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20 mil RO4003C PCB
20 mil RO4003C PCB

What's the Difference between High-Frequency Microwave PCBs and Aluminum-based PCBs?

  • July 13. 2022
What's the Difference between High-Frequency Microwave PCBs and Aluminum-based PCBs?

1. Let’s talk about high-frequency microwave printed circuit boards first

High-frequency microwave printed circuit boardsare heating up in China.

In recent years, many printed circuit board companies in East China, North China, and the Pearl River Delta have been staring at the market of high-frequency microwave boards, and are gathering together the dynamics and information of high-frequency waves and polytetrafluoroethylene (Teflon, PTFE). , regard this new variety of printed boards as a set of products necessary for the electronic information high-tech science and technology industry, and emphasize research and development. Some business executives definitely believe that the high-frequency microwave board is a new economic improvement point for the company in the future.

Senior overseas experts predict in advance that the market for high-frequency microwave boards will develop rapidly. In the fields of communication, medical treatment, military, transportation, computer, spectrograph, etc., the demand for high-frequency microwave boards is rising very fast. A few years later, high-frequency microwave circuit boards may account for about 15% of the total printed boards in the world. Many PCB companies in Taiwan, South Korea, Europe, the United States, and Japan have formulated plans for progress in this direction.

European and American high-frequency microwave sheet suppliers Rogers, Arlon, Taconic, Metclad, and GIL Toyo Chukoh have entered the latent big market in China in the past two years, looking for agents, explaining and teaching related technologies. The American GIL company held a lecture on "Application and Production Technology of High-Frequency Microwave Printed Boards" in Shenzhen. Hundreds of seats were all filled, and the corridor was also full of company representatives listening to the speech. Many executive-level figures listened to a lecture. A full day of tech talks. I really don't want domestic counterparts to have such a strong interest in high-frequency boards. European and American sheet metal suppliers have been able to supply more than 100 varieties of sheet materials with dielectric constants ranging from 2.10, 2.15, 2.17, ... to 4.5, and even higher.

In the Pearl River Delta and Yangtze River Delta studies, it is reported that many companies have been listed to order Teflon and high-frequency boards in batches. It is reported that some companies have reached the level of several thousand square meters per month. Many domestic printed board factories of radar and communication research institutes need high-frequency microwave sheets to increase year by year. Domestic Huawei, Bell, Wuhan Academy of Posts and other large communication companies need high-frequency microwave printed boards to increase year by year, and overseas companies that invest in high-frequency microwave products have also migrated to China to collect and purchase high-frequency microwave printed boards nearby. plate.

There are various indications that high-frequency microwave boards are heating up in China.

(What is high frequency? Above 300MHZ, that is, the short-wave frequency range with a wavelength of more than 1 meter, commonly referred to as high frequency.)

High Frequency Microwave Printed Circuit Board

 2. Why is it hot?

There are three reasons.

(1) The local frequency band of high-frequency communication originally used for military use was given to people's life (started in 1996), which greatly improved the high-frequency communication used in people's life. It has great ambitions in various fields such as long-distance high communication, navigation, medical treatment, transportation, transportation, and inventory.

(2) High confidentiality and high transmission quality, so that mobile phones, vehicle phones, and wireless communications will progress to high frequency, and high screen quality will enable radio and television transmission, and broadcast programs with very high frequency and ultra-high frequency. The transmission of high information volume requires high frequency of satellite communication, microwave communication and optical fiber communication.

(3) The experience of computer technology processing has increased, and the information memory volume has increased, so it is very necessary to require high-speed signal transmission.

In a word, the high frequency and high speed of electronic information products put forward high requirements for the high frequency special properties of printed boards

3. Why is low ε (Dk) required for printed boards?

ε or Dk, called the dielectric constant, is the ratio of the capacitance between the electrodes when something is charged to the capacitance of a vacuum capacitor of the same construction. Generally expresses the volume that a material has experience in storing electrical energy. When ε is large, the stored electric energy has a large experience, and the transmission speed of circuit electric shock signal transmission will become lower. The direction of the current passing through the electrical signal on the printed board is generally alternating positive and negative, which is equivalent to the process of continuously charging and discharging the substrate. In mutual exchange, the electrical volume affects the channel transport speed. And this effect is even more critical in high-speed transmission devices. The low ε means that the storage experience is small, and the charging and discharging process is fast, so the transmission speed is also fast. Therefore, in high-frequency channel transmission, a low dielectric constant is required.

