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Elementary, Mr. Watson: Slash Sheets a ‘Smorgasbord’ of Materials

Slash sheets touch on materials, one of my favorite areas of PCB design, and what I refer to as “the buffet of the PCB world.” After all, this smorgasbord of materials comes in an endless variety, with differing balances of reinforcement and resin, from different manufacturers and vendors, and from a variety of sources. Add the possibility of out-of-date or deteriorated panels, and it makes ensuring the quality of your next PCB design a real hit or miss. Think of it like looking at a variety of available restaurants. To determine whether they are “safe and clean,” and meet quality standards, the health department conducts regular inspections, and gives them ratings which help us know whether it meets our standards.

It seems that nothing has more impact on a successful PCB than its materials. In general, a working PCB comprises some essential, everyday items like fiberglass, resin, and copper. Add the physics to the mix, and you have a working PCB. The materials determine the electrical and thermal properties of every PCB. Certain types of materials are not a choice but a must, especially for high-speed or power PCBs. Choosing the wrong material can affect signal integrity (SI) and lead to potential disaster, including premature system failures and added costs to a company. Often these issues do not show up until long after the PCB has left a company.

Slash Sheets, Defined
Slash sheets provide information on specific materials used in manufacturing PCBs, including the material's properties, specifications, and usage instructions. It is essential to use the correct material in the manufacturing process to ensure that the final product meets required standards.

Basically, slash sheets answer this simple question: Does a specific material meet the requirements or standards of IPC-4101 and IPC-4103?

History and Evolution of Slash Sheets
Slash sheets were introduced in the 1980s to standardize all the different materials used in the PCB industry. The original slash sheets were simple documents that provided basic information on materials. However, as technology and manufacturing processes evolved, so did the need for more detailed information on materials.

According to IPC-4101, each slash sheet has a set of criteria to help determine whether a specific material matches the requirements. The standard has four categories which provide comprehensive requirements for the performance and properties of laminate and prepreg materials used in manufacturing PCBs. The standard also defines several vital areas used to validate a specific PCB material.

Mechanical properties: This covers the material's mechanical strength, dimensional stability, and thermal expansion. They are important because they affect the reliability and performance of the PCB, particularly in high-stress or high-temperature applications.

Electrical properties: This covers the electrical insulation, dielectric constant, and loss tangent of the material. They are important because they affect the signal integrity and reliability of the PCB, particularly in high-speed digital or high-frequency applications.

Thermal properties: This covers the material's thermal conductivity, thermal resistance, and glass transition temperature. They are important because they affect the heat dissipation and thermal management of the PCB, particularly in high-power applications.

Chemical properties: This covers the material's chemical resistance, moisture absorption, and flame retardance. They are important because they affect the durability and safety of the PCB, particularly in harsh or hazardous environments.

These areas are then broken down further into reinforcement type (usually fiberglass) and resin material system (usually an epoxy mix).

Table 1: Break down of reinforcement type

PCB Reinforcement Types

Woven E-glass Core

The most popular PCB reinforcement is woven E-glass, although, according to IPC-4101, expected reinforcements are nonwoven E-glass core, unidirectional E-glass, and nonwoven aramid paper. The go-to reinforcement is FR-4, or fiberglass PCB, which uses a woven fiberglass material as its core substrate. The fiberglass material comprises thin strands of glass that are woven into a mat, which is then coated with epoxy resin to create a rigid and durable material.

The use of woven E-glass provides excellent mechanical strength and dimensional stability to the PCB, making it suitable for applications that require high reliability and durability. Woven E-glass PCBs also have good electrical insulation properties, which make them ideal for high-speed digital and analog circuits.

Cellulose Paper Core

Cellulose paper core are PCBs that use a core material made of cellulose paper impregnated with phenolic resin. In the PCB industry, this core material is often called FR-2 or phenolic paper.

The cellulose paper core provides good mechanical strength and dimensional stability to the PCB, while the phenolic resin provides excellent electrical insulation properties. That makes cellulose paper core PCBs ideal for low-cost, low-power applications where high-frequency performance is not critical.

Resin Material Systems

The IPC-4101 standard defines several resin material systems, each with its own properties and characteristics. These include:

• Epoxy resin systems are the most used in the PCB industry due to their excellent mechanical properties, chemical resistance, and electrical insulation properties.
• Polyimide resin systems are used in high-temperature applications due to their exceptional thermal and dimensional stability.
• Cyanate ester resin systems are used in applications that require high stiffness and low dielectric constants, such as microwave and high-speed digital circuits.
• BT (bismaleimide triazine) resin systems are used in applications that require high thermal stability and low dielectric constant, such as high-speed digital and RF circuits.
• PTFE (polytetrafluoroethylene) resin systems are used in applications that require exceptional electrical insulation properties and low-loss tangents, such as microwave and RF circuits.

Each resin material system has its properties and characteristics and is tested and validated against specific requirements defined in the IPC-4101 standard. By selecting the appropriate resin material system for a particular application, PCB designers can ensure that their boards have the necessary mechanical and electrical properties to meet the application's requirements.

Tg or Glass Transition Temperature
Tg is when a material transitions from a rigid, glassy state to a softer, rubbery state. It is the temperature at which the polymer chains within the material begin to move more freely, allowing the material to become more flexible and elastic. The Tg of the material used in the PCB determines the maximum temperature at which the board can operate without experiencing mechanical or electrical failure due to thermal stress.

Dk (dielectric constant), on the other hand, refers to the ability of a material to store electrical energy in an electric field. It measures how much a material's capacitance changes relative to air (or a vacuum), which has a Dk of 1 MHz. Both are related to the thermal properties of the material.

A significant problem I've experienced with material manufacturers (especially those who are, let's say, less honest) is their claim that a particular material matches the requirement of an IPC-4101 slash sheet. It is difficult to verify the chemical breakdown of a material after the fact. It usually requires Fourier transform infrared (FTIR) spectroscopy, ion chromatography (IC), and differential scanning calorimetry (DSC) at, of course, significant cost.

John Watson, CID, is a customer success manager at Altium.

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