How To Choose PCB Materials Of Your Board

How To Choose PCB Materials Of Your Board

Abstract

Different types of PCB materials are available in the market today. However, finding good quality of these materials could be a challenge especially if you lack sufficient knowledge on the criteria to follow when selecting PCB materials to use. This article will provide you with the information you require in order to settle for the right PCB materials.

 

Table of content

 1. Introduction

 2. Criteria for selecting materials to be used in PCB Boards

    2.1. Thermal properties

      2.1.1  Thermal conductivity (k)

      2.1.2  Decomposition temperature (Td)

      2.1.3  Glass Transition temperature (Tg)

      2.1.4  The coefficient of Thermal Expansion (CTE)

    2.2 Electrical Properties to consider

      2.2.1  Electrical strength of a PCB material

      2.2.2  Volume Resistivity

      2.2.3  The surface resistivity of a dielectric material

      2.2.4  Loss Tangent or Dissipation factor.

      2.2.5  Relative permittivity or Dielectric Constant

 3. Impacts of heat and power on the selection of PCB Material

    3.1 Absorption of Moisture

    3.2 Flammability

    3.3 Methylene Chloride Resistance (MCR)

 4.  Mechanical Properties of Flex and Flex-Rigid Boards.

    4.1. Density

    4.2. Peel strength

    4.3. Time to Delamination

    4.4. Flexural strength

 5.  Other factors to consider in designing PCBs.

    5.1  Thickness of boards

    5.2  Track distance

    5.3  Condition of the vias

    5.4  Strength and Durability of the material

    5.5  Cost

 6.  Conclusion

 

1 Introduction

Printed circuit boards are physical elements of electronics and basically, have great significance on parts of electronic devices. These boards are also referred to as printed wiring cards or even printed wiring boards. PCBs play a very important role in electronic devices such as computers and calculators, and therefore care should be taken when selecting materials for use in developing them. Good knowledge of electrical devices and the necessities for different pieces is essential in selecting the right material for PCBs.

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2 Criteria for selecting materials to be used in PCB Boards

 

The factors to consider when selecting materials used in PCB can be categorized into two main classes: thermal properties and electrical properties.

2.1 Thermal properties

 2.1.1 Thermal conductivity (k)

The ability of PCB materials to conduct heat is referred to as thermal conductivity. A materials thermal conductivity level determines its ability to transfer heat. Thermal conductivity of a material is measured in terms of Watts per Meter at Kelvin (W/Mk). In most dielectric materials, thermal conductivity is within the range of 0.3 W/mk to 6W/mk. Copper has a thermal conductivity of 386 W/mk. Due to its high thermal conductivity, it has the ability to transfer heat at a higher rate than PCB’s dielectric layer.

 2.1.2 Decomposition temperature (Td)

When a PCB substrate is exposed to temperatures that exceed a given threshold level, it tends to decomposed. Decomposition temperature is the range within which decomposition of substrate occurs. Once decomposition of the substrate has occurred, it cannot be reversed even when it is exposed to low temperatures. It is, therefore, necessary to select a PCB material that can withstand the temperatures range in which you work within. It is recommendable to select a material that can withstand temperatures above 250 0 C. In order to solder PCB substrate, a temperature range of between 200 degrees Celsius to 250 degrees Celsius is required. The decomposition temperature should be above that range while the glass transition temperature should be below the range.

 2.1.3 Glass Transition temperature (Tg)

When a PCB substrate is exposed to high temperatures beyond a given threshold level, they tend to soften. When the substrate is cooled, it gains the initial state it had before heating. Glass transition temperature is the range within which a substrate can be transformed from one form to the other.

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 2.1.4 The coefficient of Thermal Expansion (CTE)

The expansion rate of PCBs is referred to as Coefficient of thermal expansion. It is measured using the units parts per million (ppm).  The coefficient of thermal expansion may increase or decrease depending on temperature variations on the substrate. For example when the temperatures to which a substrate increase beyond its glass transition temperature, there may be an increase in the material's coefficient of thermal expansion. There exists a difference in CTE between the copper layer and the substrate. The copper layer generally has a CTE lower than of the substrate. When heat is applied, there may be problems of interconnection due to the difference in CTE between the two layers. The CTE of PCB on both the X and Y axis is maintained at a range of between 10 to 20 ppm. The minimum CTE should be maintained. The recommended CTE is of about 70 ppm or below.

2.2  Electrical Properties to consider

 2.2.1 Electrical strength of a PCB material

Electrical strength of a PCB is defined as the ability of the material to resist failure or even a breakdown in the electrical system on Z direction of PCB. A PCB should have sufficient electrical strength to enable it to resist a breakdown in the electrical system. The unit for measuring electrical strength is Volts/mil.  In most cases, PCB materials will have an electrical strength ranging from 800V/mil to 1500V/mil.

 2.2.2 Volume Resistivity

Volume resistance is defined as the ability of a dielectric material to resist electricity or insulation. It is measured in Ohms-meters or ohms-centimeters.  PCB materials should have a volume resistivity of between 10 Mega ohms-centimeters. Dielectric materials with high resistivity are preferable as compared to those with lower resistivity. A material’s resistivity can be affected by external factors such as moisture, cold and heat.

 2.2.3 The surface resistivity of a dielectric material

Surface resistivity is defined as the ability of the surface of a dielectric material to resist electricity and insulation. The unit used to measure surface resistance is Mega ohms per square. Dielectric materials with high surface resistivity are preferable. A surface resistivity ranging between 10 3 and 10 9 Megaohms per square is preferred for most dielectric materials.  Surface resistivity is also affected by external factors such as moisture and high and low temperatures.

