Estimated reading time: 5 minutes

Fresh PCB Concepts: Don't Let Over-specification Come Back to Haunt You

In my career, I never imagined a shortage of FR-4 materials like the one we see today, a situation that has frustrated both PCB manufacturers and customers. FR-4 materials are highly substitutable when compared to exotic materials, so if a factory does not have one specific brand of FR-4, they will typically have an equivalent material in stock. This forces our customers to choose between waiting an unreasonable amount of time for the branded materials to arrive at the factory and waiting a slightly shorter time for the substituted materials.

This problem has left some design engineers perplexed. I don’t know if we will have the material availability enjoyed in the past. However, I do know that we can dig into datasheets and specifications to make a reasonable, confident choice when we need to substitute materials.

How to Change Materials in a Smart Way

Over the years, I have accumulated a large folder on my hard drive dedicated to PCB material datasheets. When I get a question about a PCB material, I typically refer to the datasheet before and during the meeting. PCB material manufacturers often have the datasheets available for download. The first thing I check in the datasheet is the date it was created. I recommend always using the most up-to-date datasheet available. I will admit, sometimes this is easier said than done. Some materials are tried and true, and the datasheets do not need to be updated, but I recommend always checking first. Furthermore, some variables important to a PCB design may change from one update to the next. Hence, this is the reason for the update.

A typical FR-4 callout from a fabrication drawing looks something like “Insert brand here, or equivalent that has a Tg of 170 minimum.” The truth is, Tg alone is not a reliable indicator of whether one PCB material is a good substitute for another. For example, two different materials may have the same Tg, but they also may have very different Dk. Therefore, let’s consider a typical datasheet and the information it may provide.

A Rigid Material Datasheet
How about we walk through a typical datasheet and cross-reference IPC-4101? I see many different properties. The first is Tg. It says 170°C, a typical value, which makes me wonder why so many fabrication drawings still specify material by Tg? There are so many other material properties on the datasheet that we can consider to determine whether the material will meet the performance requirements of the circuit at hand. Here are some other material characteristics I see:

• CTE, or Z-axis expansion: Measured in parts per million (PPM) and indicates how much the material expands along the Z-axis as temperature changes. Some datasheets define this figure with copper removed as the materials have two different CTE values. How is this useful? Well, if the stackup requires different materials like a hybrid or a rigid-flex stackup, it is great to compare the CTE of the different materials to determine if they have similar values. If the two materials have very different CTEs, then it may be a good idea to keep looking for an alternative material. Otherwise, the PCB may experience failures during thermal excursions. Examples of such failures include pad lifting and cracking of the hole barrels. In extreme cases delamination may occur.
• Decomposition temperature (Td): This is the temperature at which the resin system begins to decompose chemically. When the PCB reaches the threshold temperature of Tg and continues to become hotter, the resin system begins to break down and carbonize. This is important for the lead-free assembly process and thermal excursions, as carbonization introduces an unplanned conductive path.
• Dielectric constant (Dk): When you want to start learning about the signal integrity of a PCB material, this is a great place to start. Dk is a measure of how well the PCB material stores electrical energy in a vacuum. In a nutshell, the most electrically stable materials have a lower Dk. The Dk for FR-4 is about 4. Some exotic materials have a Dk value of 2.5 to 3.5, which will provide better signal integrity. However, the signal integrity gained is traded for material cost and lead time. There simply are not many factories that regularly stock exotic materials.
• Dissipation Factor (Df): The dielectric loss tangent is a measure of how much electrical energy is lost as the material heats up. The lower the Df, the less signal loss, just like the Dk. At higher operating frequencies, Df becomes an important consideration, as they generate more heat than lower frequencies.
• Comparative Tracking Index (CTI): Expressed in volts, sometimes moisture and contamination can cause current to travel across the insulating surface of a laminate. If it occurs enough, a carbonized conductive path will form, robbing the circuit of power. The higher the CTI, the better the resistance to electrical tracking. CTI becomes more important in designs with higher voltage and/or harsh environments where contamination is common.
• Moisture absorption: This parameter is the percentage of water a PCB material will absorb from the surrounding environment. When a PCB absorbs a high amount of water, the Dk, Df, and insulation resistance are all affected. Moisture may affect the material's signal integrity parameters. This will cause impedance mismatches, data corruption, and maybe a release of magic smoke from some components.
• Flammability rating: UL 94V-0 is the most common UL rating for PCBs. This means that the PCB material has passed a vertical burn test that measures many parameters, but the material’s ability to self-extinguish is of the most interest to designers. The flammability rating indicates how the PCB material will behave when exposed to a flame. Exposing a PCB assembly to a flame will cause it to catch fire. For most PCBs, this condition is not the desired condition. For the others, we want to ensure the rapid disassembly is commanded and not premature.
• Relative thermal index (RTI): By nature, some PCBs will heat up during operation. RTI is another UL material rating that measures the PCB material’s long-term thermal endurance. It is the temperature at which the PCB can operate for extended periods without changes in the material’s electrical or mechanical properties.
• IPC-410X specification: This is one of the first datasheet parameters I typically look for, as it helps determine an alternative material without over- or underspecifying the material’s performance. You may see something like IPC-4101/126. The /126 is the important part. The next step is to open IPC-4101 and look up the /126 material specifications.

The IPC-4101 Slash Sheet Method of Material Substitution
Reading through many material datasheets can take a lot of time and coffee. Instead of looking at several datasheets side by side, we can take a different approach. Remember that the datasheets will still be needed to see which slash number the specific material meets. PCB materials are available worldwide, but with some major brands not available in certain regions, prices and delivery times are often better. However, some popular materials have experienced longer lead times, which has negatively affected the supply chain. What can we do?

Here is where it all comes together in a real scenario. Let’s take another look at this material callout, “Insert brand here, or equivalent that has a Tg of 170 minimum.” The factory has provided a quote with the caveat that the material is currently unavailable or has a six-month lead time. The factory has also provided three datasheets of substitutable materials that are available and in stock. Which material do we choose? We have looked at the datasheet and learned that our desired material meets IPC-4101/126. Therefore, we simply must look at the datasheets the factory provided and find out which one meets IPC-4101/126. It is that simple.

Simplicity Is Bliss
Instead of getting locked into one specific brand of PCB material, which may not be available in certain regions of the world or may be scarce due to global challenges today, the slash sheets allow PCB designers to specify and substitute materials  based on performance. Specifying IPC-410X/xx is a bulletproof way to communicate the material properties. Simply stating the performance requirements, not the brand name, opens up more manufacturing opportunities and reduces material lead times.

As always, I encourage you to foster a relationship with your PCB supplier as they can tell you more about the materials they use. Many suppliers already have a list of FR4 materials that are readily available and substitutable.

Ryan Miller is a field applications engineer with NCAB Group, and a featured guest on I-Connect007’s On the Line with: PCB Thermal Management podcast series here.