PCB FR-4 Materials Are Not All The Same

In the PCB industry, “FR-4”.is a common designation for laminate materials.  To a certain extent, FR-4 as a specific type of laminate is one of the many myths promulgated throughout the industry. This blog will address the history the term FR-4, what it really means, the various quantifiers associated with it, and the characteristic issues of concern when selecting design-specific laminates.

The Origins of FR-4

If you research FR-4 as a term, nearly every reference defines it as being a glass-reinforced epoxy laminate. Essentially, the Wikipedia definition is: “FR-4 is a NEMA (National Electrical Manufacturers Association) grade designation for glass-reinforced epoxy laminate material. FR-4 is a composite material composed of woven fiberglass cloth with an epoxy-based resin binder that is flame resistant (self-extinguishing).”This definition of FR-4 is so ubiquitous within the industry that many people involved in PCB product development have come to use FR-4 as a laminate designation in this manner.  

The reality is that FR-4 is not and has never been a laminate material. Rather, it is a UL rating that means “flame retardant class 4.” If you go back far enough into the history of PCBs, there were only two laminate choices: polyimide and epoxy-based materials.  If you were designing an aerospace product you used polyimide because the resin system could withstand higher temperatures. But, it comes with a lot of negatives—it is expensive, difficult to manufacture, and at the top of the list of problems, it soaks up moisture. This means you must bake a board made out of polyimide dry and then have a conformal coating put on it to prevent severe leakage problems.  

The other “original” material was epoxy.  Epoxy was less expensive, easier to manufacture, and was the most commonly used resins. But, the material cannot withstand high temperatures. So, it becomes soft during the soldering process, making it prone to warping. Also, the term “epoxy” does not denote any one particular resin, but to a class of laminates.

So, based on these two original laminate choices, FR-4 came to mean “not polyimide”, but, by no means, are all FR-4 boards the same. In fact, it’s possible to make 50 different boards, all of which satisfy the“FR-4” designation, but differ in their appearance, performance, and where they fall within the cost spectrum.  

Even when the industry moved beyond purely epoxy-based materials, the FR-4 designation remained because the newer resin systems weren’t polyimide.  For instance, to address the shortcomings of epoxy-based systems, we now have several epoxy blends with a variety of temperature characteristics. One of these is known as “High-Tg FR-4” addresses the low Tg (glass transition temperature) problems associated with epoxy-based boards.

What Really Matters

Along with copper foils and woven glass reinforcement, resin systems are one of the main components of board material systems.  And, within the resin system, one of the most important properties is the dielectric (insulator). The property of the dielectric that affects impedance and wave velocity is the relative dielectric constant (er). As shown in Table 1, this property varies with both resin content and the frequency at which it is measured.  

Table 1. Properties of a Typical Hi Tg FR-4 Laminate System

When people use “FR-4” they assume that there is one common dielectric constant for all FR-4 materials. Specifically, they assume FR-4 means the material has an er of 4.7. This er has not existed for 30 years, but the assumption was the source of lots of impedance errors. Table 2 shows the er of commonly used laminates none of which are much above 4.0. Additional critical information includes Tan (f) loss tangent, DBV (dielectric breakdown voltage) and WA (water absorption).  

Table 2. Properties of Several Common PCB Materials Systems

Tg = glass transition temperature                                  WA = water absorption

DBV = dielectric breakdown voltage                              Tan (f) = loss tangent

er = relative dielectric constant

All materials with woven glass reinforcement except Teflon.

Specifying the Best Laminate for Your Design

One of the things that plagued the industry over the years is that the data sheets on materials historically did not include enough information to allow product developers to choose laminates based on the performance parameters of their designs. Within the broad category of “FR-4”, many materials are “typical” without enough specifics to ensure the right laminate was selected for a given product design.  

As designs became more complex, the need for further refinement of laminate information rose exponentially. For instance, in recent years, one common challenge was missing information on glass weave styles. Not having this information led to serious issues with jitter and skew. Fortunately, laminate manufacturers now provide information on the glass weave styles such as that shown Table 3.

Table 3. List of Glass Weave Styles Available in PCB Laminates

Table 4 is an example of the laminate information containing glass style and resin content needed to design a stackup.  This table is for the FR408HR laminate system from Isola Group.

Core

Table 4. Typical Laminate Information Needed to Design a PCB Stackup

From Prototyping to Production

As with other critical aspects of design, it’s imperative that the engineering team be very specific when identifying a particular laminate for a product, and then be equally diligent about ensuring that the same laminate is used throughout the entire product manufacturing cycle from prototyping to full production. Sometimes, a cost-conscious production manufacturer substitutes one material for another to save the customer money. This type of situation occurred about five years ago when a production manufacturer substituted a material that had the cheaper one-ply glass material rather than the design-correct, but more expensive, two-ply. This resulted in boards failing and the manufacturer having to buy all of the assembled boards, each of which had $5-6K worth of components.  

In truth, our industry got into the bad habit of letting fabricators change the artwork, the stackup,or the material either as part of their routine or as a way of “saving” their customers money. This may not have had much impact when the speeds of design were slow, but that kind of latitude is impossible with today’s high-speed designs. This is why it’s important to have test structures built into your boards and to make sure a “traveler” goes through the fabrication facility with each board. Product developers should insist on this document so they can see which materials went into their boards throughout the product development cycle.

Summary

While the use of FR-4 as a laminate designator has a basis in history, it does not mean it offers performance metrics. In reality, using  “FR-4” materials for a particular design can lead to severe consequences including product failure. To ensure your product will work as designed the first time, you must factor in the critical performance-related characteristics into the final material choice. It’s also imperative to specify that the same laminate be used throughout the entire production cycle from prototype through to full production.  

Would you like to find out more about how Altium can help you with your next PCB design? Talk to an expert at Altium.

Reference

1. Ritchey, Lee W. and Zasio, John J., “Right The First Time, A Practical Handbook on High-Speed PCB and System Design, Volumes 1 and 2.”