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Introduction of PCB Materials
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The materials used to make PCBs are very important. And because the price of different material can vary from 10% to 100%, so the price differences of PCBs made by different materials can be huge! But there is no "free lunch", usually low cost material come with low quality.


1. Glass transition temperature (Tg)


    A high Tg is very important for the PCBs to guard against barrel cracking and pad fracture during soldering operation.


2.  Coefficient of thermal expansion (Tce)


SMD assembly process exposes the printed wiring assembly to more temperature shocks than typical through-hole process. At the same time, the increase in lead density cause the designer to use more and more 2 layers, making the board more susceptible to the problems concerned with the base materials ...Tce. This can be a particular problem with regard to Z-axis expansion of the material since this induces stress in the copper-plated holes, and becomes a reliability concern.

 

3. Dissipation factor (Df)

A measurement index of the tendency of an insulating material to absorb some energy from electro-magnetic field passing through. Low values are important for RF application, but relatively unimportant for logical applications.

 

4. Electrical breakdown voltage (DBV)

    The voltage per unite thickness of an insulator at which an arc may develop through insulator.

 

5. Water absorption factor (WA)

    The amount of water an insulating material may absorb when subjected to high relative humidity, expressed as a percent of total weight.Absorbed water increases relative dielectric constant as well as reduces Electrical breakdown voltage .

 

6 Major Different Materials -FR1, FR2, FR3, FR4,CEM-1, CEM-3

 

1. FR1-FR4

 

1.1 FR1 is basically the same as FR2. FR1 has a higher TG of 130 oC instead of 105oC for FR2. Some laminate manufacturers who produce FR1 may not produce FR2 since the 

     cost and usage are similar and it is not cost effective for having both.

 

1.2 FR3 is also basically FR2. But instead of phenolic resin it uses an epoxy resin binder.

 

1.3 FR4 (FR = Flame Retardant)

It is a glass fiber epoxy laminate. It is the most commonly used PCB material 1.60mm (0.062’’) FR4 uses 8 layers glass fiber material. The maximum ambient temperature is between 120oC and 130oC depending on thickness.

 

In China, FR4 is the most widely used PCB base material, next is FR1 then FR2. But FR1 and FR2 are usually used for 1-layer PCBs because they are not good for passing through holes. FR3 is not recommended to building multi-layer PCBs. FR4 is the best selection. FR4 is widely used because it is good to make from one- layer to multi-layer PCBs. With only FR4, PCB companies can make all kinds of PCBs, which leaves the management and quality control much easier, and eventually it can reduce the cost!

 

2. CEM-3 &CEM-1

    CEM-3 (CEM = Composite Epoxy Material) is very similar to FR4. Instead of woven glass fabric a "flies" type is used. CEM-3 has a milky white color and is very smooth. It is a complete replacement of FR4. But it is not popular in China, so it is not cost effective to use it. CEM-1 is a paper-based laminate with one layer of woven glass fabric. It is not suitable for Plated Through Hole, same as FR1-3. CEM-1 can only be used for one-layer PCB, and CEM-3 can be used for 1 layer and 2 layer PCBs.


Form the analysis above, it is obvious that FR4 is the best selection to make PCBs in China market. Some materials may be cheaper but it can only be used to make one-layer boards, and the reliability of the boards made by these materials is not good. So they are only used for one-layer and very simple boards.

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