Fresh PCB Concepts: 4 Characteristics to Consider When Selecting PCB Base Materials

NCAB-JeffBeauchamp.jpgSelecting the correct material is critical if you want your circuit board to survive the assembly process or come out of the assembly process in good condition. In this column, I will explain the four main characteristics from the IPC-4101 material specification that are critical in finding the performance of your base material.

I have placed these in order of importance and draw attention to why I list Td ahead of Tg. There is an industry-wide misunderstanding that Tg is the most critical attribute to consider when selecting material, but I urge you to consider the Td as the more important material attribute, as this value determines the material’s survivability or capability to withstand the high temperatures seen during reflow.

Td: Decomposition Temperature

Td is a measure of the degradation of the material. The method used is thermogravimetric analysis (TGA), which measures when 5% of the material is lost by weight—the point at which reliability is compromised, and delamination may start to occur. It is important to understand that the value given is not a fixed value; it is tested according to IPC-TM-650 2.4.24.6 (5% weight loss) in N² gas:

  • 5.3.1: Place the sample in the TGA and measure its mass
  • 5.3.2: Heat the sample at a rate of 10°C/min from ambient to 550°C
  • 5.3.3: Record the temperature, Td (2%), at which the mass of the sample is 2.0% less than its mass measured at 50°C
  • 5.3.4: Record the temperature, Td (5%), at which the mass of the sample is 5.0% less than its mass measured at 50°C

Higher-reliability PCBs or PCBs that operate in demanding conditions will require a Td greater than or equal to 340°C (644°F).

Tg: Glass Transmission Temperature

Tg is a value of the temperature at which the material changes from a reasonably stiff, glass-like material to a more elastic and bendable plastic-like material. Above Tg, the material’s properties will change, such as the expansion of the material, meaning above Tg materials tend to expand more.

It’s important to know that materials can have the same Tg yet different CTEs, and the material changes heavily above this point (post-Tg). Of course, lower CTE is better. Equally, some materials can have higher Tg values yet also have a higher or worse coefficient of thermal expansion post-Tg.

This point is often measured by thermomechanical analysis (TMA) or differential scanning calorimetry (DSC). A common value on standard materials is 130–140°C (266–284°F). When testing using TMA methods (IPC-TM-650 2,4,25), it is very important that the difference between any Tg results (delta between Tg2 and Tg1) is very short as this means that the lamination of the multilayer took place well with regards to the transmission of pressure and temperature across the manufacturing panel and through the stack.

CTE: Coefficient of Thermal Expansion (Z-Axis)

This is a measure of how much the base material will expand when heated. When considering lead-free applications, the most referenced factor can be that of Z-axis expansion. This can be measured as PPM/°C before and after Tg, and also as a percentage over a temperature range. Typically, above Tg, we see that the expansion is greater. If the CTE is insufficient or too high in the material, then failures can occur, such as a fracture or delamination during assembly as the material expands rapidly above Tg. CTE is also a potential cause for PTH failure as excessive expansion puts undue stress on the through-holes during the thermal excursion (Figure 1).

NCAB fig 1.JPG

Figure 1: This image shows the difference between Tg ratings and also the impact of higher CTE values. Materials B and C have the same Tg (Tg1) but different CTE (material C is higher) values. Material A has a lower Tg (Tg1) but a low CTE, so while it passes Tg sooner than the other two, the lower CTE means it does not expand (Z-axis) as much as material C.

Time to Delamination (T260, T280, T300)

This is a method in which the board is tested, using a TMA, to determine the time it takes for an event to occur such that the thickness is irreversibly changed at a pre-defined temperature. The term irreversibly changed is seen as the point at which the specimen delaminates.

Common temperatures referenced within IPC-4101 are:

  • T260: 260°C (500°F)
  • T288: 288°C (550°F)
  • T300: 300°C (572°F)

Ownership of the material selection lies with the customer, the stakeholder who truly knows the processes and the demands of the project and what the boards will be exposed to (reflow time + temperature + number of reflow cycles) and what is needed to survive.

Your supplier should be able to recommend materials that meet the demands of IPC-4101 specification sheets, and these are based on some product and process parameters. To select the most suitable material for your projects, it’s important to know how you want your material to perform and use the characteristics as defined in the IPC-4101 specification, and specifically if you can define the specification sheet or slash sheet. This will help immensely.

Jeffery Beauchamp is a field application engineer at NCAB Group.

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2020

Fresh PCB Concepts: 4 Characteristics to Consider When Selecting PCB Base Materials

10-08-2020

Selecting the correct material is critical if you want your circuit board to survive the assembly process or come out of the assembly process in good condition. Jeffrey Beauchamp explains the four main characteristics from the IPC-4101 material specification that are critical in finding the performance of your base material.

