster transmission speed; the dielectric constant Dk should also be relatively small. Commonly used sheets are mainly composite high Tg materials, hydrocarbon, PTFE, etc. Below is a table of transmission loss and speed for different PCB materials.

Material selection is one of the manifestations of the PCB designer’s ability. Choosing a suitable material will reduce production costs and improve the PCB’s quality and efficiency.

For mature smartphone communication products with a relatively short cycle, high volume of mass production, and short delivery time. Therefore, when selecting materials, it should consider not only meeting the performance requirements of customers but also factors such as material procurement and warehousing. We can try to choose common specifications of CCL and PP; especially for PP, we should try to ensure the diversity of selection and reduce the type of PP, which is conducive to the versatility and consistency of materials.

We can design some common stack up suitable for our factory production standards (such as 10 layers of 0.6mm, 12 layers of 0.8mm, etc.), and on the premise of meeting customer needs, determine several specifications of CCL and PP as standing materials. Then negotiate with the customer and directly refer to the standard common stack when designing the circuit schematic diagram to reduce the preparation time and shorten the delivery time. Formulating standard common stacks and selecting common materials can reduce material control and storage costs.

For industrial communication base stations with low-volume manufacturing, various material requirements. we can consider the following:

Lower loss copper-clad laminate material

5G telecommunication PCB will require high-speed copper-clad stack-up technology, lower loss Df, lower dielectric constant Dk, higher reliability, and lower CTE technology. Correspondingly, the main components of copper-clad laminates are copper foil, resin, glass cloth, filler, etc.

Lower loss resin material

PCB fr4 material

In order to meet the high-speed requirements, the traditional FR4 epoxy resin system can no longer meet the requirements, and the Dk/Df of the CCL resin is required to be smaller. The resin system is gradually approaching the hybrid resin or PTFE material.

The high-speed and high-frequency is getting higher and higher, the aperture is getting smaller and smaller, and the aspect ratio of the PCB will be larger, which requires the copper-clad laminate resin to have a lower loss.

Lower Roughness Copper Foil Technology

High-frequency CCL materials are important for high-frequency PCBs, including substrate material Dk/Df, TCDk, dielectric thickness stability, and copper foil type.

The smaller the roughness of the copper foil, the smaller the dielectric loss. The dielectric loss of HVLP copper foil is significantly smaller than that of RTF copper foil. Considering the performance of 5G products, HVLP copper foil with lower roughness is required, but the roughness of copper foil is reduced, and the peel strength is also reduced. There is also a risk of stripping lines or small pads.

Low loss and low expansion glass cloth technology

In order to meet the high-speed PCB design and large-size chip application in 5G communication products, the Dk/Df and CTE of the high-speed CCL glass cloth are required to be smaller.

If the material CTE is too large, defects such as solder joint cracking will occur during PCB assembly and soldering. In order to develop a low-CTE high-speed copper-clad stack-up, the CTE of the glass cloth is less than or equal to 3.0ppm/℃.

In order to meet the above CTE requirements, it is necessary to innovate the glass fiber raw material formulation and drawing process technology to prepare glass cloth with lower CTE to meet the needs of 5G or 6G communication technology.

Media Thickness Stability

The uniformity and fluctuation of the dielectric layer’s structure, composition and thickness affect the characteristic impedance value. Under the same thickness of the dielectric layer, the dielectric layers composed of 106, 1080, 2116 and 1035 and resin respectively have different characteristic impedance values.

Therefore, the characteristic impedance value of each dielectric layer of the PCB is different. In the high-frequency and high-speed digital signal transmission application, choosing a thin glass fiber cloth or an open fiber flat cloth is necessary to reduce the fluctuation of the characteristic impedance value. We must control the Dk value of different batches of materials within a certain range, and the thickness uniformity of the dielectric layer should be better. Make sure the Dk change value is within 0.5.

telecommuncation PCB component

Higher thermal conductivity copper clad laminate

In order to reduce the Df value of the material, we can choose materials with higher thermal conductivity (TC). For 5G high-frequency PCB boards, we should choose a relatively thin substrate material. At the same time, material characteristics such as high thermal conductivity, smooth copper foil surface, and low loss factor are beneficial to reducing the circuit’s heating in the millimeter-wave frequency band.

