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52 DESIGN007 MAGAZINE I SEPTEMBER 2025 TA RG E T C O N D I T I O N Similarly, a basic signal-integrity-conscious stackup includes five electrical ingredients: lami- nate 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 im- pedance. 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 substi- tutes." Then the engineering queries (EQs) roll in. Your EMS provider might receive an email from the fab saying, "Can we quote an equivalent mate- rial? 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 defin- ing target impedance, trace dimensions, and toler- ances, 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 ▼ Ty p i c a l P C B st a c k u p d raw i n g i nfo r m at i o n i n c l u d e s st a r t i n g c o p p e r we i g ht , b u t l e ave s fa b r i c ato r w i g g l e ro o m w i t h i n t h e s u p p l y c h a i n .