PCB materials

Materials for printed circuit boards

The main elements of the PCB are dielectric substrate (rigid or flexible) with copper conductors on it, vias and holes both plated and non plated.

As the dielectric substrate the glass-epoxy laminates or composite materials are used. Basic types and parameters of materials used for the production of printed circuit boards are given in the table.

Type

Description

Glass transient temp. Tg

Dielectric constant Dk

Main suppliers

FR4

FR stands for Fire Retardant. FR4 is a glass fiber epoxy laminate. It is the most commonly used PCB material.

135°C

3,8-4,7

Shengyi, Isola, Nanaya, KB, Goldenmax

FR4 halogen free

This laminate type does not contain halogen, antimony, phosphorus, etc., does not emit hazardous substances when burning.

140°C

4,5 -4,9

Shengyi, Nanya

FR4 High Tg, FR5

These laminate types have excellent performance in Pb-free soldering.

170°C

3,8-4,6

Nanya, Nelco, Panasonic

RCC

RCC is electrolytic copper foil coated with a layer of special epoxy resin

130°C

4,0

SengYi, Nelco

PD

Polyimide resin with aramid basis

260°C

4,4

Arlon, Nelco

High frequency (PTFE)

High Frequency laminates are used in PCBs that require a low dissipation factor (Df) and very stable dielectric constant (Dk)

240–280°C

2,2–10,2

Taconic, Rogers

High frequency (non PTFE)

240–280°C

3,5

Rogers

Polyimide

Material for the production of flexible and rigid-flex PCB

195-220°C

3,4

Dupont, Taiflex


Most often printed circuit boards are made of standard glass-epoxy laminate namely FR4 type, with an operating temperature from -50 to + 110°C, glass transition temperature Tg of 135°C. The dielectric constant Dk can range from 3.8 to 4.6, depending on the supplier and type of material. For compliance with the lead-free technology assembly usually the laminates FR4 High Tg or FR5 are used. When there is a requirement of continuous operation at the high temperature or under extensive temperature stresses the polyimide as a base material is used. Besides polyimide has a good electric strength and often used in military products or in high endurance applications. For printed circuit boards for high frequency or microwave range the special materials are used. The cost of these materials is higher than the basic material FR4.

FR-4

The family of laminates under the name FR-4 by the NEMA (National Electrical Manufacturers Association, USA) classification. These are the most common materials for the single sided, double sided and multilayer printed circuit boards with strict requirements for mechanical strength. FR-4 is a composite material based on woven glass-epoxy compounds. As a rule, laminate has a matte yellow color, green color of the PCB gives it a solder mask which is applied to the surface. Flammability rating is UL94-V0.

Depending on the properties and applications, FR-4 laminates are divided into the following subclasses:

  • standard, with a glass transition temperature Tg of ~ 130°C, with UV blocking or without it. The most common and widely used type, at the same time, the cheapest of FR-4;

  • with a higher glass transition temperature, Tg ~ 170°C - 180°C, compatible with the lead free reflow technology;

  • halogen-free, compatible with the lead free reflow technology;

  • with normalized index of CTI ≥ 400, ≥ 600;

FR-1/FR-2

The family of laminates under the name FR-1 and FR-2  by the NEMA (National Electrical Manufacturers Association, USA) classification. These materials are made from paper and phenol compounds and are used only for the production of single side printed circuit boards. FR-1 and FR-2 has the similar parameters, the main difference is that FR-1 has a higher glass transition temperature Tg. Because of the similarity of the parameters and applications of FR-1 and FR-2, many of the material suppliers produce only one type of laminates, most often  FR-1. Laminates have a good ability to mechanical processing (milling, punching). Flammability rating is UL94-V0.

FR-1/FR-2 laminates are divided into the following subclasses:

  • standard;

  • halogen-free, without phosphorus and antimony, non-toxic;

  • with normalized index of CTI ≥ 400, ≥ 600;

  • non hydrophobic;

CEM-1

The family of laminates under the name CEM-1 by the NEMA (National Electrical Manufacturers Association, USA) classification. These materials are made from paper and two layers of woven glass epoxy and phenol compounds and are used only for the production of single sided printed circuit boards. As a rule, they have a milky white or milky yellow color. Laminates are incompatible with the process of metallization in holes, therefore, they are used only for the production of single sided printed circuit boards. Dielectric properties close to that of FR - 4, mechanical endurance is somewhat worse. CEM-1 is a good alternative to FR-4 when the price is the deciding factor. Flammability rating is UL94-V0.

CEM1 laminates are divided into the following subclasses:

  • standard;

  • with a higher Tg , compatible with the lead free reflow technology;

  • halogen-free, without phosphorus and antimony, non-toxic;

  • with normalized index of CTI ≥ 600;

  • non hydrophobic, with good dimension stability;

CEM-3

The family of laminates under the name CEM-3 by the NEMA (National Electrical Manufacturers Association, USA) classification. Composite material based on glass-epoxy compounds, usually has a milky white color. Very widely used in the production of double-sided PCBs with plated holes. The properties are very similar to that of FR-4, except the lower mechanical endurance. CEM-3 is a cheaper alternative to FR-4 for most applications. Laminates have a good ability to mechanical processing (milling, punching). Flammability rating is UL94-V0.

Depending on the properties and applications, CEM-3 laminates are divided into the following subclasses:

  • standard, with UV blocking or without it.

  • with a higher Tg , compatible with the lead free reflow technology;

  • halogen-free, without phosphorus and antimony, non-toxic;

  • with normalized index of CTI ≥ 600;

RO3000

A family of laminates developed for wide use in the early 90s of XX century. These materials have excellent electrical properties at the high frequencies and high thermal stability. The CTE (coefficient of thermal expansion) along the X and Y axes is close to the CTE of copper and FR4, therefore it is possible to produce the reliable RO3000 / FR4 hybrid PCBs. Low dielectric losses (Df = 0,0013 at a frequency of 10 GHz) provide great benefits when using these laminates in the applications for microwave range.

RO4000

This is a family of materials for a very high frequency range, which has been designed, on the one hand, to achieve the performance comparable with that of materials containing polytetrafluoroethylene (PTFE), and on the other hand, to simplify the technology of PCB production, that is, to make it more in line with the traditional technology used for reinforced laminates (FR4). Materials RO4000 contain reinforced fibreglass of a high glass transition temperature (Tg = 280 °C) with a thermosetting polymer as a bonding agent as well as additives ceramics.

Polyimide

Is a flexible polymeric film, the often used as the substrate for flexible printed circuit boards. The polyimide films produced under the trademarks Kapton, Rogers, Dupont. Advantages: excellent flexibility in the wide temperature range, good electrical properties, high chemical resistance (except for hot concentrated alkalies), very good tensile strength. Some types of polyimide have additional advantages (the coefficient of expansion in accordance with that of a copper, small internal stresses). Working temperatures from -200°C to + 300°C. Disadvantages: high water absorption (up to 3% by weight), relatively high price. Despite the high glass transition temperature (Tg ≥ 250 °C), their high temperature properties are limited by the bonding compound layers.

The polyimide film thickness can vary widely, but in practice most of the flexible materials are available with the thicknesses in a narrow range from 12 to 125 µm. When designing the flexible printed circuit boards it can be useful to remember the practical rule: the stiffness of the flexible materials is proportional to the third power of their thickness. That means if the material thickness is doubled, it becomes eight times tougher and with the same load will bend eight times less.

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