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Target Condition: Rethinking the PCB Stackup Recipe

Marie Antoinette is attributed with saying, “Let them eat cake,” but historians now agree she likely never said it. It was probably revolutionary propaganda to paint her as out of touch with the starving masses. Yet, the phrase still lingers, and oddly enough, it applies to the world of PCB design.

Today’s designers, in their well-intentioned pursuit of signal integrity and performance, are writing stackup specifications that resemble high-end pastry recipes. But are we forgetting the realities of offshore fabrication and the limitations of the PCB “bakers” who have to make these designs work? Let’s revisit the classic cake metaphor for stackups, explore why the recipe approach is falling flat, and discuss how we can return to performance-based design that satisfies everyone at the table.

The Layer Cake Stackup Metaphor
The layer cake analogy for PCB stackups isn’t new. Designers often describe the stack as a baked cake: cores and prepregs as cake layers and sticky frosting, held together through pressure and heat. It's simple and helpful until it becomes too specific. When designers call out exact materials—brand names, resin systems, weave styles—and forbid any substitutions, they’ve crossed from “baking” into “gourmet patisserie.” The result? A tasty metaphor gone stale in the fab shop.

Five Basic Ingredients of a Stackup
A standard cake recipe has five essential components: flour, eggs, liquid, leavening agent, and flavoring. Similarly, a basic signal-integrity-conscious stackup includes five electrical ingredients: laminate thickness and dielectric constant, trace width, trace thickness, prepreg thickness, and solder mask and dielectric constant.

Like any well-balanced recipe, the proportions matter more than the brands. In baking, using locally available ingredients that meet these requirements can still produce the desired outcome.

Impedance Calculators: The Measuring Cups of PCB Design
Web-based tools such as impedance calculators help designers ensure that trace widths, spacings, and dielectric constants result in the desired impedance. These tools are akin to measuring cups. You don’t need to know the cow that produced the milk, just that you added one cup. Yet, designers often overlook this simplicity and hard-code every detail into the stackup, leaving fabricators little flexibility to adapt.

When the Recipe Gets in the Way
Designers sometimes hand over stackups that read: “Use Isola 370HR with 106/2116 prepreg. No substitutes.” Then the engineering queries (EQs) roll in.

Your EMS provider might receive an email from the fab saying, “Can we quote an equivalent material? That laminate isn’t available in our region.” That’s because many offshore suppliers, particularly in Asia, don’t stock Western-brand laminates like Isola, Nelco, or Rogers. Instead, they use regionally sourced equivalents. That’s fine if the electrical performance still meets your target specs.

You Asked for Chocolate Cake, Not an Eight-Layer Sachertorte

Overly rigid stackups are like ordering a chocolate cake with these conditions:

• Non-GMO cacao from Ecuador
• Only grass-fed Irish butter in the frosting
• Must be German, not Belgian chocolate 
• Hand-delivered at 22°C by Paula Deen

Imagine asking a bakery in another country to pull that off. This is how fabricators feel when they receive stackups demanding a specific brand, resin system, and glass style with no room for substitution.

Define the Outcome, Not Every Ingredient
So how do we get a cake that rises—or a PCB that performs—without constraining the chef? By defining target impedance, trace dimensions, and tolerances, not laminate brands. Here’s an example of a smart fabrication note:

Impedance Requirements:

• L1 & L6: 60Ω ±10% single-ended, 0.0061" wide
• L3 & L4: 60Ω ±10% single-ended, 0.0051" wide
• L3 & L4: 100Ω ±10% differential, 0.0045" wide / 0.0095” spacing

(Note: SI trace widths taken to four-place decimal values to make them easily identifiable.)

Then, follow with a generic stackup—enough to communicate the intent, without locking the fab into a specific supply chain.

When Are the Exact Ingredients Necessary?
Sometimes you really do need the exact materials. The aerospace, automotive, and military sectors often require traceability and rigorous qualification. In those cases, lock down your materials but expect longer lead times and higher costs. However, for most commercial high-speed work, those constraints aren’t necessary. Letting fabricators choose equivalent materials while still meeting your electrical requirements keeps your project moving.

Conclusion: Let Them Bake the Cake
PCB design isn’t about controlling every last grain of flour; it’s about delivering performance. Trust your fabricator (your pastry chef), to use what they have locally, as long as the end result meets your specs. The real goal? A board that performs as expected, built on time, and within budget.

So, the next time you’re tempted to call out every last prepreg and dielectric, take a step back. You’re not baking from Grandma’s sacred recipe; you’re working with global manufacturing partners. Let them bake the cake, because keeping your order simple will get it on your dessert tray or your assembly line almost tout de suite.

This column originally appeared in the September 2025 issue of Design007 Magazine.