Best Practices for High-Frequency PCB Material Selection

May 18, 2017 CircuitStudio

Wave on oscilloscope

 

I tried out speed dating recently, it didn’t go well. The date was set up in a nice restaurant near my house. I wanted to stand out a bit and make a good impression. So I decided to wear my nice velour shirt. Well, apparently velour isn’t as classy as I thought it was, and I went home without a single phone number. I found out that night that material matters, something that has surprisingly translated into PCB design. Just like how your shirt’s cloth can affect the integrity of a speed date, your PCB’s material can affect the signal integrity of your high-frequency circuits. You can minimize attenuation on your board by choosing the right fiberglass, resin, and copper foil. There are a variety of best practices that will help you choose the optimal combo, though you have to watch out for price and other considerations.

 

 fat cupid
You need to be the matchmaker for your circuit and a high-frequency substrate.
 

Why Do Materials Matter?

The right PCB materials will keep your circuits from sending mixed signals. Things like losses in poor quality dielectrics or sub-optimal copper foil can have more of an effect than you might think.

 

How exactly do dielectrics introduce loss into your system? All dielectrics are made up of polarized molecules. These molecules vibrate in the magnetic fields generated by signals. The higher the frequency, the more vibration, which results in energy lost as heat. This energy loss can be mitigated by using low loss dielectrics, but more on that later.

 

Another significant loss is introduced by the copper conductors themselves. You may recall learning something about skin depth in college. Remember that electrons do not always flow through the centers of conductors. As frequency increases, currents are limited to a maximum “skin depth.” Well, if your copper is finished with nickel, that can mean most of the current is flowing through that nickel layer. Boom, loss. Even if your entire conductor is made of copper, the micro-profile of the copper can make a difference. If the copper has micro ridges, currents will travel up and down those ridges, increasing resistance and loss.

 

The dielectrics and conductors on your board can make a huge difference in signal integrity. So let’s talk about how to control those variables.

 

RF electronic circuit
You may forget about the substrate, but it’s important.
 

What Can You Choose?

There are two main variables you can control to mitigate loss: your substrate and your foil.

 

  • Substrate -  Your PCB substrate can consist of a variety of materials. From an epoxy resin and woven glass fibers to a ceramic plate. For high-frequency circuits you’ll want a substrate with the lowest dielectric constant (Dk).

  • Foil -  You know what copper foil is, but you may not have known there are several foil options. You obviously want the option with the least resistance.

 

Best Practices

There are several best practices that will help you choose the right substrate and foil for your high-frequency application.

 

  • Match Dielectric Constants - When dating you want a personality match, in PCBs you want a Dk match. If your PCB substrate is made of a resin and woven material they may have different Dks. Non-uniform Dks in your substrate will cause problems. You should check with your manufacturer to make sure you get all of your substrate’s Dks as close as possible.

  • Match Coefficient of Thermal Expansion (CTE) - There are several temperature related characteristics for substrates, but CTE is the only one I’ll mention. If you go on a double date, your friend’s compatibility will affect yours. Your substrate component’s CTE can affect its Dk. If the components of your substrate have different CTEs, they may expand at different rates during fabrication or operation. In fabrication, this can lead to defects. In operation, this could change the physical geometry of the substrate and lead to non-uniform Dks. In love this will lead to a ruined relationship.

  • Tight Substrate Weave - You want to mesh well with your partner. The woven part of your substrate also needs to mesh tightly. A loosely woven substrate will lead to vibe killing differing Dks.

  • Don’t Use FR4 - It was obvious I shouldn’t have worn velour to a speed date, but I still did it. It’s also obvious you should use a low loss substrate. However, some people out there are still using FR4 for their high-frequency circuits. FR4 is not an appropriate material for high-frequency applications, use something else.

  • Use a Smooth Foil - Beauty, and apparently current, is only skin deep. A smooth copper foil will ensure lower resistive losses at extremely high frequencies.

  • Use a Conductive Foil - If you’re going to have a low skin depth, make sure you’re not finishing copper with poor conductors. The current will flow through those poor conductors and make for a poor circuit.

 

Other Considerations

Toby Keith once sang that “a high maintenance woman don’t want no maintenance man.” Before you go off looking for the perfect PCB, make sure you can meet its requirements. Great high-frequency boards will cost more than a nice dinner with a lady. They’ll also be harder to manufacture than a fake girlfriend for family Thanksgiving (don’t ask).

 

  • Cost - Simply put, high-frequency substrates are expensive. We like to use  FR4 because it’s cheap and easy. So is McDonald's, but that won’t get you a second date. There are a few potential low-cost high-frequency substrate materials out there. Just make sure they’re not too good to be true.

  • Manufacturing Defects - Oh yeah, remember those things? Well, they happen even more often with high-frequency materials. Some materials, like PTFE, are especially vulnerable to defects. Make sure you’re getting in bed with the right manufacturer before they break your boards and your heart.

 

Speed dating was a blur of attractive women making confused faces at my shirt. This may have been a blur for you, but hang in there. Remember that you are the master of your substrate, the captain of your foil. If you follow the best practices in this article you can avoid the heartbreak of signal loss. Keep in mind the considerations of cost and manufacturing defects as well.

 

The world of PCBs is even more confusing than the world of love. PCB design software that’s the perfect match will help you beget some great boards. CircuitStudio may not be able to choose your board’s material, but it will help you design the boards. Check out its wide array of great features, you can thank me later.

 

Need some fashion advice for you or your board? Call an expert at Altium.

 
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