Higher reliability copper-clad laminates

5G communication products are getting smaller, the PCB density has been reduced from 0.55mm to 0.35mm, the PCB thickness of HDI process single board has been increased from 3.0mm to 5.0mm, and the MOT temperature requirement has been increased from 130°C to 5.0mm. 150℃, the copper-clad laminate is required to have better heat resistance and higher CAF resistance.

Process compatibility

The designed stack-up must match the PCB manufacturing process. We should first determine the core board layer and the first lamination layer according to the layer of the buried hole and then determine the lamination of the subsequent layers according to the layer of the blind hole.

At the same time, according to the aspect ratio of the copper electroplating process (hole copper, The ratio of copper to surface copper) to calculate the copper thickness that can be achieved in each layer, to determine the thickness of copper foil that needs to be used for lamination.

The horizontal direction (X, Y-axis) is the matching relationship between the copper thickness (base copper + electroplated copper) and the line width and line spacing completed in each layer. There will be a better PCB manufacturing process only with stacks that match the process.

PCB hole

impedance

Telecom PCB has higher requirements for signal transmission and higher impedance consistency requirements, especially for some signal control with higher impedance, such as 50Ω characteristic impedance; impedance tolerance requirements have been tightened from the normal ±10% to ±6 %, namely (50±3)Ω.

The main influencing factors of impedance are the thickness of the insulating dielectric layer, copper thickness, line width and line spacing. Therefore, when designing a stack up, we can calculate the impedance value according to the electrical properties of the material, as well as the copper thickness and insulating layer thickness of each layer pattern.

The theoretical impedance value is designed to the median value required by the customer by adjusting the corresponding line width and spacing.

In addition to the above considerations when designing PCB, in order to ensure the high reliability of the telecommunication PCB, the mature processing and testing technology of the PCB manufacturer is also inseparable.

For 5G communication products, the requirements for PCB production and processing are even higher, especially for PCB substrate materials, processing technology, and surface treatment.

telecomuncation PCB press machine

As the operating frequency of 5G communication products increases, it brings a new challenge to the manufacturing process of printed boards. Millimeter-wave PCBs are usually multi-layer structures, and microstrip lines and grounded coplanar waveguide circuits are usually located in the outermost layer of the multi-layer structure. Millimeter waves belong to the extremely high frequency (EHF) range in the entire microwave field. The higher the frequency, the higher the required circuit size accuracy. When processing them, we need to control the below factors:

Appearance control requirements: Microstrip lines in critical areas are not allowed to have pets and scratches because the high-frequency PCB lines transmit not current but high-frequency electrical pulse signals. The pits, gaps, and pinholes on high-frequency wires. etc. defects will affect the transmission, So any such small defects are not allowed.

Control the corners of the microstrip antenna: In order to improve the gain, direction, and standing wave of the antenna; to avoid the resonant frequency shifting to high frequencies, and to improve the margin of the antenna design, it should strictly control the corners of the microstrip antenna patch (Corner sharpness control (EA). ), such as ≤20um, 30um, etc.

For single-channel 112G high-speed products, the PCB copper-clad laminate material is required to have lower Dk and Df, and new resin, glass cloth and copper foil technologies are required. The PCB process requires to have higher back-drilling precision, stricter thickness tolerance control, and a smaller hole.

In 5G telecom PCB processing, we must face the following difficulties.

1) 5G chips require a smaller spacing between PCB holes, the minimum hole wall spacing is 0.20mm, and the minimum hole diameter is 0.15mm. Such a high-density layout challenges CCL materials and PCB processing technology, such as CAF problems, cracks between heated holes, etc.

2) 0.15mm small hole, the maximum aspect ratio exceeds 20:1, how to prevent needle breakage when drilling, improve the aspect ratio of PCB plating, and prevent the copper-free hole wall, etc.

