Summary： Technical requirements of copper clad laminate Technical requirements of epoxy copper clad laminateIn recent years, with the development of electronic technolo
Technical requirements of copper clad laminate
Technical requirements of epoxy copper clad laminate
In recent years, with the development of electronic technology, more and newer requirements have been put forward for epoxy resins used in epoxy copper clad laminates, mainly in the following aspects.
8.1 High glass transition temperature (Tg)
Tg is a physical change that reflects the epoxy resin matrix as the temperature rises and falls. At room temperature, the substrate is a rigid "glass state." When the temperature rises to a certain area, the substrate will change from "glass state" to "hyperelastic state". The temperature at this time is called the glass transition temperature (Tg) of the substrate. In other words, Tg is the maximum temperature (°C) at which the substrate remains rigid. The Tg of the matrix depends on the resin used. The traditional FR-4 copper clad laminate is a difunctional brominated bisphenol A epoxy resin, and the Tg is generally around 130°C. In order to improve the Tg of the substrate, most of the current industry uses Novolac epoxy resin. Since Novolac epoxy resin structure contains more than two epoxy groups, the cured product has a high cross-linking density and the Tg increases accordingly. The heat resistance, chemical resistance, and dimensional stability of the substrate are correspondingly improved.
Novolac epoxy resin, because the structure contains multiple epoxy groups, the heat resistance and other properties of the matrix will be significantly improved. However, the product is brittle and has poor adhesion. It is generally not used alone in the production of epoxy resin copper clad laminate, but used in conjunction with bisphenol A epoxy resin. The amount of Novolac epoxy resin is generally 20% to 30% of the total epoxy resin. Practice has proved that, in Novolac epoxy resin, the use of bisphenol A Novolac epoxy resin can get a better overall effect.
8.2 Ultraviolet light (UV) and automatic optical inspection (AOI) functions
(1) Blocking UV With the rapid development of the electronics industry, printed circuits have high precision and high density. In the manufacturing process of double-sided printed boards and multilayer printed boards, liquid photosensitive solder resists and exposure on both sides are widely used. New process. Since ultraviolet light (UV) can penetrate the substrate, the circuit patterns on both sides interfere with each other, and a ghost image (GHOST IMAGE) appears, causing waste products. In order to avoid ghosting, the epoxy resin used for the substrate must have a UV blocking function. At present, the general practice in the industry is to add a four-functional epoxy resin or UV absorber to the epoxy resin system, use its own fluorescent chromophore properties, absorb UV light, and achieve the blocking effect.
In 1995, my country successfully developed a copper-clad epoxy resin with UV and AOI blocking function, and also developed corresponding detection methods and detection instruments. This test method has been confirmed by the International Electrotechnical Commission (IEC), standard number IEC1189-2C11. The UV transmittance detector consists of a UV light source and a UV light meter. The light energy under the condition of no sample and sample is measured by the light meter, and the corresponding UV transmittance is calculated.
Where K——UV transmittance;
a——Light energy without sample;
b——The light energy of the sample.
According to the size of UV transmittance, the quality of the substrate blocking UV function is judged. The high transmittance indicates that the substrate has poor UV resistance. The small transmittance indicates that the substrate has good barrier to UV. If the UV transmittance of the substrate is below 1%, it can basically meet the requirements for use.
(2) AOI function In the quality inspection of printed circuit boards, with the expansion of output and the increase in the density of circuits, the use of traditional manual inspection methods has been unable to adapt. At present, some large enterprises have widely adopted the new technology of automatic optical inspection (AOI). The epoxy resin in the substrate must have AOI function. AOI instrument county uses argon laser as light source. The epoxy resin in the substrate must be able to absorb the argon laser and excite lower-energy fluorescence. By measuring the fluorescence on the substrate, it can realize automatic optical detection of the appearance defects of the printed circuit board.
8.3 Low dielectric constant
In recent years, with the development of communication technology and the high speed of information processing and information propagation, it is eager to provide a copper clad laminate with a low dielectric constant that can be used under high frequency conditions. In high-frequency lines, the frequency generally exceeds 300 MHz. In high-frequency lines, the signal propagation speed is related to the dielectric constant of the substrate, and its relationship is as follows:
Where: V-signal propagation speed;
C-speed of light;
ε——The dielectric constant of the substrate.