There is an additional concept, that is, medium damage. Under the effect of an alternating electric field, the energy consumed by the electric medium material due to the medium damage, which is generally expressed by the medium damage factor tanδ. ε and tanδ are proportional, high-frequency circuits also require low ε, and the medium damage tanδ is small, so the energy + ray damage is also small.

4. ε of Teflon printed circuit board

Among the substrates of printed boards, the dielectric constant ε of PTFE is the lowest, typically only 2.6~2.7, while the dielectric constant ε of FR4 of ordinary glass cloth epoxy natural resin substrate is 4.6 ~5.0, so Teflon printed board signalling is much faster than FR4 (about 40 percent). The intermediate damage factor of Teflon board is 0.002, which is 10 times lower than 0.02 of FR4, and the energy + ray damage is also much smaller. In addition, polytetrafluoroethylene is called "the king of molecular compound plastics", which has good electrical insulation performance, good chemical and thermal stability (until now there is no solvent that can dissolve it below 300 ° C), Therefore, for high-frequency and high-speed signal transmission, Teflon or other substrates with low dielectric constant must be used first. The author saw that Polyflon, Rogers, Taconic, Arlon, and Meclad can all supply substrates with dielectric constants of 2.10, 2.15, 2.17, and 2.20, and their dielectric damage factors are 0.0005~0.0009 at 10GHZ. The performance of PTFE material is good, but the process of processing it into a printed board is absolutely different from the traditional FR4 process, which will be negotiated later.

In the past two years, we have used Rogers RO4000 and GIL1000 series, such as ε3.38, 3.0, 3.2, 3.8, etc., in addition to the requirements of ε2.15 and 2.6 in practice.

5. Basic requirements for high frequency microwave circuit boards

· Because it is a high-frequency signal transmission, the special property impedance of the conductors of the finished printed board is required to be strict, and the line width of the board is generally required to be ±0.02mm (the most strict is ±0.015mm). Because of this, the etching process needs to be strictly controlled, and the master plate for photoimaging transfer needs to be paid according to the line width and copper foil thickness.

The circuit of this type of printed board does not transmit current, but high-frequency electrical and electronic pulse signals. Defects such as pits, gaps, and pinholes on the wires will affect the transmission. No matter what kind of small defects are not allowed of. Sometimes, the thickness of the solder mask is also strictly controlled, and the solder mask on the circuit is too thick or too thin by a few microns, and it will also be judged to be unsuitable for the standard.

· Thermal shock 288℃, 10 seconds, 1~3 times, no hole wall clutching occurs. For the PTFE plate, the wettability in the hole must be solved, so that there are no holes in the chemically immersed copper hole, and the copper layer plated in the hole can withstand thermal shock. One of the easy places to fix. Because of this, many substrate manufacturers have developed substitutes with higher output ε, and the chemical copper deposition process is the same as the common FR4 method, Rogers Ro4003 (ε3.38) and Xi'an 704 Factory's LGC-046 (ε3.2 ±0.1) is such a product.

· Warpage: Generally, 0.5~0.7% of the finished board is required.

6. The processing of high-frequency microwave circuit boards is not easy to solve

Based on the special physical and chemical properties ofPTFE circuit boards, the processing technology is different from the traditional FR4 process. Get products that meet standards.

(1) Drilling: The base material is soft, and the number of stacked boards for drilling should be small. Generally, the thickness of the 0.8mm board should be two stacked one; has its own special requirements.

(2) Printed solder mask: After the wrench is etched, do not use a roller brush to grind the board before printing the green oil for the solder mask, so as not to damage the substrate. It is recommended to use chemical methods for surface treatment. To do this: without grinding the board, after printing the solder mask, the circuit and the copper surface are on average exactly the same, and there is no oxygen oxide layer, which is by no means easy.