3) Pad warping: In order to reduce the signal loss on high-speed and high-frequency PCBs, we should use high-speed materials, and the whole ring should be as small as possible, from 5.0 mil to 3.0 mil, but the bonding force between high-speed materials copper foil and resin is stronger than conventional FR4 material, and then use the small hole ring. Due to thermal stress shock, the pad warping or surface PP resin cracking defects will occur when the PCB is reflowed or wave soldered.

4) Immersion copper: Due to the particularity of the high-frequency PCB board material, the whole wall is not easy to be covered with copper, causing problems such as failure to sink copper or voids in the copper sinking.

5) Control of image transfer, etching, line gaps of line width, and sand holes.

6) Green oil process: control of green oil adhesion and green oil foaming.

7) The high-frequency material is relatively soft, and each process strictly controls the board surface scratches, pits, dents and other defects.

Therefore, in order to ensure good telecom PCBs, the following processes and quality control are often used when manufacturing high-frequency PCBs with FR4.

Process and process control:

Cutting: The protective cover must be kept for cutting to avoid scratches and indentations.

Drilling:

1. A) Using a brand new drill tip.
2. B) Laminated board: 2 holes are drilled for the laminated board below 1.6mm. 1.6mm or more use 1 drill plate.
3. C) The feeding port comprises a phenolic board as a cover.
4. D) Drilling speed is 20% slower than FR4 board.
5. E) If the edges of the holes are uneven, use manual grinding and use 2000# sandpaper. Mechanical grinding is not allowed, which may cause lengthening and prevent the sandpaper from scratching the copper surface.

Pore treatment: high-frequency pore-forming agent, soak for half an hour.

Immersion copper:

1. A) Before the copper sinking, confirm the wear marks of the grinding board: 8-12mm.
2. B) The backlight cannot confirm the copper sinking, so use a nine-hole mirror of Uno to check the effect of sinking copper.
3. C) The board surface is rough and the copper particles must be treated with 2000# sandpaper.

Figure turn:

1. A) Before grinding the plate, confirm the wear scar: 8-12mm.
2. B) Line width and line gap ensure that within the range of MI compensation requirements, the line width after development is generally within ±0.01mm from the film line width.
3. C) After developing, the space between the racks and the racks is not allowed to be fully inserted to prevent scratches.

Picture and electricity:

1. A) The control clip is broken, the board surface is rough, pinholes, fingerprints and other problems.
2. B) Hole thickness: minimum 18um, average 20um.

Etch:

1. A) Line width is ±10%.
2. B) The etched board is not allowed to bare hands and the substrate inside the board to prevent contamination of the substrate surface and affect the adhesion of the green oil.

Solder Mask:

1. A) Pretreatment: use acid washing, not mechanical grinding
2. B) Bake the plate at 85°C for 30 minutes after pretreatment.
3. C) Use ink with better adhesion, such as sun: PSR -4000, PSR -2000. Stand still: 30 minutes – 1 hour.
4. D) Before the alignment, check the green oil board with poor appearance and directly develop the green oil and reprint it.
5. E) Green oil post-fix: All high-frequency boards must be segmented and baked.

The first stage: 1 hour at 50°C, and the second stage: 1 hour at 70°C.

The third stage: 100°C for 30 minutes. Fourth stage: 120°C for 30 minutes.

Fifth stage: 1 hour at 150°C.

Tin spray:

1. A) Bake the board with a solder mask before spraying the tin: 140℃ for 60 minutes.
2. B) Bake the board without a solder mask at 110℃ for 60 minutes before spraying the tin and 50℃*60 minutes.
3. C) The board should be sprayed with tin as much as possible to prevent the board from peeling off.

gong side:

1. A) Adopt a special program and special gong knife.
2. B) The gong edge speed must be 20% slower than that of FR-4.
3. C) Using a new gong knife, the service life is 10 meters.
4. D) The burrs on the back plate of the gong should be carefully scraped with a scalpel to avoid damage to the substrate and copper surface.

Package:

1. A) Because the board is soft and easy to deform, it is best to use waste cardboard to protect both sides and then vacuum the package when shipping.
2. B) Immersion silver plate must be isolated from sulfur-free paper to prevent oxidation.

In addition, although high-speed, multi-layer PCB raw materials is not difficult to get, there are also certain difficulties in manufacturing and processing. Because high-speed multi-layer PCB has more layers, more vias and lines, larger size, thinner dielectric layer, thicker and other characteristics.