The above formula shows that the lower the dielectric constant of the substrate, the faster the signal propagation speed. To achieve high-speed signal propagation, it is necessary to use low dielectric constant plates. In addition, under the action of the electric field, the substrate consumes energy due to heat generation, which reduces the transmission efficiency of high-frequency signals. The relationship is as follows:
Where: PL-signal propagation loss;
tanδ——The dielectric loss tangent of the matrix.
The above formula shows that the tanδ of the substrate is small, and the signal propagation loss is correspondingly small. It can be seen that as a copper clad laminate for high-frequency lines, a resin with a low dielectric constant and a low dielectric loss tangent must be selected. However, the epoxy resin used for FR-4 copper clad laminate is currently high, which makes it difficult to meet the use of high-frequency circuits. In high-frequency lines, most of them use PTFE. Although PTFE has excellent dielectric properties, it has the following disadvantages compared to epoxy resins: (1) poor processability; (2) poor overall performance; (3) high cost. Although epoxy resin has a high dielectric constant and a high dielectric loss tangent, it has the advantages of good processability, excellent comprehensive performance, reasonable price, and sufficient supply. If the modification method is adopted, a group with a small polarity and a large volume is introduced into the epoxy resin structure to reduce the content of the polar group in the cured product, and the dielectric properties of the resin can be improved. The modified epoxy resin may become a cost-effective high-frequency material.
Build-up Multilayer (Build-up Multilayer, abbreviated as BUM) is a new technology developed in recent years for manufacturing high-density, small-aperture multilayer printed circuit boards. With the rapid development of BUM, Resin Coated Copper Foil (RCC) as its main material has been developed accordingly.
(1) Structure of RCC RCC is composed of high-temperature elongation copper foil with surface roughened, heat-resistant, anti-oxidation, etc. and B-stage resin. RCC mostly uses epoxy resin. The resin layer of RCC should have the same process performance as FR-4 adhesive sheet. In addition, the following requirements of the multi-layer board of the build-up method must be met:
1) High insulation reliability and micro-via reliability.
2) High glass transition temperature (Tg).
3) Flame retardancy.
4) Low dielectric constant and low water absorption.
5) Good adhesion to the inner layer board.
6) The thickness of the resin layer is uniform after curing.
7) Good adhesion strength to copper foil.
(2) RCC technical requirements.
(3) RCC coating process The RCC manufacturing process requires the resin to be evenly coated on the copper foil. The thickness deviation of the resin layer is controlled within ±2 mm. Therefore, high-precision coating equipment must be used. At the same time, the production environment must be highly purified. The coating machine is mainly composed of an applicator and an oven.
(4) Advantages of RCC
1) Facilitate the weight reduction and thinning of the multilayer board.
2) Conducive to the improvement of dielectric properties.
3) Conducive to laser and plasma erosion processing.
4) It is easy to process thin copper foil such as 12um.
5) You can use ordinary printed board production lines without the need for new equipment investment.
8.5 Halogen-free products
At present, most of the flame retardant products in the production of epoxy resin clad laminates account for more than 90%. From a safety perspective, users require products to pass UL safety certification, and flame retardancy must reach V-0. In order to meet the above requirements, brominated epoxy resins are widely used in the production of flame retardant copper clad laminates. Some foreign research institutes have proposed that halogen flame retardants will produce toxic substances during the combustion process, endangering human health and polluting the environment. The international community, especially Europe, has expressed strong concerns about this issue. The European Community (EC) Environmental Protection Committee proposes to ban the use of halogen-containing flame retardant materials in electrical and electronic products within a specified period. The development of halogen-free flame retardant copper clad laminates has become an important issue in the industry and is imperative. From a chemical point of view, in addition to halogen elements (F, Cl, Br, I), there are elements of group V, N, P and other elements in addition to halogen elements. Experiments show that in the epoxy resin system, the introduction of elements such as N and P, together with appropriate flame retardant additives, can also obtain satisfactory flame retardant effects.
As we all know, phenolic resin can be used as a curing agent for epoxy resin. If phenolic resin is used to modify epoxy resin, it can increase the crosslink density of epoxy resin, further improve its heat resistance and reduce its thermal expansion coefficient, etc. . If a nitrogen-containing group is introduced into the molecular structure of the phenolic resin and this nitrogen-containing phenolic resin is used as a curing modifier to modify the epoxy resin, both the flame retardant performance of the epoxy resin can be improved, and Can improve its heat resistance and dimensional stability.