Is FR4 the Best Board Material for Your Design?

One of the greatest hand-me-downs that can be passed on or embraced is a good tradition. For example, getting together with family for traditional holiday meals provides some of my most cherished memories year after year. And it is my hope that these will continue to be passed down forever. Not all traditions should be perpetual, however. There are those that should never have begun, and then there are the ones that have run their course and need to be changed. But alas, change is sometimes hard to institute and we often continue doing the same thing, simply because it is the tradition.

Believe it or not, becoming locked into a routine despite the possibility of a better option can occur in PCB design. A good example of this is simply using the default materials for your boards, irrespective of the type of board being designed. For example, high-speed boards have different requirements from high-density ones. Most PCBs are made using FR4 as the base material. Although this choice may not result in catastrophe, it may render your design much less than optimized for its intended function. Let’s take a look at standard FR4, along with alternatives, and see when it is and is not the best material for your board design.

What is FR4?

FR4 is the most common material grade that comprises fabricated circuit boards. ‘FR’ indicates the material is flame retardant and the ‘4’ indicates woven glass reinforced epoxy resin. Single or double-sided PCB structures consist of an FR4 core and top and bottom copper layers. Multilayer boards have additional prepreg layers between the center core and top and bottom copper layers. Now, the core consists of a substrate with copper covering, also referred to as a copper clad laminate. The core, laminate, and prepreg may all be FR4 with the copper sheets between the signal and ground layers.

The properties of FR4 may vary slightly depending on the manufacturer; however, it generally has favorable strength and water resistance attributes that support its widespread usage as an insulator for many electrical applications. It serves the same purpose in PCBs, namely to isolate adjacent copper planes and provide overall bending and flexural strength for the structure. FR4 is a good general purpose material for PCB fabrication; however, alternative materials are available.

Alternative PCB Materials

Prior to the explosion of multilayer PCBs, there were many alternative board materials to FR4. These included FR2, CEM 1, and CEM 3, which were paper-based. However, the strength of FR4, especially for multilayer boards, was a major factor in separating it from alternatives to become the industry standard. Today, there are other materials that are used for single-sided, double-sided, non-plated through-hole (NPTH), and multilayer PCBs, in addition to FR4. These are compared in the table below:

FR4 VERSUS ALTERNATIVE GENERAL PURPOSE PCB MATERIALS
Properties	FR4	FR5 (High Tg FR4)	Teflon	Polyimide
Glass Transition Temperature -Tg (°C)	135	150 - 210	160	> 250
Dielectric Constant -dk	4.2 - 4.8	4.5 - 5.4	2.5 - 2.8	3.8
Lengthwise Bending Strength (N/mm)	600	600	600	110
Crosswise Bending Strength (N/mm)	450	490	490	100
Copper Adhesiveness (N/mm)	2.0	> 1.4	> 1.4	1.2

The results above clearly indicate that FR4 is a good general purpose material as its parameters are mostly comparable to the other alternatives. It excels in structural integrity with a 2.0 N/mm copper adhesiveness and matches the alternatives in bending strength. However, compared to the alternatives, FR4 has a low Tg. This means that the material may be subject to deformation or breakdown when exposed to excessive temperatures, especially over time.

When is FR4 Not the Best Material For Your Board?

As described above, FR4 is indeed a good standard or default option for your board material in most cases. However, there are cases where FR4 is not the best material for your board, as listed below.

• If lead-free soldering is required

If your boards will be distributed in Europe and must adhere to The Restriction of Hazardous Substances Directive (RoHS) or your client necessitates that lead-free soldering be used, then you may want to explore other material options. This is due to the fact that reflow temperatures for no-lead PCBA may reach as high as 250°C, which substantially exceeds the Tg for many versions of FR4.

• If high-frequency signals are used

At high frequencies, FR4 boards are not able to maintain a constant impedance and reflections may occur that negatively impact signal integrity. This is a consequence of the relatively high value for dk.

• If the board will be exposed to extremely high temperatures during operation

It is also ill-advised to use FR4 if the PCB will be required to operate in an environment under extremely high temperatures. An example would be near the engine compartment of an aerospace vehicle.