Generally, the 5G ONT transmission network single board is above 220 layers, the base station BBU telecommunication PCB is above 20 layers, and the backplane is above 40. Therefore, when manufacturing telecom PCBs, it will face the problems of impedance control, interlayer alignment and reliability.

ont transmission

1. a) Interlayer alignment

Due to the large size of the multi-layer PCB, the temperature and humidity of the workshop cause the expansion and contraction of the PCB, which brings a certain dislocation, which makes the alignment between the high-level PCB layers more difficult.

1. b) inner layer circuit production

Because telecom PCBs mostly use high-speed, high-frequency TG, thin dielectric layers and thick copper materials, it brings the difficulty of manufacturing inner layers. In addition, the particularity of the material will bring the following problems.

• The line width and spacing are small, the open and short circuits increase, the micro-shorts increase, and the pass rate is low;
• Many signal layers in fine lines and the probability of missed detection of AOI in the inner layer increases;
• The thickness of inner core board is thin, which is easy to wrinkle and cause poor exposure, and it is easy to roll the board when it is etched;
• Most of the high-end boards are system boards, large unit sizes, and the cost of scrapping the finished product is relatively high.

c)OPress-fit

Multilayer PCB lamination production is prone to defects such as slippage, delamination, resin voids and bubble residues.

d)Drilling

The special PCB materials also increase the difficulty of drilling roughness, drilling burrs and decontamination. In addition, the number of PCB layers is large, the total copper thickness and the thickness of the PCB board are thick, and the drilling tool is easy to break;

There are many dense BGAs, and the narrow hole wall spacing causes the failure of CAF; the PCB board’s thickness easily causes the oblique drilling problem.

In order to ensure accurate alignment between high-speed multi-layer PCB layers, it should design a reasonable stack structure, fully consider the heat resistance, withstand voltage, amount of glue, and dielectric thickness of the material, and set an appropriate pressing procedure. On the other hand, it should use more advanced processing equipment and strictly follow the production process.

The key production process of high-speed PCB board:

Interlayer alignment control

The interlayer alignment control must be considered comprehensively, such as:

• The compensation value of the inner layer
• The positioning method of pressing
• The parameters of the pressing process
• The material properties

Inner circuit technology

We can use a laser direct imaging machine (LDI) to improve the ability of graphic analysis; with a high-precision alignment exposure machine, the graphic alignment accuracy can be increased to about 15μm.

In order to expand the line etching ability, it should take suitable compensation to the width of the line and the pad (or solder ring) in the engineering design, and also take a complete design for the compensation amount of special graphics, such as independent lines and return lines,

Laminated structure design

Follow these main principles:

It should ensure prepreg and core board manufacturers are consistent. When the customer requires a high TG sheet, the scoreboard and the prepreg must use the corresponding high TG material.

If the inner layer substrate is 3OZ or above, we can choose the prepreg with high resin content. Suppose the customer has no special requirements; the thickness tolerance of the interlayer dielectric layer is generally controlled by +/-10%.

Lamination process

Different product structures use different positioning methods. We can use X-RAY to check the layer deviation during fusion when adjusting the machine to make the first board. According to the laminated structure of the multi-layer circuit board and the materials used, the appropriate pressing procedure is studied, and the optimal heating rate and curve are set.

Drilling process

The plate and copper layer becomes thick due to the superposition of each layer, which will cause the drill wear and drill blade failure. We also appropriately adjust the number of holes, drop speed, and rotation speed. Measure the expansion and contraction of the board accurately and provide accurate coefficients;

In order to solve the drilling burr problem of high-level thick copper plates, we should use high-density backing plates, the number of stacked plates is one, and the grinding time of the drill is controlled by 3 times.

Back-drilling technology effectively improves signal integrity for high-frequency, high-speed, and massive data transmission high-level circuit boards.

Therefore, compared with ordinary PCBs, high-frequency boards and high-speed multi-layer telecom PCBs require higher technical processes. In addition to high-precision equipment, mass production requires long-term production and processing experience accumulation.