(3) Hot air leveling: Based on the external properties of fluorine natural resin, try to prevent the sheet from heating very quickly. Before spraying tin, it should be preheated at 150°C for about 30 minutes, and then spray tin immediately. The temperature of the tin cylinder should not exceed 245℃, otherwise the adhesion of the isolated pad will be affected.

(4) Milling shape: Fluorine natural resin is soft, and the usual planer milling shape has too many burrs, which is not fair and needs to be milled with a suitable special type of planer.

(5) Inter-process transportation: It cannot be placed vertically, but can only be placed in the basket with the paper separated, and the circuit graphics in the board cannot be touched with fingers during the whole process. Avoid scratches and scratches in the whole process. Scratches, pinholes, indentations, and pits on the line will affect the signal transmission and the wrench will be rejected.

(6) Etching: Strictly control side erosion, sawtooth, and notch, and the line width tolerance is strictly controlled within ±0.02mm. Check with a 100x magnifying glass.

(7) Chemical immersion copper: The pretreatment of chemical immersion copper is the biggest and most difficult part of making Teflon boards, and it is also the most critical step.

For ε3.38 and Rogers Ro4003C high-frequency substrates, the high-frequency performance is roughly similar to that of PTFE glass fiber substrates, and the unique feature of easy processing is roughly similar to FR4 substrates. This is made of glass fiber and ceramics. Filler, high heat-resistant material with glass transition temperature Tg>280℃. Drilling this kind of base material consumes a lot of drill bits, requires the use of special drilling machine parameters, and often needs to change the planer for milling the shape; but other processing techniques are generally similar, and no special hole treatment is required, so it has won many PCB circuit board manufacturers and customers. However, Ro4003 does not contain flame retardants. When the wrench reaches 371°C, the wrench can trigger the burning phenomenon. The LGC-046 sheet of the state-owned 704 factory is a modified polyphenylene ether (PPO) type, with a dielectric constant of 3.2 and the same processing performance as FR4. This product has also obtained many single licenses in China.

RO4003C 8mil PCB

7. What is the use of high frequency microwave circuit board?

Satellite collector, base receiving antenna, microwave transmission, vehicle telephone, global positioning system, satellite communication, communication equipment material adapter, collector, signal vibrator, home appliance networking, high-speed computer, oscilloscope, IC For testing spectrographs, etc., high-frequency microwave printed boards are required in high-frequency communication, high-speed transmission, high security, high transmission quality, high memory volume processing and other communication and computer fields.

8. The general situation of high-frequency microwave sheet at domestic and abroad

In China, in addition to the LGC-046 modified polyphenylene ether sheet of the 704 factory mentioned above, Taizhou high-frequency copper-clad sheet factory TF-2, F4B, F4BK high-frequency microwave, PTFE sheet It is also selling well. It is reported that many companies in Beijing, Yangtze River Delta, and Guangdong are also starting work and starting construction.

Overseas, the main sheet suppliers are: Europe and the United States Rogers, Arlon, GIL Taconic, Metclad, Isola, Polyclad, Toyo Asaki, Hitach, ehemical, Chukok, etc., have formed about 130 different dielectric constants for high-frequency microwave paper Variety.

The gap between domestic and foreign countries so far: varieties, quality are stable and completely the same, price; large overseas customers have a certain degree of difficulty in licensing Chinese products, and so on.

The thickness of the sheet is 1.5~1.6mm, while 0.5, 0.8, and 1.0mm are relatively common. The main consideration is the cost. The price of Teflon sheet is 5~10 times that of ordinary FR4. It also costs about 100 US dollars/m2 for bulk purchases, and several hundred US dollars/m2 for sporadic purchases.

Summary: High-frequency microwave sheet should be a new variety of high-tech science and technology. With the continuous development of high-frequency and high-speed communication and computers, the future use will definitely become wider and larger. The price of sheet metal is also high, and there is a large profit margin. This product has a bright future.

