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July 24, 2020

How to Select Materials for PCBs Considering Cost and Reliability

The properties and functions of a Printed Circuit Board or PCB depend primarily on the material it uses for its substrate. Designers select the material for the PCB substrate based on the requirements set by the application of the project, with the appropriate material selection bringing an optimal balance of requirements, cost, and reliability. Additionally, the manufacturing process depends on the material chosen for the PCB.

FR-4 is the most common material for PCBs as it is ideal for most applications and it is a type of fire-resistant material. However, for special applications that require operation at different temperatures, designers select other material that may increase the cost. For instance, reliable operations at higher temperatures may require designers to select FR4-TG150, increasing the cost. For reliable operations at still higher temperatures, the designer may opt for FR4-TG175, increasing the cost by an additional factor.

Factors Affecting PCB Performance and Reliability

Several factors affect the performance of a PCB when operating under specific conditions. One of the conditions is the operating temperature. While the temperature the PCB faces during normal operations may not be very high, during assembly, the board may have to pass through temperatures high enough to soften its core material—for instance, temperatures necessary during Lead-free soldering. For reliable operations, the Tg or Glass Transition Temperature of the material must be high enough to withstand this temperature without damage.

Another factor that depends on temperature is the amount of expansion or shrinkage of the material. A PCB consists not only of the substrate material, but also Copper foils, with different rates of expansion and shrinkage, known as CTE or Coefficient of Thermal Expansion for both. Furthermore, the CTE for the material is different in X, Y, and Z directions of the board. The CTE for the Z-direction of the substrate material is more important and must be as close to that for Copper, as a mismatch of the two CTEs may cause a rupture of the wall of a plated through hole, resulting in electrical discontinuity of the circuit.

A fault in the circuit or outside it may cause a component/components to heat up and even catch fire. Therefore, both the component and the PCB material must be of the fire- and flame-retardant type, which does not allow the fire to burst into flames and spread. Simultaneously, the PCB material must also be resistant to heat, typically, to the extent of 250 °C for 50 seconds..

Copper traces on the PCB carry away a major part of the heat generated by components in normal operation. Copper traces on the two outermost surfaces can dissipate some of this heat by convection. However, heat in the copper traces in the inner layers must pass through the PCB material to reach the surface. This requires the PCB material to be thermally conductive, but electrically insulating.

For acceptable high-speed and high-frequency performance, the PCB material must uphold signal integrity. Two factors are important here, and the PCB material must exhibit high Dk or dielectric constant and a low Df or dielectric loss.

Various Types of PCB Substrate Materials

The most common type of PCB substrate material in the industry is the Copper Clad Laminate or CCL, with the basic composition consisting of the base material, resin, and copper foil. The base material may make the PCB rigid or flexible, with rigid CCLs being of various base materials such as:

  • Paper
  • Fiberglass
  • Composite

Of the above, phenolic CCL with a paper base has the oldest presence in the industry, and has innumerable classes such as FR1, FR2, X, XP, XPC, and many more. Manufacturers make very low-cost, single-sided PCBs for the consumer electronics industry with paper-based phenolic CCL. Other varieties of PCBs with paper base are epoxy CCL, from which manufacturers make singleand double-sided PCBs for consumer, telecommunications, and power industry.

Epoxy CCLs with a fiberglass base are more common for medical, telecommunications, and computer industries. It has better physical characteristics as compared to the material with paper base—better chemical resistance, dimensional stability, water resistance, heat resistance, and electrical insulation. Manufacturers make high density PCBs with epoxy CCL with a fiberglass base, not only as core material for multiple-layer PCBs with through holes, but also as prepreg.

Composite CCLs such as CEM-3 may have epoxy resin and or polyester resin. These have high reliability, moisture resistance, heat resistance, and their better dimensional stability make them suitable for making PCBs for SMT and thin PCBs.

FR-4 is a fiberglass-reinforced epoxy resin CCL standard defined by NEMA. FR means the material is flame retardant, complying with the UL94V-0 standard. All FR-4 PCBs carry the 94V-0 code, thereby guaranteeing non-propagation of fire, and its rapid extinguishing if the material starts burning. FR-4 has three classes—A3, A2, and A1. FR-4 A3 is the cheapest among the three and PCBs made with this material are suitable for game machines, calculators, and toys. Slightly more expensive than A3, PCBs made of FR-4 A2 material are useful in several industries such as electronic products, home appliances, computers, and instrumentation. For high quality products such as in automobiles, industrial instrumentation, digital circuits, telecommunication, and military service industry, manufacturers prefer to make PCBs out of FR-4 A1 material.

There are several advantages in using fiberglass-reinforced epoxy resin CCL or FR-4 material for PCBs. It is low cost, fire resistant, and resists heat up to temperatures of 140 °C to 150 °C. FR-4 has a good resistance to weight ratio, does not absorb moisture, has a high mechanical strength and good insulation in both dry or humid conditions. Tg value for FR-4 is around 180 °C, which is fairly high.

However, manufacturers cannot make thin PCBs of FR-4 material, and therefore, designers must choose from other materials such as Polyimide if they require thinner PCBs. Moreover, the FR-4 material is not suitable for high-speed and high-frequency operation.

Best Practices for Selecting Suitable PCB Material

Dielectric Constant: For PCBs meant to operate at high frequencies and with high speed signals, the designer must consider material with high dielectric constant or Dk. If they are using different materials, their Dk must match.

Coefficient of Thermal Expansion: CTE for different materials in the substrate must match, as should the Z-axis CTE for the substrate match with that of Copper. Mismatch in CTEs can affect Dk.

Substrate Weave: For proper interaction, the material for the substrates must have a tight weave, and they must mesh properly. Materials with a loose weave will impact Dk.

Use of FR-4: FR-4 is suitable only for low-frequency and low-speed use. For circuits that work at high frequencies and with high-speed signals, FR-4 is an ill-suited material.

Foil Structure: At higher frequencies, smooth copper foil performs the best.

Conclusion

Performance of PCBs is all about quality, and well-matched parts ensure this. Substrates of good quality can be a sizable investment. With a choice of proper materials, and a check on manufacturing processes, it is possible to obtain quality and good performance from PCBs at the optimum costs.