Base laminate used in the construction of Printed Circuits Boards can come in many different forms from simple Paper & Resin to exotic hybrid materials. The price of the material also varies from inexpensive to prohibitive. The cost is mainly based on a supply and demand with the cost of the raw materials and ease of manufacture. The functionality of a design will basically determine what type of material a manufacturer would use. After cheap mass production of consumer electronics the majority of PCB’s fall into the use of a material class called FR4
What is FR4
FR4 is a generic term for “Fire retardant” – 4. This complies to UL94V-0. Simply put, this is a test whereby a flame is applied to the material and the time taken for the material to sustain fire itself is determinate as to whether it passes or fails. Which by definition tells you nothing about what it is made of!? The 4 denotes laminated epoxy composite, which, in turn, can cover a large spectrum of materials.
To clarify, when we are talking about PCBs, FR-4 is a weaved glass fabric in an epoxy resin. The copper cladding on one or two sides has nothing to do with the classification. It also refers to both Laminate and Pre-Pregs used to make up multilayer PCBs .
What is in it?
- Woven Glass Fabric: From 25 %up to 60%
- Resin system: Epoxy based compound containing flame retarding compounds and UV blockers, fluorescents, extenders, and wetting agents, making up the remainder.
This is used to give the stability and rigidity to the laminate. It is predominately made from E-Glass or Electronic grade.
It is a fusion of Silica (Silicon dioxide) and other metal oxides which is drawn to form Filaments. The filaments are 5-9micron in diameter and are bundled together to form stands containing 200 to 400 filaments to form a yarn. The yarn is then woven together to form a fabric.
As with all Fabrics it has a Warp which runs the length of the roll of material and a weft which runs from side to side of the width of material. A basic 1.6mm thick laminate will have 8 layers of fabric bonded together with the resin system.
Pre-Pregs will have different fabrics depending on required thickness after bonding. Different pre-pregs will also have different resin contents to offer different properties ‘in situ’.
This is generally referred to as epoxy resin which again is a generic term and different manufacturers will have different formulae to give different characteristics. They have been refined over the years as have the processes used. The result is a consistent and predicable product. There are variations of materials employed which give alternate Dielectric properties. There are other additives included in the formulations which are for processes used for the manufacture of Printed circuits and have no effect on the finished properties of the material.
What do you look for in a material.
This will depend on your product and what you need it to do. Factors to consider include:
- Thermal stability do you need to accommodate localized heating, or is the environment in which the product goes into harsh?
- Are you running high frequency? MHz to GHz
- Does the design incorporate high layer count small land estate?
- Are blind and buried vias present?
- Is controlled impedance necessary?.
- During PCB manufacture, will Microvias need Laser drilling?
Dk and Df
These two values are listed on all data sheets for the different types of FR4 material available.
However they are stated for typical 50% resin content at room temperature. They will vary from these norms with increases in temperature and Increases in resin content.
Dk –Permittivity, relative Dielectric Constant or the property of a material that impedes the transmission of an electromagnetic wave. The lower the value the closer the performance of the material is to that of Air. Air has the value of 1.
Dk will be lowered as the resin content increases. Dk values will increase as the temperature increases.
Df-Loss Tangent The property of a material that describes how much of the energy transmitted is absorbed by the material. The greater the loss tangent the larger the energy absorption into the material. This property directly impacts signal attenuation at high speeds.
Df will be increased with increased resin content and increases with a rise in temperature.
Dk and Df are used in the calculation of controlled impedance but are calculated at room temperature with the manufacturers stating values at 50% resin.
As with all materials, Laminate has a coefficient of expansion due to temperature change. This is expressed as ppm/oC or % They are often quoted up to the Tg temperature. ( Temperature at which the resin changes from glass like physical state to an amorphous gel like state – changing its mechanical behavior ).
Tg for FR4 varies from 130 to 190oC. The coefficient tends to be lower for a higher Tg
When these properties are predictable and stable for a given material, its manufacture has a controlled manufacturing process which can safely be used to give repeatable and reliable product.
So why would you consider the more expensive materials?
If your design is not giving you problems then don’t change there is no point in over engineering a product.
However due to higher demands for speed and functionality standard FR4 has its limit. This is always being pushed as are the processes used to construct high performance boards, with more and more functionality required from the same or smaller pcb footprint.
There are only two main variables that can be changed to give different characteristics when considering performance. When selected, these have an impact on the way they laminates and therefore pcbs are manufactured.
They are the cloth or fabric type used or the epoxy system.
Let us consider Multilayer pcbs separately in an effort to try and cover the basic concepts before we introduce too many variants.
As previously stated the fabric is made up of E-glass filaments woven together. The yarn is coated in a wetting agent so that the epoxy fully coats the filaments and aids the removal of air. Each of the eight layers that go to make up a standard 1.6mm FR4 laminate is resin coated separately and then bonded together with the copper foil of the thickness required, on the top and or bottom. These are in effect 8 layers of pre-pregs (pre-impregnated epoxy). The resultant laminate is fully cured as opposed to the intermediate state of pre-pregs (as used in multilayer constructs).
The glass cloth is homogenous as is the resin system however the resultant composite is not. The sectional photos below show the various states within the laminate.
Fibre rich Area.
Resin Rich Area.
Fibre Rich at surface under track. | Resin rich at surface under track.
The above are the extremes and would vary over the distance of the material thus given rise to an average figure for Dk and Df. However as a note of caution if adesign is sensitive on the micro scale this may explain inconsistencies in results.
The introduction of different types of yarn called Flattened profiles do give different Dk and Df values without affecting the overall chemistry of the laminate. These types of laminate have come into play for laser drilling, which will be covered in a different blog along with multilayer construction.
Variants in Resin systems are predominantly made for:
- Improved Higher Tg Values
- Improved Dk and Df values
- Modified flow characteristic from no flow to full flow.( Refers to Pre-pregs.)
- Chemical resistance and thermal stability
- Moisture sensitivity.
There is always a tradeoff between improvements to physical properties and suitability for use in manufacturing processes. When we talk about standard FR4 laminate, these have been well documented and most bare board manufacturers know what is required to process the different laminates available. Once we move onto multilayer and hybrid builds, the choice of laminate material becomes more influential and important in getting the best out of the product design.
Higher Tg’s are great for extra thermal stability and some Tg’s are even given as high as 2700C for a modified epoxy system, which hides the next important consideration. This is TD temperature or the temperature at which the material breaks down. This is not to be confused with the operating temperature. It is important NOT to assume that a design can run in an environment near the stated Tg temperature and the pcb design will be stable and functional long term at this temperature..
Higher Tg materials tend to be harder which will has a dramatic effect on drilling. Which in-turn has an effect on resin smear which will also have an effect on how manufacturers de-smear which will have an effect on the rest of the material and how it will behave in the final design. Its important to be aware that there may be hidden processing costs from changing from one laminate to another.
As previously stated the cost of base materials varies due to supply and demand and ease of manufacture. If we consider the glass fiber E-glass which has a Dk value of around 6 and is the most commonly used. The D-glass has a Dk and Df value of 30%less at 3.8-4 and 0.0005-0.0026 respectively. However very little D-glass is woven into the fabrics used, simply because it is harder to produce.
D-Glass has different physical properties such as at least a 30% drop in tensile strength. This makes it more prone to moisture retention (which can lead to a load of other issues/difficulties during manufacture), and which makes it less attractive to end users.
Glass epoxy Laminate materials
|* not glass non woven aramid|
This list is not exhaustive but goes to illustrate that there is a vast array of base laminates which on paper may sound attractive when only Dk and Df values are chosen.
Suppliers should be fully aware of the different materials but will tend to only stock material from one manufacturer and will keep minimal stock. So there may be an added delivery time if it has to be ordered in. Likewise there can be minimum order values which will also affect unit price
There is a large jump in cost when you move away from epoxy glass laminate, which has to be factored in to any design. There are also gains to be made in electrical and thermal values which may outweigh the increased costs.
A halfway house is to bond different laminates together to produce a hybrid. But there may be hidden manufacturing costs involved which may be higher than buying the higher grade material in the first place.
There are many different ways of achieving the required values of a product and new products do come onto the market from time to time .
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