Second, let's talk about the metal aluminum circuit board

1. Why use metal-based printed circuit boards?

(1) Heat dissipation

Up to now, many double-sided and multi-layer boards have high density and high power, and it is difficult to emit calories. Common PCB substrates such as FR4 and CEM3 are bad conductors of heat, and the layers are insulated, so the calories cannot be emitted. The heat of the electronic equipment is not eliminated, causing the high temperature of the electronic components to lose their effectiveness, and the metal-based printed circuit board can solve this problem of difficult heat dissipation.

(2) Heating expansion

Thermal expansion and cold contraction are the side-by-side nature of things. Different things have different CTE (Coefficient of Thermal Expansion), that is, the coefficient of thermal expansion.

The printed board is a composite of natural resin + reinforcing material (such as glass fiber) + copper foil. In the X-Y axis direction of the board surface, the thermal expansion coefficient (CTE) of the printed board is 13~18 PPM/℃, and in the Z axis direction of the board thickness is 80~90PPM/℃, while the CTE of copper is 16.8PPM/℃. The CTE of the sheet ceramic chip carrier is 6PPM/℃. The CTE of the metallized hole wall of the printed board and the connecting insulating wall in the Z-axis is very different. The generated heat cannot be removed as soon as possible. Cracks and fractures appear in the chemical hole, so the machinery and facilities are unreliable.

SMT (Surface Mount Technology) has exacerbated this problem and made it an unsolvable problem. Since the surface mount interconnection is successfully achieved by the direct adjacency of the surface solder joints, the CTE of the ceramic chip carrier is 6, while the CTE of the FR4 substrate in the X-Y direction is 13~18. The point is that because the CTE is different, long-term stress tolerance will cause fatigue disconnection.

Metal-based printed circuit boards can effectively solve the problem of heat dissipation, so that the problems of thermal expansion and contraction of different components on the printed circuit board are alleviated, and the durability and reliability of the whole machine and electronic equipment are increased.

(3) Dimensional stability

The metal-based printed board is obviously much more stable than the printed board of insulating material. Aluminum-based printed boards and aluminum sandwich panels are heated from 30 °C to 140~150 °C, and the size change is 2.5~3.0%.

(4) Other end by

Iron-based printed circuit boards have shielding effect; replace brittle ceramic substrates; use surface mounting technology to stabilize the mood; reduce the size of the plane or surface of objects that are really useful for printed boards; replace components such as heat sinks, improve product heat resistance physical properties; impairs productive capital and labor.

3. Structure
Up to now, the standard metal-based copper clad laminates collected and purchased on the market are composed of three layers of different materials: copper, insulating layer, metal plate (copper, aluminum, steel plate), and aluminum-based copper clad laminate is the most common.

(1) Metal substrate

Aluminum base material, using LF, L4M, Ly12 aluminum material PCB, requires an expanded strength of 30kgf/mm2 and an elongation of 5%. American Beggs aluminum base is divided into 4 types: 1.0, 1.6, 2.0, 3.2mm, and the aluminum model is 6061T6 or 5052H3

Copper-based base material, the expanded strength is 25~32kgf/mm2, and the elongation is 15%. American Beggs copper base thickness is divided into 5 kinds: 1.0, 1.6, 2.0, 2.36, 3.2mm, which is C11000 copper alloy.

Iron-based base material, using cold-rolled rolled copper plate, low-carbon copper, with special properties of magnetic shielding, thickness 0.5~1.5mm. American Beggs uses invar copper (nickel-iron alloy), tungsten-gold alloy, and cold-rolled copper, with a thickness of 1.0 and 2.3mm.

Double Sided Aluminum PCB

(2) Insulation layer

It acts as an insulating layer, generally 50~200um. If it is too thick, it can have an insulating effect, and the effect of avoiding short circuit with the metal base is good, but it will affect the emission of calories; if it is too thin, it can dissipate heat better, but it is easy to cause a short circuit between the metal core and the component leads.

The insulating layer (or prepreg) is placed on an aluminum plate treated by anodizing and insulating, and is laminated with a solid copper layer on the surface.

The insulation layer application of Beggs in the United States reported a patent, the standard type of aluminum substrate, the insulation layer is 75 microns, and the special type is 150 microns.