View Story

Fresh PCB Concepts: Advantages of Application-Engineered PCBs

09-17-2020

When working with your PCB supplier, do you have a dedicated engineering resource to help with the design of your PCB before fabrication? How about a resource that has experience and knowledge about the different applications for PCBs? Jeffrey Beauchamp explains how this is one of the most important and valuable factors when producing high-reliability PCBs, as well as what—or who—this resource could be.

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Fresh PCB Concepts: How Do You Calculate Finished Copper?

08-13-2020

How do you calculate finished copper on a PCB? This may sound simple, but Ruben Contreras has seen copper thickness called out either on the drawing or the specification, which can lead to additional EQs and, in some cases, additional costs. In this column, he explains the unintentional results that can come from misunderstanding what was requested.

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Fresh PCB Concepts: Pros and Cons of the 6 Most Common Surface Finishes

07-23-2020

There are only two different types of surface finishes for PCBs: organic and metal. Harry Kennedy describes the pros and cons of the six most common finishes on the market.

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Fresh PCB Concepts: 7 Options for Via Treatment

06-18-2020

In some cases, it's acceptable to have via holes that are completely exposed in a PCB design. But there are many others where the hole should either be covered and/or tented, or in most cases, plugged. Jeffrey Beauchamp shares seven different via hole protection types based on IPC-4761.

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Fresh PCB Concepts: Why Design and Produce PCBs Beyond Industry Specification?

05-07-2020

While questions may be an annoyance, especially when you finally have a board designed and are ready to have it built for a product, Ruben Contreras explains the importance of asking these questions and requiring specifications.

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Fresh PCB Concepts: What You Should Know About Your Board’s Solder Mask

04-09-2020

It’s a weird time we’re all in, and for most designers, work has slowed down or even stopped. While work might have slowed, now is a good time to review some of your past PCB projects to see if you can improve functionality and reduce field failures. Harry Kennedy explains how one of the simplest ways to do that is to start from the top: solder mask.

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Fresh PCB Concepts: The Benefits of Being a PCB Producer Without Owning Any Factories

03-19-2020

With NCAB's infrastructure and factory management team based in China, many customers ask why they don’t own any factories. Jeffrey Beauchamp explains how it's part of the company's long-term strategy and an advantage to our customers that they don’t own any factories. Instead, they “own” what is most important: the relationship with the factory.

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Fresh PCB Concepts: Controlled Impedance—Design to Testing

02-12-2020

Ruben Contreras often gets questions from customers on one of his favorite subjects in PCB design: controlled impedance. Ruben explains how controlled impedance is a science, which is why he likes it; therefore, it must be designed and tested comprehensively.

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Fresh PCB Concepts: Create PCB Fabrication Notes for Success

01-09-2020

When NCAB Group receives Gerber files for an order, there are times Harry Kennedy cringes because there are missing pieces of information necessary to make sure the PCB is made with zero defects. As we come back to work for the New Year, Harry shares some key factors he wishes customers would consider before they send their files.

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2019

Fresh PCB Concepts: Designing a PCB for Telecom Applications

12-12-2019

Jeff Beauchamp and Harry Kennedy discuss PCBs for telecommunication applications, including key factors to consider, such as design and material considerations. They also recommend involving your PCB supplier at the time of design to help ensure manufacturability at the lowest possible cost.

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Fresh PCB Concepts: The Current Material Situation

11-11-2019

We have all heard about the component crisis in the circuit board industry, and maybe you heard about the CCL shortage, but how many are aware of the bare board material shortage? Ruben Contreras explains the current material situation and tips to address this issue.

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Fresh PCB Concepts: Getting It Right From the Start

10-23-2019

When faced with critical time-to-market situations, it is all too easy to say, “It doesn’t matter because this is just the prototype; we can fix this later.” However, if the design is perfected from the beginning, cost savings can be applied, and manufacturability can be ensured. Perhaps most importantly, the design can be adapted with reliability in mind, leaving a seamless transition from prototype to production. How do we get it right from the start?

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Fresh PCB Concepts: Why Material Selection Matters

10-02-2019

When you’re designing a PCB, it’s standard to call out FR-4 material, but you could be holding yourself back or even exposing your board to risk by not knowing more about PCB materials. Let’s take a small look into why. What Is FR-4, exactly? Harry Kennedy of NCAB explains.

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Fresh PCB Concepts: Qualities of Medically Applied PCBs

08-26-2019

In this inaugural column from NCAB Group, Alifiya Arastu discusses details of PCBs used in medical applications, highlighting some of the differences in terms of demands and how the design must be handled.

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