Choosing High‑Temperature PCB Laminates for Reliable Performance

Why Picking the Right Laminate Is so Important

Picking the right high-temperature PCB laminate materials can be crucial to your operation. If you are going to use your printed circuit boards in high-temperature situations, PCB laminates for high temperature are a requirement. Failing to use the proper PCB laminate materials for high temperature can be disastrous. How? Read on.

What Happens When Circuit Boards Heat Up?

We know it is a natural property of heat to cause things to expand, and circuit board materials are no exception. The circuits on a circuit board are delicate machinery and need to retain their specific shape. Microwave and millimeter-wave circuits in particular have small, delicate features. If these features are distorted by high heat, they could fail to function. Other circuit material expansion as a result of high heat can change the form of transmission lines, shift frequencies and change conductor impedance from the standard desired value of 50 Ohms.

Oxidation may also be a concern. While the dielectric material covered by your copper laminate has protection against the effects of oxidation, exposed dielectric material does not. If this material is susceptible to oxidation, you could have a problem at higher temperatures, which tend to accelerate oxidation. The result can be greater loss of transmission lines and a greater dissipation factor.

What Are Some Important Laminate Properties When It Comes to Protecting Circuit Boards?

Important laminate properties to consider when it comes to protecting circuit boards, especially from high-temperature situations, include:

• Glass Transition Temperature (TG) – The thermodynamic shift of the polymer from rigid to soft
• Z-Axis Expansion – The physical expansion along the Z-axis, expressed as a percentage or CTE, coefficient of thermal expansion.
• Decomposition Temperature (TD) – Measurement of weight loss due to resin degradation because of high heat. The point at which 5 percent of the mass is lost to decomposition.
• Moisture Absorption – How well a material can absorb moisture from the environment.
• Time to Delamination – Time it takes delamination to occur at a given temperature.

What Is a CTE?

The CTE, or coefficient of thermal expansion, is a measure of how much expansion higher temperatures cause a material, expressed in parts per million (PPM) according to the Celsius scale. The goal is to have a CTE for your printed circuit boards’ dielectric layers with a similar value as the copper laminated to those layers. This way, they expand together in a uniform fashion, rather than pulling against each other.

Where Does the Heat Come From in a PCB High-Temperature Situation?

Another reason why high-temperature laminates are so important when working with PCBs is that potentially damaging heat can come from almost anywhere, with unpredictable effects. If you assume heat will not be a problem for your PCBs, you may be in for a rude awakening. If you’re not clear on exactly where your potentially damaging heat effects may be coming from, some possible heat sources include:

• Heat created by a component mounted to the circuit board, such as gallium nitride transistors added to boost power levels of RF/microwave systems.
• Heat generated by an element external to the circuit board — for example, in automotive electronic systems.
• Heat built up due to use of the circuit board in an improperly ventilated environment.

Rather than try to anticipate where heat sources will come from, if there is any chance you will be using your PCB in a high-temperature environment, you want to make sure you are using circuit materials designed to resist the effects of high temperature, including high-temperature laminates.

What Are the Likely High-Temperature Situations for Printed Circuit Boards?

High-frequency circuits are likely to produce high-temperature situations. The more power you need, the more heat you can expect. High-temperature situations where maintaining the PCB’s optimal performance under a variety of potentially stressful conditions include the aerospace industry and medical applications. Failure in these situations under harsh conditions can be catastrophic, so using the right materials in the circuit board in general and for the laminate in particular is crucial.

Remember, making sure the CTEs line up among the laminate materials and other circuit board materials is a key element in protecting your PCB from the damaging effects of thermal expansion.

How Can I Protect My Circuit Boards From Heat Effects Besides Using the Right Laminate Materials?

As with all issues surrounding the quality and performance of printed circuit boards, it all comes down to design. When designing your circuit board, it’s important not to just look at performance characteristics in a vacuum. Designers must ask themselves how stable these characteristics are under temperature.

If a PCB is going to be in a high-temperature situation, materials that are not stable and dependable at higher temperatures must be passed over for ones that are. Designers should also be aware of a PCB material’s RTI, or relative thermal index. This measure, established by the Underwriters’ Laboratory (UL), indicates the highest temperature you can subject a particular material to so it won’t negatively affect performance or change the material’s properties.

Another rating for designers to know is the MOT, or maximum operating temperature, which is the highest temperature a circuit in a specific PCB configuration can handle without significant negative effects on performance or properties of the material. In general, when designing and manufacturing printed circuit boards, keeping temperature effects in mind from the start can make things much easier on you later.

How Can I Learn More About Thermal Expansion, High-Temperature Laminate Materials and PCBs?

The right laminate can protect your printed circuit board from extreme heat, slowing expansion, reducing distortion and allowing your circuits and circuit materials to retain the properties they need to function effectively. Millennium Circuits Limited specializes in delivering high-quality printed circuit boards to suit a variety of applications, even ones that take place in high-temperature situations.

To learn more about printed circuit boards, how heat affects printed circuit boards, laminates, how laminates protect PCBs from heat and other dangers, or anything related to PCB manufacture and use, contact MCL online today, or call us at 717-558-5975.