(3) Copper foil

The back of the copper foil is treated by chemical oxygen oxidation, and the outer surface is galvanized and brass plated to increase the peel strength. The copper thickness is generally 0.5, 1.2 cups. American Beggs companies use ED copper, and there are 5 kinds of copper thickness: 1, 2, 3, 4, and 6 cups. We use 4 cups of copper foil (140 microns) to form a set of fabricated aluminum substrates for communication power supplies.

There are two standard sizes of aluminum substrates supplied in the United States: 16"×19", 18"×24". Applicable plane or surface size: minus one inch. The aluminum base surface also has the distinction of adding and not taking care of the film.

4. Make places that are not easy to solve
The copper thickness is 4.5OZ aluminum substrate, and the following difficulties will be encountered during production:

(1) Reimbursement of the default line width of the project: Due to the thickness of copper, the line width must be repaid to a certain extent, otherwise the line width after the etch is too poor, and the customer will not collect it, and the repayment value of the line width must be accumulated through experience.

(2) The uniformity of solder mask printing: Since the copper thickness of the circuit after the pattern is etched is unreasonable, solder mask printing is very difficult, and customers who skip printing and are too thick and too thin are rejected. How to print this layer of green oil is also one of the areas that is not easy to solve.

(3) Corrosion engraving: The line width after corroding and engraving must meet the customer's drawing requirements. Residual copper is not allowed, and it is not consciously scraped with a knife. Moving a knife will scratch the insulating layer, causing sparks and leakage in the withstand voltage test.

(4) Machining: The aluminum substrate can be drilled, but no burrs are allowed on the inside of the hole after drilling, which will affect the withstand voltage test. It is very difficult to mill a curved shape. And to punch the shape, it is necessary to use advanced production models, and the production model manufacturing is very technical, which is also one of the difficult areas for aluminum substrates to be solved. After the shape is punched, the edge requirements are very neat, no burrs, and no damage to the solder mask on the edge of the board. Generally, a soldier's die is used, the hole is punched from the line, the shape is punched from the aluminum surface, and the force when the circuit board is punched is to cut up and down, etc. are all techniques. After punching the shape, the warpage of the wrench should be less than 0.5%.

(5) The whole production process is not allowed to scratch the aluminum base surface: the aluminum base surface will be discolored and blackened by touching it with your hands, or by certain chemicals, which are completely uncollectible, and the aluminum base will be re-polished. Some customers do not collect it, so the whole process does not bruise or touch the aluminum base surface, which is one of the difficult problems in the production of aluminum substrates. Some companies use the passivation process as they think it is appropriate, and some put the best care film before and after the hot air leveling (tin spraying)... There are many small techniques, and eight gods have crossed the sea, each showing their particularly brilliant skills.

(6) Over high voltage test: The aluminum substrate of the communication power supply requires 100% high voltage test. Some customers require direct current, some require alternating current, and the voltage requires 1500V and 1600V. The time is 5 seconds and 10 seconds, and the 100% printed board is tested. Dirt on the board surface, holes and burrs on the edge of the aluminum base, line saw teeth, bumps, or any little bit of the insulation layer will cause fire, leakage, and breakdown in the high-voltage test. Delamination and blistering of the pressure test wrench will be rejected.

The above are the production and process difficulties to be overcome in the manufacture of single-sided aluminum-aluminum substrates. Today, one point unit is used for aluminum-based core printed boards, that is, double-sided aluminum-based printed boards, and blind-hole multi-layer aluminum substrates, which are used in the transportation, communication, and instrument industries.

Appearance of the substrate: any surface scratches, scratches, pinholes, pits, and pattern-like grinding prints of the former officials of the aluminum base will not be recovered.

6. Application areas:

Communication power supply: voltage stabilizer, regulator, DC-AC adapter;

Electronic control: reduce the temperature of relays, junction transistor bases, and components in various circuits;

Switches, microwaves: heat sinks, insulation and heat conduction of semiconductor components, motor chokes;

Industrial vehicles: igniters, voltage regulators, semi-automatic safety control systems, lighting conversion systems;

Computer: power supply unit, floppy disk drive, motherboard;

Home Appliances: Input-Output Amplifiers, Audio, Power Equalization Amplification, etc.

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