Connector drawers and card guides were built around roughly 1.6 mm boards long before your latest stackup spreadsheet existed. That is why 1.57-1.60 mm (0.062 in) still dominates rigid FR-4 defaults - and why blindly keeping it can still be wrong for impedance, weight, or chassis gap.
Finished thickness is copper + core + prepreg + mask/finish. Layer count, copper weight, and dielectric menu move the number. Tolerance bands (often on the order of +/-10% depending on construction) matter for press-fit and tight mezzanine connectors.
Common thickness bands and why they exist
Ultra-thin constructions (about 0.4-0.8 mm) serve compact consumer and some RF modules but warp and handle more carefully. Mid values (1.0-1.2 mm) appear when profile and layer count compromise. The 1.6 mm class covers general digital, many industrial controls, and broad connector ecosystems. Thicker boards (2.0-3.2 mm+) show up in power electronics, backplanes, and mechanically harsh environments.
Availability and price favor the fab's standard menu. Exotic finished thicknesses can add lead time even when electrically elegant.
Decision drivers beyond habit
Mechanical: edge-connector engagement, enclosure clearance, vibration stiffness, and PCB weight. Electrical: dielectric height for impedance, creepage/clearance needs, and isolation between dense planes. Thermal/current: heavier copper and thicker boards can help power products but complicate drilling aspect ratios.
More layers do not automatically mean a thicker finished board - dielectrics can be chosen to hold near 1.6 mm - but aggressive isolation and stiffness often push thicker.
Impedance coupling
Trace width calculators assume a dielectric height. Changing finished thickness without revisiting stackup invalidates controlled-impedance models. If USB, Ethernet, or RF lines are on the board, thickness is an SI input, not only a mechanical callout.
Application sketches
Handheld consumer: thinner, watch warpage. Industrial power: thicker, heavier copper. Automotive control: often near standard thickness with high-Tg materials. LED modules: thickness plus metal-core choices for heat. Medical portables: thin-to-mid with reliability class notes.
How to specify without drama
State finished thickness target and tolerance, copper weights per layer, and whether connectors drive the number. Ask for a proposed stackup drawing before layout freeze. Symmetry fights bow; odd experiments fight yield.
Related reading: multilayer PCB and impedance control PCB. Include thickness and stackup targets when you place an order.