 2.2.4  Loss Tangent or Dissipation factor.

A dielectric material with low loss tangent has less power losses. In most PCB materials, loss tangent ranges between 0.02 and 0.001. Loss tangent increases with increase in frequency.

 2.2.5  Relative permittivity or Dielectric Constant

The dielectric constant of a PCB material is greatly affected by signal integrity and impedance. It also depends on frequency. An increase in frequency results to a decrease in dielectric constant. The usual range of dielectric constant of a PCB material falls within the range of 3.5 to 5.5. A material to be used in applications that utilize high frequencies should have the dielectric constant which is stable on a wide range of frequency.  This is very important for the purpose of safety. Dielectric constant varies more in some materials than in others.

 

3.Impacts of heat and power on the selection of PCB Material

Heat and power are major factors that affect the selection of PCB materials. These include;

3.1 Absorption of Moisture

The ability of a PCB dielectric material to withstand exposure to moisture is referred to as Moisture Absorption. In most cases, PCB dielectric materials will have moisture absorption ranging from 0.01% to 0.20%.

Moisture absorption of a PCB material affects the dielectric’s thermal and electric properties.

3.2  Flammability

In accordance with Standards for Safety of Flammability of Plastic Materials for Parts in Devices and Appliances (UL94), plastics flame retardant features are categorized from high plastic flammability to low plastic flammability. According to UL94, it is a requirement that specimens of PCB materials should not burn with flaming combustion for more than 10 seconds.

3.3  Methylene Chloride Resistance (MCR)

Methylene Chlorine Resistance in PCB materials is also referred to as chemical resistance. MRC is used in measuring the ability of a dielectric material to resist absorbing Methylene Chloride. In most dielectric materials, the Methylene Chloride Resistance ranges between 0.01% to 0.20%.

 

4.Mechanical Properties of Flex and Flex-Rigid Boards:

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Some of the mechanical properties considered include;

4.1 Density

The unit used to measure the density of a dielectric is grams per centimeters cubed. It can also be measured using pounds per cubic inch.

4.2 Peel Strength

Peel strength of a PCB material is defined as the ability of copper layers and dielectric material to bond. Manufacturers can test the peel strength of PCB material by exposing copper traces with the thickness of one ounce to high temperatures, chemicals, and heat stress.

4.3 Time to Delamination

This refers to the period of time a dielectric material resists delamination effects. When fiberglass and laminate materials are exposed to moisture or heat stress, they delaminate. The time duration these PCB materials take to separate from each other under moisture or thermal shock conditions is referred to as the time to delamination.

4.4 Flexural strength

The term Flexural strength refers to the tendency of dielectrics not to break when subjected to physical strength. The unit used to measure flexural strength is Pounds per square inch or Kilograms per square meter. There are two ways used in measuring the flexural strength of PCBs;

  1. By exerting force at the center of the board with the ends supported.
  2. By use of tensile modulus / Young’s modulus.

 

5.Other factors to consider in designing PCBs. FR4

 5.1 The thickness of boards is another factor to consider

Thickness influences the strength of boards. Thicker boards provide greater support when connecting heavy objects to the boards. For plane layers, the standard thickness of copper is 35 microns. Grams and ounces can also be used to indicate Copper thickness. Boards with a copper thickness greater than the ordinary are better since they support a wide range of applications. In some cases, tracks may transfer power even though they are not meant to. This happens when signals fail to handle frequencies properly, and it may lead to tracks losing high power quantities.

 5.2 Track distance

The most appropriate track distance for layer boards consisting of materials with copper tracking is two inches and the suitable time for signaling is one nanosecond. It is important to put into consideration the impacts made by transmission line on long tracks especially when the integrity of the signal is important.

 5.3  The condition of the vias

Most boards have the vias empty such that you can easily see through them. In order to create a protective barrier against impurities such as dust, the via must be filled. This is done in order to increase their capacity to conduct current.

Ball Grid Array abbreviated as BGA, and its pieces are used to fill the vias. When BGA pin comes into contact with the inner layer, solder is likely to fall and penetrate to the other layers through the via. Vias should, therefore, be filled to prevent leakage of solder and to maintain the contacts quality.

 5.4 Strength and Durability of the material

The different materials used to develop a particular board should be strong enough to stay intact even when an electronic device accidentally falls or gets knocked from the sides. Most businesses and consumers are mainly concerned with reliability in all devices that have circuit boards. All types of PCB materials used on any particular board should be of high quality and reliable.

When selecting materials to be used in developing PCB boards, it is important to consider quality as the first priority. Quality plays a major role in determining the performance and the lifespan of an electrical device. It does not matter whether the PCB is for home devices or industrial use, costly or cheap, large sized or small sized, the quality is the key factor to focus.

With long lasting PCBs, it is possible to make hardware updates even on computerized devices without interfering or damaging the materials that makeup PCB boards. Other household devices such as microwaves and electronic gadgets can last longer in good condition. For example, ATMs and other Electronic devices in public facilities remain in good working condition, with their buttons functioning correctly without fail.

 5.5 Cost

High-quality substrates with high frequencies are expensive but durable. It is, therefore, necessary to select a material of high quality bearing in mind that cheap can also be expensive.

 

6. Conclusion

With all the above-mentioned factors put into consideration, it is necessary to select materials that suitable for use with high frequency. The substrate’s dielectric constant should be low while the foil should have the capacity to maintain low resistance.