Microwave Tech - Topics of rogers high frequency board/RF microwave circuit board and metal substrate

Topics of rogers high frequency board/RF microwave circuit board and metal substrate

In previous years, in the PCB industry, the most fashionable technologies and products were HDI multilayer boards(high-density interconnects) and Build-up Multilayer (multilayer printed boards).

However, in the development trend of market economy and high-tech products, there is another branch, namely high-frequency microwave printed boards and metal-based printed boards. Today, I will talk about these two issues.

1 Let's talk about high-frequency microwave printed boards first

1. High-frequency microwave printed boards have become popular on the land of China. In recent years, many printed board companies in East China, North China, and the Pearl River Delta have been staring at the high-frequency microwave board market, collecting high-frequency waves and PTFE. The dynamics and information of ethylene (Teflon, PTFE), regard this new type of printed board as an indispensable accessory product for the electronic information high-tech industry, and strengthen research and development.

Some company bosses have identified high-frequency microwave boards as a new economic growth point for future enterprises. Foreign experts predict that the market for high-frequency microwave panels will develop very quickly.

In the fields of communications, medical care, military, automobiles, computers, and instruments, the demand for high-frequency microwave panels is rapidly rising. A few years later, high-frequency microwave boards may account for about 15% of the total global printed boards. Many PCB companies in Taiwan, South Korea, Europe, the United States, and Japan have formulated plans to develop in this direction.

European and American high-frequency microwave sheet suppliers Rogers, Arlon, Taconic, Metcold, GIL, Chukoh, Japan, have entered the potential big market of China in the past two years, looking for agents and teaching related technologies.

American GIL Company held a lecture on "Application and Manufacturing Technology of High Frequency Microwave Printed Boards" in Shenzhen. Hundreds of seats were all filled. The corridors were also full of business representatives listening to the speech. Many CEOs listened to the lecture. Days of technical lectures.

I really didn't expect domestic counterparts to have such a strong interest in high-frequency boards. European and American sheet metal suppliers can already provide more than 100 varieties of sheet series with dielectric constants ranging from 2.10, 2.15, 2.17, ... to 4.5, and even higher. In the Pearl River Delta and the Yangtze River Delta, it is understood that many companies have advertised that they can place bulk orders for Teflon and high-frequency boards.

It is said that some companies have reached the level of monthly output of thousands of square meters. The demand for high-frequency microwave plates in many domestic radar and communications research institutes printed board factories is increasing year by year.

The demand for high-frequency microwave printed boards by domestic major communications companies such as Huawei, Bell, and Wuhan Academy of Posts and Sciences has increased year by year. Foreign companies engaged in high-frequency microwave products have also moved to China to purchase printed boards for high-frequency microwaves nearby.

Various signs indicate that high-frequency microwave panels are heating up in China. (What is high frequency? Above 300MHZ, that is, the shortwave frequency range with a wavelength of more than 1 meter, generally called high frequency.)

2. Why does it heat up?

(1) Part of the frequency bands of high-frequency communications originally used for military purposes were transferred to civilian use (beginning in 1996), which greatly developed civilian high-frequency communications. It has demonstrated its skills in various fields such as long-distance high-speed communication, navigation, medical treatment, transportation, transportation, and storage.

(2) High confidentiality and high transmission quality enable mobile phones, car phones, and wireless communications to develop towards high frequency, and high picture quality enables broadcast and television transmission to broadcast programs at VHF and UHF. High-volume information transmission requires satellite communications, microwave communications and optical fiber communications to be high-frequency.

(3) The processing capacity of computer technology has increased, and the information memory capacity has increased, and there is an urgent need for high-speed signal transmission. In short, the high frequency and high speed of electronic information products put forward high requirements on the high frequency characteristics of printed boards.

3. Why is the printed board required to have a low ε (Dk)? ε or Dk, called the dielectric constant, is the ratio of the capacitance between electrodes filled with a certain substance to the capacitance of a vacuum capacitor of the same structure.

It usually indicates the capacity of a certain material to store electrical energy.

When ε is large, the ability to store electrical energy is large, and the transmission speed of electrical signals in the circuit will decrease.

The current direction of the electrical signal through the printed board. Usually positive and negative alternately change, which is equivalent to the process of continuously charging and discharging the substrate. In the interchange, the capacitance will affect the transmission speed. This effect is even more important in high-speed transmission devices.

Low ε means that the storage capacity is small, and the charging and discharging process is fast, so that the transmission speed is also fast. Therefore, in high-frequency transmission, a low dielectric constant is required.

