Fresh PCB Nuggets: How Do You Calculate Finished Copper?

NCAB_RubenContreras.jpgHow do you calculate finished copper on a PCB? This may sound simple. But we have seen copper thickness called out either on the drawing or the specification, which can lead to additional engineering questions (EQs) and, in some cases, additional costs. These are unintentional results as a misunderstanding of what is requested—not in all cases, but some. Let’s start with some terminology.

Ounces relates to the weight of copper that covers one square foot (Figure 1). The area of the board itself cannot change; it’s fixed. But as the weight of copper increases, so does the thickness of the copper deposit. What does ½ ounce, 1 ounce, or 2 ounces really mean? There is a perception that ½ ounce equals 17 microns (0.669 mils), 1 ounce equals 35 microns (1.377 mils), and 2 ounces will give you 70 microns (2.756 mils). While that’s pretty close, it’s not strictly true.

NCAB_column_0820_fig1.jpg 

Figure 1: A sheet of copper.

This brings us to IPC—specifically IPC-6012, which is the qualification and performance standard for rigid PCBs. Let’s start with the internal layer foil thickness (Table 1).

NCAB_IPC_6012-Table-3-13_650.jpg 

Table 1: Table 3-13 in IPC-6012. (Source: IPC)

If we consider 1 ounce, we can see that this actually equates to 34.3 microns, (1.350 mils) as the target for the foil manufacturer. However, this also allows for a 10% tolerance on the produced foils, which means that a 1-ounce inner layer foil could be received at the PCB factory with a thickness of 30.9 microns (1.217 mils). From there, we also see that there is a 6-micron (0.236-mil) reduction allowed for the pretreatment of the copper foil during the process at the PCB factory before imaging and bonding. After processing, the minimum copper foil thickness for 1 ounce of copper foil may be 24.9 microns (0.980 mils), which is a little different than the 35 microns perceived.

Where we see the potential challenges is when we see the thickness for inner layers specified as 35 microns minimum. This means that to achieve the 35 microns (1.377 mils) as the foil thickness after processing, we would need to start with 2 ounces, which would provide 55.7 microns (2.193 mils).

Let’s now look at external or outer layers, where this becomes a little bit more complex. As the industry standard, IPC-6012 relates to base copper weights rather than finished copper (Table 2).

NCAB_IPC_6012-Table-3-14_650.jpg 

Table 2: Table 3-14 in IPC-6012. (Source: IPC)

IPC Table 3-14 shows us that the minimum surface conductive finish of the finished copper is equal to the absolute minimum copper foil thickness, plus an average copper plating thickness of 20 microns (0.787 mils) for Class 2, and 25 microns (0.984 mils) for Class 3, minus the maximum processing reduction allowance. Let’s see what happens when a customer calls for 1 ounce finished.

We know that IPC minimum for 1-ounce copper weight equates to 30.9 microns (1.217 mils). If we start with ½ ounce, 15.4 microns (0.606 mils), plus plating of 20 microns (0.787 mils) for Class 2 or 25 microns (0.984 mils) for Class 3, and then allow for process reduction of 2 microns (0.079 mils), we can see that we will finish with 33.4 microns (1.315 mils) for Class 2 and 38.4 microns (1.512 mils) for Class 3.

This is pretty clear, and again, close to the 1-ounce (35-micron) perception. But this is a different story as copper weights increase. If we look at the 2-ounce (70-micron), this is based on the perception that we have 35 microns to start, plus an additional 35 microns or 1-ounce base plating. If we start with 1 ounce or 30.9 microns (1.217 mils), plus plating of 20 microns (0.787 mils) for Class 2 or 25 microns (0.984 mils) for Class 3, and then allow for processing reductions of 3 microns (0.118 mils), we will finish with 47.9 microns (1.886 mils) for Class 2 or 52.9 microns (2.083 mils) for Class 3.

When we see drawings that call for a specific thickness such as 70 microns (2.756 mils) or 70 microns minimum, we have to start with 2-ounce base copper foil and plate up to achieve a finished thickness of 78.7 microns (3.098 mils) for Class 2 products or 83.7 microns (3.295 mils) for Class 3.

This has a notable impact on costs as there is a difference when quoting with a 1-ounce copper base or 2 ounces. This is why you may see questions from your PCB supplier as we seek to clarify what copper thickness you need, and what base copper thickness we should be starting with. We always want to help you avoid any unneeded additional costs in your PCB design. And, as always, if there are any questions on this or anything to do with your PCB design, I am happy to help.

Ruben Contreras is a program manager and field application engineer at NCAB Group.

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2020

Fresh PCB Nuggets: 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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04-09-2020

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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

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