As a leading raw material of CCL (copper clad laminate) as substrate material of PCBs (printed circuit boards), both structure and performance of epoxy resin play a decisive role in determining performance of CCL. Plus, the constant development of epoxy resin gradually leads CCL to make progress in terms of performance. Along with fast development of electronic information industry, upgrading of electronic products and circuit assembly technology makes PCB fabrication technology move towards microvia, fine trace, high-density tracing and multiple layers, therefore laying higher requirement on CCL's thermal dissipation capability, dimensional stability and dielectric loss, which thereafter brings forward new demands on the performance of epoxy resin.
Brief Introduction on CCL
• Definition, Composition and Structure of CCL
As a type of multi-function electronic laminated composite material, CCL is a type of board-type material composed by reinforcing material (glass fabric, fiber paper, glass fiber paper etc.) that is soaked into resin (primarily epoxy resin). Then they go through baking in order to generate prepreg that will be cut, laminated and coated with copper through high temperature, high pressure and high vacuum.
CCL plays a role as a leading fundamentally raw material contributing to materials for PCB fabrication, performing four functions including conductivity, insulation, supporting and signal transmission and determining PCB performance, quality, fabrication level, fabrication cost and long-term reliability etc. Constant development of PCB boards and increasing application demands of terminal electronic products gradually bring forward new technological demands for CCL, simultaneously providing driving force to technological development and fabrication technology progress of CCL.
Up to now, majority of CCLs used for PCB fabrication belong to rigid organic resin CCL, including paper substrate, glass fiber substrate and composite substrate. Apart from the above categories, rigid CCLs also include BUM (build-up multilayer) substrate, metal substrate, ceramic substrate, thermoplastic substrate, capacitor embedded substrate etc. As far as flexible CCLs are concerned, they mainly come in the following categories: polyester base film flexible CCL, polyimide base film flexible CCL, LCP (liquid crystal polymer) base flexible CCL etc.
Nowadays, numerous types of CCLs are applied in PCB fabrication and their thickness maintain in the range from 0.05mm to 3.2mm. Click for detailed introduction in terms of CCL.
• Development Momentum and Trend of CCL
CCL technology has developed for almost one hundred years and its development can never be achieved without the progress of PCB industry. Innovative development of electronic machine products, semiconductor fabrication technology, electronic assembly technology and PCB fabrication technology has been driving the development of CCLs. Essentially, the development momentum of CCLs derives from electronic packaging engineer and technology, which means that rapid improvement on light weight, thinness, miniaturization, high performance, multiple functions, high reliability and IC chip performance plays a radical role in promoting high-speed development on electronic packaging technology.
Fast development of electronic packaging technology calls for higher and more rigorous demands and drives CCLs develop towards miniaturization, light weight, high speed, highly thermal dissipation, high temperature withstanding, greenization, CAF withstanding, CTI withstanding, high intensity, high modulus, multiple functions and high reliability.
Epoxy Resin Based CCL
Although CCLs come in numerous types, they primarily come in epoxy resin base CCL that accounts for over 70% of all CCLs. Leading categories of epoxy resin glass fiber CCL include G-10, G-11, FR-4 and FR-5; leading categories of epoxy resin paper base CCL include FR-1 and FR-3; leading categories of epoxy resin composite CCL include CEM-1 and CEM-3; leading categories of high-performance and multi-functional epoxy resin glass fiber CCLs include high-Tg FR-4 (Tg: 175°C), lead-free compatible FR-4, high CTI, CAT resistance, high thermal conductivity, FR-4 used for HDI (high density interconnect) substrate, polyimide modified epoxy, BT modified epoxy, PPO modified epoxy, CE modified epoxy glass fiber CCL and epoxy glass fiber prepreg and RCC used for multi-layer PCBs and BUM PCBs.
CCL's Requirement on Epoxy Resin
As one of key substrate materials for electronic devices, the key function of CCL lies in insulation providing to traces and electronic products. Apart from performance upgrading requirement due to constant progress of technology, epoxy resin features the following fundamental demands: high purity, low humidity and mechanical strength.
High purity is such a significant fundamental demand which mainly concerns epoxy resin alkali metal (Na+) content and chlorine content that are rigorously required. However, most electronic-grade epoxy resin in the modern market has performed excellently enough in terms of alkali metal and chlorine content control. The key issue occurs to hydrolysable chlorine ions. Due to hydrolysable chlorine ion precipitation from epoxy resin, erosion will be accelerated on electronic devices under the action of water, which dramatically reduces electronic products' life span. As a result, the overall content of chlorine in epoxy resin is generally required to be less than 500ppm and the content of hydrolysable chlorine ion fundamentally doesn't exceed 300ppm. Low humidity requirement on epoxy resin is actually a basic performance compatible with reliability requirement during the application of electronic products. Moreover, mechanical property allows CCL to play a supporting role in electronic products.
CCL's New Requirement on Epoxy Resin
To cater to fast development of electronic information industry, electronic products and circuit assembly have to climb to a new stage, which leads PCB fabrication technology move towards microvia, fine trace, high-density tracing and high and multiple layers and brings forward new requirement to CCL in terms of thermal withstanding, low CTE, high dimensional stability and low dielectric loss. As a leading raw material of CCL, epoxy resin also faces more demands of new technology.
• "Green" Epoxy Resin
Traditional CCL achieves flame resistance owing to bromide epoxy resin in glue solution and curing agent containing halogen among which bromide content in epoxy resin accounts for 12% to 50%. According to regulations of RoHS released by EU, polybrominated biphenyls and polybrominated diphenyl ethers are forbidden to be applied in electronic products. Moreover, based on research released by some international research centers, brominated fire retardant will release hazardous substances that are bad for humans and environment. Thus, electronic machines bring forward halogen-free requirement on CCL.
Up to now, halogen-free CCL is capable of resisting fire through the application of N, P and B, Al etc. Recent years has witnessed quick development of epoxy resin containing phosphorus or nitrogen among which epoxy resin containing phosphorous features relatively mature generation technology depending on reaction between phenanthrene and epoxy resin. In recent years, people become constantly aware that N and P compounds cause bad effect to the environment, so fire-resistant epoxy resin containing no P, N and Pb will become a leading technology acquired by CCL manufacturing.
• Liquid Crystal Epoxy Resin
Along with continual development of PCB on high density and multiple layers, board space for components to be assembled on dramatically shrinks. Electronic machines lay increasingly higher demands on component power and large power will lead to heat accumulation with electrical performance of components go down or even destroyed. Besides, some base boards require CCLs to be able to work under a high temperature for long time such as LED base board, new-type power module, automotive electronics and high-density IC packaging base boards. Thus, high thermal conductivity is extremely significant as far as CCL is concerned.
CCL obtains high thermal conductivity primarily in two ways. One is to add inorganic filler with thermal conductivity to resin component and thermal conductivity is achieved through heat conduction channel formed by tight piling of filler in resin. The other way is to take advantage of resin with high thermal conductivity to obtain its thermal conductivity through thermal carriers as resin's complete crystallization leads to lattice vibration. When the first method is used alone and filler volume exceeds critical volume fraction, although CCL features high thermal conductivity, other performances will be obviously decreased. Thus, resin with thermal conductivity introduction will be beneficial to its thermal conductivity achievement.
Because liquid crystal epoxy resin features polymer network, compared with ordinary epoxy resin, it features relatively higher thermal conductivity.
• UV Resistant Epoxy Resin
Later period of 1990s witnessed the application of photosensitive solder mask in PCB fabrication process for solder mask image protection and the application of UV light for imaging and curing. When CCL is used with no UV-resistant functions, UV light will go through base board. As photosensitive solder mask is coated on each side, photosensitive ghosting will be generated, badly reducing solder mask graphic quality. Therefore, when it comes to base boards using UV light for solder mask curing, it has to be capable of stopping UV light.
• High-Tg Epoxy Resin
High polymer features a glassy state when its temperature is lower than glass transition temperature, with mechanical intensity, so Tg temperature increasing can increase products' application temperature. Up to now, Tg falls within the range from 130°C to 140°C for ordinary FR-4 materials and there are a couple of phases exceeding this temperature phase. As a result, the research on high-Tg epoxy resin is quite important for CCL development.
• High-CTI Epoxy Resin
CTI, short for comparative tracking index, is an index indicating that insulating material surface becomes carbonized conductive pathways when potential difference is available. The higher CTI is, the higher insulation is. CTI of ordinary FR-4 substrate material is approximately 200V while high-CTI substrate material is generally required to be over 600V. High-CTI substrate material can be obtained through picking up special epoxy resin, high leakage tracking material and glass fiber.
• Low-Dk Epoxy Resin
To cater to rapid development of electronic technology, increasing speed of information processing and transmission and to expand communication channel, substrate material is required to feature low dielectric constant and low dielectric loss tangent or Dk. Currently-used high-Dk epoxy resin is worse than epoxy resin in terms of material cost, fabrication technology and PCB fabrication technology, so lots of manufacturers are contributing to the research on low-Dk epoxy resin. Dielectric constant of epoxy resin is correlated with its polarity. The lower polarity is, the lower dielectric constant will be.
Electronic assembly technology, HDI development and cost performance development lead CCLs to develop towards high speed, high frequency, high integration, high reliability, high density, low loss and low cost and call for higher requirement on epoxy resin performance.
Based on traditional CCLs requiring epoxy resin to feature high purity, low humidity and mechanical stress, rapid development of CCLs pushes epoxy resin towards higher requirement such as high thermal resistance, low humidity, low dielectric constant and environmental protection, manufacturability and cost performance of epoxy resin are also compatible with CCL manufacturing requirement.