Another concept is dielectric loss. Under the action of an alternating electric field, the energy consumed by the dielectric material due to heat is called the dielectric loss, which is usually expressed by the dielectric loss factor tanδ. ε and tanδ are proportional, and high-frequency circuits also require low ε and small dielectric loss tanδ, so the energy loss is also small.

4. Polytetrafluoroethylene (Teflon)

Ε of printed boards 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 of FR4 of general glass cloth epoxy resin substrate ε is 4.6~5.0, so the signal transmission speed of Teflon printed boards is much faster than FR4 (about 40%).

The intermediate loss factor of Teflon board is 0.002, which is 10 times lower than the 0.02 of FR4, and the energy loss is much smaller. In addition, PTFE is called the "Plastic King". It has excellent electrical insulation, chemical stability and thermal stability (there is no solvent that can dissolve it below 300°C), so high-frequency and high-speed Signal transmission must first use Teflon or other low dielectric constant substrates.

I have seen that Polyflon, Rogers, Taconic, Arlon, and Meclad can provide substrates with dielectric constants of 2.10, 2.15, 2.17, and 2.20. The dielectric loss factor is 0.0005 to 0.0009 at 10GHZ. The performance of PTFE vinyl material is very good, but the process of its processing into a printed board is completely different from the traditional FR4 process. This aspect will be discussed later.

In the past two years, we have often used Rogers RO4000, GIL1000 series, etc. in addition to the requirements of ε of 2.15 and 2.6 in the past two years.

5. The basic requirements of high frequency microwave board

Because it is high-frequency signal transmission, the characteristic impedance of the conductor of the finished printed board is required to be strict, and the line width of the board is usually required to be ±0.02mm (the most stringent is ±0.015mm). Therefore, the etching process needs to be strictly controlled, and the film used for light imaging transfer needs to be compensated according to the line width and the thickness of the copper foil.

The circuit of this kind of printed board transmits not electric current, but high-frequency electric pulse signal. Defects such as pits, gaps, pinholes on the wires will affect the transmission, and any such small defects are not allowed.

Sometimes, the thickness of the solder mask is also strictly controlled, and the solder mask on the circuit is too thick or too thin to be a few microns.

·Heat shock at 288 degree Celsius, 10 seconds, 1~3 times, no hole wall separation occurs. For Teflon boards, we must solve the wettability in the holes, make the electroless copper holes have no holes, and the copper layer electroplated in the holes can withstand thermal shock, which is the difficulty of making Teflon porous boards one.

Because of this, many substrate manufacturers have developed and produced a little higher ε, and the electroless copper process is the same as the conventional FR4 alternatives, Rogers Ro4003 (ε3.38) and LGC-046 (ε3.2±0.1) from Xi’an 704 Factory. ) Is this kind of product.

·Warpage: usually 0.5~0.7% of the finished board is required. 6. The processing difficulty of high-frequency microwave board is based on the physical and chemical properties of PTFE board, which makes its processing technology different from the traditional FR4 process. If it is processed under the same conditions as conventional epoxy resin glass fiber copper clad laminate, it cannot Get qualified products.

(1) Drilling: The base material is soft, and the number of drilled stacked plates is small. Usually 0.8mm plate thickness is suitable for two sheets and one stack; the speed should be slower; to use a new drill bit, the top angle and thread angle of the drill bit have their own characteristics. special request.

(2) Printed solder mask: After the board is etched, the board cannot be polished with a roller brush before the solder mask is printed, so as not to damage the substrate. It is recommended to use chemical methods for surface treatment. To achieve this: without grinding the board, after printing the solder mask, the circuit and the copper surface are uniform and there is no oxide layer, which is by no means easy.

(3) Hot-air leveling: Based on the inherent properties of fluororesin, rapid heating of the sheet should be avoided as much as possible. Before spraying tin, do a pre-heat treatment at 150°C for about 30 minutes, and then spray tin immediately. The temperature of the tin tank should not exceed 245 degree Celsius, otherwise the adhesion of the isolated pad will be affected.

(4) Milling profile: The fluororesin is soft, and the milling profile of ordinary milling cutters has many burrs and unevenness. It is necessary to mill the profile with a suitable special milling cutter.

(5) Transport between processes: It cannot be placed vertically, only placed in the basket flat with paper, and no finger is allowed to touch the circuit pattern in the board during the whole process. The whole process prevents scratches and scratches. Line scratches, pinholes, indentations, and dents will affect signal transmission, and the board will be rejected.

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

(7) Electroless copper: The pretreatment of electroless copper is the most difficult and critical step in the manufacture of Teflon boards. There are many methods for pretreatment of copper sinking, but in summary, it can stabilize the quality and is suitable for mass production.