Technical requirements for epoxy copper clad laminate
In recent years, with the development of electronic technology, more and newer requirements have been put forward for epoxy resin used for epoxy copper clad laminate, mainly in the following aspects.
8.1 high glass transition temperature (Tg)
Tg is a physical change that reflects the rise and fall of epoxy resin matrix with temperature. At room temperature, the matrix is a rigid "glassy state". When the temperature rises to a certain area, the matrix will change from "glassy state" to "high elastic state". The temperature at this time is called the glass transition temperature (Tg) of the matrix. In other words, Tg is the highest temperature (℃) at which the matrix remains rigid. The Tg of the matrix depends on the resin used. The traditional FR-4 copper clad laminate is made of bifunctional brominated bisphenol A epoxy resin, and its Tg is generally about 13℃. In order to improve the Tg of the matrix, Novolac epoxy resin is mostly used in the industry at present. Because Novolac epoxy resin contains more than two epoxy groups in its structure, the cross-linking density of the cured product is high, and the Tg is correspondingly increased. The heat resistance, chemical resistance and dimensional stability of the matrix are improved accordingly.
Novolac epoxy resin, because the structure contains polyepoxy groups, the heat resistance and other properties of the matrix will be obviously improved. However, the product is brittle and has poor adhesion. In the production of epoxy resin copper clad laminate, it is generally not used alone, but in combination with bisphenol A epoxy resin. The amount of Novolac epoxy resin is generally 2% ~ 3% of the total amount of epoxy resin. Practice has proved that in Novolac epoxy resin, bisphenol A Novolac epoxy resin can obtain better comprehensive effect.
8.2 functions of blocking ultraviolet light (UV) and automatic optical inspection (AOI)
(1) blocking UV With the rapid development of electronic industry, printed circuits are high-precision and high-density. In the manufacturing process of double-sided printed boards and multilayer printed boards, liquid photosensitive solder resist and new technology of simultaneous exposure of light on both sides are widely used. Because ultraviolet light (UV) can penetrate the substrate, the circuit patterns on both sides interfere with each other, resulting in GHOST IMAGE and waste products. In order to avoid ghosting, the epoxy resin used in the matrix must have the function of blocking ultraviolet light. At present, the general practice in the industry is to add tetrafunctional epoxy resin or UV absorber into the epoxy resin system, and use its own fluorescent chromophore properties to absorb UV light to achieve the blocking effect.
in p>1995, China successfully developed epoxy resin copper clad laminate with the function of blocking UV and AOI, and also developed the corresponding detection methods and instruments. This detection method has been confirmed by the International Electrotechnical Commission (IEC) with the 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 with sample was measured by light meter, and the corresponding UV transmittance was calculated.
K=(b/a)×1%
where k is the K——UV transmittance;
a—— light energy without sample;
b—— light energy with sample.
judge the UV blocking function of the substrate according to the UV transmittance. The high transmittance indicates that the matrix has poor UV resistance. The low transmittance shows that the matrix has good UV resistance. If the UV transmittance of the substrate is below 1%, it can basically meet the use requirements.
(2)AOI function In the quality inspection of printed circuit boards, with the expansion of output and the high density of lines, the traditional manual inspection method has been unable to adapt. At present, some large enterprises widely adopt 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 argon laser and excite low-energy fluorescence. By measuring the fluorescence on the substrate, the automatic optical detection of the appearance defects of printed circuit boards can be realized.
8.3 Low dielectric constant
In recent years, with the development of communication technology and the high speed of information processing and dissemination, it is urgent to provide a copper clad laminate with low dielectric constant that can meet the requirements of high frequency. In high frequency lines, the frequency generally exceeds 3 MHz. In high frequency lines, the signal propagation speed is related to the dielectric constant of the substrate, and the relationship is as follows:
V = K1 c/ε
where: V-signal propagation speed;
k1-constant;
c-the speed of light;
ε-the dielectric constant of the substrate.
