A Comprehensive Introduction of Copper Clad Laminate
The core of electronic circuits and devices is the board on which the electronic components are assembled. This board is called the Printed Circuit Board (PCB) or Printed Wiring Board ( PWB). The thick and hard board (PCB) made up of a combination of different types of material is what gives shape to the electronic circuit. This board has the combination of copper tracks, conducting and insulating layers, vias, holes, pads and components.
The PCB board itself is the "core" of PCB made of Copper Clad Laminate (CCL). The CCL is usually made of fiber glass base substrate laminated from top and bottom sides with copper foil. The lamination is done by soaking the sandwich of base substrate and copper foils with epoxy resin. The fiber glass is actually "pre-impregnated" with epoxy resin and hence called "Pre-preg". The fiber glass is also called reinforcing material and there are several other types like paper, wood pulp, glass, fiber cloth etc.
The pre-impregnation is done by applying heat and pressure to dry epoxy resin that will then melt and sticks to the fiber glass dielectric substrate material. The fiber glass is actually the dielectric material due to its insulating properties. The dielectric constant for glass fiber FR-4 is 4.70 max., 4.35 @ 500 MHz, 4.34 @ 1 GHz.
The FR stands for Flame Retardant that is the capability of PCB material to resist heat flames. FR-4 is by far the most commonly and widely used PCB base substrate material that has great mechanical strength, excellent flame retardant capability (self-extinguishing characteristic), both humid and dry environment it has reasonable electrical insulating properties, good strength to weight ratio. Absolutely no water absorption.
Types of CCL As per Resins:
1- Phenolic resin used in FR-1 , FR-2 and XPC substrate CCL
2- Epoxy resin used in FR-3 and FR-4 base substrate CCL
3- Polyester resin CCL
Types of CCL As per Core Dielectric Material:
1- FR-1 is hard and flat material with thin layer of copper based on Phenolic resin
2- FR-2 is also Phenolic resin based cotton paper dielectric material CCL core
3- FR-3, CEM-1 and CEM-2 are epoxy resin based cotton paper CCL
4- FR-4 and CEM-4 is glass fiber CCL based on epoxy resin
5- FR-5 is woven glass CCL based on epoxy resin
6- FR-6 is polyester resin based matte glass CCL core
7- G-10 is epoxy resin , woven glass CCL
8- CEM-3 is non woven glass and epoxy resin based CCL
9- CEM-5 is woven glass and polyester resin based CCL
10- Polytetrafluoroethylene (PTFE) based high frequency PCBs used in Microwave electronic devices. Also known as "Teflon" based PCB.
11- An amorphous thermoplastic material called Polyphenylene ether (PPE) is also extensively used as PCB base material for automotive and domestic electronic products. They have good thermal performance and good mechanical stability. The epoxy resin PPO (polyphenylene oxide) is used with it having Dk =3.60 and Df = 0.009
12- Polyimide is the base substrate material widely used as CCL in flexible PCBs. Good thermal stability and reasonable resistance to chemical effects. Polyimide has glass transition temperature Tg of 260OC hence this minimize thermal expansion
13- Bismaleimide Triazine (BT) is a type of epoxy used in PCBs requiring high temperature stability. The Tg of 300OC and very low dielectric constant make it attractive for high
14- Some other big names in high quality, high performance PCB base materials are Isola, Rogers, Arlon and Taconic. Low dielectric constant Dk, low dissipation factor Df, low coefficient of thermal expansion (CTE) and These are specialized in RF / Microwave engineering PCB circuits.
Measurement of Copper thickness on CCL and Units:
The copper foil laminated upon the core is also very important and must be precisely controlled so as to obtain the desired electrical characteristics of PCB. The thickness of copper foil in PCB is measured in units of weight i.e. "Ounce". It means that if 1 ounce of copper is taken and rolled / spread out on the surface area of 1 square feet then the copper thickness on that surface area will be 1.4mils.
1 mils = 1 x 10-3 inch 1 inch = 2.54cm 1 cm = 10mm
These units conversions are highly important and one should know these values to accurately calculate the PCB board thickness, copper foil thickness, layer stack-up thickness and thcikness of copper clad laminate.
Typical Side View of Copper Clad Laminate (CCL):
The above diagram is the side view representation of a generic PCB board. It is showing layer stack-up. The fiber glass is the core and on top and bottom is the layer of "resin". On top and bottom of resin layer is copper foils. This is the generic 2-sided PCB layer stack-up. This can be converted to multilayer PCB stack-up by stacking alternate layers of resin and copper foils upon each other. For more than 4 layers there should be two or more cores in the stack-up.
The cost of pre-preg and CCL matters most. Commonly, the thinner the pre-preg the higher the cost. Also the quality big factor, the good quality pre-preg result in good quality CCL and hence PCB. The types of pre-preg as per the applied resin amount is of three types. SR, MR and HR represents Standard Resin, Medium Resin and High Resin respectively.
The copper foils are available in high ductility, annealed and standard electrodeposited, rolled wrought, anneal-wrought.
Things required in CCL:
The four main properties desired in CCL are Low dielectric constant Dk, Low dissipation factor Df, high heat resistance and low coefficient of thermal expansion CTE.
The CCL has to be perfect in appearance, i.e. it must not have any dent, scratch, wrinkle or bubble on its surface. The surface should be smooth and flat. The CCL size should meet the requirements of PCB panel length, width and warpage. The PCB warpage is the undesired deformation in the shape/geometry of PCB. The CCL should also meet the electrical specs of the PCB design like Dk, Df factors, surface resistance and voltage breakdown ratings. The CCL must also conform to mechanical/physical factors like bending and peel strengths. CCL must also hold back the chemical reagents to show chemical resistance and other environmental conditions conformity like water absorption resistance.
RoHS Compliance in CCL:
Halogen Free CCL:
The copper clad laminate according to RoHS should be maintained the 900ppm levels of bromine (Br) and chlorine (Cl) and the aggregate contents of both should not exceed 1500ppm.
As per the RoHS compliance standards, the use of PBB and PBDE is prohibited in CCLs. Hence brominated epoxy resin is used in lead free CCLs.
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