Book Excerpt: 'The Printed Circuit Designer's Guide to... Designing for Reality,' Chapter 2

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This is an excerpt from Sunstone Circuit's book: The Printed Circuit Designer's Guide to... Designing for Reality by Matt Stevenson

Chapter 2: Variations in the Manufacturing Process

Once a CAD tool is chosen and the basics understood, the thought process should then shift toward the manufacturing of the PCB, before applying electrons to the screen. Let’s get the “bad news” out of the way first. Perfection is an unattainable goal. Don’t expect manufacturing to be perfect. There will always be variations from optimum at every step of every manufacturing process. Designers should recognize that variation and physical challenges within the PCB manufacturing process can be exacerbated by their design decisions. Not to worry, this is easily overcome with forethought during design and a good manufacturing partner.

According to the law of variation as defined in the Statistical Quality Control Handbook, 2nd Edition (January 1982):

• Everything varies. In other words, no two things are exactly alike.
• Groups of things from a constant system of causes tend to be predictable. If outcomes from systems can be predicted, then it follows that they can be anticipated and managed.

In a recent post for bizfluent, Beth Robinson identified the five main sources of process variation: raw materials, equipment, human actions, environment, and method.

Raw Materials
There will be variation in the output of all manufacturing processes and all suppliers.

The materials required in a manufacturing process come most often in either the form of a manufactured material or ore. These suppliers of manufactured material will experience variation during their production process, so there will inevitably be slight inconsistencies in the materials they supply. This can have a cascading effect, creating variations during PCB production and potentially impacting board performance. Raw materials like copper don’t always come from the same source, especially during times when supply chains are compromised (Figure 2.1). Manufacturers and designers may need to consider variations in purity and how they will affect the manufacturing outcome. There are specifications for most of these materials (copper cladding, laminate, etc.) that limit the variation to an agreed upon, acceptable range.

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