the above formula shows that the lower the dielectric constant of the matrix, the faster the signal propagation speed. In order to realize the high-speed propagation of signals, it is necessary to choose plates with low dielectric constant. In addition, under the action of electric field, the matrix consumes energy due to heating, which reduces the propagation efficiency of high-frequency signals. The relationship is as follows:
PL = K2 f tan δ
where: PL-signal propagation loss;
k2-constant;
f-frequency;
tan δ-dielectric loss tangent of the matrix.
the above formula shows that the tanδ of the matrix is small, and the signal propagation loss is correspondingly small. It can be seen that as a copper clad laminate for high frequency lines, resins with low dielectric constant and low dielectric loss tangent must be selected. However, at present, the dielectric constant of epoxy resin used for FR-4 copper clad laminate is high, which makes it difficult to meet the needs of high frequency lines. In high frequency lines, most of them use PTFE. Although PTFE has excellent dielectric properties, compared with epoxy resin, it has the following disadvantages: (1) poor processability; (2) poor comprehensive performance; (3) High cost. Although the dielectric constant and dielectric loss tangent of epoxy resin are high, it has the advantages of good processability, excellent comprehensive performance, reasonable price and sufficient supply. The dielectric properties of epoxy resin can be improved by introducing groups with small polarity and large volume into the structure of epoxy resin and reducing the content of polar groups in the cured product. The modified epoxy resin may become an ideal cost-effective high-frequency material.
8.4 RCC
Build-up Multilayer (BUM) is a new technology developed in recent years, which is used to manufacture multilayer printed circuit boards with high density and small aperture. With the rapid development of BUM, Resin Coated Copper Foil (RCC) as its main material has been developed accordingly.
(1) The structure of RCC is composed of high-temperature ductile copper foil and B-grade resin whose surface has been roughened, heat-resistant and oxidation-resistant. RCC mostly adopts epoxy resin. The resin layer of RCC should have the same process performance as FR-4 adhesive sheet. In addition, the following requirements should be met for the multilayer board by the lamination method:
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) has good adhesion with the inner plate.
6) The thickness of the cured resin layer is uniform.
7) has good adhesion strength to copper foil.
(2)RCC technical requirements.
(3)RCC coating process The resin is required to be evenly coated on the copper foil in the RCC manufacturing process. The thickness deviation of the resin layer is controlled within ±2mm. Therefore, high-precision coating equipment must be adopted. At the same time, the production environment must be highly purified, and the coater is mainly composed of coater and oven.
(4) Advantages of RCC
1) It is beneficial to the lightweight and thinning of multilayer board.
2) is beneficial to the improvement of dielectric properties.
3) It is beneficial to laser and plasma etching.
4) It is easy to process thin copper foil such as 12um.
5) Ordinary PCB production line can be used without new equipment investment.
8.5 halogen-free products
At present, flame-retardant products account for more than 9% in the production of epoxy resin copper clad laminate. From the safety point of view, users require that the products must pass UL safety certification and the flame retardancy must reach V- level. In order to meet the above requirements, brominated epoxy resin is widely used in the production of flame retardant copper clad laminate. Some foreign research institutions have proposed that halogen flame retardants will produce toxic substances in the combustion process, which will endanger human health and pollute the environment. The international community, especially Europe, has expressed strong concern about this issue. The European Commission for Environmental Protection (EC) proposed that the use of halogen-containing flame retardant materials in electrical and electronic products should be banned within a time limit. The development of halogen-free flame retardant copper clad laminate has become an important topic in the industry, and it is imperative. From the chemical point of view, the elements with flame retardant function include halogen elements (F, Cl, Br, I) and V-group elements such as N and P.. Experiments show that satisfactory flame retardant effect can also be obtained by introducing elements such as N and P into epoxy resin system and matching with appropriate flame retardant additives.
As we all know, phenolic resin can be used as curing agent of epoxy resin. If phenolic resin is used to modify epoxy resin, the crosslinking density of epoxy resin can be increased, its heat resistance can be further improved and its thermal expansion coefficient can be reduced. If nitrogen-containing groups are introduced into the molecular structure of phenolic resin, and this nitrogen-containing phenolic resin is used as curing modifier to modify epoxy resin, it can not only improve the flame retardancy of epoxy resin, but also improve its heat resistance and dimensional stability.