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?

  1. Woven Glass Fabric: From 25 %up to 60%
  2. Resin system: Epoxy based compound containing flame retarding compounds and UV blockers,  fluorescents,  extenders, and  wetting agents, making up the remainder.

Glass Fabric.

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’.

Resin system

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:

  1. Thermal stability do you need to accommodate localized heating,                                                     or is the environment  in which the product goes into harsh?
  2. Are you running high frequency? MHz to GHz
  3. Does the design incorporate high layer count small land estate?
  4. Are blind and buried vias present?
  5. Is controlled impedance necessary?.
  6. 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.

Thermal Stability.

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.

Typical 8 layers of 7628 glass cloth in a standard FR4 1.6mm laminate.

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

Manufacturer

Material ref

Tg0C

Dk/Df

 

 

 

 

AGC (Nelco)

N4000-29

185

4.5/0.016

 

N4103-13EP

210

3.6/0.008

 

N4000-12

190

3.6/0.008

 

N4000-11

175

4.3/0.016

 

N4380-13RF

210

3.8/0.009

 

N4350-13RF

210

3.57/0.009

 

N4800-20S1

200

3.4/0.006

 

N4000-7

155

4.0/0.016

 

N4000-6

175

4.3/0.023

 

MERCURYWAVE 9350

200

3.5/0.004

 

N5000-32

205

4.4/0.009

 

N4000-29NF

185

4.0/0.017

 

N4000-6NF

175

3.7/0.015

 

 

 

 

ISOLA

IS415

200

3.72/0.012

 

1SPEED

180

3.64/0.0060

 

FR408HR

190

3.68/0.0092

 

IS400HR

150

4.2/0.0160

 

370HR

180

4.04/0.0210

 

IS400

150

3.9/0.022

 

IS420

170

4.4/0.021

 

 

 

 

ARLON

47N

135

4.3/0.02

 

49N

170

4.3/0.022

 

51N

170

4.2/0.02

 

44N

175

4.6/0.025

 

45

175

4.6/0.025

 

55NT*

170

3.8/0.015

 

* not glass  non woven aramid

 

 

 

VENTEC

VT42

140

4.2/0.015

 

VT42S

150

4.2/0.015

 

VT42A

135

4.2/0.015

 

VT464

160

4.05/0.012

 

VT464D

160

3.6/0.008

 

VT461

175

3.9/0.0095

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

HITACHI

MCL-E-67

125

4.8/0.02

 

-75G

170

5.2/0.016

 

-78G

170

3.6/0.014

 

-679FTYPER

170

5.0/0.015

 

-679FG

175

5.2/0.018

 

-700G TYPER

270

4.8/0.012

 

-705G

270

4.5/0.009

 

-770G

270

4.4/0.006

 

TD-002

170

3.8/0.013

 

-775G

240

3.7/0.009

 

MCL-HS-100

240

3.6/0.004

 

 

 

 

 

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 .