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PCB TG, TD, CTI, DBV, WA explaination

adminJan13,20181012 views

Tg

The glass transition temperature (Tg) is an important normative dimension for the base material that determines the temperature at which the resin matrix converts from a glassy, brittle condition into a soft, elastic one.
When the glass transition temperature (Tg) is over 170°C, it is referred to as a high Tg material.


High Tg materials have the following properties:
 high glass flow temperature value (Tg)
 high temperature durability
 long delamination durability
 Low Z axis expansion (CTE)


TD

The decomposition temperature Td of a resin system depends on the binding energies within the polymers, and not on the glass transition temperature Tg. A good indicator for this characteristic is the T260 or T288 value, which specifies the time until delamination at 260°C or 288°C, respectively.


The Tg value of the base material sets here an upper boundary, at which the resin matrix decomposes and a subsequent delamination occurs. The Tg is thus not the value of the maximum operational temperature, but rather that which the material can endure for only a very short time. A guideline for a continuous thermal load is an operating temperature approximately 25°C below the Tg.


CTI

Stands for Comparative Tracking Index. It is expressed as that voltage which causes tracking after 50 drops of 0.1 percent ammonium chloride solution have fallen on the material. The results of testing the nominal 3 mm thickness are considered representative of the material’s performance in any thickness. It indicates the highest usable voltage for a laminate. It becomes important in application with high humidity, such as washing machines or cars. A higher index means a better protection. Similar is ‘PTI’ and ‘KC’. We have CTI PLC class 3 (> 175v), Class 2 (> 300V) and Class 0 (>600V) available.

CTI range

CTI Range (V)

CTI>=600

600>CTI>=400

400>CTI>=250

250>CTI>=175

175>CTI>=100

100>CTI>0

UL Standards(PLC)

0

1

2

3

4

5

IEC Standards

I

II

IIIa

IIIb

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Coefficient of thermal expansion (Tce)

SMD assembly process exposes the printed wiring assembly to more temperatur e 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.


Dissipation factor

A measure index of the tendency of an insulating material to absorb some of the ac energy from electromagnetic field passing through it. Low values are important for RF application, but relatively unimportant for logical applications.


Electrical breakdown voltage DBV
The voltage per unite th ickness of an insulator at which an arc may develop through insulator.


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 increa ses relative dielectric constant as well as reduces Electrical breakdown.

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