Epoxy Resin Used for CCL in PCB and Its Development Trend
In recent few years, the electronics industry has grown at a rapid pace, hence the demand of PCBs has dramatically increased accordingly. This is because the world’s top largest business making industrial sectors are automobile and electronics. According to Wikipedia, Apple and Samsung rank 2nd and 3rd in top 5 largest manufacturing companies by revenue generation. So what is the first thing comes in our mind if we talk about these two mobile industry giants..? Yes, you though it right..! It is “PCB”. The PCB is the heart of these consumer electronics and mobile manufacturing industrial sector. And the core of PCB manufacturing is based on its 3 main raw materials or ingredients i.e. PCB substrate, adhesive resin and copper foil. The topic of discussion of this article is one of the raw materials of PCB called “Epoxy Resin”. Today the competition in electronics industry is much intense and every PCB manufacturer and prototype pcb assembler is doing research to find ways to enhance their product’s quality and reduce manufacturing cost. In this context, the resin is the focus because it is the main ingredient of PCB CCL (Copper Clad Laminate).
A Brief Introduction of CCL:
The Copper Clad Laminated or CCL, is the building block of PCB. Actually the PCB is nothing but a CCL upon which the traces, pads, holes and vias are made. The CCL is basically a substrate material that is pre-impregnated (pre-preg) and soaked with epoxy resin adhesive and then the copper foil is applied on top and bottom of the pre-preg sheet and then treated under high temperature, high pressure and high vacuum pressing machine to form a “lamination”. This lamination or a sandwich of PCB base substrate, copper foil and epoxy resin is called CCL.
There are many different types of PCB substrate materials available. FR-4 is by far the commonly used substrate material. However the metal core PCB (MCPCB) use substrate like aluminum or copper to effectively dissipate heat away from the surface and commonly used in high power electronic devices like LED lights. Similarly the ceramic core PCBs are used where the substrate material is Al2O3 (Alumina) or AlN (Aluminum Nitride). The ceramic substrate materials have higher tendency to dissipate heat efficiently than FR-4 and widely used in high power devices like microprocessor chip heat sinking along with active cooling methods like fan and heat sinks. Moreover rigid CCLs also comprise of BUM (build-up multilayer) substrate, thermoplastic substrate and capacitor embedded substrate etc.
Other types of widely used rigid CCL substrate based on organic resins are paper substrate, glass fiber substrate and composite substrate. For flexible PCB substrate the core materials used nowadays are polyester base film flexible CCL, polyimide base film flexible CCL, LCP (liquid crystal polymer) base flexible CCL etc.
The Epoxy Resin based CCL Properties:
The substrate material is expected to have high electrical insulation, good thermal dissipation capability, High Tg (Glass Transition Temperature), dimensional stability, good resistant to humidity and chemicals, fire retardant capability, low dielectric loss tangent (Dk), low dissipation factor (Df) and high performance towards high speed high frequency signal transmission with low transmission losses (good signal integrity) and overall helpful towards high reliability.
The substrate material provides the physical solid structural shape of the PCB that holds SMT and THT components. Usually large sheet of CCL is taken by most PCB manufacturers and then it is cut into panels according to the size of the PCB manufacturing machines and then small individual PCBs are cut out of the panel after a long process of PCB fabrication, assembly and testing.
Substrate materials discussed above for CCL (Copper Clad Laminate) are the existing trend used in PCB market, however the future trends and developing improvement in “CCL” has some new demands in respect of raw materials specially Epoxy Resin.
Types of Epoxy Resin CCL:
Around 70% of the CCLs (Copper Clad Laminate) that is present in the market today is epoxy based resin. The popular FR-4 is the example of epoxy based CCL having Tg = 175OC. Lead Free FR-4, glass fiber CCL include G-10 G-11 and FR-4 and FR-5. FR-1 and FR-3 paper based CCL and others are CEM-1 and CEM-3 composite epoxy materials based CCLs. For flexible PCBs the base substrates are polyimide modified epoxy, BT modified epoxy and PPO modified epoxy.
The Development Trend and Requirements of Epoxy Resin CCL:
Hydrolysable Chlorine Ion Control:
The key requirements of CCL with respect to its epoxy resin is that the epoxy resin augments the CCL’s good mechanical strength, enable low humidity absorption and ensure high purity level. In order to maintain high purity of electronic grade epoxy resin the content of alkali metal i.e. Sodium (Na+) and Chlorine (Cl) must be controlled. The hydrolysable chlorine content in epoxy specially has to be controlled to the level below 300ppm because it can precipitate under water or humidity exposure causing erosion, thus reducing the life time of the end product. The overall content of Chlorine must be controlled to below 500 ppm. The low humidity environment is necessary for epoxy resin CCL to increase product reliability.
Phosphorus Based Epoxy Resin:
The curing agent of epoxy resin contains halogen and bromine is the key content in epoxy resin adhesive itself. So we all know that bromine and halogen are hazardous materials and as per the RoHS compliance standards they are prohibited to be used. These brominated substance including polybrominated diphenyl ether (PBDE) and others like PBB will emit hazardous content that if inhaled or consumed can cause serious health disorders. Keeping this in view, the safer options are being researched nowadays and is a developing trend of using “Phosphorus (P)” as epoxy resin for CCL. Although the compound of Phosphorus, Nitrogen and Lead can be helpful in fire resistance capability of CCL but it is injurious to health. People are getting awareness and seeking and researching for improved ways of using Phosphorus as compound for example in polyacrylamides epoxy resin composites thus avoiding other hazardous substances like lead (Pb), Halogen and bromine.
Liquid Crystal Epoxy Resins (LCER):
Another latest topic of research nowadays is prevalent by using an epoxy monomer 4,4′-diglycidyloxybiphenyl (BP) synthesized and cured with tetra functional amine i.e. sulfanilamide (SAA), to produce liquid crystalline epoxy resin. Through experiments it was shown that (BP) is not a liquid crystalline epoxy monomer and an irreversible crystal transition exists in the temperature range of 120 °C–140 °C. The use of SAA results in the formation of a “smectic” liquid crystalline phase. The LCERs exhibited a poly-domain structure with individual liquid crystalline domain distributed in the resin matrix, which results in better thermo-mechanical properties. These LCERs exhibits low shrinkage upon curing, excellent thermal stability and good mechanical properties. Moreover the disadvantage of brittleness found in traditional epoxy resins is overcome here in LCERs and glass transition temperature is also maintained. This makes LCER suitable for use in PCB CCLs for various applications like electronic packaging.
Ultra-Violet Resistant Epoxy Resin:
One difficulty faced by epoxy resin based CCLs in PCBs is that by time goes, the surface becomes pale yellow due to the constant exposure to sunlight in cases where PCBs are mounted in places direct exposure to sun. This can discolor, brittle or disintegrates the traditional epoxy resin. The solution to this problem is to use a combination of UV absorber with HLAS (Hindered Amine Light Stabilizer). This combination is very effective in delaying the damaging effect of UV on epoxy coatings, but not permanent. By using pigmented aliphatic polyurethane or polyaspartic acid coating on epoxy resin CCL can protect the CCL from UV radiation effects. The research in this regard is the development trend in epoxy resin CCLs.
High Tg Epoxy Resin CCL:
The glass transition (Tg) is the temperature critical to the application temperature i.e. where the PCB is being used. It is recommended that the Tg of epoxy resin CCL must be high so that it can bear high temperature. Currently the Tg for ordinary FR-4 epoxy resin CCL is around 140OC but research is still going on to increase it to substantial level.
High CTI Epoxy Resin CCL:
CTI (Comparative Tracking Index) is the measure of material providing electrical insulation when there is a potential difference present. Normally the CTI index of ordinary FR-4 based epoxy resin CCL is around 200V while it should be in the range of 600V. This is because of conductivity due to carbon paths formed. So it is desired to choose appropriate epoxy resin that has less leakage and good electrical insulation.
Low Dk and Low Df Epoxy Resin:
It is the topic of developing trend and research for engineers and scientists to lower the dielectric constant and dissipation factor (Dk and Df) respectively to enhance the signal integrity. Thus lowering the signal losses, lowering inductance in signal path and minimizing cross talks and EMI effects in high speed, high frequency PCB